STUDY OF RADIATION HARDNESS OF OPTICAL FIBERS

by

RAYBURN D. THOMAS, B.S.

A THESIS

IN

PHYSICS

Submitted to the Graduate Faculty of Texas Tech University in

Partial Fulfillment of the Requirements for

the Degree of

MASTER OF SCIENCE

Approved

Chairperson of the Committee

Accepted

Dean of the Graduate School

August, 2004

ACKNOWLEDGMENTS

1 wish to thank Dr. Nural Akchurin for giving me a chance to prove myself and for

never being satisfied. It was your constant motivation and genuine interest in Physics

which pushed me to succeed. Also, Dr. Lynn Hatfield and Dr. Richard Wigmans,

throughout my career as a student, both of you have believed in me, pushed me, and

inspired me. I would like to thank Kenneth Carrell for all of the discussions, simulations,

and most of all for your friendship. Thanks to my family who supported me in every way

possible. Dillon, you were always so understanding when I could not play and you have

helped me in more ways than you may ever know. Thank you Dawn, for embracing me so

I could fly, for taking a genuine interest in my work, and for your faith: in God, in me, and

in us. 1 could not have made it without you.

TABLE OF CONTENTS

ACKNOWLEDGMENTS ii

ABSTRACT v

LIST OF TABLES vi

LIST OF FIGURES vii

CHAPTER

I. INTRODUCTION 1

1.1 Background 1

1.2 Present Understanding of Radiation Damage 3

1.3 Applications 6

1.3.1 CMS Hadronic Forward 6

1.3.2 Dosimetry 7

1.3.3 Diagnostics 8

n. EXPERIMENTAL SET-UP 9

2.1 VandeGraaff 9

2.1.1 Dose Measurements and Estimates 14

2.3 PMT Laser 20

2.3 Spectrometer - Xenon 22

m. ANALYSES 25

iii

3.1 Optical Damage Studies 25

3.2 Optical Recovery Studies 37

IV. CONCLUSIONS 44

BIBLIOGRAPHY 48

IV

ABSTRACT

Optical fiber manufacturing is a multibillion dollar industry today, and optical

fibers have found diverse applications, such as telecommunication, medicine, nuclear and

chemical industries, and many others. There is no doubt that this trend will continue.

Fused-silica core optical fibers are being used in applications where there is high ionizing

radiation and/or neutron fluence, especially because fused-silica is a radiation hard

material. The radiation damage to optical fibers is a complicated phenomenon and the

exact nature of color centers formation is not well-known. In this study, we attempted to

address a few but fundamental questions currently studied in the community:

(1) How radiation hard is the polymer-clad, fused-silica core optical fiber? How

does the induced attenuation depend on the wavelength of light transported by

the fiber?

(2) How does the radiation dose rate affect the total optical transmission? What is

the wavelength dependence of this phenomenon?

(3) Is there a dynamic but stable transmission recovery in these types of fibers?

(4) What are the implications of the obtained results to the performance of the

CMS forward calorimeter at the Large Hadron Collider?

LIST OF TABLES

2.1 The average stopping power (MeV cm^/g) of relevant materials. 11

2.2 Summary of data used for dose estimation. 18

3.1 List of the seven different sets of data used in this study. 27

3.2 Parameters of the power law fit for spectrometer data. 31

3.3 Transmission percentages for the five dose rates and accumulated doses using two different measurement techniques. 38

3.4 Parameter values for seven different recovery data sets 41

VI

LIST OF ITGLTRES

1.1 Optical attenuation induced from radiation damage 4

1.2 Absorption and transmission of known color centers 5

2.1 Experimental setup for irradiation of optical fibers 10

2.2 Energy loss of electrons in air 12

2.3 Cherenkov threshold beam energy calibration 12

2.4 Beam current monitor calibration data 13

2.5 GEANT4 simulation of electron energy deposition in

aluminum 15

2.6 GEANT4 simulation of electron energy deposition in air 16

2.7 GEANT4 simulation of electron energy deposition in optical fiber 17

2.8 Simulation of electron beam interactions 19

2.9 Experimental beam profile 19

2.10 Diagram of the Laser-PMT experimental setup 21

2.11 Traces of OTDR pulses and gate 22

2.12 Diagram of the spectrometer - xenon setup 24

3.1 Optical damage and recovery data 25

3.2 Joined optical damage data sections 26

3.3 Optical damage curve fitfing 29

\ i i

3.4 Optical damage fit comparison 30

3.5 Optical damage extrapolations for 7 Rad/s and 230 Rad/s 32

3.6 Optical damage extrapolations for 1 kRad/s and 5 kRad/s 32

3.7 Optical damage extrapolations for 10 kRad/s 33

3.8 Combined optical damage spectra 34

3.9 Combined optical damage spectra plotted in 2 - dimensions 34

3.10 Optical damage profiles for 400 and 426 nm 35

3.11 Optical damage profiles for 450 and 500 nm 36

3.12 Optical damage profiles for 600 and 630 nm 36

3.13 Fit parameter P2 comparisons 37

3.14 Fit parameter P3 comparisons 38

3.15 PMT - laser setup damage extrapolations 39

3.16 Typical recovery data set and fit 40

3.17 Plot of the percent difference of recovery vs. total

accumulated dose for 1 kRad/s dose rate 43

3.18 Plot of the percent difference of recovery versus total accumulated dose for 5 kRad/s dose rate 44

3.19 Plot of the slope of the percent difference of recovery versus dose rate for 10 (plot a) and lO"* (plot b) seconds of recovery time 44

vm

C H A P T E R I

INTRODUCTION

1.1 Background

In 1870, John Tyndall demonstrated the principle of total internal reflection to the

British Royal Society. He demonstrated how light could be directed around curves by

directing light down a stream of poured water. In 1880, Alexander Graham Bell showed

how light could be used to transmit information with the advent of his photophone. The

concept of using light to send communication signals, the principle of total internal

reflection, and the development of high purity glass were combined in the mid-to late-

1900s into the field of fiber optics (Polymicro Technologies, 2000).

The field of fiber optics has undergone many technological advances since its

inception in the 1950s. A preform is the large stock material from which the fibers are

made and there are several different methods utilized for the manufacture of the preforms

used to draw fibers. Some of the methods are: modified chemical vapor deposition

(MCVD), Plasma Outside Deposition (POD), Plasma Chemical Vapor Deposition

(PCVD), and many others. Utilizing these methods, a large glass stock is built up from

different materials. The materials chosen depend on the qualities desired, but the bulk of

the material usually is silica dioxide (SiOi). By adding dopants, it is possible to impart

various properties to the material, such as a graded index of refraction, fluorescence, or

radiation hardness. Also it is possible to vary the Hydroxyl or OH" content of the fiber.

Once the preform is made with the desired qualities, it is next drawn into a thin

fiber. The preform is hung at the top of a drawing tower, where intense heat is applied to

the bottom of it undl it begins to melt. As the material melts it falls and is grasped and

pulled like taffy. By controlling the heat and tension on the strand, it is possible to draw

a very precise core diameter. As the fiber is drawn down the tower it passes through

several stages. The core material is usually coated with a material which is slightly less

dense to provide total internal reflecfion. This coating is referred to as the cladding of the

fiber. In many cases, it is desirable to apply a protective layer to the fiber. This layer is

referred to as the buffer. If the fiber is to be coated, it passes through a small cup where

1

material is deposited on the surface of the fiber by contact. The material is then cured

with heat or UV lamps. The process is controlled by a computer which relies upon

readings from several laser micrometers to control the temperature and drawing speed.

Once the fiber is rolled on a spool, it is ready for final qualification and shipment.

The attenuation in an optical fiber is usually expressed in decibels per kilometer

(dB/km) in the visible and IR spectra and in dB/m in the UV because UV light is

attenuated very quickly. The attenuation is expressed as.

A(^) = - ^ l o g P,M))

(1.1)

where L is the length of fiber, P,„ is the input power injected into the fiber core, and Pom

is the output optical power collected from the distal end. It is possible to manufacture

opfical fibers with attenuations of less than 1 dB/km today for infrared light. One dB/km

corresponds to 79.4% transmission of the input power after it has traveled 1 km. This is a

great improvement over the 1000 dB/km, or 0.1% transmission after 1 meter, which was

recorded in 1960 (Polymicro Technologies, 2000).

Depending upon the methods used in the manufacturing process, many defects

may be incorporated into the fiber prior to any irradiation. Due to the different thermal

methods used for the material synthesis (H2/02 flame, dry plasma torch, wet plasma

torch), different types of silica glasses will be obtained. This is due to the residual

amount of water contained in the glass, thus the terms "dry" and "wet" silica. One such

defect is a strong optical absorption band at 630 nm, which is introduced when dry silica

is drawn into a fiber. The drawing-induced absorption was reported by Kaiser et al.

(Kaiser, 1973, 1974), who observed that the band occurs only in fibers drawn from dry

silica, that it may decay at room temperature with time, and that it can be annealed out in

3 hours at 700°C (Friebele, 1976). Based on the similarity between this drawing-induced

band and a 625 nm band created in alkali silicate glasses by ionizing radiation. Kaiser

proposed that the drawing-induced absorption arises from holes trapped at the site of

broken Si-O bonds. This drawing-induced optical absorpfion band is identical in half-

width and center position with a radiation-induced band. Neither the optical absorption

band nor the Electron Spin Resonance (ESR) spectra of the drawing-induced defect

centers were observed in fibers having a core of wet silica (Friebele, 1976). UV

transmission and radiation-induced attenuation can be influenced by variations in fiber

coating, draw temperature, draw tension, and speed (Lyons, 2001). It is suggested that

the variation in radiation response may be due to a post-drawing anneal occurring during

coafing cure, which minimizes drawing-induced defects (Barnes, 1987). While the cause

of this optical absorption band is not fully understood, it is evident that the drawing

conditions can strongly impact a fiber's performance.

Other drawing defects occur due to the addition of dopants, impurities, and

solarization incurred due to intense ultraviolet (UV) light. Solarization is a change in

material characterisfics due to illuminafion of a material with UV ("solar") light. High

intensities of UV illumination can cause photo-thermal damage in quartz optical fibers,

dramatically increasing the scattering and attenuation. These drawing-induced

attenuations are generally small compared to the effects of radiation.

1.2 Present Understanding of Radiation Damage

Radiation hardness is a measure of how well an optical fiber maintains its optical

transmission properties after receiving some dose of radiation. Radiation damages an

optical fiber in many different ways. There are many different types of color centers

known to exist in irradiated and un-irradiated fibers. A center, or color center, is a lattice

defect in a crystalline solid consisting of a vacant negafive ion site and an electron bound

to the site. Such defects will absorb light and make certain, normally transparent, crystals

appear colored. This means that the light transmitted may differ in wavelength from the

input light due to a color center's presence. The so-called E'-center, comprising a silicon

atom with valence electrons, forms upon a radiation induced breakage of the Si-O-Si

chain and is characterized by luminescence at 450 nm in addition to the absorption band

at 212 nm. There is the non-bridging-oxygen radiation defect (NORD), also known as

non-bridging-oxygen hole center (NBOHC), which exhibits strong absorption bands at

260 and 630 nm and has a luminescence line at 670 nm. This 630 nm absorption band is

well evidenced by Figure 1.1. It should be noted that the absorption spectra shown in

3

Figure 1.1 were taken several days after the irradiation occurred. This is important

because the absorption at 630 nm is very long-lived, whereas many others effect

transmission on much shorter time scales. There is the Peroxyradical, oxygen vacancy,

and many other color centers which are shown in Figure 1.2 (Gavrilov, 1997). Although

many of these color centers have been quantified, the production mechanisms are still not

well understood.

A n

Rel

ativ

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ttenu

atio

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B/m

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J b

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c

u.u -3-

Spectral Attenuation QP600

70 420 470 520 570 620 6" Wavelength (nm)

— 120MRad

— SOMRad

— 20MRad

•0 7< 10 770

Figure 1.1. Optical attenuation induced from radiation damage. Plot shows the spectral attenuation of three identical Polymicro optical fibers after irradiation to three different accumulated doses. The strong absorption peak is evident around 620 nm.

The primary method to study the presence of color centers is Electron Spin

Resonance (ESR). ESR is a technique applicable to the wide variety of substances which

exhibit paramagnetism because of the magnetic moments of unpaired electrons. The

spectra obtained from this technique are useful for detection and identification, for

determination of electron structure, for study of interactions between molecules, and for

measurement of nuclear spins and moments.

As previously stated, it is possible to vary the OH' content of a fiber during the

drawing process. The OH" content directly affects the radiation hardness of optical fibers

due to the abundance, or lack of. Oxygen available in the material. The coating is

important because it can help the core material maintain this high OH" content. If the

coating is very thin, a few microns, or if it is composed of a highly diffusive material, the

Oxygen can easily diffuse out of the core material upon irradiation (Brichard, 2002).

This diffusion facilitates the production of the Nonbridging Oxygen Hole Center

(NBOHC).

E.eV

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1 Figure 1.2. Absorption and transmission of known color centers. This Figure shows the majority of the known color centers and their relative absorption and transmission energies (Gavrilov, 1997).

5

While it is possible to quantify the amount of optical damage a fiber receives as a

function of dose and also to predict how much light will be transmitted after some dose is

incurred, the mechanisms, at a molecular level, are unclear. Many workers have used

ESR data to model radiation damage using known theoretical models. While some of

these models can reproduce the optical damage profiles quite well for a given type of

fiber, none has been able to predict the optical damage for all fibers. The most widely

used formulation is the power law dependence of attenuation to accumulated dose. The

power law does well for giving the saturation level, but does not accurately describe the

optical damage during the initial dose period. The recovery phenomenon is also not well

described.

1.3 Applications

There are many diverse applications for optical fibers today. Due to the relative

infancy of the industry, people find new applications for this technology almost daily.

We started trying to understand the radiation hardness of optical fibers due to their use in

the Compact Muon Solenoid (CMS) Hadronic Forward calorimeter (HF). Radiation

hardness is of interest to many different industries that are using optical fibers in high

radiation environments for non-conventional applications. Some of these applications are

discussed here.

1.3.1 CMS Hadronic Forward Calorimeter

The CMS-HF utilizes over 1000 km of quartz optical fibers to detect particles in

the Large Hadron Collider (LHC). Quartz fibers were chosen for their radiation hardness

properties. The HF will experience unprecedented particle fluxes. At certain locations,

the HF could receive more than 1 GRad of radiation over its 10 year lifetime. More

importantly, the damage will not occur uniformly over the entire structure. Some

portions will receive far less radiation, -100 kRad in 10 years. This widely varying

degree of radiation levels requires some means of calibration to insure that the signals

recorded do not vary over time. This is one reason we have studied this phenomenon.

6

The detector will also experience times when no particle flux is present. During this time

the fibers will undergo an optical recovery which must also be understood and modeled.

An online monitoring system, utilizing a laser pulse into a sample fiber to measure the

amount of radiation damage has been installed. A laser will only give an approximate

calibration because it will only give the optical damage profile for one wavelength. The

PMT's used in the readout boxes are not only sensitive to one wavelength of light but a

wide range (350-500nm), with differing quantum efficiencies. It will be necessary to

understand the profile of the optical damage at all wavelengths to make a more accurate

calibration.

1.3.2 Dosimetry

In fission reactors, and other high radiation environments, it is necessary to monitor

the amount of radiation produced. The materials currently used for dosimetry saturate

quickly under the intense radiation conditions in these environments. Most dosimeters

saturate after few hundred Rads, or at most a few kRad. Optical fibers could be used in a

wide variety of dosimetry applications with reliability into the MRads or possibly GRads.

Fluence and dose measurements of neutrons with optical fibers can increase the security at

nuclear engineering facilities and can enable new solutions at fusion reactors where high

voltages and high, fast changing magnetic fields often prevent the use of copper cables

(Henschel, 1997). One task of fiber optic dose sensors could be the detection of radiation

dose levels along accelerator sections, caused by the dark current due to field emission in

super-conducting accelerator cavities and RF laser guns, or by beam losses at certain

positions (Henschel, 2001). If the nature of the fiber was well understood it would be

possible to measure the light transmission through the fiber at specific wavelengths to

quantify the amount of dose it had received. Some of the color centers are of particular

interest for this application because of their near linear response.

1.3.3 Diagnostics

It has been proposed to use optical fibers for the diagnostics of beam lines in

accelerator facilities. The optical fibers could be used as the sensing medium for the

detection of various problems in an accelerator facility. This method would involve

placing fibers on the outer circumference of the beam pipe. By doing so, whenever an

interaction occurs in the pipe and a charged particle is produced it will generate a signal in

the fiber through Cherenkov radiation. One could use the luminescence light

(predominantly Cherenkov light) that will be generated in optical fibers during high

radiation emission from a certain accelerator section to initiate a rapid switch off, in order

to prevent serious damage during longer adjustment periods (Henschel, 2001). This

signal could be detected at some central location where it would then be possible to discern

the location of the interaction based upon the amount of time it takes the signal to travel in

opposite directions. If the signal took equal times to travel from left and right the event

must have taken place on the exact opposite side of the accelerator and so on. This may

prove to be a useful tool to determine the location of vacuum leaks, poor focusing, etc..., in

accelerator facilities.

C H A F T E R II

EXPERMENTAL SET-UP

2.1 VandeGraaff

Texas Tech University's electron Van de Graaff accelerator produces beam currents

up to 200 \iA within the energy range of 100 keV to 2.5 MeV. The accelerator was first

installed in 1971 by High Voltage Engineering Corporation but it was not operated for

thirteen years, from 1989 to 2002. In December of 2002, it was refurbished by the High

Energy Physics Group in order to study the radiation damage phenomena in optical fibers,

and as a source for relatively high energy electrons for detector research and development.

In this section, the main features of the Van de Graaff accelerator are described, since

precision in beam energy and current play a crucial role in the studies discussed in this

thesis.

We measure the beam energy with a generating voltmeter and compare it to a

Cherenkov threshold detector. The generating voltmeter is a capacitive pickup which is

connected to a resistor across which, voltage is measured. The voltmeter located on the

control panel gives an accurate measure of the potential difference that an electron

experiences in the accelerating column. A Cherenkov threshold detector is optical fiber

coupled to a PMT. When a charged particle traverses the core material of the fiber with

sufficient kinetic energy to break the local speed of light, photons are released (Cherenkov

radiation) in the material and the captured photons propagate in the fiber to the PMT. The

index of refraction for the fiber core is 1.457 (synthetic fused silica), which gives a

Cherenkov threshold of 191 keV. It is possible to check the energy of the beam by

determining the distance away from the beam-pipe exit, at which this threshold occurs. In

our experimental setup, the beam exits the pipe and travels some distance in air to the fiber

(see Figure 2.1).

As the electrons traverse this distance, they loose energy primarily by ionization.

By calculating the energy loss in the window of the beam-pipe, air, and the buffer and clad

of the fiber, it is straight forward to determine the location along the beam direction at

which the Cherenkov threshold is crossed for a given beam energy. 300 keV electrons lose

energy at the rate of 4.99 MeV/cm in aluminum. For the 100 /^m-thick aluminum beam

pipe window, the average energy loss is 49.9 keV. Similarly electrons lose on average 2.74

MeV/cm in the acrylate buffer and cladding, and 2.7 keV/cm in air (see Table 2.1).

Figure 2.1. Experimental setup for irradiation of optical fibers. Electrons penetrate the aluminum window on the end of the beam pipe and travel to the fiber coil in air over a distance of 1 meter. The beam stop is a lead brick on a pneumatic piston which allows us to quickly start and stop the irradiation. A Faraday cup is placed at the location of the fiber coil to provide a relative measurement of beam current. Copper tubing is used to shield the length of fiber necessary to transport the signals to PMTs and spectrometers which are located under the 3-inch-thick steel table for shielding purposes.

10

Table 2.1. The average stopping power (MeV cm^/g) of relevant materials. The relative densities are used to find the amount of energy lost per cm. The relative densities (g/cm^) are as follows: aluminum = 2.6989 g/cm^ air = 1.20479E-3 g/cm^ buffer/cladding = 1.06g/cml

Material

Kinetic Energy (keV)

100 125 150 175 200 250 300 350

Aluminum (MeV cm^/g)

3.185 2.789 2.521 2.328 2.183 1.981 1.849 1.757

Air (MeV cm^/g)

3.637 3.176 2.865 2.642 2.474 2.241 2.089 1.984

Cladding/Buffer (MeV cm^/g)

4.038 3.523 3.176 2.926 2.739 2.479 2.309 2.192

As noted earlier, the Cherenkov threshold is 191 keV, however the electrons must have

sufficient energy to penetrate the buffer and cladding of the fiber. Since the cladding and

buffer absorb 29 keV, the electron kinetic energy must be 220 keV or above when they

reach the fiber to produce Cherenkov light. The threshold is reached at 11 cm from the

window for 300 keV electrons. The experimental data are in good agreement with this

calculation as illustrated in Figure 2.2 and 2.3. Figure 2.2 begins at 250 keV since 49.9

keV was lost in the aluminum window of the beam pipe. The electrons lose 29 keV in

11cm of air. This measurement provides a clear energy calibration method. Data for

stopping power of materials (£' <1.0 MeV) are taken from the ESTAR data (Agostinelli,

2003).

11

275-,

250-

225-

200-

1 ^ 175-J£, >. 150-S> 0) c 125-UJ •a 100-V c 5 75 -

50-

25-

0 -

250 keV Electron Energy in Air Denaily (g/cm") = 1.20479E-03

^. 220keV@11cm

• • • • .

1 • 1

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• • - .

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• • -' • . .

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• - .

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• \ • \

1 1 1 I 1 7 1 1 r 1 10 50 60 70 20 30 40

Distance (cm) Figure 2.2. Energy loss of electrons in air. A 250 keV electron loses energy in air as shown. The two marked points show the difference between the threshold location if the cladding and buffer were not considered (data taken from NIST-ESTAR).

1.0-|

0.9-

0.8-

0.7-

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m

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Location where signal drops to background (11 cm)

1 — 1 — 1 — [ - 1 1 1 1 ' 1 1 1 • 1 1 1 < 1

15 20 25 30 35 40 45 50 55

Distance f rom beam-pipe w indow (cm)

Figure 2.3. Cherenkov threshold beam energy calibration. Experimental data showing a sharp cutoff for 300 keV electrons. Data was recorded as the fiber was moved away from the aluminum window of the beam pipe.

12

The beam current was measured using a Faraday cup. The utilization of a Faraday

cup is destructive technique thus; we also measure the induced image current on an isolated

section of the beam-pipe (BCM). This measurement is very sensitive to beam energy and

alignment inside the vacuum pipe. The image charge is only used to provide a relative

measurement to the Faraday cup (see Figure 2.4). Similarly, it is also possible to measure

the amount of energy intercepted in the beam pipe window and infer the beam current. For

a given energy and beam current, some of the energy will be lost in the aluminum window

of the beam pipe. By measuring the amount of energy lost, it is straight forward to

calculate the number of electrons which were not intercepted by the window. The beam

current and energy are very reproducible (within 10%) if the control settings were solely

relied upon. The settings of the control panel are checked periodically to verify their

reproducibility.

Faraday Cup vs. Beam Current Monitor

3 <

O m

3 -

2 5 -

nl

^

W^i'

^

• 1.5 MeV • 1.75 MeV A 2.0 MeV

20 40 60 80 100 120 140 160 180 200 220

Faraday Cup Current (/L/A)

Figure 2.4: Beam current monitor calibration data. This data show the beam current monitor's (BCM's) linear response to the Faraday cup for three different beam energies. The BCM signal increases as the beam energy decreases because the diameter of the beam increases, which changes the capacitance.

13

2.1.1 Dose Measurements and Estimates

In order to verify our calculations and experimental results, we performed

simulations of the beam using GEANT4 (Berger, 2000). GEANT4 is a toolkit for the

simulation of the passage of particles through matter, and it is widely used in high energy,

nuclear, medical, accelerator and space physics studies. Figures 2.5 and 2.6 show the

GEANT4 results for energy loss in the aluminum window and air for electrons. The 0.2

and 0.5 MeV simulations are not in good agreement with the NIST data due to simulation

step size, geometry considerations and other corrections for low energy particles. The

GEANT4 simulation results are in good agreement for all energies at or above 1.0 MeV.

GEANT4 was mainly used to calculate the geometrical acceptance of the optical

fibers. In Figure 2.7, the number of entries shows the number of electron interactions out

of 100,000 beam electrons simulated. Mean value gives the average energy deposition in

the fiber coil. For example, the energy deposition in the fiber coil for 2.0 MeV electrons is

1.14 MeV. The fiber used was 800 |4.m in diameter which includes a 600 )xm core, 30 |j,m

clad and 70 |a,m buffer. The samples were 1.2 meters in length and wrapped into a 7 cm

coil. The fiber is placed 1 meter away from the beam-pipe window in air. All of these

dimensions were accurately modeled in the GEANT4 simulation. All electromagnetic

processes were included in the GEANT4 simulation: ionization, bremmstrahlung, Bhabha,

Moller, and multiple scattering. There are no events in the plot for 0.2 MeV Figure 2.7,

because these electrons do not have enough energy to reach the fiber. We found the

simulations to be in good agreement with experimental and NIST data for energies above 1

MeV. These comparisons were made from calculations of geometrical acceptance inferred

from the current produced in PMTs by electrons above Cherenkov threshold at several

distances. From the geometrical acceptance, beam current, and energy we can now

calculate the dose given to each fiber.

Figure 2.8 shows the simulated beam profile for electrons in air. This plot was

generated from a 2-D histogram of the interactions of the electrons with air. Figure 2.9

shows the experimental data recorded for the beam profile measurement. These data were

recorded by scanning an optical fiber through the beam in different locations along the

beam axis. The intensity of the beam was recorded at each position by measuring the

14

current induced in a PMT. From this current we estimated the geometrical acceptance of

the fiber by calculating the number of electrons required to produce such a signal and

compared these results to simulations. These data were combined to give a graphical

representation of the shape and intensity of the beam, (Figure 2.9). Figures 2.8 and 2.9 can

be compared in shape only. The data is representative of two different measurements.

Since the GEANT4 simulation represents electron interactions it is dependent upon energy

in that as the energy decreases so does the number of interactions. The experimentally

produced beam profile however, represents the amount of Cherenkov light produced in an

optical fiber. Since the same amount of photons is produced for any electron above the

Cherenkov threshold, we see a much larger region where the profile is the same.

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'"eVi*'. 333888' OLiotPsnat,: 0 f . lwn B r r . :

Q ( r 4 0 » Q c r i s o ;

b) 02 0 3 0 4 0,5

2 4 -I

22 •

2 0

IS -

1J6 •

1 4 -

12

i;a •

0.8 -

OS •

0.4 •

0 2

_

1 1

Entld. JZ66I I OLion^nQi; iaz3 { r-tejn acuaeot

2 0 MeV Energy Deposit ion fwindow)

00 0,1 OJ? 02 0.4 05

2,5 MeV Energy OepoBltion (window)

2 4 +

2.2 T

2.0 ••

1 .8-•

i.e --1 .4 -1.2 -1.0--0.8-f 0.6 -^1 0,4 -f 1 e)

1 e-twi *«Bia6 -10 i OisaiPinifi €ez

2.0 '

: . 4 -

2 0

I S -

1.6 -

1 4

1 2 -

1 0 -

0 3 -

O.O

0 4 -

01 -

1 fm . QOJBGOI

1 1 ^ * - , , 1

encigyi,l/te'.'|

OJ 04 0.5

Figure 2.5. GEANT4 simulation of electron energy deposition in aluminum. Figures show the energy deposited in 100 .m of aluminum for six different initial energies: a) 0.2 MeV, b) 0.5 MeV, c) 1.0 MeV, d) 1.5 MeV. e) 2.0 MeV, and f) 2.5 MeV.

15

0.2 MeV Energy Deposition (air)

2J600 -r

3.400 •

2 300

2.000

1 soo • •

1600

1 100

1 200 •

1dOO - •

8O0

600

40)

2(B

0 CO

E-*1«5 asBi OiXCfRanCK : 0 r.U>n : RTTL

acc9e3j a03OM1

I a)

0.5 10

1.5 MeV Energy Deposition fair)

40 000

35 000 •

30 000

2 5 0 0 0

20 0 0 0 -

15 000 -

10O00 -

SjJOO -

0 - )

'

E-*lM : 4-36+3

1 r.tedn : a'33ce9 ; JR-re : a 13369 \

•JL^ -

0 5 MeV Energy Deposition (air]

5

1.0 MeV Energy Deposition (air)

.10

1.2 -r

1.1

1.0

0.9 • •

o.a

0.-

0.6 -

0.5 - •

0.4

0.3

0.2 - •

0.1

0.0 0.0

E t 1 « : 3 n 0 3 OLtOfRangc : 0 r.lun Brr. :

a242*4 0 19634

b) 1 —

1,0

2.0 MeV Energy Deposition fair)

45.000 r E t l A : 4S-436 oaonRnoc : o r-lMH : R n t :

a3241-ai14.2l

ensig,. |/teVi

E-rtei : 43(164 I OOORjios : 0 I

r. tn: aizui j RTS: a 1*600 •

0.5 1.0 1,5

2.5 MeV Energy Deposition (air)

50 000 -

45000

40O00

35 000

XOOO

25 0 0 0 -

20000 -

15 000 -

10 000 -

5.000 -

0-1 0.

1 1 1 L 0 0.5 1,0

BtriK : 4S0G0 CUOfR JXK : 0 r. ton : Q'3266- !

R T C : 01044-3 J

f)

1.5 2 0

energy i l.l-'.'i

Figure 2.6. GEANT4 simulation of electron energy deoposition in air. Figures show the energy deposited in air for six different initial energies: a) 0.2 MeV, b) 0.5 MeV, c) 1.0 MeV, d) 1.5 MeV, e) 2.0 MeV, and f) 2.5 MeV. The number of entries varies greatly because many of the electrons do not penetrate the aluminum window at low energies.

16

1

2 6 0

2 4 0

2 2 0

2O0

IBO

l i O

1 4 0

1 2 0

1 0 0

BO

( 0

40

2 0

0

4 0 -

35

3 0

2 5

2 0

1 5 -

1 0 -

5 -

0 -0

------------

1

J

H ill ^1

~ 1 i^Biii^l -H11 Ih H H | ^ ^ ^

0 0 O S

' 1 w run i i j , i n WLir 0 0 5

2 . 5 M e V

1 0 1 5

1 M e V

1

1 1 0

Ei l t f i . ; M.aii

111,,

a)

} 0

Em HI'S 1-1.an Pnii

d)

1 5

I 4 D ' I 7

1311.

ft. j l

2 5

JES i>i:;5u : u 5 j

2 M e V

SO -

4 5 -

4 0 - -

3 5 -

3 0 -

2 5 -

2 0 - -

1 5 - h

1 0 -

^'IHILIMM i i 11 j

0 0 0 5 1 0

0 , 5 M e V

7 5 -

?o -6 5 -

6 O -

5 5 -

5 0 -

4 5 -

4 0 -

3 5

3 0 -

2 5 -

2 0 -

1 5 -

1 0 -

1 2 0

0 5 -

1

ml

II 111]

0 0 0 5 1 0

Enwi M'Fl

Rill.

b)

1 5

Ellin-

Rrii;

e)

1 5

) 1 111 ?,

1 1441

<i J|J!;45

2 0

11 i.;i:i5i iP5bii4I

2 0

5 5 -

5 0 -

45

4 0 -

3 5 -

3 0 -

2 5 -

2 0 -

1 5 -

1 0 -

5 -

0 -0

1 0 -

0 9 -

0 B -

0 7 -

0 6 -

0 5 -

0 4 -

0 3 -

0 2 -

0 1 -

0 0 ^ 0

0 0 5

0 0 5

1.5 M e V

1 0

0.2 M e V

I 0

Eii t f i*; bio

Pfii

1

c)

1 1 5

i)

1 5

'14 J 35'J

1 2 0

imii'i. 0 ('i-Aii n Pnii 11

7 0

Figure 2.7. GEANT4 simulation of electron energy deposition in optical fiber. Figures show the energy deposition in optical fiber and number of interacting particles for 100,000 events at a location of 1 meter from the beam pipe window for six different initial energies: a) 2.5 MeV, b) 2.0 MeV, c) 1.5 MeV, d) 1.0 MeV, e) 0.5 MeV, and f) 0.2 MeV. The sharp cutoff occurs because these electrons are completely stopped in the fiber and deposit a specific amount of energy. Figure f), the plot for 0.2 MeV, has no entries because the electrons do not have enough energy to traverse the aluminum window and the 1 meter of air before impacting the fiber.

17

Table 2.2. Summary of data used for dose estimation. Geometrical acceptance, or the fraction of electrons impacting the fiber at 1 meter, is taken from GEANT4 simulations.

Beam Energy(MeV)

Beam Current(//A)

Geometrical Acceptance

Average Energy Deposition(MeV)

Sample Mass(g) Number of Particles Striking Fiber per second

Energy deposited in sample per second(J)

Estimated Dose Rate Rad/sec

0.5 MeV

20.00

5.15E-05

0.10

1.53

6.43E+9

1.05E-04

7

1 .OMeV

10.00

5.70E-04

0.62

1.53

3.56E+10

3.53E-03

230

2.0MeV

10.00

1.28E-03

1.15

1.53

7.99E+10

1.47E-02

960

2.0MeV

50.00

1.28E-03

1.15

1.53

4.00E+11

7.34E-02

4800

2.0MeV

100.00

1.28E-03

1.15

1.53

7.99E+11

1.47E-01

9600

18

X Axis (cm) Figure 2.8: Simulation of electron beam interactions. This GEANT4 simulation of beam interactions is for 2.0 MeV electrons in air. The color scale units are arbitrary.

100

20 30

X Axis (cm)

40 50

Figure 2.9: Experimental beam profile. This profile is produced by passing an optical fiber through the electron beam in successive scans. The step-like features are due to the shape of the beam pipe opening. The units for the color scale are arbitrary.

19

2.2 PMT-Laser

Two different experiments were run in tandem to record as much information as

possible. The first was a laser and PMT set-up to measure the transmission of light at a

specific wavelength (423 nm Stilbene dye pumped with pulsed N2 laser at 337nm) and on

short time scales. All measurements were taken in-situ due to the dynamic nature of the

optical damage and recovery properties of optical fibers. We used four PMTs and a dye

laser with a Stilbene cuvette to give a short pulse of light at 426nm. The pulse length is 3-

5ns. This dye was chosen to provide light which is at the peak of the quantum efficiency

profile for the PMTs used. The PMTs used are Hamamatsu model R6427, which are 10

stage versions of the 8 stage ones used for the forward calorimeter of the CMS project. The

laser light is launched into one lead of a 3-way splitter. The two other ends of the splitter

are connected to PMTs. One PMT is used to provide a trigger for the data acquisition

system (DAQ) and the other is for the radiation profile which will be referred to as the

signal PMT (PMT 2). The signal PMT records the initial laser pulse and a reflected pulse

from the end of the irradiated sample of fiber. This is similar to a commonly used optical

time domain reflectometer (OTDR) set-up, (West, 1994). The difference is that we are not

interested in the time domain information but the magnitude of the reflected signal. The

coupled end of the splitter is connected to a delay line for separating the reflected signal

from the initial laser pulse for data acquisition purposes and then to the sample fiber. This

can be better understood from Figure 2.10. This leaves one end of the sample fiber free

and this is connected to a third PMT to record the transmission of light through the

irradiated portion of the fiber. A fourth PMT is used with a second sample fiber. No light

is launched into this fiber. It is used to record any background Cherenkov radiation from

the beam. The light produced from the beam is almost negligible for the PMT setup due to

the short integration time and the relative power of the laser.

20

The two sample fibers are placed in an electron beam from the Van de Graaff

electro-static accelerator. The beam current and energy are continuously recorded for dose

estimates during a run. Fibers were irradiated at various different dose rates and to

different total doses. All fibers were irradiated for some time and then allowed to recover.

Some fibers received multiple cycles of irradiation and recovery to allow us to observe the

in-situ optical damage profile and the dynamic recovery of this material. All measurements

were recorded using CAMAC, see Figure 2.10.

~4 pulses/sec.

Np/Stilbene @426 nm

[UseT Trigger

Deliy Coil Clierenkov

iDelay '-50ns

PMTs connect to ADC's and integrate signal over

330 ns.

Transmission

Shielding

Gate (330 ns)

Beam Pipe

2.0 MeV e

10-200 MA

Figure 2.10: Diagram of the Laser-PMT experimental setup. 426 nm light is launched into the fiber from the Laser which is split into three components by a 3-way splitter. Some light is directly reflected from the splitter to PMT 1, which provides a trigger, and PMT-2, which provides the signal for the OTDR method. PMT-3 gives direct transmission data. PMT-4 provides a measure of the Cherenkov background produced by the beam.

21

By delaying the two signal pulses with respect to the trigger it is possible to select

the portions of the pulse where the optical damage occurs. Pulses 2 and 3 have two peaks

(Figure 2.11). The first peak is a result of the laser being reflected off of the coupler which

connects the splitter to the sample. The second peak is the reflected laser pulse from the

opposite end of the damaged sample. By monitoring the first pulse one can determine if

the laser is firing reproducibly. This method was chosen because of the unique setup of the

CMS HF. In the HF, only one end of the fiber is accessible due to the way it was

constructed. It is not possible to place any light launching mechanism in the interior of the

HF due to the limited amount of space and the high radiation levels. It is however possible

to launch light into the front end of the calorimeter and record the reflected signals.

T G K Runn ing ; 500MS/S Sampip

1-* • *> - % > > . * • • • • I I.IIWW

i4ii|i * mvr

I O T B R I Pulse

PMVZ

•'^i^i^ Transmission

Pul^e PMT;3

Gate|33bns) PMT1I

.J_ J_

Ch3 SOOrnVQ

A" J30ns •(3> i 28ns

::=SK..?i: =ac isswa ^sp;x?=;;i';

««MX|«^

Cii4i 20GrriVO M IClOns Ch4 SOOmVO

<400inV

Figure 2.11. Traces of OTDR pulses and gate. Signal 1 shows the gate which is 330 ns. Signal 2 shows the delayed pulse from the laser and the reflected pulse from the irradiated sample fiber. Signal 3 shows the cut pulse which excludes the first pulse which comes directly from the laser. Separation between the first and second pulse is attained by the use of a 14 meter length of optical fiber used only as a delay line. Signal 4 is the direct transmission of the laser pulse to a PMT. This pulse is not reflected and therefore only makes one pass through the damaged section of fiber.

22

2.3 Spectrometer - Xenon

The second experimental setup is a pulsed Xenon light source and two

spectrometers. This set-up was used to measure the response of the fibers at wavelengths

from 350-800nm. The spectrometers used are Ocean Optics USB2000 models. The pulsed

Xenon light source is a model PX-2, also from Ocean Optics. The Xenon light source is

connected to the spectrometer with a five meter section of Polymicro QP optical fiber.

Only 1.2 meters of the fiber are exposed to the beam. The exposed section is coiled into a

7 cm diameter coil and the transport ends are placed into a copper shield to carry the fiber

to the location of the instruments. The instruments are shielded under a three-inch-thick

steel table where they are connected to a desktop computer.

The spectrometer triggers the light source and the signal is integrated over a 50 ms

time period. A second spectrometer is connected to an optical fiber which has no light

launched into it. This is to record the Cherenkov background just as in the previous setup.

The background Cherenkov radiation produced within the 50 ms integration time of the

spectrometer is appreciable. We were able to nullify this effect by using a grating in the

spectrometer (spectrometer 1), which made it much less sensitive. This allowed us to

pump much more light into the fiber, while the Cherenkov background remained

unchanged, without saturating the spectrometer. In essence we increased our signal-to-

background ratio by a factor of ten. We used the more sensitive spectrometer

(spectrometer 2), to monitor the Cherenkov radiation in the fiber without the Xenon light.

This provided the opportunity to study the Cherenkov signal produced by the beam which

proved to be a valuable tool for monitoring the beam stability. This setup can be better

understood by looking at Figure 2.12, which is run at the same time as the one in Figure

2.11.

23

By running the experiment with two different types of instruments at the same time

we are able to coirelate the data recorded by the PMT's to what is seen by the

spectrometers. This gives us the ability to study the optical damage profile for all

wavelengths of light, from 350-800nm, instead of just the integrated results of the PMT.

This is only possible if the results for the PMT agree with the results of the spectrometer

for 426nm, because this is the wavelength of the laser.

It is important to note that the signals from the laser appear different at first from

those of the spectrometer. This is due to the fact that the light from the laser makes two

passes through the damaged section of the fiber because it is a reflected signal. The Xenon

light only makes one pass through the damaged section. This is discussed later in more

detail.

Spectrometer integrates signal over 50 ms.

(Ocean Optics USB 2000)

Cherenkov Light

Spectrometers give information for 350-800nm.

For All wavelengths (ikA - 0.35 nm.)

BEAM

f Beam Pipe

2.0 MeV e

10-200 MA

Figure 2.12: Diagram of the spectrometer - xenon setup. Light is launched into a 5-meter-long optical fiber and travels through a 1.2 meter section which is exposed to the electron beam. The spectra are recorded from 350 - 800 nm. The second spectrometer, which is not connected to the xenon lamp, is used to monitor the Cherenkov background.

24

C H A P T E R I I I

ANALYSES

3.1 Optical Damage Studies

A typical set of data includes two damage sections and two recovery sections, see

Figure 3.1. This was done to investigate to what extent the recovery affected the overall

performance of the fiber. We also wanted to study the dose rate dependence of the optical

damage in addition to the total accumulated dose, thus we recorded data at several different

beam currents and for different time intervals.

400^ Spectrometer Data

Energy = 2.0 MeV

Current = 50 |JA

Dose Rate = 4.8 kRad/sec

Wavelength 500 nm

350-

300-(/)

c 3 O O 250-O O <

200-

150-

RecQvery

Recovery

T T T 1000 2000 3000 4000 5000 6000

Time(sec)

I 7000

Figure 3.1. Optical damage and recovery data. A typical data set depicting 30 minutes of damage, 30 minutes of recovery, and a second damage and recovery period is shown.

We found that the fiber does not really recover at all. The fiber maintains a

memory of its accumulated dose and retums quickly to the transmission level it was at

during the previous irradiation period. This phenomenon was noted by Brichard et al. 25

where he recognized the same type of behavior, (Brichard, 2002). This behavior is evident

from Figure 3.2. The recovery sections have been removed and the damage sections

moved over on the x-axis to join with the previous damage sections. It is clear that the

transmission level retums to the previous value within a few seconds of irradiation. This

leads to the conclusion that the instantaneous, not integrated, dose rate must be considered

when calculating the optical transmission due to radiation damage.

Spectrometer Data Energy 2.0 MeV Current 50 ^A Dose Rate 5 kRad/s Wavelength 500 nm

1.00-

0,95-

^ 0.90-

o

w 0.85 £ c 03

1^ 0.80

0.75

0.70

Joining Points

0.0 5.0x10' 1.0x10 1.5x10

Accumulated Dose (Rads)

2.0x10'

Figure 3.2. Joined optical damage data sections. In this set of data we conducted four 15 minute irradiation sessions, each followed by 15 minute recovery periods. Each point represents 0.3 seconds in time. We removed the recovery sections and shifted the data in time (or dose) to illustrate how quickly the fiber retums to the previous transmission value.

We studied five different dose rates and several different levels of accumulated

dose. In Table 3.1 you can see the sets of data used in this study. In all cases, we ran the

experiment several times in order to verify reproducibility of data.

26

Table 3.1. List of the seven different sets of data used in this study. The first column shows the designation of the file. The second column gives a description of the data set and, the third column gives the dose rate and total accumulated dose of the fiber after all irradiations had occurred.

File Data Description Rate/Accumulated Dose

20/500keV

10/1 MeV

10/2 MeV

1015/2MeV

50/2MeV

50C/2MeV

100/2MeV

Beam current at 20 pA, Energy at 500 keV: 1-60 minute optical damage section 1-60 minute recovery section

Beam current at 10 pA, Energy at 1.0 MeV: 1-60 minute optical damage section 1-60 minute recovery section

Beam current at 10 pA, Energy at 2.0 MeV: 2-30 minute optical damage sections 2-30 minute recovery sections

Beam current at 10 pA, Energy at 2.0 MeV: 4-15 minute optical damage sections 4-15 minute recovery sections

Beam current at 50 pA, Energy at 2.0 MeV: 2-30 minute optical damage sections 2-30 minute recovery sections

Beam current at 50 pA, Energy at 2.0 MeV: 4-15 minute optical damage sections 4-15 minute recovery sections

Beam current at 100 pA, Energy at 2.0 MeV: 2-30 minute optical damage sections 2-30 minute recovery sections

Dose Rate = 7 Rad/s Total Dose = 25.2 kRad

Dose Rate = 230 Rad/s Total Dose = MRad

Dose Rate = 1 kRad/s Total Dose = 828 kRad

Dose Rate = 1 kRad/s Total Dose = 828 kRad

Dose Rate = 5 kRad/s Total Dose = 18 MRad

Dose Rate = 5 kRad/s Total Dose = 18 MRad

Dose Rate = 10 kRad/s Total Dose = 36 MRad

The primary objective of the analysis was to be able to predict what the

transmission level would be at any given dose and to establish if there was any dose rate

dependence upon the optical damage. We cut the data into individual damage and recovery

sections and began with the first damage profile. For the optical damage part we use an

equation of the form.

y = PI*exp 'i/w^y (3.1)

where PI is used to verify that the normalization was done correctiy, and 10 was placed in

the denominator of the exponential to give the normalized value at 100 MRad. We exclude

27

parameter PI from Equation 3.2 and show how these parameters are related to attenuation

in Equations 3.3, 3.4 and 3.5 as follows:

V = exp P2\^ '\Q'

' - - ^ X o s ) " ,

A(/l) = -—log

r = 2.3*P2

(3.2)

(3.3)

(3.4)

(3.5).

In Equation 3.5, T represents the transmission ratio which is the same as the ratio of power

out divided by power in, in the common attenuation calculation. The parameters were

extracted from fits similar to Figure 3.3. Table 3.2 lists all of the parameters found from

fits of experimental data. Figure 3.4 illustrates the how accurately the damage can be

predicted based solely upon the first damage section. The subsequent irradiation periods

cause the predicted value to be inaccurate only to about 4% of the transmission.

28

Tran

smis

sion

(%

) p

o

o

o

-»

O

) •->

I (D

O

CD

O

y =

1

^ ^ • % ^ g j ^ - ^

• 100bUV500

PI *exp(P2*(x/100000000)'^P3)

Data: 100bUV500_100bUV500 MoiJel: GriscomFit Weighting: y No weighting

Chi"2/DoF = 0.00003 R' 2 = 0.99159

PI 1 ±0 P2 -0.78578 ±0.00201 P3 0.2918 ±0.00099

B^kkfeUif.

1 ' 1 ' 1 ' 1 ' 1

0.0 5.0x10^ 1.0x10' 1.5x10' 2.0x1 o'

Accumulated Dose (Rads)

Figure 3.3. Optical damage curve fitting. A fitting session provides the parameters PI, P2 and P3 which are entered into a table along with the error for comparison purposes. The nature of the initial decrease in transmission is remarkable, in that 20% of the optical transmission loss takes place in the first 2 minutes (~ 1 MRad).

29

c o V)

"E (/> CO

I -

"c5 o

Q.

o

1.00-

0.95

0.90-

0.85

0.80-

0.75-

0.70 0.0

S5015S500 •F6 •GriscomFit 500

y =P1*exp(P2*(x/100000000)' P3)

Data: 5015UV500_S5015S500 iVIodel' GriscomFit Weighting: y No weighting

Chi'>2/DoF = 0.00003 R'*2 0.96533

±0.0034 ±0.00209

5.0x10" 1.0x10^ 1.5x10' 2.0x10^

Accumulated Dose (Rads)

Figure 3.4. Optical damage fit comparison. The first optical damage section from was fit and this function was extrapolated to 20 MRads (red line). The four joined damage sections were fit (blue line), to show the difference. We see ~ 4% difference in the predicted and measured values after 18 MRads of accumulated dose.

30

Table 3.2. Parameters of the power law fit for spectrometer data. Parameter PI was fixed for normalization therefore there is no associated error. The x^value listed is per degree of freedom.

Optical Damage Parameters Current/X/E

10/400/2 MeV 10/426/2 MeV 10/450/2 MeV 10/500/2 MeV 10/600/2 MeV 10/630/2 MeV 50/400/2 MeV 50/426/2 MeV 50/450/2 MeV 50/500/2 MeV 50/600/2 MeV 50/630/2 MeV 100/400/2 MeV 100/426/2 MeV 100/450/2 MeV 100/500/2 MeV 100/600/2 MeV 100/630/2 MeV lOb/400/2.0 MeV 10b/426/2.0MeV 1 Ob/450/2.0 MeV 1 Ob/500/2.0 MeV 1 Ob/600/2.0 MeV 10b/630/2.0MeV 50b/400/2.0 MeV 50b/426/2.0 MeV 50b/450/2.0 MeV 50b/500/2.0 MeV 50b/600/2.0 MeV 50b/630/2.0 MeV 100b/400/2.0MeV

PI P2 -0.97405 -0.76833 -0.65356 -0.57694 -0.73658 -0.67593 -1.08367 -0.89753 -0.75617 -0.65144 -0.97982 -0.83543 -1.06138 -0.89548 -0.74891 -0.67088 -1.07329 -0.89333 -1.04416 -0.82968 -0.71144 -0.61981 -0.78434 -0.64619

-1.1715 -0.95172 -0.80145 -0.66413 -0.89848 -0.71465 -1.28024

AP, 0.00968 0.00676 0.0063

0.00623 0.00984 0.01571 0.00693 0.0045 0.0038

0.00326 0.00641 0.00875 0.00617 0.00355 0.0032

0.00279 0.00531 0.00693 0.00913 0.00679 0.00602 0.00587 0.00854 0.0088 0.0134

0.01028 0.00909 0.00723 0.00434 0.00914 0.00452

P3 0.29646 0.28015 0.2852

0.32556 0.39646 0.43886 0.29302 0.26531 0.26122 0.29582 0.37823 0.39053 0.27573 0.24689 0.24601 0.27471 0.38738 0.38207 0.30732

0.287 0.29245 0.33316 0.3937

0.40166 0.28246 0.27398 0.27278 0.29736 0.37353 0.35668 0.26579

AP3

0.00207 0.00183 0.00201 0.00228 0.00285

0.005 0.00197 0.00154 0.00155 0.00157 0.00209 0.00336 0.00222 0.00151 0.00163 0.00162 0.00198 0.00313 0.00184 0.00172 0.00178 0.00201 0.00234 0.00293 0.0029

0.00274 0.00289 0.0028

0.00237 0.00334 0.00132

1' 0.00007 0.00004 0.00004 0.00002 0.00003 0.00004 0.00016 0.00009 0.00007 0.00004 0.00008 0.00015 0.00026 0.00011 0.00011 0.00008 0.00012 0.00023 0.00005 0.00004 0.00003 0.00002 0.00002 0.00002 0.00015 0.0001

0.00009 0.00005 0.00003 0.00005 0.00013

R 0.97 0.97 0.97 0.97 0.97 0.93 0.97 0.98 0.97 0.98 0.98 0.95 0.95 0.97 0.97 0.98 0.98 0.96 0.98 0.97 0.97 0.98 0.98 0.97 0.96 0.97 0.96 0.97 0.99 0.97 0.98

From the parameters in Table 3.2, we illusti^te the data in different ways. We first

combined all of the functions with the same dose rate and plot them to compare how the

fiber behaves at different wavelengths, (Figtires 3.5-3.7). All wavelengths fi-om the

spectrometer were combined to form Figures 3.8 and 3.9. When plotted in this manner, it

is much easier to see where absorption occurs.

31

Wavelength Comparison at 7 Rad/s

1 n-

nq-

ns-

07-

nfi-

05-

. 1 _ rr ^ = ^ '

Heavy'Lines ^ ^ \ ^ V ^ Represent Data ^ V \ ^ \

- 400nm 426nm 450nm

. SOOnm 600nm 630nm

t 4 •

^

\ v \ \ \ Wvv

-)

\ \ v \

VyVx

"1 Wavelength Comparison at 230 Rad/s

10 10 10 10' 10 10' 10' 10" 10" 10" 10' 10- 10' 10' 10' 10'

Accumulated Dose (Rad) Accumulated Dose (Rad)

Figure 3.5. Optical damage extrapolations for 7 Rad/s and 230 Rad/s. Six different wavelengths are plotted to show their behavior at given dose rates. The dose rate for Figure 3.5a is 7 Rad/s and 3.5b is 230 Rad/s. The heavy lines represent experimental data whereas the lighter lines are fits and extrapolations from data.

Wavelength Comparison at 1 kRad/s Wavelength Comparison at 5 kRad/s

Accumulated Dose (Rad) Accumulated Dose (Rad)

Figure 3.6. Optical damage extrapolations for 1 kRad/s and 5 kRad/s. Six different wavelengths are plotted to show their behavior at given dose rates. The dose rate for 3.5a is 1 kRad/s and 3.6b is 5 kRad/s. The heavy lines represent experimental data whereas the lighter lines are fits and extrapolations from data.

32

Wavelength Comparison at 10 kRad/s

10 10' 10' 10® 10' 10' 10' 10 10 ^Q11 10'

Accumulated Dose (Rad) Figure 3.7. Optical damage extrapolations for 10 kRad/s. Six different wavelengths are plotted to show their behavior at given dose rates. The dose rate was 10 kRad/s for all data represented in this Figure. The heavy lines represent experimental data whereas the lighter lines are fits and extrapolations from data.

33

''o - ^ ^ , j i ^ ^ ^ ^ ^ ^ ^ ^

^ 1 WBBII^iWW ^ -l

c I B iiii -li r'7 ' '" 1 ^^ni^^^HB|y|h|g|M 1 H^^^^^^^^^ l 4 W^^^^^H 1 s^^^^i

' ^ ^ ^ ^ 1 ^ ^ ^ H H H M •S- -\ :"'^^^K' ' • .' "iH

o 1__--^Wto.

Q. V -aHHiBn^ ^ ^ \> % ^^^^HBnfe.^

^ " 350 ^°°

^

IH^nHHHhiii^^^^ss

- ^ „ - ' / ' •

HflHj lHHfeBBi /'' ^^^^^^^|^^^HHHH|^i^i-;

^^^^^^^^^^^^^^^^^^^^HBo^vf ^^^^^^I^^^IHI%? ^^^^^^^^^^^^^^^^^^^^^^HB^^^-'i ('|£l ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ H '

^^^^^^^|RES? .^^1 -'^HHHIHI^u ittiMii ^^^^B

^ ^

6 5 0

Figure 3.8. Combined optical damage spectra. The entire spectrum (from 350 to 700 nm) is recorded every second for 15 minutes at a dose rate of 10 kRad/s. The combined spectra form a continuous plot showing the relative damage at all wavelengths as a function of total accumulated dose.

700 ssion {%)

0.0 5.0x10° 1.0x10' 1.5x10'

Accumulated Dose (Rads)

Figure 3.9. Combined optical damage spectra plotted in 2 dimensions. The same data as in Figure 3.8 plotted in this way, makes the dominant absorption peaks more distinguishable.

34

The optical ti-ansmission is > 70% at 1 MRad regardless of dose rate, with the

exception of 10 kRad/s, where the transmission is only ~ 70% for all wavelengths studied.

The fits are constrained to the first damage profile (-15 min) and these data reveal the

parameters which are in turn used for extrapolations to much higher doses. The

experimentally accumulated dose varies from -10'' to 10* Rads. In Figures 3.5 to 3.7 and

3.10 to 3.12 data are represented by a heavy line. Figure 3.4 shows that the apparent

recovery in the absence of irradiation does not play a major role in determining the optical

transmission loss w/dose (-10% at 18 MRad). The a parameter varies between 0.2 and 1.2

depending on the wavelength, accumulated dose and dose rate. The P parameter ranges

between 0.2 to 0.4 also depending on the wavelength, accumulated dose and dose rate. We

will return to their implications at the end of this chapter.

We wanted to compare data recorded at the same wavelength but different dose

rates to better understand the dose rate dependence of the transmission. We plotted data for

one wavelength but at five different dose rates on one plot to show this dependence. It is

evident fi-om Figures 3.10-3.12 that the transmission is dose rate dependent.

Dose Rate Comparison at 400nm Dose Rate Comparison at 426nm

Heavy Lines Represent Data

10' 10' 10' 10' 10' 10' 10' 10" 10' 10"" 10" 10" 10" 10"

Accumulated Dose (Rad)

10° 10' 10' 10' 10' 10' 10' 10' 10' 10' 10" 10" 10" 10" 10"

Accumulated Dose (Rad)

Figure 3.10. Optical damage profiles for 400 and 426 nm. Plotted are the data and extrapolations for five different dose rates from 18 Rad/s to 25 kRad/s for a smgle wavelength. The heavy lines represent experimental data whereas the lighter Imes are fits. Figure 3.10a shows 400 nm wavelength and Figure 3.10b shows 426 nm wavelength data.

35

1 1 -,

1 0-

0 9 ^

0 8 -

§ 0 6 -

« 0 5 -

c

UJS

l

H 0 3 -

0 0 -

Dose Rate Comparison at 450nm

/Heavy Lines Represent Data

N \ \ \ ^ ^ Hn.n. J , _ - - ^ \ \ / 7 Rad/s 10 kRad/s-—^\v \ \ /

1 kRad/s'

. a) d i i i . j J III..J . . . , ^ J ......^

- ^

.^nui . . . u J . , . . J > i w l --^^ . , , , - J 1 ^

1 1 •

1 Q .

0 9

Dose Rate Comparison at SOOnm

08

g 07

O 06 'w

0.3

02

01

00

^Heavy Lines Represent Data

Rad/s^

10' 10- 10' 10' 10' 10' 10' 10' 10* 10" 10" IC" 10" 10"

Accumulated Dose (Rad)

10° 10' 10' 10' 10' 10' 10' 10' 10' 10' 10'° 10" 10" 10" 10"

Accumulated Dose (Rad)

Figure 3.11. Optical damage profiles for 450 and 500 nm. Plotted are the data and extrapolations for five different dose rates from 18 Rad/s to 25 kRad/s for a single wavelength. The heavy lines represent experimental data whereas the lighter lines are fits. Figure 3.11a shows 450 nm wavelength and Figure 3.1 lb shows 500 nm wavelength data.

Dose Rate Comparison at 600nm Dose Rate Comparison at 630nm

10° 10' 10' 10' 10' 10' 10' 10' 10' 10' 10'° 10" 10" lO" 10"

Accumulated Dose (Rad)

10° 10' 10' 10' 10' lO' 10' 10' lO' lO' 10" 10" 10" lO" 10"

Accumulated Dose (Rad)

Figure 3.12. Optical damage profiles for 600 and 630 nm. Plotted are the data and extrapolations for five different dose rates from 18 Rad/s to 25 kRad/s for a single wavelength. The heavy lines represent experimental data whereas the lighter lines are fits. Figure 3.12a shows 600 nm wavelength and Figure 3.12b shows 630 nm wavelength data.

36

We have studied how the parameters P2 and P3 are affected by the dose rate and

plotted these parameters as a function of dose rate. Parameter P2 levels off at 10" Rad/s

and appears to stay relatively constant for wavelengths 400,425,450, and 500 whereas for

600 and 630 nm P2 continues to decrease. This is thought to be at the absorption band of

the NBOH center which contributes strongly to this effect.

It is clear from Figures 3.4 to 3.7 that the optical transmission is affected differently

for the 600 and 630 nm wavelengths. A plot of the parameter P2 also illustrates this point,

(Figure 3.13). These two wavelengths continue to show dose rate dependence long after

the others have leveled off. It is also clear from Figure 3.14 that the parameter P3 shows a

different behavior for 600 and 630 nm wavelengths in that they are much higher. The

parameter P3 is thought to be a constant for a particular type of optical fiber and

wavelength. With the exception of the lowest dose rate (18 Rad/s), this would appear to be

the case. It is very difficult to maintain a low dose rate and high energy with the Van de

Graaff accelerator used in this study. It is our feeling that the data for 18 Rad/s is an

anomaly since the accuracy of the data is not considered reliable. We did not have the

opportunity to reproduce the low dose rate data several times as we had done with the

others. We intend to follow up with measurements in the regime of 1 to 100 Rad/s at a

later time.

(val

ue)

Par

amet

er

Frt

0 0 - ,

-0 2 -

-0 4 -

-0 6 -

•0 8 -

- 1 . 0 -

P2a Dose Rate Dependance

y =P1 •exp(P2'(x/100000000)''P3)

a)

- • - 4 0 0 n m P 2 - • - 4 2 6 n m P 2 - . i -450nr rP2 —T—500nrrP2 -< -600nmP2 - • - 6 3 0 n m P 2

0 0 20x10' 4Ox10' 60x10' 80x10' 10x10'

Rads/sec

P2a Dose Rate Dependance

=P1 •exp(P2*(x/100000000)' 'P3)

b)

- • - 4 0 0 n m P 2 • 426nmP2 i - 450nmP2

- T - 500nmP2 —<—600nmP2 - • - 6 3 0 n m P 2

Figure 3.13. Fit parameter P2 comparisons. The parameter P2 vs. dose rate shows the overall trend. Plot a) is linear scale in x whereas plot b) is log scale in x for the same data. It is of note that the value for P2 for 600 and 630 nm continues to decrease after the others have leveled off indicating a strong absorption center(s).

37

i 0 35

0) E 0 3 0 -

in a. ..= 0 25-

P3a Dose Rate Dependance

y =Prexp{P2*(x /100000000) '^P3) J

- • - 4 0 0 n m P 3 • 426nmP3 * 450nmP3 0 50 T 500nmP3

—«-600nmP3 - >•- 630nmP3

0 45-

0 40

a )

20x10" 40x10' 60x10' 80x10' 10x10*

Rads/sec

P3a Dose Rate Dependance

L y. -PJ * expJPr (x/IOOOOOOOOJ^sTI

• -400nmP3 • -426nmP3 *-450nmP3 r - 500nmP3 »-600nmP3 • -630nmP3

> *

b)

10'

Rads/sec

Figure 3.14. Fit parameter P3 comparisons. Parameter P3 versus dose rate general trend and, for 600 and 630 nm again shows a different behavior than wavelengths. Plot a) is linear scale in x whereas plot b) is log scale in x for data. The data for 7 Rad/s dose rate is considered an anomaly due to the unreli the Van de Graaff accelerator at low currents.

shows a the other the same ability of

The data recorded with the PMT laser experimental setup yield very similar

results as the spectrometer. The light from the laser makes two passes through the

irradiated section of fiber because we employ the OTDR method as discussed in Chapter 2.

This means that if the normalized transmission is 50% in the spectrometer - xenon setup, it

should be 25% (0.50^), in the PMT - laser setup. It is clear from Table 3.3 and Figure 3.15

that this is the case. There are slight differences in the measurements because of the

instability of the laser pulse but overall, there is good agreement.

Table 3.3. Transmission percentages for the five dose rates and accumulated doses using two different measurement techniques. This shows that there is good overall agreement between the PMT - laser experimental setup and the spectrometer - xenon experimental setup.

Dose Rate 10 kRad/s 5 kRad/s 1 kRad/s

230 Rad/s 7 Rad/s

Accumulated Dose

18 MRad 9 MRad

1.8 MRad 800 kRad 25 kRad

Spectrometer - Xenon 49.70% 66.50% 77.40%

87% 96.60%

PMT Laser

25.40% 42.70% 67.30% 78.10% 94.40%

Difference 0.60% 1.50% 7.40% 2.40% 1.10%

38

We estimate the errors to be 2.8% for the transmission percentage and 10.4% in the

dose estimates. The error in the transmission percentage stems from 2% uncertainty in the

normalization, and 2% for the fitting process. The stability of the xenon lamp and laser

were insignificant. The associated uncertainties for the dose estimates are; 10% for energy

measurement, 2% for the beam stability, 2% for current measurement, and <1% for the

geometrical acceptance calculations.

Dose Rate Comparison at 426nm Laser

27,0 fftad/K

7 Rad/s

" " I • - j • "" •

10° 10' 10= 10' 10' 10' 10' 10' 10' 10' 10'° 10" 10'= 10'^ 10"

Accumulated Dose (Rad) Figure 3 15- PMT - laser setup damage extrapolations. Plotted are the data and extrapolations for five different dose rates from 7 Rad/s to 10 kRad/s for a PMT - laser setup at 426 nm, using the OTDR method. The heavy lines represent expenmental data whereas the lighter lines are fits.

39

3.2 Optical Recovery Studies

In the same manner as we studied the damage of the optical fibers we also

investigated the optical transmission recovery. We took the first recovery sections of the

data and fit them with

V = exp PI

1 + P2 - 1 F3 /

(3.6)

where PI is the transmission percentage of the fiber after the previous irradiation and P3 is

used to check the accuracy of the normalization and PI. If PI is assigned property and the

normalization is performed properly P3 = 1. We found that the recovery process is

dependent upon the total accumulated dose and the dose rate. A typical set of data, and the

fit, is shown in Figure 3.15.

0.95-

0 90

c • i 0-85 in

E en c CO

0.80

0.75

50bUV500

•GrlscomRecoveiY 500 y=exp(P1/(1+P2*((x/P3)-1)'>P4))

Data 50bUV500_50bUV500

Model GriscomRecovery

Weighting

y Ho weighting

Chl«2/DoF = 0 00003

R»2 = 0 79205

P I -0 24461 ±0

P2 0 5 1 1 4 ± 0 0 1 0 6 P3 1 ±0.00003 P4 0 17899 ±0 0032

500 1000

Time (sec)

1500 2000

Figure 3 16. Typical recovery data set and fit. The beam current for this data was set at 50/xA, which is 5 kRad/s. The wavelength was 500 nm.

40

The fit parameters are tabulated in Table 3.3. We performed analysis on the

second, third and fourth recovery sections of two sets of data (50-15 and 10-15) to better

understand how the recovery was related to the total accumulated dose and the dose rate.

The notation 50-15 and 10-15 refers to 50pA beam current in 15 minute optical damage

and recovery intervals.

Table 3.4. Parameter values for seven different recovery data sets. The notation 50-15 and 10-15 refers to 50pA beam current in 15 minute optical damage and recovery intervals. If more than one recovery section from one set of data was used, it is notated by Reel, Rec2, Rec3, or Rec4.

Recovery Parameters Current (nA)/ Wavelength (nm)

10/400

10/426

10/450

10/500

10/600

10/630

50/400

50/426

50/450

50/500

50/600

50/630

100/400

100/426

100/450

100/500

100/600

100/630

10/400 IMev

10/426 IMev

10/450 IMev

10/500 IMev

10/600 IMev

10/630 IMev

50c/400Recl

50c/426Recl

50c/450Recl

50c/500Recl

50c/600Recl

50c/630Recl

PI

.0.319

-0.258

-0.224

-0.169

-0.172

-0,137

-0.473

-0.347

-0.295

-0.245

-0.284

-0.253

-0.766

-0.543

-0.467

-0.429

-0.648

-0.569

-0.174

-0.143

-0.12

-0.0914

-0.0814

-0.0666

-0.451

-0.299

-0.249

-0.262

-0.477

-0.444

P2

0.579

0.523

0.607

0,612

0.239

0,17

0,54

0,607

0.613

0.511

0.183

0.139

0.4

0.449

0.484

0.374

0.134

0.122

0,39

0,416

0.432

0.528

0,217

0.151

0.392

0.476

0.448

0,316

0.0994

0.0773

AP2

0,0104

0,11

0,0139

0,0134

0,00407

0,00347

0,00987

0,01403

0,0148

0,0106

0.00295

0.00238

0,00384

0,00527

0.00584

0.00407

0.00127

0.00126

0.00637

0.01

0.0117

0.0129

0,00536

0,00419

0,00387

0,00691

0,0072 •

0,00428

0,00129

0,00111

P3

1

I

1

1

1

1

1

1

1

1

1

1

1

1

I

1

1

1

1

1

1

1

1

1

1

1

1

1

1

1

AP3

0,00004

0.00003

0,00003

0,00002

0,00002

0,00002

0,00006

0.00005

0.00005

0,00003

0,00003

0,00002

0,00006

0.00005

0.00004

0.00003

0,00003

0,00002

0.00003

0.00004

0.00003

0.00003

0.00002

0.00002

0.00004

0.00004

0.00003

0,00003

0,00003

0,00003

P4

0,289

0,386

0,332

0,232

0,187

0,218

0,191

0.261

0,247

0,179

0.196

0,213

0,185

0.236

0,229

0,158

0.127

0,131

0,334

0,341

0.331

0,223

0.188

0,209

0,15

0,18

0,171

0,127

0,147

0.169

AP4

0,00282

0,00344

0.00368

0.00341

0,00256

0,00307

0.00282

0,00365

0,0038

0,0032

0,00243

0,00257

0,00146

0,00181

0.00186

0.00165

0.00142

0.00156

0,00219

0,00325

0,00365

0.00317

0,00313

0,00348

0,0015

0,00222

0,00246

0,00205

0,00194

0.00213

^ 2

3,00E-05

2,00E-05

2,00E-05

2.00E-05

8.54E-06

6,93E-06

6,00E-05

5,00E-05

5,00E-05

3,00E-05

l,00E-05

l,00E-05

4,00E-05

3.00E-05

3,00E-05

2,00E-05

7,86E-06

7,43E-06

3.00E-05

4.00E-05

3,00E-05

2,00E-05

2,00E-05

2,00E-05

3,00E-05

3,00E-05

3,00E-05

2,00E-05

l,00E-05

l,00E-05

R' 2

0,91

0,92

0,88

0,82

0,86

0,86

0.85

0,85

0,82

0,79

0,89

0,9

0,95

0,95

0,94

0,91

0,91

0,9

0,76

0,6

0,52

0.43

0,39

0,4

0,88

0,82

0,77

0.74

0,82

0,83

41

Table 3.4. Continued Current (nA)/ Wavelength (nm)

50c/400Rec2

50c/426Rec2

50c/450Rec2

50c/500Rec2

50c/600Rec2

50c/630Rec2

50-15/400Recl

50-15/426Recl

50-15/450RecI

50-15/500Recl

50-15/600Recl

50-15/630Recl

50-15/400Rec2

50-15/426Rec2

50-15/450Rec2

50-15/500Rec2

50-15/600Rec2

50-15/630Rec2

50-15/400Rec3

50-15/426Rec3

50-15/450Rec3

50-15/500Rec3

50-15/600Rec3

50-15/630Rec3

50-15/400Rec4

50-15/426Rec4

50-15/450Rec4

50-15/500Rec4

50-15/600Rec4

50-15/630Rec4

10-15/400Recl

10-15/426Recl

10-15/450Recl

10-15/500Recl

10-15/6OORecl

10-15/630Recl |

PI

-0,52643

-0.35712

-0.30839

-0.34058

-0.67605

-0.62099

-0.352

-0.252

-0.209

-0.213

-0.364

-0.342

-0.42081

-0.29087

-0.24504

-0.25736

-0.47954

-0.44205

-0.46224

-0.34058

-0.28443

-0.29503

-0.57167

-0,53353

-0,5072

-0.37433

-0.32064

-0.34479

-0.66668

-0,6098

-0,17376

-0,13

-0.111

-0.10585

-0,16809

-0.157

P2

0,25458

0,29805

0,29465

0,20359

0,06266

0.04727

0.433

0,615

0.612

0,365

0,129

0.112

0.35173

0,41505

0,40942

0,27472

0.10102

0.08681

0.25246

0.37026

0.34522

0.18626

0.06486

0.06849

0.29887

0.42621

0.42417

0.26444

0.07362

0.06603

0.48306

0.52944

0.57485

0,46267

0,11971

0,11209

erront

0.00197

0.00346

0.00377

0.00225

0.00067

0.00056

0.00821

0.0153

0.0165

0.00793

0,00223

0,00196

0,00506

0,00942

0,00987

0,00506

0,00171

0,00149

0,004

0,00684

0,00704

0,00371

0,00126

0.00117

0.00198

0.00386

0,00423

0,00226

0.00061

0.0057

0.01593

0,02547

0,02985

0,01876

0,00381

0,00332

P3 erront

4,00E-05

4.00E-05

3.00E-05

3.00E-05

2.00E-05

2.00E-05

0.00005

4.00E-05

4,00E-05

3.00E-O5

3,0OE-O5

3,OOE-05

4,00E-05

5,00E-05

4,00E-O5

3.00E-05

3.00E-05

3,00E-05

5.0OE-O5

4,O0E-05

4,00E-05

3.00E-05

3,00E-05

2.00E-05

4,00E-05

4,00E-05

3.00E-O5

3.00E-05

2.00E-05

2.00E-05

3.00E-05

3.00E-05

3,00E-05

0,00003

0,00002

2,00E-05

P4

0,18095

0.20365

0.19276

0.15012

0.14602

0.16771

0.186

0.221

0.205

0.159

0.224

0.247

0.17487

0.21021

0.19899

0.14234

0.159

0.1768

0.20665

0,2159

0,21074

0,18045

0,17653

0,1757

0,1662

0,17283

0.16165

0.12013

0,12718

0.13364

0.3183

0,33986

0.30536

0J21218

0,29729

0,32049

error

0.00116

0.00175

0.00193

0.00165

0.00157

0.00173

0.00321

0.00429

0,00464

0,00366

0,00286

0,00288

0,00237

0,00378

0,00401

0.00303

2.74E-03

0.00278

0.00268

0.00317

0.00349

0.00336

0.00325

0.00286

0.00092

0,00128

0.0014

0.00119

0,00114

0,00118

0,00601

0.00892

0.00953

0.00714

0,00542

0,00504

x z 2,00E-05

3,00E-05

2,00E-05

l,00E-05

5,90E-06

5,10E-06

4.00E-05

4,00E-05

4,00E-05

2,00E-05

2,O0E-O5

2.00E-04

3.00E-05

5.00E-05

4.00E-05

2,00E-05

l,00E-05

l.OOE-05

3,00E-O5

4,00E-05

3.00E-05

2.00E-05

l.OOE-05

8.40E-06

2.O0E-O5

3,00E-05

3,00E-05

2.00E-05

6,53E-06

5.70E-06

3,00E-05

3,00E-05

3,OOE-05

2.00E-05

2,00E-05

1,OOE-05

R' 2

0,94

0,9

0,87

0.85

0.86

0.88

0.83

0.79

0.74

0,74

0,91

0.92

0.87

0,8

0,76

0.75

0.83

0.85

0.91

0.88

0.85

0.82

0.84

0.87

0.93

0.87

0.83

0,81

0,84

0,85

0.85

0,73

0.66

0.65

0.87

0,9

42

Table 3.4. Continued Current (jtA)/ Wavelength (nm)

10-15/400Rec2

10-15/426Rec2

10-15/450Rec2

10-15/500Rec2

10-15/600Rec2

10-15/630Rec2

10-15/400Rec3

10-15/426Rec3

10-15/450Rec3

10-15/500Rec3

10-15/600Rec3

10-15/630Rec3

10-15/400Rec4

10-15/426Rec4

10-15/450Rec4

10-15/500Rec4

10-15/600Rec4

10-15/630Rec4

PI

-0,19819

-0.14096

-0.11705

-0.11273

-0.20356

-0.20356

-0.2388

-0.16785

-0.13938

-0.13344

-0.24795

-0.22308

-0.26553

-0.1876

-0,15534

-0.15572

-0.29701

-0.26838

P2

0,40751

0.43116

0.45395

0.37497

0.10814

0.16247

0.37288

0.4004

0.41317

0.31927

0.09126

0.07916

0.36927

0.41792

0.42791

0.3411

0.10813

0.10102

errori

0.01089

0O177

0.02075

001337

0.00281

0.00524

0.00888

0.0138

0.01592

0.01026

0.00247

0.00217

0.00697

0.01149

001316

0,00845

0.00225

0.00207

P3 errori

0,00003

0,00003

0,00003

0.00002

0.00002

0,08234

0.00004

0.00004

0.00003

0.00003

0.00003

0.00002

0,00004

0,00004

0,00003

0,00003

0,00003

0,00002

P4

0.30448

0.35566

0.3379

0.2337

0.26816

0.24706

0.27166

0,31144

0,29777

0,21572

0.23894

0.26616

0.23348

0.26175

0.24801

0.17866

0.18159

0.1986

error

0.00465

0.0073

0.0081

0.00608

0.0043

0.00328

0.00408

0.006

0.00669

0.00544

0.00446

0,0045

0,00311

0,00459

0,00512

0,00405

0,00334

0,00328

Y^2

3,00E-05

3,00E-05

3.00E-05

2.00E-05

2.00E-05

l.OOE-05

3,00E-05

4.00E-05

3,OOE-05

2,00E-05

2.00E-05

l.OOE-05

3.00E-05

3.00E-05

3.00E-05

2.00E-05

l.OOE-05

l.OOE-05

R' 2

0.85

0,75

0,69

0.67

0.86

0.89

0,86

0,78

0,72

0,69

0.82

0.85

0.86

0.78

0.72

0.7

0.79

0,83

Percent difference after 10^5 seconds of recovery for 1 kRad/s data Percent difference after 1D'*14 seconds of recovery for 1 kRad/s data

2x10' 3x10

Total Dose (Rads)

10x10* 15x10* 2.0x10* 25x10* 30x10* 35x10* 40x10*

Total Dose (Rads)

Figure 3 17 Plot of the percent difference of recovery versus total accumulated dose for 1 kRad/s dose rate. This shows how much the fibers transmission recovers after 10 seconds (plot a), lO'' seconds (plot b). The dose rate for this experiment was 1 kRad/s^ The most noticeable feature is that the recovery percentage continues to decrease with accumulated dose if the fiber is allowed to recover for 10 seconds but does not if it is allowed to recover for lO"* seconds (or infinitely long) for 600 and 630 mn wavelengths. This indicates that the optical transmission recovery mechanism is much slower at these wavelengths.

43

Percent difference after 10"5 seconds of recovery for 5 kRad/s data

400nm 426nm 450nm SOOnm 600nm

-630nm

0.28-

0.26-

0 24-

077-

020-

018-

016^

0 1 7 -

0 1 0 -

0 0 8 -

•

• A

W

a)

I

•

A A

- -t~

t

•

A

- •

•

H I

t

; I

1

m

f

<

Percent difference after 10'

0 42.

0 40 -

0 3 8 -

036

034

? 0 32

S 030 c <U S 028 It

B 0 26

0.24

022

0.20

018

14 seconds of recovery for 5 kRad/s data

• ^ ^

t 1 »

-m

—tr

— T -4

- • -

400nm 426nm 450nm SOOnm eoonm eSOnml

0 1 2x10'

Total Dose (Rads)

2 0x10' 0' 1 2x10'

Total Dose (Rads)

Figure 3.18. Plot of the percent difference of recovery versus total accumulated dose for 5 kRad/s dose rate. This shows how much the fibers transmission recovers after 10 seconds (plot a), lO''* seconds (plot b). The dose rate for this experiment was 5 kRad/s. The most noticeable feature is that the recovery percentage continues to decrease with accumulated dose if the fiber is allowed to recover for 10 seconds but does not if it is allowed to recover for lO'"* seconds (or infinitely long) for 600 and 630 nm wavelengths. This indicates that the optical transmission recovery mechanism is much slower at these wavelengths.

eoxia"'q

7 0x1 a ' -

6 0 x 1 0 ' -

5 0x1 o ' -

4 0x10 ' ' -

0) 3 0x10" ' -a .

W 2 0x10- ' -

1 0x10 ' ' -

0 0 -

-1 0 x 1 0 ' -

-2 0x10 ' ' -

Slope Comparison for 10^5 seconds Recovery

•

• A

<

* ^ ' ' - - - ^ • • -

"4

a ) • 1 1 1 • 1 • 1 • 1

1000 2000 3000 4000 SOOO

Dose Rate (Rad/s)

-400nm 426nm 450nm

-SOOnm 600nm 630nm

Slo

pe

6000

1 6x10'' -|

1 4x10'' -

1 2x10'*-

1 0x10^-

8 0x10' '-

6 0x10' '-

4 0x10 ' ' -

2 0x10 ' ' -

Slope Comparison for 10* 14 seconds Recovery

4

•

•

^ " = % % ^ b)

—•— 400nm —•—426nm —^r- 4 S 0 n m

—»—SOOnm < 600nm

—»—630nm

t

3 1000 2000 3000 4000 5000 6000

Dose Rate(Rad/s)

Figure 3.19. Plot of the slope of the percent difference of recovery versus dose rate for 10^ (plot a) and lO'"* (plot b) seconds of recovery time. This" gives us some insight into the dose rate dependence of the recovery.

44

C H A P T E R IV

CONCLUSIONS

The lower operating limit for the accelerator was 7 Rad/s. This is still an

appreciable amount of radiation in comparison to the CMS - HF which will experience a

dose rate of 0.003 Rad/s at the location of lowest particle flux (^ = 3). In the region of

highest Ti the detector will receive ~31.5 Rad/s. This study provides the broadest range

possible considering the limitations of our equipment.

Optical fiber manufacturing is a multibillion dollar industry today and optical

fibers have foimd diverse applications from telecommunication, medicine, nuclear and

chemical industries and many others. There is no doubt that this trend will continue.

Fused-silica core optical fibers are being used in applications where there is high ionizing

radiation and/or neutron fluence, especially because fused-silica is a radiation hard

material. The radiation damage to optical fibers is a complicated phenomenon and the

exact natvire of color center's formation is not well-known. In this study, we attempted to

address a few but fimdamental questions currently studied in the community:

(1) How radiation hard is the polymer-clad, fused-silica core optical fiber? How

does the induced attenuation depend on the wavelength of light transported by

the fiber?

(2) How does the radiation dose rate affect the total optical transmission? What is

the wavelength dependence of this phenomenon?

(3) Is there a dynamic but stable transmission recovery in these types of fibers?

(4) What are the implications of the obtained results to the performance of the

CMS forward calorimeter at the Large Hadron Collider?

Multi-mode hard-clad optical fibers are very radiation hard. At 425 nm, the

transmission loss is ~ 50% at 100 MRad of accumulated dose. Parameterization of a

yields 0.2 to 1.2 depending on the dose rate. This is comparable with fluorine-doped

fused-silica clad fibers but, these fibers are simpler to produce and much less expensive.

45

We found that the dose rate has a significant effect on the optical transmission

loss. At 100 MRad, for example, the transmission loss is 5% if the dose rate is 7 Rad/s,

whereas the loss increases to 25% if the dose rate is 10 kRad/s at 450 nm. Clearly, the

rate at which dose is delivered to the fiber plays a significant role in the dynamics of the

color center formation and recovery.

There is an apparent recovery of optical transmission at the level of 10 to 20%

after several MRads of irradiation. The recovery is swift and levels off at a stable value.

When the fiber is exposed to radiation again, the transmission is quickly lost to near the

value when the previous irradiation session had ceased. Effectively, there is irradiation

memory in the fiber. Therefore, we can conclude that there is no real recovery for these

types of fibers.

The implication of this type of radiation damage to the CMS forward calorimeter

is not as severe as it was feared. The optical transmission of the fibers will be better than

50% at 1 Grad (10 years of LHC operation) at 425 nm. This is the worst part of the

detector. The other parts are less exposed to radiation, as low as 100 kRad in the same

amount of time.

As in all scientific research, although we answered some questions fully, some

other significant questions still remain. We need to investigate the couplings in the

optical damage and recovery parameters that we quantified in these studies. There is no

doubt that they are correlated. It may be that the present formulation needs to be

modified to expose these couplings. We covered a large span of dose rates, fi-om 7 Rad/s

to 10 kRad/s. In the lower dose rate data, the P parameter seems to show a different

behavior compared to the higher dose rate data. Data may very well be correct but this

issue has to be revisited.

46

BIBLIOGRAPHY

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48

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