April 2009 1

OSL Albedo Neutron Dosimeter

Ikuo Kobayashi Ph.D.

April 2009 2

Overview

• How OSL albedo neutron dosimeter (OSLN) works

• Dosimeter Design Concept

• OSLN Type Testing and Comparison to OSL Characteristics

• Neutron Energy Response

April 2009 3

How It Works

• Al2O3:C coated with 6Li2CO3.• Neutron response is induced in the Al2O3:C

when 6Li absorbs a neutron (1n0) and produces both tritium (3H) and alpha (4He) particles.

6Li + 1n0 = 4He2 (2.05 MeV) + 3H (2.75 MeV)

• Alpha and beta particles give up their energy in absorber via coulomb interactions.

April 2009 4

Design Philosophy Maintained• Reader calibration protocols not impacted.• Sensitivity will be obtained from the slide.

– 2D barcode on slide will have the OSLN and OSL sensitivity on board

– NXXXYYY######, N = slide type, X = OSL sensitivity, Y = OSLN sensitivity, # = 1 - 999999

• YA case will be used (OW, PL, Al, and Cu). Same configuration as XA.

* Density thickness of case only.

April 2009 5

Configuration

Plastic (PL)

Aluminum (Al)

Copper (Cu)

2D Barcode – communicates both OSL and OSLN sensitivity.

OSLN Detector in position 2 (E2)

OSLN detectors have a slight pinkpink tint.

April 2009 6

OSLN Type Testing and Comparison to OSL Characteristics

• Material tested using the International Electrotechnical Commission (IEC) standard 62387-1– Material Linearity– Fade– Energy Response

• Depletion• Optical Annealing

April 2009 7

Material Linearity• OSL and OSLN material response tested from 0 – 1000 R

– Bias within 1% from 0 - 100 R, 5% from 100 - 500 R, and 10% from 500 - 1000 R.

– Readers initially tested up to 1250 R• OSLN:OSL response within ± 2% from 0 – 500 R but

shows a slightly higher variation at 1000 R (+4%).Linearity Bias Plot (OSLN and OSL)

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Delivered Dose (mrem)

Bia

s OSL Bias

OSLN Bias

Linearity of Material (OSLN and OSL)

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Delivered Dose (mR)

Resp

onse

OSL

OSLN

April 2009 8

Fade• OSLN fade estimated

to be less than 4% annually.– Actual data from 0 -

180 days extrapolated to 390 days.

• No significant variation noted between OSL and OSLN fade.

0.94

0.95

0.96

0.97

0.98

0.99

1

1.01

Normalized Response

0 60 120 180 240 300 360

Days After Irradiation

OSL and OSLN Fade

OSLN OSL

April 2009 9

Photon Energy Response

• The typical energy dependence of the OSL material was also exhibited in the OSLN material.

• The effects of the OSLN over response to photons in the 24 – 65 keV region were not as pronounced. – The 6Li2CO3 reduced the energy over

response by approximately 10 to 20% over the 24 – 65 keV photon energy range.

April 2009 10

Photon Energy Response CurvePhoton Energy Response

OSLN and OSL Material

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0.500

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16 24 33 47 65 83 117 208 662

Photon Energy (keV)

Res

po

nse

/Hs

OSLN

OSL

Photon Energy Response

OSLN and OSL Material

0.700

0.750

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10 110 210 310 410 510 610 710Photon Energy (keV)

Res

po

nse

/Hs

OSLN:OSL

April 2009 11

Depletion

• ~ 5% depletion was noted for both the OSL and OSLN detectors after 25 reads on the strong beam. – Similar depletion rate for OSLN as OSL

material (rule of thumb ~10% depletion after 50 reads).

– The re-readability of the detector materials was preserved.

April 2009 12

Depletion CurveDepletion Curve

0.920

0.940

0.960

0.980

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1.020

1.040

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Read Number

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rmal

ized

Res

po

nse

OSLN OSL

April 2009 13

Optical Annealing• Optical annealing effectiveness studied from 0 -

1000R using the InLight 50A Annealer.• Recommended anneal levels OSL = 5 converted

values and OSLN = 10 converted values both based on 99% confidence interval.

• 500 mrem can be annealed in 60 s• The annealing effectiveness of the OSLN and OSL

material can be modeled using a classic decay curve (I = I0 * e-bt). – decay constant for bOSL = 0.246/s and bOSLN = 0.240/s

April 2009 14

Optical Annealing Curves

Decay of Response from LED Annealing

Model 50A - Automatic Annealer (Exposed Dose = 10R)

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Anneal Time (s)

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sp

on

se

OSLN

OSL

OSLN Anneal Level

OSL Anneal Level

Decay of Response from LED Annealing

Model 50A - Automatic Annealer (Exposed Dose = 500 mrem)

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10.00

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0 200 400 600 800 1000

Anneal Time (s)

Re

sp

on

se

OSLN

OSL

OSLN Anneal Level

OSL Anneal Level

April 2009 15

Neutron Energy Response Testing

• NPL– Monoenergetic neutrons 40 keV to 5

MeV• PNNL

– Cf-252 bare (2 MeV), Cf-252 moderate (615 keV), and AmBe (4.4 MeV)

• PTB– 14 MeV (irradiations in progress)

• Landauer– AmBe (4.4 MeV)

April 2009 16

OSL Albedo Neutron Correction Factor

• Hn = Net OSLN / NCF

• NCF ranges from 0.08 for higher energies to 5 for moderated fields.

• Moderated 252Cf response off curve.

April 2009 17

Net OSLN Response to Various Neutron Fields

^ N. Hertel and J. McDonald, Calculations of Anisotropy Factors and Dose Equivalent for Unmoderated 252Cf Sources , Radiation Protection Dosimetry, Vol. 32 No. 2 pp. 77-82 (1990)

# N. Hertel and J. McDonald, Calculated Dosimetric Quantities for D2O Moderate 252Cf Sources, Radiation Protection Dosimetry, Vol. 35 No. 1 pp. 23-30 (1991)

#

^

April 2009 18

Comparison with Other Neutron Dosimeters

• OSLN and UD-802 same response/mrem characteristic• Significant variations noted with all albedo neutron

dosimeter for response:dose• CR-39 response:dose the same for the 2 radiation fields

+N. Stanford, Stanford Dosimetry, Three Whole Body Algorithms for the Pantex Plant Panasonic 802 TLD, February 1992

+

April 2009 19

Field Testing

• Field testing conducted at Callaway Nuclear Power Plant, Lawrence Berkeley National Laboratory, Hanford Site, Georgia Power, and US Army.

April 2009 20

Field Testing

• Data obtained from Callaway Nuclear Power Plant.• Average NCF of 1.57 as compared to CR-39 result

(relative to AmBe response)• Neutron energy ~90 keV (determined from derived

function)

April 2009 21

Using OSL Albedo Dosimeter

• Field specific neutron corrections required.– Significant errors can result in inappropriate

use of any albedo dosimeter.– Must know your neutron radiation

environment.– Similar to other Albedo products on the

market (Panasonic UD-802 and Harshaw TLD-760).

• Neutron field must be know for neutron performance testing.

April 2009 22

Conclusions

• OSLN material characteristics are similar OSL.

• Specific corrections will be required to accurately assess the neutron dose similar to Harshaw or Panasonic Albedo Technique.

• Provides a low cost alternative, quicker results, and easier dose determination than CR-39 neutron monitoring.