Decay Detector Development for Giant Resonance Studies

By: Gus OlsonMentor: Dr. Youngblood

Motivation• The energy of the Isoscalar Giant Monopole Resonance (EGMR) can

be used to deduce Knm, the incompressibility of nuclear matter.• Knm is an important parameter in several fields.

– Directly related to the curvature of the equation of state of nuclear matter.

– Helps in understanding nuclear structure and heavy ion collisions – Important value in nuclear astrophysics: supernova collapse and

neutron stars.– Provides a test for theoretical nuclear models, and nucleon-nucleon

effective interactions.• The giant resonance has been thoroughly studied in stable nuclei

over a wide range of A (12C-208Pb).• Future research directed towards the study of giant resonances in

unstable nuclei.

Giant Resonances• Collective nuclear excitations• Several oscillation modes: Monopole, Dipole, Quadrapole etc. • Isoscaler and Isovector resonances, as well as electric and

magnetic resonances exist for each resonance mode

isoscalar isoscalarisovector isovector____________magnetic___________________electric____________

Macroscopic diagrams of the giant resonances

Measuring Giant Resonances• Procedure for 28Si(α, α’):• MDM Spectrometer

– Beam of 240MeV α’s from the K500 cyclotron is inelastically scattered by target nuclei

– Momentum of scattered particles is analyzed by Dipole magnet

• Focal plane detector – Gas (isobutane) is ionized by

incoming particles– High voltage causes liberated

electrons to drift upwards– 4 resistive wires measure position – Plate at top of detector measures ΔE

for particle identification– Plastic Scintillator measures total

energy and gives a fast signal to trigger the electronics to acquire data.

• Scattering angle and energy for each particle are obtained by using position signals from each wire.

• To clearly identify the monopole resonance small angle (including 0°) measurements are necessary

Focal Plane Detector

TargetChamber

Dipole Magnet

Data Analysis

• Giant Resonances exist at about 10-40 MeV excitation energy • Lower energy peaks are single particle excitations• Large peak consists of all Giant Resonance collective excitations• Energy spectrum is separated into peak and continuum

contributions.• Continuum due primarily to knock-out and pick-up→break-up

reactions.

E=240 MeV 28Si(α,α’)

5Li p 5He n The break-up processes

Data Analysis (cont.)

• Spectrum is separated into energy “bins” (equal width energy intervals)– Angular distribution for each energy bin

• Each energy bin is fit by a weighted sum of the theoretical cross-sections for each of the resonance modes (from DWBA calculations) .– The weights give the strength

distribution of each resonance mode. – Using the strength functions of the

resonance modes we can obtain the energy of the resonance

28Si

28Si

Giant Resonance in Radioactive Nuclei

• Problem: Can’t use a radioactive target: decay products contaminate the target

• Use the inverse reaction, with a radioactive beam.– Low density of gaseous helium target means fewer interactions. Also, it is difficult

to contain the gas in the target chamber.– Beam intensity for a radioactive beam will be much lower so having a solid target

is essential.– Using solid 6Li target allows us to avoid difficulties involved with a gas target.

• We will use 28Si (which is, of course, not radioactive) as a test case to be sure the new detector gives us results consistent with previous methods.

28 28( , *)Si Si28 ( , ')Si Inverse Reaction:Normal Reaction:

28 6 6( , ')Si Li Li 6 28 28( , *)Li Si Si

Giant Resonance in Radioactive Nuclei

• Problem: The GR excited state has a very short lifetime

• Excitation energy of 28Si* can only be determined if the scattering angle and energy of both fragments are known.– Large fragments can be detected in the Focal plane detector as

before.– Small fragments require a new detector placed in the target

chamber.

28 27*Si Al p 28 24*Si Mg Two main decay channels

Decay Detector• Two 1mm thick layers of

scintillating plastic strips oriented vertically and horizontally measure the scattering angle of α’s and p’s.

• 3’’ thick scintillator blocks measure the total energy of the particles. – Together these scintillators

allow us to make particle determinations

• Scintillators will be connected to photomultiplier tubes (located outside the target chamber) via optical fibers

• Will be able to measure particles at ±35° vertically and horizontally. (each strip measures 5°)

Plastic Scintillators• Incoming charged particles lose

energy in the scintillator by exciting the molecules of the scintillator.

• Excited molecules decay by photon emission (peak output at ~420 nm for our scintillators (BC408)).

• Energy loss in the scintillator, and hence the light output, depends on the kinetic energy of the particle, its charge, and the thickness of the scintillator.

• Plastic scintillators are ideal for our needs– Very fast response (~2ns decay time)– Can be easily machined into the

shapes we need for our detector

Light Output• Calculating relative light output

– Energy loss per unit length (dE/dx, the stopping power) and range (x) estimates are obtained using a computer program (SRIM).

– Light output is related to energy loss by

– dL/dx is integrated to obtain L(x). • total light output of a particle which stops

completely in the scintillator at a range x.• This can be used for particle

determinations with the 3” scintillators.

– Light output for particles not totally stopped (as in the case of the thin scintillator strips) is obtained using the relation

og(1 )dL dE

L adx dx

225( / ) /a mg cm MeV

Light Output for Protons and Alphas

0

0.20.4

0.6

0.81

1.2

1.4

1.61.8

2

0 50 100 150 200 250 300

E(MeV)

L

Alpha

Proton

1.1415pL =0.0033E

1.2972L =0.0008E

[1] T.J. Gooding and H.G. Pugh, Nuclear Instruments And Methods 7, 189-192

[1]

( ) ( )tL L x L x t Where x=range and t=thickness of scintillator.

Optical Fibers

• Operate on the principle of total internal reflection– Most of fiber is core, surrounded

by a thin “cladding” with a lower index of refraction.

– At incident angles greater than the critical angle (θc=sin-1(nc/nf)) all light is reflected internally.

• Plastic optical fibers are flexible and can transmit light even when bent.

• We used fibers 1mm in diameter arranged in bundles to connect the scintillator to the PMT.

Claddingnc=1.49

Corenf=1.6

θ

Photomultiplier Tube

• Scintillation photons incident on photocathode.

• Photocathode emits electrons via the photoelectric effect

• High voltage accelerates electron towards dynodes

• On impacting each dynode secondary electrons are emitted

• Avalanche of electrons is converted to an electrical pulse at the anode

Test Case

• One scintillator strip connected via optical fibers to a photomultiplier tube with a beta source (90Sr) to test light output.

Fiber-bundle endsPlastic scintillator Photomultiplier tube

Testing• We must collect as much of the light as we

can to PMT to get reliable particle detection.

• Scintillation light is emitted in all directions some travels directly to the fibers but most must be reflected at the surface of the Scintillator

– Total internal reflection– External reflection by aluminum foil

• Must have good optical coupling between each of the components

– Surfaces need to be very flat and very clear– Optical cement, and optical grease are used

to make connections• Light Tight

– We must make sure that we can reliably seal off each component from any outside light leaking in or we will get false detects.

– Prevents cross-Talk between different scintillator strips.

Scint.

Al

Internal reflection

External reflection↑

↑

↑

-140

-120

-100

-80

-60

-40

-20

0

20

-20 -10 0 10 20 30

time(ns)

vo

ltag

e(m

V)

Sample PMT output: 7.3” long scintillator, 18” long fibers using a β-source (90Sr).

Testing (cont.)• We were concerned that we might not get enough light reflected in

the fibers due to the acceptance angle so we tested wrapping the fibers in Al:– We tested using 2” long fibers that had been wrapped in Al foil but this

showed no change in output amplitude. • Light attenuation in optical fibers:

– Tested with fiber lengths of 2”, 12”, and 18” with no appreciable amplitude difference.

• Light attenuation and reflection losses in scintillator:– Output shows great dependence on the position of the test-source:

~150-200mV with source close to the coupling with the fibers compared with ~40-60mV at the far end of the scintillator.

– The manufacturer’s rating indicates that light attenuation should not be a great problem at such short lengths (1/e of the original amplitude at 210cm), thus it seems that we are losing too much of the light on the multiple reflections down the scintillator.

Acknowledgments

• Department of Energy, National Science Foundation, Texas A&M University, Cyclotron Institute.

• DHY group: Dr. Dave H. Youngblood, Dr. Y.-W. Lui, Dr. Yoshiaki Tokimoto, Xinfeng Chen.