Decay Detector Development for
Giant Resonance Studies
By: Gus Olson
Mentor: 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
__________electric____________
isoscalar
isovector
____________magnetic_________
isoscalar
isovector
Macroscopic diagrams of the giant resonances
Measuring Giant Resonances
•
•
Procedure for 28Si(α, α’):
MDM Spectrometer
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–
•
Focal plane detector
–
–
–
–
–
•
•
Beam of 240MeV α’s from the K500
cyclotron is inelastically scattered by
target nuclei
Dipole
Momentum of scattered particles is
Magnet
analyzed by Dipole magnet
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.
Target
To clearly identify the monopole
resonance small angle (including 0°) Chamber
measurements are necessary
Focal Plane
Detector
Data Analysis
E=240 MeV
28Si(α,α’)
•
•
•
•
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.
The break-up processes
5
Li    p
5
He    n
Data Analysis (cont.)
28Si
• 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
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.
Normal Reaction:
Si ( ,  ')
28
6
6
Si( Li, Li ')
28
•
Inverse Reaction:
 ( Si, Si*)
28
6
28
28
28
Li( Si, Si*)
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.
Giant Resonance in Radioactive
Nuclei
• Problem: The GR excited state has a very short lifetime
28
Two main decay channels
28
27
Si*  Al  p
Si*  24 Mg  
• 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.
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
• 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 Protons and Alphas
L
– 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
[1]
dL
dE
 L og(1  a )
a  25(mg / cm2 ) / MeV
dx
dx
– dL/dx is integrated to obtain L(x).
Lp =0.0033E1.1415
2
1.8
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0
Alpha
Proton
L =0.0008E1.2972
0
50
100
150
200
E(MeV)
– Light output for particles not totally
stopped (as in the case of the thin
scintillator strips) is obtained using the
relation
Lt  L( x)  L( x  t )
Where x=range and t=thickness of scintillator.
[1] T.J. Gooding and H.G. Pugh, Nuclear Instruments And Methods 7, 189-192
250
300
Optical Fibers
Cladding
nc=1.49
• 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.
Core
nf=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
Plastic scintillator
Fiber-bundle ends
Photomultiplier tube
• One scintillator strip connected via optical fibers to a
photomultiplier tube with a beta source (90Sr) to test light
output.
Testing
•
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Scint.
↑
External reflection
Total internal reflection
External reflection by aluminum foil
Surfaces need to be very flat and very clear
Optical cement, and optical grease are used
to make connections
Light Tight
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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.
Al
20
0
Must have good optical coupling between
each of the components
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–
•
Internal reflection
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
-20
voltage(mV)
•
-40
-60
-80
-100
-120
-140
-20
-10
0
10
20
tim e(ns)
Sample PMT output: 7.3” long
scintillator, 18” long fibers using a βsource (90Sr).
30
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.