decay detector development for giant resonance studies
DESCRIPTION
Decay Detector Development for Giant Resonance Studies. By: Gus Olson Mentor: Dr. Youngblood. Motivation. The energy of the Isoscalar Giant Monopole Resonance (E GMR ) can be used to deduce K nm , the incompressibility of nuclear matter. K nm is an important parameter in several fields. - PowerPoint PPT PresentationTRANSCRIPT
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Decay Detector Development for Giant Resonance Studies
By: Gus OlsonMentor: Dr. Youngblood
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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.
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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
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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
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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
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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
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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
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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
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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°)
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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
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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.
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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
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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
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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
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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).
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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.
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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.