Decay Detector Development for Giant Resonance Studies

1
Decay Detector Development for Giant Resonance
Studies

• By Gus Olson
• Mentor 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

__________electric____________
____________magnetic_________
isoscalar
isoscalar
isovector
isovector
Macroscopic diagrams of the giant resonances
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Measuring Giant Resonances

• Procedure for 28Si(a, a)
• MDM Spectrometer
• Beam of 240MeV as 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 ?E 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
Dipole Magnet
Target Chamber
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Data Analysis
E240 MeV 28Si(a,a)

• 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
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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
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Giant Resonance in Radioactive Nuclei

• Problem Cant 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.

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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.

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 as and ps.
• 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

1
Where xrange and tthickness of scintillator.
1 T.J. Gooding and H.G. Pugh, Nuclear
Instruments And Methods 7, 189-192
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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 (?csin-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.

?
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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
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.

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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.

Sample PMT output 7.3 long scintillator, 18
long fibers using a ß-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 manufacturers 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 AM University, Cyclotron
  Institute.
• DHY group Dr. Dave H. Youngblood, Dr. Y.-W. Lui,
  Dr. Yoshiaki Tokimoto, Xinfeng Chen.