Presentation
Scintillator Characterization for Neutron Detection
Speaker
Bethany L. Goldblum
- Grad school/postdoc/staff at UCB
- Director of the Nuclear Policy Working Group
- Leader of Bay Area neutron group
- Nuclear forensics, structure, data
Organic Scintillators
- Work at the 88-in cyclotron
- Use scintillators for fast neutron detection, emit light due to elastic scattering -> organic scintilator (uses Carbon)
- Molecules are excited and de-excite to singlet bands via collisions, vibrations, etc
- Singlets de-ecite via flouresence
- Spin flip transition may lead to excited triplet state that de-excite via phosphoresence on much longer time scales. This is hindered by that required spin-flip.
- Of the properties of scintillators, will focus on light conversion, and decay time
Characterization Techniques
- Can fit decay curves with multi-componant exponentials, fit the data really well. No physical basis for this functional form. Would like a physics based model.
- Determination of light yield takes a long time, don’t indicate any error, so you can’t tell if they match. Identified the edge of the pulse-height distribution to identify the incident neutron energy (the neutron can pass at most all of its energy to the proton).
- Different sized cylinders give different answers, 5”, 2”, etc. Values are a little different based on cylinder size.
- A lot of discrepancies: models have issues, not measuring the radiation of interest directly (requires the neutrons to create protons)
Work
- Created a model-independant way of identifying light yield.
- In cave 0 at cyclotron, created an open air beam of neutrons
- Two different deuteron energies, Ed = 16 MeV and 44 MeV. Can tailor the spectrum and intensity of the source using diffrent materials for the breakup target.
- Moved a scintillator around to identify the size of the beam.
(0.5%) Stilbene Doped Bibenzyl Single-crystal Scintillator
- Intensity of light changes by orders of magnitude over nanoseconds
- Use temporal difference between the two PMTs to recreate the original waveform.
- High suppressed triplet-triplet annihilation
- Can model the flourescence rate well
- Used in nTOF diagnostics at NIF
- Created a physics-based method of identifying the light response
Light Yield Characteristics
- In the cyclotron, because it is a cycle, you have a contribution in each cycle from the previous cycle (slower neutrons). This makes TOF measurements difficult.
- Neutrons elastically scatters at scintillators and is detected in scattering cells.
- Use the angle and TOF to calculate the energy of the incident neutron and the proton energy deposited -> light yield directly.
- Don’t know what cyclotron pulse each neutron comes from
- Can use kinematics to use angle and TOF to identify which RF pulse it came from, to reduce error -> reject criteria.
- Compare nTOF and kinematically-calculated TOF to get “clean determination” of incoming nTOF.
- Can get Light Yield v. Proton Energy: bins are equally spaced in time due to resolution of ~4 ns
- Can now compare this to the data. Will use a MC approach to get systematic error in y-axis.
- This is a model independent approach, measurement can be made in 48 hours, reduces systematic bias and can go to very low proton energies.