1. Progress on Table Top Neutron Source for the Calibration of Nuclear Recoil Detectors
Dante Nakazawa
with Prof. Juan Collar
For Physics 335 with Prof. Mark Oreglia

2. Reminder of Project
Facility of Table Top Neutron Source : Calculation of neutron response of scintillating crystals. Ability to discern the direction of recoil.
Proof of Principle
Calculate neutron energy spectrum of 252Cf neutron source with time-of-flight spectroscopy.
Using this initial neutron energy scattering angle to calculate the energy of the recoiling atom (Na or I).
Measure the known scintillation response of a NaI(Tl) crystal to neutron recoils w.r.t. gamma recoils.

3. Experimental Setup NaI(Tl) Coupled to Shielding Photomultiplier TubePb
Polyethylene
Borated Silicone
NaI(Tl) Crystal
D1A
252Cf
60 cm D2
D1B
6LiI Crystal
60 cm D3

4. Experimental Setup
D1A
D1B D2 D3

5. When does a fission occur?
Each fission from 252Cf emits 20 gammas and 4 neutrons
PNNL solution : try to cover at least 3 of the source with gamma detectors (also NaI in our case) and trigger from a coincidence.
NaI(Tl) Coupled to
Photomultiplier Tube
252Cf
shielding

6. PMT voltage
D1A
D1B
time
Coincidences

7. Event Rate Estimation 252Cf source : Measured :
Decay Rate GBq (109 decays per second)
3.1% into fissions, MBq
Measured :
337.5 kBq, roughly 10% of events. This seems fine considering not all gammas, incident on the phototubes, will be above triggering threshold.

8. Logic Electronics D1A D1B 10x Amp 10x Amp discriminator discriminator
AND Logic Unit

9. Coupled the 2nd and 3rd Detectors
6LiI Crystal
NaI(Tl) Crystal
Fast PMT (response < 1 ns)
High Speed Amplifier

10. 6LiI : Ideal for 3rd Detector
Do not need PSD to discriminate neutrons from gammas. Use the (n,alpha) reaction.

11. Measured 6LiI Spectrum with a multi channel analyzer
Y-88 gamma lines at 898 keV and MeV
Co-60 gamma lines at 1.1 and 1.3 MeV
Am-Be
gamma line at 4.4 MeV, neutrons at 4-8 MeV
Cf : 80% of gammas are below 1 MeV. (n,alpha) reaction
in 6Li peaks at 3 MeV so pulse height discrimination will work.

12. Time-of-flight in 60cm (ns)
Problems
Shifting baseline from Timing PMTs (D1A and D1B). Probably need to look for coincidences offline in the data analysis phase.
6LiI Detector we have may not have the efficiency necessary for nanosecond timing needed for 252Cf spectrum measurements. Need to build a liquid or plastic scintillator and use pulse shape discrimination.
Energy
Time-of-flight in 60cm (ns)
10 keV
433
100 keV
137
1 MeV 43
5 MeV 19

13. To do :
Once we have timing coincidence nailed down and another scintillator, we can acquire spectrum of 252Cf.
Develop the Data Acquisition Software (LabVIEW):
Trigger from 3rd detector once well shielded against background neutrons. Store waveform of D2 and coincidence detectors.
Check timing information from D3, D2, and coincidence logic for energy conservation per event.
Calculate nuclear recoil energy from initial energy (time-of-flight) and scattering angle.
Integrate the pulse of 2nd detector.
Acquire NaI(Tl) gamma response and calculate the scintillation efficiency.