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Progress on Table Top Neutron Source for the Calibration of Nuclear Recoil Detectors

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### 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

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.

Experimental Setup

NaI(Tl) Coupled to

Photomultiplier Tube

Shielding

Pb

Polyethylene

Borated Silicone

NaI(Tl) Crystal

D1A

252Cf

60 cm

D2

D1B

6LiI Crystal

60 cm

D3

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

Event Rate Estimation

- 252Cf source :
- Decay Rate 0.1737 GBq (109 decays per second)
- 3.1% into fissions, 5.385 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.

Coupled the 2nd and 3rd Detectors

D2

D3

6LiI Crystal

NaI(Tl) Crystal

Fast PMT (response < 1 ns)

High Speed Amplifier

6LiI : Ideal for 3rd Detector

- Do not need PSD to discriminate neutrons from gammas. Use the (n,alpha) reaction.

Measured 6LiI Spectrum with a multi channel analyzer

Y-88 gamma lines at 898 keV and 1.836 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.

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.

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.

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