Neutron Detector Capture Gate Analysis Software

1. Software for Neutron Detector Capture Gate Analysis Quesly Daniel, Jr. Florida A&M University Dr. Calvin Howell Duke University, TUNL David Ticehurst UNC-Chapel Hill, TUNL Quesly Daniel, Jr. Florida A&M University Dr. Calvin Howell Duke University, TUNL David Ticehurst UNC-Chapel Hill, TUNL

2. Motivation & Overview • We need the capability to detect nuclear materials crossing our borders (e.g. at weigh stations) • One method requires neutron detectors that do not rely of time-of-flight to measure energy, have high efficiencies, and can detect neutrons with energies less than 10 MeV

3. Motivation & Overview • We need the capability to detect nuclear materials crossing our borders (e.g. at weigh stations) • One method requires neutron detectors that do not rely of time-of-flight to measure energy, have high efficiencies, and can detect neutrons with energies less than 10 MeV • Therefore, TUNL is developing detectors with • excellent discrimination between neutrons and γ rays • excellent γ rejection (< 1 per million) • neutron spectroscopic ability

4. Detector Overview Filled with BC523A (4.41% 10B) liquid scintillator. Hydrocarbon-based scintillator has high neutron efficiency and 10B has a high thermal neutron cross section (3840 barns) Schematic diagram of the detector built at TUNL

5. Detector Overview • Initial signal from kinetic energy deposited by neutron • Delayed signal from energy released by thermalized neutron capturing on a boron nucleus: 10B (n,α)7Li

6. Project Overview • Detector read out by a 250MHz waveform digitizer. Waveforms analyzed offline. • Code written in ROOT for: • pulse shpe discrimination to distinguish between neutron and gamma events • coincidence gating between PMTs to reduce noise • Delayed signal gating on neutron events to select full energy events CAEN NT5720: 4-channel 250 MHz Desktop Digitizer

7. PSD Analysis Light output from neutron or alpha-initiated events decays proportionally slower than gamma-initiated events, allowing particle identification Source: Radiat Prot Dosimetry (2007) 126 (1-4): 253-255

8. PSD Analysis Signal is integrated and the “head” (the start of the signal to 44 ns in) is compared to the tail (from the 44 ns mark to the end of the signal)

9. PSD Analysis Histogram of the head-to-tail ratios taken from Americium-Beryllium (AmBe) source. Gamma rays are rejected by excluding events with small ratio (somewhat poor gamma-neutron separation may be from air leaks in to the detector)

10. Capture Gating Discarding events without delayed signal from neutron capture retains only events with full energy deposit . . . but signal from neutron capture is very small, requiring a large PMT bias Large PMT bias creates afterpulses—small signals occurring randomly after prompt pulse that mimic capture pulses Afterpulses are random, so coincidence requirement between PMTs eliminates them

11. Capture Gating Waveform taken from AmBesource

12. Capture Gatingwithout PMT coincidence Taken from AmBe source

13. Capture Gatingwith PMT coincidence Taken from AmBe source

14. Future Goals • Reseal scintillator tank • Update software to make cuts and gates on the fly • Attempt deconvolution of capture-gated neutron spectra

15. Special Thanks Dr. Calvin Howell, Advisor and Program Director David Ticehurst, Graduate Student and Mentor James Esterline, TILT Director Domestic Nuclear Detection Office (DNDO) Graduate students, advisors, and staff at TUNL/DFELL/LENA

16. Questions?