Special Section Papers

Detection of Combined n/γ Fission Signatures Induced by an Epithermal Neutron Source

[+] Author and Article Information
A. Ocherashvili, A. Beck

Physics Department,
Nuclear Research Center Negev (NRCN),
P.O. Box 9001,
Beer-Sheva 84190, Israel

T. Bogucarska, M. Mosconi, E. Roesgen, J.-M. Crochemore, V. Mayorov, G. Varasino, B. Pedersen

Nuclear Security Unit,
Joint Research Centre,
Institute for Transuranium Elements (ITU),
European Commission,
Via Enrico Fermi 2749,
Ispra, VA 21027, Italy

G. Heger

Israel Atomic Energy Commission (IAEC),
P.O. Box 7061,
Tel Aviv 61070, Israel

Manuscript received October 30, 2016; final manuscript received April 13, 2017; published online May 25, 2017. Assoc. Editor: Jean Koch.

ASME J of Nuclear Rad Sci 3(3), 030911 (May 25, 2017) (6 pages) Paper No: NERS-16-1151; doi: 10.1115/1.4036698 History: Received October 30, 2016; Revised April 13, 2017

In this paper, a method is presented for the detection of special nuclear materials (SNMs) in shielded containers, which is both sensitive and applicable under field conditions. The method uses an external pulsed neutron source to induce fission in SNM and subsequent detection of the fast prompt fission neutrons. The detectors surrounding the container under investigation are liquid scintillation detectors able to distinguish gamma rays from fast neutrons by means of pulse shape discrimination method (PSD). One advantage of these detectors, besides the ability for PSD analysis, is that the analog signal from a detection event is of very short duration (typically few tens of nanoseconds). This allows the use of very short coincidence gates for the detection of the prompt fission neutrons in multiple detectors, while benefiting from a low background coincidence rate, yielding a low detection limit. Another principle advantage of this method derives from the fact that the external neutron source is pulsed. By proper time gating, the interrogation can be conducted by epithermal source neutrons only. These neutrons do not appear in the fast neutron signal following the PSD analysis, thus providing a fundamental method for separating the interrogating source neutrons from the sample response in the form of fast fission neutrons. This paper describes laboratory tests with a configuration of eight detectors in the Pulsed Neutron Interrogation Test Assembly (PUNITA). Both the photon and neutron signature for induced fission is observed, and the methods used to isolate these signatures are described and demonstrated.

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Fig. 1

Sketch of PUNITA showing the permanently mounted neutron detectors and the neutron generator mounted inside the sample cavity (left picture). The right-hand picture shows the positioning of the eight liquid scintillation detectors within the sample cavity of PUNITA.

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Fig. 2

Triggering scheme for data recording during epithermal neutron interrogation

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Fig. 3

Typical waveforms of epithermal data (a), and picture (b) is a zoom of the last signal. Time zero in picture (a) corresponds to 28 μs after the 14 MeV burst; the length of the stream is 95 μs.

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Fig. 4

Fraction of the neutron signals to all signals as a function of the time following the n-generator burst (a). The insert (b) shows the time behavior of the thermal flux in PUNITA.

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Fig. 5

Epithermal neutron cross section for 235U(n,γ), 238U(n,γ), and Al(n,γ) (top picture) and 235U(n,f) and 238U(n,f) (bottom picture). The Monte Carlo simulated source neutron spectrum at a given time after the n-generator burst is also indicated.

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Fig. 6

Distribution of detection time t0 of high energy neutron associated pulses with only the empty container present (CBNM000)

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Fig. 7

PSD spectrum from a 252Cf neutron source (a). Picture (b) shows the low amplitude part of the upper picture.

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Fig. 8

Energy distribution of signals from CBNM000 (empty container) and the CBNM446 samples with regions of interest (ROI) for future analysis

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Fig. 9

Neutron detections as a function of time for the CBNM446 sample: (a) entire picture, (b) zooming of first 400 ns, and (c) zooming of first 40 ns

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Fig. 10

Difference between PG and DG gates of photon (a) and neutron (b) pair events normalized to the neutron emission of the neutron generator

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Fig. 11

Single photon count rates as a function of 235U mass for ROI #2 (a) and ROI #3 (b)




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