A compact radiation imaging system capable of detecting, localizing, and characterizing special nuclear material (e.g. highly-enriched uranium, plutonium…) would be useful for national security missions involving inspection, emergency response, or war-fighters. Previously-designed radiation imaging systems have been large and bulky with significant portions of volume occupied by photomultiplier tubes (pMts). the prototype imaging system presented here uses silicon photomultipliers (SipMs) in place of pMts because SipMs are much more compact and operate at low power and voltage. the SipMs are coupled to the ends of eight stilbene organic scintillators, which have an overall volume of 5.74 × 5.74 × 7.11 cm 3. the prototype dual-particle imager's capabilities were evaluated by performing measurements with a 252 Cf source, a sphere of 4.5 kg of alpha-phase weapons-grade plutonium known as the BeRP ball, a 6 kg sphere of neptunium, and a canister of 3.4 kg of plutonium oxide (7% 240 Pu and 93% 239 Pu). these measurements demonstrate neutron spectroscopic capabilities, a neutron image resolution for a Watt spectrum of 9.65 ± 0.94° in the azimuthal direction and 22.59 ± 5.81° in the altitude direction, imaging of gamma rays using organic scintillators, and imaging of multiple sources in the same field of view.

An accurate model of the nonlinear detector response of organic scintillators to neutrons is required to correctly simulate fast neutron detection, as well as interpret measured pulse height data. Several empirical and semi-empirical models are available to fit measured scintillator light output data. In this work, EJ-309 light output data from neutrons depositing 1.15 MeV to 5.15 MeV on hydrogen wereanalyzed using empirical models as well as semi-empirical models based on the work of Birks and Voltz. Although all tested models fit the experimental light output data well in the measured range, the models were observed to diverge in low-energy extrapolation. The modelswerethen tested by comparing a measurement and MCNPX-PoliMi simulation of an EJ-309 detector response to fast neutrons from a 252 Cf spontaneous fission source. The agreement between the measured and simulated pulse height distributionsvaried significantly depending on the light output model used. The best agreement between simulated and measured neutron pulse height distributionswas achieved by using the Birks model. The bin-by-bin agreement was better than 5% over the range 0.08 to 2.18 MeVee, and better than 10% from 2.18 to 3.13 MeVee. The integral count rate over the range 0.08 to 3.14 MeVee differed by less than 1% in absolute units. 1.0 Introduction The IAEA is interested in high-fidelity Monte Carlo modeling of detector technologies for international safeguards applications[1]. Several ongoingsafeguards projects employ organic scintillators as fast neutron detectors, such as theLiquid-Scintillator Neutron Coincidence Collar (LS-NCC) [1],the Fast Neutron Multiplicity Counter (UM-FNMC) [2,3],radiation portal monitors (RPMs)[4,5], and the Dual Particle Imager (DPI) [6-8]. Organic scintillators are also frequently employed in a wide variety of applications including, but not limited to, nuclear physics [9], material characterization [6,3,10,2], imaging [6-8], and nuclear medicine[11,12].

He replacement technology a b s t r a c tWe present new experimental results from a radiation portal monitor based on the use of organic liquid scintillators. The system was tested as part of a 3 He-free radiation portal monitor testing campaign at the European Commission's Joint Research Centre in Ispra, Italy, in February 2014. The radiation portal monitor was subjected to a wide range of test conditions described in ANSI N42.35, including a variety of gamma-ray sources and a 20,000 n/s 252 Cf source. A false alarm test tested whether radiation portal monitors ever alarmed in the presence of only natural background. The University of Michigan Detection for Nuclear Nonproliferation Group's system triggered zero false alarms in 2739 trials. It consistently alarmed on a variety of gamma-ray sources travelling at 1.2 m/s at a 70 cm source to detector distance. The neutron source was detected at speeds up to 3 m/s and in configurations with up to 8 cm of high density polyethylene shielding. The success of on-the-fly radionuclide identification varied with the gamma-ray source measured as well as with which of two radionuclide identification methods was used. Both methods used a least squares comparison between the measured pulse height distributions to library spectra to pick the best match. The methods varied in how the pulse height distributions were modified prior to the least squares comparison. Correct identification rates were as high as 100% for highly enriched uranium, but as low as 50% for 241 Am. Both radionuclide identification algorithms produced mixed results, but the concept of using liquid scintillation detectors for gamma-ray and neutron alarming in radiation portal monitor was validated.

a b s t r a c tPulse shape discrimination (PSD) techniques can be used to discern between neutron and gamma-ray interactions in certain organic scintillators. Traditionally, photomultiplier tubes (PMTs) have been used in organic-scintillator assemblies. However, silicon photomultipliers (SiPMs) have great potential to be used in many applications in which PMTs have been predominantly used, including those utilizing PSD techniques. To evaluate the current state of the art of the SiPM technology, SensL's 6-mm B-Series and CSeries SiPMs were compared to a fast Hamamatsu PMT in conjunction with a 6 Â 6 Â 6-mm 3 stilbene organic scintillator to assess the PSD performance of the detector assemblies. Measurements with a Cf-252 source were performed and a figure of merit (FOM) for discriminating between neutron and gamma-ray pulses between 100 keVee and 200 keVee was calculated for each assembly. A digital charge-integration PSD technique was used to process all measured data. The FOM for the B-Series SiPM, PMT, and C-Series SiPM was 1.37, 1.93, and 2.13, respectively. The C-Series SiPM was shown to perform as well as the PMT in the experiments.