Artificial neural network algorithms for pulse shape discrimination and recovery of piled-up pulses in organic scintillators

A series of fluorescent silyl-fluorene molecules were synthesized and studied with respect to their photophysical properties and response toward ionizing neutron and gamma-ray radiation. Optically transparent and stable organic glasses were prepared from these materials using a bulk melt-casting procedure. The prepared organic glass monoliths provided fluorescence quantum yields and radiation detection properties exceeding the highest-performing benchmark materials such as solution-grown trans-stilbene crystals. Co-melts based on blends of two different glass-forming compounds were prepared with the goal of enhancing the stability of the amorphous state. Accelerated aging experiments on co-melt mixtures ranging from 0% to 100% of each component indicated improved resistance to recrystallization in the glass blends, able to remain fully amorphous for >1 month at 60 °C. Secondary dopants comprising singlet fluorophores or iridium organometallic compounds provided further improved detection efficiency, as evaluated by light yield and neutron/gamma particle discrimination measurements. Optimized singlet and triplet doping levels were determined to be 0.05 wt % 1,4-bis(2-methylstyryl)benzene singlet fluorophore and 0.28 wt % Ir3+, respectively.

Taking Advantage of Disorder: Small-Molecule Organic Glasses for Radiation Detection and Particle Discrimination

Abstract Large, 10 cm-size, single crystals of trans- stilbene were grown for the first time from solution by a temperature reduction technique. Their scintillation performance for fast neutron detection depended on the purity of the initial starting material. A light-yield-inhibitor-free starting material was specially synthesized and crystals from this material were compared to those grown from commercially available powders of trans -stilbene. The use of pure material made possible the growth of the large size crystals (>400 g) with excellent neutron–gamma discrimination performance, which are needed for use as a radiation detection material.

The effect of material purity on the optical and scintillation properties of solution-grown trans-stilbene crystals

This paper reports a series of measurements that characterize the directional dependence of the scintillation response of crystalline anthracene to incident DT neutrons, DD neutrons, $^{137}{\rm Cs}$ gamma rays, and, for the first time, cosmic ray muons. The neutron measurements give the amplitude and pulse shape dependence on the proton recoil direction over one hemisphere of the crystal, confirming and extending previous results in the literature. In similar measurements using incident gamma rays, no directional effect is evident, and any anisotropy with respect to the electron recoil direction is constrained to have a magnitude of less than a tenth of that present in the proton recoil events. Cosmic muons are measured at two directions, and no anisotropy is observed. This set of observations indicates that high $dE/dx$ is necessary for an anisotropy to be present for a given type of scintillation event, which in turn could be used to discriminate among different hypotheses for the underlying causes of the anisotropy, which are not well understood.

Investigating the Anisotropic Scintillation Response in Anthracene through Neutron, Gamma-Ray, and Muon Measurements

This paper reports a series of measurements that characterize the directional dependence of the scintillation response of crystalline melt-grown and solution-grown trans- stilbene to incident DT and DD neutrons. These measurements give the amplitude and pulse shape dependence on the proton recoil direction over one hemisphere of the crystal, confirming and extending previous results in the literature for melt-grown stilbene and providing the first measurements for solution-grown stilbene. In similar measurements of liquid and plastic detectors, no directional dependence was observed, confirming the hypothesis that the anisotropy in stilbene and other organic crystal scintillators is a result of internal effects due to the molecular or crystal structure and not an external effect on the measurement system.

Characterization of the scintillation anisotropy in crystalline stilbene scintillator detectors

A solution-based technique for growing large-volume stilbene scintillators was developed in 2013; crystals up to diameters of 10 cm, or larger, have been grown while preserving excellent pulse shape discrimination (PSD) properties. The goal of this study is to evaluate the PSD capabilities of 5.08 by 5.08-cm stilbene crystals grown by Lawrence Livermore National Laboratory and Inrad Optics when exposed to a 1000 to 1 gamma ray-neutron ratio and operating at a 100-kHz count rate. Results were compared to an equivalent EJ-309 liquid scintillation detector. 252 Cf neutron pulses were recorded in two experiments where 60 Co and 137 Cs sources created the high-gamma field. The high count rate created numerous double pulses that were cleaned using fractional and template approaches designed to remove double pulses while preserving neutron counts. PSD was performed at a threshold of 42 keVee (440-keV proton) for stilbene and 60 keVee (610-keV proton) for EJ-309 liquid. The lower threshold in stilbene resulted in a neutron intrinsic efficiency of approximately 14.5%, 10% higher than EJ-309 liquid, for bare 252 Cf and 13% for 252 Cf in the high-gamma field. Despite the lower threshold, the gamma misclassification rate in stilbene was approximately 3×10 −6 , nearly a factor-of-five lower than what we found with the EJ-309 liquid.

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N. Adamowicz

Papers

Neutron detection in a high-gamma field using solution-grown stilbene