Pulse Shape Discrimination Properties Research Articles

Pulse shape discrimination properties of plastic scintillators incorporating a rationally designed highly soluble and polymerizable derivative of 9,10-diphenylanthracene

A highly soluble and polymerizable derivative of 9,10-diphenylanthracene was designed and synthesized specifically to be capable of achieving very high loadings (at least 50wt.%) when copolymerized with a polyvinyltoluene (PVT) matrix. The resulting heavily crosslinked plastics are mechanically hard and robust, and were found to have exceptional clarity with no sign of dye precipitation. Samples of these plastics both with and without added wavelength shifter were characterized for light yield, scintillation decay, and pulse shape discrimination (PSD) performance for α/γ discrimination, and the results were compared to that of a commercially available PSD plastic, EJ-299-34. The best performing formulation, with a primary dye loading of 50wt.%, had a measured light yield of 9950 photons/MeV, and achieved a PSD figure-of-merit (FOM) of 1.05, the latter indicating that while the present material is not suited for practical applications, the overall approach demonstrates a proof-of-concept of PSD in highly loaded plastics stabilized through copolymerization of the primary dye, and suggests that further improvements through better dye choice/design may yet be achievable.

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

A solution-based technique for growing large-volume stilbene scintillators was developed in 2013; crystals up to diameters of 10cm, 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. 252Cf neutron pulses were recorded in two experiments where 60Co and 137Cs 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 42keVee (440-keV proton) for stilbene and 60keVee (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 252Cf and 13% for 252Cf 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.

Pulse-shape discrimination of the new plastic scintillators in neutron–gamma mixed field using fast digitizer card

Recently invented plastic scintillator EJ-299-33 enables pulse-shape discrimination (PSD) and thus measurement of neutron and photon spectra in mixed fields. In this work we compare the PSD properties of EJ-299-33 plastic and the well-known NE-213 liquid scintillator in monoenergetic neutron fields generated by the Van de Graaff accelerator using the 3H(d, n)4He reaction. Pulses from the scintillators are processed by a newly developed digital measuring system employing the fast digitizer card. This card contains two AD converters connected to the measuring computer via 10 Gbps optical ethernet. The converters operate with a resolution of 12 bits and have two differential inputs with a sampling frequency 1 GHz. The resulting digital channels with different gains are merged into one composite channel with a higher digital resolution in a wide dynamic range of energies. Neutron signals are fully discriminated from gamma signals. Results are presented.

Development and characterization of a neutron detector based on a lithium glass–polymer composite

We report on the fabrication and characterization of a neutron scintillation detector based on a Li-glass–polymer composite that utilizes a combination of pulse height and pulse shape discrimination (PSD) to achieve high gamma rejection. In contrast to fast neutron detection in a PSD medium, we combine two scintillating materials that do not possess inherent neutron/gamma PSD properties to achieve effective PSD/pulse height discrimination in a composite material. Unlike recoil-based fast neutron detection, neutron/gamma discrimination can be robust even at low neutron energies due to the high Q-value neutron capture on 6Li. A cylindrical detector with a 5.05cm diameter and 5.08cm height was fabricated from scintillating 1mm diameter Li-glass rods and scintillating polyvinyltoluene. The intrinsic efficiency for incident fission neutrons from 252Cf and gamma rejection of the detector were measured to be 0.33% and less than 10−8, respectively. These results demonstrate the high selectivity of the detector for neutrons and provide motivation for prototyping larger detectors optimized for specific applications, such as detection and event-by-event spectrometry of neutrons produced by fission.

Modeling of Time-correlated Detection of Fast Neutrons Emitted in Induced SNM Fission

Neutron multiplicity methods are widely used in the assay of fissile materials. Fission reactions release multiple neutrons simultaneously. Time-correlated detection of neutrons provides a coincidence signature that is unique to fission,which enables distinguishing it from other events. In general, fission neutrons are fast. Thermal neutron sensors require the moderation of neutrons prior to a detection event; therefore, the neutron's energy and the event's timing information may be distorted, resulting in the wide time windows in the correlation analysis. Fastneutron sensing using scintillators allows shortening the time correlation window. In this study, four EJ-299-33A plastic scintillator detectors with neutron/photon pulse shape discrimination properties were modeled usingthe MCNP6 code. This sensor array was studied for time-correlated detection of fast neutrons emitted inthe induced fission of 239Pu and (α,n) neutron sources. This paper presents the results of computational modeling of arrays of these plastic scintillator sensors as well as3He detectors equipped with a moderator.

Application of Wavelet Unfolding Technique in Neutron Spectroscopic Analysis

Nonproliferation of nuclear materials is important in nuclear power industry and fuel cycle facilities. It requires technologies capable of measuring and assessing the radiation signatures of fission events. Neutrons produced in spontaneous or induced fission reactions are mainly fast. The neutron energy information allows characterization of nuclear materials and neutron sources. It can also be applied in remote sensing and source search tasks. The plastic scintillator EJ-299-33A was studied as a fast neutron detector. The detector response to a polyenergetic flux was unfolded usingthe multiple linear regression method. It yields the intensities of neutron flux of particular energy, hence, enabling the spectroscopic analysis. The wavelet technique was evaluated for the unfolding of neutron spectrum using the scintillator's response functions between 1 MeV and 14 MeV computed with the MCNPX code. This paperpresents the computational results of the wavelet-based spectrum unfolding applied to a scintillator detector with neutron / photon pulse shape discrimination properties.

Boron-loaded plastic scintillator with neutron-γ pulse shape discrimination capability

Development of the plastic scintillator with neutron sensitivity from thermal to multi-MeV and pulse shape discrimination (PSD) has been demonstrated. Incorporation of 10B-containing compounds into the plastic scintillator with PSD capability leads to detector improvement in regard to neutron detection efficiency while preserving the discrimination between neutrons and γ-rays. Effects of boron loading on scintillation and pulse shape discrimination properties are discussed. A PSD figure-of-merit value of 1.4±0.03 has been achieved for events in a thermal neutron energy domain, 50–100keVee, for PSD plastic loaded with 5wt.% of m-carborane.

Neutron/gamma pulse shape discrimination in plastic scintillators: Preparation and characterization of various compositions

This work deals with the preparation and evaluation of plastic scintillators for neutron/gamma pulse shape discrimination (PSD). We succeeded in developing a plastic scintillator with good neutron/gamma discrimination properties in the range of what is already being commercialized. Several combinations of primary and secondary fluorophores were implemented in chemically modified polymers. These scintillators were fully characterized by fluorescence spectroscopy and under neutron irradiation. The materials proved to be stable for up to 5 years without any degradation of PSD properties. They were then classified in terms of their PSD capabilities and light yield. Our best candidate, 28.6wt% of primary fluorophore with a small amount of secondary fluorophore, shows promising PSD results and is particularly suited to industrial development, because its preparation does not involve the use of expensive or exotic compounds. Furthermore, even at the highest prepared concentration, high stability over time was observed. As a proof of concept, one sample with dimensions 109mm ∅×114mm height (≈1L) was prepared.

Target thickness dependence of the Be(p,xn) neutron energy spectrum

We report on the current status of the analysis of an experiment performed at The Svedberg Laboratory, with the aim of investigating the produced neutron field by Be(p,xn) converters of three different thicknesses with a 30 MeV proton beam. The neutron energy spectra were measured with the Time of Flight technique using a BC-501 liquid scintillator with good n-γ Pulse Shape Discrimination properties, while the detected events were recorded simultaneously by two Data AcQuisition systems. In this paper, we present the experimental setup, the analysis technique and some preliminary results.

Pulse shape discrimination with lithium-containing organic scintillators

6Li-containing organic scintillators have been prepared and characterized as a new type of transparent, single-phase materials with pulse shape discrimination (PSD) properties for simultaneous detection of thermal and fast neutrons discriminated from gamma radiation. Tests conducted with recently developed PSD-capable plastic scintillators showed that incorporation of 6Li into the aromatic matrix with fast-neutron/gamma discrimination properties offers the additional sensitivity to thermal neutrons, substantially increasing efficiency and the energy range of neutron detection. Comparative analyses of 6Li-loaded plastic, liquid and single crystal organic scintillators provide evidence that, in addition to neutron/gamma discrimination, these novel materials have the ability for discrimination between the signatures of fast and thermal neutrons.

Analysis of the pulse shape mechanism in a plastic scintillator with efficient neutron/gamma pulse shape discrimination

A sample of the new plastic scintillator recently formulated by Zaitseva et al., and shown by them to have efficient pulse shape discrimination properties, was exposed to pure gamma and mixed neutron/gamma radiation from different sources, and the pulse shapes recorded by a digitising oscilloscope. Separate average neutron and gamma pulse shapes were derived, using pulse shape discrimination where necessary.A theoretical pulse shape formula, based on the conventional two-lifetime description of pulse shape formation in organic scintillators, was then fitted to these measured shapes to obtain numerical values for the lifetimes and proportions of the scintillator excitations. It is found that, although the conventional model explains much of the observed pattern of values, there are hints of behaviour that is not consistent with its assumptions.

Pulse Shape Discrimination Properties of Neutron-Sensitive Organic Scintillators

The new plastic scintillators with n/γ pulse shape discrimination (PSD) properties being developed by the Lawrence Livermore National Laboratory (LLNL) and commercialized by Eljen Technology are addressing the toxicity and flammability issues of liquid scintillators, thus enabling a much wider range of practical applications for the detection of neutrons. These scintillation materials use multiple dyes, the concentration of which can vary, and therefore the light output and PSD properties of these new materials are expected to vary as well. In this paper, we compare the light signal time profiles of a liquid scintillator and two samples (one from LLNL and one from Eljen Technology) of new plastic scintillators with PSD properties. We acquired the light signal time profiles using both γ sources ( <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">60</sup> Co, <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">137</sup> Cs, <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">241</sup> Am) and neutrons calibrated in electron-equivalent by the gamma sources. The n/γ PSD properties for time profiles collected are analyzed and discussed with respect to charge integration time.

Pulse Shape Discrimination in Impure and Mixed Single-Crystal Organic Scintillators

Neutron/gamma pulse shape discrimination (PSD) utilized for detection of high-energy neutrons with organic scintillators was investigated using a model system of mixed diphenylacetylene-stilbene single crystals of different compositions. The results of the studies, which include experimental tools of crystal growth and characterization combined with computer simulation, showed that the presence of impurities with lower bandgap energies can be a major factor influencing PSD properties of organic materials. Depending on the concentration, an impurity may suppress or increase the rate of excited triplet state interaction leading, respectively, to a complete disappearance or enhancement of PSD, consistent with a percolation threshold. The results are applied to produce novel materials with controlled decay characteristics. Single crystals with a large fraction of delayed light and enhanced PSD have been grown for high energy neutron detection, while crystals with suppressed delayed light were produced for use as low-afterglow scintillators for energetic neutron detection in time-of-flight experiments.

Enhanced energy resolution ( 22Na, 137Cs and 241Am) and alpha–gamma ( 241Am + 60Co) and neutron–gamma ( 241Am–Be) pulse shape discrimination properties of p-terphenyl crystals prepared by modified selective self-seeded vertical Bridgman technique (SSVBT)

p -Terphenyl crystals were grown by modified selective self-seeded vertical Bridgman technique (SSVBT). This new technique has been adopted for nucleation and growth of seed crystals and the bulk crystals were grown by selective self-seeding method. The scintillation properties of the grown p -terphenyl crystal by the aforementioned method have been studied for different radioactive isotopes. Its particle-detection ability has been brought out by pulse-shape discrimination studies for alpha–gamma ( Am 241 + Co 60 ) and neutron–gamma ( 241Am–Be) sources. The results show that there is an enhancement in the scintillation characteristics of the SSVBT p -terphenyl crystals when compared to the commercially available p -terphenyl crystals.

CHELSI: a portable neutron spectrometer for the 20-800 MeV region

CHELSI is a CsI-based portable spectrometer being developed at Los Alamos National Laboratory for use in high-energy neutron fields. Based on the inherent pulse shape discrimination properties of CsI(Tl), the instrument flags charged particle events produced via neutron-induced spallation events. Scintillation events are processed in real time using digital signal processing and a conservative estimate of neutron dose rate is made based on the charged particle energy distribution. A more accurate dose estimate can be made by unfolding the 2D charged particle versus pulse height distribution to reveal the incident neutron spectrum from which dose is readily obtained. A prototype probe has been assembled and data collected in quasi-monoenergetic fields at The Svedberg Laboratory (TSL) in Uppsala as well as at the Los Alamos Neutron Science Center (LANSCE). Preliminary efforts at deconvoluting the shape/energy data using empirical response functions derived from time-of-flight measurements are described.

CHELSI: Recent developments in the design and performance of a high-energy neutron spectrometer

The intrinsic pulse shape discrimination properties of CsI(Tl) form the basis of a high-energy neutron (>20 MeV) spectrometer (CHELSI) currently being developed at LANL that shows promise in satisfying the requirements of an ideal survey meter; lightweight, portable and real time display of dose. Charged particle spallation products generated in the scintillator via neutron interactions are identified on the basis of pulse shape using digital pulse processing. Conservative estimates of dose rate can be given in real time based on count rates and pulse height distributions. More accurate dose measurements can be done off-line using unfolding methods to analyze stored pulse shape versus energy data. As a precursor to the development of a portable instrument, data has been obtained using a 1″×1″ CsI-based probe and a digital spectrometer. This system has been used to collect data on the 90 m flight path at the LANSCE/WNR facility at an average neutron energy of 335 MeV. The spectrometer has the capability, in addition to storing individual waveforms for later analysis, of recording time-of-flight data and calculating a pulse shape parameter and pulse height for each scintillation event in real time. Combining these data with traditional multichannel analyzer data has yielded a set of empirical response functions with respect to neutron energy. Analysis of the charged particle spectra has yielded an overall average count rate of 0.12±0.02 cps/μSv h for a 1″×1″ CsI(Tl) scintillator in this neutron field.

Digital pulse shape discrimination in organic scintillators for fusion applications

Stilbene and NE213 organic scintillators are commonly used for neutron and γ-ray detection in mixed n/γ fields due to their pulse shape discrimination properties. A system for n/γ digital pulse shape discrimination (DPSD) and simultaneous pulse height analysis using a commercial 12-bit 200 MHz transient recorder is presented. The results of measurements performed on the fusion experiment FTU (Frascati Tokamak Upgrade) are described. The importance of DPSD for fusion applications and its advantages with respect to analog PSD are analyzed.

A feasibility study of boron-loaded liquid scintillator for the detection of electron anti-neutrinos

Boron-loaded liquid scintillator offers some potential advantages as a detector for electron anti-neutrinos. A research program was carried out with the objective of developing such scintillators. The crucial feature is the pulse shape discrimination properties following the neutron capture by 10B. Results of the R&D efforts are presented. The feasibility and the technical difficulties of carrying out a full-scale neutrino experiment based on this approach are discussed.

Air gap effect on the properties of a long liquid scintillation detector

Light attenuation, n-γ pulse shape discrimination and timing properties of a long liquid scintillation detector (LLSD) were examined by means of Monte Carlo simulations and experiments. LLSD is a long rectangular shaped NE213 scintillator (80 × 6.5 × 10 cm 3) coupled with two photomultipliers and has been developed for neutron scattering experiments. We found that an air gap between a photomultiplier and a scintillator cell worked as a filter and reduced position dependencies of pulse height and of n-γ pulse shape discrimination which were disadvantages on the use of LLSD, whereas it deteriorated n-γ separation. This paper discusses the advantages and disadvantages of the air gap to the detector properties.

An analytical approach to the evaluation of the pulse shape discrimination properties of scintillators

In this paper a general methodology to compute the performances of different pulse shape discrimination techniques, applied to exploit the intrinsic PSD features of organic and of some inorganic scintillators, is presented. The most popular methods, i.e. the rise time technique and the charge integration approach, are evaluated and compared. As a specific example, detailed calculations are carried out for a binary organic liquid scintillator under consideration for the low energy solar neutrino experiment Borexino. It is demonstrated, in particular, that the charge integration method offers superior performances, the difference being remarkable for pulses comprising a low number of photoelectrons.