Suppression of gamma-ray sensitivity of liquid scintillators for neutron detection

Methods to reduce gamma-ray sensitivity of a liquid scintillator EJ309 have been studied. Zero-crossing pulse shape discrimination method was used to separate events generated by neutron and gamma radiation between 60− keVee and 4 MeVee. The measurements were carried out under irradiation from an intense ¹³⁷Cs source, yielding dose rate of 10 mR/h at the detector. A Pu-Be source was used to establish neutron integration window. Pile-up rejection (PUR) circuit was used to reduce gamma-ray induced events under irradiation from an intense gamma-ray source. Further, application of lead, tin and copper shields was done in order to decrease intrinsic gamma-neutron detection efficiency.     

A model for fitting peaks induced by fast neutrons in an HPGe detector

Inelastic neutron scattering in the HPGe detector produces wide, triangular-shaped peaks in the spectrum. We develop an accurate model for the peak shape and show that the inclusion of the model in the gamma spectrum analysis makes it possible to quantify fast neutron scattering in the Ge crystal and improves the estimation of the baseline. This in turn facilitates the detection of fission products present at trace levels in environmental samples. The model, together with simulations, is used to deduce some properties of the underlying neutron energy distribution. The neutron evaporation temperature of 1.1 MeV is obtained from the analysis of environmental monitoring gamma spectra.     

Response of a BaF2 scintillation detector to quasi-monoenergetic fast neutrons in the range of 45 to 198 MeV

We have studied the neutron response of a scintillation detector consisting of a 14 cm long, hexagonal-shaped BaF₂-crystal with an inner diameter of 8.75 cm coupled to an EMI9821QB photomultiplier tube. The detector was exposed to calibrated quasi-monoenergetic neutron fields obtained from ⁷Li(p,n)⁷Be reactions. The measurements were performed at neutron energies of 45, 60, 96, 147 and 198 MeV as given by the energies of the incident protons. The experimental pulse-height spectra of the BaF₂-detector are compared with Monte Carlo simulations using the FLUKA code. The detection efficiency of the BaF₂-detector in the energy range of 45–198 MeV was determined as a function of the discriminator threshold and compared to the literature data. At neutron energies above 100 MeV the detection efficiency of the BaF₂-detector was found to be a factor of two higher than that of an NE213-detector of comparable size.     

Flat-response neutron detector using spatial distribution of thermal neutrons in a moderator

We propose a novel neutron detector that realized flat-response using information of a spatial distribution of thermal neutrons in a moderator. The proposed detector consists of a ³He position sensitive proportional counter (PSPC) and a cylindrical moderator surrounding the ³He PSPC. The cylindrical detector is irradiated by neutrons along the cylinder axis. The spatial response of the ³He PSPC is used to correct the detector response into flat-response. We adopt a weighting method to achieve flat-response, in which detected neutrons weighted depending on their detected positions are accumulated as the detector response. Through Monte Carlo simulation studies, we confirm that the flat-response neutron detector can be realized by correcting the response of the proposed detector using the weights determined by a multiple least square method (MLSM). Additionally, fundamental property of the ³He PSPC is experimentally investigated to check applicability to the proposed flat-response neutron detector. We conclude that we should take account of the end effect when determining the weights and correcting the detector response.     

Comparison of PVT and NaI(Tl) scintillators for vehicle portal monitor applications

The demand for radiation portal monitor (RPM) systems has increased, and their capabilities are being further scrutinized as they are being applied to the task of detecting nuclear weapons, special nuclear material, and radiation dispersal device materials that could appear at borders. The requirements and constraints on RPM systems deployed at high-volume border crossings are significantly different from those at weapons facilities or steel recycling plants, where RPMs have been historically employed. In this new homeland security application, RPM systems must rapidly detect localized sources of radiation with a very high detection probability and low false-alarm rate, while screening all of the traffic without impeding the flow of commerce. In light of this new Department of Homeland Security application, the capabilities of two popular gamma-ray-detector materials as applied to these needs are re-examined. Both experimental data and computer simulations, together with practical deployment experience, are used to assess currently available polyvinyltoluene and NaI(Tl) gamma-ray detectors for border applications.     

Synthesis and scintillation properties of CsGd2Cl7:Ce for gamma ray and neutron detection

In this work, we report on optical and scintillation properties of a new scintillation material CsGd₂Cl₇:Ce. Crystals were grown in vacuum sealed ampoules, and the response to gamma rays and thermal neutrons was characterized. The scintillation light yield was ∼38,000 ph/MeV, and the primary decay time was 60 ns under γ-ray excitation. A thermal neutron response resulting from capture by ¹⁵⁷Gd and ¹⁵⁵Gd and subsequent emission of conversion electrons and X-rays was observed. The crystals exhibited a layered structure with cleavage planes and relatively low hygroscopicity.     

Development of multi-moderator neutron spectrometer using a pair of Li and Li glass scintillators

A multi-moderator spectrometer using a pair of ⁶Li and ⁷Li glass scintillators has been developed. This new type of neutron spectrometer can measure the neutron spectrum in a mixed field of neutrons, charged particles and gamma-rays. The particle identification capability was investigated in neutron–gamma-ray and neutron–proton mixed fields and the neutron response functions of the spectrometer were obtained by calculations and experiments up to 200 MeV. This spectrometer has been applied to measure neutron spectrum in a neutron–proton mixed field, produced by bombarding a Be target by 70 MeV protons from the cyclotron.     

A comparison of performance between organic scintillation crystals and moderated He-based detectors for fission neutron detection

Direct detection of fast neutrons using organic scintillators is one alternative to moderated thermal neutron detectors deployed to detect fission neutrons—a relevant question in light of dwindling ³He supplies. Recent developments in materials science have demonstrated the capability to grow larger crystals in reasonable times. In light of these developments, this study compares the relative performance of a ³He-based neutron module from a commercially available portal monitor with a theoretical organic scintillator of similar overall size. Stilbene serves as a benchmark with its performance estimated from a combination of energy deposition modeled by radiation transport calculations and an assumption of the lowest neutron energy at which pulse shape discrimination can effectively separate neutron and gamma-ray events. Before intrinsic detection efficiencies on par with moderated detector systems can be achieved, the results point to the need for further advances including significant increases in detector size, especially thickness, and/or lower pulse shape discrimination thresholds.     

Design and performance of vacuum capable detector electronics for linear position sensitive neutron detectors

We describe the design and performance of electronics for linear position sensitive neutron detectors. The eight tube assembly requires 10 W of power and can be controlled via digital communication links. The electronics can be used without modification in vacuum. Using a transimpedance amplifier and gated integration, we achieve a highly linear system with coefficient of determinations of 0.9999 or better. Typical resolution is one percent of tube length.     

Evaluation of large deuterated scintillators for fast neutron detection (E=0.5-20 MeV) using the D(d,n)He, C(d,n) and Al(d,n) reactions

The response of large deuterated liquid scintillators (up to 10 cm diameter by 15 cm) to neutrons 0.5-20 MeV has been studied using the 2.5 MeV neutron generator at the University of Michigan, and the d(d,n), ¹³C(d,n), ²⁷Al(d,n) and other reactions at the University of Notre Dame FN tandem accelerator. The latter utilize 9 and 16 MeV deuteron beams including a pulsed beam, which permitted time-of-flight measurements. Combining pulse-shape discrimination and time-of-flight allows gating on specific neutron energy groups to determine the detector response to specific neutron energies. This will permit accurate simulation of the detector response functions for applications of these detectors in nuclear research and homeland security applications.     

Proposal for measuring the quantum states of neutrons in the gravitational field with a CCD-based pixel sensor

An experimental setup is proposed for the precise measurement of the quantum states of ultracold neutrons bound in the earth's gravitational field. The experiment utilizes a CCD-based pixel sensor and magnification system to observe the fine structure of the neutron distribution. In this work, we analyzed the sensor's deposited energy measurement capability and found that its spatial resolution was . A magnifying power of two orders of magnitude was realized by using a cylindrical rod as a convex mirror.     

Specular reflection of thermal neutrons from Gd-containing layers and optimization of antireflective underlayers for polarizing coatings

A detailed analysis of specular neutron reflection from an absorbing medium is given. The experimental studies revealed that oxidation and roughness of the surface are the main factors that determine the neutron reflectivity of Gd-containing layers. It is concluded that the empirical approach used at present does not guarantee optimization of underlayer parameters (composition and thickness) and technological regimes. An algorithm of optimization is proposed, in which account is taken of the substrate potential, the dependence of the underlayer potential on the thermal neutron wavelength, the polarizing coating imperfections that enhance reflection of neutrons with the undesired spin. The antireflective TiZrGd underlayer for CoFe/TiZr supermirrors produced at the magnetron facility DIOGEN (PNPI, Gatchina) is optimized.     

Design and study of an enhanced Faraday cup detector

Faraday cups (FCs) have been used to measure charged particle beams currents for a wide variety of experiments since the early days of particle accelerators. The major error source in FC performance is the escape of ejected electrons produced by energetic particles striking with interior metal surfaces of the FC. To avoid this problem, electrostatic and magnetic guard rings are used. Some kinds of non-symmetrical electrostatic FC with superior performance than traditional cylindrically symmetric FC have been reported. Here, we propose a novel non-symmetrical electrostatic FC structure, and a method to compare different non-symmetrical FCs. The purpose of this design is to recapture energetic electrons with lower guard ring bias. Performance simulations of the proposed FC show considerable improvement over other reported FCs.     

Novel and efficient B lined tubelet detector as a replacement for He neutron proportional counters

This paper presents a novel and robust proportional detector which addresses the well publicized shortage of ³He gas by using a ¹⁰B lining applied to a tubelet configuration. The advantage of the tubelet structure is that it yields a detector maintaining the form factor of a conventional ³He tube whilst achieving a sensitivity of up to 75% of a 3 atm ³He device. The design and fabrication of the tubelet detector is presented and discussed with test data comparing the new detector to existing ³He and BF₃ tubes. The application of the tubelet design to security and industrial applications including retro-fitting to existing portals and suitability for high integrity oil and gas installations is addressed.     

Evolution of phase stresses in Al/SiC_p composite during thermal cycling and compression test studied using diffraction and self-consistent models

In this work,the evolutions of stresses in both phases of the Al/SiCp composite subjected to thermal cycling during in situ compression test were measured using Time of Flight neutron diffraction.It was confirmed that inter-phase stresses in the studied composite can be caused by differences in the coefficient of thermal expansion for the reinforcement and matrix,leading to a different variation of phase volumes during sample heating or cooling.The results of the diffraction experiment during thermal cycling were well predicted by the Thermo-Mechanical Self-Consistent model.The experimental study of elastic-plastic deformation was carried out in situ on a unique diffractometer EPSILON-MDS(JINR in Dubna,Russia)with nine detector banks measuring interplanar spacings simultaneously in 9 orientations of scattering vector.For the first time,the performed analysis of experimental data allowed to study the evolution of full stress tensor in both phases of the composite and to consider the decomposition of this tensor into deviatoric and hydrostatic components.It was found that the novel Developed Thermo-Mechanical SelfConsistent model correctly predicted stress evolution during compressive loading,taking into account the relaxation of thermal origin hydrostatic stresses.The comparison of this model with experimental data at the macroscopic level and the level of phases showed that strengthening of the Al/SiCp composite is caused by stress transfer from the plastically deformed A12124 matrix to the elastic SiCp reinforcement,while thermal stresses relaxation does not significantly affect the overall composite properties.     

Effects of proton escape on detection efficiency in thin scintillator elements and its consequences for optimization of fast-neutron imaging

Plastic scintillators are commonly used for neutron detection in the MeV energy range, based on n-p scattering and the subsequent deposition of recoil proton's kinetic energy in the detector material. This detection procedure gives a quasi-rectangular energy deposition distribution for mono-energetic neutrons, extending from zero to the neutron energy. However, if the detector sensitive element (DSE) is small, the energy deposition may be incomplete due to the recoil proton escape.In the application of neutron imaging, here exemplified by fast-neutron tomography, two conflicting requirements have been identified: (1) thin DSEs are required to obtain high spatial resolution and (2) energy discrimination may be required to reduce the influence of neutrons being scattered into the DSEs, which generally occurs at lower energies. However, at small DSE widths, the reduction of energy deposition due to recoil proton escape may cause a significant decrease in detection efficiency when energy discrimination is applied.In this work, energy deposition distributions in small-size DSEs have been simulated for Deuterium-Deuterium (DD; 2.5 MeV) and Deuterium-Tritium (DT; 14.1 MeV) fusion neutrons. The intrinsic efficiency has been analyzed as a function of energy discrimination level for various detector widths. The investigations show that proton recoil escape causes a significant drop in intrinsic detection efficiency for thin DSEs. For DT neutrons, the drop is 10% at a width of 3.2 mm and 50% at a width of 0.6 mm, assuming an energy threshold at half the incident neutron energy. The corresponding widths for a DD detector are 0.17 and 0.03 mm, respectively.Finally, implications of the proton escape effect on the design of a fast-neutron tomography device for void distribution measurements at Uppsala University are presented. It is shown that the selection of DSE width strongly affects the instrument design when optimizing for image unsharpness.     

Characterization and development of an active scintillating target for nuclear reaction studies on actinides

This article presents the development of a new kind of active actinide target, based on organic liquid scintillators containing the dissolved isotope. Amongst many advantages one can mention the very high detection efficiency, the Pulse Shape Discrimination capability, the fast response allowing high count rates and good time resolution and the ease of fabrication. The response of this target to fission fragments has been studied. The discrimination of alpha, fission and proton recoil events is demonstrated. The alpha decay and fission detection efficiencies are simulated and compared to measurements. Finally the use of such a target in the context of fast neutron induced reactions is discussed.