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.     

Electron-Nuclear Recoil Discrimination by Pulse Shape Analysis

In the framework of the “ULTIMA” project, we use ultra cold superfluid ³He bolometers for the direct detection of single particle events, aimed for a future use as a dark matter detector. One parameter of the pulse shape observed after such an event is the thermalization time constant τ _b . Until now it was believed that this parameter only depends on geometrical factors and superfluid ³He properties, and that it is independent of the nature of the incident particles. In this report we show new results which demonstrate that a difference for muon- and neutron events, as well as events simulated by heater pulses exist. The possibility to use this difference for event discrimination in a future dark matter detector will be discussed.      

A neutron Albedo system with time rejection for landmine and IED detection

A neutron Albedo system has been developed for imaging of buried landmines and improvised explosive devices (IEDs). It involves irradiating the ground with fast neutrons and subsequently detecting the thermalized neutrons that return. A scintillating ⁶Li loaded ZnS(Ag) screen with a sensitive area of 40 cm×40 cm is used as a thermal neutron detector. Scintillation light is captured by orthogonal arrays of wavelength-shifting fibers placed on either side of the scintillator surface and then transferred to X and Y multi-pixel PMTs. A timing circuit, used with pulsed neutron sources, records the time when a neutron detection takes place relative to an external synchronization pulse from the pulsed source. Experimental tests of the Albedo system performance have been done in a sand box with a ²⁵²Cf neutron source (no time gating) and with pulsed D-D (2.6 MeV) neutrons from the Defense R&D Ottawa Van de Graaff accelerator (with time gating). Information contained in the time evolution of the thermal neutron field provided improved detection capability and image reconstruction. The detector design is described and experimental results are discussed.     

Characteristics of Gd-loaded liquid scintillators BC521 and BC525

Characteristic features of neutron detection properties of the loaded liquid scintillators, i.e., capture time distribution, pulse shape discrimination, and time resolution have been studied for two Gd-loaded liquid scintillators, BC521 and BC525. A new technique to measure the capture time distribution with a small size neutron detector (capacity 7.3 l) in combination with BaF₂ γ-ray detectors has been demonstrated. The measured capture time distributions were in good agreement with the results obtained by Monte Carlo simulation using the computer code DENIS.     

Variation of neutron detection characteristics with dimension of BC501A neutron detector

A systematic study of the variation of neutron detection characteristics (efficiency, pulse shape discrimination) and the intrinsic time resolution with the active volume of the detector has been carried out with liquid scintillator (BC501A)-based neutron detectors of various dimensions designed and fabricated for this purpose. Energy-dependent neutron detection efficiency has been measured using associated particle technique and its dependence on detector dimension has been studied. The measured efficiencies have been compared with those obtained from GEANT4 simulations.     

A CdTe detector with a Gd converter for thermal neutron detection

A CdTe detector with a Gd converter has been developed and investigated as a neutron detector for neutron imaging. The fabricated Gd/CdTe detector with the 25 μm thick Gd was designed on the basis of simulation results of thermal neutron detection efficiency and spatial resolution. The energy resolution of the Gd/CdTe detector is less than 4 keV, which is enough to discriminate neutron capture gamma rays from background gamma emission. The Gd/CdTe detector shows the detection of neutron capture gamma ray emission in the ¹⁵⁵Gd(n, γ)¹⁵⁶Gd, ¹⁵⁷Gd(n, γ)¹⁵⁸Gd and ¹¹³Cd(n, γ)¹¹⁴Cd reactions and characteristic X-ray emissions due to conversion-electrons generated inside the Gd film. The observed efficient thermal neutron detection with the Gd/CdTe detector shows its promise in neutron radiography application.     

Gaseous detector of ionizing radiation for registration of coherent neutrino scattering on nuclei

A method for registration of the coherent scattering reactor antineutrino on nuclei using a three-section low-background proportional counter was proposed. It is planned to use argon and xenon as the working substance. As has been shown on a test bench, pulse shape discrimination can effectively suppress the background from electromagnetic interference and microphonic effects in the energy range from 20 to 100 eV where the effect of coherent scattering of neutrinos on nuclei is expected with a factor of about 10³. Problems of the neutron background generated by cosmic-ray muons are analyzed. The scheme of the experimental setup is presented.     

Comparison of n-γ discrimination by zero-crossing and digital charge comparison methods

A comparative study of the n-γ discrimination done by the digital charge comparison and zero-crossing methods was carried out for a 130 mm in diameter and 130 mm high BC501A liquid scintillator coupled to a 130 mm diameter XP4512B photomultiplier. The high quality of the tested detector was reflected in a photoelectron yield of 2300 ± 100 phe/MeV and excellent n-γ discrimination properties with energy discrimination thresholds corresponding to very low neutron (or electron) energies. The superiority of the Z/C method was demonstrated for the n-γ discrimination method alone, as well as, for the simultaneous separation by the pulse shape discrimination and the time-of-flight methods down to about 30 keV recoil electron energy. The digital charge comparison method fails for a large dynamic range of energy and its separation is weakly improved by time-of-flight method for low energies.     

Performance enhancements of compound semiconductor radiation detectors using digital pulse processing techniques

The potential benefits of using compound semiconductors for X-ray and gamma ray spectroscopy are already well known. Radiation detectors based on high atomic number and wide band gap compound semiconductors show high detection efficiency and good spectroscopic performance even at room temperature. Despite these appealing properties, incomplete charge collection is a critical issue. Generally, incomplete charge collection, mainly due to the poor transport properties of the holes, produces energy resolution worsening and the well known hole tailing in the measured spectra. In this work, we present a digital pulse processing (DPP) system for high resolution spectroscopy with compound semiconductor radiation detectors. The DPP method, implemented on a PC platform, performs a height and shape analysis of the detector pulses (preamplifier output pulses), digitized by a 14-bit, 100 MHz ADC. Fast and slow shaping, automatic pole-zero adjustment, baseline restoration and pile-up rejection allow precise pulse height measurements both at low and high counting rate environments. Pulse shape analysis techniques (pulse shape discrimination, linear and nonlinear pulse shape corrections) to compensate for incomplete charge collection were also implemented. The results of spectroscopic measurements on a planar CdTe detector show the high potentialities of the system, obtaining low tailing in the measured spectra and energy resolution quite close to the theoretical limit. High-rate measurements (up to 820 kcps) exhibit the excellent performance of the pulse height analysis and the benefits of pulse shape techniques for peak pile-up reduction in the measured spectra. This work was carried out in the framework of the development of portable X-ray spectrometers for both laboratory research and medical applications.     

Resolution tests of CsI(Tl) scintillators read out by pin diodes

Cylindrical CsI(Tl) scintillators of 38 mm diameter and 100 mm length read out with PIN diodes of 400 mm² area were tested with respect to their response to medium energy light particles (p, d, t, α). Resolutions of better than 1% were achieved for 50 MeV protons and 90 MeV α-particles. For many crystals the resolution was found to be limited to 2–3% by local crystal nonuniformities which caused variations of the light output efficiency of several percent. A bench test is described which allows the detection of inhomogeneities to better than 0.5% accuracy. The quality of particle identification obtained with ΔE-E and pulse shape discrimination techniques are investigated as a function of count rate.     

Development of a Position Sensitive Neutron Detector with High Efficiency and Energy Resolution for Use at High-Flux Beam Sources

We are developing a high-efficiency neutron detector with 1 cm position resolution and coarse energy resolution for use at high-flux neutron source facilities currently proposed or under construction. The detector concept integrates a segmented ³He ionization chamber with the position sensitive, charged particle collection methods of a MicroMegas detector. Neutron absorption on the helium produces protons and tritons that ionize the fill gas. The charge is amplified in the field region around a wire mesh and subsequently detected in current mode by wire strips mounted on a substrate. One module consisting of a high-voltage plate, a field-shaping high-voltage plate, a grid and wire strips defines a detection region. For 100 % efficiency, detector modules are consecutively placed along the beam axis. Analysis over several regions with alternating wire strip orientation provides a two-dimensional beam profile. By using ³He, a 1/v absorption gas, each axial region captures neutrons of a different energy range, providing an energy-sensitive detection scheme especially useful at continuous beam sources.     

Two-dimensional neutron detector based on a position-sensitive photomultiplier

A new type of two-dimensional neutron scintillation detector with high spatial resolution based on a position-sensitive photomultiplier has been investigated. With a ⁶Li glass scintillator a spatial resolution of 1.0 mm was measured. The integral linearity over the detection area of 55 × 45 mm² is ⩽1.5 mm. The detector homogeneity is within 10% at a discriminator level at 60% of the average peak height. The spatial resolution is expressed by a geometrical parameter which could be evaluated from light experiments and by the number of photoelectrons per neutron capture.     

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.     

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.     

Determination of the momentum distribution of scattering particles from high-Q scattering spectra of a resonance detector neutron spectrometer

We have measured high-Q neutron scattering spectra from various crystaline solids using an eV resonance detector spectrometer, and analyzed the different contributions to the line profile, namely the finite time spread of the neutron pulse, the detector resolution, the finite size of the moderator, sample and detector, the scattering function and multiple scattering processes, by first-principle calculations. The calculated spectra agree well with the experiments. The momentum distribution of scattering particles can be determined from the high-Q limit of the scattering function. However, deviations from this asymptotical form are clearly observed even for momentum transfers around 100 Å⁻¹.     

A small spherical NE213 scintillation detector for use in in-assembly fast neutron spectrum measurements

A small spherical 14 mm diameter NE213 scintillation spectrometer has been developed. Its linearity, gain stability and pulse shape discrimination were examined through experiments using a 14 MeV neutron field. The pulse responses were analyzed by a Monte Carlo code and compared with the measured ones. The detector proposed by us is superior in in-assembly measurements.     

Development of neutron-monitor detector using liquid organic scintillator coupled with 6Li + ZnS(Ag) Sheet

A phoswitch-type detector has been developed for monitoring neutron doses in high-energy accelerator facilities. The detector is composed of a liquid organic scintillator (BC501A) coupled with ZnS(Ag) sheets doped with 6Li. The dose from neutrons with energies above 1 MeV is evaluated from the light output spectrum of the BC501A by applying the G-function, which relates the spectrum to the neutron dose directly. The dose from lower energy neutrons, on the other hand, is estimated from the number of scintillations emitted from the ZnS(Ag) sheets. Characteristics of the phoswitch-type detector were studied experimentally in some neutron fields. It was found from the experiments that the detector has an excellent property of pulse-shape discrimination between the scintillations of BC501A and the ZnS(Ag) sheets. The experimental results also indicate that the detector is capable of reproducing doses from thermal neutrons as well as neutrons with energies from one to several tens of megaelectronvolts (MeV).     

Spectral measurement of angular neutron flux on the restricted surface of slab assemblies by the time-of-flight method

An experimental method of neutron angular spectrum measurement in a slab geometry with D-T neutrons has been developed and its characteristics were examined. This method separates collided flux from uncollided flux. A neutron collimator restricts the measuring area on the surface of a slab assembly and the detector-collimator response function was determined. The time-of-flight method with two discrimination levels was adopted using an NE213 scintillation detector with pulse shape discrimination to obtain an accurate efficiency. Such measurements provide useful information in fusion blanket research. The results of the spectrum measurement were compared with calculated neutron spectra. The estimated overall error of the spectrum was within 5%.     

Low-energy neutron spectrometer using position sensitive proportional counter—Feasibility study based on numerical analysis

There is no direct technique to measure a neutron energy spectrum, particularly in the lower energy region, because the reaction Q value for detection is much larger than the neutron energy to be measured. However, such techniques are becoming a necessity, for example, in medical applications such as boron neutron capture therapy. In this study, a new spectrometer to measure low-energy neutrons (from thermal to 100 eV) is investigated numerically. We propose a unique approach of estimating the neutron energy spectrum by analyzing the distribution of neutron detection depths in the detector using an exact relation between the neutron energy and nuclear reaction cross-section. The proposed spectrometer has been established to be feasible to manufacture. The conversion performance of the neutron detection depth distribution to the neutron energy spectrum has also been proven to be acceptable, with the unfolding process based on Bayes’ theorem, even though the detector response function is non-distinctive (without peaks or edges). The present spectrometer is now under development, and its practical performance will be reported as soon as the prototype detector is completed.