Neutron Counting Efficiency Research Articles

Small-angle neutron scattering geometry with ring-shaped collimation for compact neutron sources

The neutron flux of compact neutron sources is low, making it difficult to measure scattered neutrons with a typical small-angle neutron scattering (SANS) geometry. A SANS geometry with ring-shaped collimation (r-SANS) was thus developed to enable experiments to be conducted using low-flux neutron source facilities. In this geometry, circumferentially collimated beams hit a sample. The scattered neutron flux is high on the ring center axis because scattered neutrons with each nominal scattering angle are superimposed on each point of the ring center axis. A 3He point detector – which has higher neutron counting efficiency compared with that of a typical scintillation detector and can also properly distinguish neutron from gamma events – is set on the ring center axis. These features make high signal-to-noise ratio experiments possible. Monte Carlo (MC) simulation studies were conducted to evaluate the characteristics of the r-SANS geometry. The results show that oblique incident neutrons, which cause cross-talk, asymmetrically smear the scattering intensity, and increase the incident neutrons. To study the feasibility of the r-SANS geometry, it was tested at the Kyoto University Accelerator-driven Neutron Source (KUANS), which is one of the most compact and has one of the lowest neutron fluxes among available neutron beam facilities. Two types of porous silica gels, HC-N and C-500HG, were examined. As reference, the same samples were analyzed using a small-angle X-ray scattering (SAXS) spectrometer under a typical pinhole geometry. To consider the smearing effect of the r-SANS geometry, MC simulations with SAXS experimental results were conducted. The simulated results agree with the r-SANS experimental results. In addition, to show the consistency of the scattering model parameters determined via r-SANS with those via SAXS, a parameter survey was performed, revealing reasonable agreement between the results.

Survey and design of an irradiation setup for measuring the amount of heavy water in a sample

An irradiation system, which works based on photodisintegration reaction of deuterium, has been evaluated for determination of heavy water in water samples. This irradiation system was designed by simulation with MCNPX code, considering some gamma point-like sources placed in the center of sample, and then counting the created neutrons by the long cylindrical BF3 counter. For increasing the neutron counting efficiency, an optimization was performed for thickness of various moderators considered between the sample and thermal detector. To investigate the practical effectiveness of the designed system, a model apparatus was built for routine use in measurement of a wide range of heavy water concentrations. It was successfully applied for weight percentages larger than 0.5 % to observe the acceptable detection sensitivity of D2O with an error less than 0.24 %.

GEM-based thermal neutron beam monitors for spallation sources

The development of new large area and high flux thermal neutron detectors for future neutron spallation sources, like the European Spallation Source (ESS) is motivated by the problem of 3He shortage. In the framework of the development of ESS, GEM (Gas Electron Multiplier) is one of the detector technologies that are being explored as thermal neutron sensors. A first prototype of GEM-based thermal neutron beam monitor (bGEM) has been built during 2012. The bGEM is a triple GEM gaseous detector equipped with an aluminum cathode coated by 1μm thick B4C layer used to convert thermal neutrons to charged particles through the 10B(n,7Li)α nuclear reaction. This paper describes the results obtained by testing a bGEM detector at the ISIS spallation source on the VESUVIO beamline. Beam profiles (FWHMx=31mm and FWHMy=36mm), bGEM thermal neutron counting efficiency (≈1%), detector stability (3.45%) and the time-of-flight spectrum of the beam were successfully measured. This prototype represents the first step towards the development of thermal neutrons detectors with efficiency larger than 50% as alternatives to 3He-based gaseous detectors.

A Design Study for a New Neutron Counter Based on an Annular Tube-Type 3He Detector through a Monte Carlo Simulation

A new well-type neutron counter that is composed of a double-layered annular tube 3He detector, a polyethylene moderator, and a lead gamma shield has been conceptually designed through an MCNPX simulation. A computer program that generates MCNPX input simulating neutron counter performance has been developed with 28 parameters to describe the geometrical structure of the neutron counter and source position. The neutron counter has been optimized to have 72.0-cm diameter × 68.3-cm height with a double-layered annular tube 3He detector of 50-cm length and 2.5-cm thickness through repeated MCNPX simulation with the computer program. The neutron counting efficiency appeared to be >42.4%, and the spatial variance in the sample cavity of the counter was estimated to be <3% for its available space.

Microstructured semiconductor neutron detectors

Perforated semiconductor neutron detectors are compact diode detectors that operate at low power and can be fashioned to have high thermal neutron detection efficiency. Fabricated from high-purity Si wafers, the perforations are etched into the diode surface with ICP-RIE and backfilled with 6LiF neutron reactive material. The intrinsic thermal neutron detection efficiency depends upon many factors, including the perforation geometry, size, and depth. Devices were fabricated from high resistivity 10 k Ω cm n-type Si with conformal p-type shallow junction diffusions into the perforations, which demonstrate improved neutron detection performance over previous selectively diffused designs. A comparison was made to previous selectively diffused designs, and pulse height spectra show improved signal-to-noise ratio, higher neutron counting efficiency, and excellent gamma-ray discrimination. Devices with 20 ( average ) μ m wide 100 μ m deep sinusoidal trenches yielded intrinsic thermal neutron detection efficiencies of 11.94 ± 0.078 %.

Characteristics of 3D Micro-Structured Semiconductor High Efficiency Neutron Detectors

Silicon diodes with large aspect ratio perforated micro-structures backfilled with <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> LiF show a dramatic increase in neutron detection efficiency beyond that of conventional thin-film coated planar devices. Described in the following are advancements in the technology with increased perforation depths. Perforated silicon diodes with three different etched micro-structure patterns were tested for neutron counting efficiency. The etched micro-structure patterns consisted of circular holes, straight trenches, and continuous sinusoidal waves, with each pattern etched 200 mum deep. Normal incident neutron counting efficiencies were determined to be 9.7%, 12.6%, and 16.2% for circular hole, straight trench, and sinusoidal devices, respectively, at a reverse bias of 3 volts. The perforated neutron detectors demonstrate limited sensitivity to high-energy photon irradiation with a <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">60</sup> Co gamma-ray source. This work is part of on-going research to develop solid-state semiconductor neutron detectors with high detection efficiencies and uniform angular responses.

Calibration experiments of He3 neutron detectors for analyzing neutron emissivity in the hot-ion mode on the GAMMA 10 tandem mirror

Under the international fusion cooperating research, He3 neutron detectors in the GAMMA 10 tandem mirror are calibrated by the use of a Cf252 spontaneous fission neutron source (8.96×104 n/s). The calibration experiments are carried out with a “rail system” placed along the magnetic axis of the GAMMA 10 central-cell region, where hot ions in the plasma experiments with the bulk temperatures of ∼10 keV are produced. As compared to a previous neutron monitoring system with a BF3 detector in GAMMA 10, the present He3 systems are designed with about two orders-of-magnitude higher neutron-counting efficiency for analyzing a neutron emissivity from the plasmas in a single plasma discharge alone. Two He3 systems are installed near the middle and the end of the central cell so as to identify the central-cell hot-ion axial profile. The filling pressure of He3, the effective length, and the diameter of the detector are designed as 5 bar, 300 mm, and 50 mm, respectively. The detector output spectra are carefully analyzed by the use of a preamplifier, a shaping amplifier, as well as a multichannel analyzer for each He3 detector. In the present article, the neutron-counting data from the two He3 detectors due to the on-axis Cf252 scan are interpreted in terms of the d−2 intensity dependence (d being the distance between the detector and the neutron source) as well as the effects of the central-cell magnetic coils and the other machine structural components.

Integral test of a boron-10 loaded liquid scintillator for neutron detection

A neutron detection system using a boron loaded liquid organic scintillator is presented. Double photomultipliers and a coincidence circuitry are used to enhance light collection and noise rejection. The neutron/gamma separation is based on pulse shape discrimination. Evaluations on time resolution, gamma rejection, MeV neutron response, and slow neutron counting efficiency are performed using standard neutron/gamma sources and a Van de Graaff accelerator. Comparisons with an NE213 neutron spectrometer and a BF 3 counter are reported.

An associated particle technique for absolute neutron counting efficiency determination

The 7Li(p, n) 7Be reaction has been used to produce neutrons with energies ranging from 8 to 13 MeV in order to measure the absolute neutron counting efficiency of a large NE213 scintillator by the associated particle method. Recoil 7Be nuclei were detected with a ΔE (gas)- E (solid) telescope in coincidence with neutrons. The method is suitable for neutron energies greater than 1.2 MeV and could be applied to establish the neutron efficiency response of any detector.

New thermal-neutron solid-state electronic detector based on HgI2 single crystals

The use of HgI2 as a thermal-neutron solid-state electronic detector, in particular its application for neutron diffractometry, is demonstrated for the first time. A single crystal HgI2 detector is used to count prompt gamma emissions (0.2–5 MeV) from (n,γ) nuclear reactions in Gd or Cd foils. The neutron counting efficiency depends on the HgI2 detector thickness. For a 1-mm thickness of HgI2 the efficiency is about 10% compared to the efficiency of a 10BF3 gas detector.

The contribution of carbon interactions to the neutron counting efficiency of organic scintillators

The neutron counting efficiency of an NE-213 detector was measured for neutron energies up to 15 MeV using biases at 0.3 and 2.0 MeV neutron energies. For the lower bias, nonelastic carbon interactions contribute appreciably to the efficiency at energies as low as 5.5 MeV.

A Search for Naturally Occurring Superheavy Elements

IN a continuing effort to measure neutron emission in natural samples for evidence of superheavy elements, several ores, minerals, concentrates, and special samples have been examined in our neutron multiplicity counter1. This counter, containing twenty3He detectors in a paraffin matrix, enables the evaluation of the emitted neutron multiplicity spectrum of large samples (volume 20 1) with little or no chemical processing. Such measurements provide an effective tool in the search for superheavy elements, since either their decay or the decay of daughter nuclides is expected to proceed by spontaneous fission. Furthermore, the parameter v (average number of prompt neutrons emitted in the fission act) has been estimated2,3 to be very high for superheavy nuclides; for example, v (298114 or296112)≃10 in contrast to v ≲ 4 for all known spontaneously fissioning nuclides. In addition to providing a unique indicator, this predicted high value of v enhances the sensitivity for detecting superheavy elements. A neutron multiplicity counter with a single neutron counting efficiency ɛ° = 0.30 would exhibit an efficiency of ɛ(>3)=0.62 for observing multiplicities of three or greater and ɛ(>4)=0.35 for observing multiplicities of four or greater in a fission event in which ten neutrons were emitted. On the basis of a capability for detecting 1.0 fission event per day for a nuclide with a half life of 108 yr (approximately the shortest half life for a nuclide to have survived geological times), the maximum sensitivity of the counter for such nuclides can be estimated to be ˜5 × 1010 atoms or ˜10−15 g g−1 in a 25 kg sample.

Large area Li 6I(Eu) detector bank for slow neutron time-of-flight measurements

For slow neutron time-of-flight experiments a large surface detector bank was constructed which uses 20 Li 6I(Eu) crystals, 2mm thick and 3″ in diameter, mounted on 20 phototubes. After a general consideration of the properties of LiI(Eu) scintillators, the construction of the bank is described. Some typical performances are presented. Neutron counting efficiency of the bank is better than that of the normally used BF 3-counters and follows a simple 1 v - law . Thus corrections for efficiency become low and easily calculable. Unfortunately this is at the price of a non-negligible gamma-sensitivity. For short flight-times the improvement in time resolution by a factor of about 50 compared with equivalent BF 3-counters may be interesting.

A new design of a multiple-wire spark counter and some studies with the single wire-plate spark counter

Abstract A new design of multiple-wire spark counter is described which eliminates almost completely the mutual interaction between the individual counter wires. The counter shows a long plateau (3.3 kV–6 kV) for alpha particle detection with practically no slope. The new design results in a considerable improvement in the neutron counting efficiency over the earlier records. The alpha counting efficiency of a single wire-plate spark counter is studied at various partial pressures of water, methyl alcohol and ethyl alcohol A marked effect is observed which shows a close relationship with the diffusivity of these vapours in air. The effect of irradiation by strong beta, gamma and ultraviolet rays on the alpha counting behaviour of the counter shows some interesting results.

Fast Neutron Counting with Hydrogenous Liquid Phosphors

A series of controlled experiments has been undertaken to study the dependence of neutron counting efficiency on the type of liquid phosphor and on the geometry and light collection efficiency of the liquid container. Five hydrogenous scintillating liquids have been compared in various container geometries. From differential lead absorption measurements using a calibrated polonium-beryllium neutron source, neutron counting efficiencies from 5.4±2 percent to 17.9±6 percent have been obtained.

The Design of Neutron Counters Using Multiple Detecting Layers

A discussion is given of ion chambers in which a series of electrodes carry layers of solid detecting material. Counters of this type require low operating voltages, and can easily be miniaturized. Formulas are given for the correct layer thicknesses and for the neutron counting efficiency. The equations are applied to boron 10, and it is shown that there is a permissible tolerance of about 50 percent on the thickness of a boron layer, and a very considerable tolerance on the assumed values of the disintegration product ranges. Curves are given relating the optimum boron layer thicknesses and the efficiency to the number of layers. A miniature instrument is described having a sensitive volume of 0.4 cm3. It operates at a thermal neutron efficiency of 24 percent with a collecting voltage of 70.