Characteristics of a phoswich detector to measure the neutron spectrum in a mixed field of neutrons and charged particles

A phoswich detector composed of NE115 and NE213 scintillators has been developed to distinguish gamma-ray, neutron, and proton events. The detector performance was investigated in a neutron–proton mixed field at National Institute of Radiological Sciences (NIRS), which showed satisfactory particle identification.     

PiBrED—A novel bragg edge detector for neutrons

We present a single pixel prototype of a pixelated Bragg edge detector for neutron transmission measurements. The optical signal coming from a scintillator is collected by an optical fiber and is detected by an avalanche photodiode. A fast, Field Programmable Gate Array based, readout allows to obtain transmission spectra within reasonable acquisition times. The performances of the instrument have been tested by measuring the transmission spectra of iron powder samples with two different scintillators. The instrument accuracy in detecting the Bragg edges positions is comparable with the state of the art for similar devices.     

Optimization of LSO/LuYAP phoswich detector for small animal PET

LSO/LuYAP phoswich detectors for small animal PET were developed to measure the depth of interaction (DOI), and to improve the spatial resolution at the edge of the field of view (FOV). The aim of this study was to optimize the optical coupling conditions between the crystal and photomultiplier tube (PMT) to maximize the light-collection efficiency, and to develop a method for rejecting scatter events by applying an equal energy window in each crystal layer. The light yields of the phoswich detector were estimated by changing the refractive index of the optical coupling material using a DETECT simulation. The accuracy of the DOI measurement on the phoswich detector, using an optical coupling material with the optimal light yield, were evaluated experimentally and compared with the air condition. The energy window for the photopeak events cannot be applied properly because the light outputs of LSO and LuYAP are different. The LSO/LuYAP photopeaks need to be superposed in order to effectively discriminate the scattered events by applying an equal energy window. The photopeaks of the LSO and LuYAP can be superposed by inserting a reflecting material between the crystals. The optimal coverage ratio of the inserting material was derived from a DETECT simulation, and its performance was investigated. In the simulation result, optimal refractive index of the optical coupling material was 1.7. The average DOI measurement errors of the LSO/LuYAP were 0.6%/3.4% and 4.9%/41.4% in the phoswich detector with and without an optical coupling material, respectively. The photopeaks of the LSO and LuYAP were superposed by covering 75% of the contact surface between the crystals with white Teflon. The DOI measurement errors of the LSO/LuYAP were 0.2%/2.4%. In this study, the optimal condition of the optical coupling material inserted between the crystal and PMT was derived to improve the accuracy of DOI measurement, and a photopeak superposition method of the LSO and LuYAP was developed in order to reject scatter events.     

Performance of a phoswich detector array for light particles and heavy ions

A phoswich array consisting of 48 ΔE - E elements with sufficient granularity (ΔΘ = ± 2.5°) to handle high multiplicity events has been built and used in several experiments. The full angular coverage is a 35° × 35° square cross section of the 4π sphere. Unit charge resolution for Z ≤ 10 and mass resolution for Z = 1 particles has been achieved with a ±5% energy resolution. Experimental details and the performance of the array are presented and discussed.     

Temperature dependence of BGO-CsI(Tl) phoswich detector properties

We have studied the variations of the performances, and particularly the discrimination efficiency, of a BGO-CsI(Tl) phoswich detector in the temperature range 2.5–40°C.The scintillation decay time evaluation has been carried out by means of a double constant fraction discriminator and a time to amplitude converter on 1 μs shaped pulses. Good discrimination between BGO and CsI(Tl) events has been obtained over the whole temperature range, even though small efficiency variations are present because of the relative displacement of the acceptance window of the discriminator due to the decay time dependence on temperature. A criterium to minimize this effect, useful in particular for the BGO-CsI(Tl) phoswich, has been developed.     

The 12B counter: an active dosemeter for high-energy neutrons

High-energy accelerators can produce strong time-structured radiation fields. Such dose shots are generated at linear machines with low duty cycles as well as at circular machines when complete fills are instantaneously lost. The main dose component behind thick shielding is due to high-energy neutrons occurring at that time structure. Dosemeters based on Geiger-Mueller tubes or proportional counters fail here completely. The 12B counter, a novel dosemeter made of a plastic scintillator using carbon activation for event-like exposure, has been introduced. High-energy neutrons activate the carbon nuclei by three inelastic reactions. The decay patterns with half-lives between 20 ms and 20 min can be exploited depending on the time structure of the radiation field. The response of the 12B counter was measured along with some other dosemeters, both active and passive, in the radiation field behind the lateral concrete shielding of a 7.5 GeV proton transfer line.     

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.     

A simple phoswich system

Normal phoswich detector systems use a combination of NaI(Tl) and CsI(Na) scintillators and require the application of careful pulse-shape discriminator techniques to resolve the two components in the scintillation light output which have decay constants of 250 and 630 ns respectively. These techniques provide a good anticoincidence veto efficiency for a relatively narrow range in the ratio of energy deposits in the two crytals and for a detector system whose temperature is carefully controlled. This paper describes the performance of a simple phoswich which makes use of the fast UV signal from a BaF₂ crystal to provide a prompt veto signal. The performance to be expected from various combinations of a BaF₂ anticoincidence crystal with other primary detectors is presented. These simulations have been verified by simple experimental tests.     

CsI(Tl)/plastic phoswich detector enhanced in low-energy gamma-ray detection

A phoswich detector composed of a thin plate CsI(Tl) scintillator and a plastic scintillator (BC-400) has been designed and evaluated in order to improve the sensitivity in the low-energy region of a large-area plastic scintillation detector. This newly designed phoswich detector can be applied to both gross gamma measurement and energy spectrometry for low-energy gamma-ray emitters. Judging by estimations of minimum detectable activity, the lower measurable energy of a large-area plastic scintillation detector can be expanded down to a few tens of keV by adding a thin plate CsI(Tl) scintillator.     

Rotating-wheel detector system for measuring millisecond half-life high-energy β emitters

In order to detect the 11 ms ¹²N activity produced in the pion single charge exchange reaction ¹²C(π⁺, π⁰)¹²N, a high-energy β-particle telescope was designed, constructed, and used successfully in the high-background field of γ rays, neutrons, and high-energy charged particles at LAMPF. A high-speed rotating wheel served both as a target and as a means of transporting the short-lived activity to an array of telescopes for counting. Monte Carlo calculations were carried out to optimize the design parameters of the telescope and to estimate counting efficiencies.     

Design and test of a large-area scintillation detector for fast neutrons

A plastic scintillator array of 3.4 m² total area for the detection of neutrons in medium-energy nucleon–nucleon reactions was constructed and built. Calibration procedures for the detector were developed which allow the monitoring of gain shifts by means of muons from the cosmic radiation. Experiments were performed in order to calibrate the efficiency simulations and study the performance of the detector using the d+t→+n reaction at 14.7 MeV neutron energy and proton-induced deuteron breakup at 300 MeV.     

Compound detector with selective sensitivity to intermediate neutrons

A compound detector based on the²³Na(n,) reaction in a boron shield is investigated. The detector cross section was calculated assuming that the spectrum inside the shield is a sum of two components, one due to direct penetration of neutrons through the shield wall and neutrons scattered in the shield mass. Also investigated was the effect of hydrolyzing varnish in the shield and the size of the activation detector on the detector readings.Translated from Izmeritel'naya Tekhnika, No. 1, pp. 70–71, January, 1995.     

Application of low-cost Gallium Arsenide light-emitting-diodes as kerma dosemeter and fluence monitor for high-energy neutrons

Displacement damage (DD) caused by fast neutrons in unbiased Gallium Arsenide (GaAs) light emitting diodes (LED) resulted in a reduction of the light output. On the other hand, a similar type of LED irradiated with gamma rays from a (60)Co source up to a dose level in excess of 1.0 kGy (1.0 x 10(5) rad) was found to show no significant drop of the light emission. This phenomenon was used to develop a low cost passive fluence monitor and kinetic energy released per unit mass dosemeter for accelerator-produced neutrons. These LED-dosemeters were used to assess the integrated fluence of photoneutrons, which were contaminated with a strong bremsstrahlung gamma-background generated by the 730 MeV superconducting electron linac driving the free electron laser in Hamburg (FLASH) at Deutsches Elektronen-Synchrotron. The applications of GaAs LED as a routine neutron fluence monitor and DD precursor for the electronic components located in high-energy accelerator environment are highlighted.     

The phostron: A phoswich counter for neutron and charged particle detection

A phoswich detector able to detect and identify gammas, neutrons and charged particles (p, d, t, α) of 10–100 MeV is described. It consists of a CaF₂(Eu) scintillator optically coupled with a NE213 liquid scintillator.     

Spatial distribution of moderated neutrons along a Pb target irradiated by high-energy protons

High-energy protons in the range of 0.5–7.4 GeV have irradiated an extended Pb target covered with a paraffin moderator. The moderator was used in order to shift the hard Pb spallation neutron spectrum to lower energies and to increase the transmutation efficiency via (n,γ) reactions. Neutron distributions along and inside the paraffin moderator were measured. An analysis of the experimental results was performed based on particle production by high-energy interactions with heavy targets and neutron spectrum shifting by the paraffin. Conclusions about the spallation neutron production in the target and moderation through the paraffin are presented. The study of the total neutron fluence on the moderator surface as a function of the proton beam energy shows that neutron cost is improved up to 1 GeV. For higher proton beam energies it remains constant with a tendency to decline.     

A high-performance polarimeter for medium-energy neutrons

A polarimeter for medium-energy neutrons is described. This polarimeter uses the analyzing power of n–p scattering which is typically +0.50 for laboratory neutron scattering angles near 20° in the energy range from 50 MeV to 1 GeV. Hydrocarbon organic scintillators are employed as “active” scatterers. Calibration data for the analyzing power and the efficiency of the polarimeter are presented.     

A moderating assembly for spallation neutrons

An increased interest in intense pulsed neutron beams from spallation reactions using accelerator beams, caused us to study neutron moderating structures that optimize energy and intensity in a collimated external channel.We show that it is possible using inhomogeneous structures to increase by 40±20 times the neutron beam current over a homogeneous D₂O moderator with the same exterior dimensions.     

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.     

Identification of charged particles using plastic phoswich detectors

A plastic phoswich detector consisting of a 0.1 mm thick (NE102A) fast scintillator and a 10 mm thick slow scintillator (NE115 or BC-444), heat pressed together, has been tested and found to give a satisfactory charge separation of light particles.     

A directional dose equivalent monitor for neutrons

A directional dose equivalent monitor is introduced which consists of a 30 cm diameter spherical phantom hosting a superheated drop detector embedded at a depth of 10 mm. The device relies on the similarity between the fluence response of neutron superheated drop detectors based on halocarbon-12 and the quality-factor-weighted kerma factor. This implies that these detectors can be used for in-phantom dosimetry and provide a direct reading of dose equivalent at depth. The directional dose equivalent monitor was characterised experimentally with fast neutron calibrations and numerically with Monte Carlo simulations. The fluence response was determined at angles of 0, 45, 90, 135 and 180 degrees for thermal to 20 MeV neutrons. The response of the device is closely proportional to the fluence-to-directional dose equivalent conversion coefficient, h'phi (10; alpha, E). Therefore, our monitor is suitable for a direct measurement of neutron directional dose equivalent, H'(10), regardless of angle and energy distribution of the neutron fluence.