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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

A position-sensitive detector for synchrotron radiation

A two-dimensional detector that gives an electrical signal proportional to the position of a light spot on a 1 × 1 cm² area has been developed. The detector is intended for position measurements of the beam in a synchrotron radiation source.     

A high-pressure MWPC detector for crystallography

The application of the Multi-Wire Proportional Counter (MWPC) as a potential detector for protein crystallography and other wide-angle diffraction experiments is presented. Electrostatic problems found with our large area MWPC when operated at high pressure are discussed. We suggest that a solution to these problems is to use a glass micro-strip detector in place of the wire frames. The characteristics of a high-pressure Micro-Strip Gas Chamber (MSGC) tested in the laboratory are presented.     

ANDI-03: a genetic algorithm tool for the analysis of activation detector data to unfold high-energy neutron spectra

The thresholds of (n,xn) reactions in various activation detectors are commonly used to unfold the neutron spectra covering a broad energy span, i.e. from thermal to several hundreds of MeV. The saturation activities of the daughter nuclides (i.e. reaction products) serve as the input data of specific spectra unfolding codes, such as SAND-II and LOUHI-83. However, most spectra unfolding codes, including the above, require an a priori (guess) spectrum to starting up the unfolding procedure of an unknown spectrum. The accuracy and exactness of the resulting spectrum primarily depends on the subjectively chosen guess spectrum. On the other hand, the Genetic Algorithm (GA)-based spectra unfolding technique ANDI-03 (Activation-detector Neutron DIfferentiation) presented in this report does not require a specific starting parameter. The GA is a robust problem-solving tool, which emulates the Darwinian Theory of Evolution prevailing in the realm of biological world and is ideally suited to optimise complex objective functions globally in a large multidimensional solution space. The activation data of the 27Al(n,alpha)24Na, 116In(n,gamma)116mIn, 12C(n,2n)11C and 209Bi(n,xn)(210-x)Bi reactions recorded at the high-energy neutron field of the ISIS Spallation source (Rutherford Appleton Laboratory, UK) was obtained from literature and by applying the ANDI-03 GA tool, these data were used to unfold the neutron spectra. The total neutron fluence derived from the neutron spectrum unfolded using GA technique (ANDI-03) agreed within +/-6.9% (at shield top level) and +/-27.2% (behind a 60 cm thick concrete shield) with the same unfolded with the SAND-II code.     

A compact, large-diameter adiabatic spinflipper for ultracold neutrons

We report the design and test of a compact, large-diameter spinflipper for ultracold neutrons based on the principle of adiabatic spinflip. A solenoid rf coil with a high diameter-to-length ratio (d/l~1.7) is surrounded by up to three short magnet coils (d/l~0.4…1.7) to provide the static gradient field. The device is optimized for low power consumption and operation in high vacuum. The magnetic field conditions necessary for full spinflipper efficiency over the full diameter were evaluated with ultracold neutrons. The measured spinflipper efficiency vs. adiabaticity correlation is in good agreement with a calculation based on an ideally linear gradient.     

A hadron-blind detector

The development of highly efficient solid photocathodes, compatible with high-gain gaseous detectors, has opened the possibility to build threshold Cherenkov counters with a good hadron rejection, with a minimum amount of matter, and a time resolution of the order of the nanosecond. We discuss the properties of a hadron-blind detector, with a granularity of a few millimetres. The study of the background sources shows that a rejection power of the order of 99% can be achieved for high-energy hadrons. It permits instantaneous multi-hadron rejection and, combined to a fast electromagnetic calorimeter, can ensure on-line electron selection, even when they are produced close to hadronic jets. It could permit the operation of lepton-tracking detectors in a magnetic field, in very high hadronic backgrounds.