Measured Pulse Height Spectra Research Articles

Photon spectra in NPL standard monoenergetic neutron fields.

A High Purity Germanium (HPGe) detector has been used to measure the photon spectra in the majority of monoenergetic neutron fields produced at NPL (0.144, 0.250, 0.565, 2.0, 5.0 and 16.5MeV). The HPGe was characterised and then modelled to produce a response matrix. The measured pulse height spectra were then unfolded to produce photon fluence spectra. The new spectra were used to improve the photon to neutron dose equivalent ratios from some earlier work at NPL with Geiger-Muller tubes and electronic personal dosemeters.

Charge collection efficiency of scCVD diamond detectors at low temperatures

The charge collection efficiency (CCE) in electronic grade, single crystal chemical vapor deposition (scCVD) diamond detector was studied using different ions of the MeV energy range from room temperature down to 47 K. The measurements were performed by using the ion beam induced charge (IBIC) technique, with 0.8 MeV H+, 3 MeV He2+, 5.6 MeV Li2+, and 12.8 MeV C4+ ion beams. The total accumulated charge as a function of temperature was extracted from the measured pulse height spectra. A similar decreasing trend of the detector CCE with temperature was observed for all ion species. A plateau was observed from room temperature to 145 K followed by a sharp, step like, decrease in the temperature range of 65 K < T < 145 K. Below 65 K, the CCE reaches another plateau with small variations down to 47 K. The decrease in collected charge was observed in the same temperature window for each ion. However, the level of the CCE decrease varied. For He, Li and C, the CCE decreased to 24.7 ± 0.4 %, 23.6 ± 1.3 %, 25.9 ± 0.8 % respectively while 39.3 ± 0.8 % for H. The reduction of the CCE is in agreement with earlier reported results obtained by alpha particles which was associated with the changes in lifetime of excitons in diamond.

Experimental verification of real-time gamma-ray energy spectrum and dose monitor

The purpose of this study is to develop a portable monitor that can measure the energy spectrum and dose of gamma-rays simultaneously in real time for the benefit of medical staff who must work in clinical radiation environments. For this purpose, we have developed a prototype monitor using a CsI (Tl) scintillator combined with a multi-pixel photon counter (MPPC). For real-time measurement, we employed an improved sequential Bayesian estimation (k-α method) to convert the measured pulse height spectrum into an energy spectrum. Then we confirmed that reconstruction of the energy spectrum and dose estimation could simultaneously be carried out in real time by the k-α method in a radiation field composed of mixed standard gamma-ray sources. In this study, we carried out measurements in a background gamma-ray field to confirm applicability of the prototype monitor to the weakest type of radiation field. In addition, we conducted measurements in front of a nuclear fuel storage room (∼2 μSv/h) in the authors’ laboratory to evaluate practicality of the monitor for measuring fields with a complex energy spectrum. As a result, it was found that the dose could be estimated in about 20 s after start of measurements even in the background field. For the energy spectrum, it was instantly reconstructed within 60 s in front of the fuel storage room. On the other hand, it could successfully be estimated within 10 min in the background gamma-ray field. Currently, the convergence of the energy spectrum is determined visually from time dependent change of the spectrum and dose. As a next step, we will attempt to develop a more quantitative procedure for determining the convergence.

Low-energy continuous filtered X-ray measurement and dosimetry with a PIPS detector

The true fluence spectra and its conversion coefficients from the air kerma to the ICRU operational quantities for our low-energy X-ray radiation field were determined using spectrometry. The pulse height spectra were measured with a PIPS detector for the ISO reference radiations (N-15 to N-30 and L-20 to L-30). According to the calculated detector response matrices, the measured pulse height spectra were unfolded. Then the average photon energy and the first half-value layer for each radiation quality were calculated and compared with the recommended or measured values. The conversion coefficients from the air kerma to the ICRU operational quantities Hp(10), H*(10), Hp(0.07) and H′ (0.07) and the revised operational quantities ambient dose H*, personal dose Hp and personal absorbed dose in local skin Dp local skin were determined. This work presents the spectrometry and dosimetry for the filtered low-energy X-ray radiations based on a PIPS detector spectrum measurement system in detail. This portable spectrum measurement system can help calibration laborites simplify the experimental setup when performing the dosemeter calibrations according to ISO 4037–4.

Silicon Carbide characterization at the n_TOF spallation source with quasi-monoenergetic fast neutrons

Silicon Carbide (SiC) is a relatively new entry in the world of solid-state detectors. Although SiC response to neutrons is more complex than the one obtained with diamonds, the measured energy resolution (FWHM/Ed<4%) makes SiC an interesting alternative to diamond and silicon detectors for fast neutrons. The results obtained from the measurements of the response of a 100μm thick SiC detector to neutrons in the energy range between 3 and 20 MeV at the n_TOF spallation source at CERN are presented in this paper.By selecting the neutron energy by means of the time of flight, the detector response to quasi-mono-energetic neutrons was measured. The main neutron-induced nuclear reactions were identified in the measured pulse height spectrum. Detection efficiency as a function of neutron energy was measured and interpreted based on available neutron cross section and by making use of Monte Carlo simulations.

Measurement of prompt fission neutron and gamma spectra in the fast neutron induced fission of $$^{232}$$Th

Measurements of prompt fission neutrons and gamma rays spectra have been carried out in the fast neutron induced fission of $$^{232}$$Th at an average neutron energy of 2.6 MeV. The energy spectrum of neutrons is obtained from the time-of-flight spectrum measured using EJ301 detectors with respect to a twin section trigger fission chamber, while the energy spectrum of prompt gamma rays is obtained after unfolding the measured pulse height spectrum using the response matrix of the CeBr$$_{3}$$ detectors. The Maxwellian temperature obtained from the prompt fission neutron spectrum is found to be 1.22 ± 0.03 MeV. The low efficiency and limited resolution of the gamma set up, in addition to the low cross section of the reaction which result into poor statistics, made the reliable extraction of the detailed fine structure in the prompt fission gamma spectrum difficult. However, a reasonable estimate of the average gamma energy could be obtained as 0.91 ± 0.04 MeV. The extracted observables are then compared with the values predicted by the GEF model. It is found that, while the measured prompt fission neutron spectrum and the extracted Maxwellian temperature agree reasonably well when compared with the predictions of the GEF model, the average gamma ray energy from the measured prompt fission gamma spectrum is found to be lower by $$\sim $$ 17$$\%$$ from the predictions of the model. A rough estimate of average neutron and gamma multiplicities are also obtained from the measurements.

Evaluation of gamma-ray strength function based on measured gamma-ray pulse-height spectra in time-of-flight neutron capture experiments

In order to develop an evaluation method of gamma-ray strength function (GSF), neutron capture pulse-height (PH) spectrum of gold was employed, where it was measured with the NaI(Tl) spectrometer of AN-NRI installed at the Material and Life Science Experimental Facility in J-PARC. The neutron capture gamma-ray spectrum of gold was calculated using the nuclear reaction model code CCONE. In order to obtain the information on GSF from the measured data, a gamma-ray response function for the NaI(Tl) spectrometer was calculated by the Monte-Carlo particle-transport simulation code PHITS. As a result, the PH spectrum comparable with measured one was derived by applying the gamma-ray response function to the calculated gamma-ray spectrum. By evaluation with measured PH spectra, we obtained GSF which reasonably explains measured PH spectrum in the continuum region.

Tailoring the scintillation properties of β-Ga2O3 by doping with Ce and codoping with Si

Measurements of pulse height spectra and scintillation time profiles performed on Czochralski-grown β-Ga2O3, β-Ga2O3:Ce, and β-Ga2O3:Ce,Si crystals are reported. The highest value of scintillation yield, 7040 ph/MeV, was achieved for pure β-Ga2O3 at a low free electron concentration, nevertheless Ce-doped crystals could also approach high values thereof. Si-codoping, however, decreases the scintillation yield. The presence of Ce, and the more of Ce and Si, in β-Ga2O3 significantly increases the contribution of the fastest components in scintillation time profiles, which makes β-Ga2O3 a very fast scintillator under γ-excitation.

Scintillation properties and effect of thermal annealing in Lu3Al5O12:Ce and Lu3Al5O12:Pr ceramics

The impact of thermal annealing on the properties of LuAG:Ce and LuAG:Pr ceramics has been investigated. The results of measurements of pulse height spectra, scintillation time profiles, absorption spectra, radioluminescence as a function of temperature, absorption spectra, as well as low temperature thermally stimulated luminescence, performed before and after annealing of the samples in air, are compared. Pulse height spectra are used to estimate the light outputs of the samples, while scintillation time profiles to find the decay time constants. Absorption spectra are analyzed to identify the observed lines and/or bands. Radioluminescence spectra examine the changes of particular emissions as a function of temperature. Finally, parameters of traps detected in thermally stimulated luminescence are determined by decompositions of the recorded glow curves.

A Study of the Fast Neutron Response of a Single-Crystal Diamond Detector at High Temperatures

The fast neutron response of a single-crystal diamond (SCD) detector was studied up to 300 °C. The SCD detector was fabricated using a $5\,\,\text {mm}\times 5\,\,\text {mm}\times 0.4\,\,\text {mm}$ substrate with premetallization treatments for obtaining oxygen terminated surfaces. The electrical contacts to the detector were provided by Cr/Au metallization. The performance of the fabricated detector was investigated using leakage current versus bias voltage (I–V) measurements and using pulse height spectra measurements with 5.5 MeV alphas and 14 MeV fast neutrons. The leakage current was observed to be about 400–600 pA at a field of 1 V/ $\mu \text{m}$ . From the full-width half-maximum of the alpha peak at 5.5 MeV, an energy resolution of about 2.1 ± 0.1% was estimated. The fast neutron response measurements were carried out using a 14 MeV deuterium–tritium neutron source. No measurable degradation of the detector performance was observed up to the maximum studied temperature of 300 °C. The neutron spectrum obtained at 300 °C showed a well-defined peak at 8.5 MeV due to the 12C(n, $\alpha)^{9}$ Be reaction which was the same as that observed at room temperature. Considering the total integrated counts, the detector sensitivity was observed to be $10^{-2}$ count per neutron. The results presented in this paper demonstrate for the first time the high temperature operation of an SCD detector up to 300 °C. These results also suggest that such SCD detectors could be used at higher temperatures for neutron diagnostics in the future fusion plasma facilities such as International Thermonuclear Experimental Reactor.

PHOTON SPECTRA IN NPL STANDARD RADIONUCLIDE NEUTRON FIELDS.

A HPGe detector has been used to measure the photon spectra from the majority of radionuclide neutron sources in use at NPL (252Cf, 241Am-Be, 241Am-Li, 241Am-B). The HPGe was characterised then modelled to produce a response matrix. The measured pulse height spectra were then unfolded to produce photon fluence spectra. Changes in the photon spectrum with time from a 252Cf source are evident. Spectra from a 2-year-old and 42-year-old 252Cf source are presented showing the change from a continuum to peaks from long-lived isotopes of Cf. Other radionuclide neutron source spectra are also presented and discussed. The new spectra were used to improve the photon to neutron dose equivalent ratios from some earlier work at NPL with GM tubes and EPDs.

Characterization of polycrystalline lead oxide for application in direct conversion X-ray detectors

While polycrystalline lead oxide (poly-PbO) is known to be one of the most promising photoconductors for utilization in X-ray detectors, its major performance parameters such as charge yield and mobility-lifetime product (μτ) are still not well established and require further investigation. Combining the conventional X-ray induced photocurrent and pulse height spectroscopy techniques we examine the X-ray photogeneration and recombination processes in poly-PbO. The measurements indicate that the amount of energy required to release a single electron hole pair W± (inverse of charge yield) strongly depends on applied electric field and at 10 V/μm reaches ~20 eV/ehp. Fitting the measured pulse height spectra with the Hecht formula provided μτ for holes and electrons to be 4.1 × 10−8 cm2/V and 10−9 cm2/V, respectively. Obtained μτ values combined with recently reported mobility values of charge carriers in PbO suggest a new direction towards improvement of PbO technology by incorporation of Frisch grid or X-ray transistor architectures.

Deriving depth-dependent light escape efficiency and optical Swank factor from measured pulse height spectra of scintillators.

Pulse height spectroscopy has been used by investigators to deduce the imaging properties of scintillators. Pulse height spectra (PHS) are used to compute the Swank factor, which describes the variation in scintillator light output per x-ray interaction. The spread in PHS measured below the K-edge is related to the optical component of the Swank factor, i.e., variations in light escape efficiency from different depths of x-ray interaction in the scintillator, denoted ε¯(z). Optimizing scintillators for medical imaging applications requires understanding of these optical properties, as they determine tradeoffs between parameters such as x-ray absorption, light yield, and spatial resolution. This work develops a model for PHS acquisition such that the effect of measurement uncertainty can be removed. This method allows ε¯(z) to be quantified on an absolute scale and permits more accurate estimation of the optical Swank factor of scintillators. The pulse height spectroscopy acquisition chain was modeled as a linear system of stochastic gain stages. Analytical expressions were derived for signal and noise propagation through the PHS chain, accounting for deterministic and stochastic aspects of x-ray absorption, scintillation, and light detection with a photomultiplier tube. The derived expressions were used to calculate PHS of thallium-doped cesium iodide (CsI) scintillators using parameters that were measured, calculated, or known from literature. PHS were measured at 25 and 32 keV of CsI samples designed with an optically reflective or absorptive backing, with or without a fiber-optic faceplate (FOP), and with thicknesses ranging from 150-1000 μm. Measured PHS were compared with calculated PHS, then light escape model parameters were varied until measured and modeled results reached agreement. Resulting estimates of ε¯(z) were used to calculate each scintillator's optical Swank factor. For scintillators of the same optical design, only minor differences in light escape efficiency were observed between samples with different thickness. As thickness increased, escape efficiency decreased by up to 20% for interactions furthest away from light collection. Optical design (i.e., backing and FOP) predominantly affected the magnitude and relative variation in ε¯(z). Depending on interaction depth and scintillator thickness, samples with an absorptive backing and FOP were estimated to yield 4.1-13.4 photons/keV. Samples with a reflective backing and FOP yielded 10.4-18.4 keV-1 , while those with a reflective backing and no FOP yielded 29.5-52.0 keV-1 . Optical Swank factors were approximately 0.9 and near-unity in samples featuring an absorptive or reflective backing, respectively. This work uses a modeling approach to remove the noise introduced by the measurement apparatus from measured PHS. This method allows absolute quantification of ε¯(z) and more accurate estimation of the optical Swank factor of scintillators. The method was applied to CsI scintillators with different thickness and optical design, and determined that optical design more strongly affects ε¯(z) and Swank factor than differences in CsI thickness. Despite large variations in ε¯(z) between optical designs, the Swank factor of all evaluated samples is above 0.9. Information provided by this methodology can help validate Monte Carlo simulations of structured CsI and optimize scintillator design for x-ray imaging applications.

Studies of light yield as a function of temperature and low temperature thermoluminescence of Gd3Al2Ga3O12:Ce scintillator crystals

In this paper we report the measurements of pulse height spectra, radioluminescence spectra at various temperatures between 10 and 325K, and low temperature thermoluminescence glow curves of Gd3Al2Ga3O12:Ce. It is shown that the scintillation yield of the studied material correlates with the concentration of cerium and anti-correlates with the thermoluminescence intensity. However, regardless of the cerium content, the yield at room temperature is almost twice lower than below 180K.

Measurements and simulations of scattered neutrons in a water phantom

A recoil neutron spectrometer based on NE230 deuterated liquid scintillator (25 mm diameter × 25 mm) has been used to investigate neutron energy spectra of scattered neutrons at positions in and near to the beam axis in a water phantom irradiated by a quasi-monoenergetic 64 MeV neutron beam. An important feature of the spectrometer is that it is able to discriminate against backgrounds that can arise from n-p scattering in the water. Neutron energy spectra are obtained from measurements of pulse height spectra by the NE230 spectrometer using the Bayesian unfolding code MAXED. The experimentally measured energy spectra are compared with spectra calculated by Monte Carlo simulation using the code MCNPX.

First neutron spectrometry measurement at the HL-2A Tokamak

A compact neutron spectrometer based on the liquid scintillator is presented for neutron energy spectrum measurements at the HL-2A Tokamak. The spectrometer was well characterized and a fast digital pulse shape discrimination software was developed using the charge comparison method. A digitizer data acquisition system with a maximum frequency of 1 MHz can work under an environment with a high count rate at HL-2A Tokamak. Specific radiation and magnetic shielding for the spectrometer were designed for the neutron spectrum measurement at the HL-2A Tokamak. For pulse height spectrum analysis, dedicated numerical simulation utilizing NUBEAM combined with GENESIS was performed to obtain the neutron energy spectrum. Subsequently, the transportation process from the plasma to the detector was evaluated with Monte Carlo calculations. The distorted neutron energy spectrum was folded with the response matrix of the liquid scintillation spectrometer, and good consistency was found between the simulated and measured pulse height spectra. This neutron spectrometer based on a digital acquisition system could be well adopted for the investigation of the auxiliary heating behavior and the fast-ion related phenomenon on different tokamak devices.

Characterization of epitaxial 4H-SiC for photon detectors

High purity epitaxial 4H-SiC became a serious candidate for the fabrication of spectrometric radiation detectors with a high resistance to neutrons and gamma rays damage and suitable for applications in hot plasma diagnostics. The present work reports on i) the characterization of high purity epitaxial 4H-SiC grown by liquid phase epitaxy on SiC substrates and ii) the performances of metal/4H-SiC detectors fabricated on the same material. X-ray diffraction and topography as well as I-V, C-V and DLTS measurements are used for the evaluation of the material properties and device characteristics. The UV and X-ray detection abilities are evaluated by photocurrent measurements in the 3-6 eV region and pulse-height spectra measurements of the 241Am, respectively. Preliminary results of the detector hardness to fast neutron and gamma ray radiations are also reported.

Response of large area avalanche photodiodes to low energy x rays

For an experiment to study neutron radiative beta-decay, we operated large area avalanche photodiodes (APDs) near liquid nitrogen temperature to detect x rays with energies between 0.2 keV and 20 keV. Whereas there are numerous reports of x ray spectrometry using APDs at energies above 1 keV, operation near liquid nitrogen temperature allowed us to reach a nominal threshold of 0.1 keV. However, due to the short penetration depth of x rays below 1 keV, the pulse height spectrum of the APD become complex. We studied the response using monochromatic x ray beams and employed phenomenological fits of the pulse height spectrum to model the measurement of a continuum spectrum from a synchrotron. In addition, the measured pulse height spectrum was modelled using a profile for the variation in efficiency of collection of photoelectrons with depth into the APD. The best results are obtained with the collection efficiency model.

Monte Carlo simulations of multiplexed electronic grade CVD diamond for neutron detection

Abstract A novel and evolutionary multiplexing technique is introduced in this work where electronic grade single crystal chemical vapor deposition (CVD) diamond plates are multiplexed together in both a series and parallel configuration, sending electronic signals from each diamond plate to a single electronic acquisition system. Modeling of this novel multiplexing technique consisted of MCNPX simulations and significant post processing. The model developed allowed for the characterization of charge collection efficiency corrections to the location of charge creation to determine the effect of increasing detection medium size with respect to charge collection direction on the measured pulse height spectrum. This work was conducted to show that this technique is theoretically capable of replacing a single crystal diamond plate of similar size for use in neutron detection without the immediate need of advancing CVD diamond growth technologies. Further, this work indicates the expected pulse height evolution from a singular large single crystal diamond if such a crystal is produced in the future. The results of this work indicate that a 14.1 MeV neutron induced energy pulse of 8.4 MeV (due to the 12C(n,αo)9Be reaction) in the pulse height spectrum has its energy resolution broadened by a factor of two to a total value of 0.225 percent for a multiplexed array with a thickness from 0.05 to 1 cm and an intrinsic detection efficiency of 25.4 percent for a 1 cm thick diamond crystal. It is also qualitatively discussed that the number of secondary neutron interactions with the diamond detector array may be about 5 percent. The results of this work indicate the capability of multiplexing diamond plates together for spectroscopic neutron detection with a combined intrinsic detection efficiency and energy resolution greater than any other diamond-based neutron detection system reported to date.

Results for the response function determination of the Compact Neutron Spectrometer

The Compact Neutron Spectrometer (CNS) is a Joint European Torus (JET) Enhancement Project, designed for fusion diagnostics in different plasma scenarios. The CNS is based on a liquid scintillator (BC501A) which allows good discrimination between neutron and gamma radiation. Neutron spectrometry with a BC501A spectrometer requires the use of a reliable, fully characterized detector. The determination of the response matrix was carried out at the Ion Accelerator Facility (PIAF) of the Physikalisch-Technische Bundesanstalt (PTB). This facility provides several monoenergetic beams (2.5, 8, 10, 12 and 14 MeV) and a white field (Emax ∼ 17 MeV), which allows for a full characterization of the spectrometer in the region of interest (from ∼ 1.5 MeV to ∼ 17 MeV). The energy of the incoming neutrons was determined by the time of flight method (TOF), with time resolution in the order of 1 ns. To check the response matrix, the measured pulse height spectra were unfolded with the code MAXED and the resulting energy distributions were compared with those obtained from TOF. The CNS project required modification of the PTB BC501A spectrometer design, to replace an analog data acquisition system (NIM modules) with a digital system developed by the Ente per le Nuove tecnologie, l'Energia e l'Ambiente (ENEA). Results for the new digital system were evaluated using new software developed specifically for this project.