Determination of a CR-39 detector response to neutrons from an Am–Be source

A comparison of experimental and calculated responses of a CR-39 detector to neutron spectra from an Am–Be source is presented. Code named Neutron_CR-39.F90 has been used to calculate the neutron dose equivalent as well as the track density. Conversion coefficient (sensitivity), between track density in track/cm² and neutron dose equivalent in mSv, was calculated and good agreement with experimental data was found. Sensitivity increases linearly with removed layer in the range between 6 μm and 24 μm.     

Estimation of background spectrum in a shielded HPGe detector using Monte Carlo simulations

Monte Carlo simulations are powerful tools used to estimate the background γ-radiation detected by high-resolution gamma-ray spectrometry systems with a HPGe (high purity germanium) detector contained inside a lead shield. The purpose of this work was to examine the applicability of Monte Carlo simulations to predict the optimal lead thickness necessary to reduce the background effect in spectrometer measurements. GEANT4 code was applied to simulate the background radiation spectrum at different thicknesses of lead. The simulated results were compared with experimental measurements of background radiation taken at the same shielding thickness. The results show that the background radiation detected depends on the thickness, size and lining of the shield. Simulation showed that 12 cm lead thick is the optimal shielding thickness.     

Simulation of alpha particle spectra from aerosol samples

A Monte Carlo code is developed for simulating energy spectra of alpha particles from aerosol samples. Geometrical detection efficiency and energy loss of the alpha particles in the source itself and in the material between the source and detector are simulated. Different characteristics of the aerosol particles and medium material are taken into account in the computation. An excellent agreement with earlier results and measurements is found. The code can be applied for example in designing optimal aerosol sampling methods in direct alpha spectrometry.     

Detection efficiency of a disk shaped detector with a critical detection angle for particles with a finite range emitted by a point-like source

Detection efficiency of a circular detector for particles with a finite range, emitted from a point-like source was investigated, taking a critical detection angle into account. Particles emitted from the source lose some of their energy in the surrounding medium, before entering the detector material. Incident-energy dependence of the critical detection angle was taken into account. The part of the detector exposed to the particles impacting at angles greater than the critical angle (with respect to the detector surface), was determined. Several different cases were investigated, depending on the radius of the detector and the position of the detector with respect to the source. Detection probability expressions were derived for each of the cases. Results obtained using these expressions were compared with Monte Carlo calculations.     

Theoretical estimates of the solid angle subtended by a dual diaphragm–detector assembly for alpha sources

Knowledge of the solid angle subtended by a detector is essential for the determination of the activity of a radioactive source. This work investigates the influence of diaphragm and source diameter, and source to diaphragm distance, on the solid angle subtended by a dual opposed diaphragm–detector assembly. Expressions published by Segre, Ruby, and others, and the Monte Carlo method are used. The analytical expressions consistently produced higher estimates than the Monte Carlo method for the solid angle.     

Simulation study of a hydrostat design for detecting underground leakage of water supply using neutron backscattering

We have embarked upon the development of a new detection method for underground water leakage using a neutron backscattering system. We have estimated the performance capabilities of such a system using Monte Carlo simulation. It is indicated that a leak which results in 40% water content in the surrounding soils could be detected at depths of up to 40 cm from the surface to the center of the source of leakage. This new detection system could be useful as a hydrostat of underground water supply in noisy areas such as Tokyo, in place of presently-used hydrostats which are based on detection of changes in sound.     

Determination of full-energy peak efficiency at the center position of a through-hole-type clover detector between 0.05 MeV and 3.2 MeV by source measurements and Monte Carlo simulations

Full-energy peak efficiency at the center position of a through-hole-type clover detector was determined by the measurement of standard sources and by Monte Carlo simulation. The coincidence summing under the large-solid-angle condition was corrected using Monte Carlo calculation based on the specific decay scheme for ¹³³Ba, ^152,154Eu, and ⁵⁶Co. This allowed the peak efficiency to be extended from 0.05 MeV to 3.2 MeV with an approximate uncertainty of 3%.     

Numerical analysis of alpha spectra using two different codes

This work presents an intercomparison between commercial software for alpha-particle spectrometry, Genie 2000, and the new free available software, Winalpha, developed by International Atomic Energy Agency (IAEA).

In order to compare both codes, different environmental spectra containing plutonium, uranium, thorium and polonium have been analyzed, together with IAEA test alpha spectra. A statistical study was performed in order to evaluate the precision and accuracy in the analyses, and to enhance the confidence in using the software on alpha spectrometric studies.     

Development and calibration of a real-time airborne radioactivity monitor using direct gamma-ray spectrometry with two scintillation detectors

The implementation of in-situ gamma-ray spectrometry in an automatic real-time environmental radiation surveillance network can help to identify and characterize abnormal radioactivity increases quickly. For this reason, a Real-time Airborne Radioactivity Monitor using direct gamma-ray spectrometry with two scintillation detectors (RARM-D2) was developed. The two scintillation detectors in the RARM-D2 are strategically shielded with Pb to permit the separate measurement of the airborne isotopes with respect to the deposited isotopes.In this paper, we describe the main aspects of the development and calibration of the RARM-D2 when using NaI(Tl) or LaBr₃(Ce) detectors. The calibration of the monitor was performed experimentally with the exception of the efficiency curve, which was set using Monte Carlo (MC) simulations with the EGS5 code system. Prior to setting the efficiency curve, the effect of the radioactive source term size on the efficiency calculations was studied for the gamma-rays from ¹³⁷Cs. Finally, to study the measurement capabilities of the RARM-D2, the minimum detectable activity concentrations for ¹³¹I and ¹³⁷Cs were calculated for typical spectra at different integration times.     

Dead layer thickness characterization of an HPGe detector by measurements and Monte Carlo simulations

To fully characterize the front dead layer (DL) of an HPGe detector at low photon energy range, its intrinsic efficiency curve was measured using a ²⁴¹Am radioactive source in 10−60 keV energy range. A comparison between experimental efficiency and MCNPX results showed that the DL value of 0.4 μm initially quoted by the manufacturer has to be changed to 7.5 μm to reproduce measurements.     

Distribution of the 222Rn decay products from a 226Ra solution in a PTB ampoule – Implications for calibration

The spatial distribution of the activity in a flame-sealed PTB ampoule containing ²²⁶Ra solution, incompletely filling the ampoule, was studied by autoradiography and by taking well-collimated gamma-ray spectra in 23 sections of the ampoule. A complex activity profile was observed and used for the Monte Carlo simulation of the ampoule by the GESPECOR code. The computed values of the efficiency transfer factors are in agreement with the experimental data. As a byproduct, the radon partition ratio in the solution–air phases was obtained.     

Characterization of R-134a superheated droplet detector for neutron detection

R-134a (C₂H₂F₄) is a low cost, easily available and chlorine free refrigerant, which in its superheated state can be used as an efficient neutron detector. Due to its high solubility in water the R-134a based superheated droplet detectors (SDD) are usually very unstable unless the detector is fabricated using a suitable additive, which stabilizes the detector. The SDD is known to have superheated droplets distributed in a short-lived and in a relatively long-lived metastable states. We have studied the detector response to neutrons using a ²⁴¹AmBe neutron source and obtained the temperature variation of the nucleation parameters and the interstate kinetics of these droplets using a two-state model.     

Calibration curves of a PGNAA system for cement raw material analysis using the MCNP code

In large samples, the γ-ray count rate of a prompt gamma neutron activation analysis system is a multi-variable function of the elemental dry composition, density, water content and thickness of the material. The experimental calibration curves require tremendous laboratory work, using a great number of standards with well-known compositions. Although a Monte Carlo simulation study does not avoid the experimental calibration work, it reduces the number of experimental calibration standards. This paper is part of a feasibility study for a PGNAA system for on-line continuous characterisation of cement raw material conveyed on a belt (Oliveira, C., Salgado, J. and Carvalho, F. G. (1997) Optimisation of PGNAA instrument design for cement raw materials using the MCNP code. J. Radioanal. Nucl. Chem. 216(2), 191–198; Oliveira, C., Salgado, J., Gonçalves, I. F., Carvalho, F. G. and Leitão, F. (1997a) A Monte Carlo study of the influence of geometry arrangements and structural materials on a PGNAA system performance for cement raw materials analysis. Appl. Radiat. Isot. (accepted); Oliveira, C., Salgado, J. and Leitão, F. (1997b) Density and water content corrections in the gamma count rate of a PGNAA system for cement raw material analysis using the MCNP code. Appl. Radiat. Isot. (accepted).]. It reports on the influence of the density, mass water content and thickness on the calibration curves of the PGNAA system. The MCNP-4A code (Briesmeister, 1993), running in a Pentium-PC and in a DEC workstation, was used to simulate the PGNAA configuration system.     

Sulfur-33 nanoparticles: A Monte Carlo study of their potential as neutron capturers for enhancing boron neutron capture therapy of cancer

In this paper, the potential effect of enhancing BNCT near the surface of the target volume by means of the addition of the sulfur isotope ³³S is studied. By means of Monte Carlo simulations, it is found a noticeable enhancement effect (local increase of the dose at the isotope site) when it is present at local concentrations that in principle can be reached by means of sulfur nanoparticles. A neutron beam with a high component of 13.5 keV would be required to produce this effect. Some open problems are discussed.     

Determination of alpha activity in organic solvents using CR-39

Solid State Nuclear Track Detector (SSNTD) like CR-39 offers a unique opportunity to measure alpha radiations in a complex matrix like high level nuclear waste due to its sensitivity to only alpha radiations in the presence of intense beta–gamma radiation field. There is however a concern about the lower sensitivity of SSNTD when the radiation source is present in solution vis-à-vis solid medium. The sensitivity of CR-39 toward detection of alpha particles homogenously distributed in n-hexane, n-dodecane and n-octanol has been investigated and was found to be better than that obtained in 3 M HNO₃.     

Feasibility study of SPECT system for online dosimetry imaging in boron neutron capture therapy

A single collimator version of a proposed PG-SPECT system was manufactured and experimentally tested. Combining this experimental data with Monte Carlo simulation, the viability of Ge and CdTe semiconductors detectors was calculated. It was determined that the best detector of the ones compared would be a CdTe detector of 2–3 mm, aided by the benefit of adding a Compton-suppression anti-coincidence timing detector.     

Monte Carlo simulation of a PhosWatch detector using Geant4 for xenon isotope beta–gamma coincidence spectrum profile and detection efficiency calculations

A simulation tool has been developed using the Geant4 Toolkit to simulate a PhosWatch single channel beta–gamma coincidence detection system consisting of a CsI(Tl)/BC404 Phoswich well detector and pulse shape analysis algorithms implemented digital signal processor. The tool can be used to simulate the detector's response for all the gamma rays and beta particles emitted from ¹³⁵Xe, ^133mXe, ¹³³Xe, ^131mXe and ²¹⁴Pb. Two- and three-dimensional beta–gamma coincidence spectra from the PhosWatch detector can be produced using the simulation tool. The accurately simulated spectra could be used to calculate system coincidence detection efficiency for each xenon isotope, the corrections for the interference from the various spectral components from radon and xenon isotopes, and system gain calibration. Also, it can generate two- and three-dimensional xenon reference spectra to test beta–gamma coincidence spectral deconvolution analysis software.     

In-situ hydrogen in metal determination using a minimum neutron source strength and exposure time

Water is frequently present in the environment and is a source of hydrogen that can interact with many materials. Because of its small atomic size, a hydrogen atom can easily diffuse into a host metal, and though the metal may appear unchanged for a time, the metal will eventually abruptly lose its strength and ductility. Thus, measuring the hydrogen content in metals is important in many fields, such as in the nuclear industry, in automotive and aircraft fabrication, and particularly, in offshore oil and gas fields. It has been demonstrated that the use of nuclear methods to measure the hydrogen content in metals can achieve sensitivity levels on the order of parts per million. However, the use of nuclear methods in the field has not been conducted for two reasons. The first reason is due to exposure limitations. The second reason is due to the hi-tech instruments required for better accuracy. In this work, a new method using a low-strength portable neutron source is explored in conjunction with detectors based on plastic nuclear detection films. The following are the in-situ requirements: simplicity in setup, high reliability, minimal exposure dose, and acceptable accuracy at an acceptable cost. A computer model of the experimental setup is used to reproduce the results of a proof-of-concept experiment and to predict the sensitivity levels under optimised experimental conditions.     

Monte Carlo validation of the self-attenuation correction determination with the Cutshall transmission method in 210Pb measurements by gamma-spectrometry

The accuracy of estimation of the self-attenuation correction C_s with the Cutshall transmission method in ²¹⁰Pb measurements by gamma-spectrometry was assessed using the Monte Carlo method. The Cutshall method overestimates the correction for samples with linear attenuation coefficient at 46.5 keV higher than that of the standard and underestimates it in the opposite case. The highest bias was found for thick samples. C_s,Cuts/C_s ratio grows linearly with sample linear attenuation coefficient.     

Diffusion cooling of thermal neutrons in basic rock minerals by Monte Carlo simulation of the pulsed neutron experiments

The pulsed neutron experiment (the variable geometric buckling experiment) in spherical geometry has been simulated using the MCNP code. The time decay of the thermal neutron flux has been observed as a function of the sample size. The thermal neutron diffusion cooling coefficient C with the correction F has been determined for three basic rock minerals (quartz, calcite, dolomite) at the given specific densities. The corresponding density-removed parameters have also been obtained.