Isotope effects in thermal neutron transmission and backscattering processes for ε-phase zirconium hydrides and deuterides

Distributions of hydrogen isotope concentrations in ε-phase zirconium hydrides and deuterides (ε-ZrH_x and ε-ZrD_x: 1.8 < x < 2.0) were investigated by neutron radiography (NRG). The NRG images of the thermal neutron transmission and backscattering revealed hydrogen concentration dependence and isotope differences. The thermal neutron mass attenuation coefficients in relation to the hydrogen isotope concentrations were determined from the transmission NRG images. The results showed the isotope effects of the thermal neutron mass attenuation coefficients for ε-ZrH_x to be about 6–9 times higher than those for ε-ZrD_x. The neutron scattering processes for transmission and backscattering NRG images of ε-ZrH_x and ε-ZrD_x were also analyzed using a general Monte Carlo neutron-particle transport (MCNP) code.     

On the Cf primary and secondary gamma rays and epithermal neutron flux for BNCT

Monte Carlo simulation has been used to calculate the different components of neutrons and secondary gamma rays originated by ²⁵²Cf fission and also the primary gamma rays emitted directly by the ²⁵²Cf source at the exit face of a compact system designed for the BNCT. The system consists of a ²⁵²Cf source and a moderator/reflector/filter assembly. To study the material properties and configuration possibilities, the MCNP code has been used. The moderator/reflector/filter arrangement is optimised to moderate neutrons to epithermal energy and, as far as possible, to get rid of fast and thermal neutrons and photons from the therapeutic beam. To reduce the total gamma contamination and to have a sufficiently high epithermal neutron flux we have used different photon filters of different thickness. Our analysis showed that the use of an appropriate filter leads to a gamma ray flux reduction without affecting the epithermal neutron beam quality at the exit face of the system.     

Calculation of thermal neutron self-shielding correction factors for aqueous bulk sample prompt gamma neutron activation analysis using the MCNP code

In this work thermal neutron self-shielding in aqueous bulk samples containing neutron absorbing materials is studied using bulk sample prompt gamma neutron activation analysis (BSPGNAA) with the MCNP code. The code was used to perform three dimensional simulations of a neutron source, neutron detector and sample of various material compositions. The MCNP model was validated against experimental measurements of the neutron flux performed using a BF₃ detector. Simulations were performed to predict thermal neutron self-shielding in aqueous bulk samples containing neutron absorbing solutes. In practice, the MCNP calculations are combined with experimental measurements of the relative thermal neutron flux over the sample’s surface, with respect to a reference water sample, to derive the thermal neutron self-shielding within the sample. The proposed methodology can be used for the determination of the elemental concentration of unknown aqueous samples by BSPGNAA where knowledge of the average thermal neutron flux within the sample volume is required.     

Measurement of the total neutron cross-section and resonance parameters of molybdenum using pulsed neutrons generated by an electron linac

The neutron total cross-sections of molybdenum were measured in the neutron energy region from 0.01 eV to 200 eV by using the time-of-flight method at the Pohang Neutron Facility, which consists of an electron linear accelerator, a water-cooled tantalum target with a water moderator, and a 12-m long time-of-flight path. A ⁶Li-ZnS(Ag) scintillator with a diameter of 12.5 cm and a thickness of 1.6 cm was used as a neutron detector, and a high purity natural molybdenum metallic disc with a diameter of 6.2 cm and a thickness of 3 mm thickness was used for the neutron transmission measurement. Notch filters composed of Co, In, Cd were used to estimate the background level and to calculate the length of neutron flight path. In order to reduce the gamma-ray background from Bremsstrahlung and from neutron capture, we employed a neutron-gamma separation system based on their different pulse shapes. The present measurement was compared with the existing experimental and the evaluated data. The resonance parameters of Mo isotopes were extracted from the transmission by using the SAMMY code and were compared with other previous reported results.     

Measurement of neutron total cross-section and resonance parameters of xenon

We measured the neutron total cross-sections of natural xenon in the neutron energy region from 0.1 to 40 eV by using the time-of-flight method at the Pohang neutron facility, which consists of an electron linear accelerator, a water-cooled tantalum target with a water moderator, and a 12-m long time-of-flight path. A ⁶Li-ZnS(Ag) scintillator with a diameter of 12.5 cm and a thickness of 1.6 cm was used as a neutron detector. Notch filters composed of Co, In, Cd were used to estimate the background level and to calculate the neutron flight path length. The present measurement was compared with the existing experimental and the evaluated data. The resonance parameters of Xe isotopes were obtained from the transmission ratio by using the SAMMY code and were compared with other previous results.     

Measurement and calculation of high-energy neutron spectra behind shielding at the CERF 120 GeV/c hadron beam facility

Neutron energy spectra were measured behind the lateral shield of the CERF (CERN-EU High Energy Reference Field) facility at CERN with a 120 GeV/c positive hadron beam (a mixture of mainly protons and pions) on a cylindrical copper target (7-cm diameter by 50-cm long). An NE213 organic liquid scintillator (12.7-cm diameter by 12.7-cm long) was located at various longitudinal positions behind shields of 80- and 160-cm thick concrete and 40-cm thick iron. The measurement locations cover an angular range with respect to the beam axis between 13 and 133°. Neutron energy spectra in the energy range between 32 MeV and 380 MeV were obtained by unfolding the measured pulse height spectra with the detector response functions which have been verified in the neutron energy range up to 380 MeV in separate experiments. Since the source term and experimental geometry in this experiment are well characterized and simple and results are given in the form of energy spectra, these experimental results are very useful as benchmark data to check the accuracies of simulation codes and nuclear data.Monte Carlo simulations of the experimental set up were performed with the FLUKA, MARS and PHITS codes. Simulated spectra for the 80-cm thick concrete often agree within the experimental uncertainties. On the other hand, for the 160-cm thick concrete and iron shield differences are generally larger than the experimental uncertainties, yet within a factor of 2. Based on source term simulations, observed discrepancies among simulations of spectra outside the shield can be partially explained by differences in the high-energy hadron production in the copper target.     

Comparison of the analytical sensitivities for non-1/v elements in different neutron beams

Irregular nuclides such as ¹¹³Cd, and some rare-earth isotopes show different analytical sensitivities in PGAA performed at different facilities, because the cross-sections of these nuclides have strong low-energy resonances which partly overlap the energy range of typical neutron beams used for activation. A series of systematic measurements has been performed in the spectrally different cold and thermal neutron beams of the Budapest Research Reactor, Hungary and at the research reactor of NIST, Gaithersburg, USA to quantitatively study the non-1/v behaviour of irregular nuclides. Samples were prepared that contained one of the irregular nuclides and also a regular one (¹⁰B, ³⁵Cl and ⁵⁶Fe) and their activation ratios were compared as measured in five different beams. Theoretical values of the activation ratios were calculated from estimates of the actual neutron spectra and cross-section data, and show a generally good correspondence to the experimental results, although some details are still not reproduced.     

Total cross section of solid mesitylene, toluene and a mixture of them at thermal neutron energies

The total neutron cross sections of mesitylene, toluene and a solution 3:2 by volume of mesitylene and toluene were measured at the electron LINAC based pulsed neutron source of Centro Atómico Bariloche. Measurements were performed at 180 K, 120 K and 31.6 K for mesitylene and at 120 K and 31.6 K for toluene and a solution 3:2 by volume of mesitylene and toluene. The systems are potential moderator materials to be considered in the design of a cold neutron source due to their high resistance to radiation and the richness in low-energy excitations of their frequency spectra, that lead to produce an enhanced cold neutron flux.     

Determining Pu isotopic composition and Pu content of PuBe sources by neutron coincidence technique

The paper describes a pure neutron method for determining both Pu content and Pu isotopic composition of PuBe neutron sources by neutron coincidence technique, without using gamma-spectrometry. The new procedure based on the R/T-T relationship is a developed version of the R/T-method based on R/T-M_Pu calibration curve described in [C.T. Nguyen, J. Bagi, L. Lakosi, A novel method of quantitative assay of PuBe neutron sources by neutron coincidence technique, Nucl. Instr. and Meth. B 246 (2006) 409], utilizing Pu isotopic correlations; here R, T, M_Pu are double count rate, single count rate and total Pu content, respectively. Accuracy of the method was found to be about 2-3% and 15% for ²³⁹Pu component and Pu content, respectively. Measurement time as a function of detector efficiency is treated in detail. It is shown that in a system of frame, a transuranium neutron source can be characterized by a pair of co-ordinates [R/T,  T].     

束流参数对热中子照相质量影响的模拟分析

中子束的准直比、镉比、散射中子及注量率空间分布等是影响中子射线照相性能的主要因素,本文通过蒙特卡罗模拟仿真计算来分析各因素对中子照相图像质量的影响,并提出相应的应对措施。     

A sextupole ion beam guide to improve the efficiency and beam quality at IGISOL

The laser ion source project at the IGISOL facility, Jyväskylä, has motivated the development and construction of an rf sextupole ion beam guide (SPIG) to replace the original skimmer electrode. The SPIG has been tested both off-line and on-line in proton-induced fission, light-ion and heavy-ion induced fusion-evaporation reactions and, in each case, has been directly compared to the skimmer system. For both fission and light-ion induced fusion, the SPIG has improved the mass-separated ion yields by a factor of typically 4-8. Correspondingly, the transmission efficiency of both systems has been studied in simulations with and without space charge effects. The transport capacity of the SPIG has been experimentally determined to be ∼10¹² ions s⁻¹ before space charge effects start to take effect. A direct comparison with the simulation has been made using data obtained via light-ion fusion evaporation. Both experiment and simulation show an encouraging agreement as a function of current extracted from the ion guide.     

Detailed spatial measurements and Monte Carlo analysis of the transportation phenomena of thermal and epithermal neutrons from the 12-GeV proton transport line to an access maze

In order to investigate the neutron transportation from a beam-line tunnel to an access maze at a 12-GeV proton accelerator, we measured the spatial distribution of thermal and epithermal neutrons by using the Au activation method in detail. Gold foils were placed at about 70 positions in the maze in the case of the insertion (or extraction) of a copper target of 1 mm thickness into (or from) the beam axis in front of the maze. After the end of accelerator operation, relative activities of the Au foils were simultaneously measured by using an imaging plate technique and the radioactivity of one reference foil was also measured with a HPGe detector to convert to the absolute activities of all foils.It was found that the neutrons reach to the depth of the maze in the case of the insertion of the copper target. This result reflects higher proportion of high-energy particles from the copper target to that from other beam loss points and high-energy particles become the successive source of low-energy neutrons. Furthermore, it was found that several circumstances such as door walls and electric wire cables obviously affect the absorption effect of thermal neutrons. The reaction rates obtained in this study were also used for the benchmark of the Monte Carlo simulation code, MARS15 (version of February 2008). The results of the MARS15 calculations precisely reproduced experimental results and significant effects of the electric wire cables and door walls.     

Design study of a channel in the moderator on improving C and O measurement in coal

By introducing a channel in the moderator of a coal analysis setup using neutron-induced gamma technique, more fast neutrons can reach the coal. Then the characteristic gamma-rays of C and O are increased due to neutron inelastic scattering (NIS) reaction, and measurement of the two elements is sufficiently improved. Monte-Carlo simulations have been carried out in this work for the setup with different prism channels. These prism channels have different positions and sizes, but their bottom surfaces are all nearby the neutron source. The results show that, it could obtain the optimum improvement of C and O measurement, by selecting a proper position and size for the prism channel.     

The total cross section and resonance parameters for the 0.178 eV resonance of Cd

It has been suggested that the capture and scattering cross sections of natural cadmium are not well described by the resonance parameters that are given in the evaluated data files. In particular, doubts on the parameters of the first resonance of ¹¹³Cd at 0.178 eV have been raised. This resonance is of high importance in the interpretation in many integral experiments, such as neutron activation analysis, in which cadmium foils are used to shield from thermal neutrons. A new set of experiments has been designed and performed at the neutron time-of-flight facility GELINA, to determine the total cross section and to extract a set of resonance parameters. The covariance information of the experimental data is propagated and the correlation between the resonance parameters is derived. The obtained parameters are then compared to the data available in the literature. Finally a set of criticality experiments from the international handbook of evaluated critical safety benchmark experiments is used to quantify the influence of the change in the resonance parameters.     

Irradiation effects of a 10 MeV neutron beam on a Nd-Fe-B permanent magnet

The effects of irradiation of a Nd-Fe-B permanent magnet by fast neutrons was investigated. The decrease in measured magnetic flux density at the center of the magnets were 0.6%, 6.9%, 25.2% and 47.3% after continuous irradiation of 1.1 kGy, 3.7 kGy, 5.6 kGy and 7.4 kGy, respectively. On the other hand, the decrease due to non-continuous irradiation, in which the magnet was first irradiated at 3.7 kGy, then irradiated again at 3.7 kGy nine months later, was 14% smaller than that of continuous irradiation, even for the same total dose. The temperature coefficient of the magnetization did not change with irradiation. Some radioactive materials, such as ¹⁴⁷Nd, ¹⁵¹Pm, and ⁵⁴Mn, were detected in the magnet after irradiation.     

散裂中子源靶站和中子散射谱仪的概念设计

本文介绍了可应用于多学科应用的散裂中子源(CSNS)靶站和中子散射谱仪概念设计的进展。CSNS靶站将由重水冷却多片钨靶,铍/铁反射体和铁/重混凝土生物屏蔽体组成。采用三个WING型慢化器:水(室温),液体甲烷(100K)和液体氢(20K),设有18个水平中子孔道。MonteCarlo模拟显示优化的靶截面高宽比为1:2.5左右。额定的100kW核功率的质子束轰击后,慢化器处钨靶溢出的脉冲中子通量约为2.4×1016cm?2·s?1。有限元方法计算表明,钨靶体内的总发热量是47kJ/s。即使使用截面较小的钨靶,在通常的水冷速率下,靶体温度也仅略高于90°C。靶体的热应力形变最大不超过0.2mm。根据经济实用原则选择建造粉末衍射仪、小角散射仪、反射仪及直接几何非弹性散射仪等四类有代表性的中子散射谱仪,就能覆盖>80%的中子散射研究领域。     

Effect of neutron dose on the structural properties of Makrofol polycarbonate

The effect of neutron dose on the structural properties of a Makrofol polycarbonate detector has been studied. Samples of Makrofol were classified into three main groups. The first, second and third groups were irradiated with neutrons in the dose ranges of 0.01-0.08, 0.1-0.8 and 1.0-8.0 Sv, respectively. The structural modifications in the neutron irradiated samples were studied using X-ray diffraction and Fourier-transform infrared spectroscopy. In addition, the effect of neutron dose on the intrinsic viscosity of the liquid samples, as a measure of the mean molecular mass of the Makrofol polymer, was studied. An increase in the -OH groups was observed at dose ranges of 0.01-0.04 and 3.21-8.0 Sv due to the degradation of carbonate group and the -H abstraction from the polymer backbone to form hydroxyl groups. This indicates that the degradation is dominant at these dose ranges that enhance the degree of ordering in the irradiated samples. On the other hand, irradiation in the dose range 0.04-3.21 Sv was characterized by a dominant crosslinking mechanism that led to an increase of the amorphous phase and average molecular mass.     

On the comparison of three methods of assessing beam quality for broad beam in vitro cell irradiation

The interaction of charged particles with living matter has recently attracted increasing interest in the field of biomedical applications such as hadron therapy, radioprotection and space radiation biology. Particle accelerators are particularly useful in this area.In vitro radiobiological studies with a broad beam configuration require beam homogeneity. The goal is to produce a dose distribution given to a cell population that is as close to uniform as possible.In this paper, we compare the results of three devices used to assess the beam quality for broad beam irradiation: a passivated implanted planar silicon (PIPS) particle detector, a position-sensitive solid state detector, which is camera-like, and a solid state nuclear track detector (CR39).The first device is a PIPS detector of 300 μm nominal depletion depth and an entrance window with a thickness of about 500 Å. It is collimated with a 0.5 mm aperture and mounted in air on an XY moving table as close as possible to the exit window of the beam line.The second device is a CMOS position-sensitive detector (technological process 0.6 μm AMS CUA), 112 × 112 pixels, with 153 × 153 μm² pixel size. It allows the user to rapidly obtain dose uniformity over a surface of 1 × 1 cm². During uniformity and dose rate assessment it is placed in air at the PIPS location.For both detectors, beam profile was obtained for various proton fluxes (from ∼5 × 10⁴ to 10⁶ particles cm⁻² s⁻¹). Preliminary tests were made with CR39 using 4 MeV He⁺⁺ ions.Results are analysed using Poisson distribution and cell hit probability.     

Ion beam optics analysis and beam transmission simulation in the reflection magnet of the EAST NBI system

Detailed trajectories of the residual ion passing through a reflection magnet, which is installed in the Experimental Advanced Superconducting Tokamak (EAST) neutral beam injector (NBI) system to remove the residual ion from the beam path, are reported. Monte Carlo simulation has been performed to obtain estimations of the beam density on the high heat flux elements of reflection magnet. On magnet pole shielding, there are three high beam power density regions, resulting from beam line focus and corresponding to the full energy, half energy and third energy. These high heat flux regions will bring a serious challenge for steady-state operation. The simulation result indicates that the re-ionization loss is about 2.43% and the magnet pole shielding encounters a problem of beam line focus, which should be taken seriously during the steady-state operation.