The dose comparison between the THOR and HFR epithermal neutron beams

This study is a part of the beam comparison campaign, inter-center dose comparison, between boron neutron capture therapy facilities at the Tsing Hua Open-pool Reactor and the High Flux Reactor. The clinical information exchange can improve the dosimetry uncertainty for medical physics in a mixed field. The method of paired Mg(Ar) and TE(TE) ionization chambers was used to determine the gamma-ray and neutron dose rates. Furthermore, activation foils, including gold, copper, and manganese, were employed to estimate the thermal and epithermal neutron fluxes. Measurements were performed free in air and also in a PMMA phantom. All the chambers were calibrated using a ⁶⁰Co primary standard source at the Institute of Nuclear Energy Research, Taiwan. Spectrum dependent neutron sensitivity of TE(TE) chamber is one of the important parameters to evaluate dose components. The requested neutron spectra were calculated by the Monte Carlo code MCNP. The measured thermal neutron fluxes, gamma-ray and neutron dose rates of the THOR beam in the phantom were 2.6, 2.2, and 2.1 times of the HFR beam at 2.5-cm depth, respectively. The higher thermal neutron flux and neutron and gamma-ray dose rates are due to the higher epithermal neutron beam intensity of the THOR.     

The angular and spatial distributions of the thermal neutron source description of the THOR BNCT beam

This paper presents a way to determine the angular and spatial distributions of the thermal neutron source strength of a boron neutron capture therapy (BNCT) beam. The experiments applied 1) the indirect neutron radiography, 2) the cadmium difference method, and 3) the instrumental neutron activation analysis. The measured data were processed by the spectrum deconvolution technique to resolve into a proper set of angular and spatial distributions. This paper took the epithermal neutron beam of the BNCT facility at the Tsing Hua Open-pool Reactor as an example.     

中国散裂中子源反角白光中子束流参数的初步测量

中国散裂中子源(CSNS)已于2018年5月建设完工,随后进行了试运行.其中的反角白光中子束线(Back-n)可用于中子核数据测量、中子物理研究和核技术应用等多方面的实验.本文报道对该中子束的品质参数测量实验过程以及最终实验结果.实验主要采用中子飞行时间法,利用~(235)U,~(238)U裂变室和~6Li-Si探测器测量了中子能谱和中子注量率,又利用闪烁体-互补金属氧化物半导体探测系统测量了中子束斑的剖面,得到了该束线的初步实验测量结果.其中白光中子的全能谱测量范围eV—100 MeV,给出了不确定度分析;给出了中子注量率两个实验厅位置的满功率值;给出了白光中子在直径60 mm情况下的全能区束斑.通过与模拟结果的比较探讨了以上结果的合理性,并提出了改进计划.这些实验结果为以后该束线的核数据测量和探测器标定实验奠定了基础.     

A proposed investigation of parity non-conserving effects using a polarized target and an unpolarized epithermal neutron beam

Parity non-conserving (PNC) effects can be studied with unpolarized neutron beams and polarized targets. TheI·k term in the forward scattering amplitude (I=target spin, k=neutron momentum) is enhanced at a p-wave resonance in the same way as thes·k term which has been studied in previous experiments (s=neutron spin). A large number of resonances can be studied in a polarized target experiment because level densities are high in odd-odd nuclei (typically 25 s and p levels per 100 eV in¹⁶⁶Ho). The goal of the experiments is to extract an average PNC matrix element from a statistically distributed set of PNC matrix elements. A rotating cryostat for use in longitudinal analyzing power measurements is described. Suitable rare earth targets are holmium, thulium, terbium and hyperfine enhanced targets such as praesodymium compounds.     

National facility for neutron beam research

In this talk, the growth of neutron beam research (NBR) in India over the past five decades is traced beginning with research at Apsara. A range of problems in condensed matter physics could be studied at CIRUS, followed by sophisticated indegenous instrumentation and research at Dhruva. The talk ends with an overview of current scenario of NBR world-wide and future of Indian activities.     

Fine structure discussion of parity-nonconserving neutron scattering at epithermal energies

The large magnitude and the sign correlation effect in the parity non-conserving resonant scattering of epithermal neutrons from²³²Th is discussed in terms of a non-collective 2p 1h local doorway model. General conclusions are drawn as to the probability of finding large parity violation effects in other regions of the periodic table.     

Facility for parity and time reversal experiments with intense epithermal (eV) neutron beams

A facility for polarized epithermal neutrons of high intensity is set up at the Los Alamos National Laboratory for parityviolation and time reversal experiments at neutron resonances over a wide range of neutron energies. The beam is polarized with the aid of a polarized proton target used as a neutronspin filter. Total cross section measurements as well as capture gamma-ray experiments will be carried out. The main features of this system will be discussed.     

Characterisation of the high-energy neutron beam at iThemba LABS

Monoenergetic and white neutron beams with a maximum energy of 200 MeV are produced at the neutron beam facility of the iThemba Laboratory for Accelerator-Based Sciences (iTL) by (p, n) reactions on Li, Be and C targets. The iTL separated-sector cyclotron can accelerate protons from 25 MeV to 200 MeV. Time of flight (TOF) measurements can be carried out by increasing the time separation between proton bunches up to 500 ns with a beam pulse selector. In the present work, the methods of beam characterisation used at this facility are discussed. Comparisons with metrological standards are presented. Specific issues related to monitoring the neutron fields at different accelerator settings are analysed.     

Characterization of a BNCT beam using neutron activation and indirect neutron radiography

This study aims to measure the neutronic characteristics of the 14-cm diameter boron neutron capture therapy beam at the Tsing Hua Open-pool Reactor and to provide essential information for the neutron source adjustment and validation. The measurements were performed both in air and inside a cubic PMMA phantom at the beam, with and without an 18-cm long extended PE collimator. The neutron intensity was determined by neutron activation analysis; the indirect neutron radiography and the cadmium difference method were coupled to provide the two-dimensional neutron flux distributions.According to the measurements, the angular distribution is highly forward for epithermal neutrons but much more divergent for thermal ones. A PE extended collimator will modify the beam characteristics and could benefit the treatment for head and neck tumors, which often needs higher therapeutical boron dose at the shallow region.     

A pulsed neutron source on the proton beam for the Moscow Meson Factory

A new pulsed neutron source based on a beam-blanking device has been under construction and improvement at the Moscow Meson Factory of the Institute for Nuclear Research of the Russian Academy of Sciences. Neutrons are generated in the course of the spallation process in a water-cooled tungsten target by a proton beam with an energy of 209 MeV. After water moderator (3 cm), neutrons are guided in three horizontal and one vertical channels with a length varying from 4 to 50 m. The standard duration of the proton pulse from the accelerator is 60 μs. At present, the average proton current is as high as 150 μA for a repetition rate of 50 Hz. The neutron fluence in the target is equal to 0.9 × 10¹⁵ neutrons/s, which corresponds to the requirements imposed on the intensity of modern pulsed neutron sources.     

A beam divergence correction mirror for neutron resonance spin echo

Neutron resonance spin echo (NRSE) enables us to measure neutron quasi-elastic scattering with high energy resolution. Its energy resolution is limited by a path length variation due to the beam divergence. Neutron focusing technique using a neutron supermirror can be used to overcome this problem. To investigate the effect of a cylindrical mirror on the path length variation due to the beam divergence, MIEZE (modulation of intensity by zero effort) spin echo measurement was performed. The result demonstrated that the cylindrical mirror effectively corrects the path length variation and leads to high energy resolution as well as high intensity in NRSE and MIEZE measurement.     

Design of the low energy beam transport line for the China spallation neutron source

The design of the China Spallation Neutron Source (CSNS) low-energy beam transport (LEBT) line, which locates between the ion source and the radio-frequency quadrupole (RFQ), has been completed with the TRACE3D code. The design aims at perfect matching, primary chopping, a small emittance growth and sufficient space for beam diagnostics. The line consists of three solenoids, three vacuum chambers, two steering magnets and a pre-chopper. The total length of LEBT is about 1.74m. This LEBT is designed to transfer 20mA of H-pulsed beam from the ion source to the RFQ. An induction cavity is adopted as the pre-chopper. The electrostatic octupole steerer is discussed as a candidate. A four-quadrant aperture for beam scraping and beam position monitoring is designed.     

Design of the low energy beam transport line for the China spallation neutron source

The design of the China Spallation Neutron Source (CSNS) low-energy beam transport (LEBT) line, which locates between the ion source and the radio-frequency quadrupole (RFQ), has been completed with the TRACE3D code. The design aims at perfect matching, primary chopping, a small emittance growth and sufficient space for beam diagnostics. The line consists of three solenoids, three vacuum chambers, two steering magnets and a pre-chopper. The total length of LEBT is about 1.74 m. This LEBT is designed to transfer 20 mA of H-pulsed beam from the ion source to the RFQ. An induction cavity is adopted as the pre-chopper.The electrostatic octupole steerer is discussed as a candidate. A four-quadrant aperture for beam scraping and beam position monitoring is designed.     

Simulation study on the boron-coated GEM neutron beam monitor

A Monte Carlo method of the 3D GEM simulation is introduced.The physical process of the neutron detection is described with the Geant4 code and the Garfield code.The results of the optimized electric-fields,the emitted ions spectrum,the electrons transverse diffusion and the signal width are presented.Moreover,the preliminary result with a CF-252 neutron source is reported.These are useful in designing detector structures and to provide an optimized option for the development of the boron-coated GEM neutron beam monitor.     

A simple white beam neutron diffraction technique

White beam neutron diffraction by time-of-flight (TOF) technique has been studied over a number of years as it is believed to be the most convenient method to investigate solids in a fixed geometry. This technique needs a pulsed neutron source and a suitable multichannel, analysing system. The entire system is in general, mechanically quite intricate and expensive. We have investigated an alternative technique to achieve the end result of a constant geometry around the diffracting sample. This involves the use of a single crystal as an analyser to study diffraction pattern from the sample bathed in a white beam and diffracting at any fixed scattering angle. In this paper we report the results of our investigations and have compared this technique with other diffraction techniques. Taking Si, KCl and KNO₃ as typical specimens we have illus. trated the results of our technique and we find that the results are comparable to those obtained by conventional neutron diffraction and TOF diffraction. The technique is simple in mechanical design and data acquisition. It can be easily adapted for high pressure diffraction which is being attempted.     

The spatial distribution of thermal and epithermal neutrons in a graphite moderated spallation neutron field

The Gamma-3 assembly is located at the Joint Institute for Nuclear Research, Dubna, Russia. It consists of a cylindrical lead target (ø = 8 cm, L = 58.8 cm) surrounded by reactor grade graphite (110 × 110 × 60 cm). The target was irradiated with a beam of 1.6 GeV deuterons from the Nuclotron accelerator and CR-39 track detectors coupled to LR-115 2B film were used to measure the slow neutron distribution on the surface of the graphite. The detection efficiency of the CR-39 in the CR-39/LR-115 2B system was measured using a custom made calibration setup and found to be (1.12 ± 0.05) × 10⁻³ and (6.1 ± 1.2) × 10⁻⁴ tracks per neutron, for thermal and epithermal neutrons respectively, under the etching and counting procedures described in this work. The irradiation of the Gamma-3 was also simulated using MCNPX 2.7 Monte Carlo code and good agreement between the experimental and calculated track densities was found. This serves as a good validation for the computational models used to simulate spallation neutron production, transport and moderation.     

Thermal neutron calibration method using an intense neutron beam from JRR-3M

We have developed a thermal neutron calibration method using a reactor produced neutron beam in JRR-3M of the Japan Atomic Energy Agency. Neutron-induced prompt gamma ray analysis has usually been performed in this beam line. Neutron energy distributions with negligible contributions from epithermal neutrons were measured by a time-of-flight method with a chopper made of ⁶LiF powder. The thermal neutron flux was determined by a gold foil activation method. We found that the thermal neutron beam in JRR-3M was well suited for calibration, neutron detector development or neutron dosimetry.     

Spatial modulation of a neutron beam by Larmor precession

We modulated the intensity of a neutron beam using Larmor precession techniques. We simulated triangular coils by using magnetised foils in electromagnets with rectangular pole shoes. Reversing the orientation of two of the magnetised foils in the spin-echo small-angle neutron scattering (SESANS)-instrument in Delft and changing the field settings was sufficient to obtain a modulation with a period in the order of 1 mm. We expect to be able to go to a modulation with a period of .This technique can be used to measure small-angle neutron scattering, analogous to the method used in SESANS, but with the advantage that all the manipulation of the neutron spin occurs before the sample. This means that the technique is insensitive to magnetic perturbations at the sample position. By positioning several of these devices after each other it will be possible to obtain a sharper modulation, or a modulation in two directions.     

Mechanical research and development of a monocrystalline silicon neutron beam window for CSNS

The monocrystalline silicon neutron beam window is one of the key components of a neutron spectrometer. Monocrystalline silicon is brittle and its strength is generally described by a Weibull distribution due to the material inhomogeneity. The window is designed not simply according to the mean strength but also according to the survival rate. The total stress of the window is stress-linearized into a combination of membrane stress and bending stress by finite element analysis. The window is a thin circular plate, so bending deformation is the main cause of failure and tensile deformation is secondary and negligible. Based on the Weibull distribution of bending strength of monocrystalline silicon, the optimized neutron beam window is designed to be 1.5 mm thick. Its survival rate is0.9994 and its transmittance is 0.98447, which meets both physical and mechanical requirements.