Neutron Beam Monitor Research Articles

Development of an online neutron beam monitoring system for accelerator-based boron neutron capture therapy in a hospital.

Boron neutron capture therapy (BNCT) is a next-generation radiotherapy, utilizing both an external neutron beam and a -containing pharmaceutical. A compact accelerator for a high intensity neutron source was installed to conduct BNCT in a hospital. The dose administered to a patient was evaluated by measuring the proton beamcurrent. Neutron intensity should be monitored in real-time by measuring the neutrons emitted from the target during BNCT irradiation. This is crucial due to potential neutron target degradation. Online neutron beam monitoring systems are required for reliable measurements of the administered neutron dose. An online neutron beam monitoring system was developed to monitor neutron intensity irradiating on the patient at the National Cancer Center Hospital (NCCH). The neutron detector comprised a back-illuminated thin Si diode of 40- thickness and an ultrathin natural LiF neutron converter of 0.05- thickness. The neutron detector was installed on the neutron target unit, regardless of whether a patient was present, without any additional modifications to the setup. The response functions for high photon dose rates of upto 100 Gy/h were measured. The pulse heights were measured using the neutron beam monitor during BNCT neutron irradiation. Neutron temporal response measured using the online beam monitor was acquired and compared with the proton beam current and the measurements at a patient position. From this measurement at the patient position, the neutron fluence rate irradiating on a patient wasobtained. The neutron events were separated from the photon events. The neutron counting rates increased rapidly with the starting of proton beam irradiation and dropped to zero upon its termination. During intermittent drops and recoveries in the proton beam, the neutron beam monitor for counting rates responded quickly, synchronizing with the beam current. A scatter plot of the neutron counting rate and proton beam current indicated a good linear correlation. A direct relationship between the online neutron beam monitor's neutron counting rates and those of the patient neutron detector showed a good correlation coefficient of 0.84. A ratio of the both neutron counting rates showed a standard deviation of 6%. The correlation coefficient and standard deviation were improved to 0.94 and 1.5%, by re-binning the neutron temporal response with longer acquisition period than 1 s. Using the online neutron beam monitor, the neutron fluence rate was obtained from the direct relationship within 1.5%. Therefore, real-time monitoring of neutron intensity was achieved within the acceptable level as per the International Commission on Radiation Units and Measurementsreport. The online neutron beam monitoring system was developed to monitor the BNCT neutron beam intensity at NCCH. The temporal response of the neutron beam monitor was synchronized with the neutron counting rate at the patient position. Using the online neutron beam monitor, the neutron fluence rate irradiating on the patient can be monitored from the direct relationship. Fluctuation of the neutron beam intensity through BNCT irradiation and the degradation of the lithium target through the lifespan of the neutron target could be monitored using the neutron beammonitor.

Pure CaF2 crystal for fast neutron detection

Pure CaF2 crystals have been widely used for optical functional crystals due to their good optical properties, mechanical properties, and chemical stability. This study discovered the crystal's pulse shape discrimination (PSD), proving it a promising scintillator for fast neutron detection via the 40Ca neutron capture reaction. Luminescence emission spectra were studied by X-ray luminescence measurements. The maximum emission peak was observed between 250 and 300 nm, corresponding to self-trap exciton emission. The crystal response to gamma, alpha, and neutron were studied under 137Cs, 241Am, and 252Cf radiation sources, respectively. The difference in gamma and alpha decay times enabled the PSD capability of pure CaF2. Large neutron reaction cross-sections, PSD capability, and good chemical stability make pure CaF2 crystal candidates for fast neutron beam monitoring detectors. Moreover, the crystal can be used for 40Ca(n,p)40K and 40Ca(n,α)37Ag neutron capture cross-section study.

A neutron beam monitor based on ceramic gas electron multiplier with high spatial resolution for low flux measurements.

Neutron scattering instruments play an important role in studying the inner structure of materials. A neutron beam monitor is a detector commonly used in a neutron scattering instrument. The detection efficiency for most neutron beam monitors is quite low (10-4-10-6). However, in some experiments with a low neutron flux, such as small angle neutron scattering (SANS) and inelastic neutron scattering experiments, a neutron beam monitor with a higher detection efficiency (∼1% for thermal neutrons) is required to reduce the duration of the experiment. To meet this requirement, a ceramic gas electron multiplier-based neutron beam monitor equipped with a 1 µm 10B4C neutron converter was developed in this study. Its performance was determined both experimentally and in simulations. The detection efficiency in the wavelength range of 1.8-5.5 Å was measured experimentally and was confirmed by the simulation results. An algorithm based on event selection and position reconstruction was developed to improve the spatial resolution to about 1mm full-width-half-maximum. The wavelength spectrum was measured in beamline 20 (BL20) and agreed well with the results obtained using a commercial monitor. The maximum counting rate was 1.3MHz. The non-uniformity over the whole 100 × 100mm2 active area was determined to be 1.4%. Due to the excellent performance of this monitor, it has been used in several neutron instruments, such as the SANS and the High-Energy Direct-Geometry Inelastic Spectrometer instruments in the China spallation neutron source.

First evaluation of a novel ionisation chamber for thermal neutron beam monitoring

The European Spallation Source ERIC (ESS), currently under construction in Lund, Sweden is a facility established to deliver the highest integrated neutron flux originating from a pulsed source with the aim of supporting an initial fifteen neutron instruments for cutting edge science experiments. This in turn requires reliable monitoring at complex neutron beam lines: in particular, linearity, timing capability, adaptability of the design for various flux ranges (dynamic range) and sensitivity to neutrons within the range of 0.6-10Å are expected from the neutron beam monitors to be installed at the ESS beam lines. Additionally, operational stability and low attenuation are also desirable characteristics for such neutron beam monitoring. A prototype neutron beam monitor based on the ionisation chamber principle and a boron converter, designed by CDT CASCADE Detector Technologies GmbH and ESS, has been investigated at the BER-II research reactor of Helmholtz Zentrum Berlin (HZB). The effort to design and investigate a thermal neutron ionisation beam monitor was initiated by adapting the concept of ionisation chambers previously known elsewhere. So far all the characterised neutron beam monitors discriminate neutron hits on a discrete event basis (pulse mode), whereas the beam monitor prototype introduced in this paper estimates the total flux as a function of current (current mode). While most other neutron beam monitoring devices and detectors rely upon a signal amplifying gain stage, the ionisation chamber operates without any gain and is consequently robust against typical detector ageing effects that compromise the sensitivity over time. The initial tests were performed at the ESS V20 test beam line under realistic conditions resembling those of the future pulses of ESS. The linearity is demonstrated for 3Å pulses in the flux range of 2-3 × 105 n/s/cm2 and for white pulses (0.6-10Å) in the range of 1-5 × 106 n/s/cm2. The timing behaviour resembles the data previously recorded at the V20 beam lines. This novel implementation of a neutron sensitive ionisation chamber shows great promise for beam monitoring and diagnostics at ESS. As the ionisation beam monitor itself is an entirely passive device, it is adequately robust to be employed in areas of high irradiation where no regular servicing or maintenance can be provided.

Realization of a thin boron layer neutron beam monitor

A neutron beam monitor can play an important role in a neutron experiment when the spatial and temporal distribution of the incident neutrons should be exactly known. In this study, we propose a method to realize a neutron beam monitor with a thin boron layer, which is necessary for easily setting an optimal threshold to separate the neutron signals from those of gamma rays and electronic noise with the aid of atomic layer deposition (ALD) technique. By applying the B2O3 layer forming process and the ZnO layer forming process, boron layers with thicknesses ranging from 16.8 nm to 472.7 nm are successfully prepared to form the beam monitors with a multiwire proportional chamber (MWPC) as the readout. A test is conducted at the compact pulsed hadron source (CPHS) at Tsinghua University. The results show that a slope of 0.7%/100 V of the counting plateau can be achieved for the high voltage region of 250 V, with the spatial resolution being better than 3.09 mm. The B2O3 film with a minimum mass thickness of 0.38μg/cm 2 enables a maximum neutron flux of 9.7 × 107 n/cm 2/s to be monitored with this beam monitor, with an accompanying attenuation for penetrating incident neutrons of 0.35%, significantly smaller than that of typical state-of-art beam monitor.

Multitube monitors: a new-generation of neutron beam monitors

With the renewal of many neutron science instruments and the commissioning of new neutron facilities, there is a rising demand for improved neutron beam monitoring systems with reduced beam perturbations and higher counting rate capability. Fission chambers are the most popular beam monitors; however, their use on some instruments may be prevented by the background generated by fast neutrons emitted during neutron captures in 235U and by neutrons scattered in the material of the fission chamber. Multitube detectors, on the other hand, offer a good alternative with minimum beam perturbations. The purpose of this paper is to report and analyse the results of the measurements performed with several Multitubes used for beam monitoring. We show that the transparency of Multitube beam monitors is 97.6 ± 0.4 %, and that their detection efficiency is uniform, with a deviation from the mean value <0.7 %. A counting rate reduction of 10 % due to pile-up effects is measured at a rate of 550 kHz. In addition to neutron beam intensity monitoring, the Multitube can be configured for 1-dimensional or 2-dimensional localisation. We present the preliminary results of these additional functionalities.

Development of a neutron beam monitor with a thin plastic scintillator for nuclear data measurement using spallation neutron source

ABSTRACT A neutron monitoring detection system was developed for neutron capture cross-section measurement using a spallation neutron source. A combination of a plastic scintillator and a thin 6 LiF foil was adopted for the detector. The detector system was tested to study the feasibility of the system. Neutron irradiation experiments were conducted with the Accurate Neutron-Nucleus Reaction Measurement Instrument in the Materials and Life Science facility of the Japan Proton Accelerator Research Complex. A neutron time-of-flight spectrum was successfully measured without significant count loss or detector paralysis.

Development of B and BN thin films for in situ neutron beam monitoring

A B-based thin-film neutron monitor is developed for the in situ neutron beam monitoring of the Korea Atomic Energy Research Institute neutron depth profiling system. Thin-film samples are prepared via radio-frequency (RF) sputtering. The RF power, Ar partial pressure, and deposition temperature are varied to optimize the growth and thickness uniformity of the samples. The neutron transmission ratio and collision heating of the designed samples are estimated via Monte Carlo simulation. The neutron transmission ratios of B and BN films prepared in this study are 92.46% and 93.93%, respectively, and the effect of temperature increase is negligible. The displacement per atom (DPA) rates of the B and BN thin films are lower than those of other known B neutron flux monitors, based on the results of DPA calculation for estimating the defect of the sample via neutron irradiation.

Uniformity of response of Uranium fission chambers used as neutron beam monitors

Uranium fission chambers are commonly used for neutron beam monitoring in neutron research facilities. The challenges brought with the renewing or commissioning of new neutron facilities encourage to minimize the interaction between the monitor and the neutron beam. In order to characterize both their detection efficiency and transmission factor, several fission chambers, have been scanned on the CT1 monochromatic beam line of the ILL. Results show that the monitor transparency, measured at the center of the monitor, varies from 90% to 96%, and that the detection efficiency varies by as much as 50% over the active area.

Parasitic neutron beam monitoring: Proof of concept on gamma monitoring of neutron chopper phases

Neutron beam monitors are an essential diagnostic component of neutron scattering facilities. They are used to measure neutron flux, calibrating experiments performed on the instruments, allowing measurement of facility performance, understanding of the effect on the neutrons of beam-line components (such as choppers), calibration of detectors and tracking of beam stability. Ideally beam monitors should not perturb the beam. Previous work shows commercial beam monitors attenuate the beam by a few percent in the worst case due to the 1–2 mm thick aluminium entrance and exit windows and the material inside. Parasitic methods of neutron beam diagnostics, where there is no beam monitor directly in the beam, would be preferable. This paper presents the concept of a parasitic method of monitoring the beam which can be used for neutron chopper phasing. This is achieved by placing a gamma detector close to a rotating chopper and measuring a signal proportional to the flux absorbed by the chopper. Neutrons interact with the boron absorber on the chopper disc leading to gamma emission at 480 keV. Detection of these gamma rays is used to determine the chopper phasing and timing. Potentially information on the flux of the beamline can be extracted. Results from a proof of concept implementation show that diagnosis of neutron chopper phases is feasible.

Vanadium-based neutron beam monitor

A prototype quasi-parasitic thermal neutron beam monitor based on isotropic neutron scattering from a thin natural vanadium foil and standard $^3$He proportional counters is conceptualized, designed, simulated, calibrated, and commissioned. The European Spallation Source designed to deliver the highest integrated neutron flux originating from a pulsed source is currently under construction in Lund, Sweden. The effort to investigate a vanadium-based neutron beam monitor was triggered by a list of requirements for Beam Monitors permanently placed in the ESS neutron beams in order to provide reliable monitoring at complex beamlines: low attenuation, linear response over a wide range of neutron fluxes, near to constant efficiency for neutron wavelengths in a range of 0.6-10 \r{A}, calibration stability and the possibility to place the system in vacuum are all desirable characteristics. The scattering-based prototype, employing a natural vanadium foil and standard $^3$He proportional counters, was investigated at the V17 and V20 neutron beamlines of the Helmholtz-Zentrum in Berlin, Germany, in several different geometrical configurations of the $^3$He proportional counters around the foil. Response linearity is successfully demonstrated for foil thicknesses ranging from 0.04 mm to 3.15 mm. Attenuation lower than 1% for thermal neutrons is demonstrated for the 0.04 mm and 0.125 mm foils. The geometries used for the experiment were simulated allowing for absolute flux calibration and establishing the possible range of efficiencies for various designs of the prototype. The operational flux limits for the beam monitor prototype were established as a dependency of the background radiation and prototype geometry. The herein demonstrated prototype monitors can be employed for neutron fluxes ranging from $10^3-10^{10}$ n/s/cm$^2$.

A ceramic GEM-based neutron beam monitor for China Spallation Neutron Source

With the development of the new generation of high-flux pulsed neutron facilities, it is necessary that the neutron beam monitors have the characteristics of wavelength resolution and long-term stability, as well as high counting rates and two-dimensional(2D) spatial resolution. In this paper, a novel neutron beam monitor was developed using a single ceramic GEM(Gas Electron Multiplier) at China Spallation Neutron Source(CSNS). Its validation of the wavelength spectra measurement was verified via comparisons with measurements made with an ORDELA 4562N monitor. In addition, it was demonstrated that this neutron beam monitor was able to measure the beam profile of total and sliced wavelengths. The maximum counting rate was about 1.5 MHz by using an X-ray source. The RSD (the relative standard deviation) of instability fluctuations was approximately 0.5% during continuous operations of 40 h. The results show that the ceramic GEM-based neutron beam monitor can satisfy the demands of the high-flux neutrons measurement, and it has been regarded as a standard solution to the neutron beam monitor at CSNS.

Development of a novel neutron tracker for the characterisation of secondary neutrons emitted in Particle Therapy

The MONDO (MOnitor for Neutron Dose in hadrOntherapy) project addresses the technical challenges posed by a neutron tracker detector: high detection efficiency and good backtracking precision. The project main goal is to develop a tracking device capable of fully reconstructing the four-momentum of the ultra-fast secondary neutrons produced in Particle Therapy treatments via double elastic scattering interactions. The tracker – a 16×16×20cm3 matrix – is made by a matrix of thin squared scintillating fibres (250 μm) arranged in layers orthogonally oriented. A custom readout system called SBAM (SPAD-Based Acquisition readout for MONDO experiment), matched to the MONDO needs of single photon detection capability, high spatial resolution and compactness, has been developed in collaboration with Fondazione Bruno Kessler (FBK). A small detector prototype (4×4×4.8cm3) has been built and tested with a sensor prototype, SPADnet-I, in order to experimentally evaluate the light output expected and consequently optimise the final readout. The simulation characterisation of the detector response with monochromatic neutrons in the [20-300] MeV will be presented together with the expected performances of MONDO as neutron beam monitor. The preliminary measurements at electron and proton beams of the prototype with the SPAD array readout will be reported.

A neutron beam monitor based on silicon carbide semiconductor coated with 6LiF converter

A neutron beam monitor was developed for the time-of-flight neutron reflectometer (NR) instrument at the China Mianyang Research Reactor (CMRR). The SiC neutron monitor employs a wide-bandgap silicon carbide (SiC) semiconductor detector coated with a 6LiF neutron converter film evaporated on a thin ceramic sheet. Given the relatively large active area [(10 × 10) mm2] and sufficiently thick epitaxial layer (30μm), the monitor can discriminate the gamma background from the signals. Prior to assembling the monitor, the I–V characteristics and alpha responses of the SiC detector were measured. Next, the SiC detector and 6LiF neutron converter were installed in a small aluminum shielding box, and the neutron and gamma responses were measured with an Am–Li neutron source and a 137Cs plus 60Co gamma source, respectively. The fabricated SiC neutron monitor was mounted inside the slit of the NR instrument (after the focusing neutron guide), and the intensity of the transmitted neutron beam was measured. To measure the incident neutron flux, the distinct triton peaks from the 6Li (n, α) T reaction were counted as normalized data for the NR instrument. This paper describes the fabrication details of the SiC neutron monitor, and discusses the preliminary measurements.

Novel method exploration of monitoring neutron beam using Cherenkov photons in BNCT

Ensuring the stability of the neutron beam during boron neutron capture therapy (BNCT) is essential. The goal of this work was to explore a novel method using Cherenkov photons for real-time monitoring of neutron beam in BNCT. The Monte Carlo toolkit Geant4, was used to calculate the Cherenkov photons and dose based on a cubic phantom and Chinese hybrid radiation phantom under different conditions. Based on the study on the cubic phantom, the change of neutron beam characteristics indeed have an impact on dose, which needs quality assurance. The relationship between Cherenkov photons and gamma dose was favorable. The maximum relative variation of the generation of Cherenkov photons is approximately 3.51%, even when the relative change of boron concentration reaching 60%. However, comparing the results of the neutron beam with the original characteristics, the maximum relative variation of Cherenkov photons reaches 14.75% and 14.68% for the usage of Neuboron- spectrum and 5-degree rotation of neutron beam field orientation, respectively. Therefore, the intensity and distribution of Cherenkov photons can only be observably changed by neutron beam characteristics. Based on the Chinese hybrid radiation phantom, the changes in the intensity and distribution of Cherenkov photons can also be observed inside the phantom, and these changes are evident by a detector outside the phantom. Thus, Cherenkov photons has the potential application for neutron beam monitoring.

Compact lithium-glass neutron beam monitor for SANS at CSNS

A small-angle scattering neutron spectrometer for material research is under construction at the China Spallation Neutron Source. An intervening neutron beam monitor behind the sample is needed to measure the beam intensity in order to reduce the measurement uncertainty caused by beam fluctuation. Considering the mobility requirement and limited space, we proposed a compact monitor using a type of lithium-glass scintillator provided by China Building Materials Academy. Its performance was studied experimentally using $$^{252}{\hbox {Cf}}$$ and $$^{60}{\hbox {Co}}$$ sources. The neutron light yield of the selected scintillator was measured to be $$5.3\times 10^3$$ photons/neutron. The feasibility of n-gamma discrimination using the charge comparison method was verified. By using the Geant4 toolkit, the monitor was modeled with precise physical processes including neutron tracking, scintillation, and optical photon transmission. The gamma sensitivity and detection efficiency were investigated in the simulation. It was concluded that a 0.5-mm-thick lithium-glass scintillator with natural lithium is an appropriate choice to satisfy both the neutron detection efficiency and gamma elimination requirements.

Demonstration of the importance of a dedicated neutron beam monitoring system for BNCT facility

The neutron beam monitoring system is indispensable to BNCT facility in order to achieve an accurate patient dose delivery. The neutron beam monitoring of a reactor-based BNCT (RB-BNCT) facility can be implemented through the instrumentation and control system of a reactor provided that the reactor power level remains constant during reactor operation. However, since the neutron flux in reactor core is highly correlative to complicated reactor kinetics resulting from such as fuel depletion, poison production, and control blade movement, some extent of variation may occur in the spatial distribution of neutron flux in reactor core. Therefore, a dedicated neutron beam monitoring system is needed to be installed in the vicinity of the beam path close to the beam exit of the RB-BNCT facility, where it can measure the BNCT beam intensity as closely as possible and be free from the influence of the objects present around the beam exit. In this study, in order to demonstrate the importance of a dedicated BNCT neutron beam monitoring system, the signals originating from the two in-core neutron detectors installed at THOR were extracted and compared with the three dedicated neutron beam monitors of the THOR BNCT facility. The correlation of the readings between the in-core neutron detectors and the BNCT neutron beam monitors was established to evaluate the improvable quality of the beam intensity measurement inferred by the in-core neutron detectors. In 29 sampled intervals within 16 days of measurement, the fluctuations in the mean value of the normalized ratios between readings of the three BNCT neutron beam monitors lay within 0.2%. However, the normalized ratios of readings of the two in-core neutron detectors to one of the BNCT neutron beam monitors show great fluctuations of 5.9% and 17.5%, respectively.

Neutron beam monitoring for time-of-flight facilities with gaseous detectors

Triple Gas Electron Multipliers (GEM) for slow and fast neutrons were employed at the n_TOF facility at CERN as online beam imaging monitors and for energy spectra measurements via the time-of-flight technique. The detectors were exposed to the neutron spectrum ranging from thermal to 1GeV, produced by spallation of 20GeV/c protons in a lead target with a maximum intensity of 7·1012 protons per pulse. The spectrum and the 2D count distribution of the neutron beam were measured and compared at two distances from the target, 185m and 200m. The detectors showed radiation hardness, linear response and the ability to monitor the beam profile online with high spatial resolution.

Monte Carlo simulations for high-rate fast neutron flux measurements made at the RAON neutron science facility by using MICROMEGAS

RAON is a Korean heavy-ion accelerator complex that is planned to be built by 2021. Deuterons (53 MeV) and protons (88 MeV) accelerated by using a low-energy driver linac (SCL1) are delivered to the neutron production target in the Neutron Science Facility (NSF) to produce high-energy neutrons in the interval from 1 to 88 MeV with high fluxes of the order of 1012 n/cm2-sec. The repetition rate of the neutron beam ranges from 1 kHz to 1 MHz, and the maximum beam current is ~12 μA at 1 MHz. The beam width is 1 ~ 2 ns. The high-energy and high-rate fast neutrons are used to estimate accurate neutron-induced cross sections for various nuclides at the NSF. A MICROMEGAS (MICRO Mesh Gaseous Structure), which is a gaseous detector initially developed for tracking in high-rate, high-energy physics experiments, is tentatively being considered as a neutron beam monitor. It can be used to measure both the energy distribution and the flux of the neutron beam. In this study, a MICROMEGAS detector for installation at the NSF was designed and investigated. 6Li, 10B, 235U and 238U targets are being considered as neutron/charged particle converters. For the low-energy region, 6Li(n,α)t and 10B(n,α)7Li are used in the energy range from thermal to 1 MeV. 235U(n,f) and 238U(n,f) reactions are used for high-energy region up to 90 MeV. All calculations are performed by using the GEANT4 toolkit.

스퍼터링 코팅기법을 이용한 중성자 검출용 B4C 박막 개발

gas has been used for neutron monitors as the neutron converter owing to its advantages such as high sensitivity, good -discrimination capability, and long-term stability. However, is becoming more difficult to obtain in last few years due to a global shortage of gas. Accordingly, the cost of a neutron monitor using gas as a neutron converter is becoming more expensive. Demand on a neutron monitor using an alternative neutron conversion material is widely increased. has many advantages among various alternative materials, as a neutron converter. In order to develop a neutron converter using (actually ), we calculated the optimal thickness of a neutron converter with a Monte Carlo simulation using MCNP6. In addition, a neutron converter was fabricated by the Ar sputtering method and the neutron signal detection efficiencies were measured with respect to various thicknesses of fabricated a neutron converter. Also, we developed a 2-dimensional multi-wire proportional chamber (MWPC) for neutron beam profile monitoring using the fabricated a neutron converter, and performed experiments for neutron response of the neutron monitor at the 30 MW research reactor HANARO at the Korea Atomic Energy Research Institute. The 2-dimensional MWPC with boron () neutron converter was proved to be useful for neutron beam monitoring, and can be applied to other types of neutron imaging.