Spectral Neutron Flux Research Articles

On a comparison of a neutron Monte Carlo transport simulation to a criticality benchmark experiment

In this work the Water-Reflected 91-Liter Sphere of Enriched Uranium Oxyfluoride Solution experiment from the International Handbook of Evaluated Criticality Safety Benchmark Experiments is simulated with a neutron transport Monte Carlo simulator. The simulator works with continuous variables of the phase space relevant in transport theory, which allows to extract the simulated spectral neutron flux and render it in parameterized representation. The neutron effective multiplication factor is calculated and compared to the experimental findings, as well as to results from other simulation codes. Agreement between experiment and the present simulation was found to be within one standard deviation.

197Au(n,xn) reactions at The Svedberg Laboratory high-energy neutron facility in Uppsal

Cross section measurements for the197Au(n,xn) reactions have been performed at The Svedberg Laboratory (TSL) high-energy neutron facility in Uppsala,Sweden. The 45.6 and 58.3 MeVquasi-monoenergetic neutron beams were produced by means of the 7Li(p,n) reaction and were monitored with thin-film breakdown counters (TFBCs). After the end of the irradiations, the activity induced by the neutron beams in the targets and in reference foils, has been measured by a HPGe detector. In order to determine the cross sections of the (n,xn) reactions, the spectral neutron flux distribution is needed, thus the characterization of the beam is of major importance. Therefore, simulations that take into account the whole experimental setup of the irradiation have been performed with the use of MCNP5 code and the results are presented in this work. Currently, further analysis of the data is in progress.

Testing of tritium breeder blanket activation foil spectrometer during JET operations

Accurate measurement of the nuclear environment within a test tritium breeding-blanket module of a fusion reactor is crucial to determine tritium production rates which are relevant to self-sufficiency of tritium fuel supply, tritium accountancy and also to the evaluation of localised power levels produced in blankets. This requires evaluation of the time-dependent spectral neutron flux within the test tritium breeding-blanket module under harsh radiation and temperature environments. The application of an activation foil-based spectrometer system to determine neutron flux density using a pneumatic transfer system in ITER has been studied, deployed and tested on the Joint European Torus (JET) machine in a recent deuterium - deuterium campaign for a selection of high purity activation foils. Deployment of the spectrometer system has provided important functional and practical testing of the detector measurement system, associated hardware and post processing techniques for the analysis of large data sets produced through the use of list mode data collection. The testing is invaluable for the optimisation of systems for future planned testing in tritium - tritium and deuterium - tritium conditions. Analysis of the time and energy spectra collected to date and the status of the development of methods for post-processing are presented in this paper.

Monte Carlo calculation of the neutron dose to a fetus at commercial flight altitudes

Aircrew members are exposed to primary cosmic rays as well as to secondary radiations from the interaction of cosmic rays with the atmosphere and with the aircraft. The radiation field at flight altitudes comprises neutrons, protons, electrons, positrons, photons, muons and pions. Generally, 50% of the effective dose to airplane passengers is due to neutrons. Care must be taken especially with pregnant aircrew members and frequent fliers so that the equivalent dose to the fetus will not exceed prescribed limits during pregnancy (1 mSv according to ICRP, and 5 mSv according to NCRP). Therefore, it is necessary to evaluate the equivalent dose to a fetus in the maternal womb. Up to now, the equivalent dose rate to a fetus at commercial flight altitudes was obtained using stylized pregnant-female phantom models. The aim of this study was calculating neutron fluence to dose conversion coefficients for a fetus of six months of gestation age using a new, realistic pregnant-female mesh-phantom. The equivalent dose rate to a fetus during an intercontinental flight was also calculated by folding our conversion coefficients with published spectral neutron flux data. The calculated equivalent dose rate to the fetus was 2.35 μSv.h−1, that is 1.5 times higher than equivalent dose rates reported in the literature. The neutron fluence to dose conversion coefficients for the fetus calculated in this study were 2.7, 3.1 and 3.9 times higher than those from previous studies using fetus models of 3, 6 and 9 months of gestation age, respectively. The differences between our study and data from the literature highlight the importance of using more realistic anthropomorphic phantoms to estimate doses to a fetus in pregnant aircrew members.

Neutron field of accelerator-driven p(35 MeV)+Be fast neutron source at NPI Rez

The accelerator driven fast neutron sources of the white- and quasi-monoenergetic spectra are operated at the NPI Rez Fast Neutron Facility utilizing the Be(thick), D2 O(thick), and 7 Li(C) target stations and the variable energy proton beam (up to 37 MeV) from the U-120M isochronous cyclotron. Recently, the design of beryllium target station was upgraded in order to provide the higher neutron flux at the modified positions of irradiated samples. Afterwards, the thick target neutron field of the p+Be source reaction was investigated for proton energy of 35 MeV. The spectral neutron flux for several target-to-sample distances was determined using the multi-foil activation technique. From measured reaction rates, new neutron spectra were reconstructed employing the SAND-II unfolding code and validated against the MCNPX predictions. The IFMIF-like (International Fusion Material Irradiation Facility) neutron field obtained from the p(35)+Be source is suitable for the neutron cross-sections validation within the IFMIF research program, radiation hardness tests of electronics, and neutron activation analysis experiments.

Neutron Spectrum Determination of the p(35 MeV)-Be Source Reaction by the Dosimetry Foils Method

The thick target neutron field of source reaction p + Be was investigated for a proton energy of 35 MeV. The spectral neutron flux at 0∘ for two target-to-sample distances was determined by using the dosimetry foils activation method. The present p(35)-Be white neutron spectra provide the suitable basis for irradiation experiments and integral tests of nuclear data.

Preliminary Experimental Test of Activation Foil Materials with Short Half-Lives for Neutron Spectrum Measurements in the ITER TBM

We have performed preliminary experimental tests for the development of a neutron spectrometer based on activation foils with short half-lives for the European ITER TBMs. Small samples of candidate materials have been irradiated with DT neutrons from the neutron generator of Technical University of Dresden. A dedicated pneumatic sample transport system has been set up for these initial tests and further development of methods for spectral neutron flux measurements in the TBM. The mass of the samples was on the order of 0.6 g. Although the neutron flux in the irradiation position of the samples was three to five orders of magnitude below the expected flux in the TBM at full DT operation of ITER, it was possible with short irradiation time of 60 s and similar gamma-ray measurement times to obtain the induced activity with moderate uncertainty.

Spectral neutron flux oscillations of cosmic radiation on the Earth's surface

Two Bonner Sphere Spectrometers (BSS) were used – one at the Schneefernerhaus, Germany (altitude: 2650 m; geomagnetic cut‐off: 4.1 GV), the other at the Koldewey station on Spitsbergen (sea level; geomagnetic: cut‐off 0 GV) – to measure continuously the spectral flux distribution of secondary neutrons from cosmic radiation. At the Schneefernerhaus, the flux of thermal neutrons was about 75% higher in summer than in winter, that of epithermal neutrons about 80%, that of neutrons between 0.125 and 17.8 MeV about 32%, and that of neutrons above 17.8 MeV about 4%, respectively. The period of the observed oscillations was very close to one year. Similar oscillations were observed at the Koldewey station, with somewhat smaller amplitudes (40%, 45%, 22%, and 2%, respectively). At both stations, the flux of the neutrons above 17.8 MeV increased with time similar to the count rates measured by nearby neutron monitors. While this increase reflects changes in the Sun's activity, the observed oscillations are due to changes in ground albedo neutrons and their absorption due to snow. Consequently, the monthly averaged neutron ambient dose equivalent rates, H*(10), oscillated by about ±7% at the UFS and about ±4% at the Koldewey Station. The results demonstrate that BSS measurements could be used to monitor secondary neutrons from cosmic radiation above about 20 MeV. Below detailed neutron transport calculations are necessary to correct for changes in ground albedo neutrons and snow cover. The data presented here can be used as an experimental basis to perform such simulations.

Application of thin-film breakdown counters for characterization of neutron field of the VESUVIO instrument at the ISIS spallation source

The high-energy neutron field of the VESUVIO instrument at the ISIS facility has been characterized using the technique of thin-film breakdown counters (TFBC). The technique utilizes neutron-induced fission reactions of natU and 209Bi with detection of fission fragments by TFBCs. Experimentally determined count rates of the fragments are ≈50% higher than those calculated using spectral neutron flux simulated with the MCNPX code. This work is a part of the project to develop ChipIr, a new dedicated facility for the accelerated testing of electronic components and systems for neutron-induced single event effects in the new Target Station 2 at ISIS. The TFBC technique has shown to be applicable for on-line monitoring of the neutron flux in the neutron energy range 1–800MeV at the position of the device under test (DUT).

Neutronics diagnostics for European ITER TBMs: Activation foil spectrometer for short measurement cycles

An important aim of neutronics Test Blanket Module (TBM) experiments in ITER will be to check the prediction accuracy of nuclear responses in an environment closer to a future fusion power reactor than so far provided by existing facilities. The development of measurement methods suitable for the harsh environment in an ITER TBM has been addressed in several recent R&D programs supported by Euratom. Within this framework, KIT is developing an activation foil spectrometer for the measurement of local neutron flux densities in the TBM. We intend to establish a measurement method which allows to record the induced activities in small packages of activation foils simultaneously and to calculate the corresponding spectral neutron flux densities with moderate time resolution of tens of seconds immediately after extraction from the TBM. In the present work we propose a candidate set of activation foil materials which cover the neutron energy range from thermal to 14MeV. In order to assess their basic suitability for such measurements, we have computed induced gamma-ray activities in the foils using a calculated neutron spectrum in a representative position in the European HCPB TBM assuming a short irradiation time of 30s. In a further step we have investigated pulse height spectra which would be obtained in a typical gamma-ray measurement arrangement in a HPGe detector and concluded that the proposed set of activation foils should be basically suitable for such a measurement system but require improvement of relevant cross sections uncertainties.

Characterization of ANITA and QMN Neutron Beams at TSL Using Proton Recoil Techniques

Neutron beam characterization measurements have been carried out in both the AN:ITA and QMN modes using the incident proton beam of 180 MeV at the The Svedverg Laboratory (TSL) in Uppsala. The spectral neutron flux data have been obtained by measuring elastic np-scattering with the Medley setup. The experimental results in the ANITA and QMN modes are compared with an MCNPX simulation and with the systematics of QMN spectra, respectively.

A computationally simple model for determining the time dependent spectral neutron flux in a nuclear reactor core

The balance of isotopes in a nuclear reactor core is key to understanding the overall performance of a given fuel cycle. This balance is in turn most strongly affected by the time and energy-dependent neutron flux. While many large and involved computer packages exist for determining this spectrum, a simplified approach amenable to rapid computation is missing from the literature. We present such a model, which accepts as inputs the fuel element/moderator geometry and composition, reactor geometry, fuel residence time and target burnup and we compare it to OECD/NEA benchmarks for homogeneous MOX and UOX LWR cores. Collision probability approximations to the neutron transport equation are used to decouple the spatial and energy variables. The lethargy dependent neutron flux, governed by coupled integral equations for the fuel and moderator/coolant regions is treated by multigroup thermalization methods, and the transport of neutrons through space is modeled by fuel to moderator transport and escape probabilities. Reactivity control is achieved through use of a burnable poison or adjustable control medium. The model calculates the buildup of 24 actinides, as well as fission products, along with the lethargy dependent neutron flux and the results of several simulations are compared with benchmarked standards.

Monte Carlo uncertainty analyses for integral beryllium experiments

The novel Monte Carlo technique for calculating point detector sensitivities has been applied to two representative beryllium transmission experiments with the objective to investigate the sensitivity of important responses such as the neutron multiplication and to assess the related uncertainties due to the underlying cross-section data uncertainties. As an important result, it has been revealed that the neutron multiplication power of beryllium can be predicted with good accuracy using state-of-the-art nuclear data evaluations. Severe discrepancies do exist for the spectral neutron flux distribution that would transmit into significant uncertainties of the calculated neutron spectra and of the nuclear blanket performance in blanket design calculations. With regard to this, it is suggested to re-analyse the secondary energy and angle distribution data of beryllium by means of Monte Carlo based sensitivity and uncertainty calculations. Related code development work is underway.

Determination of a Deuteron-Beryllium Neutron Source Spectrum by Multifoil Activation

The spectral neutron flux from a deuteron-beryllium neutron source, which is driven by a 19-MeV cyclotron deuteron beam and serves mainly for integral activation tests of fusion reactor structural materials, was determined by the multifoil activation method. Twenty-two selected threshold activation reactions were employed. An initial guess spectrum calculated by a Monte Carlo simulation was adjusted using the SAND-II code to be consistent with the measured reaction rates. The total neutron flux averaged over a 5 x 5 mm sample was found to be 2.52 x 1011 n/s·cm-2 at 10 μA of deuteron beam with uncertainty of ~10%. The activation cross sections used were based on the FENDL/A-2.0 library. However, they were extrapolated beyond 20 MeV, the upper energy limit of that library, to the maximum neutron energy of 23.4 MeV and were modified where necessary. As a result, a self-consistent set of activation cross sections was obtained, which may also be used for the characterization of other neutron fields. The determined spectral neutron flux is to be used for analyses of integral activation tests of fusion reactor-relevant materials.

Neutron spectrum unfolding — Part II: Experimental

In Part I of this paper, we described the use of the computer code SAIPS in neutron spectrum unfolding. Here in Part II, we present our experimental work carried out to study the shape of the neutron spectrum in different experimental channels of a 5 MW light-water cooled and moderated research reactor. The spectral neutron flux was determined using various fission foils (placed in close contact with mica track detectors) and activation detectors. From the measured activities, the neutron spectrum was unfolded by SAIPS.