Neutron Energy Distribution Research Articles

Neutron yield from carbon, light- and heavy-water thick targets irradiated by 40 MeV deuterons

Angular and energy distributions of neutrons produced by the interaction of deuterons of 40 MeV in carbon, light- and heavy-water targets, in which they are stopped, have been measured by the activation method. A discrepancy with a time-of-flight measurement for d + C has been found. The results are compared with a Monte-Carlo calculation and are discussed in the frame of building a deuteron-to-neutron converter for the SPIRAL2 radioactive ion-beam facility.

Neutron Energy Distribution Function Reconstructed From Time-of-Flight Signals in Deuterium Gas-Puff $Z$-Pinch

The implosion of a solid deuterium gas-puff Z-pinch was studied on the S-300 pulsed power generator [A. S. Chernenko, , Proceedings of 11th Int. Conf. on High Power Particle Beams, 154 (1996)]. The peak neutron yield above 1010 was achieved on the current level of 2 MA. The fusion neutrons were generated at about 150 ns after the current onset, i.e., during the stagnation and at the beginning of the expansion of a plasma column. The neutron emission lasted on average 25 ns. The neutron energy distribution function was reconstructed from 12 neutron time-of-flight signals by the Monte Carlo simulation. The side-on neutron energy spectra peaked at 2.42 plusmn 0.04 MeV with about 450-keV FWHM. In the downstream direction (i.e., the direction of the current flow from the anode toward the cathode), the peak neutron energy and the width of a neutron spectrum were 2.6 plusmn 0.1 MeV and 400 keV, respectively. The average kinetic energy of fast deuterons, which produced fusion neutrons, was about 100 keV. The generalized beam-target model probably fits best to the obtained experimental data.

Neutron production from beam-modifying devices in a modern double scattering proton therapy beam delivery system

In this work the neutron production in a passive beam delivery system was investigated. Secondary particles including neutrons are created as the proton beam interacts with beam shaping devices in the treatment head. Stray neutron exposure to the whole body may increase the risk that the patient develops a radiogenic cancer years or decades after radiotherapy. We simulated a passive proton beam delivery system with double scattering technology to determine the neutron production and energy distribution at 200 MeV proton energy. Specifically, we studied the neutron absorbed dose per therapeutic absorbed dose, the neutron absorbed dose per source particle and the neutron energy spectrum at various locations around the nozzle. We also investigated the neutron production along the nozzle's central axis. The absorbed doses and neutron spectra were simulated with the MCNPX Monte Carlo code. The simulations revealed that the range modulation wheel (RMW) is the most intense neutron source of any of the beam spreading devices within the nozzle. This finding suggests that it may be helpful to refine the design of the RMW assembly, e.g., by adding local shielding, to suppress neutron-induced damage to components in the nozzle and to reduce the shielding thickness of the treatment vault. The simulations also revealed that the neutron dose to the patient is predominated by neutrons produced in the field defining collimator assembly, located just upstream of the patient.

Accelerator-Based Target Design and Optimization Driven by High-Energy Electron Beams and Proton Beams for the Analysis of Neutron Generation Characteristics

Accelerator-based target design and optimization are presented in this paper as an approach for the analysis of neutron generation and characteristics. Electron-based targets and proton-based targets driven by high-energy accelerator beams are investigated. The target plays an important role in the external neutron sources in which the target was driven by high-energy accelerator beams to generate neutrons. The optimization of target design in this work is to obtain the maximum generation of neutrons out of targets considering target material and geometry, accelerator beam energy, and beam size. A three-dimensional particle detection methodology and a surface matrix arithmetic technique were used to determine the spatial distribution of the source particles (electron and proton) and the total neutron generation from the target outer surfaces. Neutron generation and characteristics were analyzed based on the optimized targets regarding neutron spectrum, average energy, and average flux. Monte Carlo calculations were performed by using MCNPX to estimate the particle interaction inside the target and to calculate the neutrons escaping out of the target surfaces.Results in this work indicated that a high-energy (1-GeV) electron accelerator beam is capable of producing high-intensity neutron flux at the range of 1.60 × 1013 n/cm2·s of 1-mA electron. Compared to an electron accelerator beam, a proton beam (1 GeV) generates higher-intensity neutron flux at the level of 4.83 × 1013 n/cm2·s of 1-mA proton. The neutron generation ratio (neutron per incident particle escaping from the target) was computed as 0.76 neutrons per electron and 38.8 neutrons per proton for the selected targets. In the electron accelerator-based target, neutron generation was mostly through photonuclear reactions (88%), followed by prompt fission (12%). Neutron production in the target of the proton accelerator-based target was mainly due to spallation reactions (40%) and prompt fissions (48%). The optimized size of the target for the electron accelerator-based target, in terms of the volume, was about 16 times smaller than that for the proton accelerator-based target. The estimated neutron energy distribution was much narrower, with the electron accelerator target ranging from 1.0 × 10−3 to 30 MeV. In the proton accelerator target, the neutron energies ranged between 1.0 × 10−5 MeV and 1 GeV.

Measurement and analysis of energy and angular distributions of thick target neutron yields from 110 MeVF19onAl27

Energy distributions of emitted neutrons were measured for 110 MeV $^{19}\mathrm{F}$ ions incident on a thick $^{27}\mathrm{Al}$ target. Measurements were done at ${0}^{\ifmmode^\circ\else\textdegree\fi{}}$, ${30}^{\ifmmode^\circ\else\textdegree\fi{}}$, ${60}^{\ifmmode^\circ\else\textdegree\fi{}}$, ${90}^{\ifmmode^\circ\else\textdegree\fi{}}$, and ${120}^{\ifmmode^\circ\else\textdegree\fi{}}$ with respect to the projectile direction employing the time-of-flight technique using a proton recoil scintillation detector. Comparison with calculated results from equilibrium nuclear reaction model codes like PACE-2 and EMPIRE 2.18 using various level-density options was carried out. It is observed that the dynamic level-density approach in EMPIRE 2.18 gives the closest approximation to the measured data. In the Fermi-gas level-density approach the best approximation of the level-density parameter is $a=A/12.0$, where $A$ is the mass number of the composite system. The trend in the angular distribution of emitted neutrons is well reproduced by the projection of the angular momentum on the recoil axis as done in the PACE-2 code.

Measurements of angular and energy distributions of prompt neutrons from thermal neutron-induced fission

The experimental setup and methodology used to measure prompt neutron angular and energy distributions from thermal neutron-induced fission are described. The neutrons are detected using two scintillation detectors, while the fission fragments are detected by multi-wire proportional detectors in conjunction with the TOF technique. To separate events corresponding to neutrons and γ-quanta, a double discrimination by the pulse shape and the time-of-flight is applied. Some preliminary results of an experiment performed with the 235U target are presented and briefly discussed. The yield of “scission” neutrons has been estimated in the framework of a simple evaporation model and was found not to exceed 5% of the total neutron yield. Including an assumed of anisotropy of the fission neutron angular distribution in the center-of-mass system of fission fragments into the model calculation leads to an increase in the “scission” neutron yields inferred from the data.

First results studying the transmutation of 129I, 237Np, 238Pu, and 239Pu in the irradiation of an extended natU/Pb-assembly with 2.52 GeV deuterons

An extended U/Pb-assembly was irradiated with an extracted beam of 2.52 GeV deuterons from the Nuclotron accelerator of the Laboratory of High Energies within the JINR in Dubna, Russia. The lay-out of this experiment and first results are reported. The Pb-target (diameter 8.4 cm, length 45.6 cm) is surrounded by a natU-blanket (206.4 kg) and used for transmutation studies of hermetically sealed radioactive samples of 129I, 237Np, 238Pu and 239Pu. Estimates of transmutation rates were obtained as result of measurements of gamma-activities of the samples. Information about the spatial and energy distribution of neutrons in the volume of the lead target and the uranium blanket was obtained with sets of activation threshold detectors (Al, Y and Au) and solid state nuclear track detectors (SSNTD). An electronic 3He neutron detector was tested on-line. A comparison of experimental data with theoretical model calculations using the MCNPX program was performed yielding satisfactory results.

Neutron production by a 13C thick target irradiated by 20–90 MeV protons

Neutron production using an enriched 13C carbon converter has been measured during the design study of the italian RIB facility SPES. Energy and angular distributions of neutrons emitted by bombarding a 13C target of stopping length with protons in the range of 20 to 90MeV have been measured by time-of-flight and activation and compared with the prediction of a Monte Carlo code developed at Snezhinsk. At the proton energy of 100MeV, firstly envisaged for SPES, the gain with respect to a natural C target is less than a factor of two, while yields still compare well with those for 40MeV deuterons on natural carbon adopted by SPIRAL-II. At energies near 30MeV the 13C thick target is definitely more prolific than the target of natural carbon, but both yields with protons are clearly lower than the one with deuterons. At the energy of 20MeV envisaged for a first stage of SPES it might be more efficient to irradiate the uranium target with protons rather than using the two-stage method with converter.

SU‐GG‐T‐341: Energy Distributions of Particles Generated for Proton Interactions in Water: A Simulation with GEANT4 Monte Carlo Code

Purpose: To simulate using GEANT4 (version 4.8.3), interaction of proton in water with energies of 60, 100, 150, 200, and 250 MeV for accurate determination of energy distributions of particles generated such as protons and neutrons. Method and Materials: A cylindrical water phantom (thickness = 1 cm, diameter = 2 cm, density = 1 g/cm3) was placed in front of a sensitive detector having the same dimension as the phantom. A pencil beam proton directed perpendicularly towards the water slab was used with the detector on the other side recording the energy distributions of particles generated. The simulations were carried out by 4 million incident protons for each of the proton beam energy. Electromagnetic energy loss processes for hadrons, electrons and positrons are categorized in GEANT4 as either “standard” or “low energy”. To extend down particle energies below the standard process, the low energy process was used in the simulation to cover protons with 1 keV; and electrons and photons with 250 eV. Range cut is lowered from the default value of 1mm to 15μm to improve the accuracy of simulation. The Hadronic process includes low energy elastic and inelastic scattering that consists of a pre‐compound nuclear interaction below 170 MeV, and a Bertini cascade model for energies above 150 MeV. The energy distributions of protons and neutrons are then normalized per incident proton. Results: The normalized proton and neutron energy spectrum show that the statistical uncertainties are mostly lower than 10% for most of the points in the proton energy distribution curves and that 20% for neutron curves. Conclusion: We have demonstrated that the GEANT4 toolkit has the ability for the radiation therapy proton beam simulation. This code will primarily be used as a standard code for our proton facility design and planning in our institution.

Monte Carlo studies in accelerator-driven systems for transmutation of high-level nuclear waste

A spallation neutron source was modeled using a high energy proton accelerator for transmutation of 239Pu, minor actinides 237Np, 241Am and long-lived fission products 99Tc, 129I, which are created from the operation of nuclear power reactors for the production of electricity. The acceleration driven system (ADS) is composed of a natural lead target, beam window, subcritical core, reflector, and structural material. The neutrons are produced by the spallation reaction of protons from a high intensity linear accelerator in the spallation target, and the fission reaction in the core. It is used a hexagonal lattice for the waste and fuel assemblies. The system is driven by a 1 GeV, 10 mA proton beam incident on a natural lead cylindrical target. The protons were uniformly distributed across the beam. The core is a cylindrical assembly. The main vessel is surrounded by a reflector made of graphite. The axes of the proton beam and the target are concentric with the main vessel axis. The structural walls and the beam window are made of the same material, stainless steel, HT9. We investigated the following neutronics parameters: spallation neutron and proton yields, spatial and energy distribution of the spallation neutrons, and protons, heat deposition, and the production rates of hydrogen and helium, transmutation rate of minor actinides and fission products. In the calculations, the Monte Carlo code MCNPX, which is a combination of LAHET and MCNP, was used. To transport a wide variety of particles, The Los Alamos High Energy Transport Code (LAHET) was used.

MCNP simulation of neutron energy spectra for a TN-32 dry shielded container

This paper presents a numerical analysis of neutron energy spectra for a TN-32 spent fuel dry storage cask using Monte Carlo simulation. The analysis results were compared with experimental measurements to determine the suitability of using such codes for neutron flux calculations in soft-spectrum neutron environments. Complete spent fuel compositions were generated using Scale 4.4a. Variations in source definition and geometry determined that geometric and source simplifications in the computational model have negligible effect on final neutron energy distribution. Variations between experimental and computed spectra at energies above 1 MeV and below 100 keV demonstrated the shortfalls of various detection instruments used to collect the experimental neutron energy spectra data principally because these instruments were calibrated based on high neutron energy spectra. The MCNP calculations were generally in agree with the experimental data, but predicted that the detectors would over-respond to the neutron spectra around a spent fuel dry shielded container. Computed neutron energy spectra were always conservative when compared to experimental spectra.

Dosimetry of the mixed field irradiation facility CALIBAN

The dosimetry of the experimental reactor CALIBAN was established for photon and neutron components, and the kerma variations were evaluated according to position (distance from the core, height, angle with the median room axis). Dosimetry was performed with TLD (Al 2O 3) for the photon component, and with passive silicon diodes, activation detectors (Au, In, Ni, Mg, Cu) and alanine pellets measured by EPR spectrometry for the neutron component. The neutron energy distribution was experimentally evaluated at various distances based on the activities measured at the activated foil using the SNAC2 software. It was then compared with those calculated with the Monte Carlo code MCNPX.

Dosimetry in radiation fields around high-energy proton accelerators

Radiation dosimetry at high-energy proton accelerators is a difficult task because of the complexity of the stray radiation field. A good knowledge of this mixed radiation field is very important to be able to select the type of detectors (active and/or passive) to be employed for routine area monitoring and to choose the personal dosimeter legally required for estimating the effective dose received by individuals. At the same time, the response function of the detectors to the mixed field must be thoroughly understood. A proper calibration of a device, which may involve a complex series of measurements in various reference fields, is needed. Monte Carlo simulations provide a complementary—and sometimes the principal—mean of determining the response function. The ambient dose equivalent rates during operation range from a few hundreds of μ Sv per year to a few mSv per year. To measure such rates one needs detectors of high sensitivity and/or capable of integrating over long periods. The main challenges to be met by a radiation monitoring system are the capability of discriminating the various radiation types, of correctly measuring the high-energy part of the neutron energy distribution—often responsible for a large fraction of the total ambient dose equivalent—and of operating in the pulsed field sometimes encountered around high-energy accelerators. The paper first briefly reviews the most relevant techniques and then concentrates on a few devices pointing to some recent results and developments: the tissue equivalent proportional counter, the silicon-based microdosimeter, the superheated drop detector and high-pressure argon- and hydrogen-filled ionization chambers to be employed for the radiological surveillance of the Large Hadron Collider at CERN. Examples and comparisons between measurements and Monte Carlo predictions are given.

Radiation measurement in the environment of FLASH using passive dosimeters

Sophisticated electronic devices comprising sensitive microelectronic components have been installed in the close proximity of the 720 MeV superconducting electron linear accelerator (linac) driving the FLASH (Free Electron Laser in Hamburg), presently in operation at DESY in Hamburg. Microelectronic chips are inherently vulnerable to ionizing radiation, usually generated during routine operation of high-energy particle accelerator facilities like the FLASH. Hence, in order to assess the radiation effect on microelectronic chips and to develop suitable mitigation strategy, it becomes imperative to characterize the radiation field in the FLASH environment. We have evaluated the neutron and gamma energy (spectra) and dose distributions at critical locations in the FLASH tunnel using superheated emulsion (bubble) detectors, GaAs light emitting diodes (LED), LiF-thermoluminescence dosimeters (TLD) and radiochromic (Gafchromic EBT) films. This paper highlights the application of passive dosimeters for an accurate analysis of the radiation field produced by high-energy electron linear accelerators.

SU-FF-T-344: Perturbation to Dose Distribution Caused by Utilizing An MLC Instead of a Brass Aperture in Passive Scattering Proton Therapy

Purpose: To evaluate the perturbation to dose distribution caused by utilizing a tungsten MLC instead of a brass aperture in passive scattering proton therapy. Method and Materials: A four-dimensional GEANT4 Monte Carlo simulation of a proton therapy nozzle is used to simulate the behavior of a beam delivery system. Several datasets have been generated accounting for different configurations of system components including binary pre-scatterer foils, double contoured scatterer, modulator wheel, and pre-collimator jaws. The resulting beam profiles are subject to one of two different final collimator options: a tungsten MLC or a brass aperture. The performance of the beam delivery system is examined for different beam energies, intensity modulation patterns, field size, air gap between the MLC and the patient, and the introduction of a step in the design of the MLC. Results: We quantify dosimetric perturbations when conforming to a simple target volume. Variations in several factors were studied, including lateral field flatness and symmetry, penumbra, and flatness of the spread-out Bragg peak. When compared to results obtained using a brass aperture, an MLC made of tungsten seems to produce similar results. Additionally, the respective neutron energy distributions were used to calculate the neutron equivalent dose absorbed outside of the treatment field in each case. Conclusion: The ability of an MLC to conform to target volume is a function of the specific contour shape. This is not necessarily true for a custom made aperture. However, our analysis of dosimetric differences between the two indicates that there are no significant differences for simple target volume contours. The performance of an MLC as a final collimator in passive scattering proton therapy is predicted to be comparable to the performance of a brass aperture. Furthermore, secondary neutron production is reduced by using a tungsten MLC rather than a brass aperture.

Experimental comparison of 241Am-Be Neutron fluence energy distributions

(241)Am-Be(alpha,n) neutron sources provide one of the most commonly used neutron fields for routine calibration of neutron sensitive devices. The neutron energy distribution of the IRSN standard (241)Am-Be source was measured in the energy region above 1.65 MeV using a BC501A proton-recoil liquid scintillator. The experimental data were compared to the ISO-recommended neutron energy distribution for an (241)Am-Be source. Some differences in shape were observed, with large variations mainly within the energy interval 3-6 MeV and around 8 MeV. Within the framework of a collaboration between three national metrological institutes (PTB, Germany; NPL, UK and LNE-IRSN, France), the neutron energy distributions of (241)Am-Be sources at each laboratory have been compared. The IRSN-BC501A proton-recoil scintillator was used to measure all the sources. The results show different energy distributions a priori influenced by the origin of the source, i.e. the manufacturing process. The maximum deviation observed for the integral dose equivalent, in the measured BC501A energy range, is within the 4% uncertainty recommended by ISO standard 8529-2 to allow for variations of the neutron spectrum among different (241)Am-Be sources. However, knowledge of the energy distribution of an (241)Am-Be source provides a way to reduce the uncertainty in the dose equivalent rate delivered by such a source.

Energy and direction distribution of neutrons in workplace fields: implication of the results from the EVIDOS project for the set-up of simulated workplace fields

Workplace neutron spectra from nuclear facilities obtained within the European project EVIDOS are compared with those of the simulated workplace fields CANEL and SIGMA and fields set-up with radionuclide sources at the PTB. Contributions of neutrons to ambient dose equivalent and personal dose equivalent are given in three energy intervals (for thermal, intermediate and fast neutrons) together with the corresponding direction distribution, characterised by three different types of distributions (isotropic, weakly directed and directed). The comparison shows that none of the simulated workplace fields investigated here can model all the characteristics of the fields observed at power reactors.

Nanodosimetric measurements and calculations in a neutron therapy beam

A comparison of calculated and measured values of the dose mean lineal energy (y(D)) for the former neutron therapy beam at Louvain-la-Neuve is reported. The measurements were made with wall-less tissue-equivalent proportional counters using the variance-covariance method and simulating spheres with diameters between 10 nm and 15 microm. The calculated y(D)-values were obtained from simulated energy distributions of neutrons and charged particles inside an A-150 phantom and from published y(D)-values for mono-energetic ions. The energy distributions of charged particles up to oxygen were determined with the SHIELD-HIT code using an MCNPX simulated neutron spectrum as an input. The mono-energetic ion y(D)-values in the range 3-100 nm were taken from track-structure simulations in water vapour done with PITS/KURBUC. The large influence on the dose mean lineal energy from the light ion (A > 4) absorbed dose fraction, may explain an observed difference between experiment and calculation. The latter being larger than earlier reported result. Below 50 nm, the experimental values increase while the calculated decrease.

Modification of ROSPEC to cover neutrons from thermal to 18 MeV

Rotating Spectrometer (ROSPEC) is a neutron spectrometer designed to measure neutron energy distributions, and provide accurate neutron dosimetry. It is a completely self-contained unit and measures neutron energy via recoiling protons in gas proportional counters. Each of the four original gas counters is dedicated to a particular neutron energy range dictated by sensitivity to gamma rays at the low energy end of the spectrum and by proton collisions with the counter walls at the high energy end. Introduced originally in 1992, ROSPEC has a proven operational record with a program of continued upgrades. The operating range of the original ROSPEC spans 50 keV-4.5 MeV. The range of the ROSPEC has now been extended down to include epithermal and thermal neutrons by adding two 2 in. (3)He counters. Also, an optional simple scintillation spectrometer was designed to extend the upper limit of ROSPEC up to 18 MeV.

Leading neutron energy and [formula omitted] distributions in deep inelastic scattering and photoproduction at HERA

The production of energetic neutrons in ep collisions has been studied with the ZEUS detector at HERA. The neutron energy and p T 2 distributions were measured with a forward neutron calorimeter and tracker in a 40 pb −1 sample of inclusive deep inelastic scattering (DIS) data and a 6 pb −1 sample of photoproduction data. The neutron yield in photoproduction is suppressed relative to DIS for the lower neutron energies and the neutrons have a steeper p T 2 distribution, consistent with the expectation from absorption models. The distributions are compared to HERA measurements of leading protons. The neutron energy and transverse-momentum distributions in DIS are compared to Monte Carlo simulations and to the predictions of particle exchange models. Models of pion exchange incorporating absorption and additional secondary meson exchanges give a good description of the data.