Quasi-monoenergetic Neutron Field Research Articles

Charged particle induced reactions on beryllium as a fast neutron source for irradiation experiments

Nuclear Physics Institute of the Czech Academy of Sciences (NPI CAS) in Rez operates the compact accelerator-driven NG-2 fast neutron source. The NG-2 neutron source uses proton (up to 35 MeV) and deuteron (up to 20 MeV) beams extracted from the isochronous cyclotron U-120M and beryllium target station for broad neutron spectrum production and lithium target station for quasi-monoenergetic neutron field production (the p+Li(C) reaction on thin target). These sources are primarily focused on nuclear data measurement and validation, especially for the future fusion energetics (DEMO and IFMIF-DONES research programs). To extend their experimental utilization mostly towards some more traditional research reactor applications (such as the neutron activation analysis and material research), the neutron fields generated by the p+Be and d+Be source reactions for various energies of charged particle beams provided by the U-120M cyclotron have been recently studied using the multi-foil activation technique. New neutron fields based on the p(35 MeV)+Be, p(30 MeV)+Be, p(24 MeV)+Be, and p(20 MeV)+Be source reactions as well as the d(20 MeV)+Be, d(15 MeV)+Be, and d(10 MeV)+Be reactions are available at NPI CAS now.

Study of thermally and optically stimulated luminescence in LiF:Mg,Ti following neutron and beta irradiation

The results reported herein were motivated by the potential application of combined optically stimulated (OSL) and thermoluminescence (TL) measurements for separation between the beta and neutron dose components in mixed neutron-beta radiation fields. The advantages of OSL/TL are two-fold: (i) The OSL and TL readout can be carried out on the same sample and (ii) the greater efficiency of OSL to high ionization density (HID) radiation due to the excitation of F2 and F3 + centers. The beta calibration coefficients for LiF:Mg,Ti (TLD-700) were measured using a 90Sr/90Y source calibrated at the Soreq Nuclear Research Center nuclear facility. The estimation of the neutron calibration coefficients was carried out by irradiation with broad beam fast neutrons of median energy 5 MeV and a quasi-monoenergetic neutron field of mean energy 14.8 MeV at the Physikalisch Technische Bundenanstalt. The preferential excitation of OSL following HID irradiation has been demonstrated.

The methodology for validation of cross sections in quasi monoenergetic neutron field

Dosimetry cross sections are fundamental quantities for proper determination of the neutron fluences in points of interest under heavy radiation load. One of the critical applications is the Reactor Pressure Vessel aging management, related to the correct estimation of its residual lifetime or for many non-reactor applications including neutron dosimetry of accelerator-based fields or space applications. The neutron flux of neutrons above 12 MeV in reference to fission spectra is below 1%. Therefore, new reference neutron fields with average energy above 5 MeV should be developed for the validation of neutron dosimetry cross sections up to 60 MeV. This paper presents the testing of a new methodology for the use of quasi monoenergetic neutron fields, where different sensitivity allows validations of the dosimetry cross section at energies much higher than the average energy of around 2 MeV typical of fission spectra. The exact shape of the neutron spectrum in the tested fields is measured by stilbene spectrometry. The total flux is determined from Ni and Al flux monitors. The developed methodology was applied to the validation of selected reactions from the IRDFF-II library showing satisfactory agreement.

Cross section determination for TAD materials in quasi mono-energetic neutron spectrum from p(Li) reaction

Threshold activation detectors (TAD) are of great importance for a determination of neutron energy spectra and flux density. For different sources, it is necessary to choose the right combination of materials that cover the estimated spectra. Several different materials were irradiated in a quasi-monoenergetic neutron field with 29.1 MeV peak neutrons energy in the CANAM facility. Neutrons were produced in p + Li-7 reaction in a thin target and the foils were situated in the proton beam axis and close geometry to the Li target. The integral number of protons was established from accelerator telemetry and lithium target activation measurements after the experiment. During the experiment, one long irradiation was done for following foils: Al, Au, Bi, Co, Cu, Fe, In, Mn, Pb, Ta, V, Y and four short irradiations for foils: Cu, Fe, In, Ta, V, W, Y. The foils were irradiated in a sandwich configuration, sorted by cross-section where the materials with higher cross-section were placed in the back of the sandwich. Neutrons produced in the p + Li-7 reaction have a quasi mono-energetic spectrum which provides a suitable basis for cross-section determination. Experimental results were calculated for (n,xn), (n,p) and (n,α) reactions via the dosimetry foils activation method including a gamma-ray spectroscopy method. Several important spectroscopic corrections have to be applied to increase the accuracy of the obtained results, including neutron background suppression. Experimental data will be submitted to the EXFOR database.

Experimental analysis of neutron and background gamma-ray energy spectra of 80-400 MeV 7Li(p,n) reactions under the quasi-monoenergetic neutron field at RCNP, Osaka University

To develop the 100-400 MeV quasi-monoenergetic neutron field, we measured neutron and unexpected gamma-ray energy spectra of the 7 Li(p,n) reaction with 80-389 MeV protons in the 100-m time-of-flight (TOF) tunnel at the Research Center for Nuclear Physics (RCNP) cyclotron facility. Neutron energy spectra with energies above 3 MeV were measured by the TOF method, which had been reported in our previous papers, and photon energy spectra with energies above 0.1 MeV were measured by the automatic unfolding function of the radiation dose monitor DARWIN. For neutron spectra, the contribution of peak intensity to the total intensity integrated with energies above 3 MeV varied between 0.38 and 0.48 in the proton energy range of 80–389 MeV. For gamma-ray spectra, highenergetic gamma-rays at around 70 MeV originated from the decay of π0 were observed with proton energies higher than 200 MeV. For the 246-MeV proton incident reaction, the contribution of gamma-ray dose to neutron dose is negligible because the ratio of gamma-ray dose to neutron dose is 0.014.

Dose Measurements through the Concrete and Iron Shields under the 100 to 400 MeV Quasi-Monoenergetic Neutron Field (at RCNP, Osaka Univ.)

Shielding benchmark experiments are useful to verify the accuracy of calculation methods for the radiation shielding designs of high-energy accelerator facilities. In the present work, the benchmark experiments were carried out for 244- and 387-MeV quasi-monoenergetic neutron field at RCNP of Osaka University. Neutron dose rates through the test shields, 100-300 cm thick concrete and 40-100 cm thick iron, were measured by four kinds of neutron dose equivalent monitors, three kinds of wide-energy range monitors applied to high-energy neutron fields above 20 MeV and a conventional type rem monitor for neutrons up to 20 MeV, placed behind the test shields. Measured dose rates were compared one another. Measured results with the wide-energy range monitors were in agreement one another for both the concrete and the iron shields. For the conventional type rem monitor, measured results are smaller than those with the wide-energy range monitors for the concrete shields, while that are in agreements for the iron shields. The attenuation lengths were obtained from the measurements. The lengths from all the monitors are in agreement on the whole, though some differences are shown. These results are almost same as those from others measured at several hundred MeV neutron fields.

Time-of-Flight Measurements for Low-Energy Components of 45-MeV Quasi-Monoenergetic High-Energy Neutron Field from <formula formulatype="inline"><tex Notation="TeX">${^7{\rm Li}({\rm p}, {\rm n})}$</tex></formula> Reaction

A quasi-monoenergetic neutron field generated in the ${^7{\rm Li}({\rm p}, {\rm n})}$ reaction consists of a high-energy monoenergetic peak and a continuum to the low-energy region. In this study, the spectral fluence of the continuum was measured with the time-of-flight (TOF) method using a ${^6{\rm Li}}$ -glass scintillation detector and an organic liquid scintillation detector for the keV and MeV region, respectively. The neutron spectral fluence was determined down to the keV region by implementing a new beam chopping system and the results showed that the neutrons that came directly from the target had a lower energy limit about 100 keV. Discussions were made also on the effect of the time-independent neutrons which are assumed to be room-scattered neutrons. The obtained information is expected to contribute to understanding the quasi-monoenergetic high-energy neutron field and improvements of calibrating neutron detectors in the field.

Response Measurement of a Bonner Sphere Spectrometer for High-Energy Neutrons

Response measurements of a Bonner sphere spectrometer (BSS) were performed for high-energy neutrons up to 400 MeV in quasi-monoenergetic neutron fields at the Research Center for Nuclear Physics (RCNP), Osaka University. The quasi-monoenergetic neutron field was generated by a <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sup> Li(p,n) reaction with the resulting spectra consisting of a monoenergetic high-energy peak followed by an unwanted continuum tail down to the low-energy region. Two-angle differential measurements for a variety of Bonner spheres were performed to obtain monoenergetic responses by subtracting the tail contributions. Discussions were made on data analysis for tail subtraction. While each Bonner sphere had a different energy response, good agreements were seen in the responses between the MCNPX calculations and the measurements with a suitable subtraction. Response calculations were also made for the whole spectra obtained by unfolding the measured BSS data, with results that reproduce well the measured responses.

Response of a PIN Diode and SOI Microdosimeter to the TSL Quasi-Monoenergetic Neutron Field

The response of a Silicon On Insulator (SOI) Microdosimeter and a silicon PIN diode were tested at the TSL quasimonoenergetic neutron beamline at Uppsala university. The objective was to determine the response of both the SOI Microdosimeter and silicon PIN diode to a high energy quasi-monoenergetic neutron field for potential future application of the devices in aviation dosimetry. PIN diode angular response dependence was also measured to determine the suitability of PIN diodes in a multidirectional high energy neutron field as encountered in aviation environments. Simulations of the response from the SOI Microdosimeter and PIN diode to the field using GEANT4 were obtained to assist in interpreting the experimental measurements.

Measurements of the neutron activation cross sections for Bi at 287 and 370 MeV

Abstract We present cross section measurements for neutron-induced activation of Bi, at 287 and 370 MeV. These values were derived from the activation method using a quasi-monoenergetic neutron field based on the 7Li(p,n) reaction. In separate experiments, samples were irradiated with neutrons derived from 7Li(p,n) reaction at either 0º or 30º for proton beam axis. This approach allows the subtraction of the low energy neutron components. The measured cross sections are compared with the findings of other studies, and evaluated in relation to nuclear data files.

Neutron Activation Experiments on Niobium in NPI p-7Li Quasi-monoenergetic Neutron Field

Neutron Activation Experiments on Niobium in NPI p-7Li Quasi-monoenergetic Neutron Field

The cross-section data from neutron activation experiments on niobium in the NPI p-7Li quasi-monoenergetic neutron field

The reaction of protons on 7 Li target produces the high-energy quasi- monoenergetic neutron spectrum with the tail to lower energies. Proton energies of 19.8, 25.1, 27.6, 30.1, 32.6, 35.0 and 37.4 MeV were used to obtain quasi-monoenergetic neutrons with energies of 18, 21.6, 24.8, 27.6, 30.3, 32.9 and 35.6 MeV, respectively. Nb cross-section data for neutron energies higher than 22.5 MeV do not exist in the literature. Nb is the important material for fusion applications (IFMIF) as well. The variable-energy proton beam of NPI cyclotron is utilized for the production of neutron field using thin lithium target. The carbon backing serves as the beam stopper. The system permits to produce neutron flux density about 109 n/cm2 /s in peak at 30 MeV neutron energy. The niobium foils of 15 mm in diameter and approx. 0.75 g weight were activated. The nuclear spectroscopy methods with HPGe detector technique were used to obtain the activities of produced isotopes. The large set of neutron energies used in the experiment allows us to make the complex study of the cross-section values. The reactions (n,2n), (n,3n), (n,4n), (n,He3), (n,α) and (n,2nα) are studied. The cross-sections data of the (n,4n) and (n,2nα) are obtained for the first time. The cross-sections of (n,2n) and (n,α) reactions for higher neutron energies are strongly influenced by low energy tail of neutron spectra. This effect is discussed. The results are compared with the EAF-2007 library.

Development of a quasi-monoenergetic neutron field using the 7Li(p,n)7Be reaction in the energy range from 250 to 390 MeV at RCNP

A quasi-monoenergetic neutron field using the (7)Li(p,n)(7)Be reaction has been developed at the ring cyclotron facility at the Research Center for Nuclear Physics (RCNP), Osaka University. Neutrons were generated from a 10-mm-thick Li target injected by 250, 350 and 392 MeV protons and neutrons produced at 0 degrees were extracted into the time-of-flight (TOF) room of 100-m length through the concrete collimator of 10 x 12 cm aperture and 150 cm thickness. The neutron energy spectra were measured by a 12.7-cm diam x 12.7-cm long NE213 organic liquid scintillator using the TOF method. The peak neutron fluence was 1.94 x 10(10), 1.07 x 10(10) and 1.50 x 10(10) n sr(-1) per muC of 250, 350 and 392 MeV protons, respectively. The neutron spectra generated from various thick (stopping length) targets of carbon, aluminium, iron and lead, bombarded by 250 and 350 MeV protons, were also measured with the TOF method. Although these measurements were performed to obtain thick target neutron yields, they are also used as a continuous energy neutron field. These neutron fields are very useful for characterising neutron detectors, measuring neutron cross sections, testing irradiation effects for various materials and performing neutron shielding experiments.

JAERI-Universities Joint Research Project on Radiation Safety in Proton Accelerator Facilities

JAERI-Universities Joint Research Project has been carried out since 2000 in order to study the radiation safety in high-energy proton accelerator facilities. Eight research groups joined in the project to tackle three major subjects, using a quasi-monoenergetic neutron field at TIARA facility of JAERI. Cross section data of elastic scattering and light charged particle production were measured for several tens MeV neutrons incident to the elements composing the human tissue. Absorbed dose distributions in a PMMA phantom were measured to verify the results calculated by some radiation transport codes. A rem counter-type monitor with a lead breeder layer within its moderator was tested in 40-70 MeV neutron fields. A new method of dose measurement for neutrons with energies from a few MeV to 100 MeV has been developed applying the spectrum weighted function to a liquid organic scintillator. The energy response of neutron track detector and superheated drop detector was examined to develop a neutron dosimeter that is applicable to the energy region more than 20 MeV. In the study on internal dosimetry for airborne radionuclides, particle size distribution of radioactive aerosols and chemical forms of radioactive gases in the air were measured for irradiation of several tens MeV neutrons.

Deuterized Radiator for Deiection of High-Energy Neutrons with Plastic Nuclear Track Detector

In order to develop a passive-type personal dosimeter based on plastic nuclear track detector (PNTD) for high-energy neutrons, a selection of radiator material and its thickness is one of key-points as well as utilization of high-sensitive type of PNTD. The radiator effect has been preliminarily estimated by numerical calculations using the angular differential cross section and characteristic data for charged particle detection with PNTD. A hydrogen compound, commonly used as a radiator, was expected to be not so effective for energetic neutrons higher than 10 MeV, so that a deuterized material was checked as one of the alternatives. The performance of these materials has been investigated through experiments quasi-monoenergetic neutron field, and the measured results were compared with those obtained numerical1y. It was also recognized that the dependence of relative sensitivity on the radiator thickness was considerable affected by a contamination of lower energy component in the neutron field.

Radiator effect on plastic nuclear track detectors for high-energy neutrons

The effect of radiators supplying charged particles to plastic nuclear track detectors has been investigated both experimentally and theoretically in order to apply them to personal dosimeters for high-energy neutrons. Performance of four types of radiator materials, CH 2, CD 2 (deuterized hydrocarbon), LiF (lithium fluoride) and C (graphite), was checked in a quasi-monoenergetic neutron field generated by p-Li reactions. The efficiency has been numerically calculated based on a model with a special attention to the angular dependence of cross sections and data of characteristic response to light ions. The effect of respective radiator candidates has been evaluated as a function of the neutron energy. A two-layer radiator has also been proposed to adjust the energy dependence of the relative sensitivity to conversion factor for dose-equivalent.

High Energy Neutron Activation Cross Sections

A systematic study on neutron activation cross-section measurements in the energy from 20 MeV to 150 MeV has been performed over several years. For this study, we have developed the quasi-monoenergetic neutron fields using 7Li(p, n) reaction at three AVF cyclotron facilities; 1) Institute for Nuclear Study, University of Tokyo for 20 to 40 MeV protons, 2) Takasald Institute of Advanced Radiation Applications(TIARA) of Japan Atomic Energy Research Institute(JAERI) for 40 to 90 MeV protons, 3) Institute of Physical and Chemical Research(RIKEN) for 70 to 210 MeV protons. By using these neutron fields, we have measured the activation cross sections of 30Si(n, np), 47Ti(n, np), 48Ti(n, np), 52Cr(n, 2n), 58Ni(n, 2n) in the energy range of 15 to 40 MeV, and of 12C(n, 2n), 59Co(n, xn) and (n, xnp), natCu(n, xn) and (n, xnp), 209Bi(n, xn) in the energy range of 15 to 80 or 150 MeV. We are now developing a new p-Li quasi-monoenergetic neutron field in the energy range of 20 to 90 MeV using a new AVF cyclotron installed in the Cyclotron and Radioisotope Center, Tohoku University, for measurements of neutron activation cross sections of light elements such as O and N which are included in air, water and soil.

Development of a quasi-monoenergetic neutron field and measurements of the response function of an organic liquid scintillator for the neutron energy range from 66 to 206 MeV

A quasi-monoenergetic neutron field was developed using a thin 7Li target bombarded by protons in the energy range from 70 to 210 MeV at the RIKEN ring cyclotron facility. The neutron energy spectra were measured with an NE213 organic liquid scintillator using the TOF method. The absolute peak neutron yields were obtained by measurements of 478 keV γ-rays from 7Be nuclei produced in a 7Li target through the 7 Li( p,n) 7 Be (g.s.+0.429 MeV) reaction. Using the neutron field, the absolute values of the neutron response functions of a 12.7 cm diameter by 12.7 cm long NE213 organic liquid scintillator were measured, and were compared with calculations using a Monte Carlo code developed by Cecil et al. The measured response functions without any wall-effect events were also obtained, and compared with calcualtions using a modified Monte Carlo code. Comparisons between a measurement and a calculation both with and without any wall-effect events gave a good agreement.

Absolute measurements of the response function of an NE213 organic liquid scintillator for the neutron energy range up to 206 MeV

The absolute values of the neutron response functions of a 12.7 cm diameter by 12.7 cm long NE213 organic liquid scintillator were measured using a quasi-monoenergetic neutron field in the energy range of 66– 206 MeV via the 7Li(p,n) 7Be reaction in the ring cyclotron facility at RIKEN. The measured response functions were compared with calculations using a Monte Carlo code developed by Cecil et al. The measurements clarified that protons escaping through the scintillator wall induced by high-energy neutrons increase from 6% for 66 MeV neutrons to 35% for 206 MeV neutrons, and that this wall effect causes a difficult problem for n–γ discrimination. Measured response functions without the wall-effect events were also obtained by eliminating the escaping-proton events in the analysis, and compared with calculations using a modified Monte Carlo code. Comparisons between the measurements and calculations both with and without any wall-effect events gave a good agreement, but some discrepancy in the light output distribution could be found, mainly because the deuteron generation process was not taken into account in the calculation. The calculated efficiencies for 10 MeVee threshold, however, also gave good agreement within about 10% with the measurements.

Development of a quasi-monoenergetic neutron field using the [formula omitted](p, n) 7Be reaction in the 70–210 MeV energy range at RIKEN

A quasi-monoenergetic neutron field was developed using the 7 Li (p, n) 7Be reaction in the energy range from 70 to 210 MeV in the ring cyclotron facility at RIKEN. Neutrons were generated from a 10-mm-thick 7 Li target injected by protons accelerated to 70, 80, 90, 100, 110, 120, 135, 150, 210 MeV. The neutron energy spectra were measured with an NE213 organic liquid scintillator using the TOF method. The absolute peak neutron yields were obtained by measurements of 478 keV γ-rays from 7 Be nuclei produced in a 7 Li target through the 7 Li (p, n) 7Be (g.s.+0.429 MeV) reaction. Two relative neutron fluence monitors, which were calibrated to the absolute peak neutron fluences by the 7 Be measurement, were equipped along the neutron beam line during an irradiation experiment. This high-energy neutron field is very useful for neutron cross-section measurements, response measurements of neutron detectors, and shielding experiments.