High-energy Neutron Measurements Research Articles

Benchmark Experiments for Bulk and Maze Shielding Using Activation Detectors with 24-GeV/c Protons at CERN/CHARM

Measurements of high-energy neutrons through shield penetration and maze streaming were performed at the Conseil Européen pour la Recherche Nucléaire (CERN) High-energy Accelerator Mixed-Field (CHARM) facility. The protons of 24 GeV/c were injected onto a 50-cm-thick copper target and the released neutrons were transmitted through shields and a maze in the facility. The transmitted neutrons in the shield and maze were measured using activation detectors placed behind various materials and thicknesses of the shields and at several locations in the maze. From the radionuclide production rates in the activation detectors, the attenuation profiles though the shield thickness and along the maze were obtained for the reactions of 209Bi(n,xn)210-xBi(x = 4–9), 27Al(n,α)24Na, 115In(n,n’)115mIn, and 12C(n,2n)11C. Monte Carlo simulations were performed with three codes, PHITS, FLUKA, and GEANT, which had good agreement with the measurements within a factor of 2 for the production rates.

Towards the use of SDD as an absolute detector for high-energy neutron measurements

As of today, the standard method employed in tokamaks for the absolute measurement of the neutron flux (thus of the nuclear fusion power) is based on activation foils, being the most robust and unbiased technique for the absolute determination of neutron fluence. However, this technique is not able to provide real-time data useful for the control of future fusion plants like DEMO. In this paper, we present some preliminary results about the R&D activity aimed at developing the Single-crystal Diamond Detectors (SDD) used for fast neutron measurements into an absolute neutron flux monitor. Tests have been conducted at the new NILE neutron source of the Rutherford-Appleton Laboratory, a facility with compact neutron generators with a maximum yield of 109 n/s and 1010 n/s for 2.5 MeV and 14 MeV neutrons, respectively. A series of neutron spectra and flux measurements have been taken with different SDD and associated DAQ. Comparisons with standard activation foils (and namely Fe, Zr, Al and Nb foils for 14 MeV neutrons and In for 2.5 MeV neutrons) and with other reference detectors are presented and discussed. Also discussed is the stability of the SDD over time when employed at high neutron rates in realistic neutron environment, and the effects of neutron irradiation on both the counting rate and detector resolution.

Study on the measurement of high energy neutrons using Zirconium capped CR-39 (Zr-CR-39) combined detector - Monte Carlo simulation and experimental validation

Neutron measurement, be it dose or fluence is a challenging field in high energy accelerators where the neutron energy varies from thermal to high-energy neutrons usually extending beyond 20 MeV. CR-39 SSNTD (Solid State Nuclear Track Detector) used in neutron measurements has near flat energy response in the range of 500 keV to 10 MeV. By capping the CR-39 with a suitable converter such as Zirconium (Zr), it's response can be enhanced in the higher neutron energy ranges (beyond 10 MeV). Zr has significant (n,2n) cross-section of ∼1 b in the neutron energy range of 10–20 MeV. Hence, when high energy neutrons impinge on Zr, neutrons of lower energy are produced by (n,2n) reactions, which create additional recoil proton tracks in CR-39. At higher neutron energies (>20 MeV) additional tracks can also be created due to other charged secondaries such as deuteron, triton, alphas and recoil carbon and oxygen. The enhancement in track density of CR-39 in presence of Zr as compared to that of bare CR-39 is termed as the Total Enhancement Factor (TEF). Monte Carlo simulations with FLUKA and GEANT4 codes are used to optimize the Zr converter thickness with 14 MeV and 20 MeV, and are also verified with measurements. At an optimum converter thickness of 1 mm, FLUKA and GEANT4 Monte Carlo codes were used to observe the enhancement in the response of the combined detector (Zr-CR-39) against the bare detector for different mono-energetic neutron beams in the range 14 MeV - 1 GeV. The enhancement obtained for Zr converter are also compared with Lead (Pb) converter. Experimental studies were carried out to validate the simulations by irradiating Zr-CR-39 to 14 MeV, 20 MeV and high energy neutrons spectra available at CERF facility. Measured TEF values are 1.27, 2.5 and 7.85 for 14 MeV, 20 MeV and CERF neutrons spectra respectively. Results of simulations and measurements are found to be in agreement within 20%.

An Alternative Incoming Correction for Cosmic‐Ray Neutron Sensing Observations Using Local Muon Measurement

AbstractMeasuring the variability of incoming neutrons locally would be usefull for the cosmic‐ray neutron sensing (CRNS) method. As the measurement of high energy neutrons is not so easy, alternative particles can be considered for such purpose. Among them, muons are particles created from the same cascade of primary cosmic‐ray fluxes that generate neutrons at the ground. In addition, they can be easily detected by small and relatively inexpensive detectors. For these reasons they could provide a suitable local alternative to incoming corrections based on remote neutron monitor data. The reported measurements demonstrated that muon detection system can detect incoming cosmic‐ray variations locally. Furthermore the precision of this measurement technique is considered adequate for many CRNS applications.

Simulated response of a multi-element thick gas electron multiplier-based microdosimeter to high energy neutrons

The response of a microdosimeter for neutrons above 14 MeV is investigated. The mean quality factors and dose-equivalents are determined using lineal energy distributions calculated by Monte Carlo simulations (Geant4 toolkit). From 14 MeV to 5 GeV, the mean quality factors were found to vary between 6.00 and 9.30 and the dose-equivalents were in agreement with the true ambient dose-equivalent at the depth of 10 mm inside the ICRU sphere, H*(10). An energy-independent dose-equivalent response around a median value of 0.86 within 22% uncertainty was obtained. Therefore, the microdosimeter is appropriate for dose-equivalent measurement of high-energy neutrons.

Validation of zirconium isotopes (n,g) and (n,2n) cross sections in a comprehensive LR-0 reactor operative parameters set

Zirconium is an important material used in most of reactor concepts for fuel cladding. Thus the knowledge of its cross section is important for reliable prediction of fuel operation. Also 90Zr(n,2n) reaction, is included in IRDFF files as dosimetry cross section standard. Due to its very high threshold, 12.1MeV, it is suitable for measurement of high energy neutrons. One of possible interesting applications is also evaluation of prompt fission neutron spectra in 235U and 238U what is under auspices of the International Atomic Energy Agency in CIELO project. The experimental values - obtained with the LR-0 nuclear reactor - of various zirconium cross sections were compared with calculations with the MCNP6 code using IAEA CIELO, ENDF/B-VII.0, JEFF-3.1, JEFF-3.2, JENDL-3.3, JENDL-4, ROSFOND- 2010, and CENDL-3.1 transport libraries combined with the dosimetry cross sections extracted from the IRDFF library.Generally, the best C/E agreement for 90Zr(n,2n) cross section, was found with the IAEA CIELO 235U evaluation that includes an updated prompt fission neutron spectra in the evaluated data file. The cross section of this reaction averaged over LR-0 spectra was determined being 28.9 ± 1.2 µb, corrected to spectral shift, spectral averaged cross section in 235U was determined to be 0.107 ± 0.005mb. Notable discrepancies were reported in both 94Zr(n,g) and 96Zr(n,g).

(n,xn) cross section measurements for Y-89 foils used as detectors for high energy neutron measurements in the deeply subcritical assembly “QUINTA”

Study of the deep subcritical systems (QUINTA) using relativistic beams is performed within the project “Energy and Transmutation of Radioactive Wastes” (E&T – RAW). The experiment assembly was irradiated by deuteron/proton beam (Dubna NUCLOTRON). We calculated the neutron energy spectrum inside the whole assembly by using threshold energy (n,xn) reactions in yttrium (Y-89) foils. There are almost no experimental cross section data for those reactions. New Y-89(n,xn) cross section measurements were carried out at The Svedberg laboratory (TSL) in Uppsala, Sweden in 2015. In this paper we present preliminary results of those experiments.

High-energy in-beam neutron measurements of metal-based shielding for accelerator-driven spallation neutron sources

Metal-based shielding plays an important role in the attenuation of harmful and unwanted radiation at an accelerator-driven spallation neutron source. At the European Spallation Source, currently under construction in Lund, Sweden, metal-based materials are planned to be used extensively as neutron guide substrates in addition to other shielding structures around neutron guides. The usage of metal-based materials in the vicinity of neutron guides however requires careful consideration in order to minimize potential background effects in a neutron instrument at the facility. Therefore, we have carried out a combined study involving high-energy neutron measurements and Monte Carlo simulations of metal-based shielding, both to validate the simulation methodology and also to investigate the benefits and drawbacks of different metal-based solutions. The measurements were carried out at The Svedberg Laboratory in Uppsala, Sweden, using a 174.1 MeV neutron beam and various thicknesses of aluminum-, iron-, and copper-based shielding blocks. The results were compared to geant4 simulations and revealed excellent agreement. Our combined study highlights the particular situations where one type of metal-based solution may be preferred over another.

Measurement of high-energy neutrons at ISS by SEDA-AP

A new type of solar neutron detector (NEM) was launched by the space shuttle Endeavour on 16 July 2009 and it began collecting data on 25 August 2009 at the International Space Station (ISS). In this paper we introduce preliminary results obtained by the NEM.

Shielding Experiments under JASMIN Collaboration at Fermilab (III): Measurement of High Energy Neutrons Penetrating a Thick Iron Shield from the Antiproton Production Target by Au Activation Method

In an antiproton production (Pbar) target station of the Fermi National Accelerator Laboratory (FNAL), the secondary particles produced by bombarding a target with 120-GeV protons are shielded by a thick iron shield. In order to obtain experimental data on high-energy neutron transport at more than 100-GeV-proton accelerator facilities, we indirectly measured more than 100-MeV neutrons at the outside of the iron shield at an angle of 50{\deg} in the Pbar target station. The measurement was performed by using the Au activation method coupled with a low-background {\gamma}-ray counting system. As an indicator for the neutron flux, we determined the production rates of 8 spallation nuclides (196-Au, 188-Pt, 189-Ir, 185-Os, 175-Hf, 173-Lu, 171-Lu, and 169-Yb) in the Au activation detector. The measured production rates were compared with the theoretical production rates calculated using PHITS. We proved that the Au activation method can serve as a powerful tool for indirect measurements of more than 100-MeV neutrons that play a vital role in neutron transport. These results will be important for clarifying the problems in theoretical calculations of high-energy neutron transport.

A neutron multiplicity meter for deep underground muon-induced high-energy neutron measurements

We present, for the first time, the design of an instrument capable of measuring the high energy ( > 60 MeV ) muon-induced neutron flux deep underground. The instrument is based on applying the Gd-loaded liquid-scintillator technique to measure the rate of multiple low-energy neutron events produced in a Pb target and from this measurement to infer the rate of high energy neutron events. This unique signature allows both for efficient tagging of neutron multiplicity events as well as rejection of random gamma backgrounds so effectively that typical low-background techniques are not required. We present design studies based on Monte Carlo simulations that show that an apparatus consisting of a Pb target of 200 cm by 200 cm area by 60 cm thickness covered by a 60 cm thick Gd-loaded liquid scintillator (0.5% Gd content) detector could measure, at a depth of 2000 meters of water equivalent (m.w.e), a rate of 70 ± 8 (stat) events/year with a background of less than 10 events/year. We discuss the relevance of this technique to measuring the muon-induced neutron background in searches for dark matter in the form of Weakly Interacting Massive Particles (WIMP). Based on these studies, we also discuss the benefits of using a neutron multiplicity meter as a component of active shielding in such experiments.

05/01536 Defect production efficiency in metals underneutron irradiation

Zirconium is an important material used in most of reactor concepts for fuel cladding. Thus the knowledge of its cross section is important for reliable prediction of fuel operation. Also 90Zr(n,2n) reaction, is included in IRDFF files as dosimetry cross section standard. Due to its very high threshold, 12.1 MeV, it is suitable for measurement of high energy neutrons. One of possible interesting applications is also evaluation of prompt fission neutron spectra in 235U and 238U what is under auspices of the International Atomic Energy Agency in CIELO project. The experimental values - obtained with the LR-0 nuclear reactor - of various zirconium cross sections were compared with calculations with the MCNP6 code using IAEA CIELO, ENDF/B-VII.0, JEFF-3.1, JEFF-3.2, JENDL-3.3, JENDL-4, ROSFOND- 2010, and CENDL-3.1 transport libraries combined with the dosimetry cross sections extracted from the IRDFF library.Generally, the best C/E agreement for 90Zr(n,2n) cross section, was found with the IAEA CIELO 235U evaluation that includes an updated prompt fission neutron spectra in the evaluated data file. The cross section of this reaction averaged over LR-0 spectra was determined being 28.9 ± 1.2 µb, corrected to spectral shift, spectral averaged cross section in 235U was determined to be 0.107 ± 0.005 mb. Notable discrepancies were reported in both 94Zr(n,g) and 96Zr(n,g).

Cosmic-ray neutron spectrometry by solid state detectors

Extensive data have been gathered since the early 1990s on the response of different detectors based on the registration of neutron-induced fission in bismuth, gold, tantalum by the spark replica counter and the thin film breakdown counter. These detectors make it possible to exploit the excellent characteristics of the fission reactions in bismuth, gold and tantalum for the measurements of high-energy neutrons. Most of the investigations have been carried out at the quasi-monoenergetic neutron beam facility at The Svedberg Laboratory-TSL of the Uppsala University in cooperation with the Khlopin Radium Institute (KRI). The responses of different fission detectors in the intermediate range of neutron energy (35– 180 MeV ) have been evaluated: a region where the predictive power of available nuclear reaction models and codes is not reliable yet. For neutron energy greater than 200 MeV , the fission-detector responses have been derived from the data of the proton fission cross-sections. Finally, by using the ratio of the responses of these detectors, a simple and accurate way to evaluate the spectrum hardness can be obtained, thus providing a tool to obtain spectral information needed for neutron dosimetry without the need to know the entire spectrum. The experimentally measured spectra obtained to-date have different shapes and they are also different from those calculated. In this paper, a new approach will be reported to analyse the existing spectra by using response ratios of different detectors. Preliminary data have been already obtained for the high-energy neutron spectrum from the CERN concrete facility.

A study of fast neutron beam geometry and energy distribution using triple- α reactions

Spectrometric measurement of high-energy neutrons using the methods of recoil protons and triple- α reactions have been used to determine energy of neutrons as well as the differential distribution along a coaxial high-energy neutron tube generator. A comparative study between two reaction mechanisms is introduced. The study shows that these methods can be used in measuring the absorbed dose in high-energy neutron radiotherapy fields. The merits and limitations of both techniques are indicated.

Measurements of high energy neutrons penetrated through iron shields using the Self-TOF detector and an NE213 organic liquid scintillator

Neutron energy spectra penetrated through iron shields were measured using the Self-TOF detector and an NE213 organic liquid scintillator which have been newly developed by our group at the Heavy-Ion Medical Accelerator in Chiba (HIMAC) of National Institute of Radiological Sciences (NIRS), Japan. Neutrons were generated by bombarding 400 MeV/nucleon C ion on a thick (stopping-length) copper target. The neutron spectra in the energy range from 20 to 800 MeV were obtained through the FORIST unfolding code with their response functions and compared with the MCNPX calculations combined with the LA150 cross section library. The neutron fluence measured by the NE213 detector was simulated by the track length estimator in the MCNPX, and evaluated the contribution of the room-scattered neutrons. The calculations are in fairly good agreement with the measurements. Neutron fluence attenuation lengths were obtained from the experimental results and the calculation.

Measurements of High-Energy Neutrons Penetrated Through Concrete Shields Using Self-TOF, NE213, and Activation Detectors

Neutron energy spectra penetrated through concrete shields were measured using three types of high-energy neutron detectors: the Self-TOF detector, an NE213 organic liquid scintillator, and Bi and C activation detectors, which have been newly developed by a group at the Heavy-Ion Medical Accelerator in Chiba (HIMAC) facility of the National Institute of Radiological Sciences, Japan. Neutrons were generated by bombarding 400 MeV/nucleon C ions on a thick (stopping-length) copper target. The neutron spectra were obtained through an unfolding code with their response functions and compared with LAHET and MCNPX calculations combined with the LA150 cross-section library. The calculations tend to overestimate with increasing the shielding thickness compared to the experimental results. The neutron fluence measured by the NE213 detector was simulated by the track length estimator in the MCNPX code, and the contribution of the room-scattered neutrons was evaluated. The neutron fluence attenuation length was obtained from the experiment for each detector and the calculation in the energy range of 20 to 800 MeV.

Measurement of high energy neutrons by fission reactions

In most high energy radiation fields, such as those encountered around accelerators or cosmic rays in the atmosphere, neutrons produce the largest percentage of the hadron dose. In these radiation fields, the neutron spectrum is typically formed by low energy neutrons (evaporation spectrum) and high energy neutrons (knock-on spectrum). Neutron spectrometry and dosimetry are better understood for low energy neutrons (i.e. neutrons with energy below 20 MeV ) than for those of higher energies. This paper reports the study of different detectors based on the registration of fission fragments of different heavy elements (namely bismuth, gold and tantalum), which have their principal response to high energy neutrons.

Feasibility studies of the self-TOF detector for high-energy neutron measurements in shielding experiments

The feasibility of a new type of neutron detector was studied. The detector consists of a radiator, a start counter and a stop counter. The radiator is composed of 20 thin plastic-scintillation detectors and the stop counter is segmented into nine plastic-scintillation detectors. Neutrons impinging on the radiator emit charged particles. The time of flight of protons emitted at a forward angle is measured using the start and stop counters, therefore, we call the detector the “self-TOF detector”. The proton time-of-flight spectrum is converted to the energy spectrum of neutrons using a measured response function. The basic properties of the detector, such as detection efficiency, signal-to-noise ratio, particle identification capability, and energy resolution, were studied.

Polycarbonate Track Detectors with a Flat Energy Response for the Measurement of High Energy Neutrons at Accelerators and Airflight Altitudes

For the polycarbonate detector routinely used at FZK, the two-step electrochemical etching was finally optimised after changing the detector thickness from 300 μm to 500 μm and applying a field strength of 42 kV.cm -1 . On the basis of calibrations with monoenergetic neutrons in the neutron energy range from 2 to about 70 MeV the response of Makrofol is now 130 tracks.cm -2 .mSv -1 within ± 30%. The linearity of detector response has been checked. The increase of the track density due to the neutron component of the natural background radiation at ground level has been measured during 1.5 y. Additional calibrations were performed in broad neutron beams at the PSI and CERN. The detector is suitable for monitoring the long-term occupational exposure at high energy particle accelerators and the exposure of the aircrews with a lower limit of detection 0.2 mSv per year. Polycarbonate is the only neutron detector which is insensitive to the high energy proton component of cosmic radiation.

Observation of Electrons from the Decay of Solar Flare Neutrons

We have found evidence for fluxes of energetic electrons in interplanetary space on board the ISEE 3 spacecraft, which we interpret as the decay products of neutrons generated in a solar flare on 1980 June 21. The decay electrons arrived at the spacecraft shortly before the electrons from the flare and can be distinguished from the latter by their distinctive energy spectrum. The time profile of the decay electrons is in good agreement with the results from a simulation based on a scattering mean free path derived from a fit to the flare electron data. The comparison with simultaneously observed decay protons and a published direct measurement of high-energy neutrons places important constraints on the parent neutron spectrum.