Cosmic-ray Neutron Spectrum Research Articles

URANOS v1.0 – the Ultra Rapid Adaptable Neutron-Only Simulation for Environmental Research

Abstract. The understanding of neutron transport by Monte Carlo simulations led to major advancements towards precise interpretation of measurements. URANOS (Ultra Rapid Neutron-Only Simulation) is a free software package which has been developed in the last few years in cooperation with particle physics and environmental sciences, specifically for the purposes of cosmic-ray neutron sensing (CRNS). Its versatile user interface and input/output scheme tailored for CRNS applications offers hydrologists straightforward access to model individual scenarios and to directly perform advanced neutron transport calculations. The geometry can be modeled layer-wise, whereas in each layer a voxel geometry is extruded using a two-dimensional map from pixel images representing predefined materials and allowing for the construction of objects on the basis of pixel graphics without a three-dimensional editor. It furthermore features predefined cosmic-ray neutron spectra and detector configurations and also allows for a replication of important site characteristics of study areas – from a small pond to the catchment scale. The simulation thereby gives precise answers to questions like from which location do neutrons originate? How do they propagate to the sensor? What is the neutron's response to certain environmental changes? In recent years, URANOS has been successfully employed by a number of studies, for example, to calculate the cosmic-ray neutron footprint, signals in complex geometries like mobile applications on roads, urban environments and snow patterns.

Cosmogenic activation of sodium iodide

The production of radioactive isotopes by interactions of cosmic-ray particles with sodium iodide (NaI) crystals can produce radioactive backgrounds in detectors used to search for rare events. Through controlled irradiation of NaI crystals with a neutron beam that matches the cosmic-ray neutron spectrum, followed by direct counting and fitting the resulting spectrum across a broad range of energies, we determined the integrated production rate of several long-lived radioisotopes. The measurements were then extrapolated to determine the sea-level cosmogenic neutron activation rate, including the first experimental determination of the tritium production rate: $(80\ifmmode\pm\else\textpm\fi{}21)\text{ }\text{ }\mathrm{atoms}/\mathrm{kg}/\mathrm{day}$. These results will help constrain background estimates and determine the maximum time that NaI-based detectors can remain unshielded above ground before cosmogenic backgrounds impact the sensitivity of next-generation experiments.

Cosmic-ray neutron fluxes and spectra at different altitudes based on Monte Carlo simulations

Cosmic-ray neutrons (CRNs) account for about half of the radiation dose received by airline crews and passengers at aviation altitudes. CRNs also comprise important background radiation near the ground, which should be considered in neutron counts for the purpose of nuclear safeguarding and homeland security. In this study, simulations were conducted with the Geant4 toolkit to describe the air shower of cosmic rays. The latest experimental driving models for the atmosphere, geomagnetic field, and primary galactic cosmic rays were applied in these simulations. The simulation was validated by comparing the results with those measured on the NASA ER-2 aircraft. the CRN fluxes and spectra were calculated at altitudes ranging from one to tens of kilometers. We determined the characteristics of the CRN spectra and analyzed their dependency on the altitude. To consider their impact on the local environment, the CRNs near the ground were modeled with a specific equivalent approach, which allowed the simulations to be conducted with limited computer processing power. The parameters for the incident primary CRNs near the ground were calculated by simulating the cosmic ray air shower. The modeling dimensions were considered for the air and ground, and an appropriate approximation solution was obtained. The model near the ground was used to investigate the dependences of the CRN flux and spectrum on the soil moisture.

Cosmic ray neutron spectrometry and dosimetry at High Altitude Research Laboratory, Gulmarg, Kashmir, India

Cosmic ray neutrons are produced by the interaction of primary cosmic rays with the air molecules present in the atmosphere. At the ground level, these secondary neutrons are one of the important contributors of natural cosmic ray radiation background dose. With two major peaks around 1 MeV and 100 MeV, the neutron energies can extend up to 10 GeV. At the ground level, considerable presence of thermal neutrons can also be observed due to collisions and moderation of fast neutrons with the soil and other surrounding materials. A study on the evaluation of cosmic ray neutron spectra, and the resulting ambient dose equivalents was carried out at High Altitude Research Laboratory, Gulmarg, India (altitude 2650 m, N34.0484°, E74.3805°) considering the enhanced contribution of cosmic ray background in the region. Extended energy Bonner Sphere Spectrometer (BSS) was used for the continuous measurement of cosmic ray neutrons during the period of October 2018 to June 2019. Neutron spectra and ambient dose equivalents were unfolded from the BSS counts with the Monte Carlo generated response matrix and the FRUIT unfolding code. Along with Bonner sphere spectrometer, Tissue Equivalent Proportional Counter (TEPC) was also used for a limited period for independent measurement of cosmic ray dose equivalents and Linear Energy Transfer (LET) distribution. For comparison, neutron spectra and dose equivalent rates were also calculated using Monte Carlo code FLUKA and EXPACS software program. The ambient dose equivalent rates were found to vary in the range of 24–34 nSv. h−1 during the entire observation period of 9 months. No considerable seasonal variations were observed during the course of the study. Dose equivalent rates, estimated by the TEPC were slightly higher than the neutron dose equivalent rates measured by BSS. Experimentally measured values match reasonably well with theoretically simulated results.

Cosmogenic activation of silicon

The production of $^{3}$H, $^{7}$Be, and $^{22}$Na by interactions of cosmic-ray particles with silicon can produce radioactive backgrounds in detectors used to search for rare events. Through controlled irradiation of silicon CCDs and wafers with a neutron beam that mimics the cosmic-ray neutron spectrum, followed by direct counting, we determined that the production rate from cosmic-ray neutrons at sea level is ($112 \pm 24$) atoms/(kg day) for $^{3}$H, ($8.1 \pm 1.9 $) atoms/(kg day) for $^{7}$Be, and ($43.0 \pm 7.1 $) atoms/(kg day) for $^{22}$Na. Complementing these results with the current best estimates of activation cross sections for cosmic-ray particles other than neutrons, we obtain a total sea-level cosmic-ray production rate of ($124 \pm 24$) atoms/(kg day) for $^{3}$H, ($9.4 \pm 2.0 $) atoms/(kg day) for $^{7}$Be, and ($49.6 \pm 7.3 $) atoms/(kg day) for $^{22}$Na. These measurements will help constrain background estimates and determine the maximum time that silicon-based detectors can remain unshielded during detector fabrication before cosmogenic backgrounds impact the sensitivity of next-generation rare-event searches.

Cosmogenic production of Ar39 and Ar37 in argon

We have experimentally determined the production rate of $^{39}$Ar and $^{37}$Ar from cosmic ray neutron interactions in argon at sea level. Understanding these production rates is important for argon-based dark matter experiments that plan to utilize argon extracted from deep underground because it is imperative to know what the ingrowth of $^{39}$Ar will be during the production, transport, and storage of the underground argon. These measurements also allow for the prediction of $^{39}$Ar and $^{37}$Ar concentrations in the atmosphere which can be used to determine the presence of other sources of these isotopes. Through controlled irradiation with a neutron beam that mimics the cosmic ray neutron spectrum, followed by direct counting of $^{39}$Ar and $^{37}$Ar decays with sensitive ultra-low background proportional counters, we determined that the production rate from cosmic ray neutrons at sea-level is expected to be $(759 \pm 128)$ atoms/kg$_\text{Ar}$/day for $^{39}$Ar, and $(51.0 \pm 7.4)$ atoms/kg$_\text{Ar}$/day for $^{37}$Ar. We also performed a survey of the alternate production mechanisms based on the state-of-knowledge of the associated cross-sections to obtain a total sea-level cosmic ray production rate of $(1048 \pm 133)$ atoms/kg$_\text{Ar}$/day for $^{39}$Ar, $(56.7 \pm 7.5)$ atoms/kg$_\text{Ar}$/day for $^{37}$Ar in underground argon, and $(92 \pm 13)$ atoms/kg$_\text{Ar}$/day for $^{37}$Ar in atmospheric argon.

Cosmic-ray neutron simulations and measurements in Taiwan

This study used simulations of galactic cosmic ray in the atmosphere to investigate the neutron background environment in Taiwan, emphasising its altitude dependence and spectrum variation near interfaces. The calculated results were analysed and compared with two measurements. The first measurement was a mobile neutron survey from sea level up to 3275 m in altitude conducted using a car-mounted high-sensitivity neutron detector. The second was a previous measured result focusing on the changes in neutron spectra near air/ground and air/water interfaces. The attenuation length of cosmic-ray neutrons in the lower atmosphere was estimated to be 163 g cm(-2) in Taiwan. Cosmic-ray neutron spectra vary with altitude and especially near interfaces. The determined spectra near the air/ground and air/water interfaces agree well with measurements for neutrons below 10 MeV. However, the high-energy portion of spectra was observed to be much higher than our previous estimation. Because high-energy neutrons contribute substantially to a dose evaluation, revising the annual sea-level effective dose from cosmic-ray neutrons at ground level in Taiwan to 35 μSv, which corresponds to a neutron flux of 5.30 × 10(-3) n cm(-2) s(-1), was suggested.

Characterization of the IRSN neutron multisphere spectrometer (HERMEIS) at European standard calibration fields

The Institute for Radiological Protection and Nuclear Safety (IRSN) has developed a new spectrometry system for neutron energies from 10−9 MeV to 10 GeV. This high energy range multisphere extended IRSN system (HERMEIS) is a high gas pressure3He-based Bonner spheres set. It is adapted to low neutron fluence rate measurements and one of its main application concerns the determination of cosmic-ray-induced neutron spectra at ground level and mountain altitudes. The neutron fluence response matrix of the set of 13 Bonner spheres, including three extended ones with tungsten and lead shells, was calculated with the radiation transport code MCNPX-2.6f. Reliable fluence responses being mandatory for a correct evaluation of the atmospheric neutron spectra, HERMEIS was characterized at standard monoenergetic, quasi-monoenergetic and realistic neutron fields facilities. Measurements with monoenergetic neutron beams of 144 keV, 565 keV, 5 MeV and 17 MeV were performed at the NPL standard Van de Graaff facility. For the characterization of the response functions at higher energies, measurements were done at the Svedberg Laboratory, with 46 MeV and 144 MeV quasi-monoenergetic neutrons. Finally, to demonstrate the suitability of the system for broad cosmic-ray neutron spectra, measurements were performed at the TSL Atmospheric-like Neutrons from thIck TArget (ANITA) and also at the CERN European Realistic Field (CERF) facilities. Data from the realistic spectra were unfolded with the GRAVEL unfolding code and as a whole, a good agreement was found between the experimental and Monte-Carlo calculated neutron fluence energy distributions.

An Extended-Range Multisphere Neutron Spectrometer with High Sensitivity and Improved Resolution

This paper describes the design and construction of an extended-range multisphere neutron spectrometer (ERMNS), also called a Bonner sphere spectrometer, created to measure the spectrum of cosmic-ray-induced neutrons on container ships. Such measurements require a highly sensitive neutron spectrometer and can benefit from improved energy resolution. To obtain high sensitivity, spherical 3 He gas proportional counters with a diameter of 152 mm (6 in.) were developed and used. The goal for designing the moderator assemblies of the new ERMNS was to optimize its energy resolution where it is typically worst for such spectrometers—at high and medium-low energies. Absorber shells containing boron carbide were used to improve resolution at medium-low energies. The response functions of various sizes and designs of plain and modified spheres were calculated using the radiation transport code MCNPX 2.6b. The resolution of combinations of 16 spheres of candidate designs was then determined using Reginatto’s RESPOW code, and the sphere designs that gave the smallest standard deviations of the RESPOW averaging kernels were selected. Compared to an ERMNS used earlier for similar measurements, resolution improved by 7 to 17% from 10 eV to 0.1 MeV and 24 to 39% from 10 MeV to 10 GeV. Bayesian parameter estimation was also used to characterize the uncertainties in spectra measured with the old and new spectrometers. The uncertainties in the fluence rates and peak energies of a typical terrestrial cosmic-ray neutron spectrum improved by 15 to 85% and 56 to 400%, respectively. These results demonstrate the utility of RESPOW and Bayesian parameter estimation for designing ERMNSs.

Response Functions of Phoswich-Type Neutron Detector for High-Energy Cosmic Ray Neutron Measurement

A phoswich-type neutron detector was developed in order to measure high-energy cosmic ray neutron spectra in aircraft. The neutron detector consists of an EJ309 organic liquid scintillator that is 121.7mm in diameter and 121.7mm in length and is covered with a 15mm thick EJ299-13 outer plastic scintillator. The neutron response functions of the detector are required for the unfolding method to obtain the energy spectrum. The neutron response functions were created based on MCNPX simulations using an anticoincidence mode with the experimental light-output correlations with particle energies, uniformity of light collection and energy resolutions. The light-output correlation with particle energy, the uniformity of light collection and the energy resolutions were evaluated based on experiments. Measurements of neutron response functions were performed using four quasi-monoenergetic neutron beams from 40 to 80 MeV p-Li reactions to verify the calculated results. The calculated response functions show good agreement with the measurements. The angular response of the phoswich detector was confirmed to be isotropic from the calculation. The photon response functions of the detector were also calculated and agreed well with the measurements for 6.129MeV photons. Neutron and photon response matrices were created up to 300 and 50 MeV, respectively, over a wide energy range for experimental flights.

Analytical Functions to Predict Cosmic-Ray Neutron Spectra in the Atmosphere

Estimation of cosmic-ray neutron spectra in the atmosphere has been an essential issue in the evaluation of the aircrew doses and the soft-error rates of semiconductor devices. We therefore performed Monte Carlo simulations for estimating neutron spectra using the PHITS code in adopting the nuclear data library JENDL-High-Energy file. Excellent agreements were observed between the calculated and measured spectra for a wide altitude range even at the ground level. Based on a comprehensive analysis of the simulation results, we propose analytical functions that can predict the cosmic-ray neutron spectra for any location in the atmosphere at altitudes below 20 km, considering the influences of local geometries such as ground and aircraft on the spectra. The accuracy of the analytical functions was well verified by various experimental data.

Sequential Measurements of Cosmic-Ray Neutron Spectrum and Dose Rate at Sea Level in Sendai, Japan

The cosmic-ray neutron energy spectrum and dose rate were measured sequentially for two years from April 2001 up to March 2003 by using three neutron detectors, a 3He-loaded multi-moderator detector (Bonner ball), 12.7 cm diameter by 12.7 cm long NE213 organic liquid scintillator, and high-sensitivity rem (dose equivalent) counter at the Kawauchi campus of Tohoku University in Sendai, Japan of geomagnetic latitude, 29°N, and cutoff rigidity, 10.43 GV. The neutron spectrum has three major peaks, thermal energy peak, evaporation peak around 1 MeV and cascade peak around 100 MeV. The ambient neutron dose equivalent rates measured by the rem counter, and the Bonner ball keep almost constant values of 4.0 and 6.5 (nSv/h), respectively, throughout this time period, after atmospheric pressure correction, and it often decreased about 30% after a large Solar Flare, that is called as the Forbush decrease. The total neutron flux was also obtained by the Bonner ball measurements to be 7.5×10-3 (n cm−2.s−1) in average. The altitude variation of neutron flux and dose was also investigated by comparing the measured results with other results measured at Mt. Fuji area and aboard an airplane, where the cutoff rigidities are similar.

Sequential Measurements of Cosmic-Ray Neutron Spectrum and Dose Rate at Sea Level in Sendai, Japan

Sequential Measurements of Cosmic-Ray Neutron Spectrum and Dose Rate at Sea Level in Sendai, Japan

Recent results from measurements of the energy spectrum of cosmic-ray induced neutrons aboard an ER-2 airplane and on the ground

Crews of future high-altitude commercial aircraft may be significantly exposed to atmospheric cosmic radiation from galactic cosmic rays (GCR). To help determine such exposures, the Atmospheric Ionizing Radiation Project, an international collaboration of 15 laboratories, made simultaneous radiation measurements with 14 instruments on a NASA ER-2 high-altitude aircraft. The primary instrument was a sensitive extended-energy multisphere neutron spectrometer, which was also used to make measurements on the ground. Its detector responses were calculated for neutrons and charged hadrons at energies up to 100 GeV using the radiation transport code MCNPX. We have now recalculated the detector responses including the effects of the airplane structure. We are also using new FLUKA calculations of GCR-induced hadron spectra in the atmosphere to correct for spectrometer counts produced by charged hadrons. Neutron spectra are unfolded from the corrected measured count rates. using the MAXED code. Results for the measured cosmic-ray neutron spectrum (thermal to >10 GeV), total neutron fluence rate, and neutron dose equivalent and effective dose rates, and their dependence on altitude and geomagnetic cutoff generally agree well with results from recent calculations of GCR-induced neutron spectra.

Measurement for the dose-rates of the cosmic-ray components on the ground.

In this study, we aimed to measure the directly ionizing component (muons and photons) and the indirectly ionizing component (neutrons) of the cosmic-ray spectra and evaluate their dose rate contribution to the total dose rate on a ground level in Japan. Measurements were carried out in Tohoku University, Japan, from October 2000. The pulse-height spectra of the cosmic-ray photons and muons were measured with a 12.7 cm diameter and 12.7 cm long NaI(Tl) scintillation detector. In order to measure energy spectra of cosmic-ray photons and muons, response functions of the detector to photons and muons were determined by the Monte Carlo simulation codes. The cosmic-ray photon dose was evaluated directly from the measured pulse-height spectrum by using the spectrum weight function, and the cosmic-ray muon dose was evaluated by converting the measured pulse height spectrum into deposited energy within the detector. The quantity of the cosmic-ray electrons is estimated to be very small and is not taken into account in this study. The cosmic-ray neutron spectrum and the neutron dose were measured by using a multi-moderator spectrometer (Bonner ball) and a rem counter. The measurements could finally give the annual absorbed dose in tissue of the cosmic-ray muons of 315 microSv/y and annual ambient doses of the cosmic-ray photons and neutrons on the ground in Japan of 55 microSv/y and 31 microSv/y, respectively.

Measurement of the energy spectrum of cosmic-ray induced neutrons aboard an ER-2 high-altitude airplane

Crews working on present-day jet aircraft are a large occupationally exposed group with a relatively high average effective dose from galactic cosmic radiation. Crews of future high-speed commercial aircraft flying at higher altitudes would be even more exposed. To help reduce the significant uncertainties in calculations of such exposures, the atmospheric ionizing radiation (AIR) project, an international collaboration of 15 laboratories, made simultaneous radiation measurements with 14 instruments on five flights of a NASA ER-2 high-altitude aircraft. The primary AIR instrument was a highly sensitive extended-energy multisphere neutron spectrometer with lead and steel shells placed within the moderators of two of its 14 detectors to enhance response at high energies. Detector responses were calculated for neutrons and charged hadrons at energies up to 100 GeV using MCNPX. Neutron spectra were unfolded from the measured count rates using the new MAXED code. We have measured the cosmic-ray neutron spectrum (thermal to >10 GeV), total neutron fluence rate, and neutron effective dose and dose equivalent rates and their dependence on altitude and geomagnetic cutoff. The measured cosmic-ray neutron spectra have almost no thermal neutrons, a large “evaporation” peak near 1 MeV and a second broad peak near 100 MeV which contributes about 69% of the neutron effective dose. At high altitude, geomagnetic latitude has very little effect on the shape of the spectrum, but it is the dominant variable affecting neutron fluence rate, which was eight times higher at the northernmost measurement location than it was at the southernmost. The shape of the spectrum varied only slightly with altitude from 21 km down to 12 km (56–201 g cm −2 atmospheric depth), but was significantly different on the ground. In all cases, ambient dose equivalent was greater than effective dose for cosmic-ray neutrons.

Dosimetry of High Energy Neutrons and Protons by 209Bi Fission

Fission of Bi by neutrons and protons of energies above 50 MeV provides a detector for the high energy part of cosmic ray neutron and proton doses received by personnel in commercial aircraft. A stack of damage track detectors and Bi radiators, of large total area, has been tested and has adequate sensitivity. Knowledge of the fission cross sections for neutrons and protons is critical for such measurements. Contrary to a common assumption that these are nearly equal, both old and recent measurements and recent calculations show that σ Bi (p,f) is more than twice σ Bi (n,f). Calculations indicate that Bi fission can probably determine dose equivalents from high energy cosmic ray neutrons to within ±30%, and from cosmic ray protons to within ± 25%. The response is nearly the same for equal dose equivalents of cosmic ray neutrons and protons.The accuracy for neutrons could be improved if cosmic ray neutron spectra were better determined.

A study on natural background neutron dose

In order to monitor the neutron dose in the environment of the Synchrotron Radiation Research Center, the sensitivity of the monitor system to the natural background cosmic-ray neutrons must be established. Large-size BF/sub 3/ counters with and without an 8.5-cm polyethylene enclosure were used to measure the cosmic-ray neutrons in the Nuclear Engineering Building, NTHU. The response functions of the BF/sub 3/ counters were calculated by using ANISN code performed in adjoint mode. By applying the cosmic-ray neutron spectrum and the detector response functions the measured counting rates of the BF/sub 3/ counters were converted to deliver the annual dose of cosmic-ray neutrons. The shielding effect of the concrete slabs and windows of the building was determined by the measured data and compared to that predicted by neutron transport calculations. >

Measurement of cosmic ray neutron spectrum at sea level

The cosmic ray neutron spectrum at sea level has been measured by the method of pulse shape discrimination in anthracene scintillator. The exponent of the differential spectrum was found to be 2.2±0.1.

"What quality factor"?

This paper discusses the evaluation of dose-equivalent rates in mixed radiation environments such as are found, for example, around a high energy particle accelerator. It is shown that it is not possible to evaluate absorbed dose and dose-equivalent rates by any single measurement, in such complicated environments, with sufficient accuracy for purposes of radiation protection. This paper studies in detail the problems of evaluating dose equivalent from measurements of absorbed dose made with a tissue-equivalent ionization chamber. Our work supplements and is distinctly different from that of Shaw et al. They have shown that when a neutron spectrum is known one can use the ORNL (Oak Ridge National Laboratory) Monte Carlo calculation to calculate the dose equivalent and absorbed dose distributions in tissue and hence evaluate QF. On the other hand, in this paper we show that knowledge of the neutron spectrum is also necessary when one must choose an average “Quality Factor” to be applied to a single measurement of absorbed dose when only a table of (QF vs. neutron energy is available. Recently the U.S. Atomic Energy Commission has published a table of quality factors for monoenergetic neutrons as a function of neutron energy, and this has now appeared in National Bureau of Standards Handbook 107, but no values were given for the spectra spanning the large energy range typically found around high energy accelerators and reactors. By the use of the values of quality factor recommended for monoenergetic neutrons, the QF for a variety of typical neutron spectra was calculated. The neutron spectra studied include the Watt fission spectrum, the PuBe spectrum, the cosmic-ray neutron spectrum, and three typical accelerator spectra measured at CERN and the Lawrence Radiation Laboratory. In addition, calculations were made for a variety of spectra expressed in simple exponential form. These calculations indicate the quality factor to be an extremely sensitive function of neutron spectrum and maximum-energy cutoff in the range of practical interest-the average quality factor varies with these parameters by more than a factor of 3. This work indicates the potential errors in estimating dose equivalent when making absorbed dose measurements and assuming a single value of quality factor. Knowledge of such errors is a valuable guide to operational health physicists.