Development of PARMA: PHITS-based analytical radiation model in the atmosphere

Estimation of cosmic-ray spectra in the atmosphere has been essential for the evaluation of aviation doses. We therefore calculated these spectra by performing Monte Carlo simulation of cosmic-ray propagation in the atmosphere using the PHITS code. The accuracy of the simulation was well verified by experimental data taken under various conditions, even near sea level. Based on a comprehensive analysis of the simulation results, we proposed an analytical model for estimating the cosmic-ray spectra of neutrons, protons, helium ions, muons, electrons, positrons and photons applicable to any location in the atmosphere at altitudes below 20 km. Our model, named PARMA, enables us to calculate the cosmic radiation doses rapidly with a precision equivalent to that of the Monte Carlo simulation, which requires much more computational time. With these properties, PARMA is capable of improving the accuracy and efficiency of the cosmic-ray exposure dose estimations not only for aircrews but also for the public on the ground.     

DS02 fluence spectra for neutrons and gamma rays at Hiroshima and Nagasaki with fluence-to-kerma coefficients and transmission factors for sample measurements

Fluence spectra at several ground distances in Hiroshima and Nagasaki are provided along with associated fluence-to-kerma coefficients from the Dosimetry System 2002 (DS02). Also included are transmission factors for calculating expected responses of in situ sample measurements of neutron activation products such as ³²P,³⁶Cl,³⁹Ar,⁴¹Ca, ⁶⁰Co,⁶³Ni,¹⁵²Eu, and ¹⁵⁴Eu. The free-in-air (FIA) fluences calculated in 2002 are available for 240 angles, 69 energy groups, 101 ground distances, 5 heights, 4 radiation source components, 2 cities. The DS02 code uses these fluences partitioned to a prompt and delayed portion, collapsed to 58 energy groups and restricted to 97 ground distances. This is because the fluence spectra were required to be in the same format that was used in the older Dosimetry System 1986 (DS86) computer code, of which the DS02 computer code is a modification. The 2002 calculation fluences and the collapsed DS02 code fluences are presented and briefly discussed. A report on DS02, which is available on the website at the Radiation Effects Research Foundation, provides tables and figures of the A-bomb neutron and gamma-ray output used as the sources in the 2002 radiation transport calculations. While figures illustrating the fluence spectra at several ground ranges are presented in the DS02 Report, it does not include any tables of the calculated fluence spectra in the DS02 report. This paper provides, at several standard distances from the hypocenter, the numerical information which is required to translate the FIA neutron fluences given in DS02 to a neutron activation measurement or neutron and gamma-ray soft-tissue dose.     

Prompt gamma-ray analysis using cold and thermal guided neutron beams at JAERI

A highly sensitive neutron-induced prompt γ-ray analysis (PGA) system, usable at both cold and thermal neutron beam guides of JRR-3M, has been constructed. The system was designed to achieve the lowest γ-ray background by using lithium fluoride tiles as neutron shielding, by placing the samples in a He atmosphere and by using a Ge-bismuth germanate detector system for Compton suppression. The γ-ray spectrometer can acquire three modes of spectra simultaneously: single, Compton suppression, and pair modes. Because of the low-energy guided neutron beams and the low-background system, analytical sensitivities and detection limits better than those in usual PGA systems have been achieved. Boron and multielemental determination by a comparative standardization have been investigated, and accuracy, precision, and detection limits for the elements in various materials were evaluated. The system has been applied to the determination of B and multielements in samples of various fields such as medical, environmental, and geological sciences.     

LET spectra of cosmic-ray nuclei for near earth orbits

Measurements of cosmic-ray LET spectra were part of the radiobiological space research programs during the Spacelab 1 (SL-1) and the D1 missions. We analyzed CR-39 plastic nuclear track detectors of the Advanced Biostack experiment of SL-1 and of the Dosimetric Mapping and Carausius morosus experiments in the BIORACK on D1. The particle tracks in the CR-39 were detected and measured by an automatic scanning and measuring system. An in-flight calibration was derived from track measurements of minimum ionizing oxygen and iron nuclei and of stopping nuclei as a function of the residual range. LET spectra measured at different locations in the space shuttle are presented and discussed for both missions. A model describing the effects of the geomagnetic field of the earth on charged cosmic-ray particles and the shielding by matter is used to calculate LET spectra for the two missions and for typical space station orbits at low inclinations. A comparison of measured LET spectra and LET spectra calculated for different flight parameters shows that besides geomagnetic shielding the shielding by matter is most important in comparison to solar modulation and to variation of particle flux with flight altitude. Model calculations must be improved and must consider more detailed sectored shielding by matter and the influence of trapped radiation. The last item is of importance in the case of low-inclination orbits.     

Surfatron acceleration of protons by an electromagnetic wave at the heliosphere periphery

The trapping and subsequent efficient surfatron acceleration of weakly relativistic protons by an electromagnetic wave propagating across an external magnetic field in plasma at the heliosphere periphery is considered. The problem is reduced to analysis of a second-order time-dependent nonlinear equation for the wave phase on the particle trajectory. The conditions of proton trapping by the wave, the dynamics of the components of the particle momentum and velocity, the structure of the phase plane, the particle trajectories, and the dependence of the acceleration rate on initial parameters of the problem are analyzed. The asymptotic behavior of the characteristics of accelerated particles for the heliosphere parameters is investigated. The optimum conditions for surfatron acceleration of protons by an electromagnetic wave are discussed. It is demonstrated that the experimentally observed deviation of the spectra of cosmic-ray protons from standard power-law dependences can be caused by the surfatron mechanism. It is shown that protons with initial energies of several GeV can be additionally accelerated in the heliosphere (the region located between the shock front of the solar wind and the heliopause at distances of about 100 astronomical units (a.u.) from the Sun) up to energies on the order of several thousands of GeV. In order to explain the proton spectra in the energy range of ～20–500 GeV, a two-component phenomenological model is proposed. The first component corresponds to the constant (in this energy range) galactic contribution, while the second (variable) component corresponds to the heliospheric contribution, which appears due to the additional acceleration of soft cosmic-ray protons at the heliosphere periphery. Variations in the proton spectra measured on different time scales between 1992 and 2008 in the energy range from several tens to several hundred GeV, as well as the dependence of these spectra on the heliospheric weather, can be explained by surfatron acceleration of protons in the heliosphere.     

Orientational and rotational velocity correlation functions for water-hydrating B- and Z-DNA double helices

The molecular dynamics simulations reported earlier for the structure and dynamics of water molecules hydrating B- and Z-DNA double helices are analyzed for the orientational correlation functions and the proton rotational velocity autocorrelation functions. The spectra of the rotational velocity autocorrelation functions obtained from the simulation results are compared with the neutron inelastic scattering experiments on hydrated Na-DNA samples. The results predict a small frequency component associated with water molecules bound to the double helices that disappears for waters away from the double helix.     

Configuration and validation of an analytical model predicting secondary neutron radiation in proton therapy using Monte Carlo simulations and experimental measurements

PurposeThis study focuses on the configuration and validation of an analytical model predicting leakage neutron doses in proton therapy.MethodsUsing Monte Carlo (MC) calculations, a facility-specific analytical model was built to reproduce out-of-field neutron doses while separately accounting for the contribution of intra-nuclear cascade, evaporation, epithermal and thermal neutrons. This model was first trained to reproduce in-water neutron absorbed doses and in-air neutron ambient dose equivalents, H*(10), calculated using MCNPX. Its capacity in predicting out-of-field doses at any position not involved in the training phase was also checked. The model was next expanded to enable a full 3D mapping of H*(10) inside the treatment room, tested in a clinically relevant configuration and finally consolidated with experimental measurements.ResultsFollowing the literature approach, the work first proved that it is possible to build a facility-specific analytical model that efficiently reproduces in-water neutron doses and in-air H*(10) values with a maximum difference less than 25%. In addition, the analytical model succeeded in predicting out-of-field neutron doses in the lateral and vertical direction. Testing the analytical model in clinical configurations proved the need to separate the contribution of internal and external neutrons. The impact of modulation width on stray neutrons was found to be easily adjustable while beam collimation remains a challenging issue. Finally, the model performance agreed with experimental measurements with satisfactory results considering measurement and simulation uncertainties.ConclusionAnalytical models represent a promising solution that substitutes for time-consuming MC calculations when assessing doses to healthy organs.     

A round-robin determination of boron in botanical and biological samples

The accurate determination of boron (B) at trace and ultratrace concentrations is an important step toward establishing the role of B in biological functions. However, low-level B concentrations are difficult to determine accurately, especially for many botanical and biological matrices. A round-robin study was conducted to assess analytical agreement for low-level B determinations. Ten experienced research groups from analytical laboratories extending across Europe, Asia, and the US participated in this study. These groups represent a crosssection of academic, commercial, and government facilities. The researchers employed both ion-coupled plasma and neutron techniques in the study. Results from this round-robin study indicate good agreement between participating laboratories at the mg/kg level, but at the lowest levels, μg/kg, only three laboratories participated, and agreement was poor. By encouraging discussion among scientists over these data, the secondary goal of this round-robin study is to stimulate continued improvement in analytical procedures and techniques for accurate low-level B determinations. Furthermore, it is intended to encourage the development of a variety of low-level (low mg/kg and (μg/kg) B certified reference samples in biological and botanical matrices. The results from the round-robin analyses were compiled and are summarized in this article.     

Neutron-scattering studies of the structure of chromatin core particles in solution

The structure of the nucleosome core particle in solution has been studied by neutron scattering using the full-contrast variation technique, which reduces the experimental spectra to three fundamental scatter functions holding information on shape and structure. Systematic calculations of the fundamental scatter functions expected from proposed core-particle models have been compared with the observed functions and show that the neutron-scattering criteria severely restrict the number of models which can be valid for the structure in solution. The best model for the core particle in solution has a hydrophobic histone core about which 1.7 ± 0.1 turns of DNA are wrapped at a pitch between 3.0 and 3.5 nm. This core contains most of the histone and has an average thickness of 4 nm and diameter 6.4–7.5 nm. While solution scattering is not able to specify uniquely the actual shape of the core to high resolution, all models which are possible for the shape of the core to a resolution justified by the data have been considered. It is clear that cylindrical or wedge shapes compatible with the above dimensions are valid structures. A hole probably penetrates the histone core, but the data do not allow a diameter greater than 1 nm. Available evidence suggests that about a quarter of the total histone is outside the core.     

The dynamical Matryoshka model: 1. Incoherent neutron scattering functions for lipid dynamics in bilayers

Fluid lipid bilayers are the building blocks of biological membranes. Although there is a large amount of experimental data using incoherent quasi-elastic neutron scattering (QENS) techniques to study membranes, very little theoretical works have been developed to study the local dynamics of membranes. The main objective of this work is to build a theoretical framework to study and describe the local dynamics of lipids and derive analytical expressions of intermediate scattering functions (ISF) for QENS. As results, we developed the dynamical Matryoshka model which describes the local dynamics of lipid molecules in membrane layers as a nested hierarchical convolution of three motional processes: (i) individual motions described by the vibrational motions of H-atoms; (ii) internal motions including movements of the lipid backbone, head groups and tails, and (iii) molecule movements of the lipid molecule as a whole. The analytical expressions of the ISF associated with these movements are all derived. For use in analyzing the QENS experimental data, we also derived an analytical expression for the aggregate ISF of the Matryoshka model which involves an elastic term plus three inelastic terms of well-separated time scales and whose amplitudes and rates are functions of the lipid motions. And as an illustrative application, we used the aggregated ISF to analyze the experimental QENS data on a lipid sample of multilamellar bilayers of DMPC (1,2-dimyristoyl-sn-glycero-3-phosphocholine). It is clear from this analysis that the dynamical Matryoshka model describes very well the experimental data and allow extracting the dynamical parameters of the studied system.     

Correlation between the synthetic activity of lymphocytes and the intensity of neutron radiation near the Earth surface

The synthetic activity of lymphocytes from rat and ground squirrel was assessed for correlation with the rate of neutron radiation about the Earth surface. In the periods 1999–2002 for rats and 2002–2003 for ground squirrels, significant positive correlation was obtained with the neutron flux incident on Earth and the overall “terrestrial neutron field” but not with the neutron efflux from Earth. No correlation was observed in 2005–2006 when the solar activity and the variation in neutron count were minimal. It is concluded that low-energy neutrons may be bioeffective in periods of substantial variation in flux intensity.     

Activation analysis of human hair as a tool for environmental pollution monitoring

Recent development and uses of neutron activation techniques for human hair analyses are reviewed. The method of neutron activation analysis (NAA) appears to have the potential to be used as a tool for environmental pollution monitoring. Principally, two types of NAA procedure are in use nowadays for multielement analyses of human scalp hair. The more common of these is the method of instrumental neutron activation analysis (INAA), consisting of a single short-term (3-10 hours) exposure of hair to a beam of neutrons in a nuclear reactor, followed by two measurements of gamma-ray spectra at 2-3 days and 3-4 weeks after the end of irradiation. The following microelements can be commonly determined by this type of activation procedure: As, Au, Br, Cu, K, La, Na, Sb, Sm, Co, Cr, Cs, Fe, Hg, Rb, Sc, Se and Zn. The other of the two procedures involves the use of radiochemical separation techniques and is employed for quantitative determinations of elements that are not easily determined by INAA (Mo, Cd, Ni, etc.), or in cases where there is a need to achieve the lowest possible limits of analytical determination. The accuracy of NAA techniques is strongly dependent on the hair sampling and hair sample processing methods used. The analytical error of this method may vary within the range of 5-15%. Its applicability as a tool for monitoring the environmental pollution level is here demonstrated on an example of groups of individuals living in the areas differing by the degree of environmental pollution. The use of other biopsy materials, such as e.g. mammalian hair, for the purpose of environmental exposure monitoring is also considered in this review.     

The high-spin/low-spin equilibrium in cytochrome P-450--a new method for determination of the high-spin content.

A new method for determination of the population of the high-spin state (high-spin content) in ferric cytochrome P-450 is presented. Based on curve fitting the electronic absorption spectra with a linear combination of gaussian bands analytical functions for the pure high-spin and pure low-spin states were constructed. These functions were used to fit the high-spin/low-spin mixed spectra. A good fit of the absorption spectra of six different cytochrome P-450 proteins in the presence and absence of substrates was found, indicating a similar pi-electron structure of the porphyrin and a similar chemical nature of the nearest coordination sphere of the iron in all cytochrome P-450 proteins.     

High-energy neutron spectroscopy with thick silicon detectors

The high-energy neutron component of the space radiation environment in thick structures such as the International Space Station contributes to the total radiation dose received by an astronaut. Detector design constraints such as size and mass have limited the energy range of neutron spectrum measurements in orbit to about 12 MeV in Space Shuttle studies. We present a new method for high-energy neutron spectroscopy using small silicon detectors that can extend these measurements to more than 500 MeV. The methodology is based on measurement of the detector response function for high-energy neutrons and inversion of this response function with measured deposition data to deduce neutron energy spectra. We also present the results of an initial shielding study performed with the thick silicon detector system for high-energy neutrons incident on polyethylene.     

Analytical Model for Estimating Terrestrial Cosmic Ray Fluxes Nearly Anytime and Anywhere in the World: Extension of PARMA/EXPACS

By extending our previously established model, here we present a new model called “PHITS-based Analytical Radiation Model in the Atmosphere (PARMA) version 3.0,” which can instantaneously estimate terrestrial cosmic ray fluxes of neutrons, protons, ions with charge up to 28 (Ni), muons, electrons, positrons, and photons nearly anytime and anywhere in the Earth’s atmosphere. The model comprises numerous analytical functions with parameters whose numerical values were fitted to reproduce the results of the extensive air shower (EAS) simulation performed by Particle and Heavy Ion Transport code System (PHITS). The accuracy of the EAS simulation was well verified using various experimental data, while that of PARMA3.0 was confirmed by the high R ² values of the fit. The models to be used for estimating radiation doses due to cosmic ray exposure, cosmic ray induced ionization rates, and count rates of neutron monitors were validated by investigating their capability to reproduce those quantities measured under various conditions. PARMA3.0 is available freely and is easy to use, as implemented in an open-access software program EXcel-based Program for Calculating Atmospheric Cosmic ray Spectrum (EXPACS). Because of these features, the new version of PARMA/EXPACS can be an important tool in various research fields such as geosciences, cosmic ray physics, and radiation research.     

The impact of treatment parameter variation on secondary neutron spectra in high-energy electron beam radiotherapy

High-energy electron treatment procedures in radiotherapy pose a potential iatrogenic cancer risk as well as a critical health risk to patients with cardiac implantable electronic devices due to the generation of secondary neutrons in the linac head, the treatment room, and the patient. It may be argued that the neutron production from photons is well characterized, but the same is not true for electrons. Therefore, to assess the risk involved in an electron treatment, one must determine the neutron flux spectrum generated by the treatment procedure. The neutron spectrum depends on the treatment parameters used and therefore it is crucial to study its dependence on these parameters. In this work, eight experiments were devised to analyze how eight electron treatment parameters impacted the neutron spectrum. The parameters we considered were the electron beam energy, location of measurement, cutout size, collimator size, applicator size, collimator angle, choice of treatment room, and the presence or absence of a solid water phantom. For each experiment, we used a Nested Neutron Spectrometer™ (NNS) to measure the neutron flux spectra for multiple settings of the treatment parameter of interest. The resulting spectra were plotted and compared. We found that the electron beam energy and the location of measurement had the most impact on the neutron flux spectra, while the other parameters had a smaller or insignificant impact. This report may serve as a reference tool for medical physicists to help estimate the risk associated with a particular high-energy electron treatment procedure.     

Quantification of structural loading during off-road cycling.

To provide data for fatigue life prediction and testing of structural components in off-road bicycles, the objective of the research described herein was to quantify the loads input to an off-road bicycle as a result of surface-induced loads. A fully instrumented test bicycle was equipped with dynamometers at the pedals, handlebars, and hubs to measure all in-plane structural loads acting through points of contact between the bicycle and both the rider and the ground. A portable data acquisition system carried by the standing rider allowed, for the first time, this loading information to be collected during extended off-road testing. In all, seven experienced riders rode a downhill trial test section with the test bicycle in both front-suspension and full-suspension configurations. The load histories were used quantitatively to describe the load components through the computation of means, standard deviations, amplitude probability density functions, and power spectral density functions. For the standing position, the coefficients of variation for the load components normal to the ground were greater than 1.2 for handlebar forces and 0.3 and 0.5-0.6 for the pedal and hub forces, respectively. Thus, the relative contribution of the dynamic loading was much greater than the static loading at the handlebars but less so at the pedals and hubs. As indicated by the rainflow count, high amplitude loading was developed approaching 3 and 5 times the weight of the test subjects at the front and rear wheels, respectively. The power spectral densities showed that energy was concentrated in the band 0-50 Hz. Through stress computations and knowledge of material properties, the data can be used analytically to predict the fatigue life of important structural components such as those for steering. The data can also be used to develop a fatigue testing protocol for verifying analytical predictions of fatigue life.     

FLUKA capabilities for microdosimetric analysis

Delta-ray transport is important in microdosimetric studies, and how Monte Carlo models handle delta electrons using condensed histories is important for accurate simulation. The purpose of this study was to determine how well FLUKA can simulate energy deposition spectra in a tissue-equivalent proportional counter (TEPC) and produce a reliable estimate of delta-ray events produced when a TEPC is exposed to high-energy heavy ions (HZE) like those in the galactic cosmic-ray (GCR) environment. A 1.27-cm spherical TEPC with a low-pressure gas simulating a 1-μm site, typical of the one flown on the ISS, was constructed in FLUKA, and its response was compared to experimental data for an (56)Fe-ion beam at 360 MeV/nucleon. Several narrow beams at different impact parameters were used to explain the response of the same detector exposed to a uniform field of radiation. Additionally, the effect that wall thickness had on the response of the TEPC and the range of delta rays in the tissue-equivalent (TE) wall material was investigated, and FLUKA produced the expected wall effect for primary particles passing outside the sensitive volume. A final comparison to experimental data was made for the simulated TEPCs exposed to various broad beams in the energy range of 200-1000 MeV/nucleon. FLUKA overestimated energy deposition in the gas volume in all cases. The FLUKA results differed from the experimental data by an average of 25.2% for y(F) and 12.4% for y(D). It is suggested that this difference can be reduced by adjusting the FLUKA default ionization potential and density correction factors. Accurate transport codes are desirable because of the high cost of beam time for experimental evaluation of energy deposition spectra produced by HZE ions and the flexibility that calculations offer in the TEPC engineering and design process.     

Vibrational neutron spectroscopy of collagen and model polypeptides.

A pulsed source neutron spectrometer has been used to measure vibrational spectra (20-4000 cm-1) of dry and hydrated type I collagen fibers, and of two model polypeptides, polyproline II and (prolyl-prolyl-glycine)10, at temperatures of 30 and 120 K. the collagen spectra provide the first high resolution neutron views of the proton-dominated modes of a protein over a wide energy range from the low frequency phonon region to the rich spectrum of localized high frequency modes. Several bands show a level of fine structure approaching that of optical data. The principal features of the spectra are assigned. A difference spectrum is obtained for protein associated water, which displays an acoustic peak similar to pure ice and a librational band shifted to lower frequency by the influence of the protein. Hydrogen-weighted densities of states are extracted for collagen and the model polypeptides, and compared with published calculations. Proton mean-square displacements are calculated from Debye-Waller factors measured in parallel quasi-elastic neutron-scattering experiments. Combined with the collagen density of states function, these yield an effective mass of 14.5 a.m.u. for the low frequency harmonic oscillators, indicating that the extended atom approximation, which simplifies analyses of low frequency protein dynamics, is appropriate.     

Microdosimetry for boron neutron capture therapy.

Preclinical studies for boron neutron capture therapy (BNCT) using epithermal neutrons are ongoing at several laboratories. The absorbed dose in tumor cells is a function of the thermal neutron flux at depth, the microscopic boron concentration, and the size of the cell. Dosimetry is therefore complicated by the admixture of thermal, epithermal, and fast neutrons, plus gamma rays, and the array of secondary high-linear-energy-transfer particles produced within the patient from neutron interactions. Microdosimetry can be a viable technique for determining absorbed dose and radiation quality. A 2.5-cm-diameter tissue-equivalent gas proportional counter has been built with 50 parts per million (ppm) 10B incorporated into the walls and counting gas to simulate the boron uptake anticipated in tumors. Measurements of lineal energy (y) spectra for BNCT in simulated volumes of 1-10 microns diameter show a dose enhancement factor of 4.3 for 30 ppm boron, and a "y" of 250 keV/microns for the boron capture process. Chamber design plus details of experimental and calculated linear energy spectra will be presented.