A database of frequency distributions of energy depositions in small-size targets by electrons and ions

Linear energy transfer (LET) is an average quantity, which cannot display the stochastics of the interactions of radiation tracks in the target volume. For this reason, microdosimetry distributions have been defined to overcome the LET shortcomings. In this paper, model calculations of frequency distributions for energy depositions in nanometre size targets, diameters 1-100 nm, and for a 1 μm diameter wall-less TEPC, for electrons, protons, alpha particles and carbon ions are reported. Frequency distributions for energy depositions in small-size targets with dimensions similar to those of biological molecules are useful for modelling and calculations of DNA damage. Monte Carlo track structure codes KURBUC and PITS99 were used to generate tracks of primary electrons 10 eV to 1 MeV, and ions 1 keV μm(-1) to 300 MeV μm(-1) energies. Distribution of absolute frequencies of energy depositions in volumes with diameters of 1-100 nm randomly positioned in unit density water irradiated with 1 Gy of the given radiation was obtained. Data are presented for frequency of energy depositions and microdosimetry quantities including mean lineal energy, dose mean lineal energy, frequency mean specific energy and dose mean specific energy. The modelling and calculations presented in this work are useful for characterisation of the quality of radiation beam in biophysical studies and in radiation therapy.     

Radiation measured for ISS-Expedition 12 with different dosimeters

Radiation in low Earth orbit (LEO) is mainly from Galactic Cosmic Rays (GCR), solar energetic particles and particles in South Atlantic Anomaly (SAA). These particles’ radiation impact to astronauts depends strongly on the particles’ linear energy transfer (LET) and is dominated by high LET radiation. It is important to investigate the LET spectrum for the radiation field and the influence of radiation on astronauts. At present, the best active dosimeters used for all LET are the tissue equivalent proportional counter (TEPC) and silicon detectors; the best passive dosimeters are thermoluminescence dosimeters (TLDs) or optically stimulated luminescence dosimeters (OSLDs) for low LET and CR-39 plastic nuclear track detectors (PNTDs) for high LET. TEPC, CR-39 PNTDs, TLDs and OSLDs were used to investigate the radiation for space mission Expedition 12 (ISS-11S) in LEO. LET spectra and radiation quantities (fluence, absorbed dose, dose equivalent and quality factor) were measured for the mission with these different dosimeters. This paper introduces the operation principles for these dosimeters, describes the method to combine the results measured by CR-39 PNTDs and TLDs/OSLDs, presents the experimental LET spectra and the radiation quantities.     

Microdosimetry distributions for 40-200 MeV protons

Theoretical calculations have been performed to obtain microdosimetrical characteristics for protons in energy range from 40 to 200 MeV. This energy range is a representative of proton energies in tissue during radiation therapy and it also represents a large portion of the proton fluency in the South Atlantic Anomaly. Distributions of deposited energy calculated using Monte Carlo track structure code TRIOL and own-made programs were compared with experimental data obtained using spherical tissue-equivalent proportional counter. A good agreement between calculated and experimentally obtained microdosimetry spectra has been found.     

Space neutron spectrometer design with SSPM-based instrumentation

The compact, robust nature of the CMOS solid-state photomultiplier (SSPM) allows the creation of small, low-power scintillation-based radiation measurement devices. Monitoring space radiation including solar protons and secondary neutrons generated from high-energy protons impinging on spacecraft is required to determine the dose to astronauts. Small size and highly integrated design are desired to minimize consumption of payload resources.RMD is developing prototype radiation measurement and personal dosimeter devices using emerging scintillation materials coupled to CMOS SSPM’s for multiple applications. Spectroscopic measurements of high-energy protons and gamma-rays using tissue-equivalent, inorganic scintillators coupled to SSPM devices demonstrate the ability of an SSPM device to monitor the dose from proton and heavy ion particles, providing real time feedback to astronauts. Measurement of the dose from secondary neutrons introduces additional challenges due to the need to discriminate neutrons from other particle types and to accurately determine their energy deposition. We present strategies for measuring neutron signatures and assessing neutron dose including simulations of relevant environments and detector materials.     

Study of an improved Allyl Di-Glycol carbonate sheet for high energy proton detection

An allyl di-glycol carbonate (ADC) sheet which has been utilised as a neutron detector for personal dosimetry has recently been studied for its application as a device for radiation exposure control for astronauts in space, where protons are the dominant radiation. It is known that the fabrication process, modified by adding some kind of antioxidant to improve the sensitivity of ADC to high energy protons, causes a substantial increase in false tracks, which disturb the automatic counting of proton tracks using the auto-image analyser. This made clear the difficulty of fabricating ADC sheets which have sufficient sensitivity to high energy protons, while maintaining a good surface. In this study, we have tried to modify the fabrication process to improve the sensitivity to high energy protons without causing a deterioration of the surface condition of ADC sheets. We have successfully created fairly good products.     

Development of a calculation method for estimating specific energy distribution in complex radiation fields

Estimation of the specific energy distribution in a human body exposed to complex radiation fields is of great importance in the planning of long-term space missions and heavy ion cancer therapies. With the aim of developing a tool for this estimation, the specific energy distributions in liquid water around the tracks of several HZE particles with energies up to 100 GeV n(-1) were calculated by performing track structure simulation with the Monte Carlo technique. In the simulation, the targets were assumed to be spherical sites with diameters from 1 nm to 1 microm. An analytical function to reproduce the simulation results was developed in order to predict the distributions of all kinds of heavy ions over a wide energy range. The incorporation of this function into the Particle and Heavy Ion Transport code System (PHITS) enables us to calculate the specific energy distributions in complex radiation fields in a short computational time.     

Analysis of dose-LET distribution in the human body irradiated by high energy hadrons

For the purposes of radiological protection, it is important to analyse profiles of the particle field inside a human body irradiated by high energy hadrons, since they can produce a variety of secondary particles which play an important role in the energy deposition process, and characterise their radiation qualities. Therefore Monte Carlo calculations were performed to evaluate dose distributions in terms of the linear energy transfer of ionising particles (dose-LET distribution) using a newly developed particle transport code (Particle and Heavy Ion Transport code System, PHITS) for incidences of neutrons, protons and pions with energies from 100 MeV to 200 GeV. Based on these calculations, it was found that more than 80% and 90% of the total deposition energies are attributed to ionisation by particles with LET below 10 keV microm(-1) for the irradiations of neutrons and the charged particles, respectively.     

Microdosimetric characteristics of proton beams from 50 keV to 200 MeV

Proton beams are of growing interest for radiation therapy due to their special physical and radiobiological properties. Microdosimetric characteristics of proton beams have strong influence on the relative biological effectiveness for each biological system. This study focused on the microdosimetric characteristics of monoenergetic protons from 50 keV to 200 MeV. Monte Carlo techniques were used to simulate track segments of protons in water. Dose mean lineal energies were derived to characterise proton beams with changing kinetic energy and changing radiation qualities at various depths and within spread-out Bragg peaks of clinic interests.     

Results of dosimetric measurements in space missions

Detector packages consisting of thermoluminescence detectors (TLDs), nuclear emulsions and plastic nuclear track detectors were exposed in different locations inside spacecraft. The detector systems, which supplement each other in their registration characteristics, allow the recording of biologically relevant portions of the radiation field independently. Results are presented and compared with calculations. Dose equivalents for the astronauts have been calculated based on the measurements; they lie between 190 microSv d-1 and 860 microSv d-1.     

Radiations in space: risk estimates

The complexity of radiation environments in space makes estimation of risks more difficult than for the protection of terrestrial populations. In deep space the duration of the mission, position in the solar cycle, number and size of solar particle events (SPE) and the spacecraft shielding are the major determinants of risk. In low-earth orbit missions there are the added factors of altitude and orbital inclination. Different radiation qualities such as protons and heavy ions and secondary radiations inside the spacecraft such as neutrons of various energies, have to be considered. Radiation dose rates in space are low except for short periods during very large SPEs. Risk estimation for space activities is based on the human experience of exposure to gamma rays and to a lesser extent X rays. The doses of protons, heavy ions and neutrons are adjusted to take into account the relative biological effectiveness (RBE) of the different radiation types and thus derive equivalent doses. RBE values and factors to adjust for the effect of dose rate have to be obtained from experimental data. The influence of age and gender on the cancer risk is estimated from the data from atomic bomb survivors. Because of the large number of variables the uncertainities in the probability of the effects are large. Information needed to improve the risk estimates includes: (1) risk of cancer induction by protons, heavy ions and neutrons: (2) influence of dose rate and protraction, particularly on potential tissue effects such as reduced fertility and cataracts: and (3) possible effects of heavy ions on the central nervous system. Risk cannot be eliminated and thus there must be a consensus on what level of risk is acceptable.     

An analytical model for calculating microdosimetric distributions from heavy ions in nanometer site targets

The analytical model of Xapsos used for calculating microdosimetric spectra is based on the observation that straggling of energy loss can be approximated by a log-normal distribution of energy deposition. The model was applied to calculate microdosimetric spectra in spherical targets of nanometer dimensions from heavy ions at energies between 0.3 and 500 MeV amu(-1). We recalculated the originally assumed 1/E(2) initial delta electrons spectrum by applying the Continuous Slowing Down Approximation for secondary electrons. We also modified the energy deposition from electrons of energy below 100 keV, taking into account the effective path length of the scattered electrons. Results of our model calculations agree favourably with results of Monte Carlo track structure simulations using MOCA-14 for light ions (Z = 1-8) of energy ranging from E = 0.3 to 10.0 MeV amu(-1) as well as with results of Nikjoo for a wall-less proportional counter (Z = 18).     

The use of TEPC for reference dosimetry.

The radiation fields on board aircraft are quite complex and cover an energy range that is unusual in ordinary radiation protection work. Usually dosemeters measure only one radiation quality and the mixture found on board makes measurements complicated. There is also some doubt when it comes to the best choice of quantity for this application and no radiation standards exist for this kind of radiation field. For those reasons there is a need to find a standard measurement procedure that could serve as a reference for other, maybe simpler, measurements or for calculations of route doses. The only direct reading dosemeter that both measures the absorbed dose to tissue and the radiation quality (in terms of lineal energy) is the tissue-equivalent proportional counter (TEPC). The instrument was originally developed for scientific purposes and is still used as such. The detector consists of a gas filled cavity surrounded by a few mm thick wall. Both wall and gas consists of tissue-like material. The measurement principles are explained. Results observed with TEPC instruments are demonstrated. A preliminary collection of data reported by different groups from measurements on board aircraft will be shown. The results agree within +/- 20%. The conclusion is that TEPC instruments have the capacity of serving as reference instruments.     

Calculation of energy distributions of charged particles produced by neutrons from 0.14 to 65 MeV in tissue substitutes

Energy distributions of secondary charged particles were calculated in tissue substitutes irradiated by neutrons from 0.14 to 65 MeV, using the Particle and Heavy Ion Transport code System. The calculations were compared with experimental data measured by tissue equivalent proportional counters (TEPC). It is found that the calculated distributions of the lineal energy, y, generally agree well with the measured ones for neutrons from several 100 keV to 15 MeV. In the case of 40 and 65 MeV neutron irradiations, wall effects of TEPC should be considered and the fluence of alphas is underestimated by the calculations. Integrated dose contributions of the secondary charged particles are generally in good agreement with those of the measured ones.     

低能粒子辐照对铝膜反射镜光学性能的影响

地面模拟研究了低能质子和电子对铝膜反射镜光学性能的影响。结果表明,低能质子辐照后,在200~800nm波长范围内铝膜反射镜反射率随辐照剂量增加而下降。质子辐照能量越低射程越短,则反射镜表面膜层中质子浓度越大损伤也更为明显。电子辐照射程较深,辐照作用对铝膜反射镜光学性能影响很小。     

Measurements of trapped protons and cosmic rays from recent Shuttle flights

Two new charged particle detectors have been flown in five recent Shuttle flights. The tissue-equivalent proportional counter measures the lineal energy spectrum of space radiation in the 0.26-300 keV micrometer-1 range. The charged particle spectrometer is a double dE/dx x E and dE/dx x Chrenekov detector system which provides a measurement of the differential energy spectrum of protons from 13 to 350 MeV and dose rate in silicon. In this paper the dose rate, equivalent dose rate, and radiation, quality factor for trapped protons and cosmic radiation are reported on separately. A comparison of the integral LET spectra with recent transport code calculations shows significant disagreement. Using the calculated dose rate from the omnidirectional AP8MAX model with IGRF reference magnetic field epoch 1970, and observed dose rate as a function of geographic latitude and longitude, the westward drift of the south Atlantic anomaly has been determined. The east-west effect has also been studied and a 'second' radiation belt observed. A comparison of the galactic cosmic radiation (GCR) lineal energy transfer spectra with model calculations shows disagreement comparable with those of the trapped protons.     

Characterising the energy deposition events produced by trapped protons in low earth orbit

Men and equipment in space vehicles in low earth orbit are exposed to a wide variety of radiations, but the majority of the dose is due to trapped protons, which have energies of the order of 100 MeV and are low LET particles. These high energy particles produce nuclear fragmentation with high LET secondaries that may be responsible for a significant fraction of dose equivalent. In order to understand better the biological effectiveness of this radiation environment, a portable tissue equivalent proportional counter spectrometer has been developed that automatically records the distribution of energy in a small tissue-like site as a function of time. This instrument weighs about 700 g and will be flown on a number of future space shuttle flights.     

FLUKA simulation of TEPC response to cosmic radiation

The aircrew exposure to cosmic radiation can be assessed by calculation with codes validated by measurements. However, the relationship between doses in the free atmosphere, as calculated by the codes and from results of measurements performed within the aircraft, is still unclear. The response of a tissue-equivalent proportional counter (TEPC) has already been simulated successfully by the Monte Carlo transport code FLUKA. Absorbed dose rate and ambient dose equivalent rate distributions as functions of lineal energy have been simulated for several reference sources and mixed radiation fields. The agreement between simulation and measurements has been well demonstrated. In order to evaluate the influence of aircraft structures on aircrew exposure assessment, the response of TEPC in the free atmosphere and on-board is now simulated. The calculated results are discussed and compared with other calculations and measurements.     

用于空间辐射环境探测的金刚石探测器研究综述

空间辐射环境探测可减轻或避免辐射环境对航天器和宇航员的危害,已成为近年来各航天大国研究空间环境的热点.对空间辐射环境进行探测的探测器较多,包括气体探测器、闪烁体探测器和半导体探测器,半导体探测器具有能量分辨率高、探测效率高等优点,已逐渐取代其他两种探测器.金刚石辐射探测器是半导体探测器的一种,具有探测精度高、耐候性好、无需制冷、寿命长以及抗辐射能力强等优点,特别适合长周期、强辐射的深空探测.同时,金刚石禁带宽度大,不响应可见光,可实现日盲观测,已被欧洲空间局(ESA)用于太阳紫外辐射探测.俄罗斯工业技术中心在多种粒子复合探测方面正在研制宇宙射线光谱仪,尽管探测能区集中在中高能,但该光谱仪可实现电子、质子和重粒子的复合探测.基于目前金刚石辐射探测器在单粒子辐射探测中的应用及空间复杂的多种粒子辐射环境,我国的空间辐射环境探测研究应通过设计基于多层金刚石膜的单粒子辐射探测器来提高探测器的能量分辨率,再构建探测器矩阵进行多种粒子复合探测,将人工神经网络算法引入数据处理过程,以拓展探测范围到低能区,实现全能量范围粒子的探测,从而为开展金刚石探测器在空间站、深空辐射环境探测等领域的应用探索奠定基础.     

The energy spectrum of cosmic-ray induced neutrons measured on an airplane over a wide range of altitude and latitude

Crews of high-altitude aircraft are exposed to radiation from galactic cosmic rays (GCRs). 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 airplane. The primary instrument was a sensitive extended-energy multisphere neutron spectrometer. Its detector responses were calculated for energies up to 100 GeV using the radiation transport code MCNPX 2.5.d with improved nuclear models and including the effects of the airplane structure. New calculations of GCR-induced particle spectra in the atmosphere were used to correct for spectrometer counts produced by protons, pions and light nuclear ions. Neutron spectra were unfolded from the corrected measured count rates using the deconvolution code MAXED 3.1. The 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 cut-off agree well with results from recent calculations of GCR-induced neutron spectra.