Radiation environment in high-altitude Antarctic plateau: Recent measurements and model studies.

Polar regions are the most exposed to secondary particles and radiation produced by primary cosmic rays in the atmosphere, because naturally they are with marginal geomagnetic shielding. In addition, the secondary particle flux contributing to the complex radiation field is enhanced at high-mountain altitudes compared to sea level because of the reduced atmospheric attenuation. At present, there are very few systematic experimental measurements of environmental dose at high southern latitudes, specifically at high-altitude region. Here, we report a campaign of measurements with different devices, that is passive and Liulin-type dosimeters, of the radiation background at high-mountain Antarctic station Vostok (3488 m above sea level, 78° 27' S; 106° 50' E). We compare the measurements with a Monte Carlo-based model for the propagation of the cosmic rays through the atmosphere and assessment of the radiation field in the atmosphere. We employed the model to estimate the radiation dose at Vostok station during the ground-level enhancement at 28 October 2021. As in previous studies by other teams, we show that the annual dose equivalent at high-altitude Antarctic facilities can significantly exceed the limit of 1 mSv established for the general population by the ICRP.

RESPONSE OF THE CZECH RMN NETWORK TO THUNDERSTORM ACTIVITY.

Thunderstorm ground enhancement (TGE) is a phenomenon that enhances radiation background on the ground related to thunderstorm activity and charge structure of the thundercloud. On the other hand, the rise of gamma background is connected with precipitation by the washout of radon progeny from the atmosphere. In our analysis, we examined known enhancements of gamma background, previously attributed solely to radon progeny, using data from the Czech Radiation Monitoring Network (RMN) to investigate the enhancements with respect to thunderstorms and TGE phenomena. We also used radar precipitation data and data from the lightning location network to analyze their influences on the radiation background enhancement during three thunderstorm events that occurred in summer 2016 over the Czech Republic (Central Europe). We state that the RMN might have detected TGE over the Czech Republic.

Dose distribution of secondary radiation in a water phantom for a proton pencil beam-EURADOS WG9 intercomparison exercise.

Systematic 3D mapping of out-of-field doses induced by a therapeutic proton pencil scanning beam in a 300 × 300 × 600 mm3 water phantom was performed using a set of thermoluminescence detectors (TLDs): MTS-7 (7LiF:Mg,Ti), MTS-6 (6LiF:Mg,Ti), MTS-N (natLiF:Mg,Ti) and TLD-700 (7LiF:Mg,Ti), radiophotoluminescent (RPL) detectors GD-352M and GD-302M, and polyallyldiglycol carbonate (PADC)-based (C12H18O7) track-etched detectors. Neutron and gamma-ray doses, as well as linear energy transfer distributions, were experimentally determined at 200 points within the phantom. In parallel, the Geant4 Monte Carlo code was applied to calculate neutron and gamma radiation spectra at the position of each detector. For the cubic proton target volume of 100 × 100 × 100 mm3 (spread out Bragg peak with a modulation of 100 mm) the scattered photon doses along the main axis of the phantom perpendicular to the primary beam were approximately 0.5 mGy Gy-1 at a distance of 100 mm and 0.02 mGy Gy-1 at 300 mm from the center of the target. For the neutrons, the corresponding values of dose equivalent were found to be ~0.7 and ~0.06 mSv Gy-1, respectively. The measured neutron doses were comparable with the out-of-field neutron doses from a similar experiment with 20 MV x-rays, whereas photon doses for the scanning proton beam were up to three orders of magnitude lower.

COMPARISON OF RESPONSE OF PASSIVE DOSIMETRY SYSTEMS IN SCANNING PROTON RADIOTHERAPY-A STUDY USING PAEDIATRIC ANTHROPOMORPHIC PHANTOMS.

Proton beam therapy has advantages in comparison to conventional photon radiotherapy due to the physical properties of proton beams (e.g. sharp distal fall off, adjustable range and modulation). In proton therapy, there is the possibility of sparing healthy tissue close to the target volume. This is especially important when tumours are located next to critical organs and while treating cancer in paediatric patients. On the other hand, the interactions of protons with matter result in the production of secondary radiation, mostly neutrons and gamma radiation, which deposit their energy at a distance from the target. The aim of this study was to compare the response of different passive dosimetry systems in mixed radiation field induced by proton pencil beam inside anthropomorphic phantoms representing 5 and 10 years old children. Doses were measured in different organs with thermoluminescent (MTS-7, MTS-6 and MCP-N), radiophotoluminescent (GD-352 M and GD-302M), bubble and poly-allyl-diglycol carbonate (PADC) track detectors. Results show that RPL detectors are the less sensitive for neutrons than LiF TLDs and can be applied for in-phantom dosimetry of gamma component. Neutron doses determined using track detectors, bubble detectors and pairs of MTS-7/MTS-6 are consistent within the uncertainty range. This is the first study dealing with measurements on child anthropomorphic phantoms irradiated by a pencil scanning beam technique.

Measurement of stray neutron doses inside the treatment room from a proton pencil beam scanning system.

PURPOSE: To measure the environmental doses from stray neutrons in the vicinity of a solid slab phantom as a function of beam energy, field size and modulation width, using the proton pencil beam scanning (PBS) technique. METHOD: Measurements were carried out using two extended range WENDI-II rem-counters and three tissue equivalent proportional counters. Detectors were suitably placed at different distances around the RW3 slab phantom. Beam irradiation parameters were varied to cover the clinical ranges of proton beam energies (100-220MeV), field sizes ((2×2)-(20×20)cm2) and modulation widths (0-15cm). RESULTS: For pristine proton peak irradiations, large variations of neutron H∗(10)/D were observed with changes in beam energy and field size, while these were less dependent on modulation widths. H∗(10)/D for pristine proton pencil beams varied between 0.04µSvGy-1 at beam energy 100MeV and a (2×2)cm2 field at 2.25m distance and 90° angle with respect to the beam axis, and 72.3µSvGy-1 at beam energy 200MeV and a (20×20) cm2 field at 1m distance along the beam axis. CONCLUSIONS: The obtained results will be useful in benchmarking Monte Carlo calculations of proton radiotherapy in PBS mode and in estimating the exposure to stray radiation of the patient. Such estimates may be facilitated by the obtained best-fitted simple analytical formulae relating the stray neutron doses at points of interest with beam irradiation parameters.

Investigation on contribution of neutron monitor data to estimation of aviation doses.

Recently, many efforts have appeared to routinely measure radiation exposure (RE) of aircraft crew due to cosmic rays (CR). On the other hand real-time CR data measured with the ground based neutron monitors (NMs) are collected worldwide and available online. This is an opportunity for comparison of long-term observations of RE at altitudes of about 10km, where composition and energy spectra of secondary particles differ from those on the ground, with the data from NMs. Our contribution presents examples of such type of comparison. Analysis of the silicon spectrometer Liulin measurements aboard aircraft is presented over the period May-September 2005 and compared with data from a single NM at middle latitude. While extreme solar driven events observed by NMs have clearly shown an impact on dosimetric characteristics as measured on the airplanes, the transient short time effects in CR of smaller amplitude have been not studied extensively in relation to RE. For the period May-September 2005, when aircraft data become available and several Forbush decreases (FDs) are observed on the ground, a small improvement in the correlation between the dose measured and multiple linear regression fit based on two key parameters (altitude and geomagnetic cut-off rigidity), is obtained, if the CR intensity at a single NM is added into the scheme.

Overview of the Liulin type instruments for space radiation measurement and their scientific results.

Ionizing radiation is recognized to be one of the main health concerns for humans in the space radiation environment. Estimation of space radiation effects on health requires the accurate knowledge of the accumulated absorbed dose, which depends on the global space radiation distribution, solar cycle and local shielding generated by the 3D mass distribution of the space vehicle. This paper presents an overview of the spectrometer-dosimeters of the Liulin type, which were developed in the late 1980s and have been in use since then. Two major measurement systems have been developed by our team. The first one is based on one silicon detector and is known as a Liulin-type deposited energy spectrometer (DES) (Dachev et al., 2002, 2003), while the second one is a dosimetric telescope (DT) with two or three silicon detectors. The Liulin-type instruments were calibrated using a number of radioactive sources and particle accelerators. The main results of the calibrations are presented in the paper. In the last section of the paper some of the most significant scientific results obtained in space and on aircraft, balloon and rocket flights since 1989 are presented.

Measurement of stray radiation within a scanning proton therapy facility: EURADOS WG9 intercomparison exercise of active dosimetry systems.

PURPOSE: To characterize stray radiation around the target volume in scanning proton therapy and study the performance of active neutron monitors. METHODS: Working Group 9 of the European Radiation Dosimetry Group (EURADOS WG9-Radiation protection in medicine) carried out a large measurement campaign at the Trento Centro di Protonterapia (Trento, Italy) in order to determine the neutron spectra near the patient using two extended-range Bonner sphere spectrometry (BSS) systems. In addition, the work focused on acknowledging the performance of different commercial active dosimetry systems when measuring neutron ambient dose equivalents, H(∗)(10), at several positions inside (8 positions) and outside (3 positions) the treatment room. Detectors included three TEPCs--tissue equivalent proportional counters (Hawk type from Far West Technology, Inc.) and six rem-counters (WENDI-II, LB 6411, RadEye™ NL, a regular and an extended-range NM2B). Meanwhile, the photon component of stray radiation was deduced from the low-lineal energy transfer part of TEPC spectra or measured using a Thermo Scientific™ FH-40G survey meter. Experiments involved a water tank phantom (60 × 30 × 30 cm(3)) representing the patient that was uniformly irradiated using a 3 mm spot diameter proton pencil beam with 10 cm modulation width, 19.95 cm distal beam range, and 10 × 10 cm(2) field size. RESULTS: Neutron spectrometry around the target volume showed two main components at the thermal and fast energy ranges. The study also revealed the large dependence of the energy distribution of neutrons, and consequently of out-of-field doses, on the primary beam direction (directional emission of intranuclear cascade neutrons) and energy (spectral composition of secondary neutrons). In addition, neutron mapping within the facility was conducted and showed the highest H(∗)(10) value of ∼ 51 µSv Gy(-1); this was measured at 1.15 m along the beam axis. H(∗)(10) values significantly decreased with distance and angular position with respect to beam axis falling below 2 nSv Gy(-1) at the entrance of the maze, at the door outside the room and below detection limit in the gantry control room, and at an adjacent room (<0.1 nSv Gy(-1)). Finally, the agreement on H(∗)(10) values between all detectors showed a direct dependence on neutron spectra at the measurement position. While conventional rem-counters (LB 6411, RadEye™ NL, NM2-458) underestimated the H(∗)(10) by up to a factor of 4, Hawk TEPCs and the WENDI-II range-extended detector were found to have good performance (within 20%) even at the highest neutron fluence and energy range. Meanwhile, secondary photon dose equivalents were found to be up to five times lower than neutrons; remaining nonetheless of concern to the patient. CONCLUSIONS: Extended-range BSS, TEPCs, and the WENDI-II enable accurate measurements of stray neutrons while other rem-counters are not appropriate considering the high-energy range of neutrons involved in proton therapy.

Calibration of modified Liulin detector for cosmic radiation measurements on-board aircraft.

The annual effective doses of aircrew members often exceed the limit of 1 mSv for the public due to the increased level of cosmic radiation at the flight altitudes, and thus, it is recommended to monitor them. Aircrew dosimetry is usually performed using special computer programs mostly based on results of Monte Carlo simulations. Contemporary, detectors are used mostly for validation of these computer codes, verification of effective dose calculations and for research purposes. One of such detectors is active silicon semiconductor deposited energy spectrometer Liulin. Output quantities of measurement with the Liulin detector are the absorbed dose in silicon D and the ambient dose equivalent H*(10); to determine it, two calibrations are necessary. The purpose of this work was to develop a calibration methodology that can be used to convert signal from the detector to D independently on calibration performed at Heavy Ion Medical Accelerator facility in Chiba, Japan.

Radiation environment at aviation altitudes and in space.

On the Earth, protection from cosmic radiation is provided by the magnetosphere and the atmosphere, but the radiation exposure increases with increasing altitude. Aircrew and especially space crew members are therefore exposed to an increased level of ionising radiation. Dosimetry onboard aircraft and spacecraft is however complicated by the presence of neutrons and high linear energy transfer particles. Film and thermoluminescent dosimeters, routinely used for ground-based personnel, do not reliably cover the range of particle types and energies found in cosmic radiation. Further, the radiation field onboard aircraft and spacecraft is not constant; its intensity and composition change mainly with altitude, geomagnetic position and solar activity (marginally also with the aircraft/spacecraft type, number of people aboard, amount of fuel etc.). The European Union Council directive 96/29/Euroatom of 1996 specifies that aircrews that could receive dose of >1 mSv y(-1) must be evaluated. The dose evaluation is routinely performed by computer programs, e.g. CARI-6, EPCARD, SIEVERT, PCAire, JISCARD and AVIDOS. Such calculations should however be carefully verified and validated. Measurements of the radiation field in aircraft are thus of a great importance. A promising option is the long-term deployment of active detectors, e.g. silicon spectrometer Liulin, TEPC Hawk and pixel detector Timepix. Outside the Earth's protective atmosphere and magnetosphere, the environment is much harsher than at aviation altitudes. In addition to the exposure to high energetic ionising cosmic radiation, there are microgravity, lack of atmosphere, psychological and psychosocial components etc. The milieu is therefore very unfriendly for any living organism. In case of solar flares, exposures of spacecraft crews may even be lethal. In this paper, long-term measurements of the radiation environment onboard Czech aircraft performed with the Liulin since 2001, as well as measurements and simulations of dose rates on and outside the International Space Station were presented. The measured and simulated results are discussed in the context of health impact.

Microdosimetry for a carbon ion beam using track-etched detectors.

Track-etched detectors (TED) have been used as linear energy transfer (LET) spectrometers in heavy ion beams for many years. LET spectra and depth-dose distribution of a carbon ion beam were measured behind polymethylmethacrylate degraders at Heavy Ion Medical Accelerator in Chiba, Japan. The measurements were performed along monoenergetic beam with energy 290 MeV u(-1) in different positions: (1) at beam extraction area, (2) at beginning, (3) maximum and (4) behind the Bragg peak region (0, 117, 147 and 151 mm of water-equivalent depth, respectively). The LET spectra inside and outside of the primary ion beam have been evaluated. TED record only heavy charged particles with LET above 8-10 keV µm(-1), while electrons and ions with lower LET are not detected. The Geant4 simulation toolkit version 4.9.6.P01 has been used to estimate the contribution of non-detected particles to absorbed dose. Presented results demonstrate the applicability of TED for microdosimetry measurements in therapeutic carbon ion beams.

Measurement of dose equivalent distribution on-board commercial jet aircraft.

The annual effective doses of aircrew members often exceed the limit of 1 mSv for the public due to the increased level of cosmic radiation at the flight altitudes, and thus, it is recommended to monitor them [International Commission on Radiation Protection. 1990 Recommendations of the International Commission on Radiological Protection. ICRP Publication 60. Ann. ICRP 21: (1-3), (1991)]. According to the Monte Carlo simulations [Battistoni, G., Ferrari, A., Pelliccioni, M. and Villari, R. Evaluation of the doses to aircrew members taking into consideration the aircraft structures. Adv. Space Res. 36: , 1645-1652 (2005) and Ferrari, A., Pelliccioni, M. and Villari, R. Evaluation of the influence of aircraft shielding on the aircrew exposure through an aircraft mathematical model. Radiat. Prot. Dosim. 108: (2), 91-105 (2004)], the ambient dose equivalent rate H*(10) depends on the location in the aircraft. The aim of this article is to experimentally evaluate H*(10) on-board selected types of aircraft. The authors found that H*(10) values are higher in the front and the back of the cabin and lesser in the middle of the cabin. Moreover, total dosimetry characteristics obtained in this way are in a reasonable agreement with other data, in particular with the above-mentioned simulations.

Spectra of linear energy transfer and other dosimetry characteristics as measured in C290 MeV/n MONO and SOBP ion beams at HIMAC-BIO (NIRS, Japan) with different detectors.

Active mobile dosimetry unit (Liulin), passive plastic nuclear track detectors (PNTD) and thermoluminescent detectors (TLD) were exposed in a C290 MeV/n beam at HIMAC-BIO (NIRS, Japan). Two different types of beam configuration were used--monoenergetic beam (MONO) and spread-out Bragg peak (SOBP); the detectors were placed at several depths from the entrance up to the depths behind the Bragg peak. Relative response of TLDs in beams has been studied as a function of the depth, and it was re-proved that it can depend on the linear energy transfer (LET). Liulin measures energy deposition in Si; the spectra of energy deposited in Si can be transformed to the spectra of lineal energy or LET. PNTDs are able to determine the LET of registered particles directly. The limitation of both methods is in the range in which they can determine the LET-Liulin is able to measure perpendicularly incident charged particles up to ∼35 keV/µm (in water), PNTD can measure from ∼7 to 400 keV/µm, independently of the registration angle. The results from both methods are compared and combined for both beams' configuration, and a good agreement is observed.

Liulin-type spectrometry-dosimetry instruments.

The main purpose of Liulin-type spectrometry-dosimetry instruments (LSDIs) is cosmic radiation monitoring at the workplaces. An LSDI functionally is a low mass, low power consumption or battery-operated dosemeter. LSDIs were calibrated in a wide range of radiation fields, including radiation sources, proton and heavy-ion accelerators and CERN-EC high-energy reference field. Since 2000, LSDIs have been used in the scientific programmes of four manned space flights on the American Laboratory and ESA Columbus modules and on the Russian segment of the International Space Station, one Moon spacecraft and three spacecraft around the Earth, one rocket, two balloons and many aircraft flights. In addition to relative low price, LSDIs have proved their ability to qualify the radiation field on the ground and on the above-mentioned carriers.

Use of energy deposition spectrometer Liulin for individual monitoring of aircrew.

Silicon energy deposition spectrometer Liulin was primarily developed for cosmic radiation monitoring onboard spacecrafts. Nowadays, Liulin type detectors are also used to characterise radiation field on board aircraft, at alpine observatories and behind the shielding of heavy ion accelerators. In this work, experiments and calibrations performed in these radiation fields are presented and the method developed for calculation of ambient dose equivalent H*(10) on board aircraft is described. Since 2001, a simple method employing the energy deposition spectra had been used to determine H*(10) on board aircraft but, in 2004, it became clear that the resulting values were strongly biased at locations close to Earth's equator. An improved method for the determination of H*(10) on board aircraft using the Liulin detector was developed. It took into account the composition of the radiation field via the ratio of absorbed doses D(low) and D(neut) reflecting the contributions from low-LET particles and neutrons, respectively. It resulted in much better agreement with the EPCARD computer code for all aircraft locations; relative differences were within 11 % for low-LET and 20 % for neutron components of H*(10).

Characterization of the radiation environment by Liulin-type spectrometers.

Liulin-type spectrometers can characterise the type of predominant particles and their energy in the radiation environment. The results from calibrations and space and aircraft experiments revealed that the most informative is by the shape of the deposited energy spectrum. Spectra generated by galactic cosmic rays (GCR) protons and their secondaries look like straight lines in the coordinates deposited energy/deposited per channel dose rate. The position of the maximum of the deposited energy spectra depends on the incident energy of the incoming protons. Spectra generated by relativistic electrons in the outer radiation belt have a maximum in the first channels. For higher energy depositions, these spectra are similar to the GCR spectra. All types of spectra have a knee close to 6.3 MeV of deposited energy, which corresponds to the stopping energy of protons in the detector.

Routine individual monitoring of aircraft crew exposure; Czech experience and results 1998-2008.

Individual monitoring of aircrew of airline operators registered in the Czech Republic has been performed since 1998. In this work, annual effective doses and annual collective effective doses of aircrew from occupational exposure in the period from 1998 to 2008 are presented, methods used for their evaluation and verification are described, and general trends observed in the data are discussed. Annual effective doses were calculated using the computer code CARI from flight schedules provided by airline operators and typical flight profiles. The method was verified via a comparison with (i) measurements using different types of detectors and (ii) calculations using the CARI and EPCARD codes with actual flight data. It was found that average annual effective doses in the period from 1998 to 2008 were in the range from 1.2 to 2.0 mSv and followed the trend of the solar cycle. Annual collective effective doses increased from 1.4 manSv in 1998 to 4.1 manSv in 2008 as the number of aircrew increased from 857 to 2158 during this period. Combined relative uncertainties (coverage factor ) of reported individual and collective effective doses were ∼ 25 %, which is well within the range given by the factor of 1.5. More work is needed to achieve a higher accuracy of this estimate.

On the neutron contribution to the exposure level onboard space vehicles.

The neutron contribution to the spacecraft crew exposure could represent an important part of the total dose equivalent value. The determination of this contribution represents a rather complex and difficult task, both through experimental and theoretical estimation. This paper will present an attempt to determine the neutron contribution onboard the International Space Station and Foton capsule using the data measured by means of a Si-diode based energy deposition spectrometer. As such a spectrometer, the MDU-Liulin equipment, developed in one of our laboratories was used. The equipment allows the data accumulated during the passage in or out of the South Atlantic Anomaly (SAA). In this paper, only the data obtained out of the SAA were analysed, assuming that the neutron spectra are similar to those onboard aircraft and/or at the CERF high-energy radiation field. The excess of deposited energy in the region above 1 MeV, when comparing with the aircraft field, was expected to represent the primary high-energy charged particles. Total dosimetry characteristics obtained in this way are in reasonable agreement with other data, neutron contribution representing approximately 40% of the total dose equivalent for the flight duration outside of the SAA.