So-called thunderclouds, which are large dark clouds that are able to generate thunder and lightning, can act as natural particle accelerators, producing complex high-energy phenomena such as terrestrial gamma-ray flashes (TGFs) and gamma-ray glows. These events are often described through the mechanism of relativistic runaway electron avalanches (RREAs), cascades of high-energy electrons accelerated by atmospheric electric fields. Since the energies of the RREAs are up to several tens of MeV, they can also trigger nuclear reactions with atoms of the air and in the soil while entering the ground. Although these phenomena are intriguing, their lack of precise measurement and still not completely understood origins pose a significant challenge for assessing their impact on aviation safety. This paper introduces the project Research Centre of Cosmic Rays and Radiation Events in Atmosphere (CRREAT), aimed at providing measurements of TGFs, thunderstorm ground enhancements (TGEs), and other ionizing radiation phenomena during thunderstorms, as well as at aviation altitudes, stratosphere, and low Earth orbits (LEO). The paper argues that without accurate data on the origins and physical characteristics of TGEs and TGFs, it is impossible to reliably simulate their impact on aircraft crews and passengers. The paper also mentions how the general-purpose 3D Monte Carlo (MC) code PHITS can be used for future simulations and comparisons with measurements related to ionizing radiation phenomena in the atmosphere.

Measurements and theoretical studies show that thunderclouds can act as particle accelerators in nature. Terrestrial gamma ray flashes (TGFs) are bursts of gamma rays with energies ranging from below 10 keV to above 40 MeV, which last for microseconds to several milliseconds, and coincide with lightning produced in Earth's atmosphere. The gamma-ray spectra of TGFs are consistent with bremsstrahlung from energetic electrons going upward into space. More recently, similar phenomena but beamed downward have been discovered by ground-based observations: short bursts, “downward TGFs”, with durations of milliseconds and long bursts, “thunderstorm ground enhancements” (TGEs) or gamma ray glows, with durations up to several minutes. These findings have established a new academic field called “high-energy atmospheric physics”. The origin, and exact mechanisms creating the thunderstorm radiation (TGFs and TGEs), are still unclear but it has been suggested that TGFs are caused by intense electric fields produced above or inside thunderstorms. To explain TGF, electrons which are traveling at speeds very close to the speed of light collide with atomic nuclei in the atmosphere and release their energy in the form of X-rays (bremsstrahlung). Large populations of energetic electrons can be formed by avalanche growth driven by electric fields, a phenomenon called relativistic runaway electron avalanche (RREA). Airplane crew and passengers flying near thunderstorms could therefore be exposed to “dangerously” high levels of radiation in the form of short terrestrial y-ray flashes and little longer y-ray glows. To clarify the origin of TGFs, and to evaluate the absorbed dose of thunderstorm radiation, the project CRREAT (Research Centre of Cosmic Rays and Radiation Events in Atmosphere) measures both the lightning and the ionizing radiation during thunderstorms using detection instrument mounted on the roofs and inside of cars, and with ionizing radiation detectors installed on high mountains. One other important research area of CRREAT is measurements and simulations of the radiation field at aviation altitudes and at low earth orbits (LEO), since this poses a health risk for crew members and passenger's onboard aircraft and spacecraft. This paper gives an overview of possible sources of ionizing radiation and phenomena during thunderstorm.

Experiments in preparation for search for uranium ternary fission by means of nuclear track emulsion are summarized. The study will be focused on the possible involvement of the unstable nucleus ${}^{8}$Be in the suggested scenario of the collinear tri-partition in the fission.

Activities performed in preparation for the search for ternary fission of heavy nuclei and the analysis of fragment angular correlations with nuclear track emulsion and an automated microscope are detailed. Surface irradiation of nuclear emulsion by a Cf source was initiated. Planar events containing nothing but fragment triples were found and studied.

Application of the nuclear track emulsion technique (NTE) in radioactivity and nuclear fission studies is discussed. It is suggested to use a HSP-1000 automated microscope for searching for a collinear cluster tri-partition of heavy nuclei implanted in NTE. Calibrations of cr-particles and ion ranges in a novel NTE are carried out. Surface exposures of NTE samples to a Cf-252 source started. Planar events containing fragments and long-range cr-particles as well as fragment triples only are studied. Splittings induced by thermal neutrons are studied in boron-enriched emulsion. Use of the image recognition program "ImageJ" for obtaining characteristics of individual events and for events from the large scan area is presented.

This paper compares the performance of four different Tissue-Equivalent proportional counters (TEPC) first in standard radiation fields, with gamma and neutron sources, then in the mixed and complex/intense neutron and photon stray radiation field of a scanning proton therapy facility. The paper focuses on the dead time correction and introduces a new spectra processing methodology to enable the comparison of the four TEPCs while accounting for their different gas filling and gain, lineal energy range of the spectrum and the analysis methodology. Measurements with 137Cs and/or 60Co gamma sources demonstrate variable low-LET threshold for each TEPC while data acquired with a 252Cf neutron source show comparable response of the four TEPCs for high-LET particles. Meanwhile, in the scattered field of proton therapy, microdosimetric spectra measured at different positions and orientations around the patient show a majority of high-LET events at the smallest angle with respect to the beam axis while low-LET particles were mainly dominant at 90° from the beam axis. The introduced processing methodology led to good overlapping of microdosimetric spectra for the four systems.

It is shown that the acceleration efficiency of protons and multiply charged ions (and also the charge composition of the latter) accelerated backwards under irradiation of the front surface of thick solid targets by high-power femtosecond laser radiation with an intensity of 2 x 10(18) W cm(-2) is determined by the contrast of this radiation. Thus, highly ionised ions up to C6+, Si12+ and Mo14+ are recorded on polyethylene, silicon and molybdenum targets at a contrast of 10(-8), the ions with charges up to C5+, Si10+ and Mo10+ possessing an energy of more than 100 keV per unit charge. In the case of a metal target, the acceleration efficiency of protons is significantly reduced, which indicates cleaning of the target surface by a pre-pulse. The measurements performed at a contrast increased by two-to-three orders of magnitude show the presence of fast protons (up to 300-700 keV) on all targets, and also a decrease in the energy and maximum charge of multiply charged ions.

Application of the nuclear track emulsion technique (NTE) in radioactivity and nuclear fission studies is discussed. It is suggested to use a HSP-1000 automated microscope for searching for a collinear cluster tri-partition of heavy nuclei implanted in NTE. Calibrations of α-particles and ion ranges in a novel NTE are carried out. Surface exposures of NTE samples to a 252Cf source started. Planar events containing fragments and long-range α-particles as well as fragment triples only are studied. NTE samples are calibrated by ions Kr and Xe of energy of 1.2 and 3 A MeV.

Spectral fluence of photoneutrons generated in the head of the radiotherapeutic linac Varian 2100 C/D was measured by means of the Bonner spheres spectrometer whose active detector of thermal neutrons was replaced by a track detector, i.e. a sandwich of four CR-39s with the boron radiator inserted between them. Measurements with different collimator settings showed that the fluence of photoneutrons was higher for the more open collimator.

Onboard aircraft exposure to cosmic radiation has been monitored during the period May-December 2005. Mobile Dosimetry Unit (MDU) Si-spectrodosimeter has been used as an active measuring equipment, it has been installed onboard of an A310-300 aircraft. Some passive detectors were placed on the body of MDU. Three types of thermoluminescent detectors (TLD) were used as well as track etch detectors (TEDs). TEDs were treated both as a neutron dosimeter and as a spectrometer of linear energy transfer. Available navigation data permitted us also to calculate onboard exposure during more than 400 individual flights. MDU established and calculated data for each individual flight are compared and analyzed, from the data conclusions as to aircrew exposure are outlined. Particular attention is devoted to the influence of:geomagnetic characteristics and flight altitude on the calculated and MDU-Liulin measured dosimetry characteristic of onboard radiation field;some solar events (Forbush decreases) registered by MDU-Liulin on the level of aircraft crew exposure during monitoring period.Integral data on the exposure due to non-neutron and neutron-like component obtained on the base of MDU and calculated data are compared with the data obtained by TLDs and TEDs, respectively. Total effective dose during the period mentioned (494 flights and 2940 It between taking offs and landings) was found to be about 11-12 mSv for. Results obtained by means of different approaches are analyzed and discussed, it was found that in general good agreement of all data sets could be stated. (c) 2006 COSPAR. Published by Elsevier Ltd. All rights reserved.

The results of measurements performed during the year 2003 onboard aircraft, mostly during regular commercial flights of the Czech Airlines (CSA) are presented. The studies were performed during more than 30 individual flights, several dosemeters and equipments were used for both neutron and non-neutron components of the onboard radiation field. CSA colleagues submitted us for all flights with navigation data necessary for the calculation of onboard aircraft crew exposure with transport codes EPCARD and CARI. Direct readings of experimental equipments were corrected on the base of the calibration in CERN high-energy radiation fields. A reasonable agreement of measured and calculated data was observed. During one of the flights, a very deep Forbush decrease occurred. The experimental results confronted with calculation permitted to obtain new view on the influence of such events on aircraft crew exposure.

Aircrew exposure represents one of the recent subjects of occupational individual dosimetry. Since 1991 many new results have been found; there is however a need to gather further data on this exposure and its variation with geomagnetic position, solar activity and flight route parameters. Since 2001, many individual and six long-term monitoring programmes have been conducted onboard aircraft of Czech Airlines (CSA). In these programmes, a Si-diode spectrometer was fixed in an aircraft. Together with it, passive dosemeters thermoluminescent detector, track-etch based neutron dosemeter linear energy transfer and spectrometer) were exposed. More than 700 regular commercial flights were monitored in this manner. CSA supplied us also with full navigation data, which allowed us to calculate the exposure levels using EPCARD 3.2 and CARI6 codes. Direct experimental readings obtained with the detectors mentioned above were interpreted on the basis of calibrations in on-Earth reference fields and compared with calculated data. A satisfactory correlation between all sets of data was observed.

Cosmic rays contribute to the exposure on the Earth's surface as well as in its surroundings. At the surface and/or at aviation altitudes, there are mostly secondary particles created through the cosmic rays interaction in the atmosphere, which contribute to this type of exposure. Onboard a spacecraft, the exposure comes mostly from primary cosmic rays. Track-etched detectors (TED) are able to characterise both these types of exposure. The contribution of neutrons, of cosmic origin, on the Earth's surface was studied at altitudes from few hundreds to 3000 in using TED in a moderator sphere. The results obtained are compared with other data on this type of natural radiation background. The results of studies performed onboard aircraft and/or spacecraft are presented afterwards. We used TED-based neutron dosemeter, as well as a spectrometer of linear energy transfer based on a chemically etched TED. The results of studies performed onboard aircraft, as well as spacecraft, are presented and discussed, including an attempt to estimate a neutron component onboard the spacecraft. It was found that they correlate with the results of other independent investigations.

Photoneutron spectra around the treatment bed of a Varian Clinac 2100C machine were measured using a Bonner sphere spectrometer. To overcome problems with pulse pile-up and detection of non-neutron-induced events, the active detector of thermal neutrons normally used at the centre of the spheres was replaced by a sandwich of four CR-39 track detectors interleaved with 10B radiators. Track densities measured for the CR-39 detectors in Bonner spheres were used for the unfolding of neutron spectra. Neutron fluence and ambient dose equivalent for the whole energy range and partial energy intervals were derived from the neutron spectra.