1. Microdosimetry and radiation biophysics.
The long term goal of our studies is to develop both theoretical and experimental tools for modelling of radiation induced DNA damage. The results achieved are briefly summarized below.
Radiation track structure represents the spatial distribution of energy depositions which can be obtained by Monte Carlo simulation of charged particle tracks. One of the main problems in radiation track structure theory is a proper classification of tracks. We have developed a new approach characterizing the structure of radiation tracks by means of clusters of ionizations formed at nanometer regions of charged particle tracks [1.1, 1.2, 1.3]. In collaboration with Chelyabinsk university a Monte Carlo code simulating charged particle tracks in liquid water has been developed. This is a prerequisite for a detailed stochastic study of the chemical development of track structure.
Recently we have started to simulate physico-chemical and chemical stages of radiation induced effects. A package of programs simulating the formation, diffusion and reactions of reactive species in a stochastic way has been prepared and is thoroughly tested against a broad set of available experimental data. Hydroxyl radical attack on DNA in aqueous solution has been successfully simulated using this method. The results of calculations are in a good agreement with the experimental data measured in collaboration with Centre de Biophysique Moleculaire, Orleans.
Modelling of radiation damage in DNA is a complementary source of information giving an insight to the yields of different types of damage, their composition, size and possible structural consequences. We have developed a model of DNA damage induced by radiations of various qualities [1.4] based on the cluster analysis of radiation track structure mentioned above. The model has been further used to study radiation induced double strand breaks [1.5]. The problem of double strand break complexity was addressed in collaboration with University of Gottingen. For an ab initio modelling of DNA damage based on the knowledge of physical and chemical track structure the knowledge of DNA primary and higher order structure at atomic level is indispensable. We have developed a target model of DNA molecule and nucleosome particle suitable for a stochastic modelling of radiation induced DNA damage.
Molecular mechanics studies of structural consequences of radiation damages in DNA have been initiated during 1994.
[1.1] V. Michalik: Phys. Med. Biol. 36 (1991)1001.
[1.2] V. Michalik: Radiat. Res. 134 (1993)265.
[1.3] V. Michalik: Radiat. Prot. Dosim. 52 (1994)245.
[1.4] V. Michalik: Int. J. Radiat. Biol. 62(1992)9.
2. Environmental radioactivity studies.
During the last period the laboratory of environmental dosimetry aimed at studying contamination of the ground layers of the atmosphere by global gaseous contaminants tritium and 85-Kr. After the Chernobyl accident the team engaged also in problems of 137-Cs.
85-Kr, an inert gas with long half-life, accumulates in the atmosphere. The only significant mechanism of 85-Kr depletion is radioactive decay and so it may be considered a purely atmospheric contaminant. The global 85-Kr inventory, whose estimate in 1985 was about 2.8x10E18 Bq, is increasing steadily by emissions from nuclear fuel-reprocessing plants. The highest concentrations are observed in ground air layers of the Northern hemisphere at medium latitudes. Local 85-Kr levels depend on the climatic conditions of dispersion in the region. Propagation in the global atmosphere is determined by the participation of this radionuclide in process of general atmospheric circulation. The investigation of air contamination by 85-Kr in our laboratories was initiated in 1983. Since that, the volume activities of this radionuclide in ground air at the locality of the capital Prague have been measured periodically [2.1].
After the planned start-up in 1996-97, the nuclear power plant Temelin will be an important local source of contamination of the environment by tritium. In 1991 a station-network for monitoring volume activities of tritium in natural water's samples was established. The aim of this project was to create a data base on the tritium activity in tritiated water, which is radiologically the most serious tritium compound. The data on the tritium background activities, their long-term trend and the scope of their fluctuations will make it possible to evaluate the impact of the nuclear power plant on the environment. Data on the current tritium activities are important also for the study and a deeper insight into transport effects in the atmosphere and the hydrosphere. Results of monitoring have shown that the mean annual volume activities of tritium varied within 1.6 - 2.8 Bq.l^-1 and slowly decreasing trend in volume activities has been observed. During annual course characteristic spring - summer maxima were measured, which can be explained by spring arise of the tropopause at northern latitudes and by injection of stratospheric air into lower layers of the atmosphere.
Radiocesium in ground layer atmospheric aerosol and fallout in Prague has been examined after the Chernobyl accident. Results from 1986 to 1993 are presented in [2.2]. Since 1993 these measurements have been the part of CEC grant project No. FI3P-CT92-0038 ``Resuspension and deposition of the Cs-137 in urban environment'', coordinated by GSF Neuherberg. Resuspension and deposition are the most important processes determining levels of long-lived radionuclides in the atmosphere, especially in a longer time after their uncontrolled release into the environment. The aim of this project is to study resuspension and deposition of 137-Cs of Chernobyl origin in local conditions of a big town agglomeration, location concentrating greatest part of the potentially exposed population.
[2.1] L. Wilhelmova, M. Tomasek, K. Stukheil: J. Radioanal. Nucl.
Chem., Letters 144}(1990)125.
[2.2] K. Rybacek, L. Wilhelmova, M. Tomasek: J. Radioanal. Nucl. Chem., Letters 186(1994)245.
3. Metrology of high energy gamma-ray and electron beams.
Since 1977 the Institute of Radiation Dosimetry is accredited by the Office for Normalization, Measurement and Testing (ONMT) as the bearer of the primary standard of exposure from gamma-rays. This accreditation has been later extended to the unit of kerma in air.
The laboratory regularly participates in the international comparisons of standards. The measured difference - 0.08% between our standard and the standard of Bureau International du Poids et des Mesures (Sevres, France) is very satisfactory [3.1].
In cooperation with the Institute of Radiation Oncology a secondary standard dosimetry laboratory has been built. Also this laboratory has been accredited by ONMT. The accreditation governs also the unit of absorbed dose in water (a secondary standard).
A water calorimeter which should serve as a primary standard of absorbed dose in water should be finished in 1994.
[3.1] A. M. Perroche, M. Boutillon, I. Kovar, R. Wagner: Comparison of the Air Kerma Standards of the UDZ and the BIPM for Co-60 Radiation. Rapport BIPM 93/1, Sevres, January 1993.
4. High energy particles dosimetry.
The dosimetry of high energy particles represents an important problem not only at the Earths' surface (high energy accelerators radiation environment) but also in near Earths surroundings, both for air transport as well as for the orbital vehicles (stations, satellites, etc.). High energy particles interactions with the matter differ, both quantitatively and qualitatively, from the interactions of lower energies (E < 50 MeV) ionizing radiation. The dosimetry of high energy particles needs qualitatively new approaches, new interpretation procedures for common dosimetric equipment and methods have to be also elaborated.
All these problems have been investigated in the previous Institute of Radiation Dosimetry since many years, particularly in the collaboration with Joint Institute of Nuclear Research at Dubna and with the Institute of Medico-Biological Problems in Moscow, both in the Earths' and on the board and/or outside of satellites. The most important results obtained recently are following:
[4.1] F. Spurny: Radiat. Prot. Dosimetry 44(1992)397.
[4.2] A.M. Marennyj, J. Charvat, F. Spurny et al.: Nucl. Tracks Radiat. Measur. 19(1991)697.
[4.3] F. Spurny, I. Votockova: Nucl. Tracks Measur. 20 (1992)171.
5. X-ray spectrometry.
The X-ray spectrometry system using a planar Ge detector and an evaluation software had been developed. In cooperation with the National Health Institute in Prague the X-ray spectra have been measured for the beams with different filtrations and voltages up to 400 kV. Problems connected with a high photon fluence rate for the beams with lower filtrations have been solved out in cooperation with the Radiation Physics Department, University of Linkoping, Sweden. Their Compton spectrometer enabled us to measure the beams even in the case when a direct method of measurement had failed. A catalogue of the X-ray spectra has been prepared [5.1] which can be used for different applications requiring a detail knowledge of the spectral distribution of the beam. The standard X-ray beams have been developed. They fulfil the ISO specification for the beams used for the calibration of dosemeters. Conversion coefficients between the air kerma and operational quantities defined by ICRU 39 have been calculated. The results show differences up to 10% from the values obtained on the basis of the mean or effective energy for energies less than 60 keV [5.2].
[5.1] F. Pernicka et al: Catalogue of X-ray spectra,
Report IRD AS CR 339/91, Prague, August 1991.
[5.2] F. Pernicka et al: Radiat. Prot. Dosim. 40(1992)129.
6. Aircrew exposure to cosmic radiation.
ICRP 60 (1990 International Commission for Radiological Protection Recommendations, Annals of ICRP 21(1991), No. 1-3) recommends including of aircrew members between personnel whose ionizing radiation exposure has to be regularly followed and administered. Cosmic ray field at flying altitudes of civil aircraft is very complex and, consequently, there are many open problems regarding the dosimetry and correct interpretation of measured data.
All these problems have been studied since 1991, both on the board of cosmic aircraft as well as behind the shielding of high energy particle beams at JINR Dubna and CERN. On the board of CSA aircraft more than 25 in flight measurements have been performed. Flight routes extended from the equator up to 66 N, flying altitudes varied from 8.2 to 12.5 km. First measurements were performed in April 1991, not too far after the sun maximum of the 22nd cycle, they are still continuing now, when the sun minimum will be reached (end 1995). The results obtained at the period just after the solar maximum (i.e. close to minimum expected values) can be characterized in the following way [6.1, 6.2]:
[6.1] V. Michalik, F. Pernicka, F. Spurny, I. Votockova, V.D. Nguyen:
Radiat. Prot. Dosim. 54(1994)255.
[6.2] F. Spurny et al.: Radiat. Prot. Dosim. 48(1993)73.
[6.3] F. Spurny, M. Kralik, M.M. Beljukov, N.D. Smirnova: Radiat. Prot. Dosimetry 46(1993)281.
[6.4] F. Spurny, I. Votockova, V.P. Bamblevskij: Nucl. Tracks Radiat. Measur. 23(1994)251.