The response of tissue-equivalent proportional counters to heavy ions

The paper presents a theoretical model for the response of a tissue-equivalent proportional counter (TEPC) irradiated with charged particles. Heavy ions and iron ions in particular constitute a significant part of radiation in space. TEPCs are used for all space shuttle and International Space Station (ISS) missions to estimate the dose and radiation quality (in terms of lineal energy) inside spacecraft. The response of the tissue-equivalent proportional counters shows distortions at the wall/cavity interface. In this paper, we present microdosimetric investigation using Monte Carlo track structure calculations to simulate the response of a TEPC to charged particles of various LET (1 MeV protons, 2.4 MeV alpha particles, 46 MeV/nucleon 20Ne, 55 MeV/nucleon 20Ne, 45 MeV/nucleon 40Ar, and 1.05 GeV/nucleon 56Fe). Data are presented for energy lost and energy absorbed in the counter cavity and wall. The model calculations are in good agreement with the results of Rademacher et al. (Radiat. Res. 149, 387-389, 1998), including the study of the interface between the wall and the sensitive region of the counter. It is shown that the anomalous response observed at large event sizes in the experiment is due to an enhanced entry of secondary electrons from the wall into the gas cavity.     

The low dose and low dose rate cytogenetic effects induced by gamma-radiation in human blood lymphocytes in vitro. I. the results of physical and dosimetric study

Physical and dosimetric characteristics of the gamma-radiation field which is formed in the room containing the 60Co radiation source were investigated on condition of an unclosed breech mechanism and the presence of the lead layer on the beam pathway. The inverse square law was approximately found for the dependence of the dose rate vs distance on the radiation source both for the unclosed breech mechanism and lead layer (4 and 9 cm wide) in the beam pathway. This finding indicated a non-significant contribution of the radiation scattered from the walls at the point of cytogenetic experiments. The Monte Carlo calculations showed that some changes in the efficient spectrum of gamma-radiation resulted in a decreased average energy of 60Co gamma-rays to 1.03, 1.17 and 1.07 MeV for the unclosed breech mechanism, behind 4 and 9 cm lead layers, respectively.     

The comparison of molecular-biochemical and cytogenetic analyses of blood cells of patients long after the acute radiation sickness

Molecular-biochemical and cytogenetic analyses were made on blood cells of 17 radiation accident victims who, from 1.7 to 43.8 years previously, had suffered acute radiation sickness (ARS) ranging from severity grades I to IV. Molecular-biochemical data were obtained with patients' leukocytes and with mononuclear cells on their oxidative status by a) the level of an anion-radical O2*- in the 3-(4,5-dimethyl-2-thiazol-2-yl)-2,5-diphenyl-2-tetrazolium bromide (MTT) test and b) the sum of reactive oxygen species in the 2,7-dichlorodihydrofluorescein diacetate (DCFDA) test together with a test for DNA strand unwinding in alkaline conditions by measurements of fluorescence intensity of ethidium bromide. Each parameter was measured in freshly sampled cells and during a 5-hour incubation as absolute means on 5 measurements and in % change from the initial values. Cytogenetic data were obtained from the standard metaphase preparations scored for routine unstable chromosomal aberrations (us-CA)--dicentrics; and stable aberrations (sCA)--translocations. The latter ones were assayed by the FISH method using whole chromosome 2, 4 and 12 fluorescent probes and scaled up to genome equivalence. For all patients reduced oxidative status of about 25-30% was obtained by the MTT-test (p < 0.005), and the DCFDA-test (p < 0.027). The yield of usCA depended on the time after irradiation with higher yields associated with the shorter postirradiation times and reducing almost to expected background frequencies. The yield of sCA was high for all patients, correlating with the severity of ARS whilst the molecular-biochemical parameters showed no relationship with ARS. A correlation was observed between parameters of oxidative status and % of cells with usCA: by the MTT-test r = from 0.50 up to 0.61 (p = from 0.06 up to 0.003), but by the DCFDA-test the strength of correlation was smaller: r = from 0.38 up to 0.48. An inverse correlation was found between initial oxidative state of mononuclear cells and the frequency of CA in lymphocytes. Similarly a marked inverse correlation between degree of DNA unwinding by the ethidium bromide assay on leukocytes and sCA in lymphocytes was also noted. The feasibility of radiation-induced delayed genomic instability in vivo for humans long time after irradiation is discussed.     

The consideration of biological effectiveness of low energy protons using biophysical modeling of the effects induced by exposure of V79 cells

We have applied Monte Carlo track structure simulations to estimate relative biological effectiveness (RBE) of low-energy protons using biophysical modelling of radiation effects induced by exposure of V79 cells growing in mono-layer. The microscopic energy deposition in cell nucleus and sub-nucleus volumes was investigated in order to understand the reasons of enhanced biological effectiveness near Bragg peak. Theoretical estimations of RBE based on frequency/dose average lineal energy and calculated yields of initial DNA breaks were collated with experimental RBE_M data. It was found: (1) dose average lineal energy for whole cell nucleus as a function of proton energy shows a distinct peak at 550 keV; (2) the peak values for subnucleus volumes are large compared with the whole cell nucleus; (3) the yield of complex DNA breaks correlates with experimental RBE_M data.     

Comparative investigation of structural and gene somatic mutations in workers of nuclear chemical plants. I. A study of stable and unstable chromosome aberrations

A study of frequency of unstable chromosome aberrations in 50 workers of nuclear chemical plants in remote period after beginning or finishing professional contact with ionizing radiation was carried out. 14 persons from this cohort were mainly whole-body exposed to external gamma-rays and 36 were exposed to combined external and internal radiation from incorporated Pu nuclides. In results of this irradiating practically every subject had a chronical radiation sickness. In the 1-st group the frequency of unstable aberrations varied from 0.2 to 3.6 per 100 cells and exceeded reliably control level in 5 persons. In the 2-nd group the frequency of unstable aberrations varied from 0 to 11.6 per 100 cells and exceeded reliably control level in 20 examined workers. The FISH study of frequency of stable aberrations was performed in 13 subjects who were exposed to combined external and internal radiation. Total frequency of complete and incomplete translocations varied from 0.6 to 18.5 aberrations per genome per 100 cells and reliable exceeded control level in 9 subjects. Non-random participation in exchange rearrangements (translocations) was revealed for used set of chromosomes (2, 3 and 8).     

The low dose and low dose rate cytogenetic effects induced by gamma-radiation in human blood lymphocytes in vitro. II. the results of cytogenetic study

The yield of chromosome aberrations induced by gamma-radiation of 60Co in human blood lymphocytes in vitro at low doses (30 divided by 600 mGy) and low dose rates (0.70, 5.05, 59.2 mGy/min) was investigated. It was found that the observed level of chromosomal aberrations induced by gamma-irradiation was unaffected by the value of the dose rate when using constant dose rate and obtaining different doses by altering the exposure time. However, a relatively enhanced level of chromatid aberrations was found at 5.05 and 59.2 mGy/min dose rates in the dose range less than 250 mGy. We have found that the observed level of the sum of chromosomal aberrations induced by gamma-irradiation at doses less than 250 mGy and a dose rate of 59.2 mGy/min was essentially larger compared with the level extrapolated from high doses (above 300 mGy) using a linear-quadratic dose curve. This complied with our previous finding in 1976, 1977 when the enhanced level of dicentrics was only found at a high dose rate approximately 500 mGy/min. Such a non-linear cytogenetic effect does not manifest itself statistically significantly at dose rates of 0.70 and 5.05 mGy/min for the sum of chromosomal aberrations and does not manifest itself at all for dicentrics at all the examined dose rates.