Biological dosimetry of mixed neutron-γ ray field

Radiation induced unstable chromosome aberration frequency (dicentrics and centric rings) in peripheral lymphocytes is a powerful tool for early biological dose assessment in cases of accidental overexposures. Occupational accidents or overexposures of people to ionizing radiation usually involve low LET sources (X-rays or γ-rays). A criticality accident, less frequent but more complex in its evaluation, involves a radiation emission of a mixed neutron and γ-ray field. From the observed frequency of unstable chromosome aberrations it is not possible to discriminate between those aberrations due to γ-rays and those due to neutrons. However, if the γ-ray/neutron ratio is known from physical dosimetry measurements it is possible to estimate both components of the total dose applying an iterative method and assuming that both radiation qualities are additive in inducing chromosome aberrations and the aberrations do not show overdispersion compared to Poisson distribution. In order to evaluate the efficiency of inducing unstable chromosome aberration of a neutron- γ ray field and the estimation of both components of the total dose using unstable chromosome aberration frequency as end point, we have performed three in vitro blood sample irradiations in a suitable RA1 research reactor facility, near to the core, using a specially designed device. Physical γ dose assessments were performed using thermoluminescent dosimeters (TLD 700). Neutron doses were calculated using the spectra obtained by WIMS and ANISN codes. Spectrometric calculi in the fast energy zone (the one with higher Kerma factor) were validated with fluency measurements by activation using In detectors (In115(n,n)In115m). Doses corresponding to thermal, intermediate and fast energy zones were calculated using blood equivalent tissue Kerma factors from ICRU 44. The measurements indicates that fast neutrons contribute mainly to the total neutron dose. Biological dose estimation of the γ and neutron component was performed using the Co60 γ-ray (our laboratory) and fission neutrons (Lloyd, 1986) in vitro dose response curves and applying the iterative process described in the IAEA Report (1986). Physical and biological dose estimations do not differ significantly, indicating that it is appropriate the use of a fission neutron dose response curve for the biological dose assessment in this overexposure scenario. (author)