Relative Biological Effectiveness of 144 keV Neutrons in Producing Dicentric Chromosomes in Human Lymphocytes Compared with 60Co Gamma Rays under Head-to-Head Conditions.

The RBE for neutrons was assessed in a head-to-head experiment in which cultures of lymphocytes from the same male donor were irradiated simultaneously with 144 keV neutrons and with Co-60 gamma rays as the reference radiation and evaluated using matched time, culture conditions, and the end point of chromosomal aberrations to avoid potential confounding factors that would influence the outcome of the experiment. In addition, the irradiation time was held constant at 2 h for the high-dose groups for both radiation types, which resulted in rather low dose rates. For the induction of dicentric chromosomes, the exposure to the 144 keV neutrons was found to be almost equally as effective (yield coefficient alpha(dic) = 0.786 +/- 0.066 dicentrics per cell per gray) as that found previously for irradiation with monoenergetic neutrons at 565 keV (alpha(dic) = 0.813 +/- 0.052 dicentrics per cell per gray) under comparable exposure and culture conditions (Radiat. Res. 154, 307-312, 2000). However, the values of the maximum low-dose RBE (RBEm) relative to (CO)-C-60 gamma rays that were determined in the present and previous studies show an insignificant but conspicuous difference: 57.0 +/- 18.8 and 76.0 +/- 29.5, respectively. This difference is mainly due to the difference in the alpha(dic) value of the (CO)-C-60 gamma rays, the reference radiation, which was 0.0138 +/- 0.0044 Gy(-1) in the present study and 0.0107 +/- 0.0041 Gy(-1) in the previous study. In the present experiment, irradiations with 144 keV neutrons and (CO)-C-60 gamma rays were both performed at 21degreesC, while in the earlier experiment irradiations with 565 keV neutrons were performed at 21degreesC and the corresponding reference irradiation with gamma rays was performed at 37degreesC. However, the temperature difference between 21degreesC and 37degreesC has a minor influence on the yield of chromosomal alterations and hence RBE values. The large cubic PMMA phantom that was used for the gamma irradiations in the present study results in a larger dose contribution from Compton-scattered photons compared to the mini-phantom used in the earlier experiments. The contribution of these scattered photons may explain the large value of alpha(dic) for gamma irradiation in the present study. These results indicate that the yield coefficient alpha(dic) for 144 keV neutrons is similar to the one for 565 keV neutrons, and that modification of the alpha(dic) value of the low-LET reference radiation, due to changes in the experimental conditions, can influence the RBEm. Consequently, alpha(dic) values cannot be shared between cytogenetic laboratories for the purpose of assessment of RBMm without verification of the comparability of the experimental conditions

Comparison of 24.5 keV neutron fluence measurements at the PTB

Under the auspices of the Section 3 of CCEMRI (Comite Consultatif pour les Etalons de Mesure des Rayonnements Ionisants) and with the help of the BIPM (Bureau International des Poids et Mesures), an intercomparison of fluence measurements for 24.5 keV neutrons had been organized. At the PTB, 24.5 keV fields were produced by means of the nuclear reactions "7Li(p,n)"7Be and "4"5Sc(p,n)"4"5Ti at the accelerator facility and by means of an iron filter at the FMRB Research Reactor. The response of the transfer instruments consisting of three Bonner spheres, 2.5'', 3.5'' and 9.5'' in diameter, together with a spherical "3He proportional counter (Centronics, type SP9) was determined in all three neutrons fields. (orig.)SIGLEAvailable from TIB Hannover: RO 8345(37) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman