The purpose of this study was to systematically investigate how high doses of sparsely and densely ionizing radiations influence the proliferation time of lymphocytes in short-term cultures and, consequently, the observed frequencies of dicentric and centric ring chromosomes. Peripheral blood samples from five volunteers were irradiated with high doses of 200 kV X-rays and with neutrons with a mean energy of < E (n)>=2.1 MeV. First division metaphase cells were collected after different culture times of 48, 56, and 72 h and dicentrics, centric ring chromosomes, and acentric fragments were determined. The data hint at considerable mitotic delay. The main increase in the number of chromosome aberrations occurred between 48 and 72 h after an X-ray exposure and between 56 and 72 h after neutron exposure. When the data were used for a calibration of aberration frequency versus dose, subsequent dose estimations resulted, however, in comparable values. Thus, in spite of the influence of mitotic delay on observable chromosome aberrations, at least for the radiation types investigated here, a culture time of 48 h is acceptable for biological dosimetry.

Purpose : The shape of the dose-effect curve for neutrons, i.e. the question as to whether the curve is linear or supralinear in the low-dose region, is still not clear. Therefore, the mutagenic effect of very low doses of low-energy neutrons was determined.Materials and methods : Human-hamster hybrid A(L) cells contain human chromosome 11, which expresses the membrane protein CD59. This membrane protein can be detected immunologically and quantified by flow cytometry. The A L cells were irradiated with neutrons of 0.565, 2.5 or 14.8 MeV and the results were compared with those after 200 kVp X-rays. Before irradiation, cells spontaneously mutated in the CD59 gene were removed by magnetic cell sorting (MACS).Results : The relative biological effectiveness (RBE) for CD59 mutation induction was 19.8 (+/-2.7) for 0.565 MeV, 10.2 (+/-1.9) for 2.5 MeV, and 10.2 (+/-1.6) for 14.8 MeV neutrons. Linear mutation responses were obtained with all radiations except for 14.8 MeV neutrons where a supralinear curve may be a better fit. The deletion spectrum of mutated cell clones showed 29 Mbp deletions on average after irradiation with 0.069 Gy of 0.565 MeV neutrons. This scale of deletions is similar to that after 3 Gy 100 kV X-rays (=34 Mbp). For 50% cell survival, the RBE of the neutrons was 11 compared with 200 kV X-rays.Conclusion : Neutrons of low energies (0.565 or 2.5 MeV) produce a linear dose-response for mutation in the tested dose range of 0.015-0.15 Gy. The neutron curve of 14.8 MeV can be approximated by a curvilinear or linear function.

Induction of DNA double-strand breaks (dsb) and their distribution are dependent on the energy deposition pattern within the cell nucleus (physical structure) and the ultrastructure of the chromosomes and its variation by the cell cycle and gene activities (biological structure). For electron radiation very similar RBE-values are observed for mammalian and yeast cells (AlK, 1.5 keV, 15 keV/micrometer: 2.6 in mammalian cells and 2.2 in yeast; CK 0.278 keV, 23 keV/micrometer: approx. 2.5 in mammalian cells and 3.8 in yeast). In contrast, the RBE-values for the induction of dsb of 4He2+ and light ions in the LET range from about 100 keV/micrometer up to 1000 keV/micrometer are significantly higher for yeast cells compared to mammalian cells. For example, the RBE-value of alpha-particles (120 keV/micrometer) is about 1.2 for mammalian cells whereas for yeast the RBE-value is about 2.5. The yeast chromatin has less condensed fibres compared with mammalian cells. Since a single CK photoelectron can induce only one dsb, the different condensation of the mammalian and yeast chromatin has no influence. However, particles may induce more than one dsb when traversing a chromatin fibre. The probability for the induction of closely neighboured dsb is higher the more condensed the chromatin fibres are. Since small DNA fragments (50 bp up to several kbp) are lost by standard methods of lysis, the underestimation of dsb yields increases with fibre condensation, which is in accordance with the observes dsb yields in mammalian cells and yeast. In order to obtain relevant yields of dsb (and corresponding RBE-values) the measurement of all DNA fragments down to about 50 bp are needed.

Scattering cross sections were precisely measured for natural carbon and polyethylene samples at neutron energies of 6.23, 7.21, 8.16, 9.16, 11.05, 11.90, 12.84 and 13.75 MeV. A proton recoil telescope served as a neutron fluence reference detector. Measured neutron tim-of-flight spectra were compared with Monte-Carlo simulations performed on the basis of evaluated cross sections (ENDF/B-V). For the elastic and inelastic scattering on carbon the ratios of masured to expected yields ranged from 0.6 to 1.8, while the neutron-proton scattering was consistently confirmed within the uncertainties of 2.5 to 6% estimated for these experiments.

The reaction16O(n, α)13C was studied with 13.9 MeV neutrons using two different counter telescopes. Absolute differential cross sections were measured for the transitions to the ground state and to the 3.08 and 3.68+3.85 MeV levels of13C. The results are interpreted in terms of a direct reaction mechanism.

The reaction16O(n, a)13C was studied with 13.9 MeV neutrons using two different counter telescopes. Absolute differential cross sections were measured for the transitions to the ground state and to the 3.08 and 3.68+3.85 MeV levels of13C. The results are interpreted in terms of a direct reaction mechanism.

Dur ing the last years some n~-react ions on l ight nuclei have been s tudied with 14 MeV neutrons. In this mass region angular d is t r ibut ions co r respond ing to def ined states of the f inal nucleus m a y be ob ta ined . F r o m these invest igat ions in fo rmat ion on the under ly ing reac t ion mechanism and re la ted spect roscopical pa ramete r s m a y be expected. The 160(n, c 0 l a c reac t ion has been invest igated by several au thors , using c loud chambers 1,2, scint i l la t ion counters 3,4, nuclear emuls ions 5 v, and counter telescopes s-11. In mos t exper iments the angular dis t r ibut ions are found to be peaked in the fo rward and /o r backward direct ion indica t ing a direct process. However , the results are mutua l ly inconsis tent . A p a r t f rom serious d isagreement on the absolu te values of the cross sections, there are large discrepancies in the details of the observed angular dis t r ibut ions . M o r e o v e r the in te rpre ta t ion of the 160(n, c01aC