THE RELATIVE EFFECTIVENESS OF VARIOUS 



IONIZING RADIATIONS ON CHROMOSOME 



BREAKAGE IN TRADESCANTIA 



J. S. Kirby-Smith, C. W. Sheppard and Doris L. Craig 

 Biology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 



A DETERMINATION of the relative biological effectiveness (RBE) of X-rays 

 of mean energies approximately 60 and 200 kV respectively, mixed 1-17- 

 and 1 -SS-MeV gamma rays from ^"Co and high-energy beta rays from ^-P 

 on Tradescantia pollen has been reported recently by Kirby-Smith and 

 Daniels^. These studies have shown medium-energy X-rays in the 60-kV 

 range (0-20 A wavelength) to be twice as effective as 1-MeV gamma rays 

 or beta rays of approximately 500-kV mean energy in producing chromo- 

 some aberrations in this material. The effectiveness of 200-kV X-rays was 

 found to be midway between that for 60-kV X-rays and 1-MeV gamma 

 rays. This work now has been extended to cover the effects of fast neutrons 

 on dry pollen and inflorescences of Tradescantia. In the course of these 

 later investigations, the twofold increase in biological effectiveness in 60-kV 

 X-rays over 1-MeV gamma rays has been confirmed for both of these 

 biological materials. 



The source of fast neutrons in the present studies has been the Oak Ridge 

 National Laboratory 86-inch, 22-MeV proton cyclotron with the reaction 

 of protons on an internal beryllium target. Measurements of the fast 

 neutron flux and dosage have been made by means of proportional counter 

 dosimeters and tissue-equivalent ion chambers. A comparison of these two 

 different methods, as well as an independent determination of the sf'ay 

 gamma rays present with neutron insensitive ion chambers, has shown the 

 gamma-ray contamination to be approximately 10-15 per cent. 



In order to reduce the gamma-ray contamination to this figure, a bio- 

 logical exposure facility consisting of a lead box with 1 -inch-thick walls was 

 constructed outside the dee chamber. The neutron energy spectrum within 

 this facility from the proton reaction includes considerably lower energies 

 than the well-known curve for deuterons on beryllium, and there is also 

 the degradation in neutron energies and scattering produced by the lead 

 and by the other metal structures of the cyclotron. The neutron energy 

 spectrum within the exposure facility has been determined approximately 

 by means of various threshold and fission detectors. As is to be expected, 

 it is a much degraded spectrum, with a broad maximum in the 1-MeV 

 range and an appreciable thermal component. 



Both Tradescantia dry pollen and inflorescences were exposed to fast 

 neutrons in the lead-walled facility described. Following irradiation, the 

 pollen was cultured after the methods of Bishop- and Conger^. Aberra- 

 tions were scored at metaphase in the pollen tube division. In the inflor- 

 escences, four-day chromosome aberrations were scored. Results of the 



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