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DISCUSSION 



HERCiK : What is the thickness of the cells, I mean cells lying on the dish bottom in 



experiments with a-particles? 



BARENDSEN: The residual range of the a-particles is about 25ju,. The thickness in 



vertical direction of the cells attached to the bottom because of flattening, 



ranges from 5 to lHn so that each part of the cells is irradiated. 



POWERS : How do you explain differences between the survival curves for X-rays 



and a-irradiation? 



BARENDSEN : As I see it the nucleus is the most sensitive site of the cells and in 



order to answer your cjuestion we have to consider by what mechanism the 



damage is produced. Alpha particles passing through the nucleus will inevitably 



damage one or more chromosomes, the integrity of which is indispensable for 



the reproduction of the cells. X-rays will produce the same damage due to local 



deposition of sufficient energy) less efficiently because of the lower LET and other 



damage (due to less than the required number of ionizations in a small volume) 



may be partially restored, possibly by the metabolic activity of the cells. A 



possible mechanism might be that a large amount of energy deposited leads to 



changes in DNA such that the separation of the two sti'ands of the helix is made 



impossible. If the amount of energy is not large enough the DNA can still separate 



into two strands, and by this action the lesion is repaired. At higher doses the 



probability that two or more electrons pass through the same DNA molecule 



and together produce sufficient damage to prevent strand separation increases. 



This produces an increasing efficiency of sparsely ionizing radiation at higher 



doses and leads to a survival curve of the type oljserved with X-i-adiation. Thus 



the primary lesions are produced at the time of irradiation, but the final damage 



may be influenced by the metabolic activity of the cell. 



PASSYNSKY: Did the result obtained for a-irradiation depend on the mitotic 



phase? Have any differences been found between j^rophase and metaphase? 



BARENDSEN: This could not be determined since the cell divisions were not 



synchronous. 



TOBIAS : Why was the presence of 50 cells in the clone chosen as a survival criterion? 



BARENDSEN: We tried to take 20 cells, 100 and 200 cells, and found no difference. 



Every surviving cell will multiply, and from it several thousand cells will arise. 



TOBIAS : I am very hapjoy to hear that the number of cells is of no importance. 



But. in the case of yeast, colonies sometimes die out, even if they contain 50 cells. 



BARENDSEN : We did not observe this. 



SOSKA: What was the composition of the medium used for the kidney cells? 



BARENDSEN: The medium consists of Hank's solution with 0-5 per cent 



lactalliumin hydrolysate and 5 per cent calf serum to which 100 lU of penicillin 



and 0-10 mg of streptomycin per ml were added. 



ARDASHNicov: How did you calculate the dose for a-irradiation? If ynu get a 



one-hit curve why, in this case, are doses for a-irradiation so much smaller than 



for X-rays? Maybe in this case you take for a hit an a-particle, and not the 



ionization it produces? 



BARENDSEN: I usccl two methods to determine the dose. First of all I counted the 



number of particles per sc^uare mm. Secondly, I also used an ionization chamber 



placed where the layer of water is situated during the irradiation. 



