The Effects of Dose Rate Variation of Fission Neutrons and of Co 60 g-Rays on Survival in Young Chicks

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Abstract
White leghorn chicks (3 days old) were irradiated with fast neutrons at dose rates of 1 and 4 rep/min. Twenty-one-day mortality curves were constructed for each dose rate. Chicks exposed at the lower dose rate showed a marked increase in survival The 21-day LD50 increased from 236.0 + 3.9 to 306.5 [plus or minus]4.1 rep, when the 4-rep/min data were compared with the 1-rep/min response. Also, the slope of the mortality curve at the lower intensity was less than that of the 4-rep/min exposures. Similar survival curves were constructed for chicks exposed to C06O gamma-rays at 6 r/min, 12 r/min, and 22 r/min. Combined data for the 2 higher dose rates (which were not significantly different) showed a 21-day LD50 of 878.4 [plus or minus] 19.4 r which was increased to 1207.4 [plus or minus] 21.9 r when the low dose rate of 6 r/min was used. After exposure to the higher dose rates of both these ionizing radiations, an early mortality was evident during the first 2 days post-irradiation. This response was similar to that seen after X-ray exposures at dose rates of 15 r/min or higher. Autopsies indicated a high incidence of visceral hemorrhages. This early mortality and the correlated pathological findings were not commonly observed after low dose rates of X-rays, gamma-rays, or fast neutrons. The relative biological effectiveness (RBE) of fast neutrons and C06O gamma-rays for producing 21-day lethality in the chick was found to be 3.72 when the high dose rate survival curves were compared.

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