Miniature pigs were bilaterally irradiated either in a neutron field (incident neutron to gamma ratio of 5) or in a gamma ray field (incident gamma to neutron ratio of 15) from the AFRRI-TRIGA reactor. For both fields the dose rate at the midline of the pigs was 250 rads/minute and uniform (class A) irradiations to the gastrointestinal (GI) tract were achieved. Midline tissue doses from the neutron field ranged from 360 to 1970 rads; survival times were from 3.6 to 11 days. Midline tissue doses from the gamma ray field ranged from 605 to 2630 rads and survival times from 4.4 to 11.1 days. Median lethal doses for GI death <tex-math>$({\rm LD}_{50(7.5)})$</tex-math> were calculated to be 430 and 870 rads for the neutron and gamma ray fields, respectively. Relative effectiveness of the neutrons was 2.0. The ratio of the doses corresponding to 7.5 days mean survival time, calculated by the least-squares method, was used as an additional estimate of the neutron relative effectiveness. Results were in agreement with those of the <tex-math>${\rm LD}_{50(7.5)}$</tex-math> method. Similar determinations for miniature pigs irradiated in the gamma ray field at 125 rads/minute showed no gamma ray dose rate dependence between 250 and 125 rads/minute. The relative segmental radiosensitivity of the GI tract and postirradiation bacterial proliferation in the miniature pigs are discussed.

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