External exposure. At the me of detonation the almost instantaneous gamma rays released wil! be of concern out to about a mile for a nominal bamb (7) and out to a few miles for high-yield weapons. However, significant amounts of radioactive fallout may occur at more than 300 miles downwind from high-yield weapons. This material emits gamma rays similar to the instantaneous ones but with les quantum energy. About 25 roentgens of gamma radiation, deliv- ered in a short time (about a day or jews) over the whole body, are required to produce minor and burns caused by the beta radiation frem the material being in contact with their skin ti f Radiations continental tests. All were within 20 miles of zero. Cattle from the 1945 exposure have under observation at Oak Ridge. There have been no observable changes in the animals, except skin damage and low of hair in spots that regrew but was gray in color. Their offspring have been normal in all respects. These experiences have showa the need for larger warning areas when high-yield weapons are to be detonated. These are now established during operational periods; and withthe continuing improve- transitory changes in the blood; about 100 roentgens are required for some persons to show radia- tion sickness; about 450 roentgens may be lethal wo half of the exposed persons (3). (A roentgen is 8 unit for measuring the um of radiation, or dese, that has been received. For example, a normal x-ray will deliver about 1/10 roentgen or more to the chest; abou: 10 roenqgens are received im a lfetame from cosmic rave and from naturally e- Cusving radioactivity im the asr, waeer and snl, ¢). The highest radiation exposure to any individual mm the Unierd Seners outside the Nevada Ten See bas been about 7 rocmegens (about 12 gracng wear the Nevada Test Ser during the spring of 1953 The esumated radiation dose w thei thvtoads was thout 30D rrentgens 9. Owing wo the - +° -waespread dtnbunon of fallout fram the Pacwic tests in the spring of 1954, racoactive sodime was fond im the thyrosds of cattle and sheep im vanous parts of the Usted Seases 10). The high- Fallout caasevial alen ext beta rave that travel at most afew mreen m as and are of principal concern when heghly acuve fallout matenal remame est total radzatom doer tw the thvroid from ths wres was estamated to be about 40 reenteem. The 1952 aad maw’ horses in 1983 capenienced beings resudung from mtake of fallout materia) has been measured. The highest radiacon exposure measured was in mdividuals near the Nevada Test Site m the spring of 1955, but there is some un- % TRE SCIENTIFIC MONTHLY