true fall-out pattern is likely to deviate from the idealized contours. The total dose accumulated by an individual in a fall-out field varies with the time of entry. For example, a reading of 50 r per hour in an area taken 50 minutes post detonation will decrease to 22 r/br at one hour. If an individual remains in this area during the period between H+30 minutes and .Ht+l hour, he will accumulate a dose of 16 roentgens. If a reading is 50 r/hr at one hour and he enters at B+l hour and stays 30 minutes, he will accumulate 20 roentgens. This difference at an early time post detonation is characteristic of the rapid decay of a fission product field at early times. However, in another area with a reading of 50 r/hr at H+8 hours and a stay of 30 minutes, an individual will accumulate a dose of 24 roentgens. Thus, at later times with the same reading in roentgens per hour, a larger amount of radiation is accumulated for the same time of stay. The variation of the energy spectrum with time has been the subject of field evaluations at several test operations. It was found at castiett/ that a considerable fraction of the gamma energy in fall-out is in the vicinity of 0.1 Mev. After 10 days there is an accenturtion in the regions of 0.5 Mev and 1.6 Mev. - tatto radiations. This is attributed to the Barto As this decreases a concentration of the spectrim in the 0.7 Mev region is noted. The military situation requires a more detailed discussion of the immediate fall-out area for a surface burst and the area around ground zero for air bursts. The radiological problems encountered in these two areas will exert a strong influence on decisions for weapon employment, as well as on deployment of troops and exploiting the advantages of the use of nuclear weapons. Following an air burst, no radiological fall-out of consequence will be encountered.. The dust swept up into the cloud from the blast effect on the ground will fall out within a short period of time. How- 14/ Cook, C.S., Fall-out Symposium (Confidential), Armed Forces Special Weapons Project, AFSWP-895, SECRET Restricted Data. 45