GAMMA DOSE from VERY-LOW-YIELD BURSTS
OBJECTIVES
The objectives of Project 2.9 were: (1) to documentthe initial gamma dose versus ground
range and (2) to measure the total gamma dose received at a point as a function of time, at distances of military interest, for a fractional-kiloton nuclear surface burst.
The primary objectives of Project 2.12b were: (1) to provide gamma-dose measurements
in support of the biomedical Project 4.2 and (2) to document the initial gamma dose versus
ground range for Shots Hamilton and Humboldt. In addition, secondary objectives of this project were to document residual radiation intensities and to determine the field gamma-decay rate.
BACKGROUND
_ Very-low-yield weapons are being considered for use in both ground and air warfare. In
ground warfare their primary tactical use would be in close-support operations. Here, the
initial gamma and neutron radiation is considered to be the controlling criterion for safe employment of such weapons (Reference 1).
Initial-gamma radiation dose has been studied at almost all tests since Operation Sandstone
by the exposure of film badges at various distances from ground zero. These measurements
have been limited to yields greater than the very-low yields of Shots Fig, Hamilton, and Hum-
boldt. Measurements made at Operation Jangle (Reference 2) and Operation Plumbbob (Ref-
erence 3) provide the most appropriate background data for this project. These references
indicate that the initial gamma radiation from a surface burst is reduced by 50 percent compared
to an equivalent air burst. Therefore, the air burst would have a greater gamma lethality radius than an equivalent surface burst.
Since fractional-kiloton weapons will probably be employed as low air or surface bursts, the
likelihood of fallout and neutron-activated soil contamination exists.
Furthermore, since the
low-yield weapons would be inefficient in terms of fissioning of nuclear materials, the likelihood
of alpha contamination exists. Project 2.10 documented the alpha and gamma contamination levels
for the surface Shots Quince and Fig (Reference 4), but was not operational for the air-burst Shots
Hamilton and Humboldt. Project 2.12b, with little additional effort, assumed these tasks for these
events.
THEORY
With any nuclear detonation, various nuclear radiations are emitted during and after the ex-
plosion. Since this report deals with the gamma radiation, the phenomena associated with this
radiation will be discussed here briefly.
It is convenient to consider the gamma radiation as being divided into two categories, initial
and residual. For this project, the initial radiation is arbitrarily taken as that emitted during
the first minute after the explosion. This radiation results from many nuclear reactions and
effects, of which three predominate (References 5 and 8):
1. Prompt radiation accompanying the fission process, which is emitted duringthe first
few microseconds.
2.
Nitrogen-capture photons emitted from the capture of thermal neutronsby nitrogen in