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A measurement of the absorption of u quantity of any kind of
nuclear radiation in any material is termed the absorbed dose.
rad if the unit used to represent the absorption ofJ00 ergsof
The
fonizinrn radiation per gram of the absorbing material or tissuc.
Thus, dose to personnel is expressed in terms of rads.
One rocnteaen
of gamma radiation results in an absorbed dose of about % ergs per
eram of tissue; hence, for gamma radiation, the roentren and rad are
almost equivalent.
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Neutrons do not produce ionization (the process used to mensure
radiation) directly in their passage through matter.
However, they
cause it to occur indirectly by their interaction with certain
nuclei, and the number, velocity, and energy of the neutrons in-
volved determines the amount of indirect ionization produced.
The
effects of neutron radiation, measured in terms of either neutron
flux (density) or time integrated neutron flux, (now called fluence)
are expressed in terms of rods based on.calculations relating fluence
to absorbed dose.
Neutron flux, the product of the neutron density and the neutron
velocity, is numerically equal to the total number of neutrons passing
in all directions through a sphere of one square cm cross-sectional
area, per second. Instruments measure neutron flux over limited
energy bands and correlate the ionization produced indirectly by
the neutrons with the amount of energy that would be absorbed in
tissue per unit time.
Integrated neutron flux or fluence, the product of neutron flux and time, expresses the total number of incident
neutrons per sq cm of detector.
Measurements of this type have been
made for several energy groups, but particularly for high-energy
heutrons, for which the standard detector is common sulfur, because
it has been determined that the absorbed dose due to neutrons
closely follows sulfur neutron fluence. Empirically determined con-
version factors are then used to express the sulfur neutron fluence
in terms of absorbed dose.
No measurements are available of neutron
fluence over the entire energy spectrum.
Interpolation and extra-
polation have been used to calculate total neutron radiation effects,
in terms of rads.
17.3.3 Contributions to Nuclear-Radiation Exposure
Determination of nuclear radiation effects has been facilitated
by dividing the radiations into two main categories: (1) fireballplume-cloud radiations and (2) residual radiations. Fireball-plume-
cloud radiations include all those emitted by the fireball and abovesurface formations except the base surge, and occur at early times
(within or in less than the first minute).
Residual radiations in-
clude all those emitted by fission products and other bomb residues in
the base surge and fallout, as well as by elements in earth, water, or
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