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17.5.5 Theoretical Calculations for Shielded Locations
It ie desirable to know the interaction of a ship's structure with
transit radiation in order to determine to what extent a ship will
shield personnel from such radiation. Comparison of topside and belowdecks weapons-test transit-radiation data which have been obtained
eimiltaneously could provide such information.
However, test data on
vbelow-decks transit-radiation exposures are insufficient to permit
extrapolation to exposures from bursts of any yield and for any burst
condition. Therefore theoretical methods of estimating such exposures
or of calculating ship-shielding effects are necessary.
A below-decks transit-radiation exposure is due to the transmission
through the ship's structure of gamma rays emanating from the airborne
radiation sources surrounding the ship.
To predict such exposures, it
is necessary to know the source characteristics and the shielding
effectiveness of the structural components of the ship. This
effectiveness is a function of the amount and type of material between
the point of interest and the external radiation source, the sourceshield-receiver geometry, and the energy spectrum of the gamma radiation
that composes the radiation field.
Effectiveness, defined in terms of
the shielding factor, is a dimensionless ratio of the gamma dose rate
at the point of interest to that at a point of measurement in the ex-
ternal radiation field above the point of interest. A method has been
developed for calculating the shielding factor without knowledge of the
actual below-decks dose rate.
Thus, it is possible to estimate the
radiation attenuation at any below-decks location, or to calculate the
dose rate at that location as the product of the shielding factor for
the location and the topside transit-radiation dose rate, if the latter
dose rate is known.
Present information is such that neither topside transit-radiation
dose rates nor base-surge characteristics expected from water-surface
bursts can be specified, since they have never been observed, as was
noted in Section 17.52.
Therefore, it is not feasible to calculate
theoretically below-decks exposures due to such bursts. However, transit-
radiation exposures from three underwater bursts have been measured,
{fection 17.5.2) and the base-surge radioactive-source characteristics
the primary source of transit radiation) have been defined, with
limitations, for underwater bursts,
in general.
In addition, a base-
surge model exists (Section 17.54) that, for practical purposes, pre-
dicts topside exposures that agree with available data from underwater
bursts.
Therefore, it has been possible to develop theoretical methods
for calculating below-decks transit radiation exposures for such bursts.
The general problem of computing ship~shielding factors involves:
(1) specification of the geometric configuration and the radiation
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