health and safety laboratory - Digital Collections

I I I
I I
The original computation of gamma dose from total beta activity on
(7}
gummed filar ' was developed to meet this lack of suitable dose estimates.
Unfortunately, the gamma dose calculations are subject to the same errors
in evaluating multiple bursts as the Sr estimates.
The gamma dose computed is the air dose to infinite time from uniform
fallout on an infinite smooth plane. No reduction for shielding or weather
ing has been introduced, nor has there been included any build-up factor for
(14)
scattering. The usual overall reduction factor is 10 for converting to
gonad dose. It might be noted here that the thirty year dose is only slightly
^
f|
less than the infinity dose.
There is a second possible estimate of gamma dose. If we assume that
Cs is produced and deposited in a fixed ratio to Sr9^, the dose from Cs
can be computed. The calculations are based on the same assumptions as the
total beta activity^ '. The modified formulas are:
m
I where,
Dose = 6.64 x 10~10 y N in mrads
• Dose = Cs ^ infinity 7 dose in millirads
I
} N = Total Cs ^7 gamma disintegrations to infinity per sq. ft,
*-*
Accepting a half -life for Cs ^ of 26.6 years and an initial p dpm ratio
• for Cs 137/Sr90 of 1.75, there would be 2.8 x 109 Cs 137 p disintegrations
• to infinity per sq. ft. for each me/mi 2 of Sr90 . This would give
• 2.3 x: :10 Cs 7 disintegrations per sq* ft.
The final formula would be:
Dose = 1.5 (me/mi 2 Sr90 ) in mrads
137
The estimate of gamma dose based on Cs ^ will be extremely low for
"
i i
fallout occurring shortly after a test and will improve with age of the debris.
- 26 -