Watches - continued -Reply

[FRAMEP@ORAU.GOV]

>The American Nuclear Society has their version of "Compute your own
>Personal Radiation Dose" based on where and how you live.  One
>source of radiation it lists is "If you wear a luminous wristwatch
>(LCD) add 0.06 mrem to your annual dose" .  Why the reference to the
>watch being the "LCD" type? Any thoughts?

>Tom O'Brien

Tom, I guess LCDs need a light source (vs a LED). Battery operated
lights have disadvantages so they use a radioluminescent source which
provides continuous light. The ANS must assume the thing uses
tritium.

You didn't ask but it might be of interest to some that the dose is
from leaking H-3 that is inhaled or absorbed through the skin.
According to NCRP 95 Fitzsimmons et al 1972 (Health Physics 22, p514)
calculated the thing theoretically at 0.6 mrem per year. Then
measurements on tritium in the urine of wearers of these watches  by
Moghissi and Carter 1975 led to an estimate of 0.8 urem/year from 27
uCi.(DHEW publication 76-8001). This would be 0.059 mrem per year
from 2 mCi, a typical watch activity. Then Buckley et al 1980 (NUREG
CR-1775) somehow estimated 0.06 mrem per year assuming 2 mCi per
watch. This must be where the ANS number comes from.

The estimate might work like this:

H-3 escapes at a rate of 83 nCi/d. It is assumed that this is HTO and
it is released into a 30 cubic meter room with a ventilation rate of
one air change per hour. The equilibrium activity therefore is 240
pCi/cubic meter. A breathing rate of 0.78 cubic meter per hour and
assuming 7000 hrs per year in the room gives a yearly intake of 1.3
uCi. The CEDE per uCi inhaled is 0.064 mrem therefore the CEDE is
0.083 mrem from inhalation. Doubling that to take care of skin
absorption gives  0.16 mrem. I got this approach from the British
NRPB.  All these are ballpark numbers, you can make up your own. All
things considered, its pretty close to the "official" numbers.

Paul Frame