RE:Cobalt-60 dose

[Brian_Gaulke@hc-sc.gc.ca]

Ivan Luketina wrote:

Not having seen tables of SI values I have puzzled over the conversion of
the air kerma constant to SI units and came up with the following:

If the specific gamma ray constant for Co-60 is 13.2 R/mCi-hr at 1 cm from
another posting then the conversion is from units of (R.cm^2)/(mCi.h) to
(microGy.m^2)/(GBq.h)

Using the air kerma K (Gy) = 0.00876 x exposure (R)
or   K (microGy) = 8760 x exposure (R)

1 cm^2 = 10^-4 m^2

1 mCi  =  3.7 x 10^7 Bq = 0.037 GBq

then the conversion factor becomes (8760 x 10^-4)/0.037 = 23.68

and 13.2  (R.cm^2)/(mCi.h)  = 313 (microGy.m^2)/(GBq.h)

Perhaps someone with a table of SI values could confirm the 350
µSv.m²/(h.GBq) value or the accuracy of the above.


You have only calculated air kerma.  The 350 microSv-m2/(h-GBq) value is
intended for estimation of effective dose (or effective dose equivalent).  In
external dosimetry we attempt to estimate personal dose equivalent (Hp(10)).
This can be calculated by multiplying the air kerma by the appropriate
conversion factor.  For example, ISO 4037-3 lists a factor of 1.15 to convert
from air kerma to personal dose equivalent for a Co-60 field meeting their
calibration geometry requirements with the ISO slab phantom.

1.15x313=360, which is close to the value of 350 quoted previously.

Brian R. Gaulke, CHP
Head, Dosimetry Section
Radiation Protection Bureau
Health Canada
brian_gaulke@hc-sc.gc.ca


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