Re: 1 rad = 1 rem = 1 roentgen... How is this?

["Leo M. Lowe" <llowe@senes.on.ca>]

The roentgen (R) is a unit that measures the amount of ionization produced
in air by x and gamma radiation.  It is currently defined as 2.58E-04
coulombs/kg  air.  Based on the mean energy expended in air per ion pair
formed and per electron charge, one derives

1 R = 0.00876 J/kg = 0.008764 Gy = 0.876 rad (air)

In tissue, 1 R = 0.96 rad (in the 0.1 to 3 MeV energy range) which is often
approximated to 1 rad.  Given the radiation factor (quality factor) of 1
for x and gamma radiation, 1 R is also approximately equivalent to 1 rem
dose equivalent in tissue.

However, when one considers irradiation of the whole body, the various
organs of the body (with varying sensitivities to cancer induction) are
irradiated differently because of the self-shielding of the body.  The
effective dose (and the corresponding risk of radiation-induced harm) then
depends on the irradiation energy and geometry, as well as body size (age).

For example, for exposure to terrestrial gamma radiation, it has been
calculated (e.g. see UNSCEAR 2000) that 1 rad (air) = 0.69 rem effective
dose for adults, or in terms of exposure,  1 R = about 0.6 rem.    For
infants, 1 rad (air) = 0.91 rem effective dose, or in terms of exposure, 1
R = about 0.8 rem.   It is often conservatively assumed that 1 R = 1 rem
effective dose.

The distinction between tissue equivalent dose and effective dose is often
confused.


Leo M. Lowe, Ph.D.
Principal, Senior Health and
Environmental Physicist

SENES Consultants Limited
121 Granton Drive, Unit 12
Richmond Hill, Ontario
Canada L4B 3N4
Tel:	(905) 764-9380
Fax:	(905) 764-9386
email:	llowe@senes.on.ca
WWW:	www.senes.on.ca (under renovation)
************************************************************************
The RADSAFE Frequently Asked Questions list, archives and subscription
information can be accessed at http://www.ehs.uiuc.edu/~rad/radsafe.html