Dose Rate Output on New Survey Meters

[SENES Consultants Limited <llowe@senes.on.ca>]

Previous discussions on converting rad to R have noted that 1 R = 0.876 rad
(air)  from the definition of the R unit, but an interesting related
question relative to new radiation survey meters was raised:  when the
meters measure dose rate in Sv/hr or rem/hr, what are they actually
measuring.?

According to one instrument manufacturer, their instruments are calibrated
to measure R or rad in air and then they convert this to tissue dose
equivalent in rem or Sv  assuming  1 R =  1 rem = 0.01 Sv.  However,
surveyors often assume incorrectly that this is effective dose, not dose
equivalent, since they are usually interested in comparison to dose limits.

Using 1 R = 0.876 rad = 0.00876 Gy and UNSCEAR's  conversion factor  for
adults of approximately 0.7 Sv (effective dose)/Gy for terrestrial gamma
radiation from the ground , one obtains 1 R = 0.6 rem = 0.006 Sv effective
dose.  That is, if one is interested in comparison of measured dose rates
to dose limits,  the "dose" output of these instruments should be first
multiplied by 0.6. 

The Sv/Gy conversion factor of course depends on gamma energy, irradiation
geometry and age (body self-shielding) but the 0.7 Sv/Gy is a good
approximation for most environmental gamma radiation  (see also the June/98
Health Physics, p. 698 for a discussion of this issue).


Leo M. Lowe, Ph.D.
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 Address:  llowe@senes.on.ca
Web Site:  http://www.senes.on.ca/