RE: Conversion: millirad/hr to uR/hr

["Fulmer, Philip" <philip.fulmer@cplc.com>]

Andrew,

Is the millirad in air, tissue, or some other material?  If you are
estimating the exposure rate based on absorbed dose rate in air, then
the two quantities are related by the W value for air (33.97 J/C) and
the appropriate unit conversions.  If you are estimating the exposure
rate based on the absorbed dose rate in some other material (assuming
that the material is removed from the air), then the answer gets a
little more complicated because you have to use the ratio of the mass
energy absorption coefficients to calculate an estimate of the absorbed
dose rate to air and then use the W-value to calculate the exposure
rate.  Of course, we're assuming the standard conditions apply with
regard to radiation equilibrium and electronic equilibrium and no
nearness to boundaries, etc.

For the first case, the answer is (pardon the intervening conversion to
SI units but it's quicker for me):

1 mrad/hr (air)* {(1 J/kg) / 100,000 mrad}* {1/ (33.97 J/C)} * {1 R/
(2.58E-4 C/kg)} * {1E6 uR/ 1 R} = 1140 uR/hr

For the second case, the photon energy must be known because the ratio
of the mass energy absorption coefficient is not constant over all
photon energies.  However, for 1 MeV photons, assuming that you are
converting from absorbed dose rate in tissue to exposure rate in air,
the calculation would proceed as follows:

1.  Calculate the absorbed dose rate in air.
       At 1 MeV, the mass energy absorption coefficient for air and
tissue respectively is 0.0278 and 0.0306 cm^2/g
     So the absorbed dose rate in air would be {1 mrad/hr (tissue)} *
{0.0278 cm^2/g (air)}/{0.0306 cm^2/g (tissue)} = 0.91 mrad/hr (air)

2.  Calculate the exposure rate air given the absorbed dose rate in air
as shown for Case 1 above to arrive at the conclusion that 1 mrad/hr in
tissue implies (but may not be exactly equal to) an exposure rate of
1040 uR/hr.

Of course, for other materials and other photon energies, the
calculation would have to be verified, particularly if the photon energy
is of relatively low energy and if the material is relatively high-Z.

Hope this is of some use.  If not, my apologies.

Take care.

Philip





__________________________
Philip C. Fulmer, PhD, CHP 
Carolina Power & Light Company
Harris Energy and Environmental Center
3932 New Hill-Holleman Road
P. O. Box 327
New Hill, NC 27562-0327
philip.fulmer@cplc.com
(919) 362-3363      

>----------
>From: 	Andrew J. Schwartz[SMTP:aschwart@secor.com]
>Sent: 	Monday, June 15, 1998 9:49 AM
>To: 	Multiple recipients of list
>Subject: 	Conversion: millirad/hr to uR/hr
>
>Can anyone provide me with the subject conversion factor?
>
>Thanks,
>
>Andrew Schwartz
>aschwart@secor.com
>
>