iron 55 -Reply

[FRAMEP@ORAU.GOV]

These calculations are highly entertaining, so let me cast my two
centibucks worth upon the waters. Disclaimer: the x-ray energies from
Fe-55 are in the 6 keV range and most listings of these types of
constants won't include anything with energies below 10 keV. My
calculations aren't worth the cyberspace they're written on. Only mad
dogs and Englishmen (and Scotsmen) etc ....... I'll be the mad dog. 

You have two numbers so far: 

0.62517 micro Sv per hr per MBq at 1 meter which translates into
0.0625 mrem/hr at 1 meter or 23 rem/hr at 1 cm per mCi.

and  1.27 microSv /hr per MBq at 30 cm which translates into 0.114
microSv/hr per MBq at 1 meter i.e. 0.0.0114 mrem/hr at 1 meter/MBq or
4.2 rem/hr at 1 cm per mCi.

To calculate the exposure rate constant (R/hr at 1meter/Ci) we can
use  the calculation  19.53 x  E x I x uen/p

If we assume that the energies of all 3 x-rays are 0.0059 Mev and the
sum of their intensities is 0.278 and the mass energy absorption
coefficient is 22 cm sq/g this becomes

19.53 x 0.0059 x 0.278 x 22  =  0.7 R/hr at 1 meter per Ci
                               or 7 R/hr at 1 cm per mCi.

However, at this low energy, the mass energy absorption coefficient
for air is about 1.3 times that for tissue so 1R would translate to
about 0.67 rad (rem) to tissue. Hence 7 R/hr becomes 4.7 rem/hr at 1
cm  per mCi which is pretty close to the estimate of 4.2. 
  
It is also possible to use the protocols of Unger and Trubey which,
based on an old ANSI, are the source of the numbers in the "new" rad
health handbook:

The appropriate equation there to get rem/hr at 1 cm / Bq 

is (1/4 pi) x  I  x  e^(-20.477 - 1.7454 ln E)
where E is the energy in MeV

 this becomes  0.00796 x  0.278 x 9.947 E-6
 which equals 2.2 E-7 rem/hr at 1 cm per Bq
                 or 8 rem/hr at 1 cm per mCi

As Dale Boyce had mentioned earlier, at low energies the Unger and
Trubey method gives dose equivalent rates (in rem/hr) higher than the
exposure rates in (R/hr). 

To calculate an effective dose equivalent (EDE) we can use the
methods in the new ANSI 6.1.1. The equations are difficult to
transmit via  e mail but the calculation I did resulted in an
effective dose equivalent of 0.0017 rem/hr at 1 cm per mCi. 

However none of these factor in air attenuation which is considerable
at say a 1 meter distance. They also assume no shielding at the
source and the dose equivalent and effective dose equivalent
calculations assume the whole body is evenly exposed. ha ha ha ha ha 


Best wishes

Paul Frame
Professional Training Programs
Oak Ridge Institute for Science and Education
framep@orau.gov