Subject: 
         Conversion factor from Exposure to Radiation Absorbed Dose
    Date: 
         Wed, 27 Feb 2002 12:52:46 -0500
   From: 
         Rob Morrison <robpm@IX.NETCOM.COM>

What is the accepted conversion factor for Roentgen to Gy when using air as
the medium.  I have used 114.1 Gy/R in the past however in two publications
that I was checking in it was listed as 0.00873 Gy/R or 114.548 R/Gy.  The
two publications were the first edition of Kahn's "The Physics of Radiation
Therapy" and the other was "Applied Physics for Radiation Oncology" by
Stanton and Stinson.

There is a nice free conversion calculator for many different classes of
units on the following web site.  It is called Versaverter.  Just thought
some might find it useful.  It is located at the following URL:

http://www.pawprint.net/contact.php

Thank you for the input.

Sincerely,

Rob Morrison
-----------------------------------------------------------
Subject: 
         Re: Conversion factor from Exposure to Radiation Absorbed Dose in air or air
         kerma at 300 keV and below
    Date: 
         Wed, 27 Feb 2002 18:38:10 -0500
   From: 
         Rob Morrison <robpm@IX.NETCOM.COM>



To all who responded regarding my question,

Thank you all for the input.  Many of you pointed out the that the key to
the difference I was seeing was the fact that it has been determined that
there are 33.97 eV required to ionize an air ion pair versus the older
number of 33.85 eV/ion.  Once you plug this number into the equation you get
the 114.1 value for R to Gy or 0.00876 or 7 for Gy to R.  Thank you for this
clarification.  I guess I need to get some updated publications!

I thought that it would be good to list some of the answers I recieved
privatly.

The current value is 0.00876 Gy/R. This based on the average energy required
to create an ion pair (W/e)=33.97 J/C. The old value used to be 33.84 and
33.87. The TG21 used one of the older values, i believe that TG51 uses the
current value as well as NACP, IAEA etc. You can find the value in TRS-277
as well as the new code of practice.

NIST states the following:
K = 2.58e-4*(W/e)*X/(1-g)
Where: K is air kerma in grays (Gy)
W/e is the mean energy per unit charge expended by electrons in dry air in
joules per coulomb (J/C); the value used art NIST is W/e = 33.97 J/C
X is the exposure in roentgens (R)
G is the fraction of the initial kinetic energy of the secondary electrons
dissipated in air through radiative processes: the value used at NIST is g =
0.0 for x-rays with energy less than 300 keV. This gives your 114.1 Gy/R
value. For higher energies you need to consider a
different value of g.

The number can be simply calculated from knowing 2.58e-4C/kg equals 1
Roentgen and
that there are 33.97 eV required to ionize an air ion pair (versus the older
number of
33.85 eV/ion which is the source of the difference between your number and
the other
numbers).  Since an electron's charge is 1.602e-19 C and there are 1.6e-19
J/eV, you
get 33.97 J/C (33.97 Joules required to produce a Coulomb of ionization).
Therefore,
Dair(J/kg)=Xroentgen*2.58e-4(C/kg*R)*33.97(J/C), and therefore
Dair(rad)=0.876(rad/R)*Xroentgen which leads to your number of 114.1 Gy/R.
Write it
out and see for yourself (also refer to page 124 of Khan's first edition and
substitute the 33.97 number for the 33.85 number).

In the CEC publication entitled European intercomparison of diagnostic
dosemeters in Rad Prot Dosim 43 (1992) 75-91 the conversion factor is given
as 1R=8.77 mGy.  Any differences will be attributed to the use of different
values for the average energy per ion pair for air.  As this figure changes,
due to superior measurements being made, so will the conversion factor so we
can expect it to be a moving target forever, albeit slowly.

Does it not depend on the "latest" value of W/e which has been changed many
times over the years. It was 33.7 for years, then 33.85 or 33.97

The definition of a roentgen (2.58 x 10-4 C/kg air) has not changed and the
charge of an electron has not changed.  The somewhat uncertain parameter
responsible for these changes is W, the energy deposited per ion pair.


Thanks again to all of you who responded both privately and publicaly.

Sincerely,

Rob Morrison

