Re : Roentgen to Rad

[PlatoTime@aol.com]

RADSAFERs,

         A recent inquiry was made as to the conversion between roentgens in
air and rads in air :

>As an aside (and given that I currently practice diagnostic physics), I
>notice in Khans' _The Physics of Radiation Therapy_ that this factor is
>.876 rad/R while Johns and Cunninghams gives .873 rad/R.  Which witch is
>which?
>>
>Bill,
>The correct factor is 0.876 rad/R.
>The  difference you observed between the values reported in Kahns and Johns
>books stems from a change in W/e from 33.85 eV/ion pair to 33.97 eV/ion
>pair. For details refer to Attix's book on Introduction to Radiological
>physics and Radiation Dosimetry, p72.
>
>
>R.G. Virudachalam, Ph.D.
>Rush Pres. St. Luke's Med. Ctr., Chicago
>312-942-5751

The conversion uses the following equation, which includes the term "G" to
account for a small fraction of photon energy that is converted to electrons
but then not absorbed due to bremmstahlung production by electrons: 

  CFair = 2.58x10-4 C/kg/R x 33.97 J/C x 100 rad/Gy / (1 - G)

  where:  G = 0.0 for low-energy X rays
       	G = 0.0016 for Cs-137
		G = 0.0032 for Co-60

  Thus:	CFair = 0.87643 rad/R for low-energy photons
		CFair = 0.87783 rad/R for cesium-137
		CFair = 0.87924 rad/R for cobalt-60

 
      The modern way of expressing air dose is by using the quantity of "air
KERMA" (rad or Gy) instead of "exposure" (R).  The conversion from air dose
to tissue dose is then done using Ck values in units of rem/rad or Sv/Gy.
 The 5 dignificant figures for CFair shown above are needed to obtain the
same Ck values used in the HPS N13.11-1993 standard.  I hope this helps.

Phillip Plato
2819 S.E. 22 Place
Cape Coral, FL 33904
platotime@aol.com