Gy and Sv = J/kg

[liselin@planetx.bloomu.edu (Louis H Iselin)]

In article  rkathren@tricity.wsu.edu (Ron L. Kathren) writes:

>Sorry to question my old (but not elderly!) mentor Wade Patterson, but I
>note that the dose equivalent quantities as expressed in rem and Sv are not
>necessarily numerically equal to J/kg. The equality only holds for absorbed 
>dose.

Numerically?  No.  They do not have the same numerical values except in the 
trivial case where all of the modifying factors are one.

>If one were to express dose equivalent (or equivalent dose if you prefer that 
>terminology) in units of J/kg, you would be really giving the absorbed dose.

I must disagree.  Wade has expressed an idea that I have been uncomfortable 
with ever since I was introducted to dosimetry units in Chuck Roessler's 
Radiation Dosimetry class.

In all of science, except for health physics, when a quantity is mulitplied by 
a conversion factor that changes the units, the conversion factor has units of 
its own that tell you how the original quantity will be modified.  Only the Gy 
to Sv conversion is different.  Stupid might be a better description.

1 Gy == 1 J / kg.  I use the == to show a definition.

1 Gy * conversion factors to convert to Sv = x Sv.

Since we _define_ these conversion factors to be dimensionless, the only units 
left are still J / kg.  Thus Wade's point:  In terms of units alone, the Gy 
and the Sv have the same units.  In terms of SI base units, both are m^2 / s^2.

I appreciated Chuck Roessler's personal notation in this matter.  He always 
described the Sv in terms of "equivalent J / kg" so there was no question as 
to the point that the physical units has been modified to include the 
dimensionless conversion factors.

>>Dan and Group:
>>Perhaps some confusion would disappear if everything (rad, rem, Gy, Sv)
>>were expressed in joules/kilogram.
>>People often forget that both the Gy and the Sv are defined to be equal
>>to 1 j/kg.
>>Wade Patterson

Louis
_____________________________________________________________________
Louis H. Iselin, Ph.D.           * Go Gators! *                   <*>
Assistant Professor of Physics (Health Physics Program)
Bloomsburg University of Pennsylvania      liselin@planetx.bloomu.edu
Bloomsburg PA  17815-1399                             lhi@poboxes.com