Re: Gy and Sv = J/kg

[JMUCKERHEIDE@delphi.com]

Jack Couch missed the point:
 
> 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.
> <snip>
> 
> Louis Iselin:
> 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.
> <snip>

Jack Couch wrote:
 
> There are other examples, such as Circumference = 2 pi Radius. Looked 
> at dimensionally, one concludes that 6.28 m = 1 m (a mathematical 
> absurdity) for a radius of 1 m. Not unlike the way we say 20 J/kg = 1 J/kg 
> for a Q of 20 in the familiar H = QD.  

This "conversion" doesn't change the _meter_! It does not say 6.28=1. Totally
unlike saying 20=1 for Gy-Sv "conversion" (as opposed to an actual change in a 
total value of 20 vs 1 as measuring different quantities). R and C are
different vectors, they have a different number of meters. They are not
converting meters to equivalent-meters!  

> In the first case, we invent the radian to be the (dimensionless) ratio 
> of circumference-to-radius for a circle, which is numerically 2 pi. By 
> analogy, the dimensionless ratio of H-to-D is the numerical value we assign 
> to Q. I don't know why we (I include myself) are uncomfortable with 
> H = QD but perfectly at ease with C = 2 pi R. Maybe its because we 
> have the word "radian" to cling to. 

No, its because one is technically and intellectually valid, the other is not.

> David Golnick* offers a nice solution to those (including my students) 
> who agonize over this point. He endows Q with the "units" Sv/Gy, which 
> is of course dimensionless (like the radian associated with 2 pi). My 
> students like multiplying 5 Gy by 20 Sv/Gy to get H = 100 Sv. This 
> of course breaks both with tradition and the formal definition of Q, 
> where Q carries no units.

This is good; its true. But it still doesn't provide a "solution" to J/kg, nor 
help the student figure it out. 

Louis had it right.  The conversion factor _changes the units_!   The  wrong.
With Gy in J/kg, Sv is in "units" of "the biological damage equivalent to the
damage of J/kg of low LET radiation" (J-eq-damage/kg?)   

In the more disciplined areas of physical sciences, a new unit would
be/have-been defined to reflect the physical reality; except that most areas
of science would not try to create a unit that has no physical foundation -
that can not be defined. A "concept" that produces different results in
different studies because the relationship is an artifact of each study, not a 
physical relationship. (Then the even more unfounded concept of
"equivalent-dose" is "introduced" as though it had physical merit.  :-) 

To some extent, all of this reflects the basic erroneous concept of "dose" as
a concentration unit instead of a total imparted energy unit. 

> Jack Couch
> Bloomsburg University

Regards, Jim Muckerheide
jmuckerheide@delphi.com