Re: Gy and Sv = J/kg -Reply

[Keith Brown <KDB1@nrc.gov>]

>>>>>>>>>>>>>>>

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.  :-) 

Regards, Jim Muckerheide jmuckerheide@delphi.com

<<<<<<<<<<<<<<<


I suppose the real question here is what areas of the physical sciences
are "disciplined".  Certainly in nuclear physics we have this unit called a
"barn" which is fundamentally square meters (analogous to Sv which is
fundamentally J/kg).  The barn is the "equivalent area" that a target
nucleus appears covers assuming that the target (i.e., the collection of
target nuclei) is an infinite in extent and uniform, and that the projectile is
pointlike (note that "equivalent area" is analogous to "equivalent dose"). 
There is no physical diminsion in the problem, however, corresponding the
the meters or square meters of the cross section.  (We use barns because
we have become comfortable with visualizing particle interactions as
collisions of various sized billiard balls but are not comfortable visualizing
the quantum mechanical reality of the interactions.)  At times, the
"equivalent area" will be much larger than the physical diminsion of the
target and projectile nuclei.  The number of "meters" in the cross section
relate, instead, to a probability of interaction, just as the number of "J/kg"
in sieverts relate to a probability of biological effect, not to physical "J/kg". 


Of course for real fun one should visit the world of high energy physicists
in which only one unit, the eV (although usually the MeV or GeV is more
convenient), is used.  Once one recognizes that mass is energy, energy is
time (via Planck's constant),  and that the only fundamental speeds are 'c'
and zero (and high energy physicists operate exclusively at 'c'), it makes
sense to quit carrying the baggage of things like 'kg' (which is MeV in this
system), 'm' (which is reciprocal MeV), 'kg-m/s' (MeV again), 's' (must be
MeV), and the like.  This is, of course, a very different discussion than
physical "reality" of sievert or barn, but it does show that this physical
science does not hold the concept of different units for different physical
quantities near and dear to its hearts.  (On the other hand, I would not
considered the high energy physicists that I have gone drinking with to be
"disciplined".  :-)    )

Keith Brown
kdb1@nrc.gov

Opinions expressed might be mine, but are probably stolen from Preston
and Bhaduri.  In any case, they may not be those of my employer.