Re: Gy and Sv = J/kg -Reply

[Michelle Wakelam <Michelle.Wakelam@adm.monash.edu.au>]

IMHO, Bequeral and Hertz are both frequencies over time and thus have the 
same SI unit, but they are frequencies of different events.

Regards 
Michelle Wakelam 



On   Mon, 07 Oct 1996 William Prestwich wrote

> I will chip in at the risk of being shot down. Becquerel and Hertz are 
> physically identical in that their dimension is sec^-1. Does this bother 
> anyone.
> Bill Prestwich.
> 
> On Mon, 7 Oct 1996, Keith Brown wrote:
> 
> > >>>>>>>>>>>>>>>
> > 
> > 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.
> > 
> > 
> > 
> > 
> > 
> 
Michelle Wakelam
Occupational Hygienist and 
Assistant Radiation Protection Officer
Occupational Health, Safety and Environment
Monash University, 
Wellington Road,
Clayton, Victoria, 3168
Australia
email: Michelle.Wakelam@ADM.monash.edu.au