Re: Gy and Sv = J/kg -Reply

[William Prestwich <prestwic@mcmail.CIS.McMaster.CA>]

Gy and Sv both have the same SI unit, but they are different entities.
There is some similarity.
Cheers,
Bill

On Wed, 9 Oct 1996, Michelle Wakelam wrote:

> 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
>