What's today's best-accepted value for W (the mean energy expended in air
per ion pair formed)?

 MEDPHYS Listserver question from 10/99,
 Douglas J. Simpkin, Ph.D., DABR
 St. Luke's Medical Center
 Milwaukee, WI 53215
=========================

I would consider W/e =3D 33.97 J/C ( or eV/ion pair) as today's best
value. Refer to Attix's text book "Introduction to Radiological physics and
Radiation Dosimetry" 

 Ram  G. Virudachalam, 
 Ph.D.,Radiological Associates of Sacramento,
 Sat, 9 Oct 1999
------------------------------

See "Re-evaluation of the W value for electrons in dry air" in Physics in
Medicine & Biology (1987) Vol 32, No.2 pp. 213-219 by M. Boutillon and A.M.
Perroche-Roux.

They give the value as 33.97 +/- 0.05 J/C which is the latest value I have
seen, the older value being 33.85 as found in ICRU 31 (1979).

 Steve Locks
 Newcastle General Hospital
 Westgate Road
 Newcastle Upon Tyne NE4 6BE
------------------------------

re: my ignorant question of last week "What is today's best value for W in
air?" I must not be the only one wondering if the 12-year old text-book
answer (e.g. in Attix) is today's best value.  A number of you asked "Could
you post a summary of your replies on the list?" Here it is.

All of the replies answered: 33.97 (+/- 0.05 J/C); c.f. "Re-evaluation of
the W value for electrons in dry air" in Physics in Medicine & Biology
(1987) Vol 32, No.2 pp. 213-219 by M. Boutillon and A.M. Perroche-Roux.

Thanks to all who responded,
Doug
------------------------------

Date:    Tue, 12 Oct 1999 13:48:28 -0400
From:    IRS/NRC medphys account <irsnrc@IRS.PHY.NRC.CA>
Subject: Best Values of W/e

Last week Doug Simpkin asked about W/e values for dry air and people uniformly
recommended the 33.97+-0.06 J/C value of Boutilon and Perroche-Roux.
This is still the recommended international value.

However a few years back I re-analyzed the data that Boutillon
and Peroche-Roux had used and found that using more up-to-date values
of parameters such as half-lives and more accurate estimates of
the uncertainties on graphite to air stoppingr- power ratios led
to a value of 33.82+-0.13 which is 3 sigma lower than the earlier
value (but still pretty much 34!).  However, for standards labs
work this is a big change.

I never published the work outside internal reports because I still
don't believe the analysis technique that both BPR and I used.  In the
covariance analysis, it turns out that ignoring just on of the 13 or
so initial data values leads to a substantial change in the final value.
What makes no sense to me is that I removed one of the values well
below the average value and the average decreased!  The math is correct
and some people swear it all makes sense - but I am not sure it isn't all
nonesense!

The value of W/e is central in air-kerma primary standards but if we
just switch over to using absorbed dose standards, then the problem goes
away.  Just one more reason for using TG-51.

Ciao

Dave Rogers
------------------------------



