RE: N-16 and 450P

["Eric Cowdrey" <ericc@mctrf.mb.ca>]

The discussion about N-16 is an interesting one, particularly if one plans
to use a measuring instrument like the Victoreen 450P which is calibrated in
exposure rate units like roentgens per hour (R/h).

Acoording to the Health Physics and Radiological Health Handbook (p. 294,
1992 edition), N-16  emits several gammas ranging in energy from 1.75 MeV to
7.12 Mev with the highest probabilities per decay of 69% (for 6.13 Mev
gammas) and 5% (for 7.11 MeV gammas).  Further, ICRU Report 33 (Radiation
Quantities and Units) notes on page 14 that: "With present techniques it is
difficult to measure exposure when the photon energies involved lie above a
few MeV or below a few keV".  Cember (Introduction to Health Physics, Third
edition) notes on page 170 that "...the use of the exposure unit is limited
to X- or gamma rays whose quantum energies do not exceed 3 MeV".

My question, then, is: what exactly do people think they are measuring with
an exposure rate instrument and a high energy gamma source?  I suppose
measurments of scattered radiation ("skyshine") could be meaningful since
the energies of the scattered photons may be less than 3 Mev and correction
factors may be supplied by the instrument manufacturer up to this energy.
For the direct beam, however, it seems to me a special technique would be
required.    For example, an additional "build-up cap" can be fitted to
separate ion-chamber type detectors (which then connect to the meter);
off-hand, though, I don't see how this technique can be applied to the
Victoreen 450P, which has an integral detector built in to the instrument
case.

**************************************************************

Eric Cowdrey
Radiation Protection Officer
Department of Medical Physics
Manitoba Cancer Treatment and Research Foundation
100 Olivia Street
Winnipeg, Manitoba    R3E 0V9
CANADA

Tel (204) 787-2166
Fax (204) 775-1684
e-mail ericc@mctrf.mb.ca


-----Original Message-----
From: radsafe@romulus.ehs.uiuc.edu
[mailto:radsafe@romulus.ehs.uiuc.edu]On Behalf Of Chris Alston
Sent: Thursday, August 19, 1999 7:56 AM
To: Multiple recipients of list
Subject: Re: N-16 and 450P


P.S. The top end of the 450P is 5R h^-1. I don't know if that's high enough
for your needs. If not, you could try integrating dose for a brief
interval, and then correcting arithmetically.

Have fun
cja

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