Source Used for Natural Thorium -Reply

[ABELQUIE@ORAU.GOV]

Andrew-

In the past, ORISE has determined the weighted beta energy of the
natural thorium series and used a beta calibration source appropriate to
that average energy.  The source we'd end up using was Tl-204.

Lately, we have adopted the MARSSIM approach for calibrating our
instruments.  This approach allows for the total efficiency to be divided
into two components - a source (surface) efficiency and an instrument
efficiency.  When determining the instrument efficiency we look at each
of the radionuclide emissions in the series (both beta and alpha),
consider its energy and yield, and determine an overall weighted
efficiency that considers the detector's response to both beta and alpha
emissions.  The surface efficiency helps us to account for the field
effects - backscatter and self-absorption.  D. Quayle (ORISE) will
present this approach at the HPS mtg in MN.

Some helpful references include MARSSIM, NUREG-1507, and an
article we published in RPM (Considerations for Decommissioning Survey
Design and Implementation at Uranium and Thorium Sites, July/August
1997).  

Regards,
Eric Abelquist
abelquie@orau.gov

>>> D1.OUT."aschwart@secor.com" 05/15/98 03:21pm >>>
What is the typical source people use to determine instrument
efficiencies when suveying surfaces for natural thorium contamination?
Ac-228 has 3 betas.  Should I use a source that is similar to the most
abundant beta or the beta with the highest energy?

Andrew Schwartz
aschwart@secor.com
SECOR