Mis-Interpretation of Beta Source Calibration Certificates

["Leon E Brown" <lebrown@cmsenergy.com>]

The Big Rock Point (BRP) Restoration Project in Michigan recently discovered a
problem with mis-interpretation of beta source calibration certificate geometry
(i.e., 2 pi vs 4 pi).  The purpose of this posting is to share this information
and to ask if anyone else has experienced this problem.

During an Oak Ridge Associated Universities training course on implementation of
MARSSIM, the attendees were cautioned that some licensees have failed license
termination surveys due to mis-interpretation of beta source calibration
certificates (i.e., 2 pi vs 4 pi geometry).  A subsequent review of BRP gross
beta counter calibration practices disclosed that several plated beta sources
purchased in 1976  have certificates which indicate source activity, but do not
designate either 2 pi or 4 pi geometry.  An investigation determined that the
source certificates provided actually represent 2 pi beta emission rates, not 4
pi activity as they had been used in the past.  As a result, past calibrations
performed using these sources, have overestimated the instrument efficiency.

These sources are deposited radionuclides on a copper disc backing material.
The source certificates specify what the vendor calls a principal activity in
betas/minute.  The source certificates state that the calibrations are based
upon deposited activities on a copper disc, using beta efficiencies of a
proportional counting system.  None of the source certificates provide
clarification that the listed principal activity is for a 2 pi or 4 pi geometry.
Historically, since a principal activity was specified, it was assumed that the
principal activity was for a 2 pi beta emission rate converted to a source
activity in 4 pi geometry.  Comparison of these sources with secondary sources
prepared from primary liquid standards indicated that the certificates actually
provide a 2 pi beta emission rate.

The consequences of these past non-conservative calibrations have been minimal.
Because of backscatter from the copper backing material, use of a 2 pi beta
emission rate as a 4 pi source activity resulted in 20 to 40 percent
over-estimates of instrument efficiencies.  The beta counters involved were used
predominately as screening tools to establish levels of gross activity which
required follow up isotopic analysis using gamma spectrometry.

As a result of this experience, BRP has initiated a source verification program.
Upon receipt of new calibration sources, their certified activity will be
compared to previous calibrations, to other sources or verified by gamma
spectroscopy, as appropriate to nuclide type.

Since this event has received considerable interest by those who oversee our
activities, we would like to know if ours is an isolated occurrence or if others
have also experienced a similar problem with mis-interpretation of beta source
calibration certificates.

Leon E. Brown
Sr. Consultant
Big Rock Point Restoration Project
lebrown@cmsenergy.com


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