RE: Sealed sources: extension of recommended working life

["Joe Alvarez" <jalvarez@auxier.com>]

Since your question on the working life of a sealed source included mention 
of Pu-238, this answer addresses alpha emitters.  A major problem with 
alpha sources is that the source geometry changes.  The source atoms 
diffuse through the substrate.  Room temperature diffusion in solids is 
very slow at room temperature, but very important to the range of alphas 
after a few years.  In the case of a beryllium window (any window material, 
in fact)the window grows in thickness.  Oxide layers and other material 
tends to build up on the window.  These layers are impossible to clean by 
ordinary means.  The only way to extend the life of an alpha source is to 
have it recalibrated against a fresh standard.  It should be calibrated for 
the conditions of use.  This may require spectral as well as efficiency 
measurements.

Joe Alvarez
jalvarez@auxier.com

-----Original Message-----
From:	Dahlskog, Leif [SMTP:Leif.Dahlskog@health.wa.gov.au]
Sent:	Friday, March 06, 1998 1:58 AM
To:	Multiple recipients of list
Subject:	Sealed sources: extension of recommended working  life

Dear radsafers

I am seeking comments on extending the recommended working life (RWL) for
sealed sources approaching or having completed their RWL.

A source manufacturer lists a recommended working life (RWL) of 10 years 
for
their Pu-238 sealed disc sources with a beryllium window.  The period of 
the
RWL is based on:
-toxicity of nuclide,
-total initial activity,
-source construction,
-half life,
-typical application environments,
-operational experience
-test performance data, etc

Consider that it is often difficult to obtain a working history for the
sources, or knowledge of adverse environments or extremes of temperatures
that may have been experienced.

If sources are to be used past their RWL, what criteria should be 
considered
?.
If, to extend the RWL, a source capsule should be tested, would a
satisfactory test involving a visual inspection, a wipe test and immersion
test be considered sufficient to extend the working life ?
What frequency should the tests be performed ?

How have other regulators dealt with this issue ?

I am hoping to receive a response specifically from:
a) other regulators.
b) those involved in source manufacture.
c) those with experience in testing source capsules.
d) anyone who may have experience on this matter.

Please respond to me personally unless you consider this a worthy subject 
of
discussion on radsafe.

Regards

Leif Dahlskog
Scientific Officer
Radiation Health Section
Health Department of Western Australia

email:   leif.dahlskog@health.wa.gov.au