Re: Electron Capture Isotopes

[Keith Welch <welch@CEBAF.GOV>]

At 11:56 AM 8/31/96 -0500, you wrote:
>
>
>Monitoring Electron Capture Isotopes
>
>Analysis of some of our contamination containing the steel
>activation products Cr-51, Mn-54, Fe-55, Fe59, Co-58 and Co60
> show that the activity is dominated by the electron capture
> isotopes Fe-55, Cr-51 and Mn-54 ( up to a ratio of 30:1 wrt
> Co-60). 
>
>Our normal method of monitoring items out of Contamination
>Controlled Areas is to use either scintillation probes (NET
>BP4's) or pancake geigers (HP260's), however these have very
>low efficiencies for detecting the x and gamma radiation from
>electron capture isotopes( approx 0.5%) and for this reason
>we have begun using 44B X-ray detctorsas well.
>
>I would be interested to know if any other facilities are
>experiencing similar problems and how they ensure that items
>removed from Controlled Areas are kept below regulatory
>contamination levels.
>

Bruce, we have a similar situation.  When monitoring for volume activation
in materials, we use a plastic scintillator that reads out in micro-rem/hr.
But for surface contamination with isotopes like Cr-51 and the others, it
gets real touchy.  Sometimes, we have enough beta emiters in the mix to have
a shot at seeing it with standard techniques, but at other times, we have
the same problem of the e-capture stuf dominating.

Our answer so far is to take our swipes back to the lab and count on a HPGe.
We have also tried using a thin NaI xray probe in the field hooked to a
portable spectrometer (BTI Microspec).  It works pretty well, but for
checking a swipe or surface, the count times needed are very long and it
becomes impractical.  However, this does not address "frisking" for
contamination on personnel - any suggestions???  We check articles of
protective clothing and do intensive swipe surveys to make up for the fact
that we do not have a good way to monitor personnel for these isotopes.

In the meantime we have begun looking for regulatory relief.  The bottom
line is that these nuclides should not be controlled to the same limit as,
say Co-60, since the radiotoxicity is orders of magnitude smaller.  DOE
forgot about us when they wrote into the rule that "nuclides with decay
modes other than alpha" were to be controlled to 1000 dpm.  The original
basis for using 1000 dpm for a limit was that it was DETECTABLE with field
instruments.  This is true for typical nuc plant fission products but not
for us.  We need a limit for "non-beta, non-alpha emiters" which is based on
DETECTABLE levels as well.

Sorry I don't have an answer to the question.  Any suggestions you have are
welcome.
Keith Welch
Thomas Jefferson National Accelerator Facility
Newport News VA
welch@cebaf.gov