RE: Gamma Spec

[bsielaff@canberra.com]

In addition to the previous observations, I would like to add that when a
spectrum is reloaded for further counting, the stored "acquisition start
time/date" will be associated with the restart, not the initial acquisition.
This may affect decay correction back to the original sample date.  Also, if
considerable time has passed between the initial acquisition and the additional
acquisition, one needs to ensure that the system setup is identical - that is,
that neither long term drifts nor intentional readjustments have altered the
system characteristics from the initial acquisition.  Modern DSP electronics go
a long way towards reducing the effects of drift, but do not eliminate them
entirely.  If all the issues raised are taken into account, the approach should
be successful.  I believe all suppliers' equipment and solftware allow the
reload of a spectrum and restart of acquisition.

Mr. Bradshaw also touches on the possibility to avoid the problem in the first
place by using a software facility to count to a specific LLD or MDA.  This
capability has been available in Canberra Genie systems (VMS and PC platforms)
for some years.  The user sets desired MDA values for each nuclide of interest
in the nuclide library.  Then, when the feature is invoked, the system counts
for a minimal period, analyzes the spectrum and projects the count time needed
to reach all MDAs - then iterates that process until all MDAs are reached.  A
user defined maximum count time stops the acquisition in the event you have
asked it to do the impossible - i.e. count to an MDA that cannot be achieved.
Users of this capability have reported that they have essentially eliminated the
need for recounts and extended counts due to missed MDAs.  More significantly -
where labs had set their fixed counting times to be somewhat conservative to
ensure they don't miss detection limits - these labs report that the feature
considerably shortens counting times for many, if not most samples - a real
productivity gain.

More info on this is available from Canberra (end of advertisement)

Regards,
Bud Sielaff
Director of Instruments Marketing
Canberra Industries






"Bradshaw, Keith" <Keith.Bradshaw@nnc.co.uk> on 03/03/2000 10:17:17 AM

Please respond to radsafe@romulus.ehs.uiuc.edu

 To:  Multiple recipients of list <radsafe@romulus.ehs.uiuc.edu>
 cc:   (bcc: Bud Sielaff/CI/US/PBC)
 Subject:  RE: Gamma Spec






> -----Original Message-----
> From:   dpharrison@aep.com [SMTP:dpharrison@aep.com]
> Sent:   Friday, March 03, 2000 1:19 PM
> To:     Multiple recipients of list
> Subject:     Gamma Spec
>
>
>
> Question for gamma spec SMEs (which I am not):
>
> You count a sample on a HPGe for a certain amount of time, but it doesn't
> meet a
> specific radionuclide LLD.  You calculate the new time where the LLD will
> be
> met.
>
     Because of it's statistical nature, you can't really calculate this.
You can *estimate* the additional time only.

>   Can you, instead of recounting the sample for the entire duration of the
> new calculated count time, start the count on top of the original spectrum
> and
> just count for the difference in time between the original count time and
> the
> calculated new time?
>
     I'm pretty sure that at least Canberra (Genie) and EG and G software
will allow you to recall a spectrum and resume counting on top of it.  Be
careful of any 'decay correction during counting' options that may be
enabled.  I've not been into it, but apparently some software packages will
count until a preset LLD is met.

     If you want to get deeper in the mire of estimating count times for
a certain LLD, see Health Physics vol 32 (Jan 1977) pp1-14: 'The statistical
interpretation of counting data from measurements of low-level
radioactivity' by Donn J J and Wolke R L.


     keith.bradshaw@nnc.co.uk




     END



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