Re: Instrument Calibrations

[John Lobdell <lobdell@fly.HiWAAY.net>]

At 01:41 PM 7/3/96 -0500, you wrote:
>Sorry I just getting involved in radsafe very recent so I may have 
>missed some correspondance.  But anyway,  the NRC does not accept 
>Pulsing as a technique to calibrate survey instruments.  They must be 
>calibrated by using a point source and must have at least 2 points per 
>scale.  You may elect to pulse the meter first to get you close, but 
>then it must be calibrated using NRC approved cal. technique.
>
>I'd be happy to help either giving you a technique or protocol or even 
>calibrating them for you.  Just write back.
>
>Good Luck
>


-- Reply --

I believe that partially calibrating an instrument with a pulser is
acceptable under the following conditions:
1) the detector produces a pulse for every ionizing event in the detector,
2) the pulser duplicates the polarity and amplitude of the pulse supplied by
the detector, and
3) the instrument readout should be in counts or counts per minute.


The calibration of the instrument can then proceed as follows:

1) Determine the polarity and amplitude of the pulse supplied by the detector.

2) Adjust the amplitude and polarity of the pulser to match that supplied by
the detector.  Also the pulser must be able to "block out" the high voltage
normally supplied by the instrument to the detector.

3) Connect the pulser to the detector plug on the instrument.  Adjust any
thresholds, gains, and lower level and upper level discriminators in the
instrument to accept the pulse.

4) Adjust the digital or analogue readout on the instrument to read "counts"
or "counts per minute" to agree with the "pulses" or "pulses per minute"
supplied by the pulser.  If the pulser can supply many different
frequencies, it is possible to adjust many or all scales on an instrument at
two or three points on each scale.  At this point, you know that the
electronics are working satisfactorily.  BUT the total instrument is not
calibrated since the instrument and detector have not been calibrated as a
unit. 

5) After you know that the instrument and pulser agree, remove the pulser
and connect the detector.  Place the detector in a reproducible geometry
with a NIST traceable source that provides the alphas, betas, or gammas at a
known disintegration rate from the isotope you desire.  In this geometry,
you can determine the counting efficiency by dividing the count rate by the
disintegration rate.  Place a calibration sticker on the side of the
instrument stating the efficiency and geometry, and you are good to go.  If
you change the geometry, isotope, or detector, it must be recalibrated.

We have not had any regulatory problem with this procedure for alpha and
beta instruments used to detect levels of contamination.

John Lobdell, Ph.D., C.H.P.
lobdell@HiWAAY.net
These are my thoughts and are not guaranteed to keep you out of regulatory
problems.
>