Re: COUNTING EFFICIENCY DETERMINATION

[David Scherer <scherer@uiuc.edu>]

At 03:28 PM 10/3/96 -0500, you wrote:
>Carl:
>
>If you do not have a standard radionuclide, with the specific energy that
>you want, you can use the following method.
>
>Obtain three standard sources of three different energies, determine the
>efficiency of each radionuclide. Graph the efficiency VS the energy, and
>extrapolate the curve to the desire energy, and efficiency. I used this
>method several times.
>
As long as the group has been harping on precision, I'd like to insert the
additional details that we all use but forget to mention.  (Also, we have
students lurking on this list.)

If you follow this approach, you have to take the nuclides' gamma ray
intensities (or yields) into consideration.  For example, if you calibrate
with the 662 keV photon from Cs-137 (or Ba-137m), you must divide by .8998
to obtain the detection efficiency to be plotted on your calibration curve.
The counting efficiency for your new isotope will be the product of the
number from the graph and the intensity of the new isotope's yield.  For
example, I-132 emits a 668 keV gamma with 98.70 percent yield.  Since the
energies are so close, you might be tempted to use the raw cpm/dpm from the
Cs-137 calibration.  However, if the different yields are not factored in,
the I-131 measurement would be off by 9.7 percent.

Question: Would the I-131 measurement be high or low?  Send PRIVATE e-mail
for the answer.

Regards,

Dave Scherer
scherer@uiuc.edu