RE: Sr-90 Source Activity/Geometry

[Falo Gerald A <Gerald.Falo@APG.AMEDD.ARMY.MIL>]

Bill,

The activity of the source is independent of the geometry.  I think what you
are looking for are the 2 pi and 4 pi emission rates.  Furthermore, the Y-90
would be in be in secular equilibrium with it's Sr parent.  Each nuclide
emits 1 beta per decay so the total beta emission rate is twice the activity
of the sample.

Total Beta Rate = 2 (betas/decay) * 802 decays/second = 1604 betas /second

However, the total activity of Sr and Y is 1604 Bq (802 Bq of Sr and 802 of
Y) or 96,2400 dpm-(Sr/Y).

Now, half of these will be emitted in each of 2 pi steradians; ie, half exit
one side of the source and half exit the other side.  This neglects self
attenuation and backscatter within the source and mounting itself. With
these assumptions ans the assumption of equal efficiencies for each electron
emitted:

The expected counting rate <R> = source Sr-activity (dpm) * eff
(count/particle emitted) * Yield (particles emitted/decay of Sr.

<R> = A * eff * Y.

So the measured Sr-activity is give by A = R / (eff*Y).  The contibution of
the Y-beta is included in the yield.

In this case, the efficiency is the number of counts recorded per particle
emitted.  This is emitted in all directions ( 4 pi).  The product of the
yield and efficiency has the units of counts per decay of Sr.  If we
multiply the numerator and denominator by the same unit of time, eg.,
minutes, and take the reciprocal, then we have a calibration factor with
units of decays per minute of Sr / counts per minute.  This calibration
factor is easy to meaure.......just put the source in the counting geomtry,
measure the counting rate, and divide by the Sr-activity of the source.

<Phew> In summary, divide the counting rate by the activity of Sr (802 Bq)
and that is your calibration factor - with all the caveats about geometry,
backscatter, etc.

I hope that these, my humble opinions, help.

Jerry Falo, Ph.D., CHP
HMJ Professional Associate
USACHPPM
Medical Health Physics Program
gerald.falo@apg.amedd.army.mil




-----Original Message-----



We recently purchased a Sr-90 source from Amersham.
The certificate of calibration lists the activity as 802 Bq, which is for
the Sr-90 only.
Our crew is having a discussion about what the activity, in dpm, would be
for 2 pi and 4 pi geometry.
Also, if you are calibrating a CAM with this source, would you use the 2 pi
or 4 pi dpm to determine efficiency?
Anyone care to comment?

Bill Miller
INEEL


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