Re: How do i find the amount of beta particles emitted from say a 6mm source??

Kerri-Ann:

There are several ways to determine this, depending on how precise and 
accurate you would like to be.

The best way is to determine the isotope you will be measuring and obtain 
a standard source of that isotope in exactly the same configuration as the 
source you want to measure.  That is, it should have the same diameter and 
should be mounted on the same type of material (eg, plastic or stainless 
steel) as your "unknown" source.  In the United States, we obtain sources 
that are traceable to our National Institute for Standards and Technology 
(NIST).  Different isotopes can emit beta particles with vastly different 
energies, so it really can be quite important to at least use an isotope 
that emits a beta particle with approximately the same energy.  The 
detector you use can be important also, because some Geiger Counters have 
much thicker windows which the betas must penetrate than other detectors 
have.  Plastic scintillation detectors can also be used, and these 
frequently have very thin windows to keep light out.  So, in the ideal 
situation, you will be comparing your unknown to a source of exactly the 
same beta particle energy spectrum in exactly the same geometry with 
exactly the same substrate (because betas CAN be backscattered off the 
substrate into the detector) positioned exactly the same distance from the 
same detector.  Then, if you count the known and the unknown sources for 
exactly the same length of time, the ratio of the two counts will be equal 
to the ratio of the two source activities.  If the known source is 1000 
becquerels (1000 Bq) and gives you a count rate of 150 counts per second, 
then if the unknown gives you 300 counts per second, it must contain twice 
the activity, or 2000 Bq.  (A becquerel (Bq) is a unit of activity equal 
to one disinegration per second).

There is a quicker, less precise way to get an estimate of the unknown 
activity without purchasing a standard.  If you have a source of beta 
particles, perhaps an instrument check source, that emits beta particles 
of approximately the same energy and you can determine from its size and 
shape that you will probably be detecting the same percentage of emitted 
particles as from the 6 mm unknown source, you can use the above technique 
to make a pretty good estimate.

Even faster, but more prone to error, would be to use a counter of known 
efficiency for counting beta particles.  If the counting efficiency was 
determined using the exact same isotope as the unknown, you will get a 
pretty accurate answer, especially if the source geometry is similar.  But 
if the beta energies are very different, your result will be much less 
accurate.

I apologize for the lengthy answer, but I couldn't tell from your brief 
question how important the accuracy or precision of the result was to 
you.  And, speaking of that, the precision of your answer in any of the 
above examples will depend to some extent on how many counts you 
accumulate while making the measurement.  Very roughly, the percent error 
of any count will be no smaller than (the square root of the number of 
counts divided by the number of counts).  Thus, the percent uncertainty 
for 400 counts would be about 20/400, or +/- 5%.  The bottom line is: the 
more counts, the more precise the answer is.

I hope this helps.  Please write back if you have additional questions.