Half life of potassium I'm working on half life of potassium using potassium nitrate with GM Counter. When I increase the mass (1 g) to get more counts number of counts decrease as compared low mass (.5 g). With large mass decay should be large. Could you tell me whats happening here and what I have to do?
I have tried potassium nitrate using powder form as well as in liquid.
 A: The half life of Potassium-40, ~$10^9$ years, cannot be measured directly and I assume is to be found via the measured activity of a sample of known mass and composition.
The problem is that potassium-40 is a beta emitted and some of the emitted particles are absorbed by the sample.
If the sample is confined in cylindrical vessel of fixed base area and the detector is aligned along the axis of the cylinder, as more of the potassium nitrate is added the fractional increase in count rate is less than the fraction increase in mass of sample due to the increased thickness of the sample resulting in proportionately more betas being absorbed.  
So the "shape" of your sample is important as a tall thin cylindrical of the sample will result in a smaller count rate than a short fat cylinder of the sample of the same mass because in the first case the sample is thicker and hence there is more self absorption.
Because of the long half life the activity of your sample will be relatively low and so background radiation will play a significant part in the accuracy of your experiment.
You might consider shielding your sample/detector region as that might improve the accuracy of your experiment?
If $N$ counts are registered by your detector then the standard deviation is $\sqrt N$ and so increasing the time over which the counts are taken reduces the error by a factor $\sqrt{\rm time}$.
This means that you must decide on a reasonable/optimum time over which to measure the background count and the count when the sample is present.
Rather then "small count over a shortish period of time (your 300 seconds) and often (your 10 times) go for longer intervals but note that the background count rate can also change at different times of the day.
One reading taken over a period of 30 minutes might be sufficient?
Keeping the area of your sample presented to the detector the same, measure the count rate with the sample for different masses of sample making sure that the count (not count rate) recorded by your detector is accurate enough to produce a significant difference between the count with the sample present and the count due to background radiation with the sample absent.
As the mass of the sample is increased the corrected count rate should increase and if it does not it might well be due to you changing the "shape/area" of the sample or observing too few counts.
If you draw a graph of corrected count rate per gramme of sample against mass of sample it should show a downward trend.  
If you have time a way to applying a simple correction for the self absorption is to draw a graph of $\log({\text{corrected count rate}})$ against mass of sample and extrapolate the resulting graph to zero mass.  
So I suggest you repeat your readings making sure that you do this over a sufficient period of time to have a significant difference between count with sample present and count with sample absent. 
One final note is that you will need to know the detection efficiency of the detector given that approximately half the beta will be emitted in a direction away from the detector and the detector window etc will also aborb betas.
