High value of ATRACK

I was working on scintillators. I used a radio active source i.e. sodium 22(Na 22) to produce positrons.
When I used ATRACK, to find the number of optical photons coming out of plastic scintillator. We were getting very high values(in order of 10^9) of ATRACK. This is probably happening due to the half life of Na22 because half life of Na22 is 2.6 years.
Is there any alternative solution to this problem. How can we reduce the time measured by Fluka using atrack.

Dear @KD_22,

You could use a positron beam instead of sodium 22.