Hello @rachel.hen.shukrun ,
From a quick glance it seems like you are using the standard mgdraw.f routine. Hence when you are calling this routine with USERDUMP specifying “step1” as SDUM, the standard output will be collected in the file with suffix “_step1” in binary format. Depending on the case you are studying this can can indeed take up quite some memory. Please have a look at for example this post on how to reduce the MGDRAW output file size.
If you would only like to count exiting particles making the surrounding geometry blackbody material is a good approach to save time. Of course it is up to your judgment if this corresponds enough to a realistic situation.
Cheers,
Andreas