Suppress pair production process

Dear Users
I am trying to simulate a setup where a proton beam is incident on a teflon target. The resultant gamma rays (PIGE) are incident on a sample where they pair produce positrons. The annihilation of the positrons are monitored in HPGe detectors. A collimator is placed between the teflon target and the sample. The detectors are placed out of sight of proton beam.
I can record the pair production events in the sample using usdraw and usrtrak. The positron annihilation gamma (511 keV) is recorded by the DETECTOR card. the output of the DETECTOR will have contributions from - (i)annihilation gamma rays from sample and (ii) the gamma rays that originate from the teflon target and scatter from the sample and get registered in HPGe. My question is that is it possible to suppress the pair production process in the sample so that I can separate out these two contributions?
I tried a coincidence setup with two HPGe detectors which look at the 511 keV gamma pair in time coincidence. The result showed very little difference, so probably the contribution (ii) is not that big. but still I would like an alternate way to make this comparison. I am attaching the input files.
proton_SSPALS_1.flair (5.0 KB)
proton_SSPALS_1.inp (3.1 KB)
mgdraw.f (15.2 KB)

thank you for your time and attention.

Hello Saurabh,

sorry for the slight delay in the answer.
I have a possible solution for your question following the same exact logic used in this tread.

A way to suppress secondary particles produced in a specific interaction is to use in combination strupre.f (since you are interested in positron/electron pair production, we need to check in the EMF stack of FLUKA) and usrmed.f

Through stupre.f, once identified the reaction of interest, the secondaries particles can be flagged using a user-variable.
Then once the flagged particles are transported in the geometry, through usrmed.f (activated with MAT-PROP card) their weight can be put as 0 and therefore they will give no contribution.

The logic needed for the two user routines is identical to the one in the thread I linked you. The only difference is that in your case, since you are interested in removing the contribution coming from positron/electron pair produced in a specific region (region number = 5), this filter need to be implemented in the if condition for strupre.f as:

52       CONTINUE
           LOUEMF(NPNW) = LLOUSE
           IF ( LPAIRP .AND. (IREMF(NPNW) .EQ. 5) ) THEN
              LOUEMF(NPNW) = 1
           END IF


Hope this help you to overcome your problem!


Dear Giuseppe
thank you for the answer. Let me try the solution and I will get back to you.