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
500 CONTINUE
Hope this help you to overcome your problem!
Cheers,
Giuseppe