Dear @riya,
Ok, I’ll reply first your questions and then I propose you a new option that I found:
- Yes, You defined the USERWEIG properly
- Yes, It is exactly what you are saying. There is another contribution that you are not considering due to the filter used in usrrnd.f
- It is because you are reducing your statistics due to the reduction in the Mo yield from other pathways.
- The USERWEIG does not change. It is ok in the way you defined it .
- I do not get other radionuclides… or at least you should not if you applied the filter in the routine of A = 99 and Z = 42
Anyways, if you are interested in this routine, we can discuss it in another moment.
Now, in order to obtain the scoring that you are looking for, I realized that you could you the routine usrmed.f. I attach here an example for your case.
In the manual you can observe that if you want to call this routine you need to add a card called MAT-PROP. There, with the USERDIRE option you can call this routine. The routine will be called every time a particle is going to be transported in one of the materials defined in MAT-PROP.
Finally, what you will find in the usrmed routine:
- the trackr.inc was included to the default usrmed routine. This was needed to be able to call the variable IAZTRK. This variable has the information of the radioactive isotope ( the A, Z and IS) that generated the particle currently transported.
- Using the same logic of the options I proposed you in previous posts, if we are transporting a photon (IJ = 7) and A != 99 and Z != 42, then the weight is redefined to Zero ( WEE = 0).
What does it mean? It means that only the photons generated by A = 99 and Z =42 will be considered in your scoring 
I hope It is a satisfactory solution to your problem.
As I said I attach the routine and the input file with the MAT-PROP card and a USRTRACK example to score the fluence.
usrmed.f (3.0 KB)
Mo_prod_ads_lv.inp (2.9 KB)
Mo_prod_ads_lv.flair (2.7 KB)
Cheers,
André