How do i improve statistics (produce more neutrons)

Dear Fluka Users
 
I am characterising photoneutrons produced by a 15MeV Linear Accelerator. Iam using USRTRACK to score neutron fluence spectra at various locations inside and outside the Bunker as shown in my input file.I can score neutrons at these locations but my problem is that iam running 5.8 Billlion(20 Mil/cycle) histories and it takes one week to finish 4 cycles but there is no improvement on the statistics of the spectra scored on locations outside the bunker.
 
Could someone please help me on how I can improve (generate more neutrons) on the statistics of my scores in a reasonably shorter time.I have attached my input and project files to this mail. Also attached is the log-log neutron fluence spectrum behind the door outside the bunker, you can seee that the statistics is still very poor.
 
Please help
 
Lawrence Mhatiwa
 
Medical Physics Department
University Of Free State
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