# Impossiblly big cross section of reaction

Hi I want to calculate number of different channels of forming Be nuclei from reaction gamma+16O. But when calculate the cross section of this process (gamma+16O->Be+something) it obtains much more bigger than cross section of gamma+16O->neutron+15O, which is in principle impossible. I mean that number of occur reactions gamma+16O->Be+something is much more times bigger than number of gamma+16O->neutron+15O reaction. What is the problem ? Here are .inp file and file for USRDUMP.

mgdraw.f (10.2 KB)

photonuc.inp (1.3 KB)

At 70 MeV photon energy, as in your input, that’s not at all impossible. Your expectation holds at the GDR peak in the 20 to 30 MeV range.

But the same situation is for example when energy of primary gammas equal to 33 MeV. The main problem is that when calculate the cross section of gamma+16O->7Be+something reaction using the number of it there are obtained enormous big values more than 1 barn. As for total cross section of gamma+16O interaction in range 40-50 MeV is about 10 times bigger than experimental values.

This is not the case, the two just get equal above 35 MeV.

This just means that your normalization is wrong, as the reaction cross section at 40-50 MeV turns out to be of few mb in good agreement with data. Most likely you missed the fact that a biasing factor of 100 is applied (LAM-BIAS) and has to be taken into account.

Can you tell please what is the sense of this factor ? Onto what it impacts ? What happened if I set it to 0.1 ?

The answer is in the manual:

``````the hadronic inelastic interaction length of the
particle is reduced by a multiplying factor WHAT(2)
``````

meaning that the respective inelastic interaction cross section is artificially increased by a factor 1/WHAT(2) (`= 1/0.01 = 100`), in order to get more easily a suitable statistics of the interaction products (since the photons will much more likely undergo a photonuclear reaction), which is what you asked for in the first place. If one uses the built-in scoring cards, this factor is automatically taken into account in the results. However, this cannot be the case for a customized event-by-scoring, where it should be considered in your own normalization.

Based on the above, the answer to your last question should be now evident.