Neutron yield for 1 GeV proton beam on copper target

Dear experts,

For the same input as shared here Plot error/warning - #4 by riya, when I checked the .out file I found in the section : Number of secondaries generated in inelastic interactions per beam particle,

1.3820E+01 (39.6%) 3.0000E-04 ( 0.0%) NEUTRON

In a text book by A.H. Sullivan, the no of secondary neutrons is given around 6 (see the attached book page).

Can you please guide if any other physics card needs to be activated in this case?

I am attaching the .out file here for your reference.

1GeVproton001.out (172.5 KB)

DocScanner 14-Nov-2022 4-47 pm.pdf (448.7 KB)


Hi Riya, it’s not a matter of physics cards, which in principle are not meant to change physics results at will, such as altering by a factor of 2 the neutron population. Rather, you are comparing different quantities. The output counter you refer to is not designed to subtract the projectile from the sum of the products. So, a proton directly generating on average two neutrons that in turn undergo one nuclear reaction producing two neutrons each, will give 6 from the counter, while in reality you got 4 neutrons (since 2 ‘disappeared’ by nuclear reaction).
To see how many neutrons are actually emitted from the target, you could score them on the target boundary.

Thank you @ceruttif ,

As you mentioned, I also added one detector around the target to score number of neutrons (usrbdx card in the input; output unit 32, shared in the above link). There also I could see a value of around 12 particles per primary.


Hi @riya I think the problem is on the definition of the thick target. Could you see on the mentioned reference how thick was the target they are using.
Also having your target inside a concrete cage will further amplify the neutron fluence (as scored with the boundary estimators)

Actually the problem is just your USRBDX scoring: if you want to count exiting neutrons, you should ask for current, not fluence (look at the beginner scoring lecture). Doing so, one gets indeed 6.5 neutrons!

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Thank you @ceruttif for the guidance.