Re: Neutron problem

Dear Zhixi Yue,

I do not know if I'm going to turn your frustration, but let's see.
Within the framework of group transport applying in FLUKA to low energy
(<20MeV) neutrons, no explicit production and transport of charged
reaction products takes place, but - as stated in the manual - their
energy is deposited at the point of interaction (kerma approximation). The
only exceptions are represented by recoil protons in hydrogen, protons
from (n,p) reactions in nitrogen and alphas and tritons from neutron
capture in 6Li (though in the manual - Chapter 10 - also 10B is
erroneously mentioned, for which only the explicit generation of the 478
keV 7Li de-excitation gamma from 10B(n,alpha)7Li is available for
the time being. Indeed the next major release will just include also
alpha/Li7 from 10B).
Above 20MeV, the neutron induced reaction's simulation is described by a
model (PEANUT) and not through cross section libraries, and secondaries
are then explicitly generated and transported. But, as soon as the neutron
energy goes below this threshold (where in your case the alpha production
cross section on B is going to increase), you will no longer get charged
particles, as said above.
On the other hand, even below 20MeV neutron energy, protons, deuterons and
tritons in 3He, and alphas and 7Li's in 10B compounds can be found as
residual nuclei through RESNUCLEi scoring. Their contribution to energy
deposition is included in USRBIN scoring (as pointed out at the
beginning), but you will never see them through fluence scoring, like your
USRTRACK, since they are not transported.
As a side remark, I would like to recall that, as soon as 'energetic' -
i.e. far from the thermal region - neutrons are concerned, the
contribution of elastic recoils to the signal of a 3He detector cannot be
neglected.

Hope this somehow helps

Kind regards

Francesco

**************************************************
Francesco Cerutti
CERN-EN/STI
CH-1211 Geneva 23
Switzerland
tel. ++41 22 7678962
fax ++41 22 7668854
Received on Fri Feb 05 2010 - 21:11:27 CET