Re: [BLAST_ANAWARE] neutron conversion efficiencies in blastmc

From: John Calarco (jrc@einstein.unh.edu)
Date: Fri May 23 2003 - 13:43:23 EDT


Hi Vitaliy,

   I just returned from visiting family in California. Someone may have
already answered your questions, but here are my 2-cents worth:

   Regarding the fall-off of efficiency with increasing energy, one
expects it to go as 1/velocity, which is proportional to 1/momentum
which looks like exactly what it's doing.

   Regarding the threshold, most codes that calculate neutron efficiency
have some sort of energy threshold. Note that p = 100 MeV/c corresponds
to a kinetic energy of 5 MeV. That may be where the threshold is set.
We will never see these neutrons anyway. Neutron TOF is 72/sqrt(KE) ns/m
where KE is in MeV. So these neutrons are pretty slow, taking about
120 ns to traverse 4 m. That puts them just outside the TOF window.

                                                            John

On Wed, 21 May 2003, vitaliy ziskin wrote:

> I generated 100000 of neutrons in the momentum range from 0.03 to 0.5
> GeV in the theta and phi angles within the neutron was acceptance. Here
> is what I found about the efficiecy of neutron conversion in tofs and
> neutron wall
>
> p(MeV) tof wall
> total #
> p<100 0% 0% 14806
> 100<p<200 6.16% 13.9% 21375
> 200<p<300 6.87% 17.21% 21266
> 300<p<400 3.85% 10.89% 21306
> 400<p<500 3.77% 10.5 % 21247
>
> To me the efficiency numbers seem hight (p>100MeV) and I'm not sure why
> there is such a kinetic energy dependance.
> Maybe, ultimately the efficiency won't matter (it should drop out of the
> asymmentry). However, at least we should understand why there are no
> neutrons detected below 100 MeV. I hope it is some bug in monter-carlo.
> Maybe, Aaron can take a look at it since he is the mc guru around here.
>
> Cheers, Vitaliy
>

-- 
John R. Calarco
Dept. of Physics
Univ. of New Hampshire
Durham, NH 03824
phone: (603)862-2088
FAX:   (603)862-2998
email: calarco@unh.edu



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