RE: definition of liquid hydrogen?

Dear Brandon,

you say that "it is well known that liquid hydrogen shielding drastically
lowers dose". That is true for doses of neutrons with energies higher than a few
tens of MeV, but not for neutrons of much higher energy.
For a typical shield of a high energy proton accelerator, the best shielding
configuration consists of a layer of high-Z (high atomic number) material, such
as steel, followed by a low-Z shield with high hydrogen content - most often
concrete. This scheme takes advantage of high inelastic cross sections such as (n,xn) or
(n,n') in high-Z materials to reduce the neutron energy effectively. The lower
energy neutrons generated in this process are then best attenuated by moderation
and absorption down to thermal energies in hydrogenous material.

Of course, I realize that a steel shield would be difficult to set up in space.
But don't be surprised that neutrons from 1 GeV spallation are only partially
shielded by pure hydrogen.

Alberto

On Fri, 11 May 2012, REDDELL, BRANDON D. (JSC-EV511) wrote:

> Thank you for the help with the H2 definition in FLUKA. The problem I am still
> having is that it is well known that liquid hydrogen shielding drastically
> lowers dose. In the case that I am running, I am only seeing an ~18% reduction
> from what aluminum shielding provides. I've attached the two cases. Both cases
> have approximately the same shielding mass (~55 g/cm2). Whether I run a
> spectrum of protons (like the GCR protons) or mono-energetic beam of around 1
> GeV, I get similar results. Other data shows that the reduction should be ~90%
> or so. Is there something wrong with either my input file or the definition of
> the material still? If, these input files are correct, then the secondary
> particles that FLUKA generates contribute 4-5x more than the beam particles
> (at this shielding thickness)..which suggests the effectiveness of the liquid
> hydrogen shielding isn't as great as thought to be.
>
> Brandon
Received on Mon May 14 2012 - 23:44:42 CEST