Re: When to use self-shielding materials?

Dear Joachim,

Thank you for your response.
I have done some comparisons between self-shielded and non self-shielded lead. Unfortunately I don't think I have conclusive
results. Neutron spectrum changes a little bit (only in certain bins, as expected), but the differences in activation are mostly
statistically insignificant, except of a couple of isotopes in lead, and even then it's not very obvious that self-shielding should
be blamed for it.
What I still don't understand is what is the disadvantage of using self-shielded cross-section. In other words, if self-shielded
cross-section exists for certain material, why not to use it every time this material is assigned?

Thanks again,

  -George

----- Original Message -----
From: "Joachim Vollaire" <joachim.vollaire_at_cern.ch>
To: "George Kharashvili" <georgek_at_jlab.org>, fluka-discuss_at_fluka.org
Sent: Sunday, October 14, 2012 4:29:24 PM
Subject: RE: When to use self-shielding materials?

Dear George,

As a first test I would try to score the "averaged" energy dependent neutron
fluence within the dump using a USRTRACK card and both sets of cross sections
(self-shielded or not). If you have the production cross section of the isotopes
you are measuring in your foil, you could then make a sensitivity study which
would give you an hint on the effect. It is rather subjective but the size of your
dump is maybe not that "huge" for the self-shielding effect to be significant.

Also, if the thin sample is made of lead and is within the lead (if I understood correctly)
I would use the same cross section for both as fluence wise, there is no heterogeneity between the two....

Cheers
Joachim

-----Original Message-----
From: owner-fluka-discuss_at_mi.infn.it [mailto:owner-fluka-discuss_at_mi.infn.it=] On Behalf Of George Kharashvili
Sent: 11 October 2012 21:20
To: fluka-discuss_at_fluka.org
Subject: When to use self-shielding materials?

Dear FLUKA experts,

Could you please explain (again!) what is the disadvantage of using a self-shielded neutron cross section?
One of the advanced course lectures has general guidelines on this issue. It says to use self-shielded materials with
    "Bulky (huge) pieces that are very pure (containing only one isotope)" and not to use with "small iron, copper, lead,
aluminum pieces and diluted materials".
-What is considered small and what happens if one uses self-shielded material with small objects?

FLUKA manual also says that the presence of even small amounts of impurities is generally sufficient
to smooth out the effect of self-shielding and, if I understood it correctly, it should only be used with very pure,
single isotope materials.
-If so, when should we use self-shielded natural lead and natural tungsten?

I am modeling photon and neutron activation in thin foils (0.01-0.1 mm thick) that are placed inside a
15x20x40 cm^3 beam dump. The dump is made of lead and tungsten and is exposed to 3GeV electron beam.
Should I use self-shielded lead and tungsten for the bulk of the dump and non-self-shielded lead for thin lead samples?

Thank you in advance for your answer(s).

    -George
Received on Fri Oct 19 2012 - 10:05:23 CEST