Dear Lee,
your questions are based on a wrong idea of neutron dosimetry and transport.
It is difficult to give you a lecture on neutron dosimetry in a simple email.
You should read some good textbook.
I will summarize here the main flaws in your concepts.
1) 2.45 MeV neutrons do not produce any signal in a He-3 detector. To do that,
they must first be moderated and slowed down to thermal energies. FLUKA's
primary particles (BEAMPART) in your case are 2.45 MeV neutrons.
There probably remain very few of them (or none at all) after 3 cm of
polyethylene: the others have been scattered and have lost some of their
energy and are not "BEAMPART" particles anymore. However, probably 3 cm are
not sufficient to fully thermalize 2.45 MeV neutrons: most of them will have
intermediate energies, mainly too high to be detected by He-3.
It will be different for 0.5 cm polyethylene: you can certainly score
several BEAMPART neutrons, but they will have nothing to do with what you
can measure experimentally.
2) It will be very different if you score NEUTRONs instead of BEAMPART. In this
case you will score all the neutrons, independent of their energy: that will
include all the neutrons which have been scattered and moderated. Some of
them can be detected by He-3, others will have still energies too high.
Again, you cannot expect to be able to predict the response of your detector,
unless you take the energy distribution carefully into account.
I remind you that in any case the response of a real detector is
proportional to fluence (of the right energy!), and not to current.
3) To measure, and to calculate, a macroscopic cross section, you need to
set up a "good geometry". That is a geometry where both the source and
the detector are collimated, so that the detector does not "see" any
scattered neutron. It does not seem to be your case. In a "bad" geometry,
i.e. without a double collimation, the equation I=I0*e-^(Ķē*d) does
not apply, but instead of an attenuation you will have a build-up
factor due to neutrons which are not initially directed on the
source-to-detector direction, but are scattered back to the detector.
This is probably the reason why you find a fluence larger than expected.
In FLUKA, perfect collimation can be achieved by means of a "blackhole"
collimator. In the actual world, only an approximation is possible.
4) In FLUKA, as already pointed out by Francesco, for low-energy neutrons
elastic and inelastic scattering cannot be disentangled. The program only
"knows" the average energy loss, which can be a mixture of elastic and
inelastic interactions.
5) Thermal neutrons (those that are detected by a He-3 detector) and also
epithermal neutrons (of energies slightly higher) are not moving in a
definite direction. They are scattered back and forth, exactly like atoms in
a gas (that is why they are called "thermal") and the same neutron can
cross a given point more than once. Any attempt to measure or calculate
their attenuation in a given direction is meaningless.
Best regards,
Alberto
On Wed, 16 Nov 2011, lzfneu wrote:
>
> Dear Francesco and all,
> First, thanks Francesco for the kindly help. But I still have some questions
> to consult you.
>
> 1) If I choose "fluence" with "neutron" in the USRBDX card, the simulated
> emergent neutron fluence (I) is larger than the incident one (I0) when
> polyethylene is 0.5cm and the result is not the same as that of the
> experimental one. I really don't know how to calculate the neutron macro
> cross-section for 2.45MeV neutron using the equation I=I0*e-^(Ķē*d) for 0.5cm
> polyethylene.
>
>
> 2) If I choose "fluence" with "beampart" for 2.45MeV neutron, the emergent
> neutron beampart is lower than the incident one and the result is lower than
> that of the experimental one.I am a layman in this field, I don't quite
> understand what the first generation particles' meaning is when using
> BEAMPART. Could you please explain it easy to understand ? Do you mean that
> the nuclear elastic and low energy neutron elastic scattering is not
> considered to increase the generation number when using BEAMPART. Am I right?
> Thank you ! 3) By the way, it is odd, only when choosing "current" with
> "neutron" in the USRBDX card, the simulated result is nearly the same as that
> of the experimental one when using He3 proporational counter tube for 0.5cm to
> 3.0cm thickness polyethylene, I don't know why ?
>
> Thanks in advanced and any help will be appreciated !
>
>
> Regards
>
>
>
>> Date: Mon, 14 Nov 2011 18:39:18 +0100
>> From: Francesco.Cerutti_at_cern.ch
>> To: lzfneu_at_live.com
>> CC: fluka-discuss_at_fluka.org; giuseppe.battistoni_at_mi.infn.it
>> Subject: Re: Question about neutron counting
>>
>>
>> Hallo
>>
>> I would say that the relevant quantity, physically meaningful, is - as
>> almost always (with the respective particle species) - 2) "fluence" with
>> "neutron".
>>
>> BEAMPART refers to first generation (LTRACK=1) particles. Any discrete
>> event (say interaction, apart from delta/Moller/Bhabha and nuclear elastic
>> scattering - but note that for low energy neutrons elastic scattering is
>> not disentangled as an independent interaction type - ) increases the
>> generation number.
>>
>> Best regards
>>
>> Francesco
>>
>> **************************************************
>> Francesco Cerutti
>> CERN-EN/STI
>> CH-1211 Geneva 23
>> Switzerland
>> tel. ++41 22 7678962
>> fax ++41 22 7668854
>>
>> On Sat, 12 Nov 2011, lzfneu wrote:
>>
>>>
>>> Dear all,
>>> I want to calculate the theoretical transmission value (I/I0) of
>>> polyethylene for 2.45MeV neutron which range from 0.5cm to 3.0cm thickness
>>> and compared with the experimental one. The counting experiment is He3
>>> proportional counting tube. I am confused that which one I should choose in
>>> the USRBDX card to do the simulation: 1) "fluence" with "beampart";2)
>>> "fluence" with "neutron";or 3) "current" with "beampart";4) "current" with
>>> "neutron"; I am also confused that what the difference is between the
>>> "beampart" and "neutron" in the USRBDX card when neutron is scored, thank
>>> you !
>>> Thanks in advance and any help will be appreciated !
>>> RegardsZ.F.Lee
>>>
>>>
>>>
>>
>
Received on Thu Nov 17 2011 - 09:07:22 CET
This archive was generated by hypermail 2.2.0 : Thu Nov 17 2011 - 09:07:33 CET