Re: Score neutron from 16O(p,xn) reaction

From: Francesco Cerutti <>
Date: Sun, 9 Nov 2008 23:41:35 +0100

Dear Chenyen,

> I'm sorry that I was intended to model a proton beam of 20MeV, but I
> still do not know how to lower the threshold. ( Defaults -> New-DEFA )

transport cutoff can be set implicitly through the DEFAULTS card (e.g.,
moving from NEW-DEFA to PRECISIO, one lowers the threshold from 10 MeV
down to 100 keV for most of the particles) or explicitly through the
dedicated PART-THR card (and the EMFCUT card for electromagnetic
particles). It is fundamental to be aware of the thresholds used, which
are printed in the output file.

> Distribution are calculated with respect to a fixed direction in USRYIELD
> So I do not have to change the beam axis.

In your case the fixed direction IS the beam direction (which is the
default direction assumed by USRYIELD).
If now you decide to inject the beam along the z axis, as I suggested (it
is the standard assumption in FLUKA), you should of course change the
orientation of your target cylinder accordingly (XCC->ZCC) and use
appropriate planes (YZP->XYP) as well.

> Note 11 of USRBIN, the azimuthal extend from -180 to 180. phi=0 correspond
> to the x-axis.
> If I want to score the neutron production angle with respect to the
> beam projected direction,
> my understanding is to change the beam axis from z to x ,nor I would
> have a beam of z direction

i. as said in the previous message, USRBIN does not allow to score as a
function of energy
ii. you misinterpreted the use of the cylindrical mesh of USRBIN. Its axis
is the z axis (since this is the standard beam axis) and, for each
longitudinal z-bin, the transverse plane (xy) can be split into radial and
azimuthal bins. You described correctly the azimuthal angle (going from
-180 to 180 deg, with 0 deg corresponding to the x axis), but that cannot
be properly used as polar angle with respect to a beam direction parallel
to the x axis!

> Is it proper that I used the binning as my USRYIELD score with the
> WHAT(4) and WHAT(5) (upper and lower limit)
> to score with 0-1.5MeV,1.5-2.5..
> ( I hope I do not misunderstand the manual and output)

It is correct. I have a couple of remarks about.
i. you left empty WHAT(4-5) on the first line. This means that default
values are taken and this way you score neutrons as they are produced in
any nuclear reaction, neglecting their transport up to the detector. Note
that if you put instead regions 2 and 3, respectively, defining the target
surface as scoring surface (thus considering the neutron transport through
water), the energy interval used as filter will be in fact - not
apparently - (very) slightly changed due to matching with the group
structure applying to neutron energies below 20 MeV.
ii. in order to increase your statistics, you might need to bias the
interaction probability of primary protons in water (LAM-BIAS card)



Francesco Cerutti
CH-1211 Geneva 23
tel. ++41 22 7678962
fax ++41 22 7668854
Received on Mon Nov 10 2008 - 11:35:38 CET

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