Re: [fluka-discuss]: Superimposing several USRYIELD cards in USR-1D plot

From: Francesco Cerutti <>
Date: Sat, 24 Mar 2018 22:25:10 +0100

Dear Anne,

note that your second quantity, defining the filtering interval, is the
*polar* angle theta (in degrees) with respect to the beam direction.
USRYIELD results are then normalized by the respective *solid* angle (in

Considering your scoring surface, only neutrons with theta > 90 deg can be
recorded (actually neutrons coming directly from the water dump have an
angle close to 180 deg). So a theta interval from 0 to 180 will give
exactly the same spectrum as the 90-180 interval, apart from the dOmega
(solid angle) normalization, making the first lower by a factor 2 (it's
divided by 4*pi sr while the latter is divided by 2*pi sr).

To compare single differential energy spectra corresponding to different
angular windows, you should just renormalize each of them by
dOmega = 2*pi*(cos(theta1)-cos(theta2))

Now, you have selected linear energy binning, which is not a good idea
for neutron energy spectra, normally spanning several decades and anyway
subject to the predefined multigroup structure below 20 MeV (far from
being linear). Sticking to that (linear binning), your horizontal energy
scale should in principle stay linear and your value should then be Y,
i.e. dN/(dE dOmega), multiplied by dOmega as above in the normalization
factor field.

When plotting spectra/distributions, it makes no much sense to multiply by
the bin width, i.e. dE in your case, which by the way you took wrong,
since dE = 2 GeV above 20 MeV but it varies according to the multigroup
structure below 20 MeV.
(Then you wrongly multiplied by
dtheta = (theta2- theta1) = 180 or 5
instead of dOmega).

If you are interested in integrating the spectra, to get the full
population N, just look into the respective _sum.lis file and you will

Tot. response (integrated over x1)

that - in the case of USRYIELD - has still to be multiplied by the x2
(second quantity) interval, i.e. in your case the solid angle expression I
gave you above.

If you better choose logarithmic energy binning, then you should plot on
a horizontal logarithmic energy scale and the value on the vertical axis
should be <X>*Y, since in this case the quantity that has to be plotted is
dN/(dlog(E) dOmega) = E * dN/(dE dOmega), always to be renormalized by

Up to here, it was a matter of plotting (and binning choice in scoring).

Clearly, the production of a meaningful backscattered neutron statistics
is challenging. If you do not get there by increasing the number of
primaries (and patiently allowing for a long simulation time), you may
want to consider the implementation of importance biasing for low energy
neutrons (and hadrons, if neutrons > 20 MeV manage to play a role). For
this, refer to the BIASING card, in combination with your geometry region
setup, in order to favour particle splitting for neutrons coming back.

I'd also resort to the mgdraw routine (to be activated by USERDUMP in the
input), customizing the entry BXDRAW to dump in a file each neutron
(JTRACK=8) crossing your reference boundary, to be used in a second step
through the source routine. But before worrying about preparing the
latter, you need to accumulate a meaningful neutron sample.
If you go with biasing, remember to store in the mentioned file also the
statistical weight (WTRACK), in addition to the neutron position (XSCO,
YSCO, ZSCO), direction (CXTRCK, CYTRCK, CZTRCK) and energy (ETRACK =
kinetic+mass and KTRACK = energy group number for neutrons < 20 MeV).

Bon courage


Francesco Cerutti
CH-1211 Geneva 23
tel. +41 22 7678962
fax +41 22 7668854

On Fri, 23 Mar 2018, Schuetz, Anne wrote:

> Dear FLUKA experts,
> I am trying to score neutrons that escape from a water beam dump in backward direction towards the exit of the beam dump hall.
> The inp file is attached to this email.
> With USRYIELD cards, the neutrons are scored with the first quantity being the kinetic energy, and the second one being the solid angle (lab) in degrees.
> Since I want to also bin the second quantity, I have added several USRYIELD cards ranging over different angle intervals.
> When I now want to superimpose the spectra in one USR-1D plot, I chose the detector from the USRYIELD card ranging over the full angle interval (0-180 degrees), and two other detectors for 170-175 degrees and 175-180 degrees (since I am interested in the neutrons heading in the opposite direction to the beam direction).
> I am confused about the normalization factors in order to be able to compare the different spectra.
> For the 'value', I chose Y.
> For the x-axis normalization, I chose 1/MeV.
> And for the y-axis normalization, I put the beam bunch population * the energy bin size (= 0.002 GeV, since the x-axis is normalized to 1/MeV) * the angle range (180 for the first detector, and 5 for the other two detectors).
> Please find the resulting plot in the attachment.
> I don't think that I am doing the plotting right, since the x-axis is logarithmic (is the normalization factor = energy bin size then still correct?) and since th graph for the full angle range has not got the highest values, which I expected it to.
> Could you help me with understanding how to plot the USRYIELD results correctly?
> And also, would you recommend a different scoring card for my purposes?
> Thank you very much!
> Best regards,
> Anne
> --
> Anne Schütz
> Deutsches Elektronen Synchrotron (DESY)
> FLA group, 1c/01.348,
> Notkestr. 85
> 22607 Hamburg
> Germany
> email:
> phone: +49 40-8998-1465

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Received on Sun Mar 25 2018 - 00:09:31 CET

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