Re: [fluka-discuss]: fluka simulation with Pb beam hitting silicon planes

From: Francesc Salvat-Pujol <>
Date: Wed, 6 Feb 2019 13:02:32 +0100

Dear Gianluca,

In the attachment is a Flair screenshot from a quick run showing a few
electron trajectories in the XZ plane (upper right-hand viewport) and
the YZ plane (lower left-hand viewport). One sees the trajectories
nicely spiraling under the action of your B field along the X axis
(viewport XZ) and one can examine the various radii in the YZ viewport.
For screenshot clarity I killed electrons below 10 MeV.

Looking at the XZ projection one sees that the delta rays cross your
first disk many times as the electrons progress along X, however, in the
YZ projection one realizes it's basically at two well defined spots
along the Y axis: at Y~0 (disk axis, where most delta rays initiate) and
at a second Y=spiral radius. This explains why you see the fluence
decrease along Y: whereas "all" spirals cross Y~0, at finite Y>0 only
those with energy matching such a curving radius will.

Another aspect which contributes to the more intense fluence you observe
at small Y is that delta rays with lower energy (smaller spiral radius)
are much more likely to be produced: the "delta ray production" cross
section goes like 1/T^2, where T is the energy loss of the passing
charged particle which sets the delta ray in motion.

In order to try to account for the structure along the X axis, and to
see which electrons contribute where, in the lower panels of the
screenshot I scored the electron fluence (your unit 27) for various
values of the delta-ray production threshold, Tdelta: 100 keV, 1 MeV,
10 MeV, 50 MeV, and 100 MeV.

The rightmost panel (Tdelta=100 MeV) shows the expected spot around Y~0,
one at Y~20 cm (the expected 2*radius for electrons in B=3 T as per the
cheatsheet in the PS below) and some minor hits in between, which should
correspond to energetic delta rays originating in the other disks (one
can spot such situations in the YZ viewport).

Similar story for the Tdelta=50 MeV and 10 MeV plots, but at the diameters
exceeding the expected ~10 cm and ~2 cm, respectively.

Note also that when including lower energy delta rays, the pitch or step
of the spiral they fly on decreases (it should scale with the velocity
along the field axis), which could explain the occasional empty pixel
along X for Y~0 at the larger Tdelta plots. Of course, this applies only
for the presently poor statistics: with a more serious/long run you'd
eventually get a smooth picture, but the origin would be clear.

The CSDA range for 1 MeV electrons in Si (just a coarse estimate from a
quick read of the ESTAR plot and dividing by rho=2.32 g/cm^3) should be
something like ~0.24 cm and for 100 keV it's something around .008 cm
(80 micron already). The latter is comparable to your disk's 50 micron

Thus, delta rays with, say, 100 keV - 1 MeV may manage to spiral a few
times in and out of the disk, losing a bit of energy at every crossing,
but dropping below (transport) threshold quite soon and not making to
the edge of your disk. Indeed, for the Tdelta=1 MeV and 100 keV plots
you see a quite intense maximum "blob" at X=Y=0 in the fluence (many
such slow delta rays produced as per 1/Tdelta^2), with a finite extent
along X as expected, and quite intense (there are a lot of such slow
delta rays).

Considering the above, unless I am missing something essential, the
overall structure and sizeable intensity of your fluence plots could
well be reasonable.

Your initial expectation (to just see electrons close to the axis) is
indeed what one would expect in the absence of a magnetic field.

I hope the above is reasonably accurate/helpful!



PS: the cheatsheet/table. In m, the rule of thumb for the (electron)
radius was something like r = p / (0.3 B), with p in GeV/c and B in T.
For electrons in your B=3 T, we have 0.3B~1, so

  Edelta = 1 MeV ---> r ~ 0.14 cm
  Edelta = 10 MeV ---> r ~ 1 cm
  Edelta = 50 MeV ---> r ~ 5 cm
  Edelta = 100 MeV ---> r ~ 10 cm

PS2: seems like your source is precisely on the boundary between two
regions (the black hole region and vacuum). Advance it a bit into vacuum
to be on the safe side with respect to numerical hiccups.

PS3: I understand that you linked with ldpmqmd.

>Dear Fluka users,
>I would like to study the charged particle fluence produced by a 40 GeV Pb beam shooted across silicon disks of radius 20 cm and thickness 50 micron. These mimics silicon tracking stations based on a possible very large area monolithic active pixel sensor which is being discussed for future heavy ion fixed target experiments. There is no central hole in the silicon disk in this simulation. The disks are immersed in a 3 T dipole field along x. Since I would like to see the fluence along x-y at the z of the different disks, I thought to use usrbin with -30<x,y<30 (60 bins in x,y), abs(z-zdisk)<50 micron (1 bin in z).
>I attach the .inp file (there is the possibility to simulate also a Pb target system, but the target volume is filled with vacuum in this simulation).
>The scoring of the electron fluence in the first disk is puzzling me (usrbin card with output unit 27). I attach a fluence plot from a simulation of 1000 primary ions. The fluence seems rather large and there is a very long horizontal band along x extending even beyond abs(x)>20 cm which I cannot explain. I would have thought naively to see electrons more or less close around the ion crossing point. I would appreciate if you could give me some clue about what I am observing and in particular if there is something wrong in the scoring or in the settings in the .inp file.
>Thanks in advance,
>best regards,
> Gianluca Usai
>Gianluca Usai
>Professor of physics
>Department of Physics and INFN
>Cagliari - Italy
>+39 070 675 4906
>+41 22 767 5740 (CERN)
>Strada prov.le per Sestu, km 1.00
>09042 Monserrato (CA)

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><span class="Apple-style-span" style="border-collapse: separate; border-spacing: 0px;"><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;"><span class="Apple-style-span" style="border-collapse: separate; color: rgb(0, 0, 0); font-family: Helvetica; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; orphans: 2; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px; border-spacing: 0px; -webkit-text-decorations-in-effect: none; -webkit-text-stroke-width: 0px;"><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;"><div>______________________________<br><font class="Apple-style-span" size="4"><span class="Apple-style-span" style="font-size: 14px;">Gianluca Usai</span></font><div><font class="Apple-style-span" size="4"><span class="Apple-style-span" style="font-size: 14px;"><br></span></font><div><font class="Apple-style-span" size="4"><span class="Apple-style-span" style="font-size: 14px;">Professor of physics<br>Department of Physics and INFN<br>Cagliari - Italy<br><br>Phone:<br>+39 070 675 4906<br>+41 22 767 5740 (CERN)<br><br>Address:<br>Strada prov.le per Sestu, km 1.00<br>09042 Monserrato (CA)<br>Italy<br><br>web:<br><a href=""></a></span></font></div></div></div><div><font class="Apple-style-span" size="4"><span class="Apple-style-span" style="font-size: 14px;"><a href=""></a></span></font></div><div><font class="Apple-style-span" size="4"><span class="Apple-style-span" style="font-size: 14px;"><br></span></font></div></div></span><br class="Apple-interchange-newline"></div></span><br class="Apple-interchange-newline"><br class="Apple-interchange-newline">

Francesc Salvat Pujol
CH-1211 Geneva 23
Tel: +41 22 76 64011
Fax: +41 22 76 69474

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Received on Wed Feb 06 2019 - 14:56:03 CET

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