Re: Continuation of discussion: High energy backscattering at 1.4GeV beam

Hi Helmut

until you observe protons coming from elastic scattering (even double
elastic scattering) you expect an energy not far from the initial
energy unless the target is a very light nucleus. Indeed for scattering on
Iron, a proton of initial energy 1.4 GeV has a centre-of-mass momentum of
roughly 2 GeV (2.05 versus 2.14 in the lab).

Even for a (back)scattering at 180 deg in the centre-of-mass (maximum
possible energy loss) the energy loss is only 160 MeV, at 90 (cms) it is
only 80 MeV....

So having a spectrum somewhat rising towards the original energy is
perfectly sensible for elastically scattered primary protons. The part
rising with decreasing energy is likely due to INELASTIC interactions,
where the typical spectrum is indeed with that shape.

                   Ciao
                  Alfredo

+----------------------------------------------------------------------+
| Alfredo Ferrari || Tel.: +41.22.76.76119 |
| CERN-EN/STI || Fax.: +41.22.76.69474 |
| 1211 Geneva 23 || e-mail: Alfredo.Ferrari_at_cern.ch |
| Switzerland || |
+----------------------------------------------------------------------+

On Wed, 15 Dec 2010, Helmut Vincke wrote:

>
> Dear colleagues
>
>
>
> Short problem description: backscattering of high-energy protons (having
> energies close to initial beam energy) on a massive iron target.
>
>
>
> After some discussion and some time spent on this topic we found some
> answers. We extracted two backscattered high-energy particles from 3E6
> particles started and looked at them with SimpleGeo+Pipsicad (reading out
> USERDUMP): The backscattered particles with such high energies turned out to
> originate from two double elastic scattering events, changing each of which
> the angle of the incoming proton by about 90 degree or less.
>
>
>
> I attach a file showing these events. Moreover, we put a detector to score
> backwards scattered particle upstream the impact point as a function of
> energy. Persons interested in this topic please have a look otherwise ignore
> my message. On the first glance the high-energy shape of the spectrum looked
> a bit odd to me because I did not expect a dip in the range of 600 MeV to 1
> GeV followed by a fluence increase when coming to closer initial beam
> energy. If you have a dedicated clear opinion on the creation of this dip
> feel free to send me your opinion.
>
>
>
> Best regards
>
> Helmut
>
>
>
> From: Helmut Vincke
> Sent: 07 December 2010 11:42
> To: fluka-discuss_at_mi.infn.it
> Cc: Chris Theis
> Subject: High energy backscattering at 1.4GeV beam
>
>
>
> Dear colleagues
>
>
>
> In my simulation I am sending a 1.4 GeV proton beam on a thick iron target
> (l=1.55m, r=50 cm). When looking at the result I see backscattered protons
> between 1.2 GeV and 1.4 GeV. Within 6E6 protons sent on the targets I found
> 3 protons with more than 1.2 GeV going backwards. I separated these three
> tracks in a separate run introducing a proton threshold of 1.2 GeV (see the
> picture in the attachment). Looks almost like elastic scattering but I did
> not expect it at that energy with such a large scattering angle. Although it
> concerns only a very minor fraction of the final fluence, I would like to
> understand its reason. I would be grateful for an explanation concerning the
> physics behind such an event.
>
>
>
> Thanks in advance
>
>
>
> Cheers
>
> Helmut
>
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Received on Thu Dec 16 2010 - 09:39:34 CET