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GEANT3 dEdx for low energy protons [message #6306] Tue, 08 April 2008 18:02 Go to previous message
Sebastian Neubert is currently offline  Sebastian Neubert
Messages: 282
Registered: March 2006
Location: Munich
first-grade participant

From: *e18.physik.tu-muenchen.de
Hi!

We observe a remarkable behaviour with GEANT3 when we look at the energy loss distributions for 200MeV protons.

From BetheBloch we would expect something like dE/dx=22keV/cm in the TPC gas.

Please have a look to the following plots, where you see the dE/dx (in keV/cm) from MC-Points plotted against the dx (in cm) for different step limitations:

dx<2.5cm
index.php?t=getfile&id=4340&private=0

dx<3cm
index.php?t=getfile&id=4341&private=0

dx<4cm
index.php?t=getfile&id=4342&private=0

One observes that as soon as a step is smaller than 3cm the dEdx calculation gives a wrong result, which is too low by a factor of 3 to 4 (6keV/cm instead of 22keV/cm).

It is remarkable, that this behaviour is observed for any short step, regardless of if it was triggered by the step limiter or by anything else. You see this in the third plot, where the points with too low dEdx still persist!

If we shut off the straggeling and just use the energy loss tables of GEANT (LOSS=4 option in SetCuts.C) we get the exepcted value for the dE/dx.

How should we deal with this?

Cheers!
Viola and Sebastian.
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Sebastian Neubert
Technische Universität München
Department Physik E18
sneubert@e18.physik.tu-muenchen.de
tel: +49-8928912592

[Updated on: Tue, 08 April 2008 18:03]

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