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GEANT3: of energy loss and delta rays [message #7878] Fri, 13 February 2009 16:56 Go to previous message
Felix Boehmer is currently offline  Felix Boehmer
Messages: 149
Registered: May 2007
Location: Munich
first-grade participant

From: *natpool.mwn.de
Hello everyone,

first of all: sorry for the double post, accidently posted this in the event generater section first - please delete Razz

some fundamental work for my diploma thesis has led me down the troublesome path of investigating GEANT3's (and partly GEANT4's) energy loss behavior for the TPC volume.

In a nutshell, I am stuck with GEANT3's funny behavior when it comes to delta ray creation.

I am using the ALICE setting for MC point creation (GEANT is forced to create points in the volume based on the actual mean free path rather than based on an energy loss value).
I understand that there are at least 2 (coupled) parameters in GEANT3 responsible for delta creation, namely DCUTE and CUTELE. The official manual (long writeup) states:
Quote:

The variable DCUTE is the kinetic energy cut-off for the generated delta-rays. Below this threshold the soft electrons ejected are simulated as continuous energy loss by the incident e+/e-, and above it they are explicitely generated

Since I want to study the effect of delta rays in detail, I have set DCUTE (and CUTELE) to 1 keV, which I specify in the SetCuts.C for the VMC. DRAY is set so 1. Applying this low cut considerably blows up MC run time and filesize, as I would expect. However, when I loop over ALL TpcMCPoints created, I only see electron tracks in about 5% of the events, and I don't see any electron tracks below approx. 700 keV! Considering the above quote I would expect G3 to EXPLICITELY create ALL delta ray tracks above 1 keV...

I have tried this for the following settings of the LOSS parameter (see http://forum.gsi.de/index.php?t=msg&th=1594&rid=0&S=0f52b86d 3e60ad239a8e44defb67bc91]), which generally steers G3's treatment of the landau-vavilov theory. While I am not surprised that for LOSS=2 there are only electron tracks in 4.3% of the events, this number almost stays the same for LOSS=1 (5.0%), still with no elecctron tracks below approx. 700 keV. LOSS=5, the secret unofficial ALICE interpretation of this parameter, blows up MC points by a factor of 2 (ok), filesize by a factor of almost 4 (not ok!) but doesn't give me any deltas at lower energies either.

I do not understand why on the one hand G3 becomes horribly slow (as expected for explicit delta generation which such a low cut) but I don't see them in the data!
Is it possible that there is some "feature" of the VMC interface that overwrites my settings or even causes G3 to discard already simulated tracks? Are there more hidden cuts doing evil things in the dark?

Maybe someone here has some experience he wants to share, I would be very thankful!

Cheers

Felix

[Updated on: Fri, 13 February 2009 17:03]

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