# Re: how to select charged particles?

From: Alfredo Ferrari (alfredo.ferrari@cern.ch)
Date: Tue Jan 09 2007 - 21:49:35 CET

• Next message: me@marychin.org: "Re: particles in beam interacting with each other"

Hi all

Sebastien is almost right:

Zfftrk = <Z_eff^2>^1/2 (that is Zfftrk^2 is the average Z_eff^2)

Zfrttk = current (random) (Z_eff^2)^1/2 (that is Zfrttk^2 is the current
Z_eff^2)
Ichrge(Jtrack) = "bare" particle charge

Please note that Zfftrk and Zfrttk make sense only for dE/dx calculations.
Zfftrk is computed on the basis of recent <Z_eff^2> parametrizations and
should be reasonable until the energy of the ion is not very low (below
100 keV/n). Zfrttk is computed on the basis of a rough model which is
tuned to reproduce reasonably the dE/dx straggling due to charge changing
events. Since there is at least another semi-empirical parameter which
enters, Zfrttk could well be non-physical but still be perfectly
meaningful in the framework of the dE/dx fluctuation calculations.

Never forget that for a random variate r, the distribution of r is not
the one of r^2 and viceversa, and that <r^(2n)> != <r^n>^2 and similar.
In short, don't try to compute an actual particle charge out of those
variable. They make sense only for dE/dx calculations.

Ciao
Alfredo

On Thu, 21 Dec 2006, Sebastien WURTH wrote:

> Hello Konstantin,
>
> This might be confirmed but I think that :
> Zfftrk = <Z_eff> is the "average" Z_eff
> Zfrttk = actual Z_eff = real Z_eff
>
> Regards.
> Sebastien.
>
> Konstantin Batkov a ecrit :
>
>> Hello,
>>
>> I have two more questions.
>> 1. I would like to analyze only charged particles in mgdraw.f. Is it
>> correct to use the following expression?
>>
>> IF (ZFRTTK .GT. ZERZER)
>> * ....
>> END IF
>>
>> 2. What is the difference between these variables :
>> Zfftrk = <Z_eff> of the particle
>> Zfrttk = actual Z_eff of the particle
>> from TRACKR? (I do not understand what does word "actual" mean here.)
>>
>> Konstantin
>
>

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