RE: very principle question

From: Chris Theis <Christian.Theis_at_cern.ch>
Date: Tue, 19 Apr 2011 19:31:27 +0000

Alberto,

thanks a lot for the quick reply.

I fully see your point that what is called a "limit" in the manual should
rather be seen as a recommendation. Don't get me wrong, the reason why I
was asking was not that I am worried about them. There are simply two reasons
- on one hand you mention that they are based on experience as well as
physical judgment by the developers. So the main reason for my mail was to
improve my understanding of this judgment and profit from the vast experience
of the code developers by getting more information on the background of
those numbers.

The second reason is that during several collaboration meetings it was
mentioned that there is not enough discussion on this discussion list. So I
saw this as a good opportunity to have a discussion which does not turn into
a monologue while improving my understanding.

I wish you nice Easter holidays,

Chris

-----------------------------------------------------------------------------------------
Chris Theis
CERN/DGS-RP - European Organization for Nuclear Research
1211 Geneva 23, Switzerland
Phone: +41 22 767 8069 Office: 892-2A-015
e-mail: Christian.Theis@cern.ch www: http://www.cern.ch/theis
-----------------------------------------------------------------------------------------

________________________________________
From: Alberto Fasso
Sent: 19 April 2011 20:01
To: fluka-discuss (fluka-discuss_at_fluka.org)
Cc: Chris Theis; Helmut Vincke
Subject: RE: very principle question

Chris,

again...! Please see my answer to Helmut.
Why are you both so concerned about those limits?
With most objects you purchase, you get some recommendations by the
manufacturer. For the tires of a car, the owner's manual gives you
the maximum recommended pressure. If it says it 32 lbs of pressure,
does it mean that you cannot inflate them at 33 lbs? Would that
be dangerous? And why 32 and not 30? The manufacturer doesn't tell you,
but you are satisfied that his experience has shown that value to ensure
maximum safety and minimum wearing.

So, please take these recommendations in the same way. They are not
compulsory, they are just *recommendations*

Alberto

On Tue, 19 Apr 2011, Chris Theis wrote:

> Dear Alberto,
>
> maybe this was a mutual misunderstanding. But from "... it would not make
> much sense to start a primary proton of 5 keV to be almost immediately stopped."
> I inferred that you are actually referring to a range and not to an *energy*
> range at which particles can be transported with more or less good accuracy.
>
> Thanks for the explanations regarding the implementation of LPM and EPDL97.
> However, this raises another question for me because the limit for the
> secondary high-energy photons did not change. To my understanding, which of
> course might be wrong, LPM would apply to high energetic secondaries in these
> energy regions as well.
>
> This leads me to another point which I would like to have a better
> understanding of. You write:
>
>> Why different for primary and secondary? Because the whole pattern of energy deposition, nuclear reactions etc. is dominated by primaries:
>> transport of secondaries improves it, but is less critical. Therefore, it is more important to have "optimum" physics for primaries than for
>> secondaries.
>
> I fully agree with your point that primaries are more critical. However, how do you
> infer the actual values for the lower limits of the secondaries? For the photons
> for example we have 1 keV for primaries and 100 eV for secondaries.
> What is not clear to me is on which basis this difference by a factor of
> 10 has been determined. To my understanding the physics for primaries& s=
econdaries
> should be the same - it is in nature - and thus, only the limit for primaries
> should be the one down to which "particles can be transported with more or
> less good accuracy". Don't you agree?
>
> Chris
>
> -----Original Message-----
> From: Alberto Fasso
> Sent: 19 April 2011 16:53
> To: fluka-discuss (fluka-discuss_at_fluka.org)
> Cc: Chris Theis; Helmut Vincke
> Subject: RE: very principle question
>
> Dear Chris,
>
> I didn't "state that the suggested limits are related to the range of particles"!
> Where did you read this statement?
> They are related in some way to the *energy* range (not the range of the
> particles) in which particles can be transported with more or less good
> accuracy.
>
> As you have noticed, that energy range has been extended for photons in
> the latest FLUKA release, both at the upper and at the lower end.
> At the upper end, by the implementation of the LPM effect
> (Landau-Pomeranchuk-Migdal) in pair production. At the lower end, by usin=
g the EPDL97
> photon cross sections from Livermore.
> A warning: transport of these very low-energy photons does not take into
> account special effects such as reflection by mirrors or crystals.
>
> Alberto
>
> On Tue, 19 Apr 2011, Chris Theis wrote:
>
>> Dear Alberto,
>>
>> Thanks a lot for your e-mail. Reading your explanations actually made
>> me think about one point. On one hand you state that the suggested
>> limits are related to the range of particles and of course this has not changed over the time.
>> Yet, when I compare the limits of FLUKA 2008 and FLUKA 2011 for
>> photons I find the following:
>>
>> Secondaries/primaries
>> 2008: photons 1 keV-1000 TeV 7 keV-1000 TeV
>> 2011: photons 100 eV-1000 TeV 1 keV-10000 TeV
>>
>> From the second part of your answer I take it that these changes must
>> be related to substantial changes/improvements in the physics because
>> we have an increase for the upper limit by a factor of 10 and a
>> decrease by 7-10 for the lower limit, depending if you're looking at primaries or secondaries.
>>
>> Especially for the low energy part I would be really interested to
>> know which changes allowed for this improvement. I would appreciate if
>> you could provide some more background information about these changes.
>>
>> Thanks a lot
>> Chris
Received on Wed Apr 20 2011 - 09:16:38 CEST

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