RE: very principle question

From: Alberto Fasso' <>
Date: Tue, 19 Apr 2011 19:32:29 +0200

Helmut, I repeat that those limits are *indications*, *suggestions*,
*guidelines*, dictated as much by physical considerations as by the
experience of the code developers. They must not be taken as absolute hard
limits. The code "doesn't know" about them, and if you are not respecting them
nothing terrible will happen. In some cases, for instance when a difference
is made between low-Z and high-Z materials, the considerations come from a
judgment about the accuracy of some approximations, perhaps the Born
approximation or something like that.

A guideline is supposed to help the user in the most common cases. Of course
you can always think about some special exception, for instance if most of
the particle transport takes place in vacuum there are no secondaries.
This looks very similar to your example of the TEPC detector. You cannot
expect a special guideline to be given for the case of a TEPC.

I don't know about the antineutrons: I guess that 10 MeV (the lower limit
for primaries) probably correspond to the lower energy limit of the
pre-equilibrium model, and 1 keV (lower limit for secondaries) has to do with
the available cross sections. Antineutrons are particles about which there are
not many experimental data: you shouldn't give too much importance to the
difference between the two limits. Take them just as the best expert guess
about them.

The same about the upper photon limits: those limits are the the best
advice the developers can give you, probably in connection with cosmic
ray applications (the only kind of application where those limts can be

I am kind of puzzled about the hair-splitting fussiness you are showing
about this very minor issue. A factor 10 vs a factor 100: this is not
precise mathematics, it is just what is recommended by the people
who wrote the code, based on their knowledge, judgment, experience... and
feeling. You are free to disregard them - at your risk.


On Tue, 19 Apr 2011, Helmut Vincke wrote:

> Dear Alberto
> Thanks for your answers. However, your answer triggers some more detailed
> questions and comments:
> 5 keV protons: if it is almost immediately stopped depends on the
> composition and above all the density of the material. Moreover, the upper limit
> for charged protons (as primary) is 100 keV, an energy which is sufficient to
> cross the whole gas volume of a TEPC detector.
> The difference of secondaries and primaries is much stronger for anti-neutrons,
> namely a factor of 10000. Can you tell me the explanation for that one?
> We have even another case where we have a factor of 10 for the upper
> transport limit: namely photons: primary: 10000 TeV in contrary to secondaries:
> 1000 TeV. Please tell us if this is a spelling mistake. In case it isn't
> please explain the reason.
> As a user I would like to have a list telling me the limits of reliability
> (this term needs to be defined by the one who provides the list) of FLUKA
> for each particle type. No matter if any other part of the particle history
> or if other secondaries are dominating the quantities I score.
> Cheers
> helmut
> -----Original Message-----
> From: Alberto Fasso=20
> Sent: 19 April 2011 15:30
> To: fluka-discuss (
> Cc: Helmut Vincke; Chris Theis; Stefan Roesler
> Subject: Re: very principle question
> Dear Helmut,
> those are not thresholds! They are *suggested* transport limits.
> The "further transport" is the same for primaries and secondaries.
> Common sense tells you that if the transport threshold of protons (yes,
> this time I am talking about an actual threshold) is 1 keV, it would not make
> much sense to start a primary proton of 5 keV to be almost immediately stopped.
> But it can be done, nothing prevents you to do it: it would just be silly.
> On the other hand, if in the course of a hadronic cascade a 5 keV secondary
> proton is generated, transporting it makes sense.
> There is also another reason. The transport limits you are referring to
> are due to the fact that the accuracy of transport and interaction models
> becomes gradually worse as energy gets close to them. But they are not "sharp"
> limits: the physics of proton transport is not "good" at 101 keV and "bad" at
> 99 keV. The limits reported in the manual are just an indication to the
> user of *about* the lowest primary or secondary energy which can give you
> good results.
> 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.
> For the same reason, with some DEFAULTS the threshold for performing multiple
> scattering is lower for primaries than for secondaries.
> Alberto
> On Mon, 18 Apr 2011, Helmut Vincke wrote:
>> Dear FLUKA users and authors
>> When I am looking at the transport limits of FLUKA I see different
>> limits for secondary and primary particles.
>> The lower threshold for secondaries is always below the corresponding
>> lower threshold of primary particles (differences up to a factor 1E4).
>> From my understanding the origin of the particle (either produced in
>> an interaction or started from a given point by the user) should not
>> make a difference for the further transport of this particle.
>> I attach the transport limits of FLUKA, as they are listed in the manual.
>> I would be grateful if you could explain me the difference.
>> Thanks in advance
>> Best regards
>> Helmut
>> Transport limits listed in the FLUKA memory:
>> Secondary particles Primary particles
>> charged hadrons 1 keV-20 TeV (*) 100 keV-20 TeV (*) (**)
>> neutrons thermal-20 TeV (*) thermal-20 TeV (*)
>> antineutrons 1 keV-20 TeV (*) 10 MeV-20 TeV (*)
>> muons 1 keV-1000 TeV 100 keV-1000 TeV (**)
>> electrons 1 keV-1000 TeV 70 keV-1000 TeV (low-Z
>> materials) (**)
>> 150 keV-1000 TeV (high-Z materials) (**)
>> photons 100 eV-1000 TeV 1 keV-10000 TeV
>> heavy ions<10000 TeV/n<10000 TeV/n
>> (*) upper limit 10 PeV with the DPMJET interface
>> (**) lower limit 10 keV in single scattering mode
Received on Wed Apr 20 2011 - 09:15:56 CEST

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