Re: setting cutoff

Hamideh,

the case of gas bremsstrahlung is completely different from the usual
electron/photon ones.
It is necessary to use a fictitious gas at normal atmospheric pressure
and density, because the actual residual gas in the vacuum chamber
has a density so low that no interaction could be simulated by Monte
Carlo. In real life, although the density is so low, the number of electrons
crossing the gas per second is enormous, and the number of interactions
is not negligible: but with Monte Carlo you can only follow a small number
of primary electrons.

Using atmospheric pressure, or one tenth of it, allows to get a good
amount of bremsstrahlung, that must then be scaled down to the actual density
of the gas by a factor of the order of 1.E-12.
But while the number of bremsstrahlung interactions scales linearly well
with density without changing the characteristics of the beam, other effects
like multiple scattering and delta ray production tend to spread the
beam (and hence also the bremsstrahlung cone) at atmospheric pressure, but are
completely absent at quasi-vacuum pressures.
I copy here the relevant text from the example on the web (look at
"Adapting the parameters of the physical processes"):

"...difficulties in considering the low operating pressure of the machine,
mentioned in the previous section, cannot be solved completely by the trick to
work at atmospheric pressure and to scale the final results. This because the
multiple scattering is practically negligible in the vacuum chamber of a
storage ring, while it is responsible of large broadening of the beam at
atmospheric pressure.

Therefore it must be suppressed in the FLUKA simulations and this is driven by
means of the MULSOPT card. In a similar way, Moller scattering, which at
amospheric pressure would cause electron angular deflections due to the
production of d rays, has been suppressed by means of an EMFCUT card with the
ELPO-THR parameter.

The code energy cuts are set at 50 keV for charged particles and 1 keV for
photons (see again the EMFCUT cards)".

Therefore, your questions "I thought setting cutoffs helps users to get more
accurate results but this significant difference ( 5 order of magnitude) caused
the question that what is the role of EMFCUT card? Is it a necessary card
for any problem?", understandable in all other kind of problems, do not apply
here, or better apply to all other materials and regions, but not to the
residual gas inside the vacuum chamber.

Alberto

On Fri, 27 May 2011, Hamideh Jalali wrote:

> Dear FLUJA Experts
>
> I have programmed a FLUKA input for gas bremstrhlung production with
> no EMFCUT card and got some results. Then I ran it by setting the
> proposed cutoffs in gasbrem.inp (the example of fluka documentation)
> via using EMFCUT card with no sdum and sdum PROD-CUT.
> Comparison the results showed considerable difference. So some
> questions came to my mind:
> I thought setting cutoffs helps users to get more accurate results but
> this significant difference ( 5 order of magnitude) caused the
> question that what is the role of EMFCUT card?Is it a necessary card
> for any problem?
> And what is the criteria to set cutoffs and production thresholds? for
> example how have the values 0.000561 GeV for e+/e- production
> threshold for air and iron and 0.01 GeV for material GAS(air in
> target) gotten in gasbrem.inp ? or Transport cutoff 50 keV
> (electrons), 1 keV (photons) for defined regions.
>
>
> Thanks in advance
> Hamideh
Received on Mon May 30 2011 - 10:04:26 CEST