FLUKA: BEAM
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BEAM
defines several beam characteristics: type of particle, energy, divergence
and profile
See also BEAMAXES, BEAMPOSit, SOURCE, SPECSOUR
WHAT(1)
> 0.0 : average beam momentum in GeV/c
< 0.0 : average beam kinetic energy in GeV
If the particle indicated by SDUM
is HEAVYION, the kinetic
energy or momentum must be expressed PER NUCLEAR MASS UNIT.
This value is available in COMMON BEAMCM as variable PBEAM.
It can be used or modified in subroutine SOURCE if command
SOURCE is present in input.
Default
= 200.0 GeV/c momentum
WHAT(2)
> 0.0 : beam momentum spread in GeV/c. The momentum distribution is
assumed to be rectangular
< 0.0 : |WHAT(2)
| is the full width at half maximum (FWHM) of a
Gaussian momentum distribution (FWHM = 2.355 sigma)
This value is available in COMMON BEAMCM as variable DPBEAM.
It can be used or modified in subroutine SOURCE if command
SOURCE is present in input. However, in that case the
momentum/energy sampling must be programmed by the user.
Default
= 0.0
WHAT(3)
specifies the beam divergence (in mrad):
> 0.0 : WHAT(3)
is the width of a rectangular angular distribution
for a beam directed along the positive z-axis (unless a
different direction is specified by command BEAMAXES: see
Note 4).
> 2000 x PI mrad (i.e. 2 pi rad) : an isotropic distribution is
assumed (see Note 7 below).
< 0.0 : |WHAT(3)
| is the FWHM of a Gaussian angular distribution
for a beam directed along the positive z-axis (unless a
different direction is specified by command BEAMAXES: see
Note 4).
This value is available in COMMON BEAMCM as variable DIVBM
(units [rad]). It can be used or modified in subroutine SOURCE
if command SOURCE is present in input. However, in that case
the divergence sampling must be programmed by the user.
Default
= 0.0
WHAT(4)
>= 0.0: If WHAT(6)
> 0.0, beam width in x-direction in cm for a
beam directed along the positive z-axis (unless a different
direction is specified by command BEAMAXES: see Note 4).
The beam profile is assumed to be rectangular.
If WHAT(6)
< 0.0, WHAT(4)
is the maximum radius of an
annular beam spot.
< 0.0 : |WHAT(4)
| is the FWHM of a Gaussian profile in x-direction
(whatever the value of WHAT(6)
) for a beam directed along
the positive z-axis (unless a different direction is
specified by command BEAMAXES: see Note 4).
This value is available in COMMON BEAMCM as variable XSPOT.
It can be used or modified in subroutine SOURCE if command
SOURCE is present in input. However, in that case the
x-profile sampling must be programmed by the user.
Default
= 0.0
WHAT(5)
>= 0.0: If WHAT(6)
> 0.0, beam width in y-direction in cm for a
beam directed along the positive z-axis (unless a different
direction is specified by command BEAMAXES: see Note 4).
The beam profile is assumed to be rectangular.
If WHAT(6)
< 0.0, WHAT(5)
is the minimum radius of an
annular beam spot.
< 0.0 : |WHAT(5)
| is the FWHM of a Gaussian profile in y-direction
(whatever the value of WHAT(6)
) for a beam directed along
the positive z-axis (unless a different direction is
specified by command BEAMAXES: see Note 4).
This value is available in COMMON BEAMCM as variable YSPOT.
It can be used or modified in subroutine SOURCE if command
SOURCE is present in input. However, in that case the
y-profile sampling must be programmed by the user.
Default
= WHAT(4)
WHAT(6)
< 0.0: WHAT(4)
and WHAT(5)
, if positive, are interpreted as the
maximum and minimum radii of an annular beam spot. If
negative, they are interpreted as FWHMs of Gaussian
profiles as explained above, independent of the value of
WHAT(6)
.
>= 0.0: ignored
Default
= 0.0
SDUM
= beam particle name. Particle names and numerical codes are listed
in the table of FLUKA particle types (see 5}).
This value can be overridden in user routine SOURCE (if command
SOURCE is present in input) by assigning a value to variable
IJBEAM equal to the numerical code of the beam particle.
For heavy ions, use the name HEAVYION and specify further the ion
properties by means of option HI-PROPErt. In this case WHAT(1)
will mean the energy (or momentum) PER NUCLEAR MASS UNIT, and not
the total energy or momentum.
The light nuclei 4He, 3He, triton and deuteron are defined with
their own names (4-HELIUM, 3-HELIUM, TRITON and DEUTERON) and
WHAT(1)
will be the total kinetic energy or momentum.
For (radioactive) isotopes, use the name ISOTOPE and specify
further the isotope properties by means of option HI-PROPErt.
In this case WHAT(1)
and WHAT(2)
are meaningless. See Note 8) for
instructions on how to run cases where the source is a
radioactive isotope.
Neutrino interactions are activated by SDUM
= (A)NEUTRIxx.
Neutrino interactions are forced to occur at the point (or area)
defined in the BEAMPOS card.
The special SDUM
's AMBE, AMB, 252CF select neutron spectra
according to an Americium-Beryllium, Americium-Boron, and
Californium-252 source respectively.
[Not yet implemented: For optical photons, use the name OPTIPHOT
and specify further the transport properties by material by means
of option OPT-PROP.]
Default
= PROTON
Default
(command BEAM not requested): not allowed! The WHAT(1)
value of the
BEAM command is imperatively required, in order to set up the
maximum energy of cross-section tabulations.
Notes:
1) Simple cases of sources uniformly distributed in a volume can be
treated as SDUM
options of command BEAMPOSit. Other cases of
distributed, non monoenergetic or other more complex sources should
be treated by means of a user-written subroutine SOURCE as explained
in the description of the SOURCE option (see 13}), or, in some
special cases, by means of a pre-defined source invoked by command
SPECSOUR (see 15, 16, 17}). In particular, the BEAM definition
cannot handle beams of elliptical cross section and rectangular
profile. However, even when using a SOURCE subroutine, the momentum
or kinetic energy defined by WHAT(1)
of BEAM is meaningful, since it
is taken as maximum energy for several scoring facilities and cross
section tabulations.
Advice:
when a user-written SOURCE is used, set WHAT(1)
in BEAM
equal to the maximum expected momentum (or energy) of any particle
to be transported.
2) A two-dimensional distribution, Gaussian with equal variances in x
and y, results in a RADIAL Gaussian distribution with variance
sigma_r = sigma_x = sigma_y
The distribution has a form
P(r) = 1/(2pi sigma_x sigma_y) exp{-1/2[(x/sigma_x)^2 + (y/sigma_y)^2]} =
= 1/(2pi sigma_r^2) exp[-1/2(r/sigma_r)^2]
3) All FLUKA results are normalised per unit incident particle weight.
All particles defined by the BEAM command have by default a
weight = 1. A distribution of initial weights may be needed,
however, when sampling from a non-monoenergetic spectrum: in this
case, a SOURCE subroutine must be written (see 13})).
4) All options governed by WHAT(3,4,5) are meaningful only if the beam
direction is along the positive z axis, unless a command BEAMAXES is
issued to establish a beam reference frame different from the
geometry frame (see command BEAMAXES). If the beam is not in the
positive z direction and no BEAMAXES command has been given,
WHAT(3)
-WHAT(5)
must be set = 0.0 (unpredictable effects would arise
otherwise).
5) The beam momentum value as defined with the BEAM card is available
to user routines as a variable PBEAM and so is the beam particle
type IJBEAM. These variables, as well as those defining other beam
properties, are in COMMON BEAMCM which can be accessed with the
INCLUDE file (BEAMCM).
6) It is possible to track pseudoparticles by setting SDUM
= RAY. See
14} for details.
7) When an isotropic source is defined (by setting WHAT(3)
> 2000 pi),
any cosines defined by option BEAMPOS become meaningless, although
their values are still reported on standard output.
8) When the radiation source is a radioactive isotope, requested by
SDUM
= ISOTOPE and defined by command HI-PROPErt, special rules must
be observed. Note that if a stable isotope is input, nothing will
occur, and no particle will be transported. On the other hand, if
the isotope is radioactive, it will be necessary to request decay in
semi-analogue mode (command RADDECAY with WHAT(1)
> 1). If RADDECAY
is not requested, nothing will occur, and no particle will be
transported. Commands IRRPROFI and DCYTIMES are not allowed: decay
secondaries are sampled over the whole decay time from zero to
infinity, and all scoring will refer to the time integral of isotope
activity (dose, fluence, current, yield or residual nuclei PER
DECAY, not the corresponding rates at particular decay times as it
happens in the "activation study" mode).
Important: to score any quantity, command DCYSCORE must be issued
with WHAT(1)
= -1, and must be applying to all relevant estimators
and detectors. Without DCYSCORE, no scoring will occur.
For time-dependent calculations (see TCQUENCH, TIME-CUT) it is to be
noted that transport of isotope decay secondaries starts with an age
equal to the time of decay.
Examples:
BEAM -1.E-4 0.0 0.0 0.0 0.0 1.0 ELECTRON
BEAM 10.0 0.2 0.0 -2.36 -1.18 1.0 PROTON
BEAM -2.0 0.0 3.0 0.0 0.0 1.0 MUON-
BEAM -661.7E-6 0.0 1.E4 0.0 0.0 1.0 PHOTON
BEAM -14.E-3 0.0 0.0 1.2 0.7 -1.0 NEUTRON
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