defines several beam characteristics: type of particle, energy, divergence,
profile and statistical weight
See also BEAMAXES, BEAMPOS, SOURCE
WHAT(1) > 0.0 : average beam momentum in GeV/c
< 0.0 : average beam kinetic energy in GeV
This value can be overridden in user routine SOURCE by
assigning a value to variable PBEAM
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 can be overridden in user routine SOURCE by
assigning a value to variable DPBEAM
Default = 0.0
WHAT(3) specifies the beam divergence (in mrad):
> 0.0 : |WHAT(3)| is the width of a rectangular angular
distribution
< 0.0 : |WHAT(3)| is the FWHM of a Gaussian angular distribution
> 2000 x PI mrad (i.e. 2 pi rad) : an isotropic distribution is
assumed (see Note 8 below)
This value can be overridden in user routine SOURCE by
assigning a value to variable DIVBM
Default = 0.0
WHAT(4) > 0.0 : If WHAT(6) > 0.0, beam width in x-direction in cm. 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))
This value can be overridden in user routine SOURCE by
assigning a value to variable XSPOT
Default = 0.0
WHAT(5) > 0.0 : If WHAT(6) > 0.0, beam width in y-direction in cm. 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))
This value can be overridden in user routine SOURCE by
assigning a value to variable YSPOT
Default = WHAT(4)WHAT(6) : |WHAT(6)| = weight of the beam particles.
If 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)
This value can be overridden in user routine SOURCE by assigning
a new weight to the beam particles (variable BEAWEI) or assigning
a new value to the logical variable LBEAMC.
(LBEAMC = .TRUE. ==> annular beam).
Default = 1.0
SDUM = beam particle name. Particle names and numerical codes are listed
in the table of FLUKA particle types (see 5}).
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 NUCLEON, and not the total
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. If no
radioactive isotope evolution or decay is requested, or if a
stable isotope is input, nothing will occur, and no particle will
be transported.
[Not yet implemented: For optical photons, use the name OPTIPHOT
and specify further the transport properties by material by means
of option OPT-PROP.]
This value can be overridden in user routine SOURCE by assigning
a value to variable IJBEAM equal to the numerical code of the
beam particle
Default = PROTON
Default (option BEAM not requested): all the above defaults apply, unless
other values are provided by the user by means of a SOURCE
subroutine (see 13}).
Notes:
1) 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}). 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.
Advice: when a user-written SOURCE is used, set WHAT(1) in BEAM
equal to the maximum source particle momentum (or energy).
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
3) 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]
4) All FLUKA results are normalised per unit incident particle weight.
Thus, setting the starting weight to a fixed value different from 1
has no practical effect. 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})).
5) 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).
6) 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).
7) It is possible to track pseudoparticles by setting SDUM = RAY. See
14} for details.
8) 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.
Examples:
* The following BEAM card refers to a 100 keV pencil-like
* electron beam:
*...+....1....+....2....+....3....+....4....+....5....+....6....+....7....+...
BEAM -1.E-4 0.0 0.0 0.0 0.0 1.0 ELECTRON
* The next option card describes a parallel proton beam with a
* momentum of 10.0 +/- 0.2 GeV/c, with a Gaussian profile in
* the x-direction and in the y-direction described by standard
* deviations sigma_x = 1. cm (FWHM = 2.36 cm) and sigma_y = 0.5
* cm (FWHM = 1.18 cm).
*...+....1....+....2....+....3....+....4....+....5....+....6....+....7....+...
BEAM 10.0 0.2 0.0 -2.36 -1.18 1.0 PROTON
* The next example concerns a negative muon beam of 2 GeV
* kinetic energy, with a divergence of 3 mrad.
*...+....1....+....2....+....3....+....4....+....5....+....6....+....7....+...
BEAM -2.0 0.0 3.0 0.0 0.0 1.0 MUON-
* The next BEAM card describes a 137-Cs isotropic source
BEAM -661.7E-6 0.0 1.E4 0.0 0.0 1.0 PHOTON
* The last example illustrates how to define a hollow 14 MeV
* neutron beam, with an inner radius of 7 mm and an outer radius
* of 1.2 cm.
*...+....1....+....2....+....3....+....4....+....5....+....6....+....7....+...
BEAM -14.E-3 0.0 0.0 1.2 0.7 -1.0 NEUTRON
