BEAM

defines several beam characteristics: type of particle, energy, divergence, profile and statistical weight

See also BEAMAXES, BEAMPOSit, SOURCE, SPECSOUR

     WHAT(1) > 0.0 : average beam momentum in GeV/c
             < 0.0 : average beam kinetic energy in GeV
                     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
             < 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 7 below)
                     This value is available in COMMON BEAMCM as variable DIVBM.
                     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. 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 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. 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 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)).
               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 UNIT ATOMIC MASS, 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 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.
               Neutrino interactions are activated by a (A)NEUTRIxx SDUM.
               Neutrino interactions are forced to occur in the point (or area)
               defined in the BEAMPOS card.

               [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 (if command
               SOURCE is present in input) by assigning a value to variable
               IJBEAM equal to the numerical code of the beam particle.
               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 (16)). 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 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
            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. 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))).

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.

Examples:

 *         The following BEAM card refers to a 100 keV pencil-like
 *         electron beam:
 *...+....1....+....2....+....3....+....4....+....5....+....6....+....7....+....8
 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....+....8
 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....+....8
 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....+....8
 BEAM         -14.E-3       0.0       0.0       1.2       0.7     -1.0 NEUTRON

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