BEAM¶

Summary

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 \(\pi\) mrad (i.e. \(2\) \(\pi\) rad): an isotropic distribution is assumed (see Note 7).

< 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 Table 5.1 on).
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 \(^4\)He, \(^3\)He, 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 and antineutrino interactions are activated by SDUM = (A)NEUTRIxx.
Neutrino interactions are forced to occur at the point (or area) defined in the BEAMPOSit card.
The special SDUM’s AMBE, AMB, 252CF select neutron spectra according to an Americium-Beryllium, Americium-Boron, and Californium-252 source respectively.
The special SDUM’s D-D, and D-T, select neutron spectra according to a deuterium-deuterium, and deuterium-tritium, thick source respectively. “Thick” means equal or larger than the range of the incoming deuterons, WHAT(1) is the momentum or kinetic energy (if negative) of the deuterons, up to 500 keV maximum for d-d, and d-T.
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

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, or, in some special cases, by means of a pre-defined source invoked by command SPECSOUR,,,). 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 cross section tabulations and scoring facilities.

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.
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:

\[{\mathrm P}(r)\,=\,\frac{1}{2\pi\sigma_x\sigma_y}\,{\mathrm e}^ {-\frac{1}{2}\left[\left(\frac{x}{\sigma_x}\right)^2 + \left(\frac{y}{\sigma_y}\right)^2 \right]}\,=\, \frac{1}{2\pi\sigma_r^2}\,{\mathrm e}^ {-\frac{1}{2}\left( \frac{r}{\sigma_r} \right)^2 }\]
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 SOURCE: user-written source).
All options governed by WHAT(3)–WHAT(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. 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).
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).
It is possible to track pseudo-particles by setting SDUM = RAY. See Use of RAY pseudo-particles for details.
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.
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:

*         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