BEAMPOSit¶

Summary

Defines the coordinates of the centre of the beam spot (i.e. the point from which transport starts) and the beam direction. Also allows to define some spatially extended sources.

See also BEAM, BEAMAXES, SOURCE, SPECSOUR

For SDUM = blank or NEGATIVE:

WHAT(1) =: x-coordinate of the spot centre.

This value is available in COMMON BEAMCM as variable XBEAM. It can be used or modified in subroutine SOURCE if command SOURCE is present in input.
Default: = 0.0
WHAT(2) =: y-coordinate of the spot centre.

This value is available in COMMON BEAMCM as variable YBEAM. It can be used or modified in subroutine SOURCE if command SOURCE is present in input.
Default: = 0.0
WHAT(3) =: z-coordinate of the spot centre.

This value is available in COMMON BEAMCM as variable ZBEAM. It can be used or modified in subroutine SOURCE if command SOURCE is present in input.
Default: = 0.0
WHAT(4) =: direction cosine of the beam with respect to the x-axis of the beam reference frame (namely the geometry reference frame unless defined differently with BEAMAXES).

This value is available in COMMON BEAMCM as variable UBEAM. It can be used or modified in subroutine SOURCE if command SOURCE is present in input.
Default: = 0.0
WHAT(5) =: direction cosine of the beam with respect to the y-axis of the beam reference frame (namely the geometry reference frame unless defined differently with BEAMAXES).

This value is available in COMMON BEAMCM as variable VBEAM. It can be used or modified in subroutine SOURCE if command SOURCE is present in input.
Default: = 0.0
WHAT(6): : not used
SDUM: = NEGATIVE means that the direction cosine with respect to z-axis is negative.

The value of the direction cosine with respect to the z-axis can be overridden in user routine SOURCE by assigning a value to variable WBEAM in COMMON BEAMCM (make sure that the three cosines are properly normalised so that the sum of their squares is 1.0 in double precision!)
Default: : beam directed in the positive z-direction

For SDUM = SPHE–VOL:

The command defines a spatially extended source shaped as a spherical shell. The centre x,y,z of the outer and of the inner sphere, as well as the particle direction, must be defined by another BEAMPOSit command. The particle angular distribution, or lack of it, is defined by the BEAM card.

WHAT(1) ≥ 0.0:: radius in cm of the inner sphere defining the shell
< 0.0:: resets to default
Default: : 0.0
WHAT(2) > 0.0:: radius in cm of the outer sphere defining the shell
= 0.0:: ignored
< 0.0:: resets to default
Default: : 1.0
WHAT(3): – WHAT(6): not used

For SDUM = CYLI–VOL:

The command defines a spatially extended source shaped as a cylindrical shell with the height of the outer and of the inner cylinder parallel to the z axis of the beam reference frame (namely the geometry reference frame unless defined differently with BEAMAXES). The outer and the inner cylinder are centred at a x,y,z point defined by another BEAMPOSit command, which also sets the particle direction by means of a SDUM blank or = NEGATIVE. The particle angular distribution, or lack of it, is defined by the BEAM card.

WHAT(1) ≥ 0.0:: radius in cm of the inner cylinder defining the shell
< 0.0:: resets to default
Default: : 0.0
WHAT(2) > 0.0:: radius in cm of the outer cylinder defining the shell
= 0.0:: ignored
< 0.0:: resets to default
Default: : 1.0
WHAT(3) ≥ 0.0:: height in cm of the inner cylinder defining the shell
< 0.0:: resets to default
Default: : 0.0
WHAT(4) > 0.0:: height in cm of the outer cylinder defining the shell
= 0.0:: ignored
< 0.0:: resets to default
Default: : 1.0
WHAT(5),: WHAT(6): not used

For SDUM = CART–VOL:

The command defines a spatially extended source shaped as a Cartesian shell with the edges parallel to the axes of the reference frame (namely the geometry reference frame unless defined differently with BEAMAXES). The outer and the inner parallelepiped are centred at a x,y,z point defined by another BEAMPOSit command, which also sets the particle direction by means of a SDUM blank or = NEGATIVE. The particle angular distribution, or lack of it, is defined by the BEAM card.

WHAT(1) ≥ 0.0:: length in cm of the x side of the inner parallelepiped defining the shell
< 0.0:: resets to default
Default: : 0.0
WHAT(2) > 0.0:: length in cm of the x side of the outer parallelepiped defining the shell
= 0.0:: ignored
< 0.0:: resets to default
Default: : 1.0
WHAT(3) ≥ 0.0:: length in cm of the y side of the inner parallelepiped defining the shell
< 0.0:: resets to default
Default: : 0.0
WHAT(4) > 0.0:: length in cm of the y side of the outer parallelepiped defining the shell
= 0.0:: ignored
< 0.0:: resets to default
Default: : 1.0
WHAT(5) ≥ 0.0:: length in cm of the z side of the inner parallelepiped defining the shell
< 0.0:: resets to default
Default: : 0.0
WHAT(6) > 0.0:: length in cm of the z side of the outer parallelepiped defining the shell
= 0.0:: ignored
< 0.0:: resets to default
Default: : 1.0

For SDUM = FLOOD:

The command defines a source distribution on a spherical surface, centred at the x,y,z point defined by another BEAMPOSit command with SDUM blank or = NEGATIVE, such as to produce a uniform and isotropic fluence within the sphere. The value of the produced fluence will be \(1/(\pi R^2\)) cm\(^{-2}\)

WHAT(1) > 0.0:: radius \(R\) of the sphere in cm
= 0.0:: ignored
< 0.0:: resets to default
Default: : \(1/\sqrt{\pi}\) (i.e. fluence = 1 cm\(^{-2}\))
WHAT(2): – WHAT(6): not used
Default: (option BEAMPOSit not requested): beam starting at point 0.0, 0.0, 0.0 in the z-direction

Notes

To take full advantage of some tracking optimisation features, it is often a good idea to create a buffer vacuum region containing the whole geometry, which must itself be contained within the external (mandatory) black hole region. It is then suggested that the beam impact point be chosen in vacuum, slightly upstream of the actual one on a material boundary. As a general rule, anyway, it is recommended to never select the starting point exactly on a boundary.
The beam spot coordinates and the beam director cosines as defined with the BEAMPOSit card are available to user routines with names XBEAM, YBEAM, ZBEAM and UBEAM, VBEAM, WBEAM respectively. These variables, as well as those defining other beam properties, are in COMMON BEAMCM which can be accessed with the INCLUDE file (BEAMCM).
Beam divergence and transversal profile defined by option BEAM, as well as polarisation defined by option POLARIZAti, 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.
When an isotropic source is defined (by setting command BEAM with WHAT(3) > 2000 \(\pi\) in option BEAM), any cosines defined by option BEAMPOSit become meaningless, although their values are still reported on standard output.

Examples:

*         A beam parallel to the x-axis starting at a point of
*         coordinates -0.1, 5.0, 5.0 :
*...+....1....+....2....+....3....+....4....+....5....+....6....+....7...
BEAMPOS         -0.1       5.0       5.0       1.0       0.0       0.0

*         A beam perpendicular to the x-axis, with director cosines
*         0., 1/sqrt(2), -1/sqrt(2) with respect to x, y and z,
*         starting at point 0.0, 0.0, 0.0 :
*...+....1....+....2....+....3....+....4....+....5....+....6....+....7....+...
BEAMPOS          0.0       0.0       0.0       0.0 0.7071068      0.0 NEGATIVE