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BEAMPOSit

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, centred at the x,y,z point defined by another BEAMPOS command with SDUM blank or = NEGATIVE. The particle direction or angular distribution are those defined by a BEAM or by another BEAMPOS 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 cm

     WHAT(3)-WHAT(6): not used

for SDUM = CYLI-VOL:

the command defines a spatially extended source shaped as a cylindrical shell centred at the x,y,z point defined by another BEAMPOS command with SDUM blank or = NEGATIVE and with the height parallel to the z axis of the beam reference frame. The particle direction or angular distribution are those defined by a BEAM or by another BEAMPOS 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 cm

     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 cm

     WHAT(5), WHAT(6): not used

for SDUM = CART-VOL:

the command defines a spatially extended source shaped as a Cartesian shell centred at the x,y,z point defined by another BEAMPOS command with SDUM blank or = NEGATIVE and with the sides parallel to the beam reference frame axes. The particle direction or angular distribution are those defined by a BEAM or by another BEAMPOS 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 cm

     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 cm

     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 cm

for SDUM = FLOOD:

the command defines a source distribution on a spherical surface, centred at the x,y,z point defined by another BEAMPOS command with SDUM blank or = NEGATIVE, such as to produce a uniform and isotropic fluence within the sphere.

     WHAT(1) >  0.0: radius of the sphere in cm
             =  0.0: ignored
             <  0.0: resets to default
               Default: 1/sqrt(pi) cm

     WHAT(2)-WHAT(6): not used

     Default (option BEAMPOS not requested): beam starting at
            point 0., 0., 0. in the z direction)

Notes:

• 1) 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) blackhole 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 impact point EXACTLY on a boundary.

• 2) The beam spot coordinates and the beam director cosines as defined with the BEAMPOS 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).

• 3) 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

• 4) When an isotropic source is defined (by setting command BEAM with WHAT(3) > 2000 pi), any cosines defined by option BEAMPOS 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....+....8
 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....+....8
 BEAMPOS          0.0       0.0       0.0       0.0 0.7071068      0.0 NEGATIVE