Virtual Point Source model

In the Virtual Point Source model, primary particles are distributed as if they were emitted by a point source. The user can prescribe a transverse distribution using Xsec (e.g. gaussian, uniform box, etc.) at a given plane downstream. This plane is called reference plane and usually in clincal applications corresponds to the ISO plane. Particle trajectories are originating from a point on the beam axis that is located at virtual source distance (VSD) from field origin. Initial positions of particles are extrapolated to the field origin plane. The following schetch clarifies the procedure.

Input parameters

VSD(float) [0]

Virtual Source Distance from field origin along beam direction (field f), downstream if >0, upstream if <0

refPlaneDistance(float) [0]

distance from field origin of the plane where particles are distributed according to Xsec and FWHM parameters

VSDx,VSDy(float)

Virtual Source Distance from field origin along x and y directions (field left and up); the source is astigmatic, i.e. focussing down to a point at different positions along x and y

Note

If VSD == refPlaneDistance, then the source becomes a paraxial source, i.e. particles are propagating parallel to beam axis

Examples

Virtual Point Source (VPS) at the field origin, FWHM = 4 cm at ISO plane, with uniform disc transverse distribution.

region: phantom; L=[20,20,40]; voxels=[201,201,80]; pivot=[0.5,0.5,0.5]; material=air

field: 1 ; O = [0,0,-50] ; L = [20,20,2] ; pivot = [0.5,0.5,0.5]

pb: 1 1; particle=p; E = 150; Xsec=disc; FWHM=4; refPlaneDistance=50

VPS downstream of field origin

region: phantom; L=[20,20,40]; voxels=[201,201,80]; pivot=[0.5,0.5,0.5]; material=air

field: 1 ; O = [0,0,-50] ; L = [20,20,2] ; pivot = [0.5,0.5,0.5]

pb: 1 1; particle=p; E = 150; Xsec=disc; FWHM=4; refPlaneDistance=50; VSD=20

VPS upstream of field origin

region: phantom; L=[20,20,40]; voxels=[201,201,80]; pivot=[0.5,0.5,0.5]; material=air

field: 1 ; O = [0,0,-50] ; L = [20,20,2] ; pivot = [0.5,0.5,0.5]

pb: 1 1; particle=p; E = 150; Xsec=disc; FWHM=4; refPlaneDistance=50; VSD=-20

Beam focussing at ISO

region: phantom; L=[20,20,40]; voxels=[201,201,80]; pivot=[0.5,0.5,0.5]; material=air

field: 1 ; O = [0,0,-50] ; L = [20,20,2] ; pivot = [0.5,0.5,0.5]

pb: 1 1; particle=p; E = 150; Xsec=disc; FWHM=4; refPlaneDistance=0; VSD=50

Astigmatic beam focussing at +/- 10 cm with respect to ISO

region: phantom; L=[20,20,40]; voxels=[201,201,80]; pivot=[0.5,0.5,0.5]; material=air

field: 1 ; O = [0,0,-50] ; L = [20,20,2] ; pivot = [0.5,0.5,0.5]

pb: 1 1; particle=p; E = 150; Xsec=disc; FWHM=4; refPlaneDistance=0; VSDx=40; VSDy=60;