GEARS uses the Geant4 general particle source to generate the primary particles, from which a Geant4 simulation starts (see this plot to understand the general concept). The up-to-date examples of gps are shipped together with the Geant4 source code in the folder:
An outdated webpage shows most of the examples contained in the folder above. In addition, it also shows resulted distribution plots, which are very helpful to understand the real effects of the gps macro commands.
Source in a volume
# define a shape that is slightly larger than <physics_volume_name> /gps/pos/type Volume /gps/pos/shape Cylinder /gps/pos/radius 2 cm /gps/pos/halfz 3 cm # place it to a position so that it can fully contain <physics_volume_name> /gps/pos/centre 3 2 0 cm # GPS will generate particles only in <physics_volume_name> /gps/pos/confine physics_volume_name
Source on a surface
# on a plane /gps/pos/type Plane # <shape> can be Circle, Annulus, Ellipse, Square, Rectangle /gps/pos/shape <shape> # on the surface of a volume /gps/pos/type Surface # <shape> can be Sphere, Ellipsoid, Cylinder, EllipticCylinder, Para /gps/pos/shape <shape>
Am241 is a complicated source. It is an alpha, X-ray and gamma emitter at the same time. A mac file is include here to demonstrate the definition of multiple particles from a source.
The alpha partiles emitted from Am241 can be used to bonbust Be and generate neutrons up to about 12 MeV. The measured neutron energy spectrum can be found in the literature. A mac file is included here to demonstrate how to generate neutrons based on an energy spectrum.
Ar39 is a beta emitter. It is a common background for examples unitize liquid argon as detecting material. Its measured beta spectrum is implemented in this mac file.
Co57 is a common low-energy calibration source. It emits a few x and gamma-rays as shown in this mac file.
Fe55 is an other commonly used low-energy calibration source. It emits a few x-rays around 6 keV as shown in this mac file.
Retrieve information of primary particles
Co57HPGe.mac modifies some default settings of a Co57 source defined in Co57.mac and shoots the emitted photons to a HPGe detector defined in HPGe.tg. The following commands show how to retrieve information of the primary particles in this simulation and use it to make event selections:
// check primary particle energy distribution root  t->Draw("k") // show results in log scale root  gPad->SetLogy() // show penetration depth (z) of 122 keV gamma rays (trk==1) in HPGe (vlm==1) root  t->Draw("zz","k>120 && k<130 && trk==1 && vlm==1") // show penetration depth of 14 keV gamma rays on the same plot root  t->Draw("zz","k<15 && trk==1 && vlm==1","same") // show penetration depth of 692 keV gamma rays on the same plot root  t->Draw("zz","k>300 && trk==1 && vlm==1","same")