Merge pull request #72 from JeffersonLab/DEV

Dev

Generators/geantBEAM/examples/geantBEAM.cfg

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+c This card configures the simulation program to act purely as a
+c Monte Carlo event generator, and not to actually track any of the particles
+c that it generates. The events are written to the output file with only the
+c MC section filled out (reactions tag). This file can be fed back later to
+c HDGeant using the INFILE card above to carry out the actual simulation.
+c This provides access to the built-in photon beam generator of HDGeant to
+c someone who wants to study the properties of the beam apart from its
+c interactions in the target. Three keywords are currently supported.
+c    'precol'   - single-photon events starting upstream of the primary
+c                 collimator, with correlated spatial and momentum
+c                 distributions for the well-tuned GlueX beamline.
+c    'postcol'  - single-photon events starting downstream of the secondary
+c                 collimator. Beam photons have been tracked through the
+c                 system of collimators and sweep magnets but then stopped
+c                 before entry into the pair spectrometer.
+c    'postconv' - e+e- pair and e+e-/e-recoil events generated in the
+c                 TPOL target. Beam photons have been tracked through
+c                 the system of collimators and then pair-converted in
+c                 the TPOL coverter using a custom polarization-sensitive
+c                 pair/triplet production generator. They are saved as
+c                 a single vertex within the PTAR target.
+c    'BHgen' -    beam photons are followed from the source to the target
+c                 where they are forced to undergo e+e- conversion through
+c                 the Bethe-Heitler process. These pairs are saved in the
+c                 output file for possible simulation later. If the first
+c                 argument 'BHgen' is followed by a floating point value
+c                 then it is taken as the minimum mass of the generated
+c                 pair in GeV/c^2 (default zero). The event weight saved as
+c                 an attribute of the <reaction> tag in the output event
+c                 allows the user to run the generated events through an
+c                 accept-reject filter before they are simulated. The
+c                 weight is in microbarns, such that the average value
+c                 is the total cross section for Bethe-Heitler pairs
+c                 above the stated pair threshold.
+c The first two modes are supported by both HDGeant and HDGeant4, while
+c postconv and BHgen are only supported by HDGeant4.
+GENBEAM 'BHgen'