History:

10-06-2010 : Makoto Asai - merge into one example
13-05-2010 : Michel Maire - create as three examples

This example demonstrates 5 ways of the usage of G4ParticleGun shooting primary particles in different cases. These are

uniform particle direction in a given solid angle
Generate several vertices and particles per event
Show how to sample a tabulated function (eg. energy spectrum)
Divergent beam in an arbitrary direction
Shooting primaries in spherical coordinates with rotation matrix.

These usages can be chosen by a UI command

/gunExample/selectGunAction actionID

where actionID corresponds to above cases.

Geometry construction

It is a simple box which represents an 'infinite' homogeneous medium.

Physics list

PhysicsList.cc defines only geantino and transportation process.

Primary generator

There are 5 concrete primary generator action classes (PrimaryGeneratorActionN, N=0,1,2,3,4) which can be used as an independent sample code. PrimaryGeneratorAction is the class which uses and switches between these 5 concrete action classes. Each concrete generator action shoots geantinoes in a distribution decribed below.

0. Uniform particle direction in a given solid angle

spherical angles (alpha,psi) respective to z axis Histograms 5,6 show momentum direction in master frame.

1. Generate several vertices and particles per event

particle 1 : a geantino uniformly randomized on a cylinder surface.
particle 2 and 3 : symetric to particle 1. In addition, time_zero of each event is randomized.

2. Show how to sample a tabulated function (energy spectrum)

Energy is sampled from a tabulated function defined in InitFunction(). The function is assumed positive, linear per segment, continuous. Two sampling methods are illustrated : RejectAccept() and InverseCumul() (see Particle Data Group: pdg.lbl.gov --> Monte Carlo techniques)

Histogram 1 shows generated energy spectrum.

3. Divergent beam in an arbitrary direction with rotation matrix

A geantino uniformly randomized around a given direction (theta, phi). One wants to limit particle direction uniformly around this direction. First, one generates momentum direction in the master frame (eg. World). AlphaMax = opening angle around z axis. Then one rotates momentum in local frame, using rotateUz() function. RotateUz() transforms uz to newUz. It is composition of two simple rotations: theta around oy, then phi around oz (non commutative).
See: http://proj-clhep.web.cern.ch/proj-clhep/manual/UserGuide/VectorDefs/node49.html
Histograms 5,6 show momentum direction in local frame.

4. Shooting primaries in spherical coordinates with rotation matrix

a geantino uniformly randomized within a spherical shell.

a) Vertex position One wishes to shoot uniformly within a spherical shell. One works in spherical coordinates. One uses inverse cumulative method with analytical formulae.
Histograms 2,3,4 demonstrate uniform distribution of vertex position.

b) Momentum direction One wants to limit particle direction uniformly within (alphaMin, alphaMax). First, one generates momentum direction in the master frame (eg. World). Then, one rotates momentum in vertex_position frame, using rotateUz() function. RotateUz() transforms uz to ur. It is composition of two elementary rotations: theta around oy, then phi around oz (non commutative).
See: http://proj-clhep.web.cern.ch/proj-clhep/manual/UserGuide/VectorDefs/node49.html
Histograms 5,6 show momentum direction in vertex_position frame.

Visualisation

Visualization Manager is set in the main () (see particleGun.cc). Initialisation of the drawing is done via the commands /vis/.. in the macro vis.mac. This macro is automatically read from the main in case of interactive running mode.

How to start ?

Execute particleGun in 'batch' mode from macro files
```
% ParticleGun   run1.mac
```

Execute particleGun2 in 'interactive mode' with visualization

% particleGun
....
Idle>      ---> type your commands. For instance:
Idle> /gunExample/selectGunAction 1
Idle> /run/beamOn 10
....
Idle> exit

Histograms

particleGun produces several 1D histograms which are saved as particleGun.root by default.

1 : energy spectrum dN/dE = f(E)
2 : vertex position: radial distr dN/dv = f(r)
3 : vertex position: cos(theta)
4 : vertex position: phi
5 : particle direction in local frame: cos(alpha)
6 : particle direction in local frame: psi

Please note that histogram 1 will be filled only if you use PrimaryGeneratorAction2, histos 5,6 with PrimaryGeneratorAction0 and 3 and 2 through 6 will be filled with PrimaryGeneratorAction4.

The histograms are managed by the HistoManager class and its Messenger. The histos can be individually activated with the command :

/analysis/h1/set id nbBins  valMin valMax unit

where unit is the desired unit for the histo (MeV or keV, deg or mrad, etc..)

One can control the name of the histograms file with the command:

/analysis/h1/setFileName  name  (default particleGun)

It is possible to choose the format of the histogram file : root (default), xml, csv, by using namespace in HistoManager.hh

It is also possible to print selected histograms on an ascii file:

/analysis/h1/setAscii id

All selected histos will be written on a file name.ascii (default gunExample)