$Id: README,v 1.10 2009-11-21 00:22:55 perl Exp $
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Geant4 - an Object-Oriented Toolkit for Simulation in HEP
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Extended Example A01
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Example A01 implements a double-arm spectrometer with wire chambers,
hodoscopes and calorimeters. Event simulation and collection are
enabled, as well as event display and analysis.
1. GEOMETRY
The spectrometer consists of two detector arms. One arm provides
position and timing information of the incident particle while the
other collects position, timing and energy information of the particle
after it has been deflected by a magnetic field centered at the
spectrometer pivot point.
- First arm: box filled with air, also containing:
1 hodoscope (15 vertical strips of plastic scintillator)
1 drift chamber (5 horizontal argon gas layers with a
"virtual wire" at the center of each layer)
- Magnetic field region: air-filled cylinder which contains
the field
- Second arm: box filled with air, also containing:
1 hodoscope (25 vertical strips of plastic scintillator)
1 drift chamber (5 horizontal argon gas layers with a
"virtual wire" at the center of each layer)
1 electromagnetic calorimeter:
a box sub-divided along x,y and z
axes into cells of CsI
1 hadronic calorimeter:
a box sub-divided along x,y, and z axes
into cells of lead, with a layer of
plastic scintillator placed at the center
of each cell
2. PHYSICS
This example uses the following physics processes:
- electromagnetic:
photo-electric effect
Compton scattering
pair production
bremsstrahlung
ionization
multiple scattering
annihilation
- decay
- transportation in a field
and defines the following particles:
geantino, charged geantino, gamma, all leptons,
pions, charged kaons
Note that even though hadrons are defined, no hadronic processes
are invoked in this example.
3. EVENT:
An event consists of the generation of a single particle which is
transported through the first spectrometer arm. Here, a scintillator
hodoscope records the reference time of the particle before it passes
through a drift chamber where the particle position is measured.
Momentum analysis is performed as the particle passes through a magnetic
field at the spectrometer pivot and then into the second spectrometer
arm. In the second arm, the particle passes through another hodoscope
and drift chamber before interacting in the electromagnetic calorimeter.
Here it is likely that particles will induce electromagnetic showers.
The shower energy is recorded in a three-dimensional array of CsI
crystals. Secondary particles from the shower, as well as primary
particles which do not interact in the CsI crystals, pass into the
hadronic calorimeter. Here, the remaining energy is collected in a
three-dimensional array of scintillator-lead sandwiches.
Several aspects of the event may be changed interactively by the user:
- initial particle type
- initial momentum and angle
- momentum and angle spreads
- type of initial particle may be randomized
- strength of magnetic field
- angle of the second spectrometer arm
4. DETECTOR RESPONSE:
All the information required to simulate and analyze an event is
recorded in HITS. This information is recorded in the following
sensitive detectors:
- hodoscope:
particle time
particle position
strip ID
- drift chamber:
particle time
particle position
layer ID
- electromagnetic calorimeter:
particle position
energy deposited in cell
cell ID
- hadronic calorimeter:
particle position
energy deposited in cell
cell ID
5. VISUALIZATION:
Simulated events can be displayed on top of a representation of the spectrometer.
vis.mac outputs HepRep version 1 files suitable for viewing in HepRApp or WIRED4.
Change the /vis/open line from HepRepFile to DAWNFILE to instead
make .prim files suitable for viewing in DAWN.
heprep2-000-gz.mac outputs a series of gzipped HepRep version 2 files
each containing a single event, suitable for viewing in HepRApp or WIRED4
heprep2zip.mac outputs a single zip file that unzips to a series of
HepRep version 2 files, each each containing a single event (unzip
the single file by hand, then view the resulting individial HepRep files).
heprep2-000-zip.mac outputs a series of zipped HepRep version 2 files
each containing a single event (not yet viewable unless you
explicitly unzip them before viewing).
heprep2.mac outputs a HepRep version 2 file with multiple events
appended to a single file in an experimental manner
heprep2gz.mac outputs a HepRep version 2 file with multiple events
appended to a single file in an experimental manner
Any of the heprep mac files above with the name bheprep (for instance
bheprep2zip.mac) will write a Binary HepRep version 2 file, readable only
by WIRED4 (not by HepRApp).
6. ANALYSIS:
This example implements an AIDA-compliant analysis system which
creates histograms, ntuples and plotters. If you have built Geant4 with
the option to use analysis (answering yes to the appropriate question in
./Configure -build), then at the completion of a simulation run,
a file A01.aida is produced which contains these data structures.
This file can be used as an input to the Java Analysis Studio (JAS) which allows
the histograms and ntuples to examined, manipulated, saved and printed.
For further details, see also README.AIDA.
Gabriele Cosmo