Gabriele Cosmo
100 lines
3.8 KiB
Plaintext
100 lines
3.8 KiB
Plaintext
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Phonon propagation in Geant4
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=================================================================
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Daniel Brandt - SLAC
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dbrandt@slac.stanford.edu
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This example demonstrates how phonon propagation in cryogenic crystals
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can be simulated in Geant4.
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1.INTRODUCTION
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Phonon propagation is different from most other Geant4 propagation
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simulations in a number of respects:
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-Phonons are massless particles moving slower than the speed of light
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-Phonon propagation and momentum vectors are not parallel
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-Events isotropic in phonon-momentum space are not isotropic in real
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space.
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This example will simulate the propagation of acoustic phonons through
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a Germanium crystal, providing processes to simulate phonon scattering
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off isotopic impurities, mode mixing between polarization states and
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anharmonic downconversion (phonon splitting). As such it provides all
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the physics required to realistically simulate phonon propagation in
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cryogenically cold semiconductor crystals.
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2. GEOMETRY
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In this example the geometry is a cylindrical Germanium crystal
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centered at (0,0,0) with Almuninium end caps. Phonons absorbed in the
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Al end caps are counted by the sensitive detector.
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3. PRIMARY EVENT
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The primary event is a single phonon of energy 7.5 meV at the center of
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the Ge crystal. The polarization type (fast transvere, slow transverse or
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longitudinal) is determined randomly according to the density of states
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in Germanium. The direction of propagation is than determined by by the
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User Stacking Action class XPhononStackingAction.
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4. EXECUTION & OUTPUT
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The executable must be run from within the source directory of the example
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to ensure that it can find the path for crystal data files. Alternatively
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the search path for the crystal maps can be set in the setting the
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G4LATTICEDATA environment variable. If this variable does not exist, it
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defaults to ./CrystalMaps.
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Data files for each crystal material are stored in a named subdirectory
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under $G4LATTICEDATA/, along with a config.txt file which specifies the
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numerical constants for the lattice. This example includes germanium [111]
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in CrystalMaps/Ge/.
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Upon execution, the vis.mac visualization macro will automatically be
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executed. For the visualization to work, OpenGL support must be installed.
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The macro will automatically generate a single Primary Event (7.5 meV phonon)
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at the center of the crystal.
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The trajectory colour will indicate the polarization state of the phonon:
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Longitudinal: blue
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Fast Transverse: green
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Slow Transverse: red
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A small circle will be drawn wherever a phonon is absorbed into the
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Aluminium. All events within the Aluminium are written into plain-text
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space-sparated-value (ssv) files.
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timing.ssv
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------------
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COLUMN 1: Time phonon was absorbed in ns since start of run
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COLUMN 2: Energy of phonon absorbed
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caustic.ssv
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------------
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COLUMN 1: x-position of absobrtion in mm
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COLUMN 2: y-position of absobrtion in mm
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COLUMN 3: z-position of absobrtion in mm
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Every time a phonon is simulated, the information is appended to timing.ssv
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and caustic.ssv. If the files do not exist they will be created.
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5. TESTING
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In order to test the example, it can be run as
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./XGeBox run.in > test.out
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This will create a single primary event and then cause the example to
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terminate automatically, with all screen output redirected to test.out.
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If all went well, test.out should be identical to run.out provided with
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this example. Also, the files caustic.ssv and timing.ssv should have been
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created and be identical to caustic.out and timing.out respectively.
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After the first time the example runs, it will append to caustic.ssv and
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timing.ssv. If the testing should be re-run, then caustic.ssv and timing.ssv
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will have to be deleted.
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