Author: Daniel Brandt - SLAC
dbran.nosp@m.dt@s.nosp@m.lac.s.nosp@m.tanf.nosp@m.ord.e.nosp@m.du

This example demonstrates how phonon propagation in cryogenic crystals can be simulated in Geant4.

INTRODUCTION

Phonon propagation is different from most other Geant4 propagation simulations in a number of respects:

Phonons are massless particles moving slower than the speed of light
Phonon propagation and momentum vectors are not parallel
Events isotropic in phonon-momentum space are not isotropic in real space.

This example will simulate the propagation of acoustic phonons through a Germanium crystal, providing processes to simulate phonon scattering of isotopic impurities, mode mixing between polarization states and anharmonic downconversion (phonon splitting). As such it provides all the physics required to realistically simulate phonon propagation in cryogenically cold semiconductor crystals.

GEOMETRY

In this example the geometry is a cylindrical Germanium crystal centered at (0,0,0) with Almuninium end caps. Phonons absorbed in the Al end caps are counted by the sensitive detector.

PRIMARY EVENT

The primary event is a single phonon of energy 7.5 meV at the center of the Ge crystal. The polarization type (fast transvere, slow transverse or longitudinal) is determined randomly according to the density of states in Germanium. The direction of propagation is than determined by by the User Stacking Action class XPhononStackingAction.

EXECUTION & OUTPUT

The executable must be run from within the source directory of the example to ensure that it can find the path for crystal data files. Alternatively the search path for the crystal maps can be set in the setting the G4LATTICEDATA environment variable. If this variable does not exist, it defaults to ./CrystalMaps.

Data files for each crystal material are stored in a named subdirectory under $G4LATTICEDATA/, along with a config.txt file which specifies the numerical constants for the lattice. This example includes germanium [111] in CrystalMaps/Ge/.

Upon execution, the vis.mac visualization macro will automatically be executed. For the visualization to work, OpenGL support must be installed. The macro will automatically generate a single Primary Event (7.5 meV phonon) at the center of the crystal.

The trajectory colour will indicate the polarization state of the phonon:

Longitudinal: blue
Fast Transverse: green
Slow Transverse: red

A small circle will be drawn wherever a phonon is absorbed into the Aluminium. All events within the Aluminium are written into plain-text space-sparated-value (ssv) files:

timing.ssv

COLUMN 1: Time phonon was absorbed in ns since start of run
COLUMN 2: Energy of phonon absorbed

tcaustic.ssv

COLUMN 1: x-position of absobrtion in mm
COLUMN 2: y-position of absobrtion in mm
COLUMN 3: z-position of absobrtion in mm

Every time a phonon is simulated, the information is appended to timing.ssv and caustic.ssv. If the files do not exist they will be created.

TESTING

In order to test the example, it can be run as

./XGeBox run.in > test.out

This will create a single primary event and then cause the example to terminate automatically, with all screen output redirected to test.out.

If all went well, test.out should be identical to run.out provided with this example. Also, the files caustic.ssv and timing.ssv should have been created and be identical to caustic.out and timing.out respectively.

After the first time the example runs, it will append to caustic.ssv and timing.ssv. If the testing should be re-run, then caustic.ssv and timing.ssv will have to be deleted.