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1
2 =================================================================
3 Phonon propagation in Geant4
4 =================================================================
5 Daniel Brandt - SLAC
6 dbrandt@slac.stanford.edu
7
8 This example demonstrates how phonon propagation in cryogenic crystals
9 can be simulated in Geant4.
10
11 1.INTRODUCTION
12
13 Phonon propagation is different from most other Geant4 propagation
14 simulations in a number of respects:
15
16 -Phonons are massless particles moving slower than the speed of light
17
18 -Phonon propagation and momentum vectors are not parallel
19
20 -Events isotropic in phonon-momentum space are not isotropic in real
21 space.
22
23 This example will simulate the propagation of acoustic phonons through
24 a Germanium crystal, providing processes to simulate phonon scattering
25 off isotopic impurities, mode mixing between polarization states and
26 anharmonic downconversion (phonon splitting). As such it provides all
27 the physics required to realistically simulate phonon propagation in
28 cryogenically cold semiconductor crystals.
29
30 2. GEOMETRY
31
32 In this example the geometry is a cylindrical Germanium crystal
33 centered at (0,0,0) with Almuninium end caps. Phonons absorbed in the
34 Al end caps are counted by the sensitive detector.
35
36 3. PRIMARY EVENT
37
38 The primary event is a single phonon of energy 7.5 meV at the center of
39 the Ge crystal. The polarization type (fast transvere, slow transverse or
40 longitudinal) is determined randomly according to the density of states
41 in Germanium. The direction of propagation is than determined by by the
42 User Stacking Action class XPhononStackingAction.
43
44 4. EXECUTION & OUTPUT
45
46 The executable must be run from within the source directory of the example
47 to ensure that it can find the path for crystal data files. Alternatively
48 the search path for the crystal maps can be set in the setting the
49 G4LATTICEDATA environment variable. If this variable does not exist, it
50 defaults to ./CrystalMaps.
51
52 Data files for each crystal material are stored in a named subdirectory
53 under $G4LATTICEDATA/, along with a config.txt file which specifies the
54 numerical constants for the lattice. This example includes germanium [111]
55 in CrystalMaps/Ge/.
56
57 Upon execution, the vis.mac visualization macro will automatically be
58 executed. For the visualization to work, OpenGL support must be installed.
59 The macro will automatically generate a single Primary Event (7.5 meV phonon)
60 at the center of the crystal.
61
62 The trajectory colour will indicate the polarization state of the phonon:
63 Longitudinal: blue
64 Fast Transverse: green
65 Slow Transverse: red
66
67 A small circle will be drawn wherever a phonon is absorbed into the
68 Aluminium. All events within the Aluminium are written into plain-text
69 space-sparated-value (ssv) files.
70
71 timing.ssv
72 ------------
73 COLUMN 1: Time phonon was absorbed in ns since start of run
74 COLUMN 2: Energy of phonon absorbed
75
76 caustic.ssv
77 ------------
78 COLUMN 1: x-position of absobrtion in mm
79 COLUMN 2: y-position of absobrtion in mm
80 COLUMN 3: z-position of absobrtion in mm
81
82 Every time a phonon is simulated, the information is appended to timing.ssv
83 and caustic.ssv. If the files do not exist they will be created.
84
85 5. TESTING
86
87 In order to test the example, it can be run as
88 ./XGeBox run.in > test.out
89
90 This will create a single primary event and then cause the example to
91 terminate automatically, with all screen output redirected to test.out.
92
93 If all went well, test.out should be identical to run.out provided with
94 this example. Also, the files caustic.ssv and timing.ssv should have been
95 created and be identical to caustic.out and timing.out respectively.
96
97 After the first time the example runs, it will append to caustic.ssv and
98 timing.ssv. If the testing should be re-run, then caustic.ssv and timing.ssv
99 will have to be deleted.
