New Features under Development - 1 Makoto Asai On behalf of the SLAC Geant4 team
Contents Semiconductor energy transport Phonon Electron/hole drift Activation of material We expect to deliver first implementation of both of these features with Geant4 version 9.6. New Features under Development - 1 2
Semiconductor energy transport - Phonon - Electron/hole drift
Semiconductor energy transport Geant4 processes may deposit energy in semiconductor crystals Usually such deposition is just handled as sensitive detector energy deposition and not simulated in detail. In some cases, interested in the details of energy transport in the crystal Low background particle physics experiments are interested in simulating backgrounds Space projects are interested in estimating damage to semi conductor structures and also simulating single event effects. Firstly we need to introduce the concept of crystal into Geant4 Up to now, Geant4 assumes a material is homogeneous, i.e. no particular direction for channeling/interference New classes for crystal lattice G4LogicalLattice class holds information about elastic constants G4PhysicalLattice class links G4LogicalLattice to G4VPhysicalVolume G4LatticeManager manages accesses to lattices G4Track::GetVelocity() is extended to allow mapping of k-vector to group velocity New Features under Development - 1 4
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Phonons of different energies have vastly different mean free paths. Down conversion causes phonons to change from diffuse to ballistic propagation. New Features under Development - 1 7
Phonon transport in Ge crystal Phonon trajectories in a 75 mm Ge crystal, simulated with Geant4. Trajectory color indicates polarization state. New Features under Development - 1 8
Luke-Neganov effect Ge has resistance, i.e. charge carriers scatter Ohmic heating is another way of saying emission of phonons Crystal analogy of Cerenkov radiation New Features under Development - 1 9
Anisotropic electron mass The energy-vs-momentum relation in Ge is called as the band structure. The Ge conduction band is highly anisotropic. Thus, particles mass can be defined using its energy-vs-momentum relationship. Hence, the electron effective mass is anisotropic in Ge. Ge conduction band has 8 equivalent minima. The mass tensor is diagonalized in a coordinate system with one axis parallel to the symmetry axis In Ge (and Si), only two distinct components remain: m parallel and m perpendicular New Features under Development - 1 10
Charge transport in Ge The anisotropic e- mass causes e- to propagate at a 30 angle to field line. This is known as oblique propagation Since the valence band maximum is isotropic, h+ has a scalar mass. Carriers propagating in Ge. Green=electron, red=hole, blue=phonon. New Features under Development - 1 11
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Simulation of Radiation Events 63-MeV proton incident on a SiGe Heterojunction Bipolar Transistor (HBT) Iso-charge surfaces following a nuclear reaction Courtesy of R.Reed (Vanderbilt U.) New Features under Development - 1 14
Activation of material
Activation of material Simulation of activation of irradiated material has been a long-demanded feature in Geant4. It has many use-cases including : high luminosity HEP experiments satellites passing through space radiation environments (e.g. South-Atlantic Anomaly, radiation belts of Earth or Jupiter), and shielding devices for accelerators (both HENP and medical). A new convolution mechanism is being developed so that the radiation spectrum of activated material at a certain time from a given irradiation period could be predicted. Assuming irradiation profile (beam profile or path in radiation environment), generation of radioactive nuclides is convoluted to predict gamma-ray spectrum of irradiated material after given time interval. New Features under Development - 1 16
ASTRO-H (1) SXI and SXS New Features under Development - 1 17
ASTRO-H (2) Point source on HXI Left; center; right - 0arcmin; 2arcmin; 4arcmin top; bottom - 10keV; 30keV New Features under Development - 1 Kepler SNR on HXI (very preliminary) 18
Courtesy of R.Reed (Vanderbilt U.) New Features under Development - 1 19
ASTRO-H (3) New Features under Development - 1 20