This deposition generator is mostly intended for simulations of laser-TCT experiments.
It would generate charge, deposited by absorption of a laser pulse in silicon bulk.
This module is not dependent on Geant4. Instead, it implements tracking algorithms and simulations of corresponding physical phenomena by its own, using internal Allpix geometry.
Current implementation assumes that the laser pulse is a bunch of point-like photons, each traveling in a straight line. A lookup table \[[@optical_properties]\] is used to determine absorption and refraction coefficients for a given wavelength.
This module has its own tracking algorithms (no dependencies on Geant4), based on Allpix geometry
Multiple photons are produced in a *one* event, thus a *single* event models a *single* laser pulse.
Tracking features:
* photons are absorbed in detector bulk on physically correct depth
* each photon is assumed to create exactly one e-h pair
* photons refract on silicon-air interface
* tracks are terminated when a photon leaves *first encountered* sensitive volume
* tracks are terminated if a passive object is hit (the only supported passive object type is `box`)
Spatial and temporal distribution of incident photons are tuned to mimic a real laser pulse.
As a result, this module yields multiple `DepositedCharge` instances for each detector, with them having physically correct spatial and temporal distribution.
Initial direction and starting timestamp for every photon in the bunch are generated to mimic
spatial and temporal distributions of delivered intensity of a real laser pulse.
Two options for beam geometry are currently available: `cylindrical` and `converging`.
For both options, transversal beam profiles will have a gaussian shape.
For a `cylindrical` beam, all tracks are parallel to the set beam direction.
For a `converging` beam, track directions would have isotropic distribution (but with a limit on a max angle between the track and the set beam direction).
**NB**: convention on global time zero for this module contradicts the general convention of the Allpix Squared.
For this module, global t=0 is chosen in such a way that the mean value of temporal distribution is *always* positioned at *4 standard deviations* w.r.t. the global t=0.
Thus, there is not necessarily a particle that is created exactly when the global time starts.
Although, the following Allpix Squared conventions still apply:
* No particles have a negative timestamp.
* Local time zero for each detector is a moment when the first particle that creates a hit in this detectors enters its bulk.
## Parameters
*`number_of_photons`: number of photons in a *single* event, defaults to 10000
*`source_position`, a 3D position vector
*`beam_direction`, a 3D direction vector
*`wavelength` of a laser supported values are 250 -- 1450 nm
*`pulse_duration`: gaussian width of pulse temporal profile
As a result, this module yields `DepositedCharge` instances for each detector, with them having physically correct spatial and temporal distribution.
*`beam_waist`: std_dev of transversal beam profile, defaults to 20 um
*`beam_geometry`, either `cylindrical` or `converging`
*`focal_distance`(length) and `beam_convergence_angle`: both need to be specified for a `converging` beam
*`output_plots`, defaults to `false`
## Parameters
*`log_level`: follows the general rules of Allpix Squared. `INFO` will output photon counter and stats on hits for each event. `DEBUG` will also include verbose information of tracking for each photon.
*`number_of_photons`: number of incident photons, generated in *one* event. Defaults to 10000. The total deposited charge will also depend on wavelength and geometry.
*`wavelength` of the laser. Supported values are 250 -- 1450 nm.
*`pulse_duration`: gaussian width of pulse temporal profile. Defaults to 0.5 ns.
*`source_position`: a 3D position vector.
*`beam_direction`: a 3D direction vector.
*`beam_geometry`: either `cylindrical` or `converging`
*`beam_waist`: standard deviation of transversal beam intensity distribution at focus. Defaults to 20 um.
*`focal_distance`: needs to be specified for `converging` beam. This distance is *as it would be in air*. In silicon, beam shape will effectively stretch along its direction due to refraction and the actual focus will be further away from the source.
*`beam_convergence_angle`: max angle between tracks and `beam_direction`. Needs to be specified for a `converging` beam.
*`output_plots`: if set `true`, this module will produce histograms to monitor beam shape and also 3D distributions of charges, deposited in each detector. Histograms would look sensible even for one-event runs. Defaults to `false`.
## Usage
`DepositedCharge` instances created by this module are properly assigned time.
Thus, a simulation pipeline with `DepositionLaser`, `TransientPropagation` and `PulseTransfer` is expected to produce pulse shapes, comparable with experimentally obtained ones.
A simulation pipeline to build analog detector response would include `DepositionLaser`, `TransientPropagation` and `PulseTransfer`.
`PulseTransfer` is to be run with `output_pulsegraphs = true`. Usually it is enough to run a few or just a single event.
Multithreading is unsupported since it is designed to run multiple events in parallel, but for typical use cases of this module each event is computation-heavy (multithreading is also not allowed by `PulseTransfer` with `output_pulsegraphs = true`).
Such pipeline is expected to produce pulse shapes, comparable with experimentally obtained ones. An example of `DepositionLaser` configuration is shown below.
