optical photon simulation in Gate

下面来自Gate 3.1.1的 users guide:

In order to use the optical photon tracking capabilities of GATE it has to switched on first. This can be done using the following command:
/gate/physics/optical/enable 1

Before discussing how the use the optical photon tracking, it has to be mentioned that there are a few disadvantages to using optical transport. First, the simulation time will increase dramatically. For example, most scintillators used in PET generate in the order of 10,000 optical photons at 511 keV, which means that approximately 10,000 more particles have to tracked for each annihilation photon that is detected. Although the tracking of optical photons is relatively fast, a simulation with optical photon tracking can easily be a factor thousand slower than one without. Finally, in order to perform optical simulations many parameters are needed for the materials and surfaces (these will be discussed below) some of which may be difficult to determine.


When optical transport is switched on in GATE , the properties of the materials and surfaces have to be defined in order to have generation and tracking of optical photons

The optical properties of materials are stored in a material properties table. In this table each of the
properties of a material is identified by a name. There are two different kinds of properties. The first
are constant properties, these contain only one value. The second are property vectors, these contain
properties that depend on the energy of the optical photon. Such a vector is a list of energy-value pairs.
The property tables for the materials used in a simulation are to be stores in a file separate from
the materials database. This is done to make it more easy to change them without having to change the
materials database. This file should be named Materials.xml1. When GATE reads in a material
from the materials database, it also checks if the Materials.xml file contains a properties table for
this material. If the file contains one, this table is read in and coupled to the material.

After processing the hits with one of the anaysis routines, the singles should be created from the hits.
Normally this is done using the adder, but for optical photons a separate adder is defined. The normal
adder only adds hits generated by all particles except optical photons. The opticaladder adds all
hits generated by optical photons. In this way it is possible to create two digitizer chains: one containing
the traditional singles and one containing the singles generated by optical photons.
The opticaladder can be added to the digitizer chain using the following command:
/gate/digitizer/Singles/insert opticaladder

Besides only adding optical photons there is one more important difference with the traditional adder:
the opticaladder does not add the energies, but counts the number of photons. Therefore, the energy
property of the singles contains the number of photons detected and not the energy deposited. Digitizer
modules like threshold can still be used, but the energy of the threshold should be specified in a
number of photons.