Energy thresholds in ausgab media definition

[Sasa-fras]

I've already checked the section on this in the class library but I don't fully understand what it does. Do the upper and lower energy limits for the particles effect the creation of new particles from collisions? Or do they have some relation to the beam source? Or is it that if a particle from the beam source ever goes outside the thresholds it gets deleted and stops being tracked? I want to cutoff all electrons with energy less than 8MeV and have them stop being simulated and thus prevent them from impacting my energy deposition data. I was hoping this is the exact threshold I would use for that.

[ftessier]

In this case you want to set the ECUT (or PCUT) thresholds in the transport parameters.

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In EGSnrc, there exist three sets of thresholds (or cutoffs): AE, AP; UE, UP; and ECUT, PCUT.

AE and AP

AE represents the minimum electron energy in the simulation, while AP denotes the minimum photon energy. Note that these are kinetic energies, with AE including the electron rest mass (approximately 0.511 MeV).

These thresholds determine particle production. They establish the energy level below which secondary particles are not explicitly created and tracked. Instead, the energy lost to events that would produce particles below AE or AP contributes to the continuous energy loss of the incident particle, and is deposited locally. Interactions generating secondary particles above these thresholds are modelled explicitly, with secondaries tracked in full detail.

During simulation initialization, EGSnrc pre-calculates data tables for each medium based on AE and AP. No data is generated below these energies, thus no particle can exist in the simulation below AE and AP.

The minimum values in EGSnrc are 0.512 MeV for AE (1 keV kinetic) and 0.001 MeV for AP. Typical values are 0.521 and 0.010 (10 keV kinetic).

These parameters significantly impact simulation efficiency since lower thresholds result in the creation and tracking of numerous additional low-energy particles. A useful guideline is to begin with a kinetic energy of 10 keV and adjust accordingly for optimal simulation efficiency. If you can afford 1 keV, got for it!

A more refined rule of thumb is to set AE so that the range of electrons is about 1/3 of the smallest dimension in the geometry (so that at least a few electron interactions are likely to occur in the smallest region).

At any rate, it is highly recommended to test one representative simulation to a simulation with 1 keV kinetic threshold, to ensure that higher values of AE and AP are not affecting the final calculated value of interest.

UE and UP

These thresholds represent the maximum total energies for which pre-calculated data tables are generated for each medium at the simulation's onset, for electrons and photons, respectively. Consequently, no particle can exist in the simulation above these upper energies.

These thresholds have minimal impact on simulation outcome or efficiency. The only requirement is that they exceed the maximum energy of the source particles. It's advisable to set them not too much higher than the maximum source particle energy to ensure accurate interpolation of the generated data tables.

ECUT and PCUT

These thresholds serve as transport cutoff energies for electrons and photons, respectively. When a particle's energy drops to this level as it loses energy during transport, it is immediately discarded, and the remaining energy is locally deposited. By default, the transport cutoffs are set to match AE and AP. However, increasing them, for instance, to halt electron or photon transport in specific cases, may be useful, as in the scenario you mention.

When ECUT exceeds the production thresholds AE, secondary electrons with energies between AE and ECUT are generated but then stopped immediately, with their energy deposited locally. This differs from merely setting AE to the higher value of ECUT, as it accurately models the straggling of the incident particle energy.

Note that if ECUT and PCUT are set below AE and AP, respectively, EGSnrc increases them to AE and AP upon initialization, as no particle can exist below AE and AP in the simulation.

Summary

The foregoing explanation can be summarized visually as follows, in terms of the total energy of the secondary electron or photon:

Here csda stands for continuous slowing down approximation: in that range secondaries are not created at all.

[ftessier]

Ah ok, perfect, I can post the code lines shortly. Do you know c++ a little? You don't have to, but I want to know what I shall explain... 😄

[Sasa-fras]

I only know basic syntax, took an intro course back in high school. I mostly work with python.

[ftessier]

All right, so most of what you need is already there in the egs_app.cpp source file:

// ausgab
int myapp::ausgab(int iarg) {

    // stack index variables
    int np  = the_stack->np - 1;
    int npold  = the_stack->npold - 1;
    int region = the_stack->ir[np]-2;

    // stack phase space variables
    double E = the_stack->E[np];
    double x = the_stack->x[np];
    double y = the_stack->y[np];
    double z = the_stack->z[np];
    double u = the_stack->u[np];
    double v = the_stack->v[np];
    double w = the_stack->w[np];
    double weight = the_stack->wt[np];
    int iq = the_stack->iq[np];

    // latch stack variable
    int latch  = the_stack->latch[np];

    // handle the ausgab call

    return 0;
}

The ausgab callback function is called whenever anything happens to particles in the underlying EGSnrc engine. You simply need to add the following code under // handle the ausgab call:

    // handle the ausgab call
    if (E < 8.0) {
        the_stack->wt[np] = 0.0;
    }

(or 8.511 if you meant 8 MeV kinetic energy for electrons). You can add && iq == -1 in the if (...) conditional to discard only electrons (or iq == 0 for photons, iq == 1 for positrons). You can consider adding more code, for example, to track the total energy that disappears this way! Let me know if you need further help.

Once you have saved the file, issue the command make in the egs_app/ directory, which will recompile the application. Then, run egs_app as usual, and let me know how it pans out!

[Sasa-fras]

I added it and confirmed a drop in energy deposition, thanks for the overwhelming support.

[ftessier]

Another option, that would leave the physics unaffected, is to score the energy of the electron in
ausgab() when it is larger than 8 MeV, but otherwise let it continue its journey. That involves creating a new variable and a corresponding output statement in outputResults(). Or else you can score the electron energy spectrum, and post-process the data to select and integrate the energy range of interest.