46.6. Recording Transmitted Photons

Table of Contents

Capturing Photons That Pass Through the Shield

In this part of the radiation shielding example, the goal is to record only those photons that have passed through the shielding material and reached a scoring region placed behind the shield. You will use GATE actors to count and characterize these transmitted photons, so that you can later compute transmission and attenuation.

Defining a Region to Record Transmitted Photons

To record transmitted photons, you first need a dedicated volume that represents the detector region behind the shield. In the overall project, this volume sits along the beam axis, after the shielding slab or block. Its material can be air or a simple detector material, since for this example you are not modeling detailed detector physics, only the flux of photons that emerge from the shield.

Place this transmitted region so that all primary photons that cross the shield will enter it if they continue along the beam direction. Ensure that its transverse dimensions are larger than the beam size, so that it catches the full beam after the shield. The exact distance between the shield and this region is not critical for computing transmission, as long as geometry is consistent across different shielding configurations.

Once this volume exists, you will attach actors to it to measure transmitted particles. The rest of the geometry, such as the world, source volume, and shield, is already created in earlier sections of the project.

Choosing the Right Actor to Measure Transmitted Photons

There are several ways to record transmitted photons, depending on what you want to analyze later. For simple transmission and attenuation calculations, you mainly need how many photons make it through, and possibly some basic information like their energy and direction. A phase space actor is usually the most flexible choice for this purpose.

A phase space actor can record individual particle tracks that cross a plane or a volume. When attached to the transmitted region, it will save one entry per particle that enters or exits that region, including position, energy, direction, time, and particle type. This provides all the information you need to identify transmitted photons and analyze their properties.

If you only want an integrated count, a simpler particle fluence actor that records flux or fluence in the region can be used. However, for a beginner-friendly shielding project, the phase space approach is useful because it mirrors how transport data is often reused in more advanced simulations.

Configuring a Phase Space Actor for Transmission

In the Python script, once the transmitted region volume is created, you create and configure a phase space actor. The actor is attached to the transmitted region and instructed to record only gamma particles.

In outline, the steps are:

Create the actor through the simulation object.

Set the attached volume to the transmitted region.

Enable the attributes you want to record, such as position, energy, direction, time, and event identifiers.

Add a filter so that only photons are written.

Point the actor to an output file, usually a ROOT file, that will contain the transmitted photon data.

In code form, this configuration might look like a block that creates an actor, assigns its volume, selects attributes, and specifies a filename. Although the exact API details belong elsewhere in the course, the key idea is that every time a photon crosses the boundary into the transmitted region, one record will be stored in the actor output.

Using Filters to Record Only Relevant Photons

Without filters, a phase space actor can record many kinds of particles, including electrons or secondary photons created in the air. For transmission calculations you typically care about photons of the same type as the initial beam, for example primary gammas from the source and any scattered gammas that still reach the transmitted region.

To restrict recording to photons, you configure a particle filter as part of the actor. The filter checks the particle type of each track entering the volume, and only if it is a gamma does it allow the event to be stored. You can further refine this selection by requiring a minimum energy, for example excluding very low energy photons that are unlikely to contribute to detector response in a realistic experiment.

This is also the right place to decide whether you want to distinguish primary photons from scattered ones. In some applications, you may later use stored process information or parent IDs to separate photons that traveled directly through the shield from those that scattered within it. For the basic shielding example, it is usually enough to record all transmitted photons and handle any separation, if needed, during analysis.

Important Considerations for Accurate Transmission Data

To ensure that the recorded transmitted photons can be used for meaningful attenuation calculations, keep several points in mind.

First, the transmitted region must be large enough to intercept all photons that would have encountered the shield. If some photons bypass the shield geometry or miss the transmitted region due to misalignment, the measured transmission will be incorrect. Before running long simulations, verify the geometry visually.

Second, consider the orientation of the actor. If it is configured to record particles crossing a specific surface rather than the full volume, you must align that surface perpendicular to the beam. A volume based configuration is generally safer for a first example.

Third, be consistent across different shielding configurations. When you change shield material or thickness, do not move the source, shield, or transmitted region. This way, any change in the number of recorded photons is due solely to the shield properties, which makes the comparison straightforward.

Finally, confirm that units are correctly specified for all geometric distances, energies, and file outputs. An incorrect unit, such as using centimeters when you intended millimeters in the position of the transmitted region, can easily lead to a situation where no photons ever enter the scoring volume.

From Phase Space to Transmission Measurements

The phase space file produced by the actor contains one entry per transmitted photon. Each entry has a position, energy, and possibly additional information. The simplest quantity you can extract is the total number of photons recorded, which you will compare to the number of photons that were incident on the shield.

In a later section, you will divide the number of transmitted photons by the number of emitted or incident photons to obtain the transmission factor. From that, you can compute attenuation quantities and compare them with the exponential attenuation law. The careful recording of transmitted photons in this step is therefore the foundation for all subsequent shielding analysis.

Transmission factor is defined as
$$T = \frac{N_{\text{transmitted}}}{N_{\text{incident}}}$$
Accurate transmission requires:

Correct counting of transmitted photons in the scoring region.
Consistent geometry and source configuration across simulations.
Sufficient number of events to reduce statistical uncertainty.

By configuring the transmitted region and its phase space actor properly, you ensure that the raw data needed for these calculations is complete and reliable.