*3.2. Detection Principles*

To date, the only GRB polarization measurements performed have made use of Compton scattering in the detector. The majority of these measurements were performed by making use of a segmented detector concept, for example a detector consisting of many relatively small scintillators, e.g., for GAP [191] and POLAR [192], or a segmented semiconductor, such as INTEGRAL-SPI [193] and AstroSAT CZT [194]. In either design, the Compton scattering interaction can be detected in one segment of the detector while an additional interaction of the photon in a second segment can be used to reconstruct the azimuthal Compton scattering angle. This concept is illustrated in Figure 5.

**Figure 5.** Illustration of the measurement principle of a polarimeter using Compton scattering. The incoming *γ*-ray Compton scatters in one of the detector segments followed by a photo-absorption (or second Compton scattering) interaction in a different segment. Using the relative position of the two detector segments, the Compton scattering angle can be calculated from which, in turn, the polarization angle can be deduced.

At energies below approximately 10 keV the cross section for photo-absorption dominates. Although no successful GRB polarization measurements have been performed using the photo-electric effect, the detection method has been successfully used recently to perform polarization measurements of the Crab Nebula in the 3–4.5 keV energy range using the PolarLight cubesat [195]. Several large-scale polarimeter ideas have been developed in the past, such as the Low-energy Polarimeter, which was part of the proposed POET mission, which was dedicated to GRBs [196]. Currently, several missions that use the same concept are currently under development [197,198]. In these X-ray polarimeters, the photo-absorption takes place in a thin gas detector. As the produced electron travels through the gas it releases secondary electrons as it ionizes the gas. These secondary electrons can be detected using finely segmented pixel detectors in order to track the path of the electron released in the photo-absorption interaction, allowing to reconstruct its emission angle.

Polarimetry in the pair-production regime is arguably the most challenging as the photon flux is low, and the detection method requires highly precise trackers capable of separating the tracks of the electron and the positron. In spectrometry, the low photon flux is often compensated by using large detectors with a high stopping power. For example, by combining tungsten layers with silicon detectors. Here, the silicon serves to measure the tracks while the tungsten is used to enforce pair production in the detector. However, the use of high Z (atomic number) materials, like tungsten, significantly increases multiple scattering of the electron and positron. Multiple scatterings quickly change the momentum of both products, thereby making it challenging to reconstruct their original emission direction. To overcome this issue, detectors that use silicon both for conversion and detection, have been proposed in the past such as PANGU [199]. Although technically possible, the large number of silicon detectors required to achieve a high sensitivity, with minimal structural material and a potential magnet, which helps to separate the electron– positron pair, make such detectors both costly and challenging to develop for space. A

second option is to use gas-based detectors, such as in the HARPO design [200]. This detector, which was successfully tested on the ground [201], allows for precise tracking but has a low stopping power for the incoming *γ*-rays and therefore a low detection efficiency. This can be compensated with a large volume. However, as the gas volume obviously needs to be pressurized, producing and launching such an instrument for use in space is highly challenging. Despite these challenges, several projects that follow this design are still ongoing, such as the potential future space mission AdEPT [202], which aims to use a time projection chamber to measure polarization in the 5–200 MeV energy range as well as the balloon borne SMILE missions [203].

Although no dedicated pair–production polarimeters are currently in orbit, it should be noted that both the Fermi-LAT [204] and the AMS-02 [205] instruments could, in theory, be used to perform polarization measurements in this energy range. For Fermi-LAT, which is a dedicated *γ*-ray spectrometer, consisting of silicon strip detectors combined with tungsten conversion layers, the aforementioned multiple scattering induced distortion is again a challenge [206]. The polarization capabilities of Fermi-LAT, which has detected many GRBs to this day, has been studied [206], but no results from actual data have been published to date. For AMS-02, which does not suffer from the use of tungsten layers and additionally contains a magnet which separates the pairs, measurements could be easier. However, as the instrument is designed as a charged particle detector, it remains nonoptimized for this purpose, and so far no results have been published by this collaboration.
