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This answer states:

From the AstroMEV website:

Terrestrial gamma-ray flashes (TGFs) are high-energy photons originating from the Earth's atmosphere in association with thunderstorm activity. TGFs were serendipitously discovered by BATSE detectors aboard the Compton Gamma-Ray Observatory. TGFs have also been detected and further studied by the RHESSI, AGILE and Fermi satellites. Their emission extends up to 100 MeV and exhibits an e+ – e- annihilation line. TGFs were utterly unexpected and as of now they are not fully understood. They are believed to be the product of particles acceleration inside thunderstorms. As they are produced in the Earth’s atmosphere, they potentially have a tremendous impact on our understanding of thunderstorms and atmospheric electrodynamics in general.

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100 MeV is quite large for a terrestrial process. Nuclear gamma rays are typically in the 100 keV to few MeV range for nuclear transitions, though there can be larger ones from exotic processes in nuclear collisions.

These are really high energy photons!

Question: How to satellites measure the energy of such high-energy single photons with reasonable certainty?

Please answer with something more than "they use gamma ray detectors". It takes a lot of mass to stop such a high energy photon, and to stop it in such a way that you can be reasonably certain you have contained all of the energy of the resulting shower of particles and photons produced, which adds even more mass and other constraints. Making it all work on a spacecraft that can run unattended in a space environment and withstand the vibrations of launch without losing accuracy or integrity is quite a feat.

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The idea is to have incoming photons dump their energy into something else that's easier to work with. Once the energy is high enough for the creation of the lightest possible particle-antiparticle pairs, this does not take a lot of mass to arrange.

Taking the EGRET instrument on the Compton GRO as an example:

A gamma ray entering the telescope within the acceptance angle has a reasonable probability (about 35% above 200 MeV) of converting into an electron-positron pair in one of the thin plates between the spark chambers in the upper portion of the telescope.

[...]

The energy of the gamma ray is determined in large part from measurements made in an eight radiation-length thick, 76 cm x 76 cm square NaI(Tl) scintillator crystal below the lower time-of-flight scintillator plane. Spark chamber measurements of Coulomb scattering in the thin plates and position information in the spark chamber system also aid in the energy determination. The energy resolution of the experiment is about 20% (FWHM) over the central part of the energy range. The resolution is degraded to about 25% above several GeV due to incomplete absorption in the NaI calorimeter, and at energies below about 100 MeV where ionization losses in the spark chamber plates comprise an appreciable portion of the total energy. In addition, some particles may completely miss the calorimeter. These losses can be partially recovered through an analysis of the scattering characteristics.

Source: EGRET Technical Information, NASA GSFC.

(Other techniques apply at lower photon energies.)


The Radiation length of NaI (sodium iodide) is only about 2.6 cm, so the telescope would not have to be enormous to absorb most of the shower.

LBL Atomic Data and Nuclear Properties

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  • $\begingroup$ Very nice, thank you! I've added a bit of background for those unfamiliar with radiation length. $\endgroup$ – uhoh Dec 14 '18 at 15:36

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