Is the Fermi Gamma Ray Detector Capable of Detecting Individual Photons?

[anorlunda]

Today's APOD shows an animated visualization of a gamma ray flare detected by
The Fermi Space Telescope. The video says, "each circle represents one gamma ray"

That language suggests single photon detection. Is that correct?

 

[e.bar.goum]

That's normal for gamma spectroscopy, so I'd be surprised if it were otherwise here.

 

[Borg]

Why wouldn't you think that that was the case?
From the Fermi Gamma-ray Space Telescope link:

This visualization shows gamma rays detected during 3C 279's big flare by the LAT instrument on NASA's Fermi satellite. Gamma rays are represented as expanding circles reminiscent of raindrops on water. The flare is an abrupt shower of "rain" that trails off toward the end of the movie. Both the maximum size of the circle and its color represent the energy of the gamma ray, with white lowest and magenta highest.

 

[Drakkith]

Indeed. A gamma ray photon has such a huge amount of energy that you almost can't help but detect it! Assuming it interacts with your detector at least.

 

[anorlunda]

I blame language for my confusion. The Wikipedia article Gamma Spectroscopy talks only of gamma rays, never photons. It is clear from the context that they mean a stream of many photons when they say "ray". On the other hand, WIkipedia's Gamma Ray Spectrometer speaks only of photons, seldom mentioning rays. Drakkith used both ray and photon in the same sentence, which makes it clearer that a photon is one of many in a ray.

My curiosity is about the detectors but also at the faintness of the distant sources. From Wikipedia Gamma-ray astronomy "

Observation of gamma rays first became possible in the 1960s. Their observation is much more problematic than that of X-rays or of visible light, because gamma-rays are comparatively rare, even a "bright" source needing an observation time of several minutes before it is even detected, and because gamma rays are difficult to focus, resulting in a very low resolution."
​

That we have progressed from "several minutes" at low resolution to individual photons at high resolution in just a few short years is amazing. Given the 10^89 or so photons running around the universe, isolating one and identifying it with a specific distance source is doubly amazing.

 

[Drakkith]

A gamma ray is, as far as I know, usually composed of a single photon. In any case, a 'ray' doesn't actually exist. It's a shortcut for geometric optics that let's engineers design optical systems using ray tracing, which is MUCH simpler and easier than treating light as a wave. So the gamma ray detector is just picking up individual photons at very high energies.

That we have progressed from "several minutes" at low resolution to individual photons at high resolution in just a few short years is amazing. Given the 10^89 or so photons running around the universe, isolating one and identifying it with a specific distance source is doubly amazing.

I don't think our exposure time has decreased that much, but I'm not sure. Gamma rays are so high in energy that they are much rarer than visible light photons and even with 'perfect' detectors you're still looking at several minutes of exposure time just to be able to beat the shot noise down to acceptable levels. Shot noise is the inherent noise due to photons arriving at random intervals, regardless of how bright your source is. The brighter the source, the faster the photons arrive on average and the faster your signal rises above the threshold needed for detection. Shot noise it not something you can control. There are no detector improvements or new technologies that can get rid of it. It puts a literal 'hard limit' on the minimum exposure time needed to get a decent image. All other sources of noise, such as noise generated by the detector itself or interference from other sources, simply add their noise on top of shot noise.