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I'm reading about how the soon-to-be-launched NuSTAR is on the cutting edge of focusing x-rays, which captures 5 to 80 keV radiation by focusing them with optics that have a 10.15 meter focal length onto 2 sets of 4 32×32 pixel detector arrays. These are particular "hard" (high energy) x-rays, which is a part of what makes the task difficult and the NuSTAR telescope novel.

If I understand correctly, imaging gets particularly difficult with electromagnetic radiation beyond a certain energy, as true gamma rays (above 100 keV) are detected with a family of radiation detectors that sense the Compton scatter or photoelectric absorption with an electrical pulse that is (in a naive sense) insensitive to the originating direction or location within the detector. It should be obvious that imaging can still be done with the use of an array of detectors, each constituting a single pixel, and these capabilities may improve with time as semiconductor detector technology evolves.

So the critical distinction I'm trying to establish is between x-rays and gamma rays. It would seem that we focus x-rays and do not focus gamma rays. For a very good example of researchers not focusing gamma rays, consider Dr. Zhong He's Radiation Measurement Group at UM, who do actual imaging of a gamma ray environment (the UM Polaris detector). They use a grid of room temperature semiconductors laid out bare in a room and use back-processing of the signals to triangulate a sequence of scatter-scatter-absorption reactions in 3D space. This is a lot of work that would be completely unnecessary if you could focus the gamma rays like we do for a large portion of the EM spectrum.

Both of the technologies I reference, the NuSTAR telescope and the UM Polaris detector, use CdZnTe detectors. Functionally they are very very different in that the telescope uses optics to capture light from just a few arc-seconds of the sky.

My question is what is the specific limitation that prevents us from focusing photons above a certain energy? It seems this cutoff point is also suspiciously close to the cutoff between the definition of x-rays and gamma rays. Was this intended? Could future technology start using optics to resolve low-energy gamma rays?

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This article touches on the difficulties and a possible future avenue for the development of gamma ray optics: news.sciencemag.org/sciencenow/2012/05/… –  user2963 Jun 12 '12 at 19:27
The gist: normal optical index of refraction arises from interaction of the EM wave with the electrons in matter, gamma rays are too high-frequency for the electrons to respond. However it appears there may be an analogous effect from virtual electron-positron pairs in the nuclei at high frequencies. –  user2963 Jun 12 '12 at 19:39
When you start to talk about "when pixel detectors get better" you should keep in mind that the granularity of large scale semiconductor gamma imaging is already on order of 100 of micrometers even for detectors expected to have high performance in difficult environments. –  dmckee Jun 12 '12 at 19:43

2 Answers 2

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I think there are two questions here. How do you focus high energy photons and how do you detect them.

Generally detecting high energy photons isn't a problem. They have lots of energy (!), even measuring their energy directly as you collect them is pretty straightforward. So you don't need spectrographs in the same way as for visible light.

Focussing x-rays is a lot trickier. You can scatter them by either a glancing incidence reflection from a metal surface (Wolter optics) as used in most gamma-ray telescopes or by scattering in an optical material (Raleigh scattering).

But the amount of scattering is low and the absorption in an optical material is high so you can't use a lot of material and so can't get significant angles.

The new research suggests that there are different scattering mechanisms at very high energies which would allow you to divert an x-ray photon through a significant angle in a real material.

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And the higher you go in energy the more important the point-like hard scattering becomes vis-a-vis continuous effects. It becomes a bit problematic to describe a "index of refraction" for a material in which your photons initiate showers. –  dmckee Jun 12 '12 at 19:45

The distinction is that the physics for detection of x-rays (hard and soft) and gamma-rays is different. For gamma-ray detection, Compton scattering will allow you to determine the direction of the incident photon by tracking the recoil electron, combined with the scattered photons direction and energy. The Compton recoil electrons can be tracked by measuring the energy losses and X,Y coordinates as it scatters through your detector. Usually done with silicon strip detectors (SSD's) and combined with high-Z materials to absorb the gamma-ray. This is your common D1 and D2 detector configuration used in Compton telescopes. Statistical techniques like maximum likelihood can be used to obtain a graphical distribution of incident gamma-rays.

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Until your comment, I did not know there was much of a president for Compton telescopes. It's very difficult to get good images with that method as I understand it, but the number of photons available to detect will probably be less and less with higher energies to start with. Even so, the probability of detection in two detectors in the array is low so that the statistics are damaged in the first place. Wouldn't accurate imaging of small images, like 10 degree or less would be impossible with that method? –  Alan Rominger Jun 13 '12 at 14:22
"the probability of detection in two detectors in the array is low" Here is another matter in which solid state detectors have improved markedly in the last fifteen years or so. Quantum efficiencies are way up (at least for photons with enough energy, but these are exactly the photons that are hard to subject to traditional focusing...). That doesn't help with flux problem of course, but that is what it is: you build a bigger detector or you run it longer. Or both. –  dmckee Jun 13 '12 at 15:46

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