One of the problems in determining the nature and source of gamma ray bursters is that is difficult to focus gamma rays, ie. there is a large “error bar” around the source. Why is this so? I assume it has to do with the high energy of the rays since, IIRC, X-rays are also difficult to focus.
Why is it difficult to focus gamma rays?
Gamma rays have wavelengths on the order of 10[sup]-12[/sup] meters, which is in the neighborhood of the diameter of an individual atom. I suspect, given this, that diffraction effects become significant, making accurate focusing very difficult if not impossible.
Gamma rays are tricky to work with because they penetrate deep into most substances. You can’t use a mirror surface to reflect it because it doesn’t stop at the surface. Once inside the substance it does scatter, but that’s a random process so you can’t use it to form an image.
The only way to systematically bend a gamma ray beam is to cause diffraction using atoms arranged in a grid pattern; this is known as Bragg diffraction. Some crystals have the right structure and can be used for this purpose, but a regular lattice can only bend light, not focus it into a point. You have to use a many tiny crystals to do that, like using many flat mirrors to focus sunlight onto a point. It works, but you don’t get a very sharp image. Another problem is that this only works for a specific wavelength, and not very useful as an astronomical telescope.
Currently, gamma-ray and hard x-ray (>10 keV) telescopes don’t use focusing mechanisms at all. Instead they use heavy metal plates to block the gamma/X ray and use simple geometry to figure out the source position. For example, you can put a long tube on a detector so it only looks at one portion of the sky, and have a whole bunch of those to create an image just like an insect’s compound eye. Or you can make a pinhone and put an imaging detector behind it. Or instead of a pinhole, you can put a complex pattern and analyze the “shadow” cast by the pattern: this is called Coded Aperture Imaging. Or you can use a slit and rotate the telescope, just like the hospital CT scanners.
IMHO the most sophisticated gamma/X-ray observatory is the RHESSI. It uses nine detectors, each with a double grid attached. The grid gives each detector a striped view of the target, which means it’s measuring one component of the spatial Fourier transform of the target. The spacecraft spins at 15 rpm, so the nine detectors trace out nine circles on the UV plane (U,V being Fourier transforms of X,Y). That’s enough data points to reconstruct the image.
The detectors are as difficult as the optics. X-rays and gamma rays penetetrate light elements easily, and if it penetrates through your detector without stopping then you can’t detect it. You have to make the detector very thick, or make it out of heavy elements, or both. Either way, it’s not easy to make small pixels.
Since this is my area of research, I’ll be more than happy to explain any of this (or related stuff) in more detail.
Thanks to all.
Let me ask one follow up question for now (must get to bed!):
Why doesn’t the principle of “least time” apply to gamma rays in the same way it does for visible light. For example, in a simple convex lens, the focus can be explained by considering it as the point where all parallel incoming rays take the same time to reach. Is the reason that this fails for gamma rays because there’s no signficant interaction between them and the lens material? If so, what did scr4 mean by “Once inside the substance it does scatter, but that’s a random process …”?
That “principle” is just a quick and dirty method of calculation only valid for certain conditions. It doesn’t work on gamma rays because gamma rays don’t reflect or refract in the conventional sense. You can’t assume a flat surface because to a gamma ray photon, even the best mirror looks like a bumpy collection of atoms.
There can be significant interaction, but a gamma ray photon interacts with individual atoms, not surfaces. When a photon hits an atom it can get scattered in any direction[1]. The only way to control the distribution of the scattered gamma ray is by interference/diffraction, as I explained earlier.
[1]Actually the distribution depends on polarization of the photon, but usually the photons are randomly polarized so that doesn’t matter.
Here’s moer of a reason you can’t focus gamma rays. Visible light can be focussed by a lens because the lens has an index of refraction much different from 1. That is, light propagates through the lens at a much slower speed than it does through empty space. This is because light propagates through lens materials like glass and plastic by making the electron distributions vibrate and jostle one another. It’s like a sound wave propagating through the medium of the electrons only. Gamma rays cause electric fields to fluctuate much faster than electron distributions can, so they don’t manifest themselves as vibrations in this electron medium, and they travel through any possible lens material at the same speed as through empty space - so everything has an index of refraction of 1. There’s a similar reason they don’t reflect off of surfaces - metals are reflective because the electron medium in a metal will counteract nearby electric field fluctuations so as to reflect a wave, and nonmetals that reflect do so because of the change in index of refraction. Neither of these do anything for gamma rays. So the refraction and reflection mechanisms for focusing rays to make images don’t work for gamma.
BTW since gamma rays have short wavelength, their diffraction phenomena are much less pronounced than those of light, not more pronounced.
You’re correct if you are talking about macroscopic structures like holes and slits. But gamma ray wavelengths are similar to typical spacing of atoms, so Bragg diffraction is caused by many common crystals and molecules. In fact, analysis of the diffraction pattern can tell you a lot about the structure of a molecule or crystal. For example, that’s how Watson, Crick, et al. found out that DNA molecules are helical.
More on Bragg’s Law, with a Java Applet, can be found here:
Just one comment on the so-called principle of least action. In fact, if you reflect ordinary light off a sufficiently convex spoon, you will see that it takes the longest path, not the shortest (there is no shortest). In fact it takes a path such that a very small deviation from that path makes (almost) no difference in the path length (a place where the derivative vanishes–a stationary point). There is a simple explanation of all this in terms of interference. Nearby points do not interfere destructively. All this breaks down for gamma rays because at those wavelengths matter is too bumpy and there are no stationary points.
Another big problem is that once you turn into The Hulk, your hands are too big to work the measuring equipment.
So, can there ever be a gamma ray laser?
I have heard that they would spread out least of all over interstellar distances, so would be easier to detect at the destination.
Or is there too much difficulty associated with that detection and perhaps with data transfer in such a beam?
X-ray and gamma ray lasers are possible but highly inefficient vs optical lasers (orders of magnitude less efficient).
See X-ray laser
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Here’s a fairly technical review of the subject by Lawrence Livermore National Laboratory -X-Ray Lasers : (Collisional pumping)
I believe you’re thinking x-rays, not gamma rays. As Q.E.D. says, typical gamma ray wavelengths are 10[sup]-12[/sup] meters or less. Atoms and atomic separations are a couple of orders of magnitude larger.