Re: Do gamma rays leave "partice" tracks?

billb@xxxxxxxxxx wrote:
In cloud chambers (or bubble/spark/crystal detectors,) do MeV photons
leave trajectory-tracks like particles do?

No.

Charged particles lose energy continuously when traversing a material, by ionizing atoms of the material close to their trajectory. This occurs because the transient electric field of the traveling particle is strong enough to overcome the Coulomb barrier of the atom and "kick" an electron out. A few-MeV (or higher) charged particle can ionize millions of atoms per centimeter in some materials (a few tens of MeV energy loss per meter). The detectors you name are specifically designed to be exquisitely sensitive to such a tiny deposition of energy into their material.

A photon (gamma ray), on the other hand, is neutral and does not affect atoms near to its trajectory at all. It can interact with a single atom, but that obviously cannot leave a track. Its interaction can, however, either liberate an electron from the atom (photoelectric effect) or produce an e+e- pair (pair production), and these charged particles can then leave tracks.


I've been arguing with myself about the details of laser
amplification, and it doesn't make sense to me that stimulated
emission produces photons with a "trajectory" the same as the
stimulating light: atoms should produce dipole radiation.

The details work out so the stimulated emission is precisely parallel to the original radiation.


Therefore I ask if gamma-ray photons are seen to behave like
particles;

In some ways they do, but not in leaving tracks in a bubble or cloud chamber. MeV photons do interact with a single charged particle giving a very precise location of their interaction (this is in some sense "particle like"). So, for instance, detectors designed for such photons always register an integral number of photons.


Associated question/musing: if such a "photon localized trajectory"
effect exists, do rod-like pulse lasers amplify better than expected
when operated without mirrors?

Remember that visible-light lasers are using photons in the fractional eV range, not the MeV range. These optical photons behave rather differently from the higher energy ones. In particular, it is MUCH more difficult to construct a single-photon detector (but it can be done: google "visible light photon counter" -- they must be cooled to 10K or so to keep the noise down).

The mirrors in a He-Ne laser are to provide a longer path through the lasing material. The necessary condition is that the gain through the lasing material be greater than 1. Some lasers, such as the free electron laser, have gains greater than 1 in a single pass without mirrors. I believe some semiconductor lasers also work without mirrors. A He-Ne tube many meters long would presumably work without mirrors.

The mirrors also provide beam-forming, and act as a
Fabry-Perot interferometer to give much narrower line
widths in laboratory lasers.


Tom Roberts

.

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