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Natural Science Forum / Physics / Research / December 2007Do gamma rays leave "partice" tracks?
In cloud chambers (or bubble/spark/crystal detectors,) do MeV photons leave trajectory-tracks like particles do? 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. But even sphere-wave dipole radiation from the pumped atoms in a laser medium would interfere with the stimulating beam and create a large anti-node in the "downstream" direction. It's sort of like a trajectory. Vaguely. And if this narrow chain of antinodes is absorbed by atoms downstream in the beam, the energy gets localized then re-emitted, perhaps producing an EM phenomenon like a narrow trajectory. Therefore I ask if gamma-ray photons are seen to behave like particles; producing ion trails when sensed by detectors which can image the tracks of alphas/betas/etc.? (Can *EM waves* produce tracks?) Associated question/musing: if such a "photon localized trajectory" effect exists, do rod-like pulse lasers amplify better than expected when operated without mirrors? ((((((((((((((((((((((( ( ( (o) ) ) ))))))))))))))))))))))) William J. Beaty Research Engineer beaty a chem washington edu UW Chem Dept, Bagley Hall RM74 billb a eskimo com Box 351700, Seattle, WA 98195-1700 ph425-222-5066 http://staff.washington.edu/wbeaty/ billb wrote: >In cloud chambers (or bubble/spark/crystal detectors,) do MeV photons >leave trajectory-tracks like particles do? [quoted text clipped - 14 lines] >image the tracks of alphas/betas/etc.? (Can *EM waves* produce >tracks?) Gammas don't produce ion trails. What usually happens in bubble chambers is the gamma converts to one or more electron-positron pairs. It's the trajectory of these pairs that provides the information about the gamma. Lower energy photons, like those to which you refer, do not have enough energy to produce e-p pairs (you need at least 1 Mev photons to start). The cloud chamber gammas can also ionize the gas in the chamber producing secondary electrons call delta rays. The delta rays can be analyzed for information about the primary ionizing agent. This is a bit more difficult than analyzing e-p pairs (this scenario is complicated by the myriad of possibilities that occur when you have charged particles banging a round in a cloud chamber). Again, the gamma has to give up all its energy to the charged particle. So a gamma ray trajectory still isn't seen. But delta rays can be produced by gamma energies well below 1 Mev. Considering everything available to detecting low energy photons, bubble chambers are probably the poorest choice. > Gammas don't produce ion trails. Thanks! > The delta rays can be analyzed for information about the primary ionizing agent. That's more like the idea I was after. I imagine that the trajectory of the resulting deltas would be mostly unrelated to the direction of the incoming gammas which produced them. Otherwise we could provide an electron lens and end up with a gammaray telescope. ((((((((((((((((((((((( ( ( (o) ) ) ))))))))))))))))))))))) William J. Beaty Research Engineer beaty a chem washington edu UW Chem Dept, Bagley Hall RM74 billb a eskimo com Box 351700, Seattle, WA 98195-1700 ph425-222-5066 http://staff.washington.edu/wbeaty/ >> Gammas don't produce ion trails. [...] The delta rays can be >> analyzed for information about the primary ionizing agent. > > That's more like the idea I was after. I imagine that the trajectory > of the resulting deltas would be mostly unrelated to the direction > of the incoming gammas which produced them. No. For a charged particle traversing matter, the delta rays are emitted in a direction very close to the direction of the charged particle. Note, however, that they are very low energy and multiple scatter a lot (which changes their direction), and don't have much range. Note: "delta ray" is an old-fashioned term for a low-energy electron kicked out of a material (typically they have a few keV energy, but can occasionally be up to an MeV or so); the term is not normally applied to higher energy electrons. But a gamma can produce only a single electron (the term "delta ray" is not normally used for this) or an electron-positron pair. A gamma with several MeV energy would produce a several MeV electron, which would immediately shower. Tom Roberts >>> Gammas don't produce ion trails. [...] The delta rays can be analyzed >>> for information about the primary ionizing agent. [quoted text clipped - 19 lines] > > Tom Roberts An indication of the extremely high optical 'X-ray laser' energies (intensity 10^22 watt/cm^2 and electric field 10^17 volt/cm ) required for 'Schwinger' pair production are indicated: http://arxiv.org/abs/nucl-th/0511085 The latest powerful lasers (XFEL) are enlisted to study such EM pair production effects. Richard D. Saam > > The delta rays can be analyzed for information about the primary > > ionizing agent. [quoted text clipped - 3 lines] > direction of the incoming gammas which produced them. Otherwise we > could provide an electron lens and end up with a gammaray telescope. As Murray also said, What usually happens in bubble chambers is the gamma converts to one or more electron-positron pairs. It's the trajectory of these pairs that provides the information about the gamma. For gammas well above the pair production threshold, tracking the primary electron-positron pair does provide the basis of a gamma telescope--see for example GLAST, the Gamma-ray Large Area Space Telescope. -dan > In cloud chambers (or bubble/spark/crystal detectors,) do MeV photons > leave trajectory-tracks like particles do? No tracks. Electron mass is 0.511 Mev. A fat gamma photon plus a heavy nucleus could give you pair formation. Compton scattering at lowwer energies. Overall, even high energy photons are invisible absent high-Z element detectors (scintillation crystals). At lower energies scattering is visible in cloudy media. > 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. Crack a book. The quantum numbers of the stimulating photons are the quantum numbers of the emitted photons. Photons are bosons Their energy levels don't stack when populated. > But even > sphere-wave dipole radiation from the pumped atoms in a laser medium [quoted text clipped - 3 lines] > downstream in the beam, the energy gets localized then re-emitted, > perhaps producing an EM phenomenon like a narrow trajectory. [snip] OK. Lasers don't work. Now convince my CD/DvD computer drive and laser pointer.  SignatureUncle Al http://www.mazepath.com/uncleal/ (Toxic URL! Unsafe for children and most mammals) http://www.mazepath.com/uncleal/lajos.htm#a2 > 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 Thus spake billb@eskimo.com >In cloud chambers (or bubble/spark/crystal detectors,) do MeV photons >leave trajectory-tracks like particles do? As posters have said no tracks are left by uncharged particles. We do see events (e.g pair creation) triggered by high energy photons, separate from the original scattering event. Regards SignatureCharles Francis moderator sci.physics.foundations. charles (dot) e (dot) h (dot) francis (at) googlemail.com (remove spaces and braces) |
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