Given today's revelation of the detection of terrestrial gammay-ray flashes (TGF) produced by thunderstorms and the associated pair production of an electron and a positron, how feasible is it to set up ground-based gamma ray detection station on the cheap using a Geiger-Muller tube?

The electron/positron pair travels along the magnetic field of the earth and bounces back (additional question: why do they bounce back?) This is how Fermi could detect the positrons: they interacted with the satellite which happened to be in the way. The loss of energy due the interaction slow down the positrons which interact with electrons and produce two 511keV photons flying in opposite directions (source.)

My question is: is it feasible at all to capture and detect positron annihilation as the pairs bounce back using a G-M tube? Given that there are 1500 thunderstorms a day I'm assuming each location on earth has a good chance of being a "bounce site" at least a few times a year.


First, positrons heading down hit a lot of atmosphere, leading to a low probability of surviving to the ground because the positron will interact with an electron before it can get there. Up is not so bad; the tops of these storms are surprisingly high, and the atmosphere is pretty thin up there.

If some do make it (they won't) your big problem is going to be backgrounds. You face several distinct categories and they will almost certainly swamp your signal:

  • Cosmic rays, including spallation products like Li-9, C-11, ...
  • Environmental radiation from K-40, radon daughters, and long-lived products of atmospheric nuclear explosive tests...
  • C-60 or U-238 in the material that make up your detector...

To even get started you'll need a energy sensitive detector (i.e. more sophisticated than a Geiger tube) and a cosmic veto of some kind. Low rate measurements near the Earth's surface are hard. Very hard.

Just to address the question of positron detection itself, this is standard tool if you control the source. You simply put two detectors back-to-back with the source in the middle ad trigger on a coincidence. A typical unit using NaI crystals might look like:

+-----+-----+      +---------+      +-----+-----+
| PMT | NaI |      | source  |      | NaI | PMT |
+-----+-----+      +---------+      +-----+-----+
  \                                          /
   \   +------+     +-------+    +------+   /
    \--| disc |-----| coinc |----| disc |--/       
       +------+     +-------+    +------+
                   | scaler  |

Here "disc" represents a pulse discriminator, "coinc" a coincidence unit (i.e. and gate), and a scaler is a simple counting circuit.

  • $\begingroup$ You're right, I must have misread, the positrons bounce back when they encounter a "mirror point" so they probably never reach earth's surface. This seems to mean the positrons actually arrive back at the thunderstorm since the mirror point reverses their direction. $\endgroup$ – Steven Devijver Jan 11 '11 at 21:11
  • $\begingroup$ @Steven: I assume the reflection effect arises because the fields are not uniform. I think something similar is applied to generate artificial magnetic confinement of plasma (for fusion research and the like). This leaves open the possibility that positrons have a path all the way to the surface, but they will annihilate on atmospheric electrons long before they get there. $\endgroup$ – dmckee --- ex-moderator kitten Jan 11 '11 at 23:07
  • $\begingroup$ This is a well-known effect. Charged particles spiral around and along magnetic field lines of earth, between poles, reflected near the poles either by plasma effects of ionosphere or by the convergent field. (I don't know) The whole thing is named "van Allen belt(s)" $\endgroup$ – Georg Jun 3 '11 at 8:44

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