Alvarez's x-raying of the Gizeh pyramid

DrDu

I had a fascinating read about Alvarez "x-raying" the great pyramid in Gizeh in the 1960ies using cosmic muons:
www2.lns.mit.edu/fisherp/AlvarezPyramids.pdf
The technique has been refined and used even to make tomographies of a volcano in Japan.

What I wonder: To obtain the direction of the muons, one needs two ionization events. Isn't the probability for two ionizations in the spark chamber very low for muons which are so energetic that they can run unscattered trough the whole pyramid or a volcano?

mfb

They are not unscattered, but their scattering angle is very small over the whole length.

DrDu

Ok, I just read that the muons have on the mean 4 GeV of energy when they reach the surface and that they loose about 2MeV per g cm-2. So they can travel about 20 m in e. g. water. On the other hand, this makes still about 1 keV loss per cm of air. Given that ionization requires some 10 eV, there should be about one ionization event every 0.1 cm.

mfb

Every 10µm?
For pyramids or mountains, you need the high-energetic muons.