Why can positron go unobstructed through lead?

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L-I-C
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Can anyone explain why a positron, in a charged magnetic field, is able to go through a 6mm lead plate, as seen in Anderson's positron experiment?

Initially I thought that it could be quite similar to the gold foil experiments, but one of the features of the gold foil experiments is that it is using a VERY thin sheet of gold.

From what I understand lead, especially lead that thick, has the ability to not easily allow particles through.

Am I taking the wrong approach?

http://en.wikipedia.org/wiki/File:PositronDiscovery.jpg
 
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In trying to understand the results of his experiments, Anderson consulted several other prominent physicists, including Robert Oppenheimer. Anderson's discussions with Oppenheimer proved fruitless; however, other physicists were studying cosmic rays like Anderson at this time. Two of them, P.M.S. Blackett and G.P.S. Occhialini, proposed the pair-production mechanism, where high-energy photons, like the gamma rays found in cosmic radiation, will strike a nucleus and produce an electron-positron pair. These particles will annihilate one another after a brief period, turning back into two gamma rays.

It's not that positrons penetrate lead, but positrons are produced within the plate when high-energy gamma rays strike one of the lead nuclei.

For details on the pair-production mechanism, see this article:
http://en.wikipedia.org/wiki/Pair_production