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Chapter 7: Part III
Secondary Colors B
1. Strange Particles
In prior chapters I have noted that some particles were discovered in cosmic rays. The positron being an example. Someone unfamiliar with the subject might get the idea that there was a kind of back yard treasure hunt in which these particles were unearthed. Since the particles in this chapter were also found initially in cosmic rays I want to explain what this means beginning with a discussion of what a cosmic ray is.
In 1896 the French physicist Henri Becquerel made the accidental discovery that a substance containing uranium emitted charged particles. This was the discovery of radioactivity. Radioactivity was thought then to be the solution to a puzzle. Atmospheric air appeared to be ionized. It carried an electric charge. The assumption was then made that this was caused by the natural radioactivity coming from the Earth. This was tested when in 1912 the Austrian physicist Victor Hess flew in a balloon to an altitude of some 5300 meters carrying an electrometer. He found that the ionization quadrupled at this altitude which meant that it was extra-terrestrial. At first it was thought that it was emanating from the Sun. But Hess ruled this out when he flew in his balloon in a near total solar eclipse and showed that the radiation persisted. But what was it and where did it come from?
In those pre-satellite days one could only study the radiation fairly close to the Earth. There were two schools of thought. One argued that the primary radiation consisted of very high energy photons-gamma rays-and the other argued that it was positively charged particles. The two proposals could be distinguished by measuring the cosmic ray flux at different locations on the Earth. Uncharged particles like gamma rays would not be deflected by the Earth’s magnetic field while charged particles would. Because of the Earth’s magnetic field it was predicted that more positively charged cosmic rays would come from the west than from the east. By the end of the 1930’s measurements made it clear that the charged particle people were right. Now it is agreed that most of the primaries are high energy protons. Some of them are of a higher energy than can be produced in any accelerator. It is also generally agreed that they have their origin in super nova explosions. The ones that we see have been traveling for millennia in the vacuum of outer space. When they crash into our atmosphere they produce a great variety of secondaries which is what is detected. But how to detect them?