A century of beta decays
27 January 1999
In January 1899, the Philosophical Magazine* published a paper by Ernest Rutherford, where he described his discovery that there are at least two types of radiation from uranium - "one that is very readily absorbed, which will be termed for convenience the a (alpha) radiation, and the other of a more penetrative character, which will be termed the b (beta) radiation". A century later, physicists recognise that beta-decay - the process that give rise to the beta radiation - is driven at a fundamental level by the weak force. The beta-rays that Rutherford found were in fact electrons emitted when neutrons in the uranium nuclei turned spontaneously into protons, together (we know now) with almost undetectable particles known as neutrinos.
Nowadays, physicists study the same fundamental process at high energies in particle collisions, for instance at the Large Electron Positron (LEP) collider at CERN, the European laboratory for particle physics near Geneva. The collisions have sufficient energy to create the W particle - the carrier of the weak force, which is "virtual" (hidden) in low-energy processes such as beta-decay in uranium. This image from the ALEPH detector shows the decay of two W particles produced at LEP. One, the W-, decays into two quarks which immediately produce the two sprays ("jets") of particle tracks. The other, the W+, decays to a positron (anti-electron), which leaves the track towards top right, and an invisible neutrino, in a process just like the one that produces the electron in beta-decay. The white lines show the direction of the neutrino (which leaves no track) , and the directions of the W particles before they decayed.
*(see E.Rutherford, Philosophical Magazine ser. 5 xivii 109 (1899))
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