Ghostbusting: how we found the ghost particles
Henry Bichard
The neutrino was first theorized in 1930 by the physicist Wolfgang Pauli, as a solution to solve a difficult problem involving radioactive decay without violating any conservation laws. However, Pauli himself was cynical, describing his theorized neutrino as a ‘ desperate remedy ’ . He was worried that he had ‘ postulated a particle that cannot be detected ’ . Despite the fact that it is the second most abundant subatomic particle in the universe 1 after the photon, it took 26 years before two American physicists – Clyde Cowan and Frederick Reines – were able to prove experimentally the existence of the neutrino. But why are neutrinos so hard to detect? It is impossible to visually observe single particles at an atomic or subatomic level even with the most powerful microscopes; instead, we are only able to detect these particles by observing how they react with other matter or fields, just as we cannot see the wind but can see how it moves the trees. But neutrinos will only react to other matter via two processes: firstly, the weak interaction (W+, W-, Z, bosons) and, secondly, by gravitation. This is because neutrinos have zero charge – so cannot interact via the electromagnetic interaction (photons) – and are part of the lepton family, so do not interact via the strong force (gluons) either. Since neutrinos are theoretically predicted to have a very small mass – <500,000 times less massive than an electron – and the weak force is very short-range, they are able to pass through almost all matter they encounter, with negligible exchange of force-carrying particles. In fact, 100 trillion neutrinos pass through the human body every second with no effect. Therefore, experiments involving neutrino detection and interactions must be both large-scale and extremely precise in order to produce accurate results. Two of the most prolific and successful methods of detection in the last half-century are the Davis Experiment and the Super-Kamiokande detector.
The Davis Experiment
In 1969 Raymond Davis Jr. first detected neutrinos emitted from the sun, deep in the Homestake mineshaft in South Dakota, USA. He built a 450 cubic metre cylindrical tank, 1.5km below the ground in order to shield the experiment from any other radiation which could potentially cause anomalous results. The neutrinos would pass straight through the ground reaching the tank below, filtered by the 1500m of rock above. Davis filled the tank with perchloroethylene - also known as tetrachloroethylene 2 (see molecular structure fig.1 ). He knew that in the extremely rare case that a neutrino collided with a chlorine atom in the tetrachloroethylene molecule, it would undergo beta Fig.1. A Perchloroethylene molecule containing Cl 37 isotopes
1 Most neutrinos were created in the early universe, microseconds after the big bang; they are also continuously produced by nuclear reactions in the stars and supernovae and now in reactors and accelerators on earth. 2 Tetrachloroethylene can be readily obtained in quantity as it is a common cleaning fluid
148
Made with FlippingBook Digital Publishing Software