The Matter of Everything by Suzie Sheehy

Author:Suzie Sheehy
Language: eng
Format: epub
Publisher: Bloomsbury Publishing

9

Mega-detectors: Finding the Elusive Neutrino

Of the three basic types of radioactive decay – alpha, beta and gamma – one was oddly distinct from the others. Beta decay had troubled physicists since the early 1900s, as it seemed to violate one of the foundational laws of physics. The mystery of beta decay would take more than fifty years to resolve, forcing physicists to build a series of extraordinary underground experiments to hunt down a theoretical new particle which leading experts believed could never be detected. That particle was the neutrino: the most abundant, yet elusive, particle in the Universe.

From the early 1900s, experiments showed that beta radiation seemed to produce electrons with a range of different energies. This wasn’t particularly worrying at the time, but after the atomic nucleus was revealed, problems began to surface. When an element undergoes beta decay, it is not left unchanged: the element shifts one place to the right in the periodic table. This is not the same as losing an electron from its atomic orbit, as this only changes the electric charge on the atom, not the type of atom. Beta decay, on the other hand, produces electrons from inside the nucleus. Detailed measurements by James Chadwick and colleagues showed that beta electrons could emerge with a continuous spectrum of energies ranging from very small up to some maximum energy, seemingly at random. This presented a profound challenge. Beta decay defied the most basic principles of physics.

In an atom undergoing beta decay there is at first one object, the atom. Afterwards, there are two objects, the atom and the electron. One of the key laws of physics, the law of conservation of momentum, dictates that the kinetic energy carried away by the projectiles in a simple two-body system like this should take a predictable, unique value. Alpha and gamma radiation obeyed this law nicely, but in beta radiation the energies were random and unpredictable. Breaking such a fundamental scientific principle is a sure sign that your experiment is flawed or your measurements are incorrect. Yet try as they might, anyone who did such an experiment couldn’t get the data to come out any other way.

Every physicist had a different opinion on what was going on. Some, like Niels Bohr, contemplated throwing out the idea of momentum conservation, or at least sneaking around it by proposing that on the tiny scales inside atoms, energy might only be conserved on average, not in every single decay. One theorist in particular, Wolfgang Pauli, was unable to set the mystery aside. Pauli was well known for his critical and rational approach, which led to his nickname ‘the scourge of God’. He wasn’t happy with the suggestion of Dutch-American physicist Peter Debye, who told him at a meeting in Brussels to simply not think about beta decay at all. Pauli was determined to save momentum conservation and managed to come up with a theoretical solution, but to his horror it made the situation even worse. ‘I have done a terrible thing,’ he said.

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