Meanwhile, he worked at the Cavendish Laboratory while working towards his doctorate in theoretical physics. At the time, the laboratory was the centre of a number of scientific breakthroughs. James Chadwick had discovered the neutron, John Cockcroft and Ernest Walton transmuted lithium with high-energy protons, and Patrick Blackett and Giuseppe Occhialini used cloud chambers to demonstrate the production of electron pairs and showers by gamma radiation.