Louis Harold Grey obtained his PhD at the Cavendish Laboratory under Rutherford at a time when the laboratory was a world centre for fundamental research in nuclear physics.

Gray had an assured future in nuclear physics, but at that time the practical applications of this knowledge were not apparent. Preferring to devote his life to the public good, he turned his attention to medical physics and radiobiology.

In 1936 he developed the Bragg-Gray principle, which provides the basis for the cavity-ionization method for measuring gamma-ray energy. Although W H Bragg had stated the principle in 1912, Gray worked out the consequences in far greater detail.

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Gray was interested in the biological effects of neutrons. Realising that more powerful sources were required, Gray, together with John Read and technician J G Wyatt constructed a neutron generator at the Mount Vernon Hospital, where Gray had been hospital physicist since 1933. With this tool, Gray and his colleagues made important contributions to understanding the relative biological effectiveness (RBE) of neutrons, discovering that it depended on dose, dose rate and level of biological damage.

In a 1940 paper, Gray and Read employed their energy unit, "that amount of neutron radiation which produces an increment of energy in unit volume of tissue equal to the increment of energy produced in unit volume of water by one röntgen of radiation".

After World War II, Gray joined the new radiotherapeutic research unit at Hammersmith where a cyclotron for radiobiology research and radioisotope production was built. As Deputy Director, he oversaw important research on radiobiology and DNA.

Leaving the Hammersmith group, Gray constructed a laboratory at the Mount Vernon Hospital, the nucleus of the present Gray Laboratory, which attracted many important researchers. The unit became known as a centre for radiation chemistry, and studies were carried on the irradiation of bacteria, and of tumours. Gray himself worked with Eleanor Deschner on the oxygen effect, and with Dewey developed the Hersch cell for the measurement of oxygen.

Gray was Vice Chairman of the International Commission on Radiation Units and Measurements (ICRU) from 1956 to 1962, and assisted in the formation of the IARR. He received many awards for his work, notably the Betrner Medal in 1964. He was elected Fellow of the Royal Society (FRS) in 1961.