Professor John Holt: Physicist who increased our understanding of matter at its most fundamental level

John Holt was a dedicated experimental physicist whose work had a major impact on our understanding of the physics of matter at its most fundamental level. With his passing we lose one of the few remaining links with the momentous days of the 20th century following the discovery of nuclear fission. He was later to become one of the pioneers of elementary particle physics research which blossomed after the Second World War.

John Holt was born in Runcorn, Cheshire, in 1918. His father worked in a boat-builder's yard, as his grandfather and great grandfather had done before him, and his mother's family owned a bakery and confectionery shop. He attended Runcorn County Secondary School, where his interest in science was awakened by constructing radios with his school friends.

He left school at 16 and went straight on to higher education, becoming an undergraduate in the Physics Department at Liverpool University in 1934. This department was propelled on to the world stage of physics with the arrival of James Chadwick in 1935, the year he won the Nobel Prize for his discovery of the neutron. Chadwick brought the resources into the department to build a cyclotron, a newly invented state-of-the-art research tool.

Holt graduated in 1938, winning the Oliver Lodge Prize for the best undergraduate, and was taken on by Chadwick as a research student. Chadwick later described him as the best research student he had ever supervised. Holt was set to work to study artificially produced radioactive isotopes. In this work he began to acquire the superb experimental skills which he was to put to good use in later life.

In 1939 two events were to change the direction of his research: nuclear fission was discovered and war was declared, causing the scientific community to turn its attention to defence. It was soon realised that nuclear fission could be used to make a powerful weapon. Frisch and Peierls in Birmingham were the first to realise that a weapon could be made using a few pounds of the 235 isotope of uranium rather than the tons referred to in Einstein's 1939 letter to President Roosevelt. This simplified both the production and delivery for both sides in the conflict. The memorandum they produced proposed that the best defence against such a weapon was to be the first to develop it, and this approach was to be adopted.

However, verifying the Frisch-Peierls calculations needed the available information to be improved, requiring measurements and tests of many different kinds. The centre for these tests moved to Liverpool, and Chadwick set up a team to perform them. The cyclotron was pressed into service and a frenetic series of experiments began. Holt made a significant contribution to these, working with Frisch, the two becoming known affectionately around the laboratory as "Frisch and Chips".

The experiments were carried out under difficult conditions because at this time air raids on Liverpool were heavy and frequent. A bomb destroyed the building adjacent to the cyclotron, although luckily it did no serious harm to the research. Holt recalled doing experiments on spontaneous fission using apparatus which needed to be shielded from cosmic radiation. These were done overnight in a cubicle in a Liverpool underground station with people on the platforms sheltering from the air raids.

Holt's PhD thesis described his work on artificial radioactivity but did not mention his work on uranium. Instead, the thesis contained a brief note explaining that some results could not be released owing to the Official Secrets Act. The culmination of this work was the proof of the feasibility of the production of a nuclear weapon, information which Chadwick used to press the urgency of the case to be first to produce it. This helped to pave the way for the Manhattan project to develop the weapon.

Holt continued with this work first at Liverpool and then at Cambridge. After the War he returned to Liverpool as a member of staff and immediately became involved in the design of a much larger cyclotron. He also took over the responsibility for the ongoing research programme on the smaller cyclotron, embarking on a systematic experimental study of the properties of nuclei, using a technique known as deuteron stripping. This made a significant contribution to the understanding of nuclear spectroscopy.

When the large, higher-energy cyclotron became operational Holt set up and led a research group to investigate the properties of sub-atomic particles produced in the higher-energy interactions initiated by beams from this machine. A far-reaching and highly significant result obtained by his group demonstrated that the weak interaction responsible for the decay of the muon, one of the sub-atomic particles, did not follow the expectation of "charge conjugation invariance", namely, that muons and their antiparticles did not decay in an identical manner. This had a critical influence on the development of our understanding of the weak interaction, leading the way to our present perspective of this interaction, nowadays known as the Standard Model.

The relentless pursuit by physicists of an understanding of matter, its origin and the laws of physics governing its behaviour, coupled with his reputation as a distinguished pioneer in this field, meant that he continued to work on further generations of experiments at accelerator laboratories. In the early 1960s an electron synchrotron, NINA, was proposed on a new site at Daresbury in Cheshire. He was intimately involved in the development of this machine, taking charge of the design of the electromagnets. His sang-froid is illustrated by his comment: "We will have to do something about that" when one day a test magnet failed catastrophically.

Once NINA was commissioned, Holt established a new group with colleagues from the Universities of Glasgow and Sheffield to study the properties of the proton and its excited states. Following the discovery that the proton has a sub-structure made of quarks, interest shifted to even higher energies to investigate this phenomenon. He led the Liverpool group into the European Muon Collaboration (EMC) to study this sub-structure using a muon beam from the Super Proton Synchrotron at the International Laboratory, Cern, in Geneva. A highly specialised polarised target was developed by his group to study the distribution of the spin of the proton amongst its quarks. The results obtained showed that the proton spin was not simply distributed among its quarks as expected, overturning all the preconceived ideas of the time. This led to an avalanche of papers interpreting the results and spawned a whole new series of experiments at laboratories around the world.

Holt was elected to a Fellowship of the Royal Society in 1964 and appointed Professor of Experimental Physics in 1966. His research lifetime of more than 50 years saw the growth of experiments from the small scale to the very large which we see today. His deep understanding of the experimental method meant that his opinions were valued when important decisions were needed. He was keen to communicate his enthusiasm for physics within and outside the academic world. He was a kindly, unassuming, considerate and sympathetic man. He led by example, never seeking the limelight, preferring to work with colleagues old and young at the physics he enjoyed. In meetings he was a man of few words but every one counted.

His main relaxation was gardening, where he enjoyed his collection of exotic shrubs and trees. He was a keen amateur photographer and also enjoyed painting in his later years, combining both with his love of all things horticultural.

John Riley Holt, physicist: born Runcorn, Cheshire 15 February 1918; Lecturer in Physics, Liverpool University 1945-53, Senior Lecturer 1953-56, Reader 1956-66, Professor 1966-83 (Emeritus); FRS 1964; married 1949 Joan Silvester Thomas (died 2001; two sons); died Wirral, Cheshire 6 January 2009.