John Holt was a dedicated experimental physicist whose work had a deep impact on our understanding of the physics of matter at its most fundamental level and contributed to the development of the atom bomb.

He was one of the few remaining links with the momentous days of the 20th century after the discovery of nuclear fission and was later one of the pioneers of elementary particle physics research which blossomed after the Second World War.

John Riley Holt was born in 1918 in Runcorn, Cheshire. His father worked in a boat builder’s yard, and his mother’s family owned a bakery and confectionery shop that his father eventually took over.

Holt attended Runcorn County Secondary School, leaving at 16 with excellent matriculation results, and he proceeded straight to the University of Liverpool’s physics department in 1934. This department was propelled on to the world stage 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 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 described him later as the best research student he had ever supervised. Holt was set to work to study artificially produced radioactive isotopes, during which activity he began to acquire the superb experimental skills that 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 the outbreak of war caused the scientific community to turn its attention to defence. It was soon realised that nuclear fission could be used to make a very powerful weapon.

Otto Frisch and Rudolf Peierls in Birmingham, using the rudimentary information available, 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 that approach was duly adopted.

But to verify the Frisch-Peierls calculations the information needed to be improved, requiring measurements and tests of many 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 being known affectionately as “Frisch and Chips”.

The experiments were carried out under difficult conditions because air raids on Liverpool were heavy and frequent. A bomb destroyed the building adjacent to the cyclotron which, luckily, did not cause serious harm to the research. Holt recorded doing experiments on spontaneous fission using apparatus that needed to be shielded from cosmic radiation. These were done overnight in a cubicle in a Liverpool Underground station while people on the platforms sheltered from the air raids.

Holt’s PhD thesis describes his work on artificial radioactivity but does not mention his work on uranium. Instead, there is 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 that 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 in the United States.

Holt continued with this work first at Liverpool and then at Cambridge. After the war he returned to Liverpool and became involved in the design of a much larger cyclotron. He also took over responsibility for the research programme on a 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 he 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 a 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 our present understanding of this interaction, nowadays known as the Standard Model.

He continued to work on experiments at accelerator laboratories. In the early 1960s an electron synchrotron, NINA, was proposed on a new site at Daresbury in Cheshire. Holt took 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, he established a new group with colleagues from Glasgow and Sheffield universities to study the properties of the proton and its excited states.

After 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 among its quarks. The results obtained from the experiments with this target showed that the proton spin was not simply distributed among its quarks as expected, overturning all the preconceptions of the time.

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 marked the growth of experiments from the small scale to the very large. His deep understanding of the experimental method meant that his opinions were highly valued when important decisions were needed.

He was a kindly, unassuming, considerate and sympathetic man. He led by example, never seeking the limelight. In meetings he was a man of few words but every one counted.

His wife, Joan, died in 2001, and he is survived by his two sons.

Professor John Holt, FRS, Professor of Experimental Physics, University of Liverpool, 1966-83, was born on February 15, 1918. He died on January 6, 2009, aged 90