Chadwick’s path-breaking discovery
85 years to this day, James Chadwick published “Possible Existence of a Neutron”, which, as the name suggests, delves into the existence of a neutron. By May same year, Chadwick went on to publish “The Existence of a Neutron”. A.S.Ganesh traces through one of the most fundamental discoveries in Physics.
If you found the iterative development of paper-bags impressive, then the story behind the discovery of neutron is sure to blow you off your feet. For the number of people behind this discovery and the collective effort that led up to it is surely another instance of building progressively.
The man who stood on the shoulder of giants and is finally credited with the discovery is James Chadwick. Born in Manchester in 1891, Chadwick was a shy child but immensely talented. That didn’t go unnoticed as he quickly rose in his field, working with Ernest Rutherford on radioactive studies as a student with the University of Manchester.
Chadwick decided to travel to Germany in order to continue his studies with Hans Geiger in 1914. Alas, the World War I was just around the corner and as an enemy alien, Chadwick was interned at Germany. Undeterred, he continued doing small experiments, using a radioactive toothpaste that was available in the market and making an electroscope using tin foil and wood.
James Chadwick | Photo Credit: Wikimedia Commons
Post-war Chadwick continued with Rutherford in England, who had, by this time, made significant progress. Having discovered the atomic nucleus in 1911 and observed the proton in 1919, Rutherford posited in 1920 that an electron and proton could actually combine to form a new particle that is neutral in nature. Evidence for this particle had to wait for another 12 years.
Now working with more sophisticated equipment than tin foil and toothpaste, Chadwick completed his PhD in Cambridge in 1921 and was appointed assistant director at Cavendish Laboratory by 1923. Chadwick’s work now involved numerous other projects as well, but his thoughts kept coming back to the neutral particle that Rutherford predicted.
Around this time, in the 1930s, several other people were working on problems pertaining to radioactivity. German physicist Walter Bothe, along with his student Becker, bombarded beryllium with alpha particles, studying the emitted radiation. Chadwick couldn’t stop thinking that the radiation released might well consist the predicted particle.
Chadwick next took notice of the experiment done by Frederic and Irene Joliot-Curie, who studied the unidentified radiation, as it hit a paraffin wax target. While Joliot-Curie believed that the radiation must be high energy gamma photons, Chadwick was sure there must a better explanation.
He repeated similar experiments himself in 1932 and was convinced that the ejected radiation was in fact composed of neutral particles, with the mass of each particle slightly more than that of a proton. He published “Possible Existence of Neutron” on February 27, 1932, and confirmed the interpretation in his more definite paper, “The Existence of Neutron”, on May 10, 1932.
By 1934, the fact that a neutron was a fundamental particle, and not an electron and proton bound together as Rutherford originally predicted, was established. Chadwick won the Nobel Prize in Physics for this discovery in 1935 and scientists soon realised that as fairly massive particles possessing no charge, neutrons would be useful to probe other nuclei. Atomic bombs were just a step away…