The story starts in the 1920s. Scientists were having a really hard time solving the mystery surrounding the beta decay, a radioactive decay that results in the emission of beta particles (basically high-speed electrons or positrons). And what was the mystery? Let me explain!
Mystery of the missing energy
It was understood by the 1920s that beta decay happens in an unstable nucleus (also called radioactive nucleus). In this nucleus, either a neutron converts to a proton or a proton to a neutron, both of which result in the shedding off of the excess energy of the unstable nucleus by emitting out a beta particle. And therein lay the mystery.
You see, the law of conservation of energy meant that the energy before and after the decay must remain the same. Since the final products were the daughter nucleus and the beta particle, it is to be expected that beta particles emitted in a certain decay would all have the same energy. It is as natural as expecting the bullets fired by the same gun to come out with the same energies!
But that wasn’t the case! The beta particles had a wide range of energies for a given decay. And therein lay the mystery. Why was the energy not constant? This had scientists scratching their heads for a decade before Wolfgang Pauli, an Austrian physicist, came up with a solution. A very clever solution at that!
Pauli's suggestion of a new particle
He suggested that maybe the beta particle isn’t alone! I mean, logically, if it is being accompanied by another particle, that means it is sharing energy with that particle as well. So, it would make sense that if sometimes a beta particle is coming out with less than the expected energy, the other particle is responsible for carrying that remaining energy.
But the big question was, why wasn’t the particle getting detected? Pauli thought of that as well. He believed that since the particle isn’t getting detected, it means it’s not interacting with anything in its environment. So, it should have no electric charge, which would explain why it wouldn’t interact with matter around it via electromagnetic force; very little to no mass and should have an extremely high penetrating power. All the characteristics explaining why a particle is going undetected.
Pauli had then named it the ‘neutron’ but Enrico Fermi, an Italian-American physicist, changed it to ‘neutrino’ after the discovery of what we now call neutron- the neutral counterpart of proton. Working on Pauli’s idea, Fermi put forth what he called a ‘tentative’ theory of beta decay which is now called Fermi interaction. He very correctly explained that in this process a neutron would decay into a proton, electron and a neutrino. But when he sent it to publication, he first experienced a rejection from ‘Nature’ but later got the revised version published in Italian and German publications in 1933 and 1934. Here is the translated version of this historic paper.
How to catch a ghost
Due to its properties, Pauli believed that we would never be able to detect and capture a neutrino. Ever. He even bet on that! (A case of champagne for those of you who are wondering)
But of course, now we know that someone did! Let’s see who and how.
Years later, in the 1950s, two bomb scientists met by chance. They were Frederick Reines and Clyde Cowan. Both of them had worked on exploding nuclear bombs, had grown tired of it and were looking for something else to work on. They were looking for a challenge and what better challenge than to detect a neutrino – the particle that can never be detected?
They first came up with an idea to detect it in a nuclear explosion. They thought that maybe keeping a detector some 50 metres from ground zero would work. They then decided on another plan which seemed more viable – using a nuclear reactor for the detection. Of course, the number of neutrinos in a reactor is far lesser than a nuclear bomb but while a bomb only gives us a few moments for detection, a reactor can work for a long duration of time.
Project Poltergeist
Finally, the Savannah River Plant, used for making tritium and plutonium nuclear bombs, was chosen for their project. They used a large tank filled with 75 gallons of chemical liquids including cadmium compounds. This was with an aim that the neutrinos would react with the liquid, however feebly, and leave a signal. This detector was, by that time’s standards, ‘thousands of times more sensitive than any other device to reactions caused by neutrinos’.
Reines and Cowan called this project the ‘Project Poltergeist’. And rightfully so, since the idea was to catch this disturbing spirit among the sub-atomic particles. And on 14th June 1956, this poltergeist of modern physics was finally caught. Or more technically, detected! This is their paper of confirmation.
The particle detected was actually the anti-matter counterpart of neutrino. Meaning they actually discovered antineutrino. But it was an astoundingly big discovery nonetheless. Later on, in 1995, Reines was honored with the Nobel Prize in Physics for his work on neutrino physics. Cowan had passed away at the age of 54 by then.
So this was the story behind the discovery of neutrinos. But did you know that there are actually three types or flavors of neutrinos? Do you know that they can oscillate from one flavor to another? Head over to Neutrino Flavors to learn more!
Commentaires