 One of my favorite particles is the neutrino. You'll recall from our segment on radiation that the beta rays were ejected electrons. What's happening here is that a neutron inside the nucleus of an atom is spontaneously decaying into a proton and ejecting an electron in the process. The mass of the proton plus the mass of the electron is less than the mass of the neutron. And because energy is conserved, some energy must be released to make up for the difference. From Einstein we know that energy equals mass times the speed of light squared. So if the lost mass is turned into energy, we can calculate the amount. Just under a million electron volts per atom. It was assumed that this energy was accounted for by the kinetic energy of the ejected electrons. In 1927, two physicists, C. D. Ellis and W. A. Worcester, set out to measure this energy. They used bismuth 210, a product of radium decay that itself decays into polonium, the rate at which unstable radioactive nuclei decay in a sample is called the activity of the sample. The greater the activity, the more nuclear decays per second. This is rather easily measured with devices like a Geiger counter. Given the number of radiating molecules in a sample and measuring the activity, we can calculate the probability for any one molecule to decay in a second. This is called the decay constant. We find that the decay constant is always a small number, constant over time, and different for different materials. Both the activity rate and the number of radioactive nuclei vary over time. As a sample decays, the number of radioactive nuclei decreases. With fewer radioactive nuclei, the activity rate also decreases. In this we get the exponential law of radioactive decay. It tells us how the number of radioactive decay in a sample decreases with time. The half-life is the time it takes for the material and activity to be reduced by half. Bismuth 210 has a half-life of five days, meaning it takes five days for half of any amount to transform into polonium. We cover half-life in more detail in how old is the Earth-Moon system segment and how old is it video book. The experiment was simple. Place the bismuth into a calorimeter. The calorimeter keeps the energy of the beta radiation inside the container. Over the five days, each and every ejected electron's kinetic energy is converted to heat as they collide with the water molecules and come to rest in the calorimeter. Measuring the change in temperature allows us to calculate the amount of energy absorbed. The results showed that each bismuth atom naturally emits 0.36 million electron volts. But here we had a significant discrepancy. Conservation of energy and Einstein's equations called for 0.8 million electron volts. That's more than twice as much as was measured. This was a real problem. Niels Bohr thought that the conservation of energy didn't hold in this case, while Wolfgang Pauli thought that it did and proposed that there must be another particle that doesn't interact much with its surroundings and carried away the missing energy. In 1931 Enrico Fermi named Pauli's particle the neutrino, or a small neutral particle. The neutrino's predicted mass was around a third of an electron volt. This is over a million times smaller than the electron. This predicted spin was one half and its predicted speed was almost the speed of light. This would have beta radiation look like this with both an electron and a neutrino being ejected. This particle was finally observed in a hydrogen bubble chamber captured in 1970. The invisible neutrino enters from the lower right and strikes a proton where the three particle tracks originate. The proton is kicked into motion and the neutrino is converted into a muon and a pion by the power of the collision with the proton.