September 18 is Edwin Mattison McMillan’s birthday. McMillan was an American physicist who discovered the first transuranium element.
McMillan joined Ernest Lawrence‘s Berkeley Radiation Laboratory team in 1934. He was attracted to the possibilities of Lawrence’s cyclotron. Over the course of his association with the cyclotron, McMillan made several contributions to the enhancement of the device. He improved nearly every aspect of the device such as magnetic field shaping, control systems, ion sources and beam extraction. His largest contribution was the development of the synchrocyclotron.
As the kinetic energy of the particles produced by cyclotrons increased, they began to travel at speeds high enough to experience relativistic effects. The faster the particle moves, the larger the apparent mass of the particle. This would cause the particles to move out of phase with the alternating magnetic field used to accelerate them. This meant the cyclotron had a practical upper speed limit unless some method could be found to fix the problem. McMillan added a way to synchronize the magnetic field’s frequency as the particles gained energy. This is the basis of the synchrocyclotron.
McMillan discovered the first transuranium element while investigating the decay products of uranium bombarded by neutrons. He observed two different decays. One was the decay of U-239 with half-life 23 minutes and an unknown beta-decay with a half-life of 2.3 days. Beta decay is produced when a neutron is converted to a proton in the nucleus of the atom. This means McMillan’s uranium atoms were absorbing the neutron to form U-239. The detected beta particle meant some of the neutrons were being converted to protons. Adding a proton increases the position on the periodic table. There had to be an element one spot higher than uranium if this was true. McMillan had trouble isolating this possible new element and enlisted the help of Philip Abelson. Together, they successfully isolated the new element and named it Neptunium. Because of the war, this discovery was kept secret. Much of the world would learn of their discovery when McMillan would receive half the 1951 Nobel Prize in Chemistry.
McMillan would go on to find other new isotopes with the cyclotron. He produced the first evidence of the radiation known as pair-production where a gamma ray collides with a nucleus and produces a positron and electron. He identified the isotopes oxygen-15 and beryllium-10. If he had continued one experiment, he would have been the discoverer of carbon-14. He was on the trail of another possible new element when World War II broke out. He left the Radiation Laboratory to work with the US Navy on radar and sonar systems and ultimately on the Manhattan Project. Glenn Seaborg’s team would continue his work and discover plutonium.
Notable Science Events for September 18
1968 – Zond 5 spacecraft makes the first orbit around the Moon.
The Soviet spacecraft, Zond 5, became the first spacecraft to fly around the moon and return to Earth. It was designed to launch from Earth orbit, reach the moon and return as a precursor to a possible manned mission. Besides taking photographs of Earth and the Moon, the probe carried a biological payload of insects, bacteria, plants and two Russian steppe tortoises. The Zond 5 returned to Earth to splash down in the Indian Ocean. The probe was recovered and the tortoises survived. They lost some weight and the trip did not affect their appetite.
These tortoises were the first tortoises in space and also became the first tortoises to fly around the Moon. Not only did tortoises beat hares to the Moon, they beat humans to the moon by three months. Apollo 8 astronauts orbited the Moon on December 24, 1968.
1967 – John Cockcroft died.
Cockcroft was a British physicist who shares the 1951 Nobel Prize in Physics with Ernest Walton for the invention of the linear accelerator. The linear accelerator generates a beam of charged particles used to smash into the nuclei of target atoms and is an essential tool to the study of subatomic physics. Cockcroft and Walton’s accelerator generated a proton beam and collided with and split lithium atoms to create the first nuclear reaction that did not involve naturally radioactive elements.
1907 – Edwin Mattison McMillan was born.
1896 – Armand Hippolyte Fizeau died.
Fizeau was a French physicist who was the first to measure the speed of light without using astronomical observations. He also showed the wave nature of light by demonstrating the speed of light was slower in water than in air. He showed the heat rays from the sun also have wave properties by demonstrating interference and suggested moving stars would shift their spectra relative to the direction of motion now known as red-shift. Fizeau attempted to measure the speed of electricity in wires and determined a value of one-third the speed of light.
1819 – Jean Bernard Léon Foucault was born.
Foucault was a French physicist who is best known for the Foucault pendulum. This pendulum demonstrated the Earth’s rotation for the first time.
Foucault devised a method to accurately measure the speed of light using a rotating mirror. He shined a light through a slit towards a distant rotating mirror. The mirror would reflect the light back towards the slit at an angle relative to the rotational speed of the mirror. The amount of deflection could be used to determine the speed of light. Foucault’s measurement of the speed of light in 1862 was within 0.6% of the accepted value today. He used this same technique to determine the speed of light in water.
He also discovered eddy currents in metals. An eddy current is an electrical phenomenon where a conductor is exposed to a changing magnetic field relative to the motion of the conductor. This causes a circular flow of electrons that oppose the changing magnetic field.