Carl David Anderson ( )

Transcription

1 Carl David Anderson ( ) Carl David Anderson was born on September 3 rd, 1905, in New York City. At the age of 7, his family moved to Los Angeles where he attended public school. Shortly after the move his parents divorced. Upon graduation from high school, Carl enrolled in the California Institute of Technology, where 7 years later, in 1930, received his Ph. D in Physics Engineering. He was a very good student, winning one of two scholastic achievement awards in his junior year. This allowed him a grant to travel to Europe for six months where he visited several museums and cathedrals, also getting the opportunity to meet a number of well known scientists including Kamerlingh- Onnes and Lorentz. Growing up during the depression was tough, and Carl found himself struggling to finish his research and support his family at the same time. Carl Anderson is known for his 1932 discovery of the positron, a positive electron. His work was published in 1932 in the Proceedings of the Royal Society, along with Chadwick s discovery of the neutron. The Royal Society in London called his discovery one of the most momentous of the century because Anderson s work put to rest the idea that protons and electrons were the only constituents of matter, thereby expanding theoretical possibilities. Carl was awarded the Nobel Prize in 1936 for this achievement, at the age of 31. Money was so tight that he had to borrow $500 to get to Sweeden to accept the award, but he was granted $20,000 with his Nobel Prize. Maurice Dirac set the stage for the discovery of the positron, when in 1928 he predicted the existence of antimatter, later winning a Nobel Prize for his work in Anderson s work triggered an era of research in modern particle physics in an attempt to understand the atomic nucleus, among other things. Other physicists used the same apparatus as Anderson to find other strange forms of matter, and by 1940 most of the stable and unstable nuclei known today had been discovered. Carl s work also led to studies of radioactivity and nuclear fission. Carl spent his entire career at Caltech; serving as a professor from 1937 until He loved to teach, but enjoyed research as an individual effort. After receiving his Nobel Prize he was called to make many public appearances, which he would have rather done away with. During the second world war, Carl worked for the US Navy developing artillery rockets, which were very successful. Carl was married in 1946 to Lorraine Bergman, and had two boys, Marshall and David. Carl s personal interests were geared towards automobiles and racing, but he also enjoyed tennis, hiking, and music. He died on January 11 th, 1991 Carl s early work had to do with X-rays and the effect they had on various gasses. As a graduate student in 1930, Millikan became Carl s advisor. Millikan encouraged Carl to turn his attention to cosmic ray research, which would ultimately lead to his discovery of the positron 2 years later, in 1932.

2 The cloud chamber Carl used a Wilson cloud chamber for his experiments, designed by C.T.R. Wilson, who was awarded a Nobel Prize in 1927 for his invention. The chamber consisted of a short cylinder, 6 inches in diameter, with glass ends, containing gas saturated with water vapor. When the pressure is dropped suddenly, the paths of ions leave a trail behind them which can be photographed. Carl improved the design of the chamber over time to get much better quality photographs. The chamber was surrounded by a strong electromagnet which was used to identify positive and negative charges by their curvature in the chamber. Also, the relative densities of the particles could be seen by the density of the water droplets. In 1932, one photograph contained two particles, each with the same droplet density but opposite curvatures, suggesting opposite charges. At first Anderson and Millikan thought that they were electrons moving in opposite directions. However, upon the use of a lead plate to slow the velocities, it was confirmed that it was indeed a positively charged electron. If it had been a proton it wouldn t have had the same penetrating ability through the lead plate, therefore it would loose a much larger fraction of its velocity. The characteristics of this particle were; a mass equal to an electron, a charge with equal magnitude but opposite polarity, always created in pairs with an electron, and when the particle collided with an electron, both were annihilated. It was determined that the positron had a very short life because it wouldn t take long before colliding with another stray electron, where the interaction would create 2 gamma ray photons of 511 kev energy each (by Einstein s equation E= mc 2 ). Anderson s discovery was a big surprise because his experiments were not actually searching for antimatter at all. Today most work in particle physics is done with huge particle accelerators where large energies are possible. This is a diagram showing pair production and subsequent annihilation of the positron. Light energy comes in from the lower left. It creates a positron-electron pair; this is a conversion of the light energy into mass energy (by E=mc2). The positron goes along until it collides with some other electron, which causes both to annihilate; the mass energy is converted into light energy. BIBLIOGRAPHY

3 FUNNY STORY This was the period when physics was being done with "love and string and sealing wax." Brilliant scientists with very little money or other support were pushing back the frontiers of physics and, in the process giving us new concepts of the world. Anderson's "anti-matter" was the first step that led to an understanding of the atomic nucleus. Millikan had carried electroscopes and Geiger counters to various places so that he could use the earth as a giant magnet to analyze the energies of primary cosmic rays. Carl and his graduate student, Seth Neddermeyer, were determined to follow Millikan's lead and take their cloud chamber to high altitudes and various latitudes. The cloud chamber was mounted on an old flatbed truck and, with great difficulty, driven to the summit of Pike's Peak. In the Depression days of the 1930s financing scientific research was difficult. The Pike's Peak trip was done on a shoestring budget. First they found a 1932 ton-and-a-half Chevrolet truck, which they purchased for $400. They then found a flat bed trailer and improvised a hitch. Some old packing cases were made into a housing for the equipment. Arriving in Colorado Springs, they had the motor overhauled and the clutch replaced. Even so, they couldn't quite make it to the toll gate up the mountain. The Pike's Peak Company towed them up, probably to clear the road. Their troubles were just beginning. They had an old Cadillac engine generator set that wouldn't produce adequate power at 14,000 feet. When they took the generator down to Colorado Springs to be fixed, the Chevrolet truck broke down because the repair work had not been done properly. However, as luck would have it, a new Chevrolet test truck appeared. It carried as a passenger a General Motors vice-president, who stopped to inquire about the scientist's problem. He then very kindly had the truck towed up the hill and replaced the engine free of charge. But their troubles weren't over. They couldn't afford to pay the $2.50 per night for the six-week stay in the shed on top of the mountain, so they bought a Chevy Roadster for $50 and stayed at a road crew bunk house half way down the mountain. They were obviously two dedicated scientists and they were first-class experimenters.

4 PAIR PRODUCTION

5 This is a diagram showing pair production and subsequent annihilation of the positron. Light energy comes in from the lower left. It creates a positron-electron pair; this is a conversion of the light energy into mass energy (by E=mc2). The positron goes along until it collides with some other electron, which causes both to annihilate; the mass energy is converted into light energy.