THE DiSCOVERY OF THE ATOMiC NUCLEUS

By 1906, Rutherford was still at McGill University in Montreal puzzling over Philipp Lenard’s conjecture from 1903 concerning the void between atoms, and he was studying his newly discovered alpha particles. He was measuring the degree of deflection he could obtain using a strong mag­netic field with alpha particles streaming through it. They were moving fast and were heavy, and to get a barely measurable deflection he had to use the most powerful magnet he could devise in the laboratory. His results were recorded on photographic film, showing where in space his beam of alphas landed after traversing the face of the magnet. He defined the beam using a narrow slit through a sheet of metal, and at one point he tried to improve the quality of the beam by putting a thin sheet of mica over part of the slit.

The mica was thin enough to allow alpha particles through, but the particles that came through the mica made an odd, blurred image on the film. As hard as it was to believe, the thin piece of mica was deflecting alpha particles through two degrees, and that was better than he could get using his best magnet. Rutherford made a calculation. To deflect alpha particles by two degrees would take an electrical field of 100 million volts per centimeter of mica. It was clear to him that the center of an atom had to be the source of very intense electrical forces. Alpha particle scattering required further study.

Back in Manchester in 1910, Rutherford set up his colleague Hans Gei­ger (1882-1945) and an undergraduate Ernest Marsden (1889-1970) to study this business of deflection of alpha particles through thin materials.

It would turn out to be a life-changing experience to be enshrined and known to physicists forevermore as “the Gold Foil Experiment.”

Geiger and Marsden were going to try Rutherfords scattering experi­ment with a lot of other materials besides mica. They planned to try alu-

Rutherford’s "Gold Foil" Experiment

 

Some alpha particles

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Zinc sulfide screen

 

Surrounding

electrons

 

Nucleus

 

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Geiger and Marsden, under Rutherford’s supervision, directed alpha particles through a foil of gold to find the attenuating properties of a thin metal shield. To their astonishment, some particles were reflected backward out of the shield.

 

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minum, silver, and platinum, all made thin enough for alpha particles to go through the samples, but first they would try gold because it was easiest to obtain in very thin samples. A vertical sheet of gold foil was set up. To count the alpha particles deflecting through the gold and note their posi­tions they used a glass plate painted with zinc sulfide. It would glow or scintillate when hit with an alpha particle, and they would view it using an attached microscope with the lights turned off.

Next they needed a source of a beam of alpha particles. Radium was a convenient source, but it radiated alpha particles in all directions and they needed a tight beam. They built a special alpha source using a speck of radium at the end of a metal tube. The alpha particles would be absorbed in all directions in the tube except the direction leading right down the center. It seemed like a design that could not fail, but there was a problem. The tube was set so that it was aimed at the gold foil at a 45 degree angle. The pencil-thin beam was expected to deflect, going through the foil and coming out the other side in a spray four degrees wide, but there were alpha particles where there should be none, wide of the opening in the end of the alpha source tube. It appeared that the tube setup was faulty, and that alphas were somehow being emitted at odd angles by the tube. The two scientists tried to fix it. Nothing they tried seemed to work.

Rutherford wandered into the room to find out how it was going. Mars — den reported unsatisfactory results. The beam was too wide, and they were detecting alpha particles scattered widely. Rutherford had an idea. He told Marsden to look for alpha particles in front of the foil, instead of in back of the foil, where the beam was supposed to emerge. Marsden slid a thick, lead shield between the viewing screen and the alpha tube to make sure he was not looking at stray alphas out of the source and put his eye to the microscope, mounted at a 90-degree angle on the front of the gold foil. Marsden was astonished at what he saw in the eyepiece. Instead of simply being deflected by as much as two degrees by going through the gold, the alpha particles were being deflected backward, by an astonishing 90 degrees or more. He met Rutherford on the steps leading to his pri­vate room and broke the news. Rutherford was overjoyed. A piece of gold 0.00002 inches (0.00006 cm) thick was deflecting alpha particles through an angle that would require one enormous magnet. As Lord Rutherford recalled the event later, “It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you.”

Lenard’s observation concerning the extreme lack of substance in matter had been absolutely correct, and Rutherford quickly adjusted his

Подпись: Two Simple Models of the Atom
Подпись: О Infobase Learning
Подпись: The left diagram shows Thomson’s “plum pudding” model of the atom, consisting of protons evenly distributed through a round blob of negative charge. On the right is Rutherford’s modified concept, a tiny, hard nut of protons at the center of orbiting electrons.

concepts to match it. The alpha particles trying to run through the gold atoms in the thin foil were like comets approaching a thin galaxy of stars in outer space. Get too close to a star, and the comet will whip around it and come back in nearly the same direction in a tight, parabolic trajec­tory. The astronomical analogy was obvious, and Lord Rutherford pro­ceeded to model his atoms as Sunlike nuclei having planetlike electrons spinning around them in elliptical orbits. It made a certain poetic sense that matter would be composed of tiny solar systems. The universe in its tiniest form was the same as the universe in its largest form.

Monumental

Подпись:Theories

In the normal day-to-day world, quantities such as speed, mass, and dis­tance are smooth and infinitely divisible. Actions are predictable and reproducible. In the submicroscopic world of quantum mechanics, quan­tities are jerky and make sudden jumps from one value to another. Actions seem governed by probability, and randomness prevails.

Imagine a block of radioactive cobalt metal, for example. It is an abso­lutely certain prediction that in 5.272 years the rate of radiation produc­tion in this sample of cobalt will decrease by half, as the metal block produces a continuous stream oigamma rays. Cut the block in half, and lay aside one of the pieces. The size and the radiation production of the sample are now smaller, but it is still a piece of cobalt and the radiation will still decrease by exactly half in 5.272 years. Now cut the remain­ing piece in half, and push one half aside. Cut this new piece in half. In theory, this block of cobalt can be cut in half a seemingly infinite num­ber of times, but eventually the block of cobalt is so small it is just two atoms of cobalt stuck together. Cut that in half, and the block consists of one atom of cobalt. That atom cannot be cut in half, or at least if it were then the results would no longer be two, smaller pieces of cobalt. Cut an atom of cobalt in half, and all the characteristics of cobalt are lost in the process. The atom cannot be evenly divided, as the nucleus contains 27 protons. The atom is therefore the smallest quantity in which cobalt can exist, and the metal is not infinitely divisible. This smallest possible bit is

the quantum of cobalt. Every quantity of cobalt is some integer multiple of this quantum, but in the macroscopic world, or the world at a scale with which we are familiar, the digital graininess of this material is so small it is not noticeable. There is a threshold. Above the threshold is the continuum of classical physics. Below the threshold is the discontinuous region of probabilistic action and quantum mechanics. There is no way to predict when one atom of radioactive cobalt will decay and send off a burst of radiation. The best prediction is that within 5.272 years the odds are 50-50 that the atom will undergo radioactive decay.

Quantum mechanics would provide a firm, theoretical basis and expla­nation for how the energy release from nuclear decay could be achieved. At full force, quantum mechanics would predict the energy-produc­ing qualities of nonexistent elements and the properties of previously unknown subatomic particles. Eventually, quantum mechanics would be used to back step time in a forensic study of the beginning of the universe. This chapter will reveal some of the most crucial beginnings of quantum mechanics as it was used to push forward an understanding of the atomic nucleus. As the physicist who invented it once said, “If quantum mechan­ics hasn’t profoundly shocked you, you haven’t understood it yet.”

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