The Geiger–Marsden experiments (also called the Rutherford gold foil experiment) were a series of landmark experiments by which scientists discovered that every atom contains a nucleus where its positive charge and most of its mass is concentrated. They deduced this by observing how alpha particles are scattered when they strike a thin metal foil. The experiments were performed between 1908 and 1913 by Hans Geiger and Ernest Marsden under the direction of Ernest Rutherford at the Physical Laboratories of the University of Manchester.
- 1 Summary
- 2 Timeline of events
- 3 Legacy
- 4 See also
- 5 References
- 6 External links
Contemporary theories of atomic structure
The popular theory of atomic structure at the time of Rutherford's experiment was the "plum pudding model". This model was developed in 1904 by J. J. Thomson, the scientist who discovered the electron. This theory held that the negatively-charged electrons in an atom are distributed in a uniform sea of positive charge, like plums in a bowl of pudding. A competing theory was proposed by Hantaro Nagaoka. Nagaoka rejected Thomson's model on the grounds that opposing charges cannot penetrate each other. He proposed instead that the positive charge of the atom is concentrated in a nucleus, with the electrons orbiting it like the rings around Saturn.
Implications of the plum pudding model
Since electrons are so very light, we can neglect their influence on the trajectory of an alpha particle that approaches an atom (they will be swept aside effortlessly) and we can thus treat an atom as a heavy sphere of positive charge. Coulomb's Law states that the electrostatic force that a point charge will exert on another point charge is inversely proportional to the square of the distance between them. What Coulomb's Law means for a sphere of electric charge is that the more concentrated it is, the more intense a force it will exert on any charged particle that comes near it. Thomson's model assumed that the atom's sphere of positive charge was large—the size of the atom itself, in fact—and thus its charge was diffuse. They calculated that its electric field at its surface would be so weak that, given how fast and heavy alpha particles are, it shouldn't be able to deflect a passing alpha particle by more than a fraction of a degree.
The outcome of the experiments
Geiger and Marsden performed a series of experiments where they pointed a beam of alpha particles at a thin foil of metal and measured the scattering pattern by the use of a fluorescent screen. They found that atoms could deflect an alpha particle by as much as 150°. This meant those particles had encountered an electrostatic force far greater than Thomson's model suggested they would, which in turn implied that the atom's positive charge was concentrated in a much tinier volume than Thomson imagined.
When Geiger and Marsden shot alpha particles at their metal foils, they noticed only a tiny fraction of the alpha particles were deflected by more than 90°. Most just flew straight through the foil. This suggested that those tiny spheres of intense positive charge were separated by vast gulfs of empty space. Imagine you are standing on the edge of a copse of trees with a large bag full of tennis balls. If you were to blindly throw tennis balls at the trees, you would notice that most of the balls would fly through hitting nothing, while a few would strike tree trunks and bounce off in all directions. This analogy illustrates what Rutherford saw in the scattering pattern of the alpha particles. Most particles went straight through the metal foil because its matter was mostly empty space, but a few had "struck" some small but strong obstacle: the nuclei of the atoms.
Timeline of events
Ernest Rutherford was a physics professor at the University of Manchester. He had already received numerous honors for his studies of radiation. He had discovered the existence of alpha rays, beta rays, and gamma rays, and had proved that these were the consequence of the disintegration of atoms. In 1906, he received a visit from a promising young German physicist named Hans Geiger, and was so impressed that he asked Geiger to stay and help him with his research. Ernest Marsden was a physics undergraduate student studying under Geiger.
Alpha particles are tiny, positively-charged particles that are spontaneously emitted by certain substances such as uranium and radium. Rutherford himself had discovered them in 1899. In 1908 he was trying to precisely measure their charge-to-mass ratio. To do this, he first needed to know just how many alpha particles his sample of radium was giving off (after which he would measure their total charge and divide one by the other). Alpha particles are too tiny to be seen even with a microscope, but Rutherford knew that alpha particles ionize air molecules, and if the air is within an electric field, the ions will produce an electric current. On this principle, Rutherford and Geiger designed a simple counting device which consisted of two electrodes in a glass tube. Every alpha particle that passed through the tube would create a pulse of electricity that could be counted. It was an early version of the Geiger counter.
The counter that Geiger and Rutherford built proved unreliable because the alpha particles were being too strongly deflected by their collisions with the molecules of air within the detection chamber. The highly variable trajectories of the alpha particles meant that they didn't all generate the same number of ions as they passed through the gas, thus producing erratic readings. This puzzled Rutherford because he had thought that alpha particles were just too heavy to be deflected so strongly. Rutherford asked Geiger to investigate just how much matter could scatter alpha rays.
The experiments they designed involved bombarding a metal foil with alpha particles to observe how the foil scattered them. Since alpha particles are so tiny, they used a fluorescent screen to measure their trajectories. Each impact of an alpha particle on the screen produced a tiny flash of light. To count these tiny flashes of light, Geiger and Marsden had to work in a darkened lab, peering through a microscope for hours on end. Rutherford lacked the endurance for this work, which is why he left it to his younger colleagues. For the metal foil, they tested a variety of metals, but they preferred gold because they could make the foil very thin, as gold is very ductile. As a source of alpha particles, Rutherford's substance of choice was radium and its decay products. Radium is a substance several million times more radioactive than uranium. Because only a very tiny fraction of the alpha particles would experience great angles of deflection, a truly prodigious emitter of alpha particles was needed to obtain statistically meaningful results.
The 1908 experiment
A 1908 paper by Geiger describes the following experiment. Geiger constructed a long glass tube, nearly two meters in length. At one end of the tube was a quantity of "radium emanation" (R) that served as a source of alpha particles. The opposite end of the tube was covered with a phosphorescent screen (Z). In the middle of the tube was a 0.9mm-wide slit. The alpha particles from R passed through the slit and created a glowing patch of light on the screen. A microscope (M) was used to count the scintillations on the screen and measure their spread. When the air was pumped out of the tube and the alpha particles were unobstructed, they left a neat and tight image on the screen that corresponded to the shape of the slit. When there was some air in the tube, the glowing patch became more diffuse. Geiger then pumped out the air and placed some gold foil over the slit at AA. This too caused the patch of light on the screen to become more spread out. This experiment demonstrated that both air and solid matter could markedly scatter alpha particles. The apparatus, however, could only observe small angles of deflection. Rutherford wanted to know if the alpha particles were being scattered by even larger angles—perhaps larger than 90°.
The 1909 experiment
In a 1909 paper, Geiger and Marsden described their eponymous experiment by which they proved that alpha particles can indeed be scattered by more than 90°. In their experiment, they prepared a small conical glass tube (AB) containing "radium emanation" (radon), "radium A" (actual radium), and "radium C" (bismuth-214); its open end sealed with mica. This was their alpha particle emitter. They then set up a lead plate (P), beneath which they placed a fluorescent screen (S). The tube was held above the plate, such that the alpha particles it emitted could not directly strike the screen. They noticed a few scintillations on the screen—this was because some alpha particles could circumvent the lead plate by bouncing off air molecules (the experiment was not done in a vacuum). They then placed a metal foil (R) to the side of the lead plate. They pointed the tube at the foil to see if the alpha particles would bounce off it and strike the screen on the other side of the plate, and that is indeed what they saw. Counting the scintillations, they noticed that metals with higher atomic mass, such as gold, reflected more alpha particles than lighter ones such as aluminium.
Geiger and Marsden then wanted to estimate the total number of alpha particles that were being reflected. The previous setup was unsuitable for doing this because the tube contained several radioactive substances (radium plus its decay products) and thus the alpha particles emitted had varying ranges, and because it was difficult for them to ascertain at what rate the tube was emitting alpha particles. This time, they placed a small quantity of radium C (bismuth-214) on the lead plate, which bounced off a platinum reflector (R) and onto the screen. They found that only a tiny fraction of the alpha particles that struck the reflector bounced onto the screen (in this case, 1 in 8000).
The 1910 experiment
A 1910 paper by Geiger describes an experiment by which he sought to measure how the most probable angle through which an a-particle is deflected varies with the material it passes through, the thickness of said material, and the velocity of the alpha particles. He constructed an airtight glass tube from which the air was pumped out. At one end was a bulb (B) containing "radium emanation" (radon-222). By means of mercury, the radon in B was pumped up the narrow glass pipe whose end at A was plugged with mica. At the other end of the tube was a fluorescent zinc sulfide screen (S). The microscope which he used to count the scintillations on the screen was affixed to a vertical millimeter scale with a vernier, which allowed Geiger to precisely measure where the flashes of light appeared on the screen and thus calculate the particles' angles of deflection. The alpha particles emitted from A was narrowed to a beam by a small circular hole at D. Geiger placed a metal foil in the path of the rays at D and E to observe how the zone of flashes changed. He could also vary the velocity of the alpha particles by placing extra sheets of mica or aluminium at A. From the measurements he took, Geiger found that the most probable angle of deflection increases with the thickness of the material, is proportional to the atomic mass of the substance, and decreases with the velocity of the alpha particles, and that in any case the probability that a particle will be deflected by more than 90° is vanishingly small.
The 1913 experiment
Considering the results of the above experiments, Rutherford published a landmark paper in 1911 where he proposed the existence of a strong electrical charge concentrated in a very small volume at the center of the atom. Rutherford developed a mathematical equation that modeled how the foil should scatter the alpha particles according to the foil's substance and thickness and the velocity of the alpha particles (see the Rutherford scattering article for a detailed explanation). To confirm his predictions and validate his model, they would have to accurately measure the full scattering pattern produced by the foil. In a 1913 paper, Geiger and Marsden described an experiment by which they did just that. The apparatus they built consisted of a hollow metal cylinder mounted on a turntable. Inside the cylinder was the metal foil and the radiation source (containing radon), mounted on a detached column which allowed the cylinder to rotate independently. A microscope with its objective lens covered by a fluorescent zinc sulfide screen penetrated the wall of the cylinder and pointed at the metal foil. By turning the table, the microscope could be moved a full circle around the foil, allowing Geiger to observe and count alpha particles deflected by up to 150°.
When Geiger reported to Rutherford that he had spotted alpha particles being strongly deflected, Rutherford was astounded. In a lecture Rutherford delivered at Cambridge University, he said:
It was quite the most incredible event that has ever happened to me in my life. 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. On consideration, I realized that this scattering backward must be the result of a single collision, and when I made calculations I saw that it was impossible to get anything of that order of magnitude unless you took a system in which the greater part of the mass of the atom was concentrated in a minute nucleus. It was then that I had the idea of an atom with a minute massive centre, carrying a charge.—Ernest Rutherford
Accolades soon flooded in. Hantaro Nagaoka, who had once proposed a Saturnian model of the atom, wrote to Rutherford from Tokyo in 1911: "Congratulations on the simpleness of the apparatus you employ and the brilliant results you obtained". The astronomer Arthur Eddington called Rutherford's discovery the most important scientific achievement since Democritus proposed the atom ages earlier. By his discovery of the nucleus, Rutherford began the new science of nuclear physics.
- Rutherford backscattering spectroscopy
- Rutherford scattering
- Atomic theory
- List of experiments
- 1909 in science
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