The Geiger-Marsden experiment (s) (also called Rutherford gold foil experiment ) is a series of landmarks from experiments where scientists discovered that each atom contains a nucleus in which all its positive charges and most of the mass is concentrated. They deduce this by measuring how the alpha particle's beam is dispersed when it attacks a thin metal foil. The experiments were conducted between 1908 and 1913 by Hans Geiger and Ernest Marsden under the direction of Ernest Rutherford at the University of Manchester Physical Laboratory.
Video Geiger-Marsden experiment
Summary
Contemporary atomic structural theory
The popular theory of atomic structures at Rutherford's experiment was the "plum pudding model". This model was designed by Lord Kelvin and further developed by J. J. Thomson. Thomson is a scientist who invented electrons, and that is a component of every atom. Thomson believes that atoms are the sphere of positive charge in which electrons are distributed, a bit like a plum in a Christmas pudding. The existence of protons and neutrons is unknown at this time. They know very small atoms (Rutherford assumes that they are in the order of 10 -8 m in radius). This model is entirely based on classical physics (Newtonian); the current accepted model uses quantum mechanics.
Thomson's model was not universally accepted even before Rutherford's experiments. Thomson himself was never able to develop a complete and stable concept model. Japanese scientist Hantaro Nagaoka rejects the Thomson model on the grounds that opposing charges can not penetrate each other. Instead, he proposed that electrons orbit a positive charge such as a ring around Saturn.
Implications of plum pudding model
Alpha particles are sub-microscopic particles, positively charged particles. According to the Thomson model, if an alpha particle collides with an atom, it will only fly straight, its course being deflected by at most a fraction of a degree. At the atomic scale, the concept of "solid matter" is meaningless, so that alpha particles will not bounce atoms like marble. This will be affected only by the atomic electric field, and the Thomson model predicts that the electric field within the atom is too weak to affect many passing alpha particles (alpha particles tend to move very fast). Both the negative and positive charges in Thomson atoms are scattered throughout the atomic volume. According to Coulomb's Law, the less concentrated the ball of electrical charge, the weaker the electric field on its surface.
As an example of work, consider a passive alpha particle passing to Thomson's gold atom, where it will experience an electric field at its strongest and thus experience a maximum deflection ? . Because electrons are very light compared to alpha particles, the effect can be neglected and the atoms can be seen as a heavy, positive charge ball.
- Q n = gold atom's positive charge = 79Ã, e = 1.266 ÃÆ' - 10 -17 Ã, C
- Q ? = particle cost alpha = 2Ã, e = 3,204 ÃÆ' - 10 -19 Ã, C
- r = gold atom radius = 1,44 ÃÆ' - 10 -10 m
- v ? = particle velocity alfa = 1,53 ÃÆ' - 10 7 m/s
- m ? = mass of particle alpha = 6,645 ÃÆ' - 10 -27 Ã,kk
- k = Constant Coulomb = 8,998 ÃÆ' - 10 9 Ã, NÃ, Â · m 2 /C 2
The above calculation is only an estimate of what happens when an alpha particle approaches a Thomson atom, but it is clear that most deflections will be in the order of a small fraction of a degree. If an alpha particle passes through a gold foil about 400 atoms thick and suffers a maximum deflection in the same direction (unlikely), it will remain a small deflection.
Results of experiment
At Rutherford's orders, Geiger and Marsden conducted a series of experiments in which they showed a bunch of alpha particles on thin metal sheets and measured scattering patterns using a fluorescent screen. They see the alpha particles bounce off the metal foil in all directions, partly back to the source. This should not be possible according to the Thomson model; alpha particles must be all straight. Obviously, the particles have experienced far greater electrostatic forces than the Thomson models suggesting them, which would in turn imply that the atomic positive charge is concentrated in volume smaller than Thomson imagined.
When Geiger and Marsden shoot alpha particles on their metal foil, they see only a small part of the alpha particle being deflected by more than 90 °. Most fly directly through the foil. This suggests that small, positively charged balls are separated by vast empty spaces. Most particles pass through empty space and experience negligible drift, while a handful strikes the atomic nucleus and bounces back.
Rutherford thus rejects the model of Thomson atoms, and instead proposes a model in which the atom consists of most of the empty space, with all its positive charges concentrated at its center in very small volumes, surrounded by a cloud of electrons.
Maps Geiger-Marsden experiment
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Ernest Rutherford is Langsworthy Professor of Physics at the University of Victoria Manchester (now University of Manchester). He has received many awards for his studies on radiation. He has discovered the existence of alpha rays, beta rays, and gamma rays, and has proven that this is a consequence of the disintegration of atoms. In 1906, he received a visit from a German physicist named Hans Geiger, and was so impressed that he asked Geiger to stay and help him in his research. Ernest Marsden is 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 had discovered them in 1899. In 1908, he tried to precisely measure their payload-to-mass ratio. To do this, he first needs to know how many alpha particles are given radium (after which he will measure the total charge and divide one with the other). The alpha particle is too small to be seen with a microscope, but Rutherford knows that alpha particles ionize the air molecules, and if the air is in an electric field, the ions will produce an electric current. In this principle, Rutherford and Geiger designed a simple counting device consisting of two electrodes in a glass tube. Any alpha particle passing through the tube creates a calculated electrical pulse. It was an early version of the Geiger counter.
Counters built by Geiger and Rutherford proved unreliable because alpha particles were too strongly deflected by their collisions with air molecules inside the detection chamber. The trajectory of highly variable alpha particles means that they do not all produce the same amount of ions as they pass through the gas, resulting in erratic readings. Rutherford is confused because he thinks the alpha particle is too heavy to bend so strong. Rutherford asks Geiger to investigate how much matter can disperse alpha rays.
The experiments they designed involve bombarding metal foils with alpha particles to observe how the foil spread them in relation to their thickness and material. They use a fluorescent screen to measure the particle trajectory. Any impact of alpha particles on the screen produces a small flash of light. Geiger worked in the dark lab for hours, counting this tiny luster using a microscope. Rutherford has no endurance for this job, which is why he handed it over to his younger colleagues. For metal foils, they test a variety of metals, but they prefer gold because they can make very thin foils, because gold is so easily formed. As a source of alpha particles, Rutherford's preferred substance is radium, a substance that is several million times more radioactive than uranium.
Experiments 1908
A 1908 paper by Geiger, On the Banning? -particles by Content , describes the following experiment. He built a long glass tube, almost two feet long. At one end of the tube is the quantity of "radium emanation" (R) that serves as the source of alpha particles. The tip of the tube is covered with a fluorescent screen (Z). In the center of the tube is a 0.9 mm slit. The alpha particles from R pass through the gap and create a glowing glow of light on the screen. Microscope (M) is used to calculate the luster on the screen and measure its spread. Geiger pumps all the air out of the tube so that the alpha particles will be blocked, and they leave a neat and tight image on the screen that relates to the shape of the gap. Geiger then allows the air inside the tube, and the blazing spots become more diffuse. Geiger then pumps air and puts some gold foil over the crack in AA. This also causes the light plot on the screen to become more diffuse. This experiment shows that both air and solids can actually disperse alpha particles. However, the apparatus can only observe the small corners of the deflection. Rutherford wanted to know if the alpha particle was being spread out with a larger angle - probably larger than 90 °.
1909 Experiments
In a 1909 paper, On the Diffuse Reflections from -Particles , Geiger and Marsden describe experiments in which they prove that alpha particles can indeed be spread over 90 °. In their experiments, they prepared small conical glass tubes (AB) containing "radium emanation" (radon), "radium A" (actual radium), and "radium C" (bismuth-214); the end is closed with mica. This is their alpha particle emitter. They then attached the lead plate (P), behind it they put the fluorescent screen (S). The tube is held at the opposite side of the plate, so that the emitted alpha particles can not directly attack the screen. They see some glow on the screen, as some alpha particles surround the plate by reflecting air molecules. They then place the metal foil (R) to the side of the lead plate. They point the tube to the foil to see if the alpha particle will bounce and hit the screen on the other side of the plate, and observe the increase in the amount of glow on the screen. Counting the sheen, they observed that metals with higher atomic masses, such as gold, reflect more alpha particles than lighter ones such as aluminum.
Geiger and Marsden then wanted to estimate the number of reflected alpha particles. Earlier adjustments are not suitable for doing this because the tubes contain some radioactive substances (radium plus decay products) and thus emitted alpha particles have varying ranges, and because it is difficult for them to ascertain at what level the tubes emit alpha particles. This time, they place a small amount of radium C (bismuth-214) on the tin plate, which bounces off the reflector platinum (R) and onto the screen. They found that only a small part of the alpha particle that engulfs the reflector bounces off the screen (in this case, 1 in 8000).
Trial 1910
A 1910 paper by Geiger, The Scattering of -Particles by Matter , describes an experiment by which he attempted to measure how the most likely angle through which a particle-deflected varies with the material passing through, the thickness of the material, and the velocity of the alpha particle. He built an airtight glass tube from which the air was pumped out. At one end is a bulb (B) containing "radium emanation" (radon-222). By using mercury, radon in B is pumped onto a narrow glass pipe whose edges at A are plugged with mica. At the other end of the tube is a fluorescent zinc sulfide screen (S). The microscope he uses to calculate the glow on the screen is affixed to a vertical millimeter scale with a vernier, allowing Geiger to precisely measure where flashes of light appear on the screen and thus calculate the angle of particle deflection. The alpha particles emitted from A are narrowed into beams by small circular holes in D. Geiger placing metal foils in the ray paths on D and E to observe how the flash zones change. He can also vary the velocity of alpha particles by placing an extra mica or aluminum sheet on A.
From the measurements he took, Geiger came to the following conclusion:
- the most likely angle of deflection increases with the thickness of the material
- The deflection angle is most probably proportional to the atomic mass of the substance
- the most likely deflection angle decreases with the velocity of alpha particles
- The probability that the particles will be deflected is more than 90 Â ° very small
Rutherford mathematically modeled the scattering pattern
Considering the results of the above experiments, Rutherford published a famous paper in 1911 entitled "The Scattering and Particles by Matter and Atomic Structures" in which he proposed that atoms contain at its center a very small and intense electrical charge volume (in fact, Rutherford treats it as a point charge in the calculation). For the purposes of his mathematical calculations, he assumes that the central charge is positive, but he admits that he can not prove this and that he has to wait for another experiment to develop his theory.
Rutherford developed a mathematical equation modeling how foils should propagate alpha particles if all positive charges and most atomic mass are concentrated at a point in the center of the atom.
- s = jumlah partikel alfa yang jatuh pada area unit pada sudut defleksi ?
- r = jarak dari titik kejadian? sinar pada materi hamburan
- X = jumlah total partikel yang jatuh pada bahan hamburan
- n = jumlah atom dalam volume satuan materi
- t = ketebalan foil
- Q n = muatan positif dari inti atom
- Q ? = muatan positif dari partikel alfa
- m = massa partikel alfa
- v = kecepatan partikel alfa
From scattering data, Rutherford estimates the center cost of Q n to about 100 units (see Rutherford model)
Trial 1913
In the 1913 paper, The Laws of Deflexion of? The particles through the Large Angles, Geiger and Marsden describe the series of experiments they are trying to test experimental above equations developed by Rutherford. The Rutherford equation predicts that the number of luster per minute s will be observed at a certain angle ? should be comparable to:
Their paper of 1913 describes four experiments proving each of these four relationships.
To test how scattering varies with the deflection angle (ie if s? Csc 4 (?/2) ) Geiger and Marsden build equipment consisting of a hollow metal cylinder mounted on the turntable. Inside the cylinder there is a metal foil (F) and radiation source containing radon (R), which is mounted on a separate column (T) that allows the cylinder to rotate independently. The column is also a tube where the air is pumped out of the cylinder. The microscope (M) with its objective lens is covered by a sulfide screen of zinc (S) penetrating the cylindrical wall and pointing to a metal sheet. By turning the table, the microscope can be moved full circle around the foil, allowing Geiger to observe and count the alpha particles deflected to 150 Â °. Correcting experimental error, Geiger and Marsden found that the number of alpha particles deflected by the given angle ? is indeed proportional to csc 4 (?/2) .
Geiger and Marsden then test how the scattering varies with the thickness of the foil (ie if s? T ). They make disk (S) with six holes drilled in it. The holes are covered with sheet metal (F) with various thickness, or none for control. This disc is then sealed in a brass ring (A) between two glass plates (B and C). The disk can be rotated using a rod (P) to bring each window in front of the alpha (R) particle source. On the rear panel glass is the screen of zinc sulphide (Z). Geiger and Marsden found that the amount of glare that appears on the screen of zinc sulfide is proportional to the thickness as long as the thickness of the word is small.
Geiger and Marsden reuse the above apparatus to measure how the scattering pattern varies with the square of the nuclear charge (ie if s? Q n 2 ). Geiger and Marsden do not know the positive charge of their metal nuclei (they have just discovered the existing nucleus), but they assume it is proportional to the atomic weight, so they test whether scattering is proportional. to the atomic weight of the square. Geiger and Marsden cover the disc holes with gold, tin, silver, copper and aluminum foils. They measure the power of each foil by equating it with an equivalent air thickness. They calculate the amount of sparkle per minute produced every foil on the screen. They divide the number of lumines per minute with each foil equivalent, then subdivided by the square root of the atomic weight (Geiger and Marsden know that for the same stopping foil, the number of atoms per unit area is proportional to the square root of the atomic weight). Thus, for every metal, Geiger and Marsden obtain the amount of luster that produces a fixed number of atoms. For each metal, they then divide this number by the square of the atomic weight, and find that the ratio is more or less the same. Thus they prove that ? Q n 2 .
Finally, Geiger and Marsden test how scattering varies with the velocity of alpha particles (ie if s <1/v 4 ). Using the same tools again, they slow down the alpha particles by placing extra mica sheets in front of the alpha particle source. They found that, within the range of experimental error, that the number of scinitillations was indeed proportional to 1/v 4 .
Rutherford determines a positively charged nucleus
In his 1911 paper (see above), Rutherford assumes that the central charge of the atom is positive, but the negative charge will fit the scattering model as well. In a 1913 paper, Rutherford stated that the "nucleus" (as it is now called) is positively charged, based on experimental results that explore the scattering of alpha particles in various gases.
In 1917, Rutherford and his assistant, William Kay began to explore parts of alpha particles through gases such as hydrogen and nitrogen. In an experiment in which they fired alpha particle beam through hydrogen, alpha particles tapped the hydrogen nuclei forward in the direction of the rays instead of backwards. In an experiment in which they shoot alpha particles through nitrogen, he found that alpha particles tapped the nuclei of hydrogen (ie protons) from the nitrogen core.
Legacy
When Geiger reported to Rutherford that he had seen a highly deflected alpha particle, Rutherford was shocked. In a lecture delivered by Rutherford at Cambridge University, he said:
It was the most incredible event that ever happened in my life. It's almost as amazing as if you fired a 15-inch shell on a piece of tissue paper and it came back and hit you. Considering, I realize that this backward scattering must be the result of a single collision, and when I make the calculations I see that it is impossible to get anything out of an order of magnitude unless you take a system where the bulk of the atomic mass is concentrated in the core one minute. That's when I had the idea of ​​an atom with a large center of minutes, carrying a load.
The awards soon flooded. Hantaro Nagaoka, who once proposed the Saturn atom model, wrote to Rutherford of Tokyo in 1911: "Congratulations on the simplicity of the tools you use and the brilliant results you have." The conclusion of this experiment reveals how all the material on Earth is structured and thus influences every discipline of science and engineering, making it one of the most important scientific discoveries of all time. Astronomer Arthur Eddington calls Rutherford's discovery the most important scientific achievement since Democritus proposed an atom of the previous era.
Source of the article : Wikipedia
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