In atomic physics, Rutherford scattering (English: Rutherford scattering) is a scattering experiment. It was designed and researched by Ernest Rutherford. It was successfully confirmed in 1909 that there is an atomic nucleus in the center of the atom, which led to the creation of Rutherford model (planetary model) and later the proposal of Bohr model.Applying Rutherford scattering technology and theory,Rutherford backscattering(Rutherford backscattering) is a special technology for analyzing materials.Rutherford scattering is sometimes called coulomb scattering because the potential it involves is coulomb potential.Deep inelastic scattering(deep inelastic scattering) is a similar kind of scattering. In the 1960s, it was often used to detect the interior of atomic nuclei.
Chinese name
Rutherford scattering
Foreign name
Rutherford scattering
Presenter
Rutherford scattering
Proposed time
1909
Applicable fields
Detect the heavy metal impurities in the semiconductor.
From 1909 to 1911, British physicist Rutherford (1871-1937) and his collaborators conducted the experiment of bombarding gold foil with particles. The experimental procedure is as follows:
In a small lead box, there is a small amount of radioactive element polonium, which emits alpha particles from the small hole of the lead box, forming a very thin beam of radiation onto the gold foil.After passing through the gold foil, alpha particles hit the fluorescent screen to produce flashes, which can be observed with a microscope.The whole device is placed in a vacuum container, and the fluorescent screen and microscope can rotate around the gold foil in a circle.
According to the results calculated by Thomson model, the angle of alpha particles passing through the gold foil from the original direction is very small.Because the mass of the electron is very small, less than one thousandth of that of the alpha particle, the alpha particle will encounter it just like a flying bullet encounters a particle of dust, and the direction of motion will not change significantly;The positive charge is evenly distributed. When the alpha particle passes through the atom, it receives a considerable part of the repulsion force of the positive charges on both sides of the atom to offset each other, so that the force of the alpha particle to deflect is not very large.
experimental result
However, the experiment got unexpected results.The majority of alpha particles still move in the original direction after passing through the gold foil, while a few particles have a large deflection, and a few particles have deflection angles exceeding 90 degrees, and some even bounce back, with deflection angles almost reaching 180 degrees.This phenomenon is called alpha particle scattering.Rutherford was surprised by the phenomenon of large angle scattering of particles generated in the experiment, because it requires a strong interaction force. Unless most of the atomic mass and charge are concentrated on a very small nucleus, large angle scattering is impossible.
The large angle scattering can not be explained by the cumulative result of accidental small angle scattering, so it can not be explained by the popular Thomson atomic model at that time. Rutherford once said that this was the most surprising event he met in his life, "It is almost like a 15 inch cannon shot fired, hit a piece of thin paper, and then bounced back and hit.
Put forward a hypothesis
Rutherford thought over and over again, analyzing and considering this amazing result for 18 months. In early December 1910, Rutherford got his important conclusion. Geiger recalled that,"One day, Rutherford rushed to my laboratory and told me that he now knew what atoms looked like and how to explain the large angle deflection of these alpha particles. On the same day, I started the experiment on the relationship between scattering particles and scattering angles that Rutherford expected.".
In order to explain this experimental result, Rutherford put forward the following nuclear structure theory in 1911: there is a very small nucleus in the center of the atom, called the nucleus. All the positive charges and almost all the mass of the atom are concentrated in the nucleus, and the negatively charged electrons rotate around the nucleus in the space outside the nucleus.The number of positive charges per unit of the nucleus is equal to the number of electrons outside the nucleus, so the whole atom is neutral.The centripetal force required for an electron to rotate around the nucleus is to check its Coulomb gravity.
Rutherford described the atomic world to people, some of which are like the planets in the solar system rotating around the sun, with electrons rotating around the atomic nucleus.
In May 1911, Rutherford published the article "Scattering of matter on alpha and beta particles and atomic structure", giving a complete explanation of the nuclear structure of atoms.In the formula given by him, the number of particles in the unit area where the α particle falls at the deflection angle of Φ (see Figure 2 drawn by reference for Φ orientation, where CD is the scattering foil, O is the incident point, OA is the direction of incident α particle, OB is the direction of scattering α particle) is proportional to 1) csc Λ 4 (Φ/2);2) The thickness t of scattering foil material (the thickness t is not large);3) The square of the central charge Ne of the atom of the scattering foil material;4) The velocity of incident alpha particle u is - 4 power[1]
Figure 2
Post validation
Geiger first made a preliminary test on this, which proved that the experiment was promising. Marsden, who had already worked at the University of London, was invited back to undertake this task. Marsden and Geiger carried out a year long continuous hard test,They finished this work in July 1912. They clearly pointed out at the beginning of the paper: "All the results we observed are in good agreement with Professor Rutherford's theoretical derivation, which provides strong evidence for the correctness of his central hypothesis that the atom has a strong charge and its size is much smaller than the atomic diameter."
The experimental device they used is shown in Figure 3, which mainly includes a solid cylindrical metal box B, the alpha particle beam emitted by the radioactive source R through the small hole D hits the scattering foil F, the microscope M is equipped with a zinc sulfide screen S, and the scattered alpha particles hit the S screen and flash, which can be seen from the microscope M. The bottom of the metal box B is fixed with the graduated disc A;Disk A can rotate around the axis with the help of the conical interface C of the sealing air. At this time, the metal box and the microscope rotate with it, while the scattering foil and the radioactive source do not move. They are connected to the tube T and fixed together with the support L. The upper end face of the metal box B is sealed with a frosted glass plate P, which can be evacuated through the tube T[1]。
application
Announce
edit
Now, using the scattering principle and technology accumulated over the years,Rutherford backscattering spectroscopyIt can detect the heavy metal impurities in the semiconductor.In fact, this technology is also the first field analysis technology used on the moon.staysurveyor mission After landing on the moon surface, Rutherford backscattering spectroscopy experiment was used to collect geological data.
