What are alpha scattering experiments used for?

Rutherford spreading test

Here you will learn what the Rutherford spreading test is all about and how to interpret it. You will also get to know the strengths and weaknesses of Rutherford's atomic model.

It's best to have a look at our video. We have already prepared the most important points for you audiovisually.

  • Rutherford spreading test simply explained
    in the text
  • Rutherford scattering test setup and observation
    in the text
  • Interpretation of the Rutherford scattering test
    in the text
  • in the text
  • in the text
  • Loopholes in the Rutherford atomic model
    in the text

Rutherford spreading test simply explained

In the Rutherford scattering test, an alpha beam is directed onto a gold foil and the reflection of the alpha particles is observed. You will notice that most of the alpha particles penetrate the film and only a few are reflected.

At the beginning of the 19th century, the New Zealand physicist Ernest Rutherford, together with his two colleagues Hans Geiger and Ernest Marsden, wanted to experimentally test the atomic model devised by Joseph John Thomson. Accordingly, the theory was that an atom consists of a homogeneous positive mass in which the electrons are evenly distributed. A substance would then have consisted of tightly packed “atomic spheres”.

The three physicists tested the theory with the Rutherford scattering test. The observations contrasted with Thomson's theory, which predicted that most of the alpha particles would reflect off the foil and that hardly any penetration would take place.

This served as a motivation to formulate the theory of Rutherford's atomic model. According to this, most of the mass of the atom is concentrated in a small positive nucleus, which is orbited by the electrons. Hence an atom is mostly "empty".

Rutherford scattering test setup and observation

The three physicists tested Thomson's theory by building an experiment in which they observed the reflection behavior of alpha particles on a gold foil - the Rutherford scattering test. To do this, they filled a container made of lead with radioactive atoms. This has a small opening on one side through which the radiation can escape. Since most of the decays not only produce alpha radiation, but also beta radiation and gamma radiation, you guide the deflected beam through an electric field. The negative beta particles then fly to the positive cathode while the positive alpha particles drive the negative anode. The uncharged gamma quanta cross the field unaffected.

In this way they filtered the beam for the alpha radiation and aimed it specifically at the gold foil. The approximately 0.5 m (micro-meter) thick gold foil is surrounded by a fluorescent screen. Alpha particles that hit this screen leave visible points and create a small flash of light. The screen itself also has a small opening to allow the beam to pass through to the gold foil.

Rutherford and his colleagues expected that most of the alpha particles would be reflected off the gold foil. At the same time, only a few or no alpha particles should reach the fluorescent screen on the other side.
Contrary to this expectation, however, it was observed that most of the alpha particles penetrated the film and only about every ten thousandth particle was deflected. It could be observed that the deflection became rarer the larger the deflection angle was. Therefore, only a few alpha particles were reflected back exactly against the direction of the radiation.

We have already prepared the corresponding videos for the different types of radiation. It is best to have a look at the videos on alpha radiation, beta radiation and gamma radiation.

Interpretation of the Rutherford scattering test

The observations on the Rutherford scattering experiment refuted Thomson's atomic model. So a new interpretation was needed.
Rutherford was able to calculate the actual structure of the atom based on the observations and the angular distributions of the reflected particles. This enabled him to determine that the mass of the atom was concentrated in a small positive nucleus. The size of the atom is determined by its shell. This shell consists for the most part of free space, filled with small and light electrons. The negative charge of the electrons shields the positive atomic nucleus. Thus the atom has an electrically neutral effect on the outside. This is Rutherford's atomic model.

This atomic structure fully explains the observations. Since most of the atoms consist of free space, most of the alpha particles pass through the gold foil unhindered. If the positive alpha particles come close to the positive nuclei, a distraction occurs. The closer the alpha particles come to an atomic nucleus, the greater the scattering angle. However, the probability that a radiation particle will hit a nucleus directly is correspondingly low. In these rare cases, however, a 180 ° reflection can be observed.

Rutherford spreading formula

Rutherford derived the formula named after him from his observations on the experiment. It gives the differential cross section as a function of the scattering angle (Theta) on.

The symbols have the following meanings:

: Dielectric constant with
: Charge of the scattered particle

: Charge of the atomic nucleus
: The elementary charge with
: Initial energy of the scattered particle

With this formula you determine the probability with the one around the angle scattered particles in the solid angle hit.

Rutherford atomic model

The Thomson atomic model could be refuted by the Rutherford scattering experiment. With the help of the scattering formula, Rutherford's atomic model could then be described.

Accordingly, the large electric field strength that is necessary to deflect alpha particles can only be achieved if all of the positive charge is concentrated in a small point. This corresponds to the atomic nucleus you know. In this almost the entire mass and the positive charge of the atom are united. Compared to the atomic radius, this nucleus is about 3000 times smaller. This means that an atom consists mostly of empty space.
Only light and small electrons fill this empty space around the atomic nucleus. The number of these corresponds to the atomic number of the atom. The electrons in the atomic shell thus shield the positive charge of the nucleus and make the atom appear electrically neutral to the outside world. Rutherford assumed that the electrons orbit the nucleus, much like planets orbit the sun.

Loopholes in the Rutherford atomic model

Although a lot can be explained with Rutherford's atomic model, it quickly reaches its limits. So it is not possible to describe emissions and absorptions of energy quanta. But this is necessary in order to be able to understand how the spectral lines of various gases arise. Furthermore, it is not possible to use Rutherford's atomic model to explain why the electrons do not fall into the nucleus. Charged particles on a circular orbit, constantly give off energy and should therefore emit a continuous spectrum. However, this cannot be observed and, due to quantized energy levels, it is also not possible.
Therefore, the Rutherford atomic model was later replaced by the Bohr atomic model and then the shell model.