Rutherford Scattering Experiment
- Updated by Scienly
- On September 10, 2024
In this chapter, we will understand Rutherford’s scattering experiment and its observations and conclusions. In order to understand the arrangement of charged particles like electrons and protons in an atom, the British scientist, Ernest Rutherford and his co-workers in 1911 carried out a series of experiments using alpha particles known as Rutherford’s alpha scattering experiments. This experiment is based on the alpha particles (helium nuclei) experiment.
The Rutherford’s alpha particle scattering experiment is one of the most important experiments in the history of atomic physics. It laid the foundation for the modern understanding of the atom, challenging earlier models and introducing the concept of an atomic nucleus.
Before Rutherford’s experiment, J. J. Thomson had given the first model of an atom in 1904, also known as “plum pudding model of atom”. In this model, he proposed that an atom may be considered as a sphere of positively charge particles in which the negatively charged particles called electrons are embedded to make the atom as a whole neutral.
This model was discarded in 1911 on both theoretical and experimental due to the Rutherford’s atomic model. Now let’s understand Rutherford scattering experiment step by step.
Rutherford’s Alpha Particle Scattering Experiment
In this experiment, Ernest Rutherford bombarded a beam of alpha particles with a very thin gold foil. The thickness of gold foil is approximately 0.00006 cm. The alpha particles were emitted from the radioactive substances such as radium or polonium. An alpha particle is a doubly ionized helium atoms or ions with two units of positive charge, mass number is equal to 4 and missing two electrons. It is represented as +2 He 4 or +2 α 4 .
The pictorial representation of Rutherford’s scattering experiment is shown in the below figure.
From the above figure, it is clear that he produced a narrow beam of alpha particles from a radioactive substance like radium placed in a lead block and then passed it through a thin sheet of gold foil. A movable circular screen coated with Zinc Sulphide (ZnS) screen is placed around the gold foil in order to detect alpha particles after scattering.
When these alpha particles hit the screen, it produced a tiny flash of light (scintillation) on the screen, which could be observed through movable a microscope.
Observations from Alpha Particle Scattering Experiment
The following observations were made from the alpha particle scattering experiment:
- Most of the alpha particles (nearly 99%) passed through the gold foil with little to no deflection. In other words, most of the alpha particle moved in the straight path with no deflection. This means that most of the space inside the atom is empty, as shown in the above figure.
- Some of the alpha particles deflected with small angle, meaning that the positive charge of an atom occupies a tiny space.
- A very few alpha particles (approx. 1 in 20,000) deflected back through the deflection of 180 degrees. This means that the whole positive charge and mass of the atom is concentrated in a very small volume inside the atom.
Thomson’s atomic model could not explain these observations. According to Thomson’s model of atom, positive charge and mass inside an atom are uniformly distributed throughout its volume.
Conclusion from Rutherford Scattering Experiment
After performing a series of scattering experiments, Rutherford concluded that:
- Most of the space inside an atom is empty because most of the alpha particles passed through the gold foil undeflected.
- There must be the presence of a heavy positively charged mass at the center of an atom because some of the alpha particles are deflected by a certain angle.
- A heavy positively charged mass at the center of the atom is very small, which he named as nucleus.
On the basis of these observations, Sir Ernest Rutherford proposed the nuclear model of atom, which you will study in the next chapter.
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The rutherford-geiger-marsden experiment.
April 11, 2017 Alpha Spectroscopy , English Posts 89,256 Views
What made by Rutherford and his assistants Geiger and Marsden is perhaps one of the most important experiments of nuclear physics.
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.
In the experiment, Rutherford sent a beam of alpha particles (helium nuclei) emitted from a radioactive source against a thin gold foil (the thickness of about 0.0004 mm, corresponding to about 1000 atoms).
Surrounding the gold foil it was placed a zinc sulfide screen that would show a small flash of light when hit by a scattered alpha particle. The idea was to determine the structure of the atom and understand if it were what supposed by Thomson (atom without a nucleus, also known as pudding model ) or if there was something different.
In particular, if the atom had an internal nucleus separated from external electrons, then they would have been able to observe events, or particles, with large angle of deviation . Obtained, actually, these results, the New Zealand physicist concluded that the atom was formed by a small and compact nucleus , but with high charge density, surrounded by an electron cloud. In the image below it is depicted the interaction of the alpha particles beam with the nuclei of the thin gold foil; one can see how the majority of the particles passes undisturbed, or with small angles of deflection, through the “empty” atom, some particles, however, passing close to the nucleus are diverted with a high angle or even bounced backwards.
The interaction between an alpha particle and the nucleus (elastic collision) is also known as Coulomb scattering , because the interaction in the collision is due to the Coulomb force. In the diagram below it is shown the detail of the interaction between an alpha particle and the nucleus of an atom.
Experimental Setup
In the PhysicsOpenLab “laboratory” we tried to replicate the famous Rutherford experiment. With the equipment already used in alpha spectroscopy we built a setup based on an alpha solid-state detector , a 0.9 μCi Am 241 source and a gold foil as a scatterer. In these post we describe the equipment used : Alpha Spectrometer , Gold Leaf Thickness . The main purpose is not to make precision measurements but to make a qualitative assessment of the scattering as a function of deflection. The images below show the experimental setup:
The alpha source is actually 0.9 μCi of Am 241 (from smoke detector) which emits alpha particles with energy of 5.4 MeV. The alpha particle beam is collimated by a simple hole in a wooden screen. Source and collimator are fixed on a arm free to rotate around a pivot, which hosts the gold foil that acts as a scatterer. The whole is placed inside a sealed box that acts as a vacuum chamber with the help of an ordinary oil rotary vacuum pump. The images below show the “vacuum chamber” and the electronic part for amplification and acquisition connected to the PC for counting events.
Linear Scale :
Semilog Scale
The results obtained in our experiment approach, albeit with obvious limitations, to the expected theoretical results, represented in the following graph:
For completeness, we report also at the side the formula that describes the distribution of the number of the counted particles in function of the scattering angle. Interestingly, this depends on the power of two the atomic number of the target and is inversely proportional to the fourth power of the sin (θ/2).
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Tags Alpha spectrometer Rutherford
Detection of beta and alfa radiation with KC761B
Abstract: in this article, we continue the presentation of the new KC761B device. In previous posts, we described the device in general terms and its functionality as a gamma spectrometer. In this post, we describe its use as a beta and alpha radiation detector. To detect beta and alpha particles, the device uses a PIN-type semiconductor sensor positioned on the back of the device.
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A replica of an apparatus used by Geiger and Marsden to measure alpha particle scattering in a 1913 experiment. The Rutherford scattering experiments were a landmark series of experiments by which scientists learned that every atom has a nucleus where all of its positive charge and most of its mass is concentrated. They deduced this after measuring how an alpha particle beam is scattered when ...
How did Rutherford figure out the structure of the atom without being able to see it? Simulate the famous experiment in which he disproved the Plum Pudding model of the atom by observing alpha particles bouncing off atoms and determining that they must have a small core.
The nucleus was postulated as small and dense to account for the scattering of alpha particles from thin gold foil, as observed in a series of experiments performed by undergraduate Ernest Marsden under the direction of Rutherford and German physicist Hans Geiger in 1909. A radioactive source emitting alpha particles (i.e., positively charged particles, identical to the helium atom nucleus and ...
The Rutherford Scattering Experiment Tony Tyson, Maxwell Chertok, Chris Brainerd, Joseph Levine March 17, 2023 1 Introduction The foundations of modern ideas about atomic structure are considered to have been laid by Sir Ernest Rutherford in 1911, with his postulates concerning the scattering of alpha particles by atoms.
The Scattering of α and β Particles by Matter and the Structure of the Atom E. Rutherford, F.R.S.* Philosophical Magazine Series 6, vol. 21 May 1911, p. 669-688. ... The form of experiment is not very suited for accurate calculation, ...
This experiment is based on the alpha particles (helium nuclei) experiment. The Rutherford's alpha particle scattering experiment is one of the most important experiments in the history of atomic physics. It laid the foundation for the modern understanding of the atom, challenging earlier models and introducing the concept of an atomic nucleus.
The scattering foil is an annulus located coaxially with the -source and detector with inner and outer diameters, 46.0 and 56.7 mm respectively. The angle is determined by a fixed distance from source to scattering foil. The scattering angle is varied by changing the distance from the scattering plane to the plane of the detector.
In the experiment, Rutherford sent a beam of alpha particles (helium nuclei) emitted from a radioactive source against a thin gold foil (the thickness of about 0.0004 mm, corresponding to about 1000 atoms). Surrounding the gold foil it was placed a zinc sulfide screen that would show a small flash of light when hit by a scattered alpha particle.
Rutherford Scattering Formula The scattering of alpha particles from nuclei can be modeled from the Coulomb force and treated as an orbit. The scattering process can be treated statistically in terms of the cross-section for interaction with a nucleus which is considered to be a point charge Ze. For a detector at a specific angle with respect to the incident beam, the number of particles per ...
Simulate Rutherford's experiment to understand atomic structure by observing alpha particles bouncing off atoms and determining they must have a small core.