rutherford backscattering experiment explained

Rutherford Backscattering

Physical principles.

Rutherford Backscattering (RBS) primarily provides a profile of concentration versus depth for heavy elements in a light material, e.g. titanium in alumina.

Typically, a beam of 2-3 MeV He + ions is directed perpendicularly on the sample's surface. As energetic ion penetrates the material, it loses energy mainly in collisions with electrons and only occasionally with nuclei.

When the positively charged He + ion comes close to the nucleus of an atom, it will be repelled by the positively charged nucleus. The repulsion force is increasing with the mass of the target atom. For very heavy atoms such as lead or gold, the He+ ion can be repelled backwards with nearly the same energy as it had before the collision.

By measuring the energy spectrum of the recoiled ions, information on the composition of the elements, and their depth within the sample can be obtained.

Figure 1: Energy spectrum of ions scattered from surface atoms (full curve) and from atoms in a thin layer (dashed curve).

In RBS, only backscattered ions are detected, and backscattering can only occur if the target atom's mass is heavier than that of the incident ion. Conventional RBS is done with 4 He ions.

There are however several advantages using heavier ions such as 12 C, 16 O, 28 Si or 35 Cl, where, for example, a better mass resolution is required for heavy elements or to eliminate large amounts of backscatter from oxygen when studying ceramic oxides. With its tandem accelerator, ANSTO has both conventional and heavy-ion backscattering capabilities.

Applications

RBS has been used to study the degradation of surfaces due to weathering of the protective coating (polymer-based bronze paint) on steel surfaces. Figure 2a) below shows an RBS spectrum of a coated steel sample, measured with 2 MeV 4 He.

The experimental spectrum is shown with a dotted line, while the solid line shows the fit used to derive the elemental profiles.

The spectrum exhibits a typical staircase structure with each step associated with an element in the sample. The flat part between different steps indicates a constant concentration for each element throughout the surface.

Figure 2 a) RBS spectrum of a freshly coated metal surface. b) shows a spectrum of the same sample after 1000 hours exposure to ultraviolet light. Figure 2b) shows a spectrum of the same sample after 1000 hours exposure to ultraviolet light. It can be seen that the composition has changed dramatically. The individual profiles now show an increase of iron (Fe) and titanium (Ti) at the surface,while the carbon (C) content at the surface has dramatically decreased.

Figure 2 also shows the calculated profiles for each element in the coating. In each individual profile, the step where the concentration changes from zero to a constant value indicates the sample surface. The experimental curve is the sum of the individual profiles. This indicates that the light has broken down the structure of the polymer film, which subsequently allowed the different components of the polymer to diffuse.

Also, large amounts of Ti and Fe from colour pigments have migrated to the surface. Measurements like these help to understand the processes involved in the degradation of polymer films and thus help to make more weather resistant polymer coatings.

In addition to typical RBS applications, such as thin-film analysis, RBS is often used in conjunction with PIXE and PIGE to determine concentrations of low Z elements such as C, O, N

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Rutherford back scattering.

November 9, 2016 Alpha Spectroscopy , English Posts 8,663 Views

Rutherford scattering is the elastic scattering of charged particles by the Coulomb interaction. It is a physical phenomenon explained by Ernest Rutherford in 1911 that led to the development of the planetary Rutherford model of the atom and eventually the Bohr model. It is now exploited by the materials analytical technique Rutherford backscattering. Rutherford scattering was first referred to as Coulomb scattering because it relies only upon static electric (Coulomb) forces, and the minimal distance between particles is set only by this potential. The classical Rutherford scattering of alpha particles against gold nuclei is an example of “elastic scattering” because the energy and velocity of the outgoing scattered particle is the same as that with which it began.

The initial discovery was made by Hans Geiger and Ernest Marsden in 1909 when they performed the gold foil experiment in collaboration with Rutherford, in which they fired a beam of alpha particles (helium nuclei) at layers of gold leaf only a few atoms thick. At the time of the experiment, the atom was thought to be analogous to a plum pudding (as proposed by J. J. Thomson), with the negative charges (the plums) found throughout a positive sphere (the pudding). If the plum-pudding model were correct, the positive “pudding”, being more spread out than in the current model of a concentrated nucleus, would not be able to exert such large columbic forces, and the alpha particles should only be deflected by small angles as they pass through.

However, the intriguing results showed that around 1 in 8000 alpha particles were deflected by very large angles (over 90°), while the rest passed straight through with little or no deflection. From this, Rutherford concluded that the majority of the mass was concentrated in a minute, positively charged region (the nucleus/ central charge) surrounded by electrons. When a (positive) alpha particle approached sufficiently close to the nucleus, it was repelled strongly enough to rebound at high angles. The small size of the nucleus explained the small number of alpha particles that were repelled in this way. Rutherford showed, using the method below, that the size of the nucleus was less than about 10 −14 m (how much less than this size, Rutherford could not tell from this experiment alone; see more below on this problem of lowest possible size).

The image below shows the layout of the famous Rutherford – Geiger – Marsden experiment on the scattering of alpha particles by a thin gold foil.

With the DIY alpha spectrometer we tried to replicate, in a qualitative way, the famous Rutherford experiment on the scattering of alpha particle. In particular we have tried to make the detection of back scattering, that is, those alpha particles that are spread to angles greater than 90 °, in practice bounce backwards. In the image below is presented the experiment setup: the alpha source of americium, collimated through a wooden screen, is placed in the upper part of the vacuum chamber and is directed toward the target, in our case a lead plate, the target is placed directly below the detector.

In the picture below you see the basic diagram of the experimental setup.

The alpha spectrometer was run for about an hour, and it has obtained the following spectrum:

In the energy range from 2,000 KeV to 5500 KeV we note about 30 pulses which can be attributed with reasonable certainty to the same number of back scattering events  originated from the lead target, the energy variation depends on the depth within the target where the alpha particle has been scattered. There is then a maximum of events around the energy of 500 keV, the interpretation of which has yet to be analyzed.

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Rutherford Backscattering Spectrometry

• First Online: 11 August 2023

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Part of the book series: Lecture Notes in Nanoscale Science and Technology ((LNNST,volume 23))

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The origin of Rutherford backscattering spectrometry (RBS) constitutes one of a handful of paradigm breaking physics experiments done in the early twentieth century that paved the way to modern physics and our conception of matter. From fundamental research in nuclear physics, RBS developed into a very useful material characterization technique for investigation of thin film composition, depth profiling of impurities, and thickness measurement in the nanometer range. This chapter introduces the foundation for the technique and how it is applied in modern materials analysis.

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Raman Scattering

Study of Multilayer Thin Film Structures by Rutherford Backscattering Spectrometry

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Hallén, A. (2023). Rutherford Backscattering Spectrometry. In: Analytical Methods and Instruments for Micro- and Nanomaterials. Lecture Notes in Nanoscale Science and Technology, vol 23. Springer, Cham. https://doi.org/10.1007/978-3-031-26434-4_5

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Rutherford Backscattering Spectrometry (RBS)

Home » Our Techniques » Spectroscopy » RBS

Rutherford Backscattering Spectrometry (RBS) is an ion scattering technique used for compositional thin film analysis.  RBS is unique in that it allows quantification without the use of reference standards. During an RBS analysis, high-energy (MeV) He 2+  ions (i.e. alpha particles) are directed onto the sample, and the energy distribution and yield of the backscattered He 2+  ions at a given angle is measured. Since the backscattering cross section for each element is known, it is possible to obtain a quantitative compositional depth profile from the RBS spectrum obtained for films that are less than 1 μm thick.

In addition to elemental composition, RBS can be used to obtain information on the crystalline quality of single crystal samples.  This technique, called “channeling,” can probe the degree of damage in a crystal or determine the amount of substitutional or interstitial species in a lattice.

EAG has world-class experience in analyzing thin films using Rutherford Backscattering Spectrometry, with both pelletron and tandetron instrumentation.  EAG’s experience in the analysis of all types of   thin films  (oxides, nitrides, silicides, high and low-K dielectrics, metal films, compound  semiconductors  and dopants) enables fast turnaround times, accurate data, and high quality person-to-person service.

Ideal Uses of Rutherford Backscattering Spectrometry

• Thin film composition/thickness
• Determine areal concentrations (atoms/cm 2 )
• Determine film density (when thickness is known)
• Non-destructive compositional analysis
• Quantitative without standards
• Conductor and insulator analysis
• Hydrogen measurements (in HFS mode)
• Low-Z element sensitivity (in NRA mode)

Limitations

• Large analysis area (~2 mm)
• Useful information limited to top ~1 μm of samples

Rutherford Backscattering Spectrometry Technical Specifications

• Signal Detected:  Backscattered He atoms
• Elements Detected:  B-U
• Detection Limits:  0.001-10 at%
• Depth Resolution:  100-200 Å
• Imaging/Mapping:  No
• Lateral Resolution/Probe Size:  ≥ 2 mm

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• Rutherford Scattering

Nowadays, the concept that matter is made of small entities called atoms is widely accepted, which seems very natural to us. However, the description of atoms has changed significantly over history. Ernest Rutherford was the first scientist who attempted to carry out experiments to gain insight into building a well-founded atomic model . He explained the physical phenomenon known as Rutherford scattering .

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Rutherford scattering is a type of experiment based on the scattering of particles due to electric interactions with the atoms of a foil.

In the fifth century BC in Ancient Greece, a Greek philosopher named Democritus proposed that matter was made of indivisible entities, which he termed “atoms”. However, his ideas were not accepted, and other models based on universal elements (water, fire) or similar non-scientific features prevailed.

With the advancements of chemistry during the seventeenth century, an English scientist named John Dalton recovered the idea of atoms as basic constituents of matter and developed a set of properties that atoms should have. Although he did not provide further information on the structure of atoms, he paved the way for chemists and physicists to study the different properties of elements and the existence of subatomic particles , such as electrons and protons.

In particular, J.J. Thomson discovered electrons in 1897, and the existence of protons was found shortly after. It was Thomson who proposed that matter is made of atoms. He offered what was known as the “plum pudding model”, which attempted to explain two known properties of atoms. Rutherford came up with an experimental setup (along with Hans Geiger and Ernest Marsden) to investigate this theory further, which is now known as the Rutherford scattering experiment or gold foil experiment.

What is the Rutherford scattering experiment?

The Rutherford scattering experiment was designed to prove Thomson’s model of the atom. Since Thomson proposed a model where matter was made out of atoms that would fill almost all the space available with a positive charge, the goal of Rutherford's experiment was to fire positively-charged particles to study the scattering and test the model.

Rutherford scattering diagram and description

This is what Rutherford used in the experiment:

• A gold foil . For this experiment, Rutherford used a very thin gold foil. (Since gold is very malleable, it is possible to reduce its thickness to 0.00004cm.)
• A beam of alpha particles. The alpha particles were the nuclei of helium (two protons and two neutrons), which, back in the 1910s, were known to have only a positive charge. Given that Rutherford wanted to test the structure of atoms, he considered small positively charged particles he could fire at the gold foil. This is because according to Thomson ’ s model, atoms were relatively big, positively charged bodies with electrons embedded in them.
• A screen detecting the alpha particles. The screen's purpose was to measure what happened to the alpha particles (their point of impact) after being scattered by the gold foil.

Below is a diagram of the Rutherford scattering experiment:

What were the results of the Rutherford scattering experiment?

The mechanism in the experiment is relatively simple. By firing alpha particles against the gold foil and detecting where they end up, we can extract important conclusions about the atomic structure of the gold ’ s atom. When conducting this experiment, it is important to reduce the thickness of the foil as much as possible because this prevents multiple scattering events that could compromise the conclusions of the experiment. Ideally, each alpha particle is supposed to interact with only one gold atom.

Conclusions from the experiment

When Rutherford did the experiment, he expected to detect most of the alpha particles on the side closer to the alpha emitter. This was because Thomson's model was based on large chunks of positive charge that would repel the alpha particles (backwards). However, the experiment yielded the opposite results. M ost alpha particles travelled through the gold foil and were not scattered , with a few particles scattering slightly. Only a small number of particles were scattered strongly, and Rutherford observed that the bigger the scattering angle was, the lower the number of alpha particles.

This is what the experiment concluded:

• Matter is almost empty . Since most of the alpha particles did not deviate from their original trajectory, Rutherford deduced that the spaces between atoms are big compared to the size of atoms themselves.
• Since only a few particles were strongly scattered (repelled) by the gold atoms, Rutherford concluded that atoms must have a part where the positive charge is concentrated . He called this part the nucleus .
• Since the fraction of alpha particles that were strongly scattered was very small, Rutherford also deduced that the size of the nucleus has to be very small . He based this conclusion on the electric properties of charged bodies and the forces they exert on others.
• After several scattering experiments, Rutherford created a theoretical model in which most of the atom ’ s mass was concentrated in the nucleus . Since his observations fitted the predictions of this model, he was able to deduce this feature of atoms.

In addition, Rutherford reached the following conclusion by using previous knowledge and the results of his experiments:

• Electrons orbit the nucleus . Since atoms are neutrally charged, electrons must balance the positive charge of the nucleus. Also, the electrons cannot be too close to the nucleus, otherwise they would not scatter alpha particles. Since they cannot be close to it, electrons are forced to orbit the nucleus (and are thus moving). If they didn ’ t orbit the nucleus, they would fall towards the nucleus due to electrostatic attraction.

Problems with the experiment

These characteristics were very different from the ones of the Thomson atomic model , and Rutherford ’ s model was the first atomic model fully based on experimental evidence. However, problems with both the experimental method and the model itself needed to be solved. Here are some of those problems:

• The thickness of the foil could affect the scattering pattern significantly, therefore offering biased conclusions.
• After Einstein developed his Theory of Relativity, it was discovered that charges in movement continuously radiate energy . If electrons orbit the nucleus, how can they not lose all their energy due to radiation and fall towards the nucleus?

The first issue was irrelevant: the scales of the thickness of the foils used were enough to obtain reliable scattering results. However, the second issue raised a lot of concerns, which were later solved with the introduction of quantum physics , the Bohr atomic model, and the quantum atomic model.

There is another issue with the Rutherford scattering experiment that was not known back then. Since the existence of protons was intuited but not known, the models provided no further structure of the nucleus apart from charge and mass considerations. Later, it was discovered that subatomic particles called protons carry a positive electric charge.

Due to electric repulsion among protons, a type of particle was predicted to shield their interactions in the nucleus. This particle is the neutron. The shielding of the interaction had to be because of another force between neutrons and protons (what we now know as strong force).

This force plays a role in scattering alpha particles because alpha particles are themselves made of neutrons and protons. However, until the model of this force was fully established, it was not known that most of the effects observed in Rutherford scattering are actually due to the electric force and not the strong force .

The force causing the scattering is the electric force of repulsion between gold nuclei and alpha particles. However, we must still consider the usually irrelevant (but present) effect of the strong force.

Rutherford Scattering - Key takeaways

• Rutherford conducted a series of experiments of scattering to obtain experimental evidence on the characteristics of an atomic model.
• The experiment was based on the scattering of alpha particles due to the presence of a gold foil. The scattering was produced by the electrostatic interaction between alpha particles and gold nuclei.
• The results of the experiments contradicted the atomic model developed by Thomson and yielded the existence of a small nucleus.
• The model developed by Rutherford had some inconsistencies that were solved afterwards thanks to quantum physics . Nevertheless, it was an excellent atomic model that was fully based on experimental evidence for the first time in history.

Flashcards in Rutherford Scattering 5

Throughout history, the idea of what matter is made of has changed significantly.

A thin gold foil, a beam of alpha particles, and a screen.

Rutherford deduced from the results of his experiments that matter is almost empty.

Quantum physics solved the issue of the loss of energy of electrons due to radiation.

Rutherford expected most of the alpha particles to bounce back when encountering the gold foil.

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Frequently Asked Questions about Rutherford Scattering

What is Rutherford scattering?

Rutherford scattering is a type of experiment that is based on the scattering of particles due to electric interactions with the atoms of a foil. 

How did the alpha particles scatter in Rutherford’s experiment?

Most alpha particles in Rutherford’s experiment travelled through the gold foil and were not scattered, with a few particles scattering slightly. Only a small number of particles were scattered strongly, and Rutherford observed that the bigger the scattering angle was, the lower the number of alpha particles. 

How did Rutherford scattering contribute to physics?

Rutherford’s scattering experiment showed that matter is almost empty and that the positive charge and most of the mass of atoms are concentrated in a small region called the nucleus.

What did Rutherford’s scattering experiment show?

What force is responsible for Rutherford scattering?

The force causing the scattering is the electric force of repulsion between gold nuclei and alpha particles. However, one has to take into account the usually irrelevant (but present) effect of the strong force

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