Rutherford scattering experiments explained

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 it strikes a thin metal foil. The experiments were performed between 1906 and 1913 by Hans Geiger and Ernest Marsden under the direction of Ernest Rutherford at the Physical Laboratories of the University of Manchester.

The physical phenomenon was explained by Ernest Rutherford in a classic 1911 paper that eventually lead to the widespread use of scattering in particle physics to study subatomic matter. Rutherford scattering or Coulomb scattering is the elastic scattering of charged particles by the Coulomb interaction. The paper also initiated the development of the planetary Rutherford model of the atom and eventually the Bohr model.

Rutherford scattering is now exploited by the materials science community in an analytical technique called Rutherford backscattering.

Summary

Thomson's model of the atom

See main article: Plum pudding model. The prevailing model of atomic structure before Rutherford's experiments was devised by J. J. Thomson. Thomson had discovered the electron through his work on cathode rays^[1] and between 1897 and 1904 he developed a model for atoms containing electrons arranged in concentric shells. To explain why atoms are electrically neutral, he proposed the existence of a commensurate amount of positive charge that balanced the negative charge of the electrons. Having no idea what the source of this positive charge was, he tentatively proposed that the positive charge was everywhere in the atom, adopting a spherical shape for simplicity.^[2] Thomson thought of the positive sphere as being akin to a liquid in that the electrons could move about in it, their arrangement and movements being determined by the balance of electrostatic forces.^[3]

Thomson was not quite satisfied with this simplistic idea and hoped to dispense with it as he refined his model. Thomson was never able to develop a complete and stable model that could predict any of the other known properties of the atom, such as emission spectra and valencies.^[4] The Japanese scientist Hantaro Nagaoka rejected Thomson's model on the grounds that opposing charges cannot penetrate each other.^[5] He proposed instead that electrons orbit the positive charge like the rings around Saturn.^[6] However this model was also known to be unstable.

Alpha particles and the Thomson atom

An alpha particle is a positively-charged particle of matter that is spontaneously emitted from certain radioactive elements. Alpha particles are so tiny as to be invisible, but they can be detected with the use of phosphorescent screens, photographic plates, or electrodes. Rutherford discovered them in 1899.^[7] In 1906, by studying how alpha particle beams are deflected by magnetic and electric fields, he deduced that they were essentially helium atoms stripped of their electrons because they had the same charge-to-mass ratio and atomic weight.^[8] Thomson and Rutherford knew nothing about the internal structure of alpha particles; prior to 1911 they were thought to have a diameter similar to helium atoms.

Thomson's model was consistent with the experimental evidence available at the time. Thomson studied beta particle scattering which showed small angle deflections modeled as interaction of the particle with many atoms in succession. Each interaction of the particle with the electrons of the atom and the positive background sphere would lead to a tiny deflection, but many such collisions could add up. The scattering of alpha particles was expected to be similar. Rutherford's team would show that the multiple scattering model was not needed: single scattering from a compact charge at the center of the atom would account for all of the scattering data.

Rutherford, Geiger, and Marsden

Ernest Rutherford was Langworthy Professor of Physics at the Victoria University of Manchester^[9] (now the University of Manchester). He had already received numerous honours for his studies of radiation. He had discovered the existence of alpha rays, beta rays, and gamma rays, and had proved that these were the 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 with his research. Ernest Marsden was a physics undergraduate student studying under Geiger.^[10]

In 1908, Rutherford sought to independently determine the charge and mass of alpha particles. To do this, he wanted to count the number of alpha particles and measure their total charge; the ratio would give the charge of a single alpha particle. Alpha particles are so tiny as to be individually invisible, but Rutherford knew from work by J S Townsend in 1902 that alpha particles ionize air molecules, and if the air is within a strong electric field, each ion will produce a cascade of ions giving a pulse of electric current. On this principle, Rutherford and Geiger designed a simple counting device which consisted of two electrodes in a glass tube. (See

1. 1908 experiment

.) Every alpha particle that passed through the tube would create a pulse of electricity that could be counted. It was an early version of the Geiger counter.^[11]

The counter that Geiger and Rutherford built proved unreliable because the alpha particles were being too strongly deflected by their collisions with the molecules of air within the detection chamber. The highly variable trajectories of the alpha particles meant that they did not all generate the same number of ions as they passed through the gas, thus producing erratic readings. This puzzled Rutherford because he had thought that alpha particles were just too heavy to be deflected so strongly. Rutherford asked Geiger to investigate just how far matter could scatter alpha rays.^[12]

The experiments they designed involved bombarding a metal foil with a beam of alpha particles to observe how the foil scattered them in relation to its thickness and material. They used a phosphorescent screen to measure the trajectories of the particles. Each impact of an alpha particle on the screen produced a tiny flash of light. Geiger worked in a darkened lab for hours on end, counting these tiny scintillations using a microscope.^[13] For the metal foil, they tested a variety of metals, but they favored gold because they could make the foil very thin, as gold is the most malleable metal.^[14] As a source of alpha particles, Rutherford's substance of choice was radium, a substance thousands of times more radioactive than uranium.^[15]

Scattering theory and the new atomic model

They had discovered that the metal foils could scatter some alpha particles in all directions, sometimes more than 90°.^[16] This should have been impossible according to Thomson's model;^[16] it relied on electron scattering and the electrons are too light to turn the heavier alpha particle to the side. This forced Rutherford to revise the model of the atom. In Rutherford's new model, the positive sphere is at least 10,000 times smaller than what Thomson imagined: it does not fill the entire volume of the atom but instead a tiny nucleus, and is surrounded by a cloud of electrons that fills the greater volume of the atom. Rutherford proposed a compact nucleus that would allow scattering particles to approach close to the concentrated charge. As Rutherford related in later years:^[17]

To verify his model, Rutherford developed a scientific model to predict the intensity of alpha particles at the different angles they scattered coming out of the gold foil, assuming all of the positive charge was concentrated at the center of the atom. He also showed that Thomson's scattering model was not adequate to explain the observations of Geiger and Marsden. This work was published in his now famous^[11] 1911 paper "The Scattering of α and β Particles by Matter and the Structure of the Atom".

Legacy

There was little reaction to Rutherford's now-famous 1911 paper in the first years.^[18] The paper was primarily about alpha particle scattering in an era before particle scattering was a primary tool for physics. The probability techniques he used and confusing collection of observations involved were not immediately compelling.^[11]

Nuclear physics

See main article: Nuclear physics and Scattering. The first impacts were to encourage new focus on scattering experiments. For example the first results from a cloud chamber, by C.T.R. Wilson shows alpha particle scattering and also appeared in 1911.^{[19] [11]} Over time, particle scattering became a major aspect of theoretical and experimental physics; Rutherford's concept of a "cross-section" now dominates the descriptions of experimental particle physics.^[20] The historian Silvan S. Schweber suggests that Rutherford's approach marked the shift to viewing all interactions and measurements in physics as scattering processes.^[21] After the nucleus -- a term Rutherford introduces in 1912^[9] -- became the accepted model for the core of atoms, Rutherford's analysis of the scattering of alpha particles created a new branch of physics, nuclear physics.^[9]

Atomic model

See main article: Rutherford model and Rutherford–Bohr model. Even Rutherford's new atom model caused no stir. Rutherford explicitly ignores the electrons, only mentioning Hantaro Nagaoka's Saturnian model of electrons orbiting a tiny "sun", a model that had been previously rejected as mechanically unstable. By ignoring the electrons Rutherford also ignores any potential implications for atomic spectroscopy for chemistry.^[9] Rutherford himself did not press the case for his atomic model: his own 1913 book on "Radioactive substances and their radiations" only mentions the atom twice; other books by other authors around this time focus on Thomson's model.^[22] The impact of Rutherford's nuclear model came after Niels Bohr arrived as a post-doctoral student in Manchester at Rutherford's invitation. Bohr dropped his work on the Thomson model in favor of Rutherford's nuclear model, developing the Rutherford–Bohr model over the next several years. Eventually led Bohr to incorporates early ideas of quantum mechanics into the model of the atom, allowing prediction of electronic spectra and concepts of chemistry.^[11]

Hantaro Nagaoka, who had once proposed a Saturnian model of the atom, wrote to Rutherford from Tokyo in 1911: "I have been struck with the simpleness of the apparatus you employ and the brilliant results you obtain."^[23] The astronomer Arthur Eddington called Rutherford's discovery the most important scientific achievement since Democritus proposed the atom ages earlier. Rutherford has since been hailed as "the father of nuclear physics".^{[24] [25]}

In a lecture delivered on October 15, 1936 at Cambridge University,^{[26] [27]} Rutherford described his shock at the results of the 1909 experiment:

Rutherford's claim of surprise makes a good story but by the time of the Geiger-Mardsen experiment the result confirmed suspicions Rutherford developed from his many previous experiments.^[11]

The experiments

Alpha particle scattering: 1906 and 1908 experiments

Rutherford's first steps towards the his discovery of the nature of the atom come from his work to understand alpha particles.^[28] In 1906, Rutherford noticed that alpha particles passing through sheets of mica were deflected by the sheets by as much as 2 degrees. Rutherford placed a radioactive source in a sealed tube ending with a narrow slits followed by a photographic plate. Half of the slit was covered by a thin layer of mica. A magnetic field around the tube was altered every 10 minutes to reject the effect of beta rays, known to be sensitive to magnetic fields.^[29] The tube was evacuated to different amounts and a series of images recorded. At the lowest pressure the image of the open slit was clear, while images of the mica covered slit or the open slit at higher pressures was fuzzy. Rutherford explained these results as alpha-particle scattering^[11] in a paper published in 1906.^[30] He already understood the implications of the observation for models of atoms: "such a result brings out clearly the fact that the atoms of matter must be the seat of very intense electrical forces".^{[30] [31]}

A 1908 paper by Geiger, On the Scattering of α-Particles by Matter, describes the following experiment. He constructed a long glass tube, nearly two meters in length. At one end of the tube was a quantity of "radium emanation" (R) that served as a source of alpha particles.^[31] The opposite end of the tube was covered with a phosphorescent screen (Z). In the middle of the tube was a 0.9 mm-wide slit. The alpha particles from R passed through the slit and created a glowing patch of light on the screen. A microscope (M) was used to count the scintillations on the screen and measure their spread. Geiger pumped all the air out of the tube so that the alpha particles would be unobstructed, and they left a neat and tight image on the screen that corresponded to the shape of the slit. Geiger then allowed some air in the tube, and the glowing patch became more diffuse. Geiger then pumped out the air and placed one or two golds foils over the slit at AA. This too caused the patch of light on the screen to become more spread out, with the larger spread for two layers.^[31] This experiment demonstrated that both air and solid matter could markedly scatter alpha particles.^{[32] [31]}

Alpha particle reflection: the 1909 experiment

The results of the initial alpha particle scattering experiments were confusing. The angular spread of the particle on the screen varied greatly with the shape of the apparatus and its internal pressure. Rutherford suggested that Ernest Marsden, a physics undergraduate student studying under Geiger, should look for diffusely reflected or back-scattered alpha particles, even though these were not expected. Marsden's first crude reflector got results, so Geiger helped him create a more sophisticated apparatus. They were able to demonstrate that 1 in 8000 alpha particle collisions were diffuse reflections.^[31] Although this fraction was small, it was much larger than what the Thomson model of the atom could explain.^[11]

These results where published in a 1909 paper, On a Diffuse Reflection of the α-Particles, where Geiger and Marsden described the experiment by which they proved that alpha particles can indeed be scattered by more than 90°. In their experiment, they prepared a small conical glass tube (AB) containing "radium emanation" (radon), "radium A" (actual radium), and "radium C" (bismuth-214); its open end sealed with mica. This was their alpha particle emitter. They then set up a lead plate (P), behind which they placed a fluorescent screen (S). The tube was held on the opposite side of plate, such that the alpha particles it emitted could not directly strike the screen. They noticed a few scintillations on the screen because some alpha particles got around the plate by bouncing off air molecules. They then placed a metal foil (R) to the side of the lead plate. They tested with lead, gold, tin, aluminum, copper, silver, iron, and platinum. They pointed the tube at the foil to see if the alpha particles would bounce off it and strike the screen on the other side of the plate, and observed an increase in the number of scintillations on the screen. Counting the scintillations, they observed that metals with higher atomic mass, such as gold, reflected more alpha particles than lighter ones such as aluminium.^[31]

Geiger and Marsden then wanted to estimate the total number of alpha particles that were being reflected. The previous setup was unsuitable for doing this because the tube contained several radioactive substances (radium plus its decay products) and thus the alpha particles emitted had varying ranges, and because it was difficult for them to ascertain at what rate the tube was emitting alpha particles. This time, they placed a small quantity of radium C (bismuth-214) on the lead plate, which bounced off a platinum reflector (R) and onto the screen. They concluded that approximately 1 in 8,000 of the alpha particles that struck the reflector bounced onto the screen. By measuring the reflection from thin foils they showed that the effect due to a volume and not a surface effect.^[29] When contrasted with the vast number of alpha particles that pass unhindered through a metal foil, this small number of large angle reflections was a strange result^[20] that meant very large forces were involved.^[29]

Dependence on foil material and thickness: the 1910 experiment

A 1910 paper^[33] by Geiger, The Scattering of the α-Particles by Matter, describes an experiment by which he sought to measure how the most probable angle through which an alpha particle is deflected varies with the material it passes through, the thickness of said material, and the velocity of the alpha particles. He constructed an airtight glass tube from which the air was pumped out. At one end was a bulb (B) containing "radium emanation" (radon-222). By means of mercury, the radon in B was pumped up the narrow glass pipe whose end at A was plugged with mica. At the other end of the tube was a fluorescent zinc sulfide screen (S). The microscope which he used to count the scintillations on the screen was affixed to a vertical millimeter scale with a vernier, which allowed Geiger to precisely measure where the flashes of light appeared on the screen and thus calculate the particles' angles of deflection. The alpha particles emitted from A was narrowed to a beam by a small circular hole at D. Geiger placed a metal foil in the path of the rays at D and E to observe how the zone of flashes changed. He tested gold, tin, silver, copper, and aluminium. He could also vary the velocity of the alpha particles by placing extra sheets of mica or aluminium at A.^[33]

From the measurements he took, Geiger came to the following conclusions:^[16]

• the most probable angle of deflection increases with the thickness of the material
• the most probable angle of deflection is proportional to the atomic mass of the substance
• the most probable angle of deflection decreases with the velocity of the alpha particles

Rutherford's Structure of the Atom paper (1911)

Considering the results of the above experiments, Rutherford published a landmark paper in 1911 titled "The Scattering of α and β Particles by Matter and the Structure of the Atom" wherein he showed that single scattering from a very small and intense electric charge predicts primarily small-angle scattering with small but measurable amounts of backscattering.^{[20] [34]} For the purpose of his mathematical calculations he assumed this central charge was positive, but he admitted he could not prove this and that he had to wait for other experiments to develop his theory.^[34]

Rutherford developed a mathematical equation that modeled how the foil should scatter the alpha particles if all the positive charge and most of the atomic mass was concentrated in a point at the center of an atom.From the scattering data, Rutherford estimated the central charge q_n to be about +100 units.

Rutherford's paper does not discuss any electron arrangement beyond discussions on the scattering from JJ Thomson's plum pudding model and from Hantaro Nagaoka's Saturnian model.^[11] He shows that the scattering results predicted by Thomson's model are also explained by single scattering, but that Thomson's model does not explain large angle scattering. He says that Nagaoka's model, having a compact charge, would agree with the scattering data. The Saturnian model had previously been rejected on other grounds. The so-called Rutherford model of the atom with orbiting electrons was not proposed by Rutherford in the 1911 paper.^[11]

Confirming the scattering theory: the 1913 experiment

In a 1913 paper, The Laws of Deflexion of α Particles through Large Angles, Geiger and Marsden describe a series of experiments by which they sought to experimentally verify the above equation that Rutherford developed. Rutherford's equation predicted that the number of scintillations per minute s that will be observed at a given angle should be proportional to:^[16]

1. cosec⁴
2. thickness of foil t
3. magnitude of the square of central charge Q_n

Their 1913 paper describes four experiments by which they proved each of these four relationships.^[35]

To test how the scattering varied with the angle of deflection (i.e. if s ∝ csc⁴). Geiger and Marsden built an apparatus that consisted of a hollow metal cylinder mounted on a turntable. Inside the cylinder was a metal foil (F) and a radiation source containing radon (R), mounted on a detached column (T) which allowed the cylinder to rotate independently. The column was also a tube by which air was pumped out of the cylinder. A microscope (M) with its objective lens covered by a fluorescent zinc sulfide screen (S) penetrated the wall of the cylinder and pointed at the metal foil. They tested with silver and gold foils. By turning the table, the microscope could be moved a full circle around the foil, allowing Geiger to observe and count alpha particles deflected by up to 150°. Correcting for experimental error, Geiger and Marsden found that the number of alpha particles that are deflected by a given angle Φ is indeed proportional to csc⁴.

Geiger and Marsden then tested how the scattering varied with the thickness of the foil (i.e. if s ∝ t). They constructed a disc (S) with six holes drilled in it. The holes were covered with metal foil (F) of varying thickness, or none for control. This disc was then sealed in a brass ring (A) between two glass plates (B and C). The disc could be rotated by means of a rod (P) to bring each window in front of the alpha particle source (R). On the rear glass pane was a zinc sulfide screen (Z). Geiger and Marsden found that the number of scintillations that appeared on the zinc sulfide screen was indeed proportional to the thickness as long as said thickness was small.

Geiger and Marsden reused the above apparatus to measure how the scattering pattern varied with the square of the nuclear charge (i.e. if s ∝ Q_n²). Geiger and Marsden did not know what the positive charge of the nucleus of their metals were (they had only just discovered the nucleus existed at all), but they assumed it was proportional to the atomic weight, so they tested whether the scattering was proportional to the atomic weight squared. Geiger and Marsden covered the holes of the disc with foils of gold, tin, silver, copper, and aluminum. They measured each foil's stopping power by equating it to an equivalent thickness of air. They counted the number of scintillations per minute that each foil produced on the screen. They divided the number of scintillations per minute by the respective foil's air equivalent, then divided again by the square root of the atomic weight (Geiger and Marsden knew that for foils of equal stopping power, the number of atoms per unit area is proportional to the square root of the atomic weight). Thus, for each metal, Geiger and Marsden obtained the number of scintillations that a fixed number of atoms produce. For each metal, they then divided this number by the square of the atomic weight, and found that the ratios were more or less the same. Thus they proved that s ∝ Q_n².

Finally, Geiger and Marsden tested how the scattering varied with the velocity of the alpha particles (i.e. if s ∝). Using the same apparatus again, they slowed the alpha particles by placing extra sheets of mica in front of the alpha particle source. They found that, within the range of experimental error, that the number of scintillations was indeed proportional to .

Positive charge on nucleus: 1913

In his 1911 paper (see above), Rutherford assumed that the central charge of the atom was positive, but a negative charge would have fitted his scattering model just as well. In a 1913 paper, Rutherford declared that the "nucleus" (as he now called it) was indeed positively charged, based on the result of experiments exploring the scattering of alpha particles in various gases.

In 1917, Rutherford and his assistant William Kay began exploring the passage of alpha particles through gases such as hydrogen and nitrogen. In this experiment, they shot a beam of alpha particles through hydrogen, and they carefully placed their detector—a zinc sulfide screen—just beyond the range of the alpha particles, which were absorbed by the gas. They nonetheless picked up charged particles of some sort causing scintillations on the screen. Rutherford interpreted this as alpha particles knocking the hydrogen nuclei forwards in the direction of the beam, not backwards.

Rutherford's model for Coulomb scattering

Rutherford begins his 1911 paper^[34] with a discussion of Thomson's results on scattering of beta particles, a form of radioactivity that results in high velocity electrons. Thomson's model had electrons circulating inside of a sphere of positive charge. Rutherford highlights the need for compound or multiple scattering events: the deflections predicted for each collision are much less than one degree. He then proposes a model which will produce large deflections on a single encounter: place all of the positive charge at the center of the sphere and ignore the electron scattering as insignificant. The concentrated charge will explain why most alpha particles do not scatter at all – they miss the charge altogether – and yet particles that do hit the center scatter through large angles.^[11]

Maximum nuclear size estimate

Rutherford begins his analysis by considering a head-on collision between the alpha particle and atom. This will establish the minimum distance between them, a value which will be used throughout his calculations.^[34]

Assuming there are no external forces and that initially the alpha particles are far from the nucleus, the inverse-square law between the charges on the alpha particle and nucleus gives the potential energy gained by the particle as it approaches the nucleus. For head-on collisions between alpha particles and the nucleus, all the kinetic energy of the alpha particle is turned into potential energy and the particle stops and turns back.^[16]

Where the particle stops, a distance

r _{min} the potential energy matches the original kinetic energy:^{[36] [37]}

$$\backslash frac\; mv^2\; =\; k\; \backslash frac$$

where

$$k\; =\; \backslash frac$$

Rearranging:^[34] $$r\_\backslash text\; =\; k\; \backslash frac$$

For an alpha particle:

• (mass) = =
• (for helium) = 2 × =
• (for gold) = 79 × =
• (initial velocity) = (for this example)

The distance from the alpha particle to the center of the nucleus at this point is an upper limit for the nuclear radius.Substituting these in gives the value of about, or 27 fm. (The true radius is about 7.3 fm.) The true radius of the nucleus is not recovered in these experiments because the alphas do not have enough energy to penetrate to more than 27 fm of the nuclear center, as noted, when the actual radius of gold is 7.3 fm.Rutherford's 1911 paper^[34] started with a slightly different formula suitable for head-on collision with a sphere of positive charge:

$$\backslash fracmv^2\; =\; NeE\; \backslash cdot\; \backslash left\; (\backslash frac\; -\; \backslash frac\; +\; \backslash frac\; \backslash right)$$

In Rutherford's notation, e is the elementary charge, N is the charge number of the nucleus (we now know this to be equal to the atomic number), and E is the charge of an alpha particle. The convention in Rutherford's time was to measure charge in electrostatic units, distance in centimeters, force in dynes, and energy in ergs. The modern convention is to measure charge in coulombs, distance in meters, force in newtons, and energy in joules. Using coulombs requires using the Coulomb constant (k) in the equation. Rutherford used b as the turning point distance (called r_min above) and R is the radius of the atom. The first term is the Coulomb repulsion used above. This form assumes the alpha particle could penetrate the positive charge. At the time of Rutherford's paper, Thomson's plum pudding model proposed a positive charge with the radius of an atom, thousands of times larger than the r_min found above. Fig. 1 shows how concentrated this potential is compared to the size of the atom.Many of Rutherford's results are expressed in terms of this turning point distance, simplifying the results and limiting the need for units to this calculation of turning point.

Single scattering from heavy nuclei

From his results for a head on collision, Rutherford knows that alpha particle scattering occurs close to the center of an atom, at a radius 10,000 times smaller than the atom. Therefore he ignores the effect of "negative electricity" (i.e. the electrons). Furthermore he begins by assuming no energy loss in the collision, that is he ignores the recoil of the target atom. He will revisit each of these issues later in his paper.^[34] Under these conditions, the alpha particle and atom interact through a central force, a physical problem studied first by Isaac Newton.^[38] A central force only acts along a line between the particles and when the force varies with the inverse square, like Coulomb force in this case, a detailed theory was developed under the name of the Kepler problem. The well-known solutions to the Kepler problem are called orbits and unbound orbits are hyperbolas.Thus Rutherford proposed that the alpha particle will take a hyperbolic trajectory in the repulsive force near the center of the atom as shown in Fig. 2.

Step 1: Conservation of angular momentum

To apply the hyperbolic trajectory solutions to the alpha particle problem, Rutherford expresses the parameters of the hyperbola in terms of the scattering geometry and energies. He starts with conservation of angular momentum. When the particle of mass

m and velocity v _{0} is far from the atom, its angular momentum around the center of the atom will be m b v _{0} where b is the impact parameter, which is the lateral distance between the alpha particle's path and the atom. At the point of closest approach, labeled A in the Fig. 2, the angular momentum will be m r _{A} v _{A} . Therefore^[11]

$$m\; b\; v\_0\; =\; m\; r\_A\; v\_A$$

$$v\_A\; =\; \backslash frac$$

Step 2: Conservation of energy

Rutherford also applies the law of conservation of energy between the same two points:

$$\backslash tfracm\; v\_0^2\; =\; \backslash tfrac\; m\; v\_A^2\; +\; \backslash frac$$

The left hand side and the first term on the right hand side are the kinetic energies of the particle at the two points; the last term is the potential energy due to the Coulomb force between the alpha particle and atom at the point of closest approach (A). q_a is the charge of the alpha particle, q_g is the charge of the nucleus, and k is the Coulomb constant.

The energy equation can then be rearranged thus:

$$v\_A^2\; =\; v\_0^2\; \backslash left\; (1\; -\; \backslash frac\; \backslash right)$$

For convenience, the non-geometric physical variables in this equation^[34] can be contained in a variable

r _{min} , which is the point of closest approach in a head-on collision scenario^[34] which was explored in a previous section of this article:

$$r\_\backslash text\; =\; \backslash frac$$

This allows Rutherford simplify the energy equation to:

$$v\_A^2\; =\; v\_0^2\; \backslash left\; (1\; -\; \backslash frac\; \backslash right)$$

This leaves two simultaneous equations for

v ^{2} _{A} , the first derived from the conservation of momentum equation and the second from the conservation of energy equation. Eliminating v _{A} and v _{0} gives at a new formula for r _{min} :

$$v\_A^2\; =\; \backslash frac\; =\; v\_0^2\; \backslash left\; (1\; -\; \backslash frac\; \backslash right)$$

$$r\_\backslash text\; =\; r\_A\; -\; \backslash frac$$

Step 3: Geometry of hyperbolic orbit

The next step is to find a formula for

r _{A} . From Fig. 2, r _{A} is the sum of two distances related to the hyperbola, SO and OA. These distances can be expressed in terms of angle \Phi and impact parameter b .

The eccentricity of a hyperbola is a value that describes the hyperbola's shape. It can be calculated by dividing the focal distance by the length of the semi-major axis, which per Fig. 2 is . The eccentricity is also equal to

\sec\Phi , where

\Phi is the angle between the major axis and the asymptote,^[39] which as can be seen in Fig. 2 is the limit of \varphi . Therefore:

$$\backslash frac\; =\; \backslash sec\backslash Phi$$

As can be deduced from Fig. 2, the focal distance SO is

$$\backslash text\; =\; b\; \backslash csc\backslash Phi$$

and therefore

$$\backslash text\; =\; \backslash frac\; =\; b\; \backslash cot\backslash Phi$$

With these formulas for SO and OA, we can now find a formula for

r _{A} . The distance r _{A} can be written in terms of \Phi using a half-angle formula:

$$r\_A\; =\; \backslash text\; +\; \backslash text\; =\; b\; \backslash csc\backslash Phi\; +\; b\; \backslash cot\backslash Phi\; =\; b\; \backslash cot\backslash frac$$

Using the previous equation for

r _{A} we can now find the relationship between the physical and geometric variables:

$$r\_\backslash text\; =\; r\_A\; -\; \backslash frac$$

$$=\; b\backslash cot\backslash frac\; -\; \backslash frac$$

$$=\; b\; \backslash frac$$

$$=\; 2\; b\; \backslash cot\; \backslash Phi$$

where the last step uses the cotangent double angle formula.

Step 4: The scattering angle

The scattering angle of the particle is

\theta = \pi - 2 \Phi and therefore \Phi = \tfrac{\pi - \theta}{ 2 } . With the help of a known reflection formula, the relationship between θ and b becomes:^[34]

$$r\_\backslash text\; =\; 2b\backslash cot\; \backslash left\; (\backslash frac\; \backslash right)$$

$$=\; 2b\backslash tan\; \backslash frac$$

$$\backslash cot\backslash frac\; =\; \backslash frac$$

which can be rearranged to give

$$\backslash theta\; =\; 2\; \backslash arctan\; \backslash frac\; =\; 2\; \backslash arctan\; \backslash left\; (\backslash frac\; \backslash right)$$

Rutherford gives some illustrative values as shown in this table:^[34]

Rutherford's angle of deviation table

b/r _{min}	10	5	2	1	0.5	0.25	0.125
\theta	5.7°	11.4°	28°	53°	90°	127°	152°

Rutherford's approach to this scattering problem remains a standard treatment in textbooks^{[40] [41] [42]} on classical mechanics.

Intensity vs angle

To compare to experiments the relationship between impact parameter and scattering angle needs to be converted to probability versus angle. The scattering cross section gives the relative intensity by angles:$$\backslash frac(\backslash Omega)\; d\; \backslash Omega\; =\; \backslash frac$$

In classical mechanics, the scattering angle

\theta is uniquely determined the initial kinetic energy of the incoming particles and the impact parameter . Therefore, the number of particles scattered into an angle between \theta and \theta + d\theta must be the same as the number of particles with associated impact parameters between and . For an incident intensity, this implies:$$2\backslash pi\; I\; b\; \backslash left|db\backslash right|\; =-2\; \backslash pi\; \backslash sigma\; (\backslash theta)\; I\; \backslash sin(\backslash theta)\; d\backslash theta$$ Thus the cross section depends on scattering angle as:$$\backslash sigma\; (\backslash theta)\; =\; -\; \backslash frac\backslash frac$$ Using the impact parameter as a function of angle,, from the single scattering result above produces the Rutherford scattering cross section:

$$s\; =\; \backslash frac\; \backslash cdot\; ^2$$

• s = the number of alpha particles falling on unit area at an angle of deflection Φ
• r = distance from point of incidence of α rays on scattering material
• X = total number of particles falling on the scattering material
• n = number of atoms in a unit volume of the material
• t = thickness of the foil
• q_n = positive charge of the atomic nucleus
• q_a = positive charge of the alpha particles
• m = mass of an alpha particle
• v = velocity of the alpha particle

This formula predicted the results that Geiger measured in the coming year. The scattering probability into small angles greatly exceeds the probability in to larger angles, reflecting the tiny nucleus surrounded by empty space. However, for rare close encounters, large angle scattering occurs with just a single target.^[43]

At the end of his development of the cross section formula, Rutherford emphasizes that the results apply to single scattering and thus require measurements with thin foils. For thin foils the amount of scattering is proportional to the foil thickness in agreement with Geiger's measurements.^[34]

Comparison to JJ Thomson's results

At the time of Rutherford's paper, JJ Thomson was the "undisputed world master in the design of atoms".^[11] Rutherford needed to compare his new approach to Thomson's. Thomson's model, presented in 1910,^[44] modeled the electron collisions with hyperbolic orbits from his 1906 paper^[45] combined with a factor for the positive sphere. Multiple resulting small deflections compounded using a random walk.^[11]

In his paper Rutherford emphasized that single scattering alone could account for Thomson's results if the positive charge were concentrated in the center.Rutherford computes the probability of single scattering from a compact charge and demonstrates that it is 3 times larger than Thomson's multiple scattering probability. Rutherford completes his analysis including the effects of density and foil thickness, then concludes that thin foils are governed by single scattering, not multiple scattering.^[11]

Later analysis showed Thomson's scattering model could not account for large scattering.The maximum angular deflection from electron scattering or from the positive sphere each come to less than 0.02°; even many such scattering events compounded would result in less than a one degree average deflection and a probability of scattering through 90° of less than one in 10³⁵⁰⁰.^[46]

Target recoil

Rutherford's analysis assumed that alpha particle trajectories turned at the center of the atom but the exit velocity was not reduced.^[20] This is equivalent to assuming that the concentrated charge at the center had infinite mass or was anchored in place. Rutherford discusses the limitations of this assumption by comparing scattering from lighter atoms like aluminum with heavier atoms like gold. If the concentrated charge is lighter it will recoil from the interaction, gaining momentum while the alpha particle loses momentum and consequently slows down.

Modern treatments analyze this type of Coulomb scattering in the center of mass reference frame. The six coordinates of the two particles (also called "bodies") are converted into three relative coordinates between the two particles and three center-of-mass coordinates moving in space (called the lab frame). The interaction only occurs in the relative coordinates, giving an equivalent one-body problem just as Rutherford solved, but with different interpretations for the mass and scattering angle.

Rather than the mass of the alpha particle, the more accurate formula including recoil uses reduced mass:$$\backslash mu\; =\; \backslash cfrac.$$ For Rutherford's alpha particle scattering from gold, with mass of 197, the reduced mass is very close to the mass of the alpha particle:$$\backslash mu\_\backslash text\; =\; \backslash cfrac\; =\; 3.92\; \backslash approx\; 4.$$For lighter aluminum, with mass 27, the effect is greater:$$\backslash mu\_\backslash text\; =\; \backslash cfrac\; =\; 3.48,$$a 13% difference in mass. Rutherford notes this difference and suggests experiments be performed with lighter atoms.^[34]

The second effect is a change in scattering angle. The angle in the relative coordinate system or center of mass frame needs to be converted to an angle in the lab frame.^[47] In the lab frame, denoted by a subscript L, the scattering angle for a general central potential is$$\backslash tan\; \backslash Theta\_L\; =\; \backslash frac.$$ For a heavy particle like gold used by Rutherford,

m _{1/m} _{2} = 4 / 197 ≈ 0 . 02 \ll 1 and at almost all angles we can neglect this factor: the lab and relative angles are the same, \Theta _{L} ≈ \Theta .

The change in scattering angle alters the formula for differential cross-section needed for comparison to experiment. For any central potential, the differential cross-section in the lab frame is related to that in the center-of-mass frame by^[47] $$\backslash frac\_L\; =\; \backslash frac\; \backslash frac$$where

s = m _{1/m} _{2.}

Why the plum pudding model was wrong

How scattering should work according to the Thomson model

In a 1910 paper, Thomson presented equations that modeled how beta particles scatter in a collision with an atom.^{[48] [11]} On average the positive sphere and the electrons alike provide very little deflection in a single collision.Thomson's model combined many single-scattering events from the atom's electrons and a positive sphere. Each collision may increase or decrease the total scattering angle. Only very rarely would a series of collisions all line up in the same direction. The result is similar to the standard statistical problem called a random walk. If the average deflection angle of the alpha particle in a single collision with an atom is

\bar{\theta} , then the average deflection after n collisions is

$$\backslash bar\backslash theta\_n\; =\; \backslash bar\backslash sqrt$$

This correction gets applied twice, once for the individual electron collisions inside the atom, and again for the case of multiple atom collisions.

The average deflection caused by the atom's electrons was calculated by matching a hyperbolic orbit to the collision geometry and then multiplied by a factor proportional to

\sqrt{N} for encounters with N electrons:^[11]

$$\backslash bar\backslash theta\_2\; =\; \backslash frac\; \backslash cdot\; \backslash frac\; \backslash cdot\; \backslash frac\; \backslash cdot\; \backslash sqrt\; \backslash approx\; 0.00007\; \backslash text\; 0.004\; \backslash text$$where

• q_g = positive charge of the gold atom = =
• q_a = charge of the alpha particle = =
• q_e = elementary charge =
• R = radius of the gold atom =
• v = speed of the alpha particle =
• m = mass of the alpha particle =
• k = Coulomb constant =
• N = number of electrons in the gold atom = 79

The average angle by which an alpha particle should be deflected by the positive sphere of the atom was simply given by Thomson as:

$$\backslash bar\backslash theta\_1\; =\; \backslash frac\; \backslash cdot\; \backslash frac\; \backslash cdot\; \backslash frac\; \backslash approx\; 0.00013\; \backslash text$$

Analysis of Rutherford's notes on Thomson's work suggests this formula is a result of averaging the deflections of a beta particle crossing the sphere, assuming a straight trajectory suitable for a small deflection.^[11]

The net deflection for each atom combines the two contributions:

$$\backslash bar\backslash theta\; =\; \backslash sqrt\; \backslash approx\; 0.008\; \backslash text$$

and after

n collisons \bar\theta _{n} = 0 . 008 \sqrt{n} degrees .

In a 1911 paper, Rutherford developed similar equations for alpha particle scattering and showed that they did not agree with experimental results of Geiger and Marsden when applied to Thomson's atom model.^[49] The critical issue was large angle scattering. A gold foil like the one Geiger and Marsden experimented with would be around 10,000 atoms thick.The probability that an alpha particle will be deflected by a total of more than 90° after n deflections is given by:

$$e^$$

where e is Euler's number (≈2.71828...). Assuming an average deflection per collision of 0.008°, and therefore an average deflection of 0.8° after 10,000 collisions, the probability of an alpha particle being deflected by more than 90° will be^[50]

$$e^\; \backslash approx\; e^\; \backslash approx\; 10^$$

While in Thomson's "plum pudding" model it is mathematically possible that an alpha particle could be deflected by more than 90° after 10,000 collisions, the probability of such an event is so low as to be undetectable. This extremely small number shows that Thomson's model of 1906 cannot explain the results of the Geiger-Mardsen experiment of 1909.

When Thomson initially proposed the plum pudding model, he believed that all the mass of an atom was carried by its electrons. This would mean that even small atoms would have to contain thousands of electrons. The atomic weight of gold is 197 and an electron is 1,837 times smaller than a hydrogen atom, which means that a gold atom would have to contain 361,889 electrons. An alpha particle passing through a gold foil 10,000 atoms thick would probably experience millions of collisions before emerging, which per the equations above would produce a high probability of a net deflection in excess of 90°. But in 1906, after studying the effects of beta ray scattering, Thomson concluded that the number of electrons in atom was a small multiple of its atomic weight, so a gold atom would perhaps have 200 to 300 electrons. In this case a gold foil could not present enough collisions to produce a large deflection.

Deflection by the positive sphere

In Thomson's model of scattering the average angle by which an alpha particle should be deflected by the positive sphere of the atom is^{[48] [11]}

$$\backslash bar\backslash theta\_1\; =\; \backslash frac\; \backslash cdot\; \backslash frac$$

Neither Thomson nor Rutherford explain how this equation was developed, but here an educated guess is made.^[51]

In 1906, Thomson provided an equation which models how a beta particle should be deflected by an atomic electron in a close encounter:^[52]

$$\backslash tan=\; \backslash frac$$

Thomson probably arrived at this equation using hyperbolic geometry because Rutherford used hyperbolic geometry to produce a related equation in his 1911 paper^[53] (a full explanation is available in the article on the Rutherford scattering experiments). The factor

m' , the reduced mass equal to \tfrac{m _{1} m _{2}{m} _{1} + m _{2}} where m₁ and m₂ are the masses of the two colliding particles, enters the model when the two-body coordinates are written as the equivalent one-body problem.^[54] q_e is the elementary charge and b is the impact parameter.

An alpha particle passing by the positive sphere with a radius R equal to that of a gold atom, just close enough to graze its edge, will experience the sphere's electric field at its strongest.^[50] This occurs for an impact parameter b equal to the radius R as shown here:

Using Thomson's equation from above to model this collision gives:

$$\backslash theta\_1=\; 2\backslash arctan\; \backslash left\; (\backslash frac\; \backslash right)\; \backslash approx\; 0.02\; \backslash text$$

Unlike Thomson's electron-electron collision, no correction for recoil is needed here because the gold atom is nearly 20 times as heavy as the alpha particle. The equation shows that the maximum deflection caused by the positive sphere will be very small. But what of the average deflection

\bar\theta _{1} over all possible values of b?

Consider an alpha particle passing through the positive sphere of a gold atom, with its initial trajectory at a lateral distance b from the center.

Inside a sphere of uniformly distributed positive charge the force exerted on the alpha particle at any point along its path through the sphere is^{[55] [50]}

$$F\; =\; \backslash frac\; \backslash cdot\; \backslash frac$$

The lateral component of this force is

$$F\_y\; =\; \backslash frac\; \backslash cdot\; \backslash frac\; \backslash cdot\; \backslash cos\backslash varphi\; =\; \backslash frac\; \backslash cdot\; b$$

The lateral change in momentum p_y is therefore

$$\backslash Delta\; p\_y\; =\; F\_y\; t\; =\backslash frac\; \backslash cdot\; b\; \backslash cdot\; \backslash frac$$

The deflection angle

\theta _{1} is given by

$$\backslash tan\backslash theta\_1\; =\; \backslash frac\; =\; \backslash frac\; \backslash cdot\; b\; \backslash cdot\; 2L\; \backslash cdot\; \backslash frac$$

where p_x is the average horizontal momentum, which is first reduced then restored as horizontal force switches direction as the alpha particle goes across the sphere. Since we already know the deflection is very small, we can treat

\tan\theta _{1} as being equal to \theta _{1} .

To find the average deflection angle

\bar\theta _{1} , we must average b and L across the entire sphere:

$$\backslash bar\backslash theta\_1\; =\; \backslash frac\; \backslash int\_0^R\; \backslash frac\; \backslash cdot\; b\; \backslash cdot\; 2\backslash sqrt\; \backslash cdot\; \backslash frac\; \backslash cdot\; 2\backslash pi\; b\; \backslash cdot\; \backslash mathrmb$$

$$=\; \backslash frac\; \backslash cdot\; \backslash frac$$

This matches Thomson's formula in his 1910 paper.

Contemporary reactions

Rutherford's 1911 paper on alpha particle scattering contained largely the same points as described above and yet in the years immediate following its publication few scientists took note.^[9] The scattering model predictions were not considered definitive evidence against Thomson's plum pudding model. Thomson and Rutherford had pioneered scattering as a technique to probe atoms, its reliability and value were unproven. Before Rutherford's paper the alpha particle was considered an atom, not a compact mass. It was not clear why it should be a good probe. Rutherford's paper did not discuss the atomic electrons vital to practical problems like chemistry or atomic spectroscopy.^[11] Rutherford's nuclear model would only become widely accepted after the work of Niels Bohr.

Limitations to Rutherford's scattering formula

Very light nuclei and higher energies

In 1919 Rutherford analyzed alpha particle scattering from hydrogen atoms,^[56] showing the limits of the 1911 formula even with corrections for reduced mass.^[57] Similar issues with smaller deviations for He, Mg and Al^[58] lead to the conclusion that the alpha particle was penetrating the nucleus in these cases. This allowed the first estimates of the size of atomic nuclei.^[20] Later experiments based on cyclotron acceleration of alpha particles striking heavier nuclei provided data for analysis of interaction between the alpha particle and the nuclear surface. However at energies that push the alpha particles deeper they are strongly absorbed by the nuclei, a more complex interaction.^{[57] [35]}

Quantum mechanics

Rutherford's treatment of alpha particle scattering seems to rely on classical mechanics and yet the particles are of sub-atomic dimensions. However the critical aspects of the theory ultimately rely on conservation of momentum and energy. These concepts apply equally in classical and quantum regimes: the scattering ideas developed by Rutherford apply to subatomic elastic scattering problems like neutron-proton scattering.^[47]

See also

• Atomic theory
• Rutherford backscattering spectroscopy
• List of scattering experiments

Bibliography

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• Web site: Rutherford scattering . . . 2014-08-13 . .
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External links

• Description of the experiment, from cambridgephysics.org

Notes and References

1. J. J. Thomson . Cathode rays . Philosophical Magazine . 44 . 269 . 293-316 . 1897.
2. J. J. Thomson (1907). The Corpuscular Theory of Matter, p. 103: "In default of exact knowledge of the nature of the way in which positive electricity occurs in the atom, we shall consider a case in which the positive electricity is distributed in the way most amenable to mathematical calculation, i.e., when it occurs as a sphere of uniform density, throughout which the corpuscles are distributed."
3. J. J. Thomson, in a letter to Oliver Lodge dated 11 April 1904, quoted in Davis & Falconer (1997):
   "With regard to positive electrification I have been in the habit of using the crude analogy of a liquid with a certain amount of cohesion, enough to keep it from flying to bits under its own repulsion. I have however always tried to keep the physical conception of the positive electricity in the background because I have always had hopes (not yet realised) of being able to do without positive electrification as a separate entity and to replace it by some property of the corpuscles.
   When one considers that, all the positive electricity does, on the corpuscular theory, is to provide an attractive force to keep the corpuscles together, while all the observable properties of the atom are determined by the corpuscles one feels, I think, that the positive electrification will ultimately prove superfluous and it will be possible to get the effects we now attribute to it from some property of the corpuscle.
   At present I am not able to do this and I use the analogy of the liquid as a way of picturing the missing forces which is easily conceived and lends itself readily to analysis."
4. Thomson (1907). The Corpuscular Theory of Matter, p. 106: "The general problem of finding how n corpuscles will distribute themselves inside the sphere is very complicated, and I have not succeeded in solving it"
5. [#refDaintithGjertsen1999|Daintith & Gjertsen (1999)]
6. Hantaro Nagaoka . 1904 . Kinetics of a System of Particles illustrating the Line and the Band Spectrum and the Phenomena of Radioactivity . . Series 6 . 7 . 41. 445–455 . 10.1080/14786440409463141 . refNagaoka1904.
7. Ernest Rutherford . 1899 . Uranium Radiation and the Electrical conduction Produced by it . Philosophical Magazine . 47 . 284 . 109–163.
8. Ernest Rutherford . 1906 . The Mass and Velocity of the α particles expelled from Radium and Actinium . Philosophical Magazine . Series 6 . 12 . 70 . 348–371 . 10.1080/14786440609463549 . refRutherford1906 .
9. Book: Pais, Abraham . Inward bound: of matter and forces in the physical world . 2002 . Clarendon Press [u.a.] . 978-0-19-851997-3 . Reprint . Oxford.
10. [#refHeilbron2003|Heilbron (2003)]
11. Heilbron . John L. . 1968 . The Scattering of α and β Particles and Rutherford's Atom . Archive for History of Exact Sciences . 4 . 4 . 247–307 . 10.1007/BF00411591 . 41133273 . 0003-9519.
12. [#refHeilbron2003|Heilbron (2003)]
13. .
14. Book: Gary Tibbetts . 2007 . How the Great Scientists Reasoned: The Scientific Method in Action . . 978-0-12-398498-2.
15. Heilbron (2003)
16. Book: Belyaev, Alexander . The Basics of Nuclear and Particle Physics . Ross . Douglas . 2021 . Springer International Publishing . 978-3-030-80115-1 . Undergraduate Texts in Physics . Cham . en . 10.1007/978-3-030-80116-8.
17. Rutherford E (1938) Forty years of physics. Revised by John Ratcliffe. MacMillan Company andCambridge University Press, New York and Cambridge as quoted in
18. Book: Pais, Abraham . Inward bound: of matter and forces in the physical world . 2002 . Clarendon Press [u.a.] . 978-0-19-851997-3 . Reprint . Oxford.
19. Book: On an expansion apparatus for making visible the tracks of ionising particles in gases and some results obtained by its use . 1912-09-19 . 87 . 277–292 . en . 10.1098/rspa.1912.0081 . 0950-1207.
20. Book: Giliberti, Marco . Old Quantum Theory and Early Quantum Mechanics. Challenges in Physics Education. . Lovisetti . Luisa . 2024 . Springer Nature Switzerland . 978-3-031-57933-2 . Cham . 229–268 . en . Rutherford’s Hypothesis on the Atomic Structure . 10.1007/978-3-031-57934-9_6 . The idea of using the scattering of particles against a target to determine the internal structure of matter, as Rutherford did, turned out to be one of the most prolific ideas of experimental physics of the twentieth century and continues today in particle colliders to be one of the basic methods we have for determining the nature of things..
21. Book: Schweber, S. S. . QED and the men who made it: Dyson, Feynman, Schwinger, and Tomonaga . 1994 . Princeton University Press . 978-0-691-03685-4 . Princeton series in physics . Princeton, N.J.
22. Andrade, Edward Neville Da Costa. "The Rutherford Memorial Lecture, 1957." Proceedings of the Royal Society of London. Series A. Mathematical and Physical Sciences 244.1239 (1958): 437-455.
23. Letter from Hantaro Nagaoka to Ernest Rutherford, 22 February 1911. Quoted in Eve (1939), p. 200
24. Web site: Ernest Rutherford . Environmental Health and Safety Office of Research Regulatory Support . Michigan State University . 23 June 2023 . 22 June 2023 . https://web.archive.org/web/20230622163634/https://ehs.msu.edu/lab-clinic/rad/hist-figures/rutherford.html . live .
25. Web site: Ernest Rutherford: father of nuclear science . New Zealand Media Resources . https://web.archive.org/web/20210612184534/https://media.newzealand.com/en/story-ideas/ernest-rutherford-father-of-nuclear-science/ . 12 June 2021 . en . dead.
26. Report on the Activities of the History of Science Lectures Committee 1936–1947, Whipple Museum Papers, Whipple Museum for the History of Science, Cambridge, C62 i.
    The report lists two lectures, on October 8 and 15. The lecture on atomic structure was likely the one delivered on the 15th.
27. Cambridge University Reporter, 7 October 1936, p. 141
    The lecture took place in the lecture room of the Physiological Laboratory at 5 pm.
28. Barrette . Jean . 2021-10-02 . Nucleus-nucleus scattering and the Rutherford experiment . Journal of the Royal Society of New Zealand . en . 51 . 3-4 . 434–443 . 10.1080/03036758.2021.1962368 . 0303-6758.
29. Leone . M . Robotti . N . Verna . G . May 2018 . 'Rutherford's experiment' on alpha particles scattering: the experiment that never was . Physics Education . 53 . 3 . 035003 . 10.1088/1361-6552/aaa353 . 2018PhyEd..53c5003L . 0031-9120.
30. Rutherford . E. . August 1906 . XIX. Retardation of the α particle from radium in passing through matter . The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science . en . 12 . 68 . 134–146 . 10.1080/14786440609463525 . 1941-5982.
31. Baily . C. . January 2013 . Early atomic models – from mechanical to quantum (1904–1913) . The European Physical Journal H . en . 38 . 1 . 1–38 . 10.1140/epjh/e2012-30009-7 . 2102-6459.
32. [#refGeiger1908|Geiger (1908)]
33. [#refGeiger1910|Geiger (1910)]
34. Rutherford . E. . Ernest Rutherford . LXXIX. The scattering of α and β particles by matter and the structure of the atom . The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science . 21 . 125 . 1911 . 1941-5982 . 10.1080/14786440508637080 . 669–688.
35. Barrette . Jean . 2021-10-02 . Nucleus-nucleus scattering and the Rutherford experiment . Journal of the Royal Society of New Zealand . en . 51 . 3-4 . 434–443 . 10.1080/03036758.2021.1962368 . 0303-6758.
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42. Webber . B.R. . Davis . E.A. . February 2012 . Commentary on 'The scattering of α and β particles by matter and the structure of the atom' by E. Rutherford (Philosophical Magazine 21 (1911) 669–688) . Philosophical Magazine . en . 92 . 4 . 399–405 . 10.1080/14786435.2011.614643 . 2012PMag...92..399W . 1478-6435.
43. Karplus, Martin, and Richard Needham Porter. "Atoms and molecules; an introduction for students of physical chemistry." Atoms and molecules; an introduction for students of physical chemistry (1970).
44. Thomson . Joseph J. . 1910 . On the scattering of rapidly moving electrified particles . Proceedings of the Cambridge Philosophical Society . 15 . 465–471.
45. Thomson . J.J. . 1906 . LXX. On the number of corpuscles in an atom . The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science . en . 11 . 66 . 769–781 . 10.1080/14786440609463496 . 1941-5982.
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48. J. J. Thomson . 1910 . On the Scattering of rapidly moving Electrified Particles . Proceedings of the Cambridge Philosophical Society . 15 . 465-471 .
49. Rutherford (1911). p. 677
50. Beiser (1969). Perspectives of Modern Physics, p. 109
51. Heilbron (1968). p. 278
52. Heilbron (1968). p. 270
53. Ernest Rutherford . 1911 . The Scattering of α and β Particles by Matter and the Structure of the Atom . . Series 6 . 21 . 125. 669–688 . 10.1080/14786440508637080 . refRutherford1911.
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