The History Of The Atom Part 6

Other Particles

By Aslam, Matthew, Wing Chung, Pameer, Alfie

Rooks Heath Year 13 Physics Students

The Positron

So the image of the atom before the Second World War was a central nucleus made up of protons and neutrons surrounded by a cloud of electrons but where did the other sub-atomic particles fit in?

Paul Dirac first suggested that a positron might exist in 1928 as a result of an equation he devised that unified quantum mechanics, special relativity and electron spin.

The positron or antielectron is the antiparticle or the antimatter counterpart of the electron. It has an electric charge (+1.602 x E-19 coulombs) of +1e, a spin of ½, and has the same mass as an electron (9.103826 x E31 kg. It is written as β+ or e+ in nuclear equations.

Richard Feynman, and Ernst Stueckelberg, proposed an interpretation of the positron as an electron moving backward in time, reinterpreting the negative-energy solutions of the Dirac equation.

Dmitri Skobeltsyn first observed the positron in 1929 while using a Wilson cloud chamber to investigate cosmic rays and Carl D. Anderson discovered the positron on August 2, 1932, for which he won the Nobel Prize for Physics in 1936. Anderson also coined the term positron. The positron was the first evidence of antimatter and was discovered when Anderson allowed cosmic rays to pass through a cloud chamber and a lead plate. A magnet surrounded this apparatus, causing particles to bend in different directions based on their electric charge. The ion trail left by each positron appeared on the photographic plate with a curvature matching the mass-to-charge ratio of an electron, but in a direction that showed its charge was positive (see image below left).



The positron is emitted (positron emission) in beta plus decay (see the picture above right), which is a form of radioactive decay. Pair production, the “conversion” of electromagnetic energy into a positron and an electron, is also a source of positrons. Regardless of the source, the positron will always seek to “combine” with any nearby electron with the mass of both particles being converted into electromagnetic energy (a pair of gamma rays).


Positrons are important in medical imaging in that they can actually show pictures of processes going on in the body.

image      image

Anderson, Carl D., “The Positive Electron”, The Physical Review, Volume 43, Number 6, pp. 491-49

The Muon

Like an electron a muon is a sub-atomic particle with a negative charge however it is 200 times heavier and very unstable. Muons make up much of the cosmic rays reaching the Earth. They are very unstable and decay to an electron and a positron.

image    image

Muons were discovered by Carl D. Anderson and Seth Neddermeyer at Caltech in 1936, while studying cosmic radiation. Anderson had noticed particles that curved differently from electrons and other known particles when passed through a magnetic field. They were negatively charged but curved less sharply than electrons, but more sharply than protons, for particles of the same velocity. It was assumed that the magnitude of their negative electric charge was equal to that of the electron, and so to account for the difference in curvature, it was supposed that their mass was greater than an electron but smaller than a proton.

The existence of the muon was confirmed in 1937 by J. C. Street and E. C. Stevenson’s cloud chamber experiment.


Why there was an apparent need in nature for what is essentially a duplicated but heavier electron was not known at the time and physicist I.I Rabi famously remarked “Who ordered that?” upon hearing of its discovery.‎

The neutrino

A neutrino is an electrically neutral, weakly interacting elementary subatomic particle with half-integer spin that is not affected by the electromagnetic force but is affected by the weak force. All evidence suggests that it has mass but that its mass is tiny even by the standards of subatomic particles. Its mass has never been measured accurately.

Neutrinos are created as a result of certain types of radioactive decay, or nuclear reactions such as those that take place in the Sun, in nuclear reactors, or when cosmic rays hit atoms. Most neutrinos passing through the Earth come from the Sun. About 65 billion solar neutrinos per second pass through every square centimetre perpendicular to the direction of the Sun in the region of the Earth.

The neutrino was first suggested by Wolfgang Pauli in 1930 to explain how beta decay could conserve energy, momentum, and angular momentum (spin).

There are three classes of neutrinos.



The study of neutrinos may enable scientists to understand our Sun better and because neutrinos are so fantastically numerous, if they have even a tiny mass, they may outweigh all the stars and galaxies, all the visible matter in the universe. They might make up as much as one fifth of the dark matter that physicists and astrophysicists have been seeking so assiduously.

The Standard Model


The Standard Model of elementary particles, with the three generations of matter, gauge bosons in the fourth column and the Higgs boson in the fifth.

A quark is an elementary particle and a fundamental constituent of matter. Quarks combine to form composite particles called hadrons, the most stable of which are protons and neutrons,

In the beginnings of particle physics (first half of the 20th century), hadrons such as protons, neutrons and pions were thought to be elementary particles. However, as new hadrons were discovered, the ‘particle zoo’ grew from a few particles in the early 1930s and 1940s to several dozens of them in the 1950s. The relationships between each of them were unclear until 1961, when Murray Gell-Mann and Yuval Ne’eman (independently of each other) proposed a hadron classification scheme called the Eightfold Way.

The Standard Model includes 12 elementary particles of spin-½ known as fermions.

There are six quarks (up, down, charm, strange, top, bottom), and six leptons (electron, electron neutrino, muon, muon neutrino, tau, tau neutrino). Pairs from each classification are grouped together to form a generation, with corresponding particles exhibiting similar physical behaviour.

The remaining six fermions do not carry colour charge and are called leptons. The three neutrinos do not carry electric charge either, so their motion is directly influenced only by the weak nuclear force, which makes them notoriously difficult to detect. However, by virtue of carrying an electric charge, the electron, muon, and tau all interact electromagnetically.

In the Standard Model, gauge bosons are defined as force carriers that mediate the strong, weak, and electromagnetic fundamental interactions.

The Standard Model falls short of being a complete theory of fundamental interactions because it makes certain simplifying assumptions and it does not incorporate the full theory of gravitation.’eman

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