PAUL DIRAC (1902- 84)

1928 – UK

Every fundamental particle has an antiparticle – a mirror twin with the same mass but opposite charge

PAUL DIRAC

‘It appears that the simplest Hamiltonian for a point-charge electron satisfying the requirements of both relativity and the general transformation theory leads to an explanation of all duplexity phenomena without further assumption’

1931 – UK

A magnetic monopole is analogous to electric charge

A magnetic monopole is a hypothetical particle that carries a basic magnetic charge – in effect, a single north or south magnetic pole acting as a free particle.

Until recently no one has observed a monopole.

TIMELINE

 

QUANTUM ELECTRODYNAMICS

Related articles

• Freezing magnetic monopoles: How dipoles become monopoles and vice versa (phys.org)
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SIR JAMES CHADWICK (1891-1974)

1932 – Manchester, England

Discovery of neutrons – elementary particles devoid of any electric charge

In contrast with the Helium nuclei (alpha rays) which are charged, and therefore repelled by the electrical forces present in the nuclei of heavy atoms, the neutron is capable of penetrating and splitting the nuclei of even the heaviest elements, creating the possibility of the fission of ²³⁵uranium.

JAMES CHADWICK

Assistant to ERNEST RUTHERFORD, Chadwick’s earlier work involved the showering of elements with alpha particles. The picture that gradually emerged was one of a nucleus that contained a very heavy particle with a positive electric charge. This particle was christened the proton, the hydrogen building block envisaged by WILLIAM PROUT.
A spin-off of this was the deduction that the nucleus of the hydrogen atom, the positively charged proton with an atomic weight of one was present in larger quantities in the nucleus of every other atom.

Rutherford and Geiger had shown that a helium atom and an alpha particle were the same thing, apart from the positive electric charge carried by the alpha particle.

A helium atom seemed to consist of a nucleus of a pair of protons circled by two electrons. However, a helium nucleus seemed to weigh as much as four protons. The mass of the known components of an atom did not add-up. Protons seemed to account for around half of the weight and were matched in number by an equal amount of negatively charged electrons to counter their positive charge. But the weight of an electron was one-thousandth that of a proton, so approximately half of the atomic weight of the element was unaccounted for.
Chadwick solved the conundrum in 1932 when he re-interpreted the results of an experiment carried out by IRENE and FREDERIC JULIOT-CURIE (Irene was the daughter of PIERRE and MARIE CURIE).
The couple had found in 1932 that when beryllium was showered with alpha particles, the resultant radiation could force protons out of substances containing hydrogen. Chadwick suggested that neutrally charged sub-atomic units, which he named neutrons, with the same weight as protons, could force this reaction and therefore were what made up the radiation that the Curies called gamma rays. Rutherford had hinted at the existence of such a particle in 1920.

The explanation was widely accepted and the riddle of `atomic weight’ had been solved: a similar number of neutrons to protons in the nucleus of an element would make up the remaining fifty per cent of the previously ‘missing’ mass.

FREDERICK SODDY (more)

The discovery of the neutron made sense of the observation that many elements come in a variety of forms, each with differing radioactive properties such as decay rate. Each form consisted of atoms with a different mass. Frederick Soddy christened these variants ‘isotopes’ in 1911. The idea that each element might be a mixture of atoms of different atomic weights explained why the atomic weights of a handful of elements were not simple multiples of the atomic weight of hydrogen, the most notorious example being chlorine whose atomic weight was 35.5 times that of hydrogen. Most of the variant forms of each element turned out to be radioactively unstable. An element such as chlorine, with more than one stable isotope, is rare.

The various isotopes of an element were merely atoms with the same number of protons in their nucleus but with a different number of neutrons.

Thus every atom was composed of electrons, protons and neutrons. The protons and neutrons clung together in a central clump – the atomic nucleus – while the electrons circled in a distant haze. The neutrons were responsible for increasing the weight of the elements without adding any electrical charge. Two protons and two neutrons made a helium nucleus; eight protons and eight neutrons an oxygen nucleus; 26 protons and 30 neutrons an iron nucleus; 79 protons and 118 neutrons a gold; and 92 protons and 146 neutrons a nucleus of uranium. When a radioactive nucleus expelled an alpha particle, it lost two neutrons and two protons and consequently became a nucleus of an element two places lower in the periodic table. When a radioactive nucleus emitted a beta particle, however, a neutron changed into a proton, transforming the nucleus into that of an element one place higher in the periodic table.

TIMELINE

 

THE ATOM

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ENRICO FERMI (1901- 54)

1942 – USA

Radioactive beta decay, the theory that a proton could be created from a neutron via the shedding of an electron (a beta particle) and an antineutrino

ENRICO FERMI

The Joliot-Curies had announced their discovery that radioactive isotopes could be generated artificially by showering certain elements with alpha particles in 1934. Fermi realised that the newly discovered neutrons would be even better suited to this purpose as their lack of charge would allow them to slip into elements’ nuclei without resistance.

Fermi established the concept of ‘slow-neutrons’ by placing a piece of solid paraffin in front of the target element during bombardment. Working his way through the elements he created a number of new radioactive isotopes.

He was awarded the 1938 Nobel Prize for physics and later the significance of his work when applied to uranium was realised. Using his neutron-bombarding technique in a series of experiments with ²³⁵uranium, Fermi and NIELS BOHR confirmed that a nuclear chain reaction could almost certainly be created as the basis of an atomic bomb.

Fission chain reaction

By Dec 2 1942 his team had created an ‘atomic pile’ of graphite blocks, drilled with uranium, which went on to produce a self-sustaining chain reaction for nearly half an hour.

TIMELINE

EDWARD TELLER (1908-2003)

1952 – USA

EDWARD TELLER

In 1950 the H-Bomb project was begun in earnest, with Teller in a key role. Collaborative work between Teller and Stanislaw Marcin Ulam (1906-86) resulted in a thermonuclear device being ready by late 1951, with a public testing in 1952. This is a hydrogen-fusion device, as opposed to the atomic nuclear bomb. The latter works by essentially splitting the nucleus of the heavy, uranium atom; the former as an offshoot of forcing the conversion of hydrogen to helium. It was ENRICO FERMI (1901-54) who pointed out the possibility that an atomic explosion could cause enough heat and pressure to force a thermonuclear reaction of a hydrogen isotope, unleashing an even greater force.

Scientific theory had hinted at this possibility ever since it was realised that a helium atom was slightly lighter than it should be given its component parts. An application of Einstein’s E=mc² equation explained that the mass ‘lost’ in the fusion was being converted into huge amounts of energy, the basis upon which the Sun works, fusing hydrogen atoms into helium under great temperature and pressure and giving off the difference as radiation.

Teller testified against ROBERT OPPENHEIMER during the investigation of alleged ‘disloyalty’.

TIMELINE

MURRAY GELL-MANN (b.1929)

1964 – USA

Neutrons and protons are made up of particles called quarks. Like electrons, they cannot be subdivided further

MURRAY GELL-MANN

The standard model of particle physics divides all elementary particles into three groups:

six types of leptons

electron, electron neutrino, muon, muon neutrino, tau and tau neutrino

six types of quarks

up, down, charm, strange, top and bottom

and

four types of bosons

      

Ordinary matter is made up of 

protons

each an up-up-down quark triplet

neutrons

each an up-down-down quark triplet

and

electrons

 

Gell-Mann predicted the existence of three quarks; up, down and strange. Other scientists predicted another three.
Quarks cannot exist singly.

TIMELINE

 

STANDARD MODEL