The Dominance of Particle Physics

Rob Iliffe

Atomic Theory before 1860

• Natural philosophy had long been divided between people who were committed to the existence of atoms (indivisibles),

• And those who believed that the universe was ‘full’ of some fluid, and that matter was continuously divisible.

• By mid-C19th, for reasons associated with chemistry, most scientists believed in the existence of indivisible atoms

• but it was also thought that atomic theory could not explain the behavior of gases in evacuated tubes, increasingly popular in scientific experiments.

Cathode Ray Tube

• 1850s – experiments with discharge tubes (in which electrical currents were passed through attenuated gases) produced different kinds of ‘glow’ –

• E.g. ‘Geissler tubes’ from late 1880s, originally invented in 1857 by Heinrich Geissler –

• sealed and partially evacuated glass cylinders with cathode and anode at either end, containing gases like argon, neon or ionizable substances such as sodium.

• When high voltage electrical current flowed through tube, ‘rays’ were projected from Cathode,

• and the gas emitted light by fluorescence.

The Cavendish laboratory

• From 1880s, ‘negative electrode’ or ‘Cathode’ Ray Tubes became major part of physics equipment, especially at Cavendish Laboratory Cambridge.

• This was set up in 1874 under directorship of James Clerk Maxwell, the first Prof. of Experimental Physics at the Cavendish Laboratory.

• Maxwell died in 1879 and was replaced by Lord Rayleigh, who was himself replaced by J.J. Thomson in 1884.

• Thomson would begin pioneering studies with Cathode Ray Tubes culminating in discovery of electron.

J.J. Thomson’s ‘electron’

• Joseph John Thomson (1856-1940) – b. nr. Manchester and attended Owens college from age 14, moving to Cambridge in 1876.

• Showed great talent as a mathematician, but after moving to the Cavendish he turned to experimental work,

• Found in 1897 that cathode rays could be deflected electrically when discharge tube was evacuated to extremely low pressures;

• he could measure the deflection, giving an accurate mass to charge ratio

• the substance of cathode rays was over 1000 times lighter than a hydrogen atom, and were negatively charged particles (or ‘corpuscles’) which other scientists termed ‘electrons’.

Thomson’s ‘Plum-pudding’ Atom

• Thomson’s work showed that there were smaller elements of an atom that were negatively charged;

• If this was true, then there had to be a source of positive charge inside the atom that left the overall charge of the atom neutral.

• In 1904 he proposed that the atom was made up of ‘small areas of negative charged corpuscles [I.e. electrons] enclosed in a sphere of uniform positive electrification’.

• The name ‘Plum pudding’ stuck as a description of this idea, with plums as electrons and the positive outside being the pudding.

Budino di Prugne

Rutherford’s Nucleus

• In 1911 Thomson’s student Ernest Rutherford performed a number of experiments at the Cavendish Laboratory

• demonstrating that the mass of an atom was largely concentrated in its ‘nucleus’, a positively charged core around which electrons orbit like planets around the Sun.

• He knew that alpha-particles (positively charged particles) scattered when fired at thin gold foil, producing scintillations on detectors;

• Rutherford knew that very few scintillations occurred on detectors not immediately behind the gold foil, so the atom was mainly empty.

Rutherford and the Proton

• In 1910s Rutherford continued to do collision experiments, i.e. the bombardment of nuclei using other particles.

• At some point he found that the light hydrogen nucleus could be removed from nitrogen nucleus by collision

• And in 1919 he announced that the hydrogen nucleus was present in all other nuclei, calling it ‘proton’.

• Discovery of ‘neutron’ by James Chadwick in 1932 provided basic ‘classical’ model of atom,

• With the number of protons in the nucleus defined as the Atomic Number (Z) of any element.

CTR Wilson’s Clouds

• Charles Thomson Rees (‘CTR’) Wilson (1869-1959) went to Cambridge to study medicine but turned to physics and chemistry,

• Studying physics at Sidney Sussex and at Cavendish under J.J. Thomson, and was fascinated by properties of clouds,

• especially by the ‘glories’ (coloured rings around shadows cast on mist and cloud) he saw while working at Ben Nevis.

• Wilson tried to reproduce these phenomena at the Cavendish in a ‘cloud chamber’ from 1895.

Fogs

• Wilson expanded (cooled) some moist, dust-free air in an evacuated container to a greater degree than previous workers had attempted,

• finding that some water droplets appeared each time he did this –

• he reproduced effects seen on Ben Nevis and suggested that these ‘fogs’ were condensation forming on charged atomic nuclei.

• Wilson next exposed the air in the ‘chamber’ to X-Rays, recently discovered by German Wilhelm Roentgen.

• With X-Rays present, the expansion produced a slowly falling fog – X-Rays seemed to produce numerous particles on which vapour could condense.

X-Rays and the chamber

• In 1897 Wilson experimented with rays emanating from uranium, then called ‘Becquerel rays’;

• Henri Becquerel had discovered in 1896 that uranium salts spontaneously emitted a sort of ray very like X-rays.

• Wilson concluded that the particles in the chamber were ions (a particle), that produced droplets when charged.

The Cloud Chamber

• Wilson perfected cloud chamber in 1911, allowing paths of subatomic particles to be tracked, photographed and classified in vapour cloud.

• In 1927 Wilson won Nobel prize for Physics (with Arthur Compton) for invention and improvement of cloud chamber.

• This was superseded by Bubble Chamber, invented by Donald Glaser in 1952 –

• These were much larger than cloud chambers and used a superheated liquid as arena for discovering and tracking particles of much higher energies.

Marie Curie (Maria Sklodowska, 1867-1934)

• b. Warsaw; her father taught physics and mathematics, and also taught Maria and siblings science at home.

• Maria spent some years as a governess but began to do serious scientific training in 1890,

• Following her sister to Paris in 1891, enrolling at Univ. Paris.

• Got degree in physics in 1893, soon meeting Pierre Curie, an instructor at the Higher School of Physics and Chemistry –

• They got married in July 1895

The Active Element

• Fascinated by Becquerel Rays, Marie used an electrometer designed earlier by Pierre to measure the charge of air that surrounding the uranium.

• In a makeshift shed next to the School she did measurements on effects of pitchblende (the mineral uraninite), which was about 4 times as active as uranium.

• Her research 1896-8 involved finding similar sorts of substances, such as thorium, which emitted Becquerel rays.

• By 1898 Pierre was sufficiently interested to join her in her work, and both Curies surmized that there had to be an element in pitchblende that was much more active than uranium.

Radium• Their work involved grinding pitchblende into powder, and idolating

the compound into fractions such as bismuth and barium

• In July 1898 they published a paper announcing the discovery of polonium (in the bismuth), named after her home country.

• At the end of the year they published a paper on the existence of another element (isolated from the barium), which they named ‘radium’, devising the term ‘radioactivity’.

• Isolating radium was much more difficult than polonium, achieving the production of one tenth of a gram of radium chloride in 1902.

• A new industry immediately arose on the basis of their discoveries, though they did not benefit financially.

Fame and misfortune

• After some discussion about whether Marie deserved recognition, the Curies were awarded the Nobel prize for Physics in 1903, on the basis of over 30 papers published on radioactive materials.

• Pierre was made professor of Physics at the Ecole, and was given a new laboratory in 1906, but was killed by a carriage in April.

• Marie was offered his position, which she accepted.

• She finally isolated pure radium in 1910 but was still prevented from entry into the Academy of Sciences –

Second Nobel prize

• In 1911 she was awarded a second Nobel, this time for chemistry, in recognition for her study of radium and polonium, the isolation of radium, and her studies of the effects of radioactivity.

• Only one of two people to win two Nobel prizes

• This finally prompted France to create the Radium Institute in her honour, but her reputation was damaged in 1911 when it was revealed that she had had an affair with a married man.

• Unit of radioactivity was named the ‘curie’ after the two Curies,

• And she became instrumental in having radioactivity used safely in hospital environments.

• Obviously – an icon for female scientists.