Aug 302017
 

Today is the birthday (1871) of Ernest Rutherford, 1st Baron Rutherford of Nelson, OM, PRS, a New Zealand-born British physicist who is one of the great pioneers of nuclear physics, yet, like so many other great experimental and theoretical physicists, his name is mostly unknown among the general public because it is not “Einstein.” In early work, Rutherford discovered the concept of radioactive half-life, proved that radioactivity involved the nuclear transmutation of one chemical element to another, and also discovered, differentiated and named alpha and beta radiation. This work was the basis for the Nobel Prize in Chemistry he was awarded in 1908. After the Nobel he performed his most famous work when he theorized that atoms have their charge concentrated in a very small nucleus, and thereby pioneered the Rutherford model of the atom. He conducted research that led to the first “splitting” of the atom in 1917 in a nuclear reaction between nitrogen and alpha particles, in which he also discovered (and named) the proton. Under his leadership the neutron was discovered by James Chadwick in 1932 and in the same year the first experiment to split the nucleus in a fully controlled manner was performed by students working under his direction, John Cockcroft and Ernest Walton.

Rutherford was the son of James Rutherford, a farmer, and his wife Martha Thompson, a school teacher originally from Hornchurch, Essex in England. James had emigrated to New Zealand from Perth, Scotland, “to raise a little flax and a lot of children.” Ernest was born at Brightwater, near Nelson, New Zealand. His first name was mistakenly spelled ‘Earnest’ when his birth was registered.

Rutherford studied at Havelock School and then Nelson College and won a scholarship to study at Canterbury College, University of New Zealand. After gaining his BA, MA and BSc, and doing two years of research during which he invented a new form of radio receiver, in 1895 Rutherford was awarded a research fellowship from the Royal Commission for the Exhibition of 1851, to travel to England for postgraduate study at the Cavendish Laboratory, University of Cambridge. He was among the first of the ‘aliens’ (those without a Cambridge degree) allowed to do research at the university, under the leadership of J. J. Thomson, and the newcomers aroused jealousies from the more conservative members of the Cavendish fraternity. With Thomson’s encouragement, he managed to detect radio waves at half a mile and briefly held the world record for the distance over which electromagnetic waves could be detected, though when he presented his results at the British Association meeting in 1896, he discovered he had been outdone by another lecturer: his name was Marconi.

In 1898 Thomson recommended Rutherford for a position at McGill University in Montreal. He was to replace Hugh Longbourne Callendar who held the chair of Macdonald Professor of physics and was coming to Cambridge. In 1907 Rutherford returned to Britain to take the chair of physics at the Victoria University of Manchester. During World War I, he worked on a top secret project to solve the practical problems of submarine detection by sonar. In 1919 he returned to the Cavendish succeeding J. J. Thomson as the Cavendish professor and Director.

When he first went to Cambridge as a research student, Rutherford started to work with J. J. Thomson on the conductive effects of X-rays on gases, work which led to the discovery of the electron which Thomson presented to the scientific world in 1897. Hearing of Becquerel’s experience with uranium, Rutherford started to explore its radioactivity, discovering two types that differed from X-rays in their penetrating power. Continuing his research in Canada, he coined the terms alpha ray and beta ray in 1899 to describe the two distinct types of radiation. He then discovered that thorium gave off a gas which produced an emanation which was itself radioactive and would coat other substances. He found that a sample of this radioactive material of any size invariably took the same amount of time for half the sample to decay – its “half-life” (11½ minutes in this case).

From 1900 to 1903, he was joined at McGill by the young chemist Frederick Soddy (Nobel Prize in Chemistry, 1921) for whom he set the problem of identifying the thorium emanations. Once he had eliminated all the normal chemical reactions, Soddy suggested that it must be one of the inert gases, which they named thoron (later found to be an isotope of radon). They also found another type of thorium they called Thorium X, and kept on finding traces of helium. They also worked with samples of “Uranium X” from William Crookes and radium from Marie Curie.

In 1902, they produced a “Theory of Atomic Disintegration” to account for all their experiments. Until then atoms were assumed to be the indestructable basis of all matter and although Curie had suggested that radioactivity was an atomic phenomenon, the idea of the atoms of radioactive substances breaking up was a radically new idea. Rutherford and Soddy demonstrated that radioactivity involved the spontaneous disintegration of atoms into other types of atoms (one element spontaneously being changed to another). The Nobel Prize in Chemistry 1908 was awarded to Ernest Rutherford “for his investigations into the disintegration of the elements, and the chemistry of radioactive substances”.

In 1903, Rutherford considered a type of radiation discovered (but not named) by French chemist Paul Villard in 1900, as an emission from radium, and realized that this observation must represent something different from his own alpha and beta rays, due to its very much greater penetrating power. Rutherford therefore gave this third type of radiation the name of gamma ray. All three of Rutherford’s terms are in standard use today – other types of radioactive decay have since been discovered, but Rutherford’s three types are among the most common.

In Manchester, he continued to work with alpha radiation. In conjunction with Hans Geiger (of radioactive counter fame) he developed zinc sulfide scintillation screens and ionization chambers to count alphas. By dividing the total charge they produced by the number counted, Rutherford determined that the charge on the alphas was 2 (suggesting it was helium nuclei). In late 1907, Ernest Rutherford and Thomas Royds allowed alphas to penetrate a very thin window into an evacuated tube. As they sparked the tube into discharge, the spectrum obtained from it changed, as the alphas accumulated in the tube. Eventually, the clear spectrum of helium gas appeared, proving that alphas were at least ionized helium atoms, and probably helium nuclei.

Rutherford performed his most famous work after receiving the Nobel prize in 1908. Along with Hans Geiger and Ernest Marsden in 1909, he carried out the Geiger–Marsden experiment, which demonstrated the nuclear nature of atoms by deflecting alpha particles passing through a thin gold foil. Rutherford was inspired to ask Geiger and Marsden in this experiment to look for alpha particles with very high deflection angles, of a type not expected from any theory of matter at that time. Such deflections, though rare, were found, and proved to be a smooth but high-order function of the deflection angle. It was Rutherford’s interpretation of this data that led him to formulate the Rutherford model of the atom in 1911 – that a very small charged nucleus, containing much of the atom’s mass, was orbited by low-mass electrons.

Before leaving Manchester in 1919 to take over the Cavendish laboratory in Cambridge, Rutherford became, in 1919, the first person to deliberately transmute one element into another. In this experiment, he had discovered peculiar radiations when alphas were projected into air, and narrowed the effect down to the nitrogen, not the oxygen in the air. Using pure nitrogen, Rutherford used alpha radiation to convert nitrogen into oxygen through the nuclear reaction 14N + α → 17O + proton. The proton was not then known. In the products of this reaction Rutherford simply identified hydrogen nuclei, by their similarity to the particle radiation from earlier experiments in which he had bombarded hydrogen gas with alpha particles to knock hydrogen nuclei out of hydrogen atoms. This result showed Rutherford that hydrogen nuclei were a part of nitrogen nuclei (and by inference, probably other nuclei as well). Such a construction had been suspected for many years on the basis of atomic weights which were whole numbers of that of hydrogen. Hydrogen was known to be the lightest element, and its nuclei presumably the lightest nuclei. Now, because of all these considerations, Rutherford decided that a hydrogen nucleus was possibly a fundamental building block of all nuclei, and also possibly a new fundamental particle as well, since nothing was known from the nucleus that was lighter. Thus, Rutherford postulated the hydrogen nucleus to be a new particle in 1920, which he dubbed the proton.

In 1921, while working with Niels Bohr (who later postulated that electrons moved in specific orbits), Rutherford theorized about the existence of neutrons, (which he had christened in his 1920 Bakerian Lecture), which could somehow compensate for the repelling effect of the positive charges of protons by causing an attractive nuclear force and thus keep the nuclei from flying apart from the repulsion between protons. The only alternative to neutrons was the existence of “nuclear electrons” which would counteract some of the proton charges in the nucleus, since by then it was known that nuclei had about twice the mass that could be accounted for if they were simply assembled from hydrogen nuclei (protons). But how these nuclear electrons could be trapped in the nucleus, was a mystery.

Rutherford’s theory of neutrons was proved in 1932 by his associate James Chadwick, who recognized neutrons immediately when they were produced by other scientists and later himself, in bombarding beryllium with alpha particles. In 1935, Chadwick was awarded the Nobel Prize in Physics for this discovery.

There you have it.  The current model of the atom – much refined by later physicists – of a dense nucleus consisting of protons and neutrons surrounded by electrons (along with the nature of radioactive decay as a nuclear process) is owed to the work of Rutherford. Rutherford died too early to see Leó Szilárd’s idea of controlled nuclear chain reactions come into being. However, a speech of Rutherford’s about his artificially-induced transmutation in lithium, printed in 12 September 1933 in The Times, was reported by Szilárd to have been his inspiration for thinking of the possibility of a controlled energy-producing nuclear chain reaction. Szilard had this idea while walking in London, on the same day.

Rutherford’s speech touched on the 1932 work of his students John Cockcroft and Ernest Walton in “splitting” lithium into alpha particles by bombardment with protons from a particle accelerator they had constructed. Rutherford realized that the energy released from the split lithium atoms was enormous, but he also believed that the energy needed for the accelerator, and its essential inefficiency in splitting atoms in this fashion, made the project an impossibility as a practical source of energy (accelerator-induced fission of light elements remains too inefficient to be used in this way, even today). Rutherford’s speech in part, read:

We might in these processes obtain very much more energy than the proton supplied, but on the average we could not expect to obtain energy in this way. It was a very poor and inefficient way of producing energy, and anyone who looked for a source of power in the transformation of the atoms was talking moonshine. But the subject was scientifically interesting because it gave insight into the atoms.

Well, his work on releasing energy from the nuclei of light elements was, as he predicted, not practical, but his theorizing on radioactive elements led in the right direction.

Given that Rutherford was born in New Zealand and always thought of himself as a Kiwi first, a New Zealand recipe is in order, but, like Australian cooking, there’s not a lot to choose from. Australia and New Zealand continue to battle it out over who invented the pavlova (meringue shell with fruit and cream) as their signature dish, but I’ve given a recipe already. If you like you can make one with kiwi fruit.

I’ll go for something more modern, but very popular throughout New Zealand nowadays: Southland cheese rolls. They originated in the Southland region of the South Island but are now more or less universal. They are basically cheese and onion rolled in toasted bread and lightly baked. This recipe gives just one version, but you can vary the types of bread and cheese any way you want.  I recommend a hearty whole wheat for the bread. It’s common to use a commercial spread on top after broiling but I prefer butter.

Southland Cheese Rolls

Ingredients

1 loaf bread cut in slices
200 gm Colby or Cheddar cheese (grated)
150 gm Parmesan cheese (grated)
1 6 oz can evaporated milk
1 cup heavy cream (optional)
1 packet onion soup
1 onion, peeled and finely chopped
2 tsp dry English mustard
butter (or nasty commercial spread) for topping (optional)

Instructions

Mix the cheeses, onion, soup powder, mustard, cream (if used), and evaporated milk together in a small saucepan. Stir over low heat until the cheeses are melted and you have a thick smooth mixture. Let cool slightly.

Spread the cheese mixture generously over the bread slices and roll them into logs.  Place them on a baking tray and broil them, turning until all sides are evenly toasted.

Serve hot with a knob of butter on top.

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