ERNEST RUTHERFORD (1871-1937)

1911 Manchester, England

‘The atom contains a core or nucleus of very high density and very concentrated positive charge. Most of the atom is empty space, with the electrons moving about the tiny central nucleus’

Early photograph of ERNEST RUTHERFORD

ERNEST RUTHERFORD

Working under JJ THOMSON (1856-1940) at the Cambridge Cavendish Laboratory and later at the McGill University in Montreal, in 1898 Rutherford put forward his observation that radioactive elements give off at least two types of ray with distinct properties, ‘alpha’ and ‘beta’ rays.

In 1900 he confirmed the existence of ‘gamma’ rays, which remained unaffected by a magnetic force, whilst alpha and beta rays were both deflected in different directions by such an influence. Although both displayed the ability to stab through solid matter, alpha rays were far less penetrating than beta rays.
He proved through experimental results that they were helium atoms missing two electrons.

Alpha Beta Particles, Gamma Rays in a Magnetic Field

Alpha Beta Particles, Gamma Rays in a Magnetic Field

Alpha rays are in fact positively charged helium atoms that become true helium when they slow down and their charge is neutralised by picking up electrons.
Beta rays were later shown to be made up of electrons, and gamma rays to have a shorter wavelength than X-rays.

diagram showing comparative penetrations of Alpha Beta Gamma radiation

Alpha Beta Gamma radiation

In Montreal, Rutherford worked with Frederick Soddy and showed that over a period of time, half of the atoms of a radioactive substance could disintegrate. During the process the substance spontaneously transmuted to other elements. During radioactive decay, one kind of atom (radium) was ejecting another kind of atom (helium).

Working with other elements, Rutherford and Soddy found that each radioactive element had its own characteristic ‘half-life’. After one half-life, a sample retained only half its original radioactivity, after two half-lives a quarter, after three half-lives an eighth. The half-life of thorium emanation, now known as radon, was close to a minute. The half-lives of other radioactive elements ranged from a split-second to many billions of years. That of radium was 1620 years, while uranium had a half-life of 4.5 billion years.

The concept of half-life provides a way of measuring the age of rocks. As radioactive atoms decay they emit alpha particles. As these are essentially helium atoms, the amount of helium gas accumulates within the pores and fissures of a sample of a uranium mineral as a measure of how many atoms have decayed. Heating samples to drive off their helium and measuring the amount gives an indication of their age.
In order to provide more reliable dates, measuring the amount of lead, the ultimate decay product, compared with the amount of uranium, eliminates the errors introduced by the escape of some of the helium decay product to the air.

Dating rocks in this way gives an estimate of the age of the Earth, and by implication also the Sun, of around 4.5 billion years.

A radioactive atom is simply a heavy atom, which happens to be unstable. Eventually it disintegrates by expelling an alpha, beta or gamma ray. What remains is an atom of a slightly lighter element. A radioactive atom may decay more than once. Uranium, for instance, transforms itself into a succession of lighter and lighter atoms, one of which is radium, until it achieves stability as a non-radioactive atom of lead.

English: Radioactive decay modes

Working with HANS GEIGER (1882-1945), Rutherford developed the Geiger counter at Manchester University in 1908. This device measured radiation and was used in Rutherford’s work on identifying the make-up of alpha rays.

While he was at McGill, Rutherford had experimented firing alpha particles at a photographic plate. He had noticed that, while the image produced was sharp; if he passed the alpha particles through thin plates of mica, the resulting image on the photographic plate was diffuse. The particles were clearly being deflected through small angles as they passed close to the atoms of mica.
In 1910 his team undertook work to examine the results of directing a stream of alpha particles at a piece of platinum foil. While most passed through, about one in eight thousand bounced back – that is, deflected through an angle of more than 90 degrees.

Deflection of alpha Particles by Thin Metal Foil

Deflection of alpha Particles by Thin Metal Foil

In 1911 he put forward the theory that the reason for the rate of deflection was because atoms contained a minute nucleus that bore most of the weight, while the rest of the atom was largely ’empty space’ in which electrons orbited the nucleus much as planets orbit the Sun. The reason that one in eight thousand alpha particles bounced back was because they were striking the positively charged nucleus of an atom, whereas the rest simply passed through the spacious part.

But what was an atomic nucleus made of?
At 100,000th the size of the atom, it would take decades of painstaking experiments to discover.

In 1919, working in collaboration with other scientists, Rutherford artificially induced the disintegration of atoms by collision with alpha particles. In the process the atomic make-up of the element changed as protons were forced out of the nucleus. He transmuted nitrogen into oxygen (and hydrogen) and went on to repeat the process with other elements.

(image source)

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WILLARD LIBBY (1908- 80)

1946 – USA

‘Radiocarbon can be used to estimate the age of any organic material. The radioactive isotope of carbon,14C (carbon-14) is present in all living things. When life stops 14C begins to decay. From the rate of decay the age (or time of death) of an organism can be calculated’

The two most common forms of carbon 12C and 13C, make up virtually all types of carbon and are stable – 12C is the simplest form and is made up of 6 protons and 6 neutrons; 13C is slightly heavier because it has one more neutron. 14C, known as radiocarbon has the unstable combination of 6 protons (defining it as carbon) and 8 neutrons.

In the late 1940s Libby led the team at the University of Chicago, USA, that developed radiocarbon dating using the radioactive isotope 14C.

Living things go on absorbing 14C until the time of their death. The half-life of 14C is 5730 years – once an organism dies, 14C begins to decay. As a result the ratio of 12C to 14C changes with time. By measuring this ratio, it can be determined when the organism died.

Libby suggested that minute amounts of radiocarbon come from the upper part of the atmosphere. He put forward the idea that when high-energy particles formed in deep space – cosmic rays – reach the atmosphere, they interact with nitrogen gas to form radiocarbon. He argued that the newly formed radiocarbon is rapidly converted to carbon-dioxide, CO2, and is taken up by plants during photosynthesis; with the result that the radiocarbon enters the food chain. Everything alive should therefore have the same radiocarbon concentration as the atmosphere.

Once an individual dies, some of the 14C atoms begin to disintegrate and give off an electron to reform nitrogen. Libby argued that if the original radiocarbon content is known. it should be possible to measure the remaining 14C in a sample of tissue to back-calculate its age, in a similar way to estimating how much time has passed by measuring the amount of sand left in the top of an egg timer.
By the end of the 1940s, Libby and his team had shown that the radiocarbon content of the air was the same around the world and that 14C could be used to date anything organic.

The crucial principle is the half-life of the unstable atom, the rate at which it will break down. The longer the half-life of a material, the further back in time a dating method can go. With radiocarbon, the dating range is 40,000 to 60,000 years.

When Libby originally measured the half-life of radiocarbon, he calculated it to be just over 5720 years. During the 1950s a new estimate of 5568 years was made by other researchers, who assumed that Libby had got his figures wrong and the 5568-year half-life was adopted by the scientific community.
It is now known that the half-life of radiocarbon is 5730 years, virtually identical to Libby’s original estimate. As a result of the large number of samples that had already been dated, the incorrect value of 5568-years is used in estimates – confusingly this is now termed the ‘Libby half-life’. As all labs use the same half-life value, all ages are directly comparable.

With radiocarbon dating the assumptions made are:

  1. that the atmosphere has had the same 14C content in the past as today
  2. that all things alive have the same radiocarbon content as one-another and as the atmosphere
  3. that no more radiocarbon is added to a sample after death

To obtain a final radiocarbon age, we have to use a point in time to compare against. 1950 is used as year zero and all ages are described relative to this as ‘before present’ (BP). Radiocarbon dating does not give a precise date and estimates are given within a range of uncertainty.

Libby received numerous awards for this work,including the 1960 Nobel Prize for Chemistry. Libby also worked on the Manhattan Project during World War II, helping to enrich the uranium used in the atomic bombs.

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