Frederick Soddy and the World Set Free

by Nicholas Mee on September 13, 2020

Frederick Soddy was a pioneer of nuclear physics and radio-chemistry. He was appointed Lecturer in Physical Chemistry at Glasgow University in 1904. Five years later he published The Interpretation of Radium, a best-selling and very influential book based on six public lectures delivered at the university. In one lecture Soddy addresses the audience holding aloft a bottle containing a uranium oxide solution:

Frederick Soddy’s Plaque at Glasgow University.

Is it not wonderful to reflect that in this tiny bottle there lies asleep and waiting to be evolved the energy of at least one hundred and sixty tons of coal? The energy in a ton of uranium would be sufficient to light London for a year.

The radioactive decay of uranium releases energy at a slow and steady pace. But Soddy considered what would happen

if only it were under our control and could be harnessed to do the world’s work in the same way as the energy in coal has been harnessed and controlled.

He believed that if a way were found to speed up the release of energy from radioactive decay, then the world would be transformed. Soddy’s visionary musings would inspire a remarkable and prophetic novel.

The World Set Free

The cover of the first American edition of The World Set Free by H.G. Wells.

H.G. Wells wrote The World Set Free in 1913 on the eve of the First World War. Wells speculated about where future developments in physics might lead. He imagined that physicists would meet the challenge set by Soddy by developing ways to extract nuclear energy at will, and invented a character named Holsten who would make the critical breakthrough. In the words of the novel:

The problem which was already being mooted by such scientific men as Ramsay, Rutherford, and Soddy, in the very beginning of the twentieth century, the problem of inducing radio-activity in the heavier elements and so tapping the internal energy of atoms, was solved by a wonderful combination of induction, intuition, and luck by Holsten so soon as the year 1933.

Holsten’s great discovery is a new element named carolinium that could be induced to release the energy locked within its atoms. This opened the door to military applications, bombs of hitherto inconceivable destructive power, atomic bombs, as Wells calls them in the first ever use of this term. Although Wells’s method of delivery for the new weapons was of its time—they were lobbed over the side of aircraft by intrepid airmen, as depicted on the front cover of the American edition—the novel would prove uncannily accurate about future developments of nuclear weapons and their military consequences. But that was still some decades ahead.

Decay Chains

Just a few years earlier, Soddy and Rutherford had discovered that atoms change their identity when they emit radiation. They realised that radioactive atoms often change into other radioactive atoms and this transmutation continues until eventually a stable atom is formed that persists indefinitely. Soddy and his colleagues now began the laborious process of mapping out the identity of each atom in these radioactive decay chains. 

Soddy’s example of a disintegration series or decay chain in his book The Interpretation of Radium.

In The Interpretation of Radium, Soddy presented the still incomplete example of a decay chain shown above. The atoms in each chain could be identified by their distinct radioactive decay patterns. They were labelled with names such as uranium II, uranium X, emanation, mesothorium, ionium and radium A to radium G. Eventually, over forty radioactive species were identified. But clearly they could not all be new elements.

Isotopes

Soddy found the answer in 1913, when he conclusively demonstrated that although radium G, thorium D and actinium D might each be the stable endpoints for three different radioactive decay chains, they were all chemically identical. All three of these different atomic species were, in fact, atoms of the same rather familiar element lead, but with different atomic masses. We know them today as lead-206, lead-207 and lead-208. It is perhaps ironic that whereas the alchemists famously aimed to create gold as the culmination of a process beginning with lead, the radio-chemists had discovered that lead is actually the endpoint of these natural radioactive decay series.

At a family dinner party, Soddy described his research to a very learned family friend Margaret Todd who was a medical doctor. She suggested that chemically identical atoms with different masses might be called isotopes from the Greek ‘iso’ meaning ‘the same’ and ‘topos’ meaning ‘place’, because they would all share the same place in the Periodic Table. Soddy gratefully accepted her suggestion and this is still the term that is used today.

Protons and Neutrons

We now know that the atomic nucleus is composed of two types of particle: positively-charged protons and electrically-neutral neutrons. (These two types of particle are collectively known as nucleons.) Chadwick’s discovery of the neutron in 1932 provided the explanation for how elements can have different isotopes. It would also bring about the possibility of nuclear power and nuclear weapons, as envisaged by H.G. Wells.

The number of protons in the nucleus determines the chemical properties of the atom, so a nucleus containing two protons is a helium nucleus, for instance, whereas a nucleus containing ninety-two protons is a uranium nucleus. But the nucleus also contains neutrons. The helium-3 nucleus contains two protons and a neutron, whereas a helium-4 nucleus contains two protons and two neutrons. So nuclei with the same number of protons and different numbers of neutrons form atoms of the same element, but they have different atomic masses—they are different isotopes of that element.

Schematic diagram of the two stable helium isotopes. Left: helium-3. Right: helium-4. (In reality the diameter of the electron orbitals is vastly greater than the nuclear diameter.)

Alpha and Beta

Soddy’s decay chain shown above indicates whether each radioactive decay is due to the emission of an alpha particle or a beta particle. (All the atoms in the diagram emit alpha particles except the second which emits a beta particle.) Soddy was the first to understand how the emission of these particles affects the position of an atom in the Periodic Table. 

Alpha particles are composed of two protons and two neutrons—they are identical to helium-4 nuclei. So each alpha decay transforms the nucleus into one with two fewer protons and two fewer neutrons. On the chart below, alpha decays move the nucleus two steps diagonally to the left.

Beta decay is rather different. When a nucleus undergoes beta decay one of its neutrons changes into a proton and an electron is emitted, along with another particle known as an antineutrino. (The beta particle is the emitted electron.) The nucleus therefore retains the same total number of nucleons, but it has one more proton and one less neutron. So on the chart below, beta decays move the nucleus one step diagonally to the right. 

The Nuclear Chequerboard

The chequerboard diagram shown here plots a sequence of nuclear transmutations. It shows the same radioactive decay chain as illustrated in Soddy’s diagram above. Following the arrows we see that uranium-238 undergoes alpha decay and transforms into thorium-234, which then undergoes beta decay to form protactinium-234. This is followed by a further beta decay to form uranium-234, and a sequence of five alpha decays. Eventually, after a few more beta and alpha decays the nucleus that results is lead-206, which is stable. The chart can be used to complete Soddy’s diagram. For example, Soddy’s Uranium X is clearly thorium-234 and his parent of radium is thorium-230.

We now know there are eighty-eight naturally occurring elements, of which eighty are stable. These elements are formed of 339 naturally occurring isotopes, including 252 that are stable. If we include isotopes created artificially in particle accelerators over 3,300 have been identified, although many of these are so unstable that they decay within a fraction of a second.

The Misuse of Science

Soddy had seen the rapid advance of science as the way towards a wonderful future in which human civilisation would be transformed. The First World War had a devastating psychological effect on him. He was sickened by the mass slaughter resulting from the industrialisation of war and the use of science to kill as efficiently as possible. The death of Henry Moseley, the rising star of nuclear physics, hit him particularly hard. He wrote:

When Moseley was killed at Gallipoli I felt enraged. Sometimes I think that something snapped in my brain. . . . I felt that governments and politicians, or man in general, was not yet fitted to use science. 

Soddy believed that scientists should take responsibility for the profound social consequences that would follow from their discoveries.

In 1919 Soddy became Professor of Chemistry at Oxford University, and two years later he was awarded the Nobel Prize for Chemistry. But his attention had turned to politics and economics, topics that seemed more relevant for the future of humanity. He challenged the thinking of conventional economists and campaigned for a radical new approach to finance and international monetary relations. Although his physics-inspired approach to economic theory was never fully accepted by mainstream economists, many of his once contentious proposals are now standard practise in the management of modern economies.

During the interwar years Soddy was almost alone in calling for the scientific community to recognise the social implications of its research. Further advances in nuclear physics and the outbreak of another global war would bring these issues into sharp focus. Many physicists would now agonise over their role in the development of nuclear weapons, and the looming threat of annihilation such weapons pose, realising that the consequences of their use would be the total devastation depicted by H.G. Wells in The World Set Free.



Further Information

There is more about Frederick Soddy’s early work here: Rutherford this is Transmutation!

There is more about the work of Henry Moseley in this article: Henry Moseley and the Nuclear Treasure Chest.

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Nuclear Time Shift

by Nicholas Mee on September 3, 2020

The White Horse of Uffington has been galloping over the rolling hills of southern England for many years. But for just how long? 

The White Horse of Uffington.

The age and origin of this elegant abstract horse cut into the Oxfordshire chalk downs provoked a heated debate that dragged on for centuries. Some claimed it was carved on the instructions of Alfred the Great to celebrate his victories over the Danes. Some claimed it was the work of an even earlier Anglo-Saxon king. Others argued it must date to the century before the Roman invasion, and pointed to its similarity to images on pre-Roman gold coins found nearby. But without contemporary written records, how could such a question be answered?

Radiocarbon dating has revolutionised archaeology. But it is only suitable for dating organic material. If we had a sample of the white horse’s droppings, then it would certainly help. Unfortunately, chalk horse droppings are as rare as rocking horse droppings. The answer lies elsewhere in nuclear physics and it is tied to a handy little device designed to protect workers in the nuclear industry.

Radiation Exposure

Many occupations involve an increased risk of exposure to potentially damaging radiation. These include workers at nuclear power plants or nuclear fuel reprocessing facilities and some research physicists, but also dentists, radiographers and other medical staff who operate X-ray machines, and doctors and nurses who deliver nuclear medicines. Many mine workers are exposed to increased levels of radon gas, and this does not just apply to those mining uranium. The technique of neutron logging used in the oil industry also has associated risks of exposure to radiation. Even airline pilots and their cabin crew receive additional exposure to cosmic rays when flying above the protective cloak of the atmosphere. Radiation exposure has a cumulative effect, but it is invisible, so monitoring this exposure is a serious challenge.

In 1954 Farrington Daniels at the University of Wisconsin-Madison invented an ingenious device that would solve the problem. Crystal lattices are formed of regular arrays of vast numbers of atoms, but they always include some impurities and defects that disrupt the regularity of the crystal. When a gamma ray photon hits a crystal it collides with numerous electrons within the crystal boosting them into excited states associated with impurity atoms where they remain trapped. Daniels realised that subsequent heating of the crystal to somewhere in the region of 100-200 degrees Centigrade enables the trapped electrons to fall back into lower energy states releasing energy as photons of visible light. By counting these photons it is possible to determine how many electrons were trapped and therefore the amount of ionising radiation the crystal has been subjected to. This is the physical basis for the thermo-luminescent dosimeter (TLD) invented by Daniels that is used by workers in many industries. The TLD is usually supplied as a small clip-on device, as shown here.

Clip-on thermo-luminescent dosimeter (TLD).

The TLD uses crystals such as calcium fluoride that are doped with small quantities of manganese, magnesium or other atoms. Different crystals may be used to monitor exposure to specific types of radiation. (Lithium fluoride crystals are used to monitor exposure to neutrons. A lithium-6 nucleus will absorb a neutron and split into a helium-4 nucleus and a tritium nucleus, which subsequently decays into helium-3. The energy released in these reactions promotes electrons into trapped states.)

Crystal Clocks

Farrington Daniels

Farrington Daniels and his colleagues Charles A. Boyd and Donald F. Saunders suggested in the 1950s that thermo-luminescence might also help determine the age of pottery shards and other long-buried archaeological artefacts. Applying such methods to the dating problem faced by archaeologists is like the flip-side of monitoring modern-day radiation hazards. When a worker is exposed to radiation we want to know the total radiation dose over a period of time. But if we could measure the total radiation exposure of an archaeological specimen and we knew the rate at which radiation damage occurred, then the time taken to reach the measured total could be calculated. Exposure to daylight resets the clock as photons in sunlight de-excite any trapped electrons. So this method could potentially determine the time that has elapsed since an artefact was buried.

By the 1980s this idea was developed into a technique known as Optically Stimulated Luminescence Dating. This method uses light rather than heat to de-excite the electrons in sample rock crystals such as quartz and feldspar. The number of emitted photons is then measured with a photo-multiplier tube to determine the accumulated radiation dose of the archaeological artefact. 

The background rate at which radiation damage has accumulated is determined by measuring the radiation due to radioactive elements, such as uranium, thorium and potassium, within the artefact and its surroundings, and by estimating the additional contribution due to cosmic rays. Dividing the accumulated total by the background rate gives the length of time since the artefact was last in direct sunlight. Exposure to daylight for as little as one hundred seconds will reset the clock, so the samples must be taken using opaque core tubes and then analysed in dark room conditions to determine their radiation exposure. This technique now has widespread archaeological applications and is reliable for materials that are up to 150,000 years old.

Looking Back in Time

In 1995 the Oxford Archaeological Unit decided to see whether Optical Stimulated Luminescence Dating would settle the debate about the White Horse of Uffington. Over the centuries, the horse has been cleaned and maintained by many generations of local people, so samples were taken from the lowest disturbed layers of chalk beneath the horse.

When the results came back they were a surprise to everyone. The white horse is much older than had been imagined. The laboratory analysis revealed that the chalk horse was originally dug some time between 1400 BC and 600 BC, so 3000 years ago plus or minus 400 years. This places its construction some time in the late Bronze Age or early Iron Age, making it easily the oldest chalk figure in England.


 

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Rutherford this is Transmutation!

August 26, 2020

Soddy discovered that no sooner had he removed the radon gas from his thorium sample that the radon would reappear. It was as though thorium was continually giving birth to the emanation as it emitted radiation. Then he found that the emanation was generating another new radioactive substance that was deposited on the walls of the glass vessel in which it was being collected. Soddy was stunned when he understood what was was happening: Rutherford, this is transmutation!

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William Ramsay’s Noble Quest

August 21, 2020

Ramsay had not just discovered one or two new elements, he had appended an entire new Group to the Periodic Table. In 1904 Ramsay became the first Briton to receive a Nobel Prize for Chemistry.

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Beyond the Alchemists’ Wildest Dreams

August 15, 2020

All this alchemical magic produces a potion that is more valuable than gold and has saved more lives than the philosopher’s stone or the elixir of life.

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Scrap Metal from the Proton Merry-Go-Round

August 11, 2020

Lawrence’s machines would project Berkeley into the position of world leaders in the creation of artificial elements, stretching the outer edges of the Periodic Table beyond uranium. It is rather surprising therefore that the first new element created in one of Lawrence’s machines in Berkeley was actually tracked down ten thousand kilometres away by an intrepid Italian who knew the value of good piece of scrap metal when he saw it.

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Atom Heart Mother

August 6, 2020

The advantage of a plutonium-238 battery is that it would outlast the patient and never need to be replaced. There are still a few people walking around with a tiny quantity of plutonium in their chest. But today’s cardiac pacemakers are usually fitted with conventional lithium-ion chemical batteries and have to be replaced every ten years or so.

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Henry Moseley and the Nuclear Treasure Chest

July 30, 2020

Moseley’s brilliant experimental work had shown that the nucleus of an atom carries a charge that is a multiple of the charge on the hydrogen nucleus. This multiple is known as the atomic number. And he had conclusively demonstrated that the elements should be ordered by atomic number not atomic mass. 

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The Atomic Bomb Ring

July 25, 2020

Incredibly, the Atomic Bomb Ring really did allow the children of the atomic age to witness the decay of atomic nuclei as they munched their breakfast cereal. Attached to the ring was a bullet-shaped capsule that separated into two halves. As the advert says, one half contained a secret message compartment, but the other half ‘the warhead’ contained a hidden atom chamber. This silver chamber was a device invented four decades earlier by renowned British physicist Sir William Crookes.

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Julia Dream

July 20, 2019

Many naturally occurring objects have a complicated appearance, and when we look closer the complexity only increases. As we zoom in we see what appear to be miniature replicas of the original object. For instance, a branch of a fir tree might resemble a scaled down version of an entire fir tree, and a broccoli […]

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