ATOMIC NUCLEUS: Energy and politics

France produces 75 per cent of its electrical power from nuclear power stations and only 12 per cent from fossil fuels. This compares with just 28 per cent nuclear power in Britain, and 70 per cent from fossil fuels.^source Fossil fuels contribute to the greenhouse effect and global warming. They also pollute the atmosphere, and most developed countries are searching for ways to reduce their dependence on fossil fuels. This suggests that making more use of nuclear power should be a good thing.

^{Bruce Nuclear Generating Station, the largest nuclear power facility. Chuck Szmurlo, CC BY 2.5}

But in the United States (the world’s largest producer of electricity from nuclear power), electricity suppliers decided in 1987 not to build any more nuclear power stations. Britain, too, cancelled plans in 1995 for building two large nuclear power stations. The main reason for these decisions was economic. In the debate on energy from nuclear or fossil fuel sources, economic and environmental arguments compete. Since then, politicians have had to think harder about how we use and obtain energy. Looming on the not too distant horizon is global warming. The pollutants produced by burning fossil fuels make it easier for the atmosphere to trap solar thermal radiation. The main culprit seems to be carbon dioxide.

At the atomic level, the thermal energy we can get from a nuclear fission event is 200 MeV, compared with just a few eV from breaking up a molecule of a carbon-based fuel. It takes 1.5 tonnes of coal to produce as much energy as we can get from a piece of nuclear fuel about 15 mm long. For the UK, North Sea gas and oil are running out, so they are becoming more dependent upon foreign suppliers of fossil fuel.

Chernobyl Nuclear Power Station was badly designed and badly maintained. The first generation nuclear power stations were designed in the 1950s – they are now building Generation 3 power stations in Finland and in France, and Generation 4 designs are in the pipeline. The main benefits from these new designs are much-increased efficiency for safer operation and much less radioactive waste.

People are scared of anything called nuclear, and of radiation. The utterly safe and effective medical diagnostic tool Magnetic Resonance Imaging (MRI) had to change its original name of Nuclear Magnetic Resonance (NMR) because of the N-word. People, in general, don’t realise that the dangerous stuff is ionising radiation. You can sit in front of an electric fire without coming to much harm.

The ideas in this post

The atomic nucleus was not known about until the early twentieth century. Yet, only thirty years on, in 1945, two nuclear bombs were to kill an estimated 200 000 people in Japan, an event which ended the Second World War in a matter of days. This chapter deals with the evidence we have about the nature and structure of the nucleus, and why some nuclei break up and others are radioactive. It was the chance discovery of radioactivity at the end of the nineteenth century that began the series of investigations that led to an understanding of what an atom was like: a tiny but massive nucleus surrounded by a cloud of electrons. The evidence came from using the emissions that came from unstable nuclei: alpha, beta and gamma radiations.

The nucleus can provide useful energy through a process that occurs only with certain rare, very large nuclei, when they break up into two smaller nuclei – nuclear fission. Radioactivity itself is a minor source of energy, but the radiations are useful tools for industrial and medical use. The radiations are able to knock electrons from, or ionise, ordinary atoms. The ionisation effects allow the radiation to be easily detected – but also cause harm to living tissue.

Life on Earth depends on the energy from nuclear reactions in the Sun known as nuclear fusion, in which two hydrogen nuclei combine to form a helium nucleus. But attempts to use nuclear fusion as a steady, controlled source of cheap and almost pollution-free energy have so far been unsuccessful.

^{Part of CNO cycle diagram, made just to be illustrative for nuclear reactions in general. Kjerish, CC BY-SA 4.0}

RADIOACTIVITY

Radioactivity was discovered in 1896 by Henri Becquerel (France, 1852-1908). He was investigating the property that some natural minerals have of glowing in the dark – fluorescence. He detected the faint light emitted by fluorescent substances using photographic plates. One day he found (by accident) that a salt of uranium emitted invisible radiation which blackened the photographic plates.

Soon, three types of radiation were discovered, and because they were then unidentified they were simply labelled alpha (α), beta (β) and gamma (γ) rays, after the first three letters of the Greek alphabet. The table below lists the nature and properties of the radiations.

Properties of alpha, beta and gamma radiations

Radiation	Alpha	Beta	Gamma
Nature	helium nucleus (2n + 2p)	electron or positron	photon of electromagnetic radiation
Symbol	Нe	e	γ
Charge	2e⁺	e^- or e⁺	γ
Range in air	1-5 cm	10-100 cm	infinite – obeys the inverse square law
Range in matter	stopped by e.g. a sheet of paper	stopped by e.g. a thin (1 mm) sheet of aluminium	intensity reduced to half by e.g. 1 or 2 cm of lead
Mass in atomic mass units	4 u	5 × 10^-4 u	zero
Relative ionising ability	very high: 1	high: 1/100	low: 1/10000

Marie Curie (Poland/France, 1867-1934) and her husband Pierre (France, 1859-1906) investigated radioactivity and developed ways for measuring the activity of sources, i.e. how much radiation a source emitted per second. In doing so they discovered other elements that also emitted energetic radiation, identifying completely new ones such as polonium and radium. Later, the physicist Ernest Rutherford (New Zealand/Britain, 1871-1937) and the chemist Frederick Soddy (Britain, 1877-1956) worked together to establish the nature of the radiations and track down the changes of the elements that were linked to them. Their results began to make sense when Rutherford’s team discovered the nucleus in the years 1909-1911.

^{Graphic showing relationships between radioactivity and detected ionizing radiation. Doug Sim, CC BY-SA 3.0}

DETECTING THE RADIATIONS

THE GEIGER COUNTER: The most common device – and one of the oldest – for detecting and monitoring ionising radiation is the Geiger counter. The key part of the counter is the Geiger-Müller tube, the GM tube. A sealed metal tube has a thin wire down the middle and is filled with a non-reactive gas (e.g. argon) at low pressure. The wire is kept at a voltage of about +400 V with respect to the metal case, which is earthed. The radiation enters the tube through a thin window in the front. Alpha particles are easily absorbed by matter so the window in an alpha detector is a thin film of mica – but strong enough to withstand the pressure difference between the atmosphere and the gas inside the tube.

^{Geiger-Müller radiation detector. Boffy b, CC BY-SA 3.0}

Radiation entering the tube ionises argon atoms. The positive argon ions are attracted to the metal case and electrons to the central wire. These charged particles are accelerated by the voltage and will collide with other argon atoms, If they are moving fast enough, the collisions will ionise the atoms. The new particles are in turn accelerated and will cause further ionisation, producing an avalanche effect. The sudden large flow of charge in the tube causes a pulse of current in the central wire which is amplified electronically.

The amplified pulses may be counted by a scaler or averaged out to give a current reading on a ratemeter. The pulses may also be fed into an audio circuit to give a series of clicks, so providing an audible indication of the radiation level. The charge avalanche takes tìme to be cleared out of the gas, so there is a dead time (of about 200 ms) in which the tube cannot react to any incoming radiation. Beta and gamma detectors are similar: windows for beta detectors can be made of glass, and gamma detectors can be made entirely of metal.

Working voltage of a GM tube

The voltage on the central rod has to be set high enough to accelerate ions and electrons and cause the avalanche effect. But the voltage should not be so high as to produce a continuous discharge. This happens when a continuous supply of ions hitting the metal walls have enough energy to eject electrons from it, which are then accelerated to cause further ionisation of the argon atoms. There is a plot of the count rate, which is proportional to the mean current in a GM tube, against voltage when the ionising radiation enters the tube at a constant rate. The operating plateau shows the range of applied voltage for which the tube works well. At plateau voltages, all ionising particles of the same type will produce the same size pulse of charge.

^{Diagram of a Geiger counter using an "end window" tube for low penetration radiation. A loudspeaker is also used for indication. Svjo-2, CC BY-SA 3.0}

SOLID STATE DETECTORS

In industry and research laboratories, the most common kind of radiation detector is ‘solid-state’. The energy of the incoming radiation (particle or photon) liberates electrons from semiconductor material. This produces a pulse of current and so indicates the arrival of the radiation. Such detectors can both count individual events and also measure the energy of the radiation.

PHOTOGRAPHIC DETECTION

Photography works because photons of light have an ionising effect on certain chemicals – generally salts of silver such as silver nitrate. Radiation badges, use this effect. Such a badge has to be worn by anyone who works in an area where they might be exposed to ionising radiation, such as a nuclear power plant, an X-ray department in a hospital, or a radioactivity laboratory. More advanced particle detectors will be described later.

BACKGROUND RADIATION

A Geiger counter set up anywhere on Earth will always register a count. This count is due to background radiation, produced by tiny fragments of radioactive elements present in all rocks and soil, the atmosphere and even in living material itself. Also, the Earth is continuously bombarded by high-speed particles from outer space and from the Sun called cosmic rays. Cosmic rays smash up the nuclei of atmospheric molecules and produce other high-speed particles which can cause ionisation. Many of these reach ground level.