Nuclear Chemistry and Applications of Radioactivity

Carbon-14 is formed continuously in the upper atmosphere, and living plants and animals take it in with food and carbon dioxide, so the ratio of \({}^{14}C\) to \({}^{12}C\) in a living body stays constant. When the organism dies the intake stops, and the \({}^{14}C\) present decays (half-life 5730 years) while \({}^{12}C\) stays the same. By measuring the \({}^{14}C\) left in the dead sample, its age can be found. So carbon-14 is used for dating once-living (biological) materials like wood, bones and fossils.

An alpha particle is a helium nucleus (2 protons and 2 neutrons) with mass 4 amu and charge +2, and it moves slowly (about 1/10 the speed of light). Because of its size and charge it ionises the atoms in its path very strongly and loses its energy in a short distance. So it is stopped by a sheet of paper or a few centimetres of air and has the least penetrating power.

Fusion needs extremely high temperature (millions of degrees) and high pressure to overcome the strong electrostatic repulsion between the positively charged nuclei. At this temperature matter exists as plasma, which must be held (confined) for long enough, and no material container can touch it. Creating and holding these conditions needs a lot of energy and very costly technology, and at present the energy put in is about equal to or more than the energy obtained. Unlike fission, fusion also does not proceed as a self-sustaining chain reaction. So fusion is still at the experimental stage, and it is used in the sun and in weapons (hydrogen bomb) but not yet in power plants.

Radioactivity is the natural process in which unstable atomic nuclei spontaneously disintegrate and emit radiations (\(\alpha\) particles, \(\beta\) particles and \(\gamma\) rays) to become more stable. It was discovered by Henri Becquerel in 1896 from uranium salts.

The substances (elements or isotopes) whose atoms have unstable nuclei and emit radiations like \(\alpha\), \(\beta\) and \(\gamma\) rays by themselves are called radioactive substances. They are generally heavier elements. Examples: uranium, thorium, radium, polonium and plutonium (and the isotope carbon-14).

Plutonium-239 absorbs a neutron and forms the unstable compound nucleus plutonium-240, which at once splits. For the products named in the question (Xe-140 and Sr-94) the mass number is conserved when 6 neutrons are released:

Mass number: \(239+1=240=140+94+6\times1\). Note: the atomic numbers on the right add up to \(54+38=92\), not 94, so the equation with exactly these two products does not balance in charge, and the question data is not fully consistent. A fully balanced fission of Pu-239 with xenon-140 uses zirconium-97 (Z = 40):

(check: \(A: 240=140+97+3\); \(Z: 94=54+40\)). The released neutrons can hit other Pu-239 nuclei and continue the chain reaction.

Both mass number and atomic number are conserved, so the equation is balanced.

Cosmic rays striking the upper atmosphere produce neutrons. A neutron is captured by nitrogen-14 (78% of air), which emits a proton and becomes carbon-14:

\(A: 14+1=14+1\); \(Z: 7+0=6+1\). The \({}^{14}C\) is then oxidised to \(CO_2\) and absorbed by plants and animals.

Natural radioactivity is the spontaneous breakdown of an unstable atomic nucleus, with emission of radiation, without any external trigger. It occurs in elements of the earth’s crust like uranium-238, thorium-232, radium-226, polonium-210 and carbon-14.

Example:

Application: carbon dating of fossils (also uranium dating of rocks, smoke detectors and tracing groundwater movement).

Artificial (induced) radioactivity is the process in which a stable nucleus is made radioactive by bombarding it with neutrons or other high-energy particles. It is a man-made process.

Example: stable aluminium-27 bombarded with a neutron gives radioactive sodium-24, which decays by beta emission:

Application: cobalt-60 is used in radiotherapy to destroy cancer cells (iodine-131 is used for thyroid disorders and plutonium-239 as fuel in nuclear reactors).

Nuclear power is the energy generated by nuclear reactions, mainly nuclear fission, in which a heavy nucleus like uranium splits into smaller nuclei and releases a large amount of energy. It is used to generate electricity in nuclear power plants, with uranium-235 and plutonium-239 as the fuel, and is the second largest low carbon energy source in the world.

Nuclear weapons are explosive devices that cause massive destruction as a result of a nuclear reaction, either nuclear fission or nuclear fusion (or both). Examples: the atomic bomb (fission based, e.g. ‘Little Boy’ dropped on Hiroshima and ‘Fat Man’ on Nagasaki in 1945) and the hydrogen bomb or thermonuclear bomb (fusion based).

Radioisotopes are radioactive isotopes of an element: atoms with the same atomic number but unstable nuclei (different number of neutrons), which emit radiation. Examples are \({}^{131}I\), \({}^{60}Co\), \({}^{24}Na\) and \({}^{18}F\).

Medical uses: (1) diagnostic imaging (PET and SPECT scans, technetium-99m for bone, heart, liver and kidney); (2) cancer treatment by radiotherapy (I-131, Co-60, Bi-213); (3) brachytherapy, placing Ir-192 or Ra-223 near a tumour; (4) studying iron metabolism with Fe-59; (5) blood flow measurement with Na-24; (6) tracing proteins and nucleic acids in research with S-35 and P-32.

Controlled nuclear fission: the chain reaction is kept under control by absorbing the extra neutrons with control rods (boron or cadmium), so that only one neutron of each fission goes on to cause another fission. Energy is released slowly and steadily. It is used in nuclear reactors to produce electricity.

Uncontrolled nuclear fission: the neutrons are not absorbed, so each fission triggers two or three more and the number of fissions grows very quickly in a fraction of a second, releasing a huge amount of energy as an explosion. It is the principle of the atomic bomb.

Radioisotopes are used in nuclear medicine for diagnosis and for treatment.

Radiocarbon dating is the process of finding the age of once-living materials (wood, bones, fossils, shells) by measuring the carbon-14 left in them. The method was invented by Willard F. Libby in 1949, and he received the Nobel Prize in Chemistry in 1960.

Principle: Cosmic-ray neutrons convert atmospheric nitrogen into carbon-14, which is oxidised to \(CO_2\) and absorbed by plants and then animals. As long as the organism lives, its \({}^{14}C\) content is constant. After death the intake stops and \({}^{14}C\) decays (\({}^{14}_{6}C\rightarrow{}^{14}_{7}N+{}_{-1}e^{0}\)) with a half-life of 5730 years. The age is found from

where \(N_0\) is the original amount of \({}^{14}C\) and \(N\) is the amount now.

Medicine: diagnosis by PET and SPECT imaging (Tc-99m, F-18), treatment of cancer and thyroid disease (Co-60, I-131), brachytherapy (Ir-192), and study of iron metabolism and blood flow (Fe-59, Na-24).

Industry: non-destructive testing of welds and pipelines by radiography, thickness gauging of paper and metal sheets with beta rays, leak detection in underground pipes with tracers, sterilisation of food and medical equipment by cobalt-60 gamma rays, smoke detectors (Am-241), and power for space missions from RTGs using Pu-238 (Voyager spacecraft and Mars rovers).

Ionising radiation damages cells and DNA and can cause cancer, radiation burns and sickness, infertility and birth defects, and it weakens the immune system. Radioactive waste can contaminate air, water and soil for a long time, and accidents or misuse of radioactive material can harm many people.

Use proper shielding (lead, concrete) and keep the exposure time and dose as low as possible; handle radioisotopes only by trained people wearing badges to monitor the dose; use isotopes with short half-lives; follow strict government rules for licensing, storage, transport and use; store the waste safely in sealed containers; keep the public informed and educated; and prefer non-radioactive methods where they work as well.

The process of finding the age of once-living materials (wood, charcoal, bones, shells, fossils) by measuring the amount of carbon-14 remaining in them is called radiocarbon dating. It was developed by Willard Libby (Nobel Prize in Chemistry, 1960).

1. Formation: Cosmic rays produce neutrons in the upper atmosphere, and these neutrons bombard nitrogen-14 to form carbon-14.

2. Uptake: \({}^{14}C\) is oxidised to \({}^{14}CO_2\), absorbed by plants in photosynthesis, and passes to animals through food. So, in all living things, the ratio of \({}^{14}C\) to \({}^{12}C\) is constant.

3. Decay after death: When a plant or animal dies, intake of carbon stops. The \({}^{14}C\) present decays by beta emission, but \({}^{12}C\) is stable, so the \({}^{14}C\) fraction falls with time.

4. Age calculation: The half-life of \({}^{14}C\) is 5730 years, and the decay follows first-order kinetics, so

where \(N_0\) is the \({}^{14}C\) present when the organism was alive and \(N\) is the amount in the sample now. For example, if a sample has one-fourth of the original \({}^{14}C\), then \(t=\dfrac{5730}{0.301}\times\log 4=5730\times 2=11460\) years (two half-lives).