In a nuclear reactor, heavy water is used mainly for which purpose?

The correct answer is C, carbon-14. Carbon-14 is the radioactive isotope of carbon used for radiocarbon dating, and its half-life is close to 5,700 years, which is the figure quoted in the question. A living plant or animal takes in carbon-14 with ordinary carbon; after death the intake stops and the carbon-14 decays by beta emission, so the proportion remaining in wood, bone, cloth or parchment gives the age of the specimen. Option A is wrong because carbon-9 is a very short-lived isotope of no use in dating. Option B is wrong because carbon-12 is the ordinary stable isotope of carbon and is the standard for atomic masses, so it does not decay at all. Option D is wrong because carbon-17 is an extremely unstable isotope that survives only for a fraction of a second.

The correct answer is A, nuclear fission and nuclear fusion. A hydrogen bomb, also called a thermonuclear bomb, works in two steps. A fission explosion of uranium or plutonium produces the temperature of millions of degrees needed to force light nuclei together, and the fusion of those nuclei then releases far more energy than the fission stage alone. So both processes contribute to the explosion. Option B is wrong because fission alone describes the atom bomb of the earlier type, which is less powerful. Option C is wrong because cryogenic hydrogen, that is hydrogen liquefied at very low temperature, is rocket fuel and has nothing to do with the bomb's energy. Option D is wrong because merely compressing hydrogen releases no nuclear energy; the nuclei must be made to fuse.

The correct answer is A, Henri Becquerel. In 1896 Becquerel found that a uranium salt kept in the dark fogged a wrapped photographic plate, which showed that the salt emitted its own penetrating radiation without being exposed to light. Option B is wrong, although Marie Curie, with her husband Pierre, discovered polonium and radium in 1898, coined the word radioactivity and shared the Nobel Prize with Becquerel; the discovery of the effect itself is his. Option C is wrong because Rutherford came later and separated the emissions into alpha, beta and gamma rays, proposed the nuclear model of the atom and achieved the first artificial transmutation. Option D is wrong because Roentgen discovered X-rays in 1895, which are produced outside the nucleus and are not a form of radioactivity.

The correct answer is B, helium. An alpha particle consists of two protons and two neutrons bound together, which is exactly the nucleus of a helium-4 atom, so it carries a charge of plus two and a mass number of four. Being heavy and doubly charged it ionises the air strongly but loses energy quickly, and a sheet of paper or a few centimetres of air stops it. Option A is wrong because the hydrogen nucleus is a single proton, which is emitted in some nuclear reactions but is not an alpha particle. Option C is wrong because a lithium nucleus has three protons. Option D is wrong because the deuterium nucleus, the deuteron, has one proton and one neutron and is used in heavy water, not in alpha decay.

The correct answer is B, the mass number falls by 4 and the atomic number by 2. An alpha particle takes away two protons and two neutrons, so the parent loses four units of mass number and two units of atomic number and becomes an element two places earlier in the periodic table; uranium-238, for instance, becomes thorium-234. Option A is wrong because no decay removes two units of mass with one of charge. Option C is wrong because that describes beta minus decay, in which a neutron turns into a proton and an electron is emitted, so the charge rises by one while the mass number stays the same. Option D is wrong because that is gamma emission, in which the nucleus only sheds surplus energy and neither number changes.