Radioactivity and Nuclear Energy: Decay, Fission, Reactors

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.

The correct answer is C, gamma rays. Gamma radiation is electromagnetic, carries no charge and no rest mass, and therefore passes through matter far more easily than charged particles; thick lead or concrete is needed to reduce it. Option A is wrong because the alpha particle, though the most strongly ionising of the three, is stopped by a sheet of paper or a few centimetres of air precisely because it interacts so strongly. Option B is wrong because beta particles, being light electrons, penetrate more than alpha but are stopped by a few millimetres of aluminium. Option D is wrong because the three differ widely. The useful rule is that penetrating power increases from alpha to beta to gamma while ionising power decreases in the same order.

The correct answer is A, as a moderator. Heavy water, in which the hydrogen of ordinary water is replaced by deuterium, slows the fast neutrons released in fission to low speeds at which they are far more likely to split further uranium nuclei, and it absorbs very few neutrons in doing so, which is why Indian pressurised heavy water reactors can use natural uranium. In such reactors heavy water also serves as coolant. Option B is wrong because the fuel is uranium or plutonium. Option C is wrong because control rods are made of neutron absorbers such as cadmium or boron, whose purpose is the opposite, to take neutrons out of the reaction. Option D is wrong because shielding is provided by thick concrete and lead around the core.

The correct answer is B, cadmium and boron. Both absorb neutrons strongly, so pushing the rods into the core removes neutrons and slows the chain reaction, while withdrawing them allows the reaction to build up; this is how the power of a reactor is controlled and how it is shut down. Option A is wrong because graphite and heavy water are moderators, which slow neutrons without absorbing them, the opposite requirement. Option C is wrong because lead and concrete form the biological shield that keeps radiation from escaping the reactor building. Option D is wrong because uranium and plutonium are the fissile fuels that supply the neutrons in the first place. Candidates should fix moderator, control rod, coolant and shield to their own materials. A reactor therefore holds four distinct materials in four distinct roles, and a question that names any one of them is really asking which role it fills.

The correct answer is B, the nuclear fusion of hydrogen nuclei. In the core of the Sun, at temperatures of millions of degrees, hydrogen nuclei fuse in stages to form helium, and the small amount of mass lost in the process appears as energy according to Einstein's mass-energy relation. Option A is wrong because fission needs heavy nuclei such as uranium, which are not the fuel of a star. Option C is wrong because chemical burning releases only a few electron volts per reaction, nowhere near enough to keep the Sun shining for billions of years, and there is no oxygen in the core to burn with. Option D is wrong because radioactive decay of heavy elements warms the interior of the Earth to some extent but is not the source of solar energy.

The correct answer is A, the becquerel. One becquerel is one disintegration per second, and it is the SI unit of the activity of a radioactive source, named after the discoverer of radioactivity. Option B is wrong because the curie is the older unit of activity, based on the activity of a gram of radium, and one curie equals 3.7 into ten to the power ten becquerel; it is still quoted but is not the SI unit. Option C is wrong because the gray measures absorbed dose, that is the energy deposited in a kilogram of matter, and it replaced the rad. Option D is wrong because the sievert measures equivalent dose, which weights the absorbed dose by the harm the type of radiation does to tissue, and it replaced the rem.

The correct answer is A, Tarapur in Maharashtra. The Tarapur Atomic Power Station was the first nuclear power station in India and also the first in Asia, built with American collaboration and using boiling water reactors, unlike the pressurised heavy water reactors of the later Indian stations. Option B is wrong because Kalpakkam, the Madras Atomic Power Station, came later and is known as the site of the prototype fast breeder reactor of the second stage. Option C is wrong because Rawatbhata in Rajasthan was the second station, built with Canadian collaboration. Option D is wrong because Narora in Uttar Pradesh is later still. Candidates should keep Apsara, the first research reactor at Trombay in 1956, separate from Tarapur, the first power station. Apsara was a research reactor for experiments and isotope production, while Tarapur generated electricity for the grid, and that difference is what the question turns on.

The correct answer is C, thorium. Homi Bhabha designed the programme around the fact that India has modest uranium but very large thorium reserves in the monazite sands of the Kerala coast. The third stage is to convert thorium into uranium-233 and burn it in reactors designed for that fuel, which would give the country a fuel supply of its own for centuries. Option A is wrong because natural uranium is the fuel of the first stage, burnt in pressurised heavy water reactors. Option B is wrong because plutonium, recovered by reprocessing the spent fuel of the first stage, is the fuel of the second stage in fast breeder reactors such as the prototype at Kalpakkam. Option D is wrong because beryllium is used as a neutron reflector and moderator, not as fuel.