Atomic and Nuclear Structure, Part 3

Question 1: Define radioactive decay and half-life. Discuss decay of isotopes and artificial radioactivity.

Answer 1: Radioactive decay, or radioactivity, is a set of processes that allow unstable atomic nuclei, or nuclides, to emit subatomic particles, or radiation. The decay is a random process and it is not possible to predict an individual atom's decay. Radioactive decay occurs at an exponential rate. This means that the amount of radioactive material, A, present after time t is given by the equation A = A0ekt, where A0 is the amount of material present at time t = 0, and k is the activity of the material. The activity may be given, but it can also be determined if the amount of material present at two different times is known.The time required for half the mass of a radioactive material to decay is known as its half-life. Half-life is the most commonly used measure of a material’s rate of decay. A half-life can be as short as a fraction of a second or as long as millions of years.Certain isotopes that are known to be unstable and spontaneously decay are referred to as radioisotopes. One common radioisotope used to determine the age of an object is carbon-14. Over time, this isotope decays to nitrogen-14. The half-life of carbon-14 is about 5,730 years.An ordinarily stable isotope may be artificially made radioactive by bombarding it with a stream of neutrons. When these neutrons are captured by a previously stable nucleus, it can become unstable and begin to decay.

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Question 2: Define alpha decay.

Answer 2: Alpha decay is a type of radioactive decay in which an alpha particle is ejected from the nucleus. An alpha particle is the nucleus of a helium atom, consisting of 2 protons and 2 neutrons. Ejection of an alpha particle reduces the parent nuclide’s atomic number by 2 and its mass number by 4. An example of alpha decay is given by:Alpha decay can be looked at as nuclear fission, in which the parent nucleus splits into a pair of daughter nuclei. Alpha decay is governed by the strong nuclear force, or the force between two or more nucleons. Alpha particles are ejected from the nucleus at speeds around 15,000 km/s with a typical kinetic energy of 5 MeV.

Question 3: Define beta decay.

Answer 3: Beta decay is radioactive decay in which there is an emission of a beta particle, a high-energy electron or positron. For an electron emission, it is known as beta minus (ß-) decay. For a positron emission, it is known as beta plus (ß+) decay. In ß- decay, a neutron is converted into a proton, an electron, and an antineutrino, ?e:This is because a down quark is converted to an up quark, emitting an electron and an antineutrino. In ß+ decay, a proton is converted into a neutron by means of an up quark becoming a down quark, emitting a positron and a neutrino:Unlike ß- decay, ß+ decay cannot occur spontaneously. It requires an input of energy.In both types of beta decay, the atomic number changes, but the mass number remains constant.

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