Artificial Radioactivity

Artificial Radioactivity

Not all nuclear reactions are spontaneous. These reactions occur when stable isotopes are bombarded with particles such as neutrons. This method of inducing a nuclear reaction to proceed is termed artificial radioactivity. These meant new nuclear reactions, which wouldn't have been viewed spontaneously, could now be observed. Since about 1940, a set of new elements with atomic numbers over 92 (the atomic number of the heaviest naturally occurring element, Uranium) have been artificially made. They are called the transuranium elements.

All naturally occurring elements with atomic numbers 83 or greater have no stable isotopes meaning they are radioactive. Other stable isotopes of other elements can also be made radioactive by bombarding stable nuclei with high energy neutrons. Changes brought about by bombarding the nucleus with high-energy particles is called artificial transmutation or nuclear transmutation. Elements in the periodic table with higher atomic numbers above 92 are called transuranic elements, none of which occur in nature and all of them are radioactive. These elements have been synthesized in nuclear reactors and nuclear accelerators, which accelerate the bombarding particles to very high speeds

Examples are:

²⁷₁₃Al  +  ¹₀n  →  ²⁴₁₁Na  +  ⁴₂He

The normally stable nuclei of aluminum are bombarded by neutrons, some of the aluminum nuclei are changed to a radioactive isotope of sodium

⁹₄Be  +  ¹₁H  →  ⁶₃Li  +  ⁴₂He

The bombarding of beryllium by protons (hydrogen nuclei) forming radioisotope lithium

Mass Defect and Binding Energy

The mass of a nucleus is always less than the combined masses of the nucleons (protons and neutrons).

Atomic mass of a proton  =  1.00728

Atomic mass of a neutron  =  1.00867

A helium nucleus contains 2 protons and 2 neutrons. So the expected mass of a ⁴₂He nucleus is:

            2(1.00728)  +  2(1.00867)  =  2.01456  +  2.01734  =  4.03190

            However, the actual mass of a ⁴₂He nucleus is 4.00150. Thus, there is a decrease of 0.03040 when 2 protons and 2 neutrons are formed as shown below.

            4.03190  –  4.00150  =  0.03040

            This difference between the total mass of the free nucleons and their mass when united in a ⁴₂He nucleus is known as mass defect, which is due to the strong forces of attraction that bind nucleons together. When protons and neutrons combine in the nucleus, energy is released. The mass defect is equivalent to the energy released as explained in E=mc².

            The amount of energy to reverse the process, to break down a nucleus into free nucleons, is known as the binding energy of the nucleus. It is equal to the energy of the mass defect and is usually expressed in megaelectron volts (MeV).