Nuclear Chemistry and Applications of Radioactivity

Radioactivity is the spontaneous emission of radiation (alpha particles, beta particles or gamma rays) from the unstable nucleus of an atom, accompanied by the disintegration of the nucleus and release of energy. It is a nuclear property that does not depend on temperature, pressure or chemical state.

Units of radioactivity:

• Becquerel (Bq) — SI unit; 1 Bq = 1 disintegration per second.
• Curie (Ci) — 1 Ci = $3.7 \times 10^{10}$ disintegrations per second.

Artificial radioactivity (also called induced radioactivity) is the phenomenon in which a stable nucleus is made radioactive by bombarding it with high-energy particles such as $\alpha$-particles, protons, neutrons or deuterons. The resulting new nucleus is unstable and emits radiation to become stable.

Example: When aluminium-27 is bombarded with an $\alpha$-particle, radioactive phosphorus-30 is formed, which decays by positron emission to silicon-30:

${}^{27}_{13}\text{Al} + {}^{4}_{2}\text{He} \rightarrow {}^{30}_{15}\text{P} + {}^{1}_{0}\text{n}$

${}^{30}_{15}\text{P} \rightarrow {}^{30}_{14}\text{Si} + {}^{0}_{+1}\text{e} \ (\text{positron})$

This was first demonstrated by Irène Curie-Joliot and Frédéric Joliot in 1934.

Nuclear fusion requires extremely high temperature (about (10^7\ \text{K})) to overcome the electrostatic repulsion between nuclei and initiate the reaction. Since the reaction is initiated and sustained only at very high temperature (thermal energy), it is called a thermonuclear reaction.

Nuclear fusion is the process in which two light nuclei combine (fuse) to form a heavier nucleus, with the release of a large amount of energy. For example:

$_1^2\text{H} + _1^3\text{H} \rightarrow _2^4\text{He} + _0^1\text{n} + \text{energy}$

It is called a thermonuclear reaction because fusion can occur only at extremely high temperatures (of the order of $10^7$ K). At such temperatures the nuclei acquire enough kinetic energy to overcome the electrostatic repulsion between them and come close enough for the short-range nuclear force to bind them together.

Uses of radioisotopes in the medical field:

1. Diagnosis (imaging): Radioisotopes such as technetium-99m and iodine-131 are used as tracers. They are introduced into the body and their emitted radiation is detected to locate tumours, study the functioning of organs like the thyroid, kidney and brain, and detect blockages.

2. Treatment (radiotherapy): Radioisotopes such as cobalt-60 and iodine-131 are used to destroy cancerous (malignant) cells. Cobalt-60 emits high-energy gamma rays that are directed at tumours to kill the cancer cells.

Nuclear fission vs nuclear fusion

Artificial vs natural radioactivity

The key difference is that natural radioactivity is spontaneous, while artificial (induced) radioactivity is man-made.

Nuclear radiations ($\alpha$, $\beta$, $\gamma$ rays and neutrons) are ionising: they knock electrons out of atoms and damage living cells.

• They break DNA and cause mutations, which can lead to cancers such as leukaemia and thyroid cancer.
• Large doses cause radiation sickness (vomiting, hair loss, burns on the skin, damage to bone marrow and a falling blood count) and can be fatal.
• Damage to reproductive cells causes genetic defects in later generations, and exposure of a pregnant woman can deform the foetus.
• Other effects include cataract of the eye and a shortened life span.

(d) Alpha rays deviate towards C, beta rays deviate towards A and gamma rays direct towards B.

Reason: Alpha particles carry a positive charge, so they are attracted towards the negatively charged plate (below) and bend downwards to strike C. Beta particles carry a negative charge, so they are attracted towards the positively charged plate (above) and bend upwards to strike A. Gamma rays are neutral (uncharged) electromagnetic radiation, so they are not deflected by the electric field and go straight to strike B.