Show/Hide Sub-topics (Radioactivity and the nuclear atom | O Level)

Radioactive decay refers to the process in which α-particles and β-particles are emitted by an unstable nuclei (contains too many neutrons or protons) of an element in order to form a more stable nuclei of another element.

The decaying nucleus is called the ‘parent’ nucleus; the resulting nucleus is called the ‘daughter’ nucleus.

Decay processes are written in the form of an equation. In the following equations, parent nuclide ‘X’ (unstable) changes into a daughter nuclide ‘Y’ (more stable) with the emission of α-particles or β-particles or γ-rays.

Alpha Decay

An alpha particle can be represented as  $^{4}_{2} He$ or $^{4}_{2} \alpha$. When a nucleus decays by alpha emission, proton number or atomic number ‘Z’ decreases by 2 and its mass number or nucleon number ‘A’ decreases by 4.

$^{A}_{Z}X \rightarrow ^{A-4}_{Z-2}Y + ^{4}_{2}He + energy$

Example: $^{226}_{88}Ra \rightarrow ^{222}_{86}Rn + ^{4}_{2}He + energy$

Beta Decay

In nuclear equation, β-particles is written as $^{0}_{-1} \beta$ or $^{0}_{-1} e$. In beta decay, nucleon number ‘A’ of the nucleus remains unchanged but the atomic number ‘Z’ increases by one.

During this process, a neutron splits into a proton, an electron and a positron (which decays rapidly into pure energy). The proton number now increases. The new electron is expelled as β-particles.

$^{A}_{Z}X \rightarrow ^{A}_{Z+1}Y + ^{0}_{-1}e + energy$

Example: $^{24}_{11}Na \rightarrow ^{24}_{12}Mg + ^{0}_{-1}\beta + energy$

Gamma Emission

The emission of gamma rays has no effect on nucleon number or proton number of the nucleus.

γ-rays are usually emitted at the same time as α-particles and β-particles. With some nuclides, the emission of α-particles and β-particles from a nucleus leaves the electrons and neutrons in an excited arrangement with more energy than normal.

These protons and neutrons rearrange themselves to become more stable and release the excess energy as a photon of gamma radiation.

Existence: The G-M tube usually detects 20 to 60 counts per minute without a radioactive source. This is due to background radiation.

Origin: Background radiation is produced by tiny fragments of radioactive elements present in all rocks and soil, the atmosphere and even in living material itself. Also, the earth is continuously bombarded by high-speed particles from the outer space and from the sun (cosmic rays).

Significance: Natural radioactive elements produce a radioactive gas, radon, which may accumulate in buildings, so increasing the local background count. Whenever taking readings with GM tube, the background count should be established at the start and deducted from subsequent readings to avoid systematic error.