Published by Patrick Mutisya · 14 days ago
The half‑life (\$t_{1/2}\$) of a radioactive isotope is the time required for half of the original nuclei to decay. It is a characteristic property of each isotope and determines how quickly the activity (decays per second) decreases:
\$\$
N(t)=N0\left(\frac{1}{2}\right)^{\frac{t}{t{1/2}}}
\$\$
where \$N_0\$ is the initial number of nuclei and \$N(t)\$ is the number remaining after time \$t\$.
| Radiation | Charge | Penetration | Typical shielding |
|---|---|---|---|
| Alpha (α) | +2 | Very low – stopped by a sheet of paper or skin | Paper, plastic |
| Beta (β) | −1 (electron) or +1 (positron) | Moderate – a few millimetres of aluminium | Aluminium, acrylic |
| Gamma (γ) / X‑ray | 0 (photon) | High – many centimetres of lead or several metres of concrete | Lead, concrete, steel |
| Isotope | Radiation | Half‑life | Typical activity (Ci or Bq) | Reason for selection |
|---|---|---|---|---|
| Americium‑241 | α (with some γ) | 432 years | \overline{37} kBq (1 µCi) per alarm | Long half‑life gives a stable source for decades; α particles ionise air but are stopped by the alarm’s detector window. |
| Cobalt‑60 | γ (1.17 MeV & 1.33 MeV) | 5.27 years | 10⁴–10⁶ Ci for industrial irradiators | High‑energy γ rays penetrate food and medical equipment but are easily shielded for safety. |
| Cobalt‑60 (again) | γ | 5.27 years | 10⁴–10⁶ Ci for sterilisation | Same as above – sufficient penetration to sterilise packed items. |
| Iridium‑192 | γ (0.3–1.4 MeV) | 73.8 days | \overline{10} Ci for industrial radiography | Short half‑life gives high activity for brief inspections; γ energy suitable for thickness measurement. |
| Cobalt‑60 (again) | γ | 5.27 years | Therapeutic beams of 1–2 MeV | Penetrates deep tumours while allowing precise dose control. |
Smoke detectors use a small sealed source of Americium‑241. The α particles ionise the air in a chamber, creating a small electric current. When smoke enters, it attaches to the ions, reducing the current and triggering the alarm.
Food irradiation employs high‑energy γ rays, most commonly from Cobalt‑60. The photons penetrate the food pack, breaking DNA bonds in bacteria and other microorganisms, rendering them inactive.
Medical and laboratory equipment are sterilised in “gamma chambers” using the same Cobalt‑60 source. The high‑energy photons destroy bacterial spores without heating the items, preserving heat‑sensitive materials.
Industrial radiography uses isotopes that emit γ rays of suitable energy to assess material thickness. The principle is based on exponential attenuation:
\$\$
I = I_0 e^{-\mu x}
\$\$
where \$I\$ is the transmitted intensity, \$I_0\$ the initial intensity, \$\mu\$ the linear attenuation coefficient, and \$x\$ the thickness.
In radiotherapy, external beam machines (e.g., linear accelerators) generate high‑energy photons, but in some facilities sealed sources of Cobalt‑60 are used (Cobalt‑60 teletherapy units). The γ rays deposit energy in tumour tissue, damaging DNA and killing cancer cells.
| Application | Isotope (radiation) | Half‑life | Key reason for choice |
|---|---|---|---|
| Smoke alarm | Americium‑241 (α) | 432 years | Very long life, α particles safe inside sealed chamber |
| Food irradiation | Cobalt‑60 (γ 1.17 & 1.33 MeV) | 5.27 years | High‑energy γ penetrates bulk food; manageable decay rate |
| Equipment sterilisation | Cobalt‑60 (γ) | 5.27 years | Uniform dose, deep penetration, long service life |
| Thickness measurement (radiography) | Iridium‑192 (γ 0.3–1.4 MeV) | 73.8 days | High activity for quick imaging; energy suited to moderate thicknesses |
| Cancer radiotherapy | Cobalt‑60 (γ) | 5.27 years | Stable, high‑energy photons for deep tumours; well‑understood dosimetry |