Radiation Properties and Nuclear Instability

Alpha radiation consists of helium nuclei, carries charge +2e and produces dense ionisation over a short range. It is stopped by paper or a few centimetres of air. Beta-minus particles are electrons emitted in a nuclear process, with charge −e, a greater range and typically less dense ionisation; a few millimetres of aluminium can stop many beta particles.

Gamma photons are uncharged electromagnetic radiation, generally more penetrating and less directly ionising per unit path than alpha particles. Lead or concrete reduces gamma intensity rather than providing a sharp absolute stopping thickness. Charged alpha and beta particles deflect oppositely in fields; gamma does not. Beta is deflected much more strongly because of its much smaller mass.

Stable light nuclei have roughly equal neutron and proton numbers; stable heavier nuclei need an increasing neutron excess because neutrons add strong-force binding without adding proton–proton electrostatic repulsion. Too many neutrons commonly leads to beta-minus decay: a neutron becomes a proton, emitting an electron and electron antineutrino.

Proton-rich nuclei may undergo beta-plus decay or electron capture, lowering proton number and increasing neutron number without changing nucleon number. Alpha decay removes two protons and two neutrons and is common among heavy unstable nuclei. Gamma emission removes excitation energy without changing A or Z.

A metastable nucleus remains in an excited state long enough for that state to have a measurable half-life. Technetium-99m is useful because its de-excitation emits gamma radiation that can escape the body for detection, while its short physical half-life limits prolonged activity.

Background includes natural sources such as radon, rocks and cosmic radiation and artificial contributions from human activities. Exposure depends on location and circumstances. Count rate alone does not determine risk: radiation type, energy, absorbed dose and whether the source is internal or external matter. Alpha radiation is especially damaging internally even though it has low external penetration.

Try each problem before opening the worked solution. State the physical model, show substitutions with consistent units, and explain what your answer means.