Physics · Chapter 24
Study notes aligned to the official NEB syllabus.
Some atomic nuclei are unstable: they spontaneously break apart or rearrange themselves, emitting radiation and turning into a different nucleus in the process. This chapter covers the law that describes this decay, the kinds of radiation emitted, and the two nuclear reactions (fission and fusion) that release energy on a much larger scale than chemical reactions.
Radioactivity is the spontaneous emission of radiation (particles or high-energy photons) from an unstable atomic nucleus, as it transforms into a more stable nucleus. It was first observed by Henri Becquerel, and studied further by Marie and Pierre Curie.
| Radiation | Nature | Charge | Penetrating power | Stopped by | Effect of magnetic/electric field |
|---|---|---|---|---|---|
| Alpha ($\alpha$) | Helium nucleus, $^4_2\text{He}$ (2 protons + 2 neutrons) | $+2e$ | Least penetrating; range of a few cm in air | A sheet of paper, or the outer layer of skin | Deflected, but only slightly (heavy, so large radius of curvature) |
| Beta ($\beta$) | High-speed electron (or, for $\beta^+$, a positron) | $-e$ (or $+e$ for $\beta^+$) | Moderately penetrating; range of a few metres in air | A few mm of aluminium | Deflected strongly, in the opposite sense to alpha |
| Gamma ($\gamma$) | High-energy electromagnetic photon | Neutral | Most penetrating | Several cm of lead, or thick concrete | Not deflected (uncharged) |
Alpha decay reduces the mass number by 4 and the atomic number by 2; beta-minus decay converts a neutron into a proton (emitting an electron and an antineutrino), increasing the atomic number by 1 with no change in mass number; gamma emission carries away energy alone, with no change in mass number or atomic number, and typically follows an alpha or beta decay to let the daughter nucleus drop from an excited state to its ground state.
Alpha rays are stopped by a sheet of paper, beta rays pass through paper but are stopped by a few millimetres of aluminium, and gamma rays pass through both paper and aluminium and are only significantly attenuated by a thick layer of lead.
Some atomic nuclei are unstable: they spontaneously break apart or rearrange themselves, emitting radiation and turning into a different nucleus in the process. This chapter covers the law that describes this decay, the kinds of radiation emitted, and the two nuclear reactions (fission and fusion) that release energy on a much larger scale than chemical reactions.
Radioactivity is the spontaneous emission of radiation (particles or high-energy photons) from an unstable atomic nucleus, as it transforms into a more stable nucleus. It was first observed by Henri Becquerel, and studied further by Marie and Pierre Curie.
| Radiation | Nature | Charge | Penetrating power | Stopped by | Effect of magnetic/electric field |
|---|---|---|---|---|---|
| Alpha () | Helium nucleus, (2 protons + 2 neutrons) | Least penetrating; range of a few cm in air | A sheet of paper, or the outer layer of skin | Deflected, but only slightly (heavy, so large radius of curvature) | |
| Beta () | High-speed electron (or, for , a positron) | (or for ) | Moderately penetrating; range of a few metres in air | A few mm of aluminium | Deflected strongly, in the opposite sense to alpha |
| Gamma () | High-energy electromagnetic photon | Neutral | Most penetrating | Several cm of lead, or thick concrete | Not deflected (uncharged) |
Alpha decay reduces the mass number by 4 and the atomic number by 2; beta-minus decay converts a neutron into a proton (emitting an electron and an antineutrino), increasing the atomic number by 1 with no change in mass number; gamma emission carries away energy alone, with no change in mass number or atomic number, and typically follows an alpha or beta decay to let the daughter nucleus drop from an excited state to its ground state.
Alpha rays are stopped by a sheet of paper, beta rays pass through paper but are stopped by a few millimetres of aluminium, and gamma rays pass through both paper and aluminium and are only significantly attenuated by a thick layer of lead.