Nuclear Emissions
Section: Nuclear Physics | Syllabus: Cambridge AS Level Physics 9702
The Three Types of Nuclear Emission Unstable nuclei emit three main types of ionising radiation: alpha (α), beta (β), and gamma (γ). Key Characteristics The emission of radiation from a nucleus is spontaneous (happens by itself without external influence) and random in direction (we cannot predict which direction the radiation will travel).
Exam Tip Always describe radioactive emission as "spontaneous" and "random in direction" - these are key syllabus terms. FIG 5.9: Random direction of radioactive emission Radioactive nucleus in center with arrows pointing in all random directions showing that emission is random in direction.
Alpha (α) Radiation Alpha (α) Particle A particle consisting of 2 protons and 2 neutrons (same as a helium nucleus). Nature and Structure Composition: 2 protons + 2 neutrons Symbol: 4 2 He or α Charge: +2 (due to 2 protons) Mass: 4 atomic mass units (relatively heavy) Identity: Same as a helium-4 nucleus FIG 5.10: Alpha particle structure 2 red protons (marked +) and 2 grey neutrons clustered together, with +2 charge labeled.
Properties of Alpha Radiation Ionising Effect: STRONG Most ionising of the three types Large charge (+2) pulls electrons off atoms easily Creates many ion pairs along its path Loses energy quickly due to frequent collisions Penetrating Power: WEAK Least penetrating of the three types Stopped by: A few centimetres of air, thin paper, or human skin Range in air: Only a few centimetres (typically 2-10 cm) Cannot penetrate through dead skin cells FIG 5.11: Alpha particles stopped by paper Source emitting alpha particles (thick arrows), all stopping at a sheet of paper.
Distances labeled in cm. Speed Relatively slow (compared to beta and gamma) Typical speed: ~5% of the speed of light Remember Alpha particles are highly ionising but weakly penetrating because they interact strongly with matter and lose energy quickly.
Beta (β) Radiation Beta (β⁻) Particle A fast-moving electron emitted from the nucleus when a neutron decays into a proton. Nature and Structure Composition: High-speed electron Symbol: 0 -1 e or β or β⁻ Charge: -1 Mass: Negligible (1/1840 of a proton) Origin: Comes from inside the nucleus , not from electron shells Important Beta particles are electrons, but they come from the nucleus (from neutron decay), not from the electron shells around the nucleus.
Formation of Beta Particles Inside the nucleus, a neutron transforms into a proton and an electron: Example: Beta Particle Formation neutron → proton + electron (β⁻ particle) 1 0 n → 1 1 p + 0 -1 e The proton stays in the nucleus; the electron is ejected at high speed as a beta particle.
FIG 5.12: Beta particle formation through neutron decay Before: neutron (grey sphere marked 'n'), Arrow labeled "decay", After: proton (red sphere marked 'p+') staying in nucleus + electron (small particle with arrow) shooting away.
Properties of Beta Radiation Ionising Effect: MODERATE Moderately ionising (less than alpha, more than gamma) Smaller charge (-1) than alpha particles Creates fewer ion pairs than alpha radiation Penetrating Power: MODERATE Moderately penetrating Stopped by: A few millimetres of aluminium or thin metal sheet Range in air: Up to about 1 metre Can penetrate skin but not thick metal FIG 5.13: Beta particles penetration Beta particles penetrating paper but stopped by aluminium sheet (3mm).
Source, particles passing through paper, stopped by metal. Speed Very fast (much faster than alpha) Typical speed: up to ~90% of the speed of light Remember Beta particles are in the middle for both ionising effect and penetrating power.
Gamma (γ) Radiation Gamma (γ) Ray High-energy electromagnetic radiation emitted from an unstable nucleus. Nature and Structure Composition: Electromagnetic wave (like light, but much higher energy) Symbol: γ Charge: 0 (no charge) Mass: 0 (no mass) Part of spectrum: High-frequency electromagnetic radiation Key Difference Unlike alpha and beta radiation (which are particles), gamma radiation is a wave with no mass or charge.
Emission of Gamma Rays Released when a nucleus reorganises to a more stable state Often emitted after alpha or beta decay Carries away excess energy from the nucleus Doesn't change the proton or nucleon number FIG 5.14: Gamma ray emission Nucleus in excited state (shown with energy level diagram) emitting wavy gamma ray arrows as it drops to ground state.
Excess energy released as γ ray. Properties of Gamma Radiation Ionising Effect: WEAK Least ionising of the three types No charge means less interaction with atoms Creates very few ion pairs Penetrating Power: STRONG Most penetrating of the three types Stopped/reduced by: Several centimetres of lead or metres of concrete Range in air: Unlimited (follows inverse square law) Can penetrate skin, tissue, and most materials Never completely stopped, only reduced in intensity FIG 5.15: Gamma ray penetration Gamma rays penetrating through paper, aluminium, but partially stopped by thick lead.
Intensity decreasin…
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