Cambridge IGCSE Physics · Syllabus 0625 · Nuclear Physics
Beta Particle
What is Beta Particle?
A beta particle is a high-speed electron emitted from the nucleus of an unstable atom. It has relative charge minus one and a very small mass, and has moderate ionising effect and moderate penetrating power.
This definition is part of the Nuclear Physics chapter in Cambridge IGCSE Physics.
Beta Particle in context
An atom is a very small, positively charged nucleus with negatively charged electrons in orbit around it, and it becomes an ion when electrons are lost or gained. The nucleus holds protons and neutrons; the proton number fixes which element it is, and the nucleon number fixes which nuclide. Some nuclides are unstable, and an unstable nucleus changes spontaneously and at random, emitting an alpha particle, a beta particle and/or gamma radiation. Those three emissions differ in ionising effect and penetrating power, which is exactly what determines how we detect them, which isotope suits which job, and how we protect ourselves. Because individual decays are random but a large sample is not, the sample has a fixed half-life — and that single number carries activity calculations, isotope selection and waste handling.
An αalpha particle is two protons and two neutrons — a helium nucleus — with relative charge \(+2\). A βbeta particle is a high-speed electron emitted from the nucleus, with relative charge \(-1\). γGamma radiation is a high-frequency electromagnetic wave with no charge. Alpha is the most ionising and the least penetrating; gamma is the least ionising and the most penetrating; beta is intermediate in both. Emission from a nucleus is spontaneous and random in direction.
Radioactive decay is a change in an unstable nucleus that can result in the emission of an alpha particle or a beta particle and/or gamma radiation; the change is spontaneous and random. During alpha decay the nucleus loses two protons, and during beta decay it gains one, so in both cases the number of protons changes — and because the number of protons is what identifies the element, the nucleus changes to that of a different element. Gamma emission changes no protons, so the element stays the same.
Common mistakes with Beta Particle
- Error Repair Common-Mistake Clinic Twenty-six errors, each with the sentence you can safely write instead and a check that it has stuck. This is a register: it is built to be scanned, so that you recognise a wrong claim when it turns up in your own writing. The full argument for each one — why it is wrong, and the model that replaces it — is in the section that teaches the concept, and the tier column points you at it. The tier column tells you which route each one matters on: a Core candidate can skip the Supplement rows. The wrong statements are quoted so that you recognise them; every quoted misconception below is false. The full misconception register for Topic 5 The false claim Tier Write this instead — the exam-safe sentence Retrieval check 1. "The nucleus occupies most of the atom." Core "Most of the atom is empty space; the nucleus is very small but contains nearly all the mass." Which two quantities does the model treat oppositely? Volume and mass. 2. "An ion forms by changing its nucleus." Core "An atom becomes a positive ion by losing electrons and a negative ion by gaining electrons; the nucleus is unchanged." An aluminium atom, \(13\) protons, \(14\) neutrons, forms a \(3+\) ion. Protons, neutrons, electrons? 13, 14, 10. 3. "Electrons and protons have equal mass because they have equal and opposite charge." Core "The proton and electron have equal and opposite relative charges, \(+1\) and \(-1\), but the electron's mass is far smaller." Which of the two does the syllabus ask you to state? The relative charges. 4. "Isotopes have different proton numbers." Core "Isotopes are atoms of the same element with the same proton number but different numbers of neutrons." Are \({}^{40}_{18}\mathrm{Ar}\) and \({}^{40}_{20}\mathrm{Ca}\) isotopes? No — different elements. 5. "The nucleon number counts the electrons too." Core "The nucleon number is the total number of protons and neutrons in the nucleus." Does \(A\) change when an atom becomes a \(2-\) ion? No — nor does \(Z\). 6. "The counter tells you which radiation it is." Core "The count rate was unchanged by paper but fell to background with a few millimetres of aluminium, so the source emits beta particles." What extra evidence identifies a radiation? Which absorbers stop it — and, on the Extended route, how it deflects in a field. 7. "Background radiation can be ignored." Supp "The background count rate was measured with the source removed and subtracted from each reading to give the corrected count rate." Corrected rate \(200\), background \(20\). What does the counter read? \(220\). 8. "Radioactive decay can be triggered on demand." Core "Radioactive decay is spontaneous: it is not caused or controlled by anything outside the nucleus." A source is heated to \(300\ ^{\circ}\mathrm{C}\). Effect on its activity? None attributable to the heating. 9. "An individual nucleus's decay time can be predicted." Core "The decay of an individual nucleus is random and cannot be predicted, but the behaviour of a large sample is statistically predictable." Give both halves from memory. Individual random; population predictable. 10. "Alpha is the most penetrating radiation." Core "Alpha is the most strongly ionising and the least penetrating; it is stopped by paper or a few centimetres of air." Unaffected by paper, stopped by \(3\ \mathrm{mm}\) of aluminium. Which radiation? Beta. 11. "Penetration and ionisation mean the same thing." Core "Gamma has the weakest ionising effect but the greatest penetrating ability, so it passes through packaging while alpha cannot." Which property makes alpha dangerous inside the body? Its strong ionising effect. 12. "Gamma is completely stopped by thin lead." Core "Thick lead or concrete greatly reduces the intensity of gamma radiation; it is not completely absorbed." Is a thin lead sheet adequate gamma shielding? No — only a small fraction is absorbed. 13. "Alpha ionises most because it has the most energy." Supp "Alpha has the largest charge, so it exerts the largest force on nearby electrons, and being massive it moves slowly and gives up its kinetic energy over a very short path, so it makes the most ion pairs per unit length." Which factor explains why gamma ionises least? Charge — it has none. 14. "Deflection directions in a field diagram can be assigned freely." Supp "With the upper plate positive, the alpha is deflected downwards towards the negative plate and the beta upwards, and further because its mass is much smaller." Swap the plate polarities. What changes? Both charged paths reverse; gamma still goes straight. 15. "Most alpha particles hit the nucleus." Supp "Most alpha particles passed straight through undeflected; a very small number were deflected through large angles because they passed close to a nucleus." Name the force and its sense. Electrostatic; repulsive. 16. "A nucleus is unstable because it is radioactive." Supp "This isotope is radioactive because its nucleus has an excess of neutrons and/or is too heavy for the repulsion between its protons to be balanced." Carbon-12 is stable and carbon-14 is not. Which cause applies? Excess neutrons — carbon-14 has two more, and neither nucleus is heavy. 17. "A beta particle is an orbital electron that escaped." Supp "In beta decay a neutron in the nucleus changes into a proton and an electron; the electron is emitted as the beta particle." If beta were an orbital electron, what would happen to \(Z\)? Nothing — contradicting the observed change of element. 18. "Gamma emission changes \(A\) or \(Z\)." Supp "In gamma emission neither the nucleon number nor the proton number changes; the nucleus loses surplus energy." Alpha then gamma: total change in \(A\) and \(Z\)? \(-4\) and \(-2\). 19. "Fission and fusion are identical." Supp "Fission splits a heavy nucleus after it absorbs a neutron; fusion joins two light nuclei. In both, the products have slightly less total mass and energy is released." Which one needs a very high temperature, and why? Fusion — both nuclei are positive and repel. 20. "The equation balances, so no mass can have been lost." Supp "The nucleon number and proton number are conserved, but the total mass of the products is slightly less than that of the reactants, and the difference is released as energy." In \({}^{235}\mathrm{U} + \mathrm{n}\), how many nucleons before and after? \(236\) both times — and mass has still fallen. 21. "Half the sample's mass disappears each half-life." Core "Half-life is the time for half the nuclei of that isotope in the sample to decay; the daughter nuclei remain in the sample." After one half-life, has the sample lost half its mass? No. 22. "After a few half-lives no nuclei remain." Core "The activity continues to halve and approaches zero, but does not reach exactly zero after a finite number of half-lives." Fraction after \(5\) half-lives? \(\tfrac{1}{32}\), about \(3.1\%\). 23. "The raw count rate always halves." Supp "Background was subtracted from each reading before the half-life was determined, because only the corrected count rate halves." Background \(30\), corrected \(240\), \(T_{1/2} = 4\ \mathrm{h}\). Counter reading at \(8\ \mathrm{h}\)? \(90\). 24. "Food irradiation makes the food radioactive." Supp "The food is exposed to gamma radiation which kills bacteria; the food does not become radioactive, because no radioactive material is added." Is a patient radioactive after an external gamma scan? No — they were exposed to radiation, not given radioactive material. 25. "External alpha radiation is always harmless." Supp "An alpha source is a relatively low hazard from outside the body, but alpha-emitting material inside the body is especially damaging because of its strong ionising effect." Why does preventing ingestion matter more than shielding for an alpha emitter? Shielding handles the external hazard easily; nothing helps once it is inside. 26. "A radioactive source can be described as completely safe." Core "The precautions reduce the dose received to a very low level, so the risk is small; the source is not without risk." Rewrite "gamma is dangerous, alpha is safe". "Gamma is the greater external hazard, alpha the greater internal one; neither is without risk." Four things that are not on this register, because they are not on the syllabus. If you have met cloud chambers or spark counters as detection methods, chain reactions, nuclear reactors, moderators, control rods or coolant, or carbon-14 dating, those come from a different syllabus. They are not errors — they are simply outside Topic 5, so no mark is available for them and no mark is lost by not knowing them.