Radioactivity and particles flashcards

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Describe the structure of an atom.
A tiny, dense nucleus of protons and neutrons, with electrons orbiting in shells around it. The atom is mostly empty space, and overall neutral.
State the relative mass and charge of a proton, neutron and electron.
Proton: mass 1, charge +1. Neutron: mass 1, charge 0. Electron: mass about 1/2000, charge −1.
What are the atomic number and mass number?
Atomic (proton) number Z is the number of protons. Mass (nucleon) number A is the total number of protons and neutrons. Neutrons = A − Z.
What are isotopes?
Atoms of the same element with the same number of protons but different numbers of neutrons. They have the same chemical properties but different masses.
How many protons, neutrons and electrons are in carbon-14 (atomic number 6)?
Protons 6, electrons 6 (neutral atom) and neutrons 14 − 6 = 8.
How did the alpha-particle scattering experiment change the model of the atom?
Most alpha particles went straight through, some deflected, and very few bounced back. So the atom is mostly empty space with a small, dense, positive nucleus.
What is radioactive decay and why is it random?
An unstable nucleus emits radiation to become more stable. It cannot be predicted when a given nucleus will decay, but the rate for a large sample is predictable.
What are the three types of ionising radiation and what is each?
Alpha (α): a helium nucleus, 2 protons and 2 neutrons. Beta (β⁻): a fast electron from the nucleus. Gamma (γ): electromagnetic radiation of high frequency.
Compare the ionising power of alpha, beta and gamma.
Alpha is the most ionising, beta is moderate and gamma is weakly ionising. Ionising radiation knocks electrons off atoms, creating ions.
Compare the penetrating power and range in air of alpha, beta and gamma.
Alpha is stopped by paper and travels a few cm in air. Beta is stopped by a few mm of aluminium and travels about a metre. Gamma is reduced by thick lead or concrete and travels far.
How are alpha, beta and gamma affected by electric and magnetic fields?
Alpha (positive) and beta (negative) are deflected in opposite directions, beta more because it is lighter. Gamma is uncharged so is not deflected.
What is background radiation and give two sources.
Low-level radiation always present. Sources include radon gas from rocks, cosmic rays, food, buildings, medical sources and nuclear waste.
How is radiation detected?
A Geiger–Müller tube and counter measure count rate in counts per second or minute. Photographic film also darkens with radiation.
How should you correct a measured count rate?
Measure the background count rate with no source present and subtract it from the count rate measured with the source.
What happens in alpha decay to the mass number and atomic number?
Mass number falls by 4 and atomic number falls by 2. Example: radium-226 → radon-222 + alpha (²²⁶₈₈Ra → ²²²₈₆Rn + ⁴₂α).
What happens in beta decay to the mass and atomic numbers?
A neutron changes into a proton and an electron is emitted. Mass number is unchanged and atomic number rises by 1. Example: ¹⁴₆C → ¹⁴₇N + ⁰₋₁e.
What happens to the nucleus in gamma emission?
Gamma emission releases energy from an excited nucleus, so the mass number and the atomic number do not change.
Define half-life.
The time taken for the number of undecayed nuclei, or the count rate or activity of a sample, to fall to half its original value.
A sample has 80 g of a radioactive isotope with half-life 6.0 h. How much remains after 18 h?
18 h is 3 half-lives. 80 → 40 → 20 → 10, so 10 g remains.
A source has count rate 400 counts/min with half-life 5 days. What is the count rate after 10 days?
10 days is 2 half-lives: 400 → 200 → 100 counts per minute (after background correction).
How can you find a half-life from a decay graph?
Read the time for the activity (or count rate) to fall from any value to half of it. Repeat for several starting values and average.
Give two uses of radioactive isotopes and say why the type of radiation chosen suits each use.
Smoke detectors use alpha (strongly ionising, short range). Thickness monitoring uses beta (partly absorbed). Medical tracers and sterilising use gamma (penetrating), with short half-lives.
Why is a source with a short half-life used for medical tracers?
It gives enough activity to detect while in the body but decays away quickly, so the patient receives a small total dose.
What are the dangers of ionising radiation?
It can damage or kill cells and cause mutations leading to cancer. Higher exposure gives higher risk.
How should radioactive sources be handled safely?
Use tongs, store in lead-lined containers, minimise exposure time, keep a distance from the source and never point it at people. Wear dosimeter badges if exposed regularly.
Why can alpha emitters be more dangerous inside the body than outside?
Alpha is stopped by skin outside, but inside the body it is absorbed by tissue over a short range and is the most ionising, so it delivers high damage to nearby cells.
What is nuclear fission?
A large unstable nucleus, such as uranium-235, splits into two smaller nuclei after absorbing a neutron, releasing energy and more neutrons.
How does a chain reaction occur in fission?
Neutrons released by one fission cause further nuclei to split, releasing more neutrons. In reactors, control rods absorb neutrons to keep it steady.
What is nuclear fusion and where does it happen?
Two light nuclei join to form a heavier nucleus, releasing energy. It powers stars, but needs extremely high temperature and pressure to overcome electrostatic repulsion.

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