C25 - Radioactivity Flashcards
What’s a cloud chamber?
A detector of ionising radiation consisting of a chamber filled with air saturated with vapour at a very low temperature so that droplets of liquid condense around ionised particles left along the path of radiation.
What does alpha radiation consist of?
Positively charged particles.
Each alpha particle comprises 2 protons and 2 neutrons and has a +2e charge.
What does beta radiation consist of?
Fast moving electrons (if beta-) or positrons (if beta+) with a charge of either e- or e+.
What does gamma radiation consist of?
High energy photons with wavelengths less than 10 ^-13 m.
They travel at speed of light and carry no charge.
What happens when alpha, beta and gamma radiation enter an electric field?
Alpha: are deflected (less than beta) towards the negative plate.
Beta: β+ (positrons) are deflected towards the negative plate. β- (electrons) are deflected towards the positive plate.
Gamma: not deflected.
What happens when alpha, beta and gamma radiation enter an magnetic field?
Alpha and beta are deflected. (Their direction can be determined with Fleming’s left hand rule).
Gamma aren’t deflected.
How easily are alpha, beta and gamma radiation absorbed?
Alpha: particles have a large mass and charge so interact with surrounding particles to produce strong ionisation.
Therefore they have a short range in air.
Beta: small mass and charge makes it less ionising than alpha particles. They have a larger range in air.
Gamma: have no charge. They are the least ionising and travel the farthest range.
What happens in the nuclear transformation equation for alpha decay?
A parent nucleus decays into a daughter nucleus and emits an alpha particle.
Loss of an alpha particle removes 2 protons and 2 neutrons from a parent nucleus (nucleon number drops by 4).
Total nucleon and proton numbers before and after are the same (equation is balanced).
Energy is released in the decay.
What happens in the nuclear transformation equation for beta- decay?
It’s characterised as having too many neutrons for stability.
The weak nuclear force is responsible for one of the neutrons decaying into a proton. In the process, an electron and electron anti-neutrino are emitted.
What happens in the nuclear transformation equation for beta+ decay?
It’s characterised as having too many protons for stability.
The weak nuclear force is responsible for one of the protons decaying into a neutron. In the process, a positron and electron neutrino are emitted.
What happens in the nuclear transformation equation for gamma decay?
Gamma photons are emitted if a nucleus has surplus energy following an alpha or beta emission.
The nucleus composition remains the same but a gamma photon is emitted.
What makes radioactive decay random and spontaneous?
Random:
- can not predict when a nucleus will decay
- each nucleus has the same chance of decaying per unit time
Spontaneous:
- decay is not affected by presence of other nuclei
- external factors e.g. pressure
What is a becquerel?
The unit for radioactive decay per second.
1 Bq = one decay per second
What is the decay constant?
The probability of decay of an individual nucleus per unit time.
What does the formula A = λN show?
The activity A of the source is equal to the number of nuclei decreasing over time N * the decay constant.
