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.
What are the features of the formula, N = N0e^-λt?
N0 is the number of undecayed nuclei at time t=0.
N is the number of undecayed nuclei at time t.
λ is the decay constant
What are the features of the formula, A = A0e^-λt?
A0 is the number of undecayed nuclei at time t=0.
A is the number of undecayed nuclei at time t.
λ is the decay constant
How is exponential decay modelled?
1) Start with given number N0 of undecayed nuclei in the sample.
2) Choose a small time interval (very small compared to the half life of the isotope).
3) Calc number of decaying nuclei ΔN within the source during the time interval (using ΔN/Δt = λN).
4) Calc number of undecayed nuclei at end point of time period by doing N - ΔN.
5) Repeat
What is used for carbon dating?
Radioactive isotope - carbon-14.
It has a half life of approx. 5700 years and is produced by upper atmosphere cosmic rays.
How is carbon-14 formed?
High speed protons in cosmic rays from space colliding with atoms in the upper atmosphere produce neutrons.
These neutrons then collide with nitrogen-14 nuclei to form carbon-14 nuclei (which emit electrons and become/replenish nitrogen 14).
What are the limitations to carbon dating?
-It assumes that the ratio of carbon-14 to carbon-12 atoms has remained constant over time.
Increased emission of CO2 due to fossil fuels may have reduced this ratio.
The ratio would also be affected by solar flares from the sun and by testing nuclear bombs.
C14 amounts in organisms are also very small, compared to the background count rate.
What is used for dating rocks?
Rb 87 - half life of carbon 14 is not long enough. (Rb 87 half life is approx. 49 bil yrs).
