Chapter 14 - Radioactivity Flashcards
When is gamma emitted?
After alpha or beta decay to release excess energy from the nuclei.
Define positron?
A positively charged particle with mass equal to that of an electron (form of antimatter).
Learn on the table in notes?
Now
Define neutrino?
An uncharged particle with almost no mass emitted during beta decay.
Word equation for beta minus decay?
Neutron -> proton + electron + antineutrino
When are alpha or beta particles emitted?
When a nucleus with a large imbalance between its protons and neutrons becomes stable it emits alpha or beta.
Word equation for beta plus decay?
Proton -> neutron + positron + neutrino
Define weak interaction/nuclear force?
The nuclear force responsible for beta decay.
Acts on both quarks and leptons
What is ionisation?
When fast moving particles (alpha or beta) drag electrons away from atoms when they collide.
What happens to Gamma Alpha and beta particles when they pass through an electric field?
- alpha particles are pulled slightly towards the negative plate.
- beta particles are pulled heavily towards the positive place (less mass than alpha therefore deflect more)
- gamma rays move in a straight line and are unaffected.
What happens to Gamma Alpha and beta particles when they pass past a magnet?
- beta particles are attracted towards the magnet.
- alpha particles are deflected away from the magnet.
- gamma rays are unaffected by the magnetic field so continue in a straight line.
What materials and thicknesses will block Alpha beta and gamma radiation?
Alpha - paper
Beta - lead (few mm)
Gamma - lead (few cm) of concrete (few m)
What piece of equipment can be used to detect Alpha beta and gamma radiation?
Geiger-müller tube
Name two other piece of equipment that can detect alpha radiation?
Cloud chamber or solid-state detector.
Two words to describe decay?
Spontaneous and random
What makes decay spontaneous?
- that decay of a nucleus isn’t affected by the presence of other nuclei
- that decay of nuclei can’t be affected by external factors such as temp. or pressure
What makes decay random?
- that it’s impossible to predict when a nucleus will decay
- that each nucleus in the sample has same chance of decaying per unit time
Define decay constant (λ)?
The probability that an individual nucleus will decay per unit time interval (s^-1 or h^-1 etc.).
Define activity?
The rate of decay or disintegration of nuclei in a radioactive sample (in decays/second = Bq).
Activity equation? (2)
A=λN
N = number of undecayed nuclei in sample
A=ΔΝ/Δt
Define count rate?
The number of particles or gamma-ray photons detected per unit time by a Geiger-Müller tube.
Two reasons why the count rate is not the true value?
- some radiation absorbed by sample
- some radiation not from sample due to background radiation
Define half life?
The mean time taken for half the number of active nuclei in a radioactive sample to decay.
Exponential decay equations for count rate (R), number of undecayed nuclei (N) and activity rate (A)?
N = N0e^(-λt)
R = R0e^(-λt)
Α = Α0e^(-λt)
0 means original/initial value
Define carbon dating?
A technique used to date relics using the carbon-14 isotope.
Explain how carbon dating works?
All living organisms absorb small amounts of carbon-14. It has a half life roughly 5800 years. Once an organism dies it does not absorb any more carbon-14, and so the remaining radioactive isotope decays gradually. Therefore the time since death can be determined using a mass spectrometer to measure the activity.
Explain four problems with carbon dating?
- recently dead organisms mean not enough change in carbon-14 content therefore it won’t work (min 100years).
- modern organisms may have less carbon-14 sent so many fossil fuse have been burnt.
- such a small amount of carbon-14 means there is a low activity rate so it can take a while.
- +-40 years uncertainty on carbon-14 half life.
What are the two half life equations?
N/N0 = e^(-λt(1/2)) = 0.5
Therefore
λ = ln(2)/t(1/2)
t(1/2) is half life
Why is the activity for very old samples measured over a period of time and then averaged?
Activity is very small/decay is random
Why can carbon dating not be used for extremely old samples?
The activity is so low it cannot be differentiated from the background radiation
