Radioactivity Flashcards
What are the 3 types of radiation
Alpha, Beta and Gamma
What is alpha radiation
The emission of an alpha particle
What is an alpha particle
2 protons and 2 neutrons
What is beta radiation
The emission of a beta particle
What is a beta particle
A fast moving electron
What is gamma radiation
A gamma wave
What is a gamma wave
A high energy EM wave
Properties of alpha radiation
Range of 3cm in air
Stopped by paper/skin
Heavily ionising
Slow
Deflected by E and B fields
Properties of Beta radiation
Range of 1m in air
Stopped by 1-3mm aluminium
Moderately ionising
Fast
Deflected by E and B fields
Properties of Gamma radiation
Infinite range in air
Stopped by thick lead
Lightly ionising
Speed of light
Not deflected by E and B fields
How do alpha beta and gamma radiation interact with E fields
Opposite charges attract
How do alpha beta and gamma radiation interact with B fields
Fleming’s Left Hand Rule
Conventional current flows in the same direction as positive charge motion
Dangers of ionising radiation
Damages living cells
Cell DNA is damaged either directly or by creating ions/free radicals that react with DNA
Damaged DNA may cause cells to divide and grow uncontrollably, causing a tumour that may be cancerous
High doses kill living cells outright
Cell mutations and cancerous growth occur at both high and low doses
Precautions to observe with ionising radiation
No source must be allowed to come into contact with skin
Solid sources must be transferred using tongs, a glove-box or robot - minimum exposure
Liquid, gas and solids in powder form must be in sealed container - can’t be inhaled. spilt on hands or drunk
Radioactive sources should not be used for longer than necessary
What are the 2 methods of detection
Cloud Chamber and Geiger Tube
How does a cloud chamber work
Contains air saturated with water at a very low temperature
Alpha/Beta particles will ionise the air and the water molecules will be attracted to the ions, creating a track of minute condensed water droplets
Describe alpha particle tracks in a cloud chamber
Straight, radiating from source
Tracks from given isotope of same length
Same range/ionising power/energy
Describe beta particle tracks in a cloud chamber
Wispy tracks because beta particles are easily deflected by air molecules
Less easy to see than alpha particle tracks
because beta particles are less ionising
How does a Geiger Tube work
Sealed tube that contains argon gas at low pressure
Thin mica window at end allows alpha and beta particles to enter
A metal rod down the centre of tube is at a positive potential with respect to the tube walls - E field in tube
When an alpha or beta particle enters the tube, it ionises all gas particles along its track
Negative ions attracted to rod - positive to tube walls
Ions accelerate - collide with more gas particles - creating more ions
How does detection work in a Geiger Tube
Many ions created in short time and discharged at electrodes
Pulse of charge travels around circuit for each discharge - causing a voltage pulse across R - which is recorded as a single count
How does absorption work in a Geiger Tube
Geiger tube and counter used to investigate absorption by different materials
Number of counts in a given time measured and used to calculate count rate by dividing by time
Before the count rate is examined, the count rate due to background ration must be measured
Count rate is then measured with absorber between source and tube, and background count rate subtracted from it
By using absorbers of the same material but different thickness, the effect of absorber thickness can be investigated
Number of counts in a given time measured and used to calculate count rate by dividing by time
Define Transmutation
Name given to the process by which an unstable nucleus decays and becomes a new nucleus
Parent nucleus becomes a daughter nucleus
The daughter nucleus itself may be unstable and decay to another nucleus
The process continues until a stable nucleus is formed
Nucleon number, atomic number, mass/energy are all conserved during a transmutation
Desribe Alpha Decay
Energy is released during an alpha decay owing to the mass deficit between mass of parent nucleus, and the sum of masses of daughter nucleus and alpha particle
Helium nucleus is emitted
What happens during Beta Minus decay
An electron and anti neutrino are emitted
What emits Beta Minus Radiation
Nuclei that have too many neutrons for stability
What happens during Beta plus decay
A positron and neutrino are emitted
What emits Beta plus radiation
Nuclei that have too many protons for stability
What happens during Gamma decay
The emission of a y photon
Doesn’t change the number of protons or neutrons but allows the nucleus to lose energy
Only happens if daughter nucleus is formed in an excited state - following an alpha or beta emission or electron capture
If daughternucleus stays in the excited state for one time - it can be separated from the parent nucleus
Thus a source of only gamma radiation is produced
Look at NZ graphs
Ok
Define Half Life
Average time taken for half the number of unstable nuclei in a sample to decay
Formula for mass remaining after half life
0.5^n x Initial mass
Where n is number of half lives
Define radioactive activity
Number of nuclei that decay every second l
Unit Bq
1 Bq = 1 decay per second
Formula for Activity (A)
A = lambda x N
Lambda is decay constant
N is number of nuclei
Nature of decay
Random and spontaneous
Alpha Decay Equation
A A-4 4
X -> Y + He
Z Z-2 2
Beta Decay Equation
A A 0
X -> Y + e + anti electron neutrino
Z Z+1 -1
Gamma Decay Equation
A A
X -> X + Gamma ray
Z Z
Formula for half life
lambda t1/2 = ln(2)
Model Exponential decay
Delta N / Delta t = -lambda x N
N = N0 x e^-lambda x t
- Start with a given number of undecayed nuclei N0
- Choose a very small interval of time, which must be very small compared with the half-life of the isotope so that we can assume activity does not change significantly
- Calculate number of nuclei decaying with the equation Delta N = Delta t x lambda x N
- Calculate the number of undecayed nuclei in the source at the end of the period by subtracting delta N from the previous value of N
Limitations to carbon dting
Assumes ratio of carbon-14 atoms:carbon-12 atoms has stayed constant over time
Explain carbon dating
Carbon-14:Carbon-12 nuclei in atmospheric carbon is almost constant at 1.3x10^-12 - same ratio in all living things
Once an organism dies, it stops taking in carbon, whilst the amount of carbon-14 it contains continues to decay
Activity from carbon-14 is proportional to undecayed carbon-14 nuclei
Why can we not use carbon-14 to dare rocks on the Earth to meteors
Formed during the creation of the Solar System
Half-life is not long enough for these ages
Use rubidium-87 instead which emits beta minus particles
Formula for decrease in activity with time
A = A0 x e^-lambda x t
A0 is the activity at time t
Formula for decrease in undecayed nuclei with time
N = N0 x e^-lambda x t
N0 is the number of undecayed nuclei at time t
