Radioactivity Flashcards
When are atomic nuclei unstable?
When there’s an imbalance for forces in the nucleus.
What is radioactive decay?
When unstable nuclei emit radiation to become more stable. This takes energy way from the nucleus, making it more stable. It results in the emission of an alpha, beta or gamma particle.
Radioactive decay is….
Spontaneous and Random
What does it mean when radioactive decay is random?
-Each nuclei has equal probability of decaying
-You don’t know which nucleus will decay next
-You can’t predict a time of decay
-Rate of decay is unaffected by surroundings
-It’s only probable to estimate the proportion of nuclei decaying in a given time period.
What does it mean when radioactive decay is spontaneous?
The decay of nuclei isn’t affected by the presence of other nuclei.
How can random nature be demonstrated?
Through the use of a Geiger-muller tube. When the GM tube is placed near a source the counts are irregular. Each count represents the decay of an unstable nucleus.
What is an alpha particle + its properties?
- Helium
- 2 protons, 2 neutrons. (4,2)
-Low penetrating
-High energy
-Range of a few cm in air.
What is an beta particle + its properties?
-Electrons/Positrons
-Charge +1e
-Moderately ionising
-Moderately penetrating
-Can be stopped by a few mm of aluminium foil.
Range of 20cm-3m in air.
What is an gamma ray + its properties?
- High energy electromagnetic waves.
-Highly penetrating
-Highly ionising (Causes damage and death of cells)
-Range of 1-10cm in lea or several metres in concrete.
When is an alpha particle emitted?
When nuclei are too large.
When is a beta particle emitted?
When nuclei have too many neutrons
When is a gamma ray emitted?
When a nucleus has too much energy.
How to investigate the absorption of alpha, beta and gamma radiation?
Independent- absorber material
Dependant- count rate
Control- radioactive sources,
Distance of GM tube to source,
Location/ background radiation.
1- Connect a GM tube to a Geiger counter without any sources present, measure background radiation for1 min, 3 times nd find average.
2- Place radioactive source 3 cm away from tube and take another 1min reading.
3- Place the absorbers (paper, different thicknesses of aluminum foil & different thicknesses of lead) one at a time between the source and the tube and take 1 min readings for each.
4- Repeat for all radioactive sources.
Results and safety considerations for the practical to investigate the absorption of alpha, beta and gamma radiation.
Results;
- Alpha is absorbed by paper
-Beta is absorbed by aluminum
-Some gamma is absorbed by lead.
Safety;
- Keep sources in lead lined container when not in use.
- Keep at least 1m between the sources and yourself.
-Use tongs and point sources away from yourself when handling.
What is equal in decay equations?
The sum of the nucleon numbers and the sum of the proton numbers before and after the arrow.
What happens in alpha decay?
-Alpha particle emitted, new element formed.
Alpha decay equation.
Nucleon number decreases by 4 and proton numbers decreases by 2.
What happens in beta decay?
Neutron turns into proton + electron. Proton remains in nucleus ad electron is emitted as a beta particle. New element formed.
Beta decay equation.
Nucleon umber stays the same, proton numbers increases by one for beta minus and decreases by 1 for beta plus. An electron/anti-electron neutrino is also emitted.
what is the decay constant?
The probability per second that a given nucleus will decay,
What is the activity?
The number of decays per unit time.
Activity units.
Becquerels (Bq)
1Bq= 1 decay per second.
Activity equation(s).
A = ΔN/Δt = -λN
Where λ is the decay constant.
ΔN is the number of undecayed nuclei
N is the number of remaining nuclei.
What is half-life?
The time taken for the initial number of nuclei to half.
Relactionship shown by half life.
Activity is directly proportional to the number of nuclei remaining
λ and half-life are inversely proportional to
Derive the equation for half-life.
N=N0e^(-λt)
1/2N0=N0e^(-λt1/2)
1/2=e^(-λt1/2)
λt1/2 =ln2
t1/2= ln2/λ
t1/2= 0.693/λ
What equations are used in radioactive decay equations?
N=N0e^(λt) (no of nuclei)
A=A0e^(λt) (activity)
C=C0e^(λt) (count rate)
What type of decay is radioactive decay?
Exponential.
What is carbon dating?
-A method of predicting age in samples ranging 1000-40,000 years old.
-Very reliable unless very young or very old.
-Uses carbon-14
