Chapter 25 - Radiation Flashcards
Ionising Radiation
Radiation with enough energy to remove electrons from an atom
Alpha radiation
2 protons, 2 neutrons
Q: +2e
Beta plus radiation
Fast moving positron
Q: +e
Beta minus radiation
Fast moving electron
Q: -e
Gamma radiation
High energy photons (λ < 10^-13m)
Q: 0
Radiation in EM fields
Gamma passes straight through
Beta minus attracted to the positive electrode/pole
Beta plus and alpha attracted to the negative electrode/pole, alpha has a weaker attraction
Parent nucleus
The nucleus before a radioactive decay
Daughter nucleus
The nucleus after the radioactive decay
Transmutation
The process of a nucleus changing from one element to another
Conservation in nuclear decay
Nucleon number, atomic number, charge, baryon and lepton numbers
Alpha nuclear equations
A A-4 4
X —> Y + He/α
Z Z-2 2
Beta minus nuclear equations
A A 0
X —> Y + e/β-
Z Z+1 -1
Beta plus nuclear equations
A A 0
X —> Y + e/β+
Z Z-1 +1
Gamma nuclear equations
A A
X —> X + γ
Z Z
What is gamma decay
Emitted photons if there is excess energy after alpha, beta decay
Decay chain
2D grid with Z along the top and A down the left. Draw a line and label with the type between each isotope it decays to
Random decay
We cannot predict when a particular nucleus will decay or which nucleus in a sample will decay next. Each nucleus has an equal chance of decaying per unit time
Spontaneous
The decay of the nuclei is not affected by the presence of other nuclei in the sample or other factors such as pressure
Half-life definition
The average time it takes for half the number of active nuclei in a sample to decay
t
1/2
Activity
The rate at which nuclei decay or disintegrate (decays per second/Bq)
Depends on the number of undecayed nuclei and the half-life of the source
Decay constant formula
A = λN
Activity = decay constant x number of undecayed nuclei
For a sample with a very high number of undecayed nuclei
Decay constant definition
The probability of decay of an individual nuclei per unit time
Units s^-1 or hrs^-1 or yrs^-1 but not Bq
Radioactive decay exponential equations
N = N0 e^-λt
A = A0 e^-λt
Half-life, decay constant relationship
λt = ln(2)
1/2
Carbon dating
All life on Earth contains carbon
Atmospheric carbon is mostly stable Carbon-12 with some Carbon-14
The ratio of C-14 to C-12 is almost constant at 1.3x10^-12
When an organism dies it stops absorbing carbon and C-14 decays over time
Carbon-14 half life
5700 years
Carbon-14 formation
A cosmic ray hits N-14, causing a neutron to be absorbed and a proton emitted, leaving C-14
Carbon-14 decay
β- to N-14
Limitations of carbon dating
It is assumed that the ratio of C-14 to C-12 is constant
Increased emissions may have reduced the ratio
Very small activities of C-14
Rubidium-87 is used for rocks on Earth as the half-life of C-14 is too small
Rubidium 87 half life
49 billion years
