chapter 25 - Radioactivity Flashcards
What are the three types of radiation?
alpha, beta, gamma
What is ionising radiation?
Radiation that removes electrons from atoms, creating positive ions
What is alpha decay?
When an alpha particle (2 protons and 2 neutrons) is emitted from the nucleus
What are the properties of alpha radiation?
- Very ionising
- Very short range
- Very low penetrating
- Charged (+2e)
What’s beta- decay?
Decomposition of down quark into up quark (neutron into proton) to make an electron and antineutrino
What’s beta+ decay?
Decomposition of up quark into a down quark (proton into neutron) to make a positron and neutrino
What are the properties of beta radiation?
- Ionising
- Short range
- Low penetrating
- Charged (-1e or +1e)
Whats gamma decay?
Emission of high energy gamma photons from an unstable atoms nuclei
What is the wavelength of gamma rays?
Less than 10^-13
What are the properties of gamma radiation?
- Low ionising
- Long range
- Highly penetrating
- No charge
How can you use electric fields to distinguish between radiations?
- Beta- radiation (electrons) will be deflected towards +ve plate because they’re -ve
- Beta+ radiation (positrons) and alpha radiation (helium nuclei) will be deflected towards -ve plate as they’re both +ve ( alpha deflected less due to its heavier mass)
- Gamma radiation (photons) won’t be deflected at all as they have no charge
How can you use magnetic fields to distinguish between radiations?
- Beta- radiation (electrons) will be deflected downwards as they’re -ve
- Beta+ radiation (positrons) and alpha radiation (helium nuclei) will be deflected upwards as they’re both +ve
- Gamma radiation (photons) won’t be deflected at all as they have no charge
How do you stop alpha?
A few centimetres of air or a thin sheet of paper
How do you stop beta?
1 meter of air or 1-3 mm of aluminium
How do you stop gamma?
A few centimetres of lead
Why is radiation dangerous?
Ionising which causes damage to living cells
What is the alpha decay equation
A: X :Z → A-4: X :Z-2 +4He2
226Ra88→222Rn86+4He2
What kind of nuclei undergoes beta minus decay?
One with too many neutrons to be stable
What kind of nuclei undergoes beta positive decay?
One with too many protons to be stable
What kind of nuclei undergoes beta positive decay?
One with too many protons to be stable
What’s the beta- decay equation?
A: X :Z → A: X :Z+1 + e− + anti-neutrino
14C6→ 14N7 + e− + anti-neutrino
What’s the beta+ decay equation?
A: X :Z → A: X :Z-1 + e+ + νe
23Mg12 → 23Na11 + e+ + νe
What kind of nucleus emits gamma radiation?
One with a surplus of energy (especially just after alpha or beta emission)
What’s the gamma decay equation?
A: X :Z → A: X :Z + γ (photon)
137 56Ba→137 56Ba+γ
What is a decay chain?
A parent nuclei decays into a radioactive daughter nuclei, which then becomes the
parent for a subsequent decay process until it has a stable nucleus
How do you describe radioactive decay?
As a random and spontaneous event
Why is radioactive decay random?
- Cannot predict when a particular nucleus in a sample will decay
- Cannot predict which nucleus will decay next
- Each nucleus in a sample has the same chance of decaying per unit time
Why is radioactive decay spontaneous?
- Not affected by presence of other nuclei in sample
- Not affected by external factors (like temperature or pressure)
What’s half life?
The time taken for the radioactivity of a specified isotope to fall to half its original value
How do you calculate half life?
Half life = ln(2)/λ
How can you simulate half life with dice?
- Throw a large number of dice at the same time (e.g. 100 dice)
- Remove all the dice showing a particular number from the group and record the number removed
- Throw the lower number of dice again and repeat
- Once you’ve gotten down to one or so dice you stop
- Plot a graph of number dice threw against the number of throws
- You can use the graph to see that you should have a relatively stable half life
What is activity?
The rate at which a source of unstable nuclei decays
What is the unit of activity?
Becquerel (Bq) = s^-1
What is activity reliant on?
Number of undecayed nuclei and half life
How do you calculate activity?
A = λN
What’s the decay constant?
The probability of decay of an individual nucleus per unit time
What’s the decay constant?
The probability of decay of an individual nucleus per unit time
What are the units for decay constant?
s^-1 or h^-1 or y^-1
What is the exponential decay equation for activity?
A = Ao x e^(-λt)
A = activity Ao = initial activity λ = decay constant t = time
What is the exponential decay equation for number of undecayed nuclei?
N = No x e^(-λt)
N = number of undecayed nuclei No = initial number of undecayed nuclei λ = decay constant t = time
Why does activity decrease exponentially?
Because activity is directly proportional to the number of undecayed nuclei. Since the number of undecayed nuclei is decreasing exponentially, the activity also has to decrease exponentially
How can you determine the half life of protactinium?
- Find the background count
- Shake protactinium bottle for 15 seconds to dissolve the protactinium
- Place a GM tube directly in front of the bottle without touching it
- Take a ten second count of decaying protactinium every 30 seconds
- Make sure to zero the count right before every 10 second count
- Take the background count away from each of the 10 second counts
- Plot graph of corrected count rate against time
- You can then find half life by seeing time it takes for count rate to half
Why is protactinium difficult to use and transport?
It has a very short half life so has decayed before you get to use it
How can you make protactinium?
Get a bottle filled with Uranyl(VI) nitrate and organic solvent
- Uranium-238 in uranyl nitrate decays into Thorium-234
- Thorium-234 decays into Protactinium-234
- Protactinium-234 is soluble in organic solvent so dissolves and rises to top of the bottle
What is carbon dating?
The process of using the radioactive isotopes in carbon 14 to calculate the age of an artifact
How does carbon dating work?
- Living organisms take in carbon-12 and carbon-14 in known ratios while alive
- After it dies the carbon-14 isotopes continue to decay while carbon-12 stays same
- Therefore the ratio of C-12 to C-14 in fossils can be measured and their age calculated
What are the limits of carbon dating?
- Assumes ratios of carbon-12 and carbon-14 have stayed same over time
- Carbon-14 in small amounts so activity is comparable to the background rate
