Nuclear Physics Flashcards
What happened in the Rutherford scattering experiment?
- Most alpha particles passed through gold leaf undeflected
- Some were slightly deflected
- A tiny proportion (1 in 8000) reflected
What did Rutherford’s experiment reveal about the atom?
- Atom is mostly empty space
- Centre of atom is small, dense and positively charged (nucleus)
Compare the ionising power of the three main types of radiation
- *Alpha most ionising**
- *Beta medium ionising**
- *Gamma least ionising**
Compare the penetrative power of the three main types of radiation
- *Gamma most penetrating**
- *Beta medium penetrating**
- *Alpha least penetrating**
Compare the range of the three main types of radiation
Alpha = 3-7cm Beta = 0.2-3m Gamma = Very long distance
What materials can shield the three main types of radiation?
- *Alpha = Paper**
- *Beta = Aluminum**
- *Gamma = Lead or several meters of concrete**
What are the main sources of background radiation?
How does a Geiger-Muller tube detect radiation?
- Radiation ionises gas in tube
- Negative ions attracted to metal rod
- Positive ions attracted to casing
- Small current generated in circuit
NOTE: The Geiger-Muller tube is not useful in areas with high levels of background radiation
How is radiation used to control the thickness of paper?
Why can’t alpha radiation be used?
If detector count too low -> metal too thick -> Rollers move closer together
If detector count too high -> metal too thin -> Rollers move apart
Alpha won’t be detected
How is radiation used in a smoke detector?
Alpha radiation ionizes air between detector
Ionized air causes current to flow
Smoke blocks ionization of air
Current stops
Alarm sounds
If the detector is moved 3x further away what will happen to the count rate?
9x smaller
Gamma radiation follows inverse square law
What are 5 safety precautions when using radiation?
- Minimise exposure time
- Maximize distance from source
- Store in shielded containers
- Don’t consume food or drink near source
- Wear protective equipment
How is radiation used for medical imaging?
Medical tracer with short half life injected
Tumors absorb radionuclides and emits gamma rays
Gamma rays detected outside body
How is radiation used to destroy tumors?
Gamma radiation focused on tumor
High energy breaks apart tumor
Low levels through other tissue
NOTE: For safety, the radiation should have a short half life
Sketch the graph of nuclear stability
On this graph of nuclear stability highlight regions of the decays…
- α
- β-
- β+
- Proton emission
- Neutron emission
What makes a nucleus unstable? (and radioactively decay)
- An incorrect balance of protons and neutrons (off line of stability)
- Too many nucleons
- Nucleus in excited state
What is electron capture and what is it’s equation?
Proton captures inner shell electron and becomes neutron
What two forms of radiation are released after electron capture?
X-ray → electron de-excites to fill inner shell
γ → Nucleus reorders and de-excites
How does distance of closest approach work?
KE at distance → PE closest
Use to get rough size of nucleus
What two graphs could you plot to prove this relationship?
What does r0 represent?
Average radius of each nucleon
How do you calculate the average density of a nucleus?
Why is the average nucleus density so large? (∼2.3x1017kgm-3)
Atom is mostly empty space
How was electron scattering used to determine nuclear diameter?
Graph plotted
First minima used to calculate diameter
(Don’t need to know equation)
How is electron scattering better than alpha recoil to determine nuclear radius?
Alpha Recoil
- Closest approach so only an estimate
- Recoil of nucleus not considered
- Effect of strong force not known
Electron Scattering:
- Not affected by strong force (leptons)
- Electron λdb tunable
Define the decay constant λ
The probability that an unstable isotope decays in one second
What’s wrong with this calculation?
Activity and time must be the same units
Define the activity, A, of a radioactive sample
The total number of unstable isotopes that decay after one second
What does the activity, A, of a radioactive sample depend on?
- The decay constant λ
- The number of unstable isotopes N
Define the half life, T½, of a radioactive sample
Time taken for either…
- Activity of sample to halve
- Number of unstable isotopes remaining to halve
How do you derive the half life T½ equation?
Set N as 0.5N0
What do the gradient and y-intercept of this graph represent?
where r0 → average radius of nucleon
- Derive the equation of this graph
- What is the gradient and y-intercept?
How do you prove this graph is exponential?
Find multiple T½ and compare
What is the gradient of this graph?
r0 → average radius of nucleon
If this unstable isotope of caesium decays by α emission where does it end up on the graph?
N → neutron number
Z → Proton Number
If this unstable isotope of caesium decays by β- emission where does it end up on the graph?
N → neutron number
Z → Proton Number
If this unstable isotope of caesium decays by β+ emission where does it end up on the graph?
N → neutron number
Z → Proton Number
What is the corrected count rate?
The measured count rate minus background radiation
What does radiation do to cells?
Ionises atoms in cells leading to dead or cancerous tissue
What unit of time should you convert to when doing nuclear equations?
Seconds
Why is half-life important?
Different uses will require differing half-lives
- Medical uses require short half-lives
- Smoke alarms and nuclear reactors require long half-lives
Also, when disposing of nuclear waste it is important to know the half-life
Describe carbon dating
If you are given the half-life of the subject, find the decay constant
Use these values in the formula A = A0e-λt
A - current count rate
A0 - initial count rate
λ - decay constant
t - time the subject has decayed
Can the formula N = N0 e-λt be written in any other way?
Yes, can use A (activity) or C(Count rate) instead of N
(A = A0e-λt)
(C = C0e-λt)
(M = M0e-λt)*
For Nuclear questions, how do you work out the number of particles flowing through area A at distance D from a source of count rate C?
- Find the fraction of the area of a sphere that the cross-section represents (proportion of particles that will hit that area compared to the whole sphere)
- Multiply this by the count rate of the source
( C * A/4πr2)
