Nuclear Physics Flashcards
Discovery of atomic energy levels
1913
Niels Bohr
Emission spectra
Discovery of the neutron
1932
James Chadwick
Bombarded beryllium with alpha particles
Discovery of the nucleus
1907
Ernest Rutherford
Gold foil experiment
Discovery of the electron
1897
J. J. Thompson
Cathode ray tubes
What was already known about the alpha particles Rutherford used in the gold foil experiment
Positive
Very fast moving
High energy
Plumb pudding model suggestion
Positive charge was evenly distributed throughout
So anticipated it wouldn’t be enough to repel alpha particles back
Expecting most to pass through
Explain what Rutherford found int the gold foil experiment and what conclusions this lead him to
Most alpha particles passed straight through and were undeflected…
Mostly empty space in atoms
Some alpha particles deflected when passing close to positive charge…
Central charged nucleus
Some (1/8000) reflected back/scattered…
Nucleus itself is small and dense
What is alpha radiation
Helium nucleus
What is beta radiation
Fast moving electron
What is gamma radiation
High energy radiation
Explain the penetration power for alpha
Very low
3-7cm in air
Explain the penetration power for beta
Medium (beta minus)
0.2-3m in air
Very low (beta plus) due to annihilation
Explain gamma penetration power
Very high
Most penetrating
Explain the ionisation of alpha
Most ionising
Due to its +2 charge
More likely to attract and strip away electrons
Explain the ionisation of beta
Has to collide with electrons to strip them from atoms
So medium
Explain gammas ionisation
Has to collide with electrons to strip them from atoms
Lowest
Explain the effect of alpha in an electric field
Deflected towards the negative plate due to its positive charge
Smaller specific charge than electron due to larger mass so less deflection for alpha
Explain the effect of beta in an electric field
Deflected towards the negative plate if its plus or positive plate if its minus
Larger deflection than alpha since a smaller specific charge
Explain the effect of gamma in an electric field
No deflection
No specific charge
Passes straight through
Explain alpha in a magnetic field
Smaller specific charge so larger radius
Since r is inversely proportional to specific charge
Positive charge means moves the same direction as current
Explain beta in a magnetic field
Larger specific charge means smaller radius
Since r is inversely proportional to specific charge
Minus will move in the opposite direction to current
Plus will move in the same direction as current
Explain gamma in a magnetic field
No specific charge
Will pass straight through
How do you stop alpha radiation
Its large so very easy to stop in air or paper
Very bad once in the body ass it cannot get out
How do you stop beta radiation
Aluminium
How do you stop gamma radiation
Lots of concrete (metres)
Or a bit of lead
Has a potential infinite range and a very large penetrating power so you can’t entirely stop it just absorb as much as possible
How do you detect radiation
Geiger counter
What is a Geiger counter
Used to detect radiation
How does a Geiger counter work
Radiation enters the Geiger-Muller tube
Passes through the inert gas
Leaving behind a trail of ions
These ions create a charged path between the metal rod in the centre of the detector and the metal casing
Allowing for a brief current to flow and the circuit to be complete
Each time the circuit is complete it registers a count on the screen
Gas inside a Geiger counter
Argon
Potential problems with a Geiger counter
2 or more ionisations get registered as one
Gamma radiation is the least ionising so some may pass straight through the tube without ionising any of the gas
Alpha has a short range so may not be picked up since some absorbed in the air or not able to get close enough to the argon gas
What is background radiation
A measure of the level of ionising radiation present in the environment at a particular location which is not due to deliberate introduction of radiation sources
List sources of background radiation and their approximate amounts
Radon and Thoron - 51 Food and drinks - 16 Gamma rays from rocks - 14 Medical - 12 Cosmic rays - 10
Explain absorption tests
Alpha particles stopped by a few cm of air or a few mm of paper
Beta particles stopped by a few mm of aluminium
Gamma rays never completely stopped, but their amplitude/intensity can be reduced a lot by a few cm of lead or a few m of concrete
How could you determine the nature of an unknown radioactive source
Absorption experiment
Measure background radiation by doing a background count with a Geiger counter with no radiation source
Over an hour to get an average per minute
Count with source
Counts with source and paper, aluminium, lead and concrete
Subtract background count
See how much count decreases by each time
If it doesn’t decrease then the absorbing material had no effect
Discuss how a beta source may be used to control the thickness of a sheet of metal or paper
Absorption
If the count rate increases so higher than normal then it means it is too thin
So primary rollers push down less
If the count rate decreases below the normal it means the paper is too thick
So move secondary rollers down and apply a force than thins out the material
How can a smoke alarm use an alpha source to detect smoke
Alpha ionises air and strips it of electrons leaving air with an overall negative charge
Meaning it can conduct electricity since the ions are moving and transferring charge
e- flow from - to + so current + to -
Current through the circuit is normal
If there is smoke, the same principle of ionisation occurs but the smoke increases collisions so harder for current to flow
Circuit breaks so alarm sounds
Activity
Actual number of nuclei in a source that decay per unit time
Intensity
Power per unit area
Count rate
The number of ionisation pulses recorded per unit of time by a detector
Usually a small fraction of the overall activity
Why is the count rate not accurate to activity for gamma
Radiation is emit in all directions and detector only picks up and registers radiation in a small area
Not all of the gamma radiation causes ionisation
Intensity relation to separation
I∝1/r^2
Intensity relationship to count rate
I∝C
Count rate relationship to separation
C∝1/r^2
2 ways to show inverse square law
By equation
By graph
Explain how to test the inverse square law with equations
Constant will be the same
Can use ratio equations
How do you test the inverse square law with a graph
Graph of 1/r^2 on x and C on Y
Crosses y axis at the background radiation
draw a dotted line through the origin with the same gradient and draw arrows up from this line to the actual line to show the systematic error is the same for all
So must subtract the background radiation to get accurate readings
Every data value will be too large otherwise due to B.R
Surface area and count equation
SA of sphere/SA of detector = Count/Actual
Immediate effects of radiation on body
Cell damage, especially fast growing cells
Brain fatigue and nausea
Hair follicles and hair loss
Intensive lining causing diarrhea and malnutrition
Skin cells, sores and peeling
White blood cells and bone marrow lead to immune system failure
Later effects of radiation on the body
DNA damage in cell nucleus
Egg and sperms cell with damaged DNA can produce babies with birth defects
Body cells develop tumours, blood cell damage can lead to leukemia
0.01mSv
Dental x ray
2mSv
Radiation most people are exposed to per year
10mSv
CT scan of full body
16mSv
CT scan of heart
100mSv
Recommended limit for radiation workers every 5 years
1000mSv
Single dose could cause radiation sickness or nausea
Not death
5000mSv
Single dose would kill half of those exposed within a month
10000mSv
Single dose fatal within weeks
Do’s for handling radioactive sources in the lab
Use tongs to pick things up
Increase distance from source
Store in lead lines boxes and label with correct safety info
Limit time of exposure
Don’t for handling radioactive sources in the lab
Look directly at source or point it at anyone
Run with source
Eat or ingest source
Explain medical diagnosis using tracers
Use gamma radiation
Identify blockage where stopped
Identify cancer or tumour since it absorbs the radioactive isotopes
Least ionising
Least damaging
Most penetrating
Explain radiotherapy using gamma sources/gamma knifes
Greatest gamma concentration on tumour
Breaks it apart
Tiny amounts pass through the rest
What must you consider when choosing an isotope for medical imaging or diagnosis
The majority of the radiation must be gamma only
Must have a short half life to decay quickly so decrease time in the body (initially very radioactive but quickly comes down)
Why isn’t alpha used in medical imaging or diagnosis
Least penetrating so can’t get out of the body
So hard to defect
Most ionising means most damaging inside the body
3 main reasons why a nucleus may be unstable and what it emits
Too many neutrons for protons = emit a fast moving electron or neutron emission if way to big
Too few neutrons for protons = emit a fast moving proton or proton emission if way too big
Too many nucleons = emit alpha particle/helium nucleus
Explain the distance of closest approach for alpha decay
Positively charged particle approaching the nucleus of an atom head on will naturally be repelled by the electrostatic force
The closest distance it will get to the nucleus is when all the kinetic energy of the incoming particle is transferred into potential energy
At this point the particle will have become stationary before being repelled away in the opposite direction to its original direction
Explain alpha emission
Very large nuclei with too many nucleons to be stable
Electrostatic repulsion between the large number of protons is too great for the short range strong nuclear force that holds nucleus together
So seeks to lose nucleons by emitting an alpha particle
Consisting of a very stable combination of two protons and two neutrons
What do you need to remember at the end of emission equations
Q
energy released
What is beta minus emission
Too many neutrons for the number of protons
Neutron decays into a proton, and electron and an electron antineutrino
High energy electron is known as a beta minus particle
4 statements about the neutrino
Fundamental particle
No charge
Very small/zero mass
Interacts with other matter very weakly
Beta minus equation on an atomic level
n —> p + e- + anti(Ve) + Q
Beta plus emission
Too few neutrons
Proton in the nucleus decays into a neutron, a positron and an electron neutrino
Electron capture
Another way a proton is turned into a neutron
Electron captured from the electron cloud
Combines with a proton in the nucleus to form a neutron
Emitting an electron neutrino
Beta plus equation on an atomic level
p —> n + e+ + (Ve) + Q
Electron capture on an atomic level
p + e- —> n + Ve
How do you do a decay chain
Write out every emission and then the final element produced last
Or
Write out each emission once with the number of times it occurs along with the number of times its corresponding particle is produced then the end element
How does 22Rn86 decay into 206Re75 as a decay chain
222Rn86 —> 4 (4a2) + 3 (0B+1) + 3(0Ve0) + 206Re75
Equation for distance of closest approach
EK=EP
1/2mv^2=Q1Q2/4πƐ0d
So the distance of closest approach (d)…
d=Q1Q2/4πƐ0x0.5mv^2
What can you use alpha to estimate
Upper limit of the size of the nucleus
Explain the equation R=r0A^¹`³
R is radius of nucleus
r0 is a constant and is the radius of one nucleotides
A is the mass number of the nucleus (number of nucleons and NOT 1.67x10^-²⁷)
What does r0 represent
Since when A is 1 (hydrogen) R=r0, r0 represents the radius of 1 nucleon
4 ways to use R=r0A^⅓
By calculation: ratios
Graph: R³ against A is a straight line through origin
Graph: R against A^⅓ is a straight line through origin
Graph: lnR against A is a straight line with y intercept ln(r0) and gradient 1/3
How do you prove the density of a nucleus is constant
- Approximate nucleus to a sphere (V=4/3piR³)
- Since R=r0A^⅓, V=4/3pi(r0A^⅓)³
=4/3pir0³A
- p=m/V
- m=A x 1.67 x 10-²⁷
- p=(Ax1.67x10-²⁷)/4/3pir0³A
- P=1.67x10-²⁷/4/3pir0³
So since all nucleons have the same mass and r0 is a constant, the density of a nucleus is constant
Two methods for finding the nuclear diameter
Alpha scattering
Electron scattering
Explain aloha scattering to find the rough size/upper limit of nucleus radius
Electrostatic repulsion between alpha particles and nucleus due to their similar positive charge
Fe=Q1Q2/4pie0r²
As the alpha approaches the nucleus their kinetic energy is converted into potential energy
At the point of closest approach
Explain electron scattering experiment to find the rough size/upper limit of the size of the nucleus
Electrons behave like waves with a De Broglie wavelength (h/p or h/mv)
First minimum user calculate the diameter
Works due to wave particle duality
If they are travelling fast enough and their db wavelength is appromatly the nucleus size it behaves like a wave passing through a gap
To get a diffraction pattern
Explain the accuracy of alpha scattering to find the size of the nucleus
Calculations only produce the distance of closest approach of the alpha particles not the diameter
Experiment can’t always detect alphase scattered 180°
Alphas have their own size which must be taken into account
What do both the electron and alpha scattering experiments need
Monoenergetic beams and a thin sample of target material
Why do electrons need high speeds for electron diffraction
C=flambda
Need a wavelength similar to the diameter of the nucleus
Advantages and disadvantages of alpha scattering to find the radius of a nucleus
Upset by nuclear recoil (if collisions are not perfectly elastic, but approximate to elastic)
Upset by the strong force since alpha contains hadrons
Alphas only affected by protons not neutrons
Advantages and disadvantages of electron diffraction to find the radius of a nucleus
Not affected by the strong force since they are leptons
1st minimum in the scattered intensity can be difficult to detect
Explain the equation to find the necessary voltage for a given velocity for an electron in electron diffraction
W=qV
W=Kinetic energy gained through accelerating plate
q=e (charge of an electron)
eV=1/2mv²
v=root(2eV/m)
So increasing the voltage by 4 increases the velocity by 2
Important to remember about radioactive decay
Random process
Can never certainly predict when any single unstable nucleus will decay
So rate cannot be increased or decreased using external factors such as catalysts heat stirring
Which of the following increase the rate of radioactive decay
Catalysts
Stirring
Heat
None
Random process
So can’t be increased or decreased by external factors
Activity
A
Total number of decays per second
Measured in Becquerel’s
Bq (decays per second)
N
Total number of active nuclei in a sample
So has no unit
λ
Decay constant
The probability that a single nucleus decays in a second
Units are therefore per second (s^-1)
AKA fraction of unstable isotopes that have decayed per second
Probability of decay after one second
λ
Amount of nuclei that decay after 1 second
A
Graph of A against N
Same as N against T
Exponential decay
Time on x axis
Asymptotes to zero
Equation linking activity, N and λ
A=λN
In formula booklet
How can you write activity when thinking of as the rate of change of number of nuclei
A=-∆N/∆t
Since the change in N is negative in order to make activity positive you must make the ∆N/∆t negative
Leads to ∆N/∆t=-λN
What can the given equation N=N0e^-λt get you
A=A0e^-λt
N=N0e^-λt variables
N; the number of unstable isotopes remaining after a period of time t
N0; initial number of unstable isotopes in sample
λ; decay constant
t; time since the initial number of nuclei were recorded
A=A0e^-λt variables
A; the activity of a sample of unstable isotopes after time period of t
A0; initial activity of unstable isotopes in sample
λ; decay constant
t; time since the initial activity was recorded
How do you get the formula for half life, ln(2)/λ
N=N0e^λt After 1 half life, N=N0/2 1/2=e^-λt ln(0.5)=-λt -ln(0.5)=λt
POWER SLIDE
ln(2)=λt
t=ln(2)/λ
For a ln(N) against t graph what is the gradient and y intercept
Gradient = -λ
Y intercept = ln(N0)
What type of half life do you want in medicine and why
Short
So sample doesn’t remain in patients body for longer than necessary
Relationship between λ and half life
Inversely proportional
Increasing decay constant decreases half life
Why do you need to know the half life when disposing of nuclear waste
To evaluate what length of time the materials remain a danger
What type of half life do isotopes used in nuclear reactors need to have and why
Large enough to make sure they have a viable life span
Would be useless if the half life was too small since they would have to be replaced regularly
Slowing down production and costing money to replace
What type of half life do smoke detectors have and why
Large half life
Enough to make sure it doesn’t decay to untraceable levels during its lifetime
Otherwise it would become redundant and pose a danger to occupants of buildings
What is carbon dating
Process used to determine the age of living materials such as plants animals and wood
Explain the process of carbon dating
All living things contain a proportion of carbon 14 (radioactive isotope of carbon with 2 neutrons too many)
So all living things emit radiation to some extent
The activity of a living organism remains constant during their lifetime as the carbon 14 that decays is replaced
Once it dies it is no longer replaced so the activity of the sample starts to decrease
By comparing the activity of a dead organism with its activity whilst living you can deduce how long ago it died
