nuclear physics Flashcards
© What was J.J Thomson’s model of the atom?
‘Plum Pudding Model’
spheres of positive charge, with negative electrons embedded, overall uniformly distributed positive charge
© Describe the Rutherford scattering experiment setup
● A beam of alpha particles was directed at a thin gold foil.
● Occurs in a vacuum so that no collisions between air particles and alpha particles can occur.
● fluorescent screen
© what was the expected result of the alpha scattering experiment?
all flashes on the fluorescent screen should’ve been seen within a small scattering angle.
all the positively charged alpha particles would be deflected by a small amount
© what were they recording in the alpha scattering experiment
the number of alpha particles scattered at different angles
★ In experiments carried out to determine the nature of atoms, alpha particles were fired at thin metal foils. Describe how the alpha scattering experiments provide evidence for the existence, charge and size of the nucleus [5]
- Most of the alpha particles went straight through
- Hence most of the atom is empty space
- A small proportion of the particles were scattered through large angles
- This showed the existence of a tiny positive nucleus
- approx. 10^-15m
★ Describe the nature and range of the three forces acting on the protons and neutrons in the nucleus [5]
- The Electromagnetic force acts between the similarly charged protons causing them to repel one another
- This has an infinite range
- The gravitational force is attractive (acts between the nucleons due to their mass)
- This has a long range
- The strong nuclear force is attractive (acts between nucleons to overcome the repulsive and force)
- short-ranged (Repulsive <0.5fm and attractive up to 3fm, negligible after)
★ Explain the term binding energy of a nucleus [2]
The minimum energy required to separate nucleons from one another in a nucleus
© How can you estimate the radius of an atomic nucleus?
Rutherford’s scattering experiment
When the particle is deflected by 180
Reaches closest to nucleus when Initial Kinetic energy = Electric potential
✪✪ Electric potential energy
= (Qnucleus*Qalpha)/4πε0r
ε0 - permitivity of free space - 8.85*10^-12Fm^-1
© what is the more accurate method for measuring nuclear radius and why?
using electron diffraction
electrons are leptons and so don’t interact with strong force
✪ De Broglie wavelength
λ = h/mv
or
( E = hc/λ )
λ = hc/E
★ An alpha particle moves directly towards a nucleus of Aluminium-27.
The alpha particle comes to rest instantaneously a short distance away from the aluminium nucleus. It then reverses its direction of travel.
Describe and explain the motion of the aluminium nucleus at the instant the alpha particle is at rest. [2]
- The aluminium nucleus moves to the right,
- There is a repulsive force on the nucleus ( conservation of momentum )
© How small must the de broglie wavelength of the electrons be while investigating nuclear radius?
How is this achieved?
similar order of magnitude to the nucleus
10^-15m
The electrons must have have a very high energy (high velocities)
© Graph of intensity against angle for electron diffraction by a nucleus
intensity of maxima decreases as angle of diffraction increases
✪✪ First minimum for electron diffraction by a nucleus
sinθ ≈ 1.22λ / 2R
R -radius of the nucleus the electrons have been scattered by
☢ approx. size of an atom
radius of 10^-15m
© What is nucleon number known as?
A
number of neutrons and protons in a nucleus
© How does nuclear radius increase with increasing nucleon number? (graph)
as number of nucleons increase, radius increases
__ _______
/
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© How can the relationship between A and and R be made linear?
Plotting R against A^1/3
Shows a linear relationship between R and A^1/3
R ∝ A^1/3
✪ nuclear radius
𝑅 = 𝑅0 * 𝐴1/3
R0 - constant - approx 1.4fm
A - nucleon number
© What does the equation for nuclear radius suggest?
𝑅 = 𝑅0 * 𝐴1/3
R ∝ A^1/3
R^3 ∝ A
V ∝ A
V = 4/3(πr^3)
m = A * m.nucleon
ρ = m/v = constant
provides evidence that the density of nuclear matter is constant, regardless of A (nucleon number)
© What does nuclear density suggest?
- Nuclear density»_space;> atomic density
- This suggest most of an atom’s mass is in its nucleus
- Nucleus is small compared to the atom
- Atom must be mostly empty space
© the four types of nuclear radiation
beta minus
beta plus
alpha
gamma
© What is radioactive decay?
The emission of particles from an unstable nuclei to become more stable
© penetration power of the different types of radiation, range in air
alpha - weakly penetrating, stopped by a piece of paper
a few cm range in air
beta minus- moderately penetrating
stopped by few millimetres of aluminium
a metre in air
beta plus - immediately annihilate with electrons so effectively zero range
gamma - strongly penetrating
several cm of lead stops gamma
long range
© Describe how you would identify nuclear radiation
record background count rate when no source is present
place unknown source near a geiger muller tube and record the count rate
place various materials and thicknesses between the tube and the source
record count rate
subtract background radiation
amount by which count rate decreases determines which kind of radiation the source was emitting
© what are two applications of gamma radiation in medicine
causes least damage to body tissue as it is weakly ionising and highly penetrating so passes straight through body causing minimal damage
radioactive tracers
treating cancer
rotating beams of gamma rays,
radiation damages all cells - both cancer and non-cancerous
damage to healthy cells - side effects - tiredness etc.
How do radioactive tracers work
The patient is injected with a radioactive isotope with a short half-life.
A detector is used to detect the emitted gamma rays
© describe an application of beta radiation
controlling the thickness of material production
source and detector placed opposite one another with material in between
the thicker the material, the more radiation it absorbs, so less is detected. Adjustable rollers change position accordingly
© Why is alpha radiation especially dangerous in the body?
Cause localised damage
ionise body tissue in a small area causing a great amount of damage
weakly penetrating meaning it can’t escape the body easily so remains
© describe an application of alpha radiation
smoke alarms
alpha particles are strongly positive, so can easily pull electrons off atoms (ionise)
strongly ionising - loses its energy quickly
allows current to flow and won’t travel far out of detector
when smoke is present the alpha particles can’t reach the detector and this breaks the flow of current and sets the alarm off
© What happens when you place a radioactive source in magnetic fields
charged particles emitted by different radiations travel through magnetic fields differently
charged particles moving perpendicular to a uniform magnetic field are deflected in a circular path, direction depends on charge
© What is background radiation and how is it dealt with?
the radiation that is always present from sources such as rocks, cosmic radiation, radon gas in the air, living things (c-14)
take 3 readings of count rate with no radioactive source present
Take average and subtract from each measurement taken with source present
© What is the intensity of radiation
amount of radiation per unit area
© What does the inverse square law state
a gamma source will emit gamma radiation in all directions
As distance from the source increases,
the intensity of radiation decreases proportional to the square of the distance
✪ inverse square law formula
I = k/x^2
intensity (Wm^-2) = K(constant of proportionality (W))/x^2 (distance^2)
© graph of number of remaining unstable nuclei against time
decreases exponentially
\
\
\
\_\_\_
-\_\_\_\_\_\_\_\_\_\_\_\_\_
plot ln(N) against time
y - intercept = ln(N0)
gradient = -λ
✪ the decay equation
number of unstable nuclei remaining
N = N0e^-λt
number of unstable nuclei remaining = Original number of unstable nuclei * e^-decayconstant(s^-1)*time(s)
© Decay constant
constant of proportionality
probability of a specific nucleus decaying per unit time
any sample of a particular isotope has the same rate of decay - each nucleus will have a constant decay probability
© Activity of a radioactive source
The number of nuclei that decay per second
ΔN
- _____ΔT
( negative since number of nuclei is decreasing)
✪ Activity formula
A = λN
activity(Bq) = decay constant (s^-1) * number of unstable nuclei in a sample
© Number of atoms in a sample
N = n*Na
number of atoms in a sample = moles*avogadro constant
✪✪ Activity and the decay equation formula
A = A0 * e^-λt
activity(Bq) = activity (at time = 0)* e^-decayconstant*time
✪ Energy
E = mc^2
Energy(j) = mass(kg) * c(ms^-1)^2
© What is mass defect
the mass of a nucleus is less than the combined mass of its constituent nucleons
the difference between these two values = mass defect
© Binding energy per unit mass defect
1u ≈ 931.5Mev
© How do you calculate average binding energy per nucleon?
Binding energy / Nucleon number
© Describe the graph of average binding energy per nucleon against Nucleon number
_Fe(56) —-_______
/ —–U(238)-
/
|
|
H(2)
average binding energy per nucleon increase rapidly for light nuclei
Iron is the most stable nucleus
Average binding energy per nucleon decreases gently for heavier nuclei
©Nuclear fission
Large nuclei are unstable and sometimes randomly split into two smaller, less heavy nuclei
spontaneous process
releases energy since new, smaller nuclei have a higher average binding energy per nucleon (more energy
© What is a condition for nuclear fusion to occur
Need to get close enough for attractive strong force (0.5fm - 3fm)
all nuclei are positively charged, resulting in electrostatic force of repulsion
Nuclei can only fuse if they can overcome this electrostatic force. Requires an initial energy input
© Nuclear fusion
When two light nuclei combine to create a larger nucleus
Releases a lot of energy because new, heavier nuclei have a higher average binding energy per nucleon.
© in fission and fusion, when is energy released
when the average binding energy per nucleon increases
© For which elements are nuclear fission and fusion energetically favourable by looking at graph
elements to the right of Fe(56), fission releases energy
to the left - fusion release energy
© in fission and fusion, when is energy released
when the average binding energy per nucleon increases
© Which process do thermal nuclear reactors take advantage of
nuclear fission
© What type of reaction occurs in nuclear reactors
chain reaction
fission reactions produce more neutrons which then induces other nuclei to fission
neutrons will only cause a chain reaction if they are slowed down so they can be ‘caught’ by uranium nuclei
© what are the slowed down neutrons in nuclear reactors called
thermal neutrons
© Fuel rods - moderator
rods of uranium that are rich in U-235 (or plutonium rods rich in Pu-239) as ‘fuel’ for fission reactions
fuel rods are places in a moderator e.g water
this slows down/ absorbs neutrons
needs to slow the neutrons down by right amount to keep chain reaction going at a steady rate
© How does a moderator slow down neutrons?
Through elastic collisions with nuclei of the moderator material.
When neutrons collide with particles of similar mass, they are slowed down most efficiently, Water works because of H-1
© control rods - what do they do and how?
ensuring chain reaction continues on its own at a steady rate
limits number of neutrons in the reactor
they absorb neutrons so rate of fission is controlled
made of materials that absorbs neutrons (e.g boron)
© what is critical mass
amount of ‘fuel’ needed to ensure chain reaction continues on its own at a steady rate
where one fission follows another
any less than the critical mass and the reaction dies out
supercritical mass - where several new fissions normally follow each fission
© What is half-life?
The half life of an isotope is the average time it takes for the number of unstable nuclei in the sample to halve
© Describe how you’d obtain a half-life value from a graph
graph of number of unstable nuclei remaining, (N) against time(t)
- read off N0, - N at time t = 0
- Divide this by two and go to this value on the y-axis
- draw a line across to the curve and draw a vertical line down where it meets the curve, read off x-value, half-life
- repeat at least thrice and calculate an average half-life
✪ Half-life formula
𝑇½ = ln2 / λ
half-life = ln2 / decay constant
© How is radioactive waste stored, and what is an example of when this is implemented
radioactive waste has to be stored carefully for hundreds of years, only then would their activity have fallen to safe levels
radioactive waste has a very long-half life, stays highly radioactive for a long time
U-235 used in fission reactors
Decays into several different radioactive isotopes with different half-lives
© How are radioactive isotopes used in medical diagnosis
Technetium - 99m
medical tracers
they have a short half-life, therefore suitable.
Also decays to a much more stable isotope
radioactive substance injected/ingested by a patient. Radiation emitted is then recorded and image produced
© What are some possible problems with radioactive dating
For man-made objects, only gives the age of the material not the object itself
may have been contaminated by other radioactive sources
A high background count rate may skew readings
Uncertainty in amount of carbon-14 that existed thousands of years ago
© How are radioactive isotopes used in radioactive dating
All living things contain a small percentage of Carbon-14
When they die the activity of the carbon-14 begins to fall
