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nuclear physics Flashcards

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1
Q

what was the plum pudding model?

A

model of the atom that envisaged a number of negative electrons (“plums”) suspended in a positively charged “dough”

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2
Q

why was gold foil used in Rutherford’s alpha scattering experiment?

A

gold is very malleable, so could be hammered into very thin, 1 atom thick sheets

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3
Q

how were the alpha particles detected in Rutherford’s alpha scattering experiment?

A

the alpha particles collided with a fluorescent phosphor screen, which excited the atoms in the fluorescent coating, which emit a photon in a visible wavelength when it de-excites.

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4
Q

what are the 3 control variables in Rutherford’s alpha scattering experiment?

A
  1. experiment must be preformed in a vacuum, other alpha particles would be absorbed when passing through the air
  2. alpha particles all need the same initial kinetic energy, otherwise the speed and initial trajectory would affect the path they took through the foil
  3. the screen (/detector) should be at a constant distance from the gold at all angles.
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5
Q

what was observed in Rutherford’s Alpha scattering experiment?

A

most of the alpha particles were undeflected, or were deflected through small angles
about 1/8000 particles were deflected by more than 90 degrees
about 1/10000 particles were deflected through very large angles, back towards the source

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6
Q

what was interpreted from Rutherford’s alpha scattering experiment

A

very few particles are deflected as very few interact with the nucleus, as the volume of the nucleus is very same relative to the volume of the atom
most of the mass of the atom is contained within the nucleus, which is dense and positively charged

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7
Q

when using the distance of closest approach is being used to approximate nuclear radius, what is true about the kinetic energy of the incident alpha particle.

A

loss in kinetic energy=gain in potential energy

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8
Q

what is the distance of closest approach?

A

gives an upper bound for the radius of a nucleus.
can assume that an alpha particle incident on the nucleus when far away has a potential energy of zero. as it gets closer to the nucleus it has to overcome the electrostatic repulsion between it and the nucleus. It slows down, and its kinetic energy is converted to electric potential energy. when the alpha particle reaches the smallest distance to the nucleus before it turns around, all kinetic energy gas been converted to electric potential energy. so if we know the initial kinetic energy, we can work out the distance of closest approach to the nucleus, which gives an upper limit to the radius of the nucleus

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9
Q

what is a drawback of using the distance of closest approach

A

only gives an upper limit of the nucleus, not an actual estimate of the nucleus.

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10
Q

what are the 2 methods used to approximate nuclear radius?

A

distance of closest approach and high-energy electron diffraction

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11
Q

how does high-energy of electron diffraction work?

A

electrons are accelerated through a potential difference until they are very fast moving, and are directed at a very thin sheet of crystalline material.
due to the wave-like nature of electrons, they diffract through the crystalline material, forming an interference pattern of concentric rings on a screen

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12
Q

how can a nuclear radius be estimated using the results of high-energy electron diffraction?

A

the intensity of scattered electrons varies with the angle at which they were scattered.. the position of the first minimum can be used to calculate nuclear radius.

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13
Q

what is approximate nuclear spacing

A

10 ^ -10 m

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14
Q

what is approximate nuclear radius

A

10 ^ -15 m

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15
Q

how would you show that nuclear density is (approximately) constant for all nuclei

A

density = mass/volume
mass ~ Au
V = 4/3 x pi x R
R=R0A^1/3

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16
Q

what are the 3 types of radiation?

A

alpha, beta and gamma

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17
Q

what is the relative ionising power of and alpha particle?

A

highly ionising
they collide frequently with the electrons in the air. they transfer energy to the electrons, freeing them from their atoms - ionisation

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18
Q

what is the relative penetrating power of an alpha particle?

A

have very low penetrating power
alpha particles are easily stopped by paper or skin, and by a few cm of air

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19
Q

what is an alpha particle?

A

a helium nucleus. consists of 2 protons and 2 neutrons

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20
Q

what is a use of alpha radiation.

A

smoke detectors

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21
Q

what is a beta particle

A

beta - = high speed electron
beta + = high speed positron

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22
Q

what is the relative ionising power of beta radiation

A

beta particles are high speed electrons, but they are smaller and fast than alpha particles so are less likely to collide with electrons to ionise them, so are less ionising than alpha particles.

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23
Q

what is the relative penetrating power of beta radiation?

A

beta particles are blocked by a few mm of aluminium foil or a few meters of air

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24
Q

what is a use of beta radiation?
why must beta be used for this purpose, and not alpha or gamma?

A

to control the thickness of aluminium foil/paper/plastic/steel whilst in production.
a detector and emitter is either side of the sample. if the count rate increases or decreases, you know the thickness had decreased/increased.
beta must be used, as alpha would be absorbed by all thicknesses of the material, and gamma would just pass straight through

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25
what is gamma radiation?
high energy electromagnetic radiation
26
what is the relative ionising power of gamma radiation?
they produce very little ionisation, as the gamma photons are very unlikely to collide with electrons. they are much less ionising than alpha or beta radiation
27
what is the relative penetrating power of gamma radiation?
gamma radiation is stopped by a few meters of concrete or a few centimetres of lead. gamma rays can travel over a kilometre in air.
28
explain how dangerous different types of radiation are inside the body
alpha is the most dangerous inside the body, as it is the most ionising and cannot pass out of the body gamma is the least dangerous inside the body as it is the least ionising and can pass easily out of the body.
29
what is the relationship between the the intensity of gamma radiation and the distance from the source?
inverse square law
30
describe a simple experiment to determine the type of radiation a source emits.
using Geiger counter, take background reading of count rate without source read count rate directly in front of source put paper in-between the source and the detector (if count rate decreases, alpha radiation is being emitted) put a few millimetres of aluminium between source and detector (if count rate decreases, beta radiation is being emitted) if count rate is still higher then background radiation with the aluminium foil present, then gamma is being emitted.
31
what are some natural forms of background radiation?
radon gas that escapes from rocks cosmic rays rocks soil food
32
what are some non-natural forms of background radiation?
medical procedures radioactive fallout from nuclear weapons radioactive fallout from nuclear meltdowns radioactive waste from power stations.
33
what must you do to recorded count rate for the data to be analysed?
must subtract the background count rate, to find the corrected count rate
34
what is the Sievert a measure of?
dose of radiation
35
give examples for safety precautions when handling radioactive substances
Keeping radioactive sources shielded when not in use, for example in a lead-lined box Wearing protective clothing to prevent the body from becoming contaminated Keeping personal items outside of the room to prevent them from becoming contaminated Limiting exposure time so less time is spent with radioactive materials Handling radioactive materials with long tongs to increase the distance from them
36
briefly describe an experiment to investigate the inverse square law for gamma radiation
before bringing radioactive source to lab, measure background count rate place the front of the GM tube window 60cm away from the source, and take the count rate over 5 minutes move the tube to 50 cm away, and take the new count rate over 5 minutes. repeat for 40, 30, 20 and 10cm away from the source calculate the corrected count rate (ccr) for each reading plot a graph of 1/sqrt(ccr) on y axis and distance from source on x axis
37
how could your reduce the uncertainty in reading taken when investigating inverse square law for gamma radiation?
when taking count rate, measure the count over a longer period of time, so percentage uncertainty in time and count reading is reduced.
38
how could you analyse a graph of 1/sqrt(CCR) against d to prove gamma radiation obeys an inverse square law?
graph should be directly proportional however, in real life will not actually be directly proportional, as the position inside GM tube where ionisation takes place is not known, and is inside the detector, not on the very edge. the distance measured is the distance between the source and the window of the detector, not the distance between where the ionisation actually takes place, so there is a small systematic error.
39
what are some examples of radiation used in medicine?
radioactive tracers using gamma radiation to sterilise medical equipment using gamma radiation to treat cancer cells
40
give and example of a radioactive substance that is substance that is used in medicine and what it is used as
technetium - 99m used as a medical tracer
41
what does the "m" stand for in Tc-99m? what does this mean?
m = metastable metastable = exists in a long-lived excited state (long enough that the sample can be separated from the parent nuclei and used as (for example) a medical tracer)
42
what makes Tc-99m a good source of radiation for medical tracers?
only emits gamma radiation, not alpha or beta gamma radiation is weakly ionising, so causes less damage to the body than other sources that emit other types of radiation gamma radiation can escape the body easily Tc has low toxicity half life is short enough to last long enough in the body for the scan, but doesn't last overly long can be prepared in/close to the hospital
43
after an unstable nucleus decays, why might the daughter nucleus emit gamma photons?
gamma emission doesn't change the number of nucleons in the nucleus, but allows the nucleus to lose energy nucleus needs to lose energy as it may have been formed in an excited state. this excited state is usually short lived, as it moves to its ground state via one or more photon emissions.
44
what type of unstable nucleus typically emits beta- radiation?
neutron rich nuclei
45
what type of unstable nucleus typically emits beta+ radiation or decays by electron capture?
proton rich nuclei
46
what type of unstable nucleus typically emits alpha radiation?
nuclei that are very large, and have too many nucleons (mostly above Z=80, N=120)
47
what type of unstable nucleus typically emits gamma radiation?
nuclei with too much energy
48
what words can be used to describe radioactive decay?
exponential decay random constant half-life
49
what does "radioactive decay is exponential decay" mean?
mass of the isotope decreases by the same factor in regular time intervals
50
define activity.
the number of nuclei of the isotope that decay per second measured in Becquels (Bq)
51
if you know the activity of the source, how can you work out the power of the source?
P = AE A = activity E = energy of the particles/photons emitted
52
what is the decay constant?
the constant decay probability the probability that any one nucleus out of N nuclei will decay in a given time
53
why can't you use carbon dating to estimate the age of dinosaur bones?
they're too old. amount of carbon-14 remaining after such a long time would be negligible
54
could you use carbon dating to find the age of a skeleton from a person who died within the last 20 years?
no - too young carbon has a long half life, not enough C-14 will have decayed between the persons death and the time the skeleton was being investigated
55
where do beta emitters lie on an NZ plot?
to the left of/above the line of stability
56
where to alpha emitters lie on and NZ plot
at the top - typically above Z = 80, N=120
57
where do nuclei that decay via beta plus/ electron capture lie on an NZ plot?
below/to the right of the line of stability
58
how would you describe an nucleus that has a lot of neutron/protons
neutron/proton rich
59
describe Rutherford's model of the atom
atoms are mostly empty space, and have a small, positively charged nucleus containing most of the mass of the atom. surrounded by negative electrons. nuclear radius is much smaller then atomic radius.
60
what is the range of the strong force?
repulsive below 0.5 fm attractive between 0.5 and 3 fm
61
define the atomic mass unit
1 twelfth the mass of 1 carbon-12 atom
62
for stable nuclei, as the nuclei gets larger the ratio of the number of neutrons to number of protons increases. why?
in stable nuclei, the strong force holding the nuclei together is balanced with the electrostatic force of repulsion between the positively charged protons. strong force is attractive between 0.5 and 3 fm, and acts on both protons and neutrons electromagnetic force is repulsive between protons and has an infinite range. as number of neutrons increases, strong force increases and electrostatic force does not. so as number of nucleons increases, more neutrons means nucleus is stable, as neutrons don't repel each other.
63
what is an isotope?
atoms of the same element (so have the same number of protons) that have a different number of neutrons.
64
define the binding energy of a nucleus
the work that must be done to separate a nucleus into its constituent nucleons.
65
what is the difference between the mass of a nucleus vs the combined mass of the protons and neutrons in the nucleus
the mass of the nucleus is less than the mass of the individual nucleons
66
what is the binding energy per nucleon?
the average work done per nucleon to separate a nucleus into its constituent neutrons and protons = (binding energy of nucleus)/(number of nucleons)
67
how can you work out the binding energy?
binding energy = mass defect x c^2
68
explain why the total mass of nucleons is greater then the mass of the nucleus.
work must be done to overcome the strong force to separate the nucleons, so energy must be inputted the separate nucleons have more energy than they did in the nucleus E = mc^2, so they have more mass
69
if a nucleus has a greater binding energy per nucleon, is it more or less stable?
higher binding energy per nucleon = more stable
70
compare the range of the strong force to the range of the electrostatic force
electrostatic - infinite range (repulsion of protons in the nucleus) strong - repulsive below 0.5fm, attractive between 0.5-3 fm
71
for nuclei heavier than iron-56, why do nuclei become less stable as nucleon number increases?
above Fe, nuclear radius is greater than the range of the strong force, as as nucleon number increases, strangth of electrostatic repulsion increases more then the strong force
72
for nuclei lighter than iron-56, why do nuclei become more stable as nucleon number increases
as you add more nucleons, strong force increases (more than the electrostatic repulsion) so more energy would be required to overcome this force, so binding energy is greater, so more stable
73
in nuclear decays, are the products more or less stable than the parent nuclei?
more stable
74
do the products of nuclear decay have a greater or smaller total binding energy than the parents?
greater binding energy
75
are nuclear decays exothermic or endothermic?
exothermic
76
what is the binding energy of a lone neutron?
zero
77
what is fission?
when a large unstable nucleus splits into 2 fragments which are more stable than the original nucleus. the binding energy per nucleon increases in the process.
78
what is nuclear fusion?
small nuclei fuse together to form a larger nucleus. the product nucleus has a larger binding energy per nucleon than the original nucleus
79
which process (fission or fusion) has a greater change in binding energy per nucleon? (include approximate numbers)
fusion is higher the change in binding energy per nucleon is about 0.5MeV in fission, but can be 10x that in fusion.
80
what is usually used as the fuel in a nuclear reactor?
uranium-235 or plutonium-239
81
what is name for a sample of uranium containing more uranium-235 than normal uranium?
enriched uranium
82
why is uranium-235 used instead of uranium-238?
235 is more likely to undergo fission than 238. 238 just absorbs the neutron and doesn't undergo fission
83
what are the product of fission of uranium-235?
2 daughter nuclei/"fragment pieces", that are more stable than the original uranium nucleus, at least one high speed neutron. energy is also released
84
how does a chain reaction occur in a nuclear reactor?
when neutrons are produced in a fission reaction, those neutrons are capable of going on to cause another fission event.
85
how does the rate of a nuclear chain reaction increase?
if more than 1 neutron from a reaction goes on to trigger another reaction, rate increases.
86
what is a thermal neutron?
a slow moving neutron, that is at thermal equilibrium with its surroundings
87
why are thermal neutron necessary?
more likely to be absorbed by a nucleus than a fast moving neutron, so is more likely to cause fission. also causes less damage to the structure of the reactor
88
what happens when a non-thermal neutron is incident on a fissionable nucleus?
rebound away from the uranium-235 nucleus after a collision and dont cause a reaction.
89
what is the moderator used for in a nuclear reactor?
fission neutrons need to be slowed down to be able to cause another fission event. neutrons collide with the molecules in the moderator, which transfers kinetic energy to the moderator molecules, which slows the neutrons down, till they become thermal neutrons.
90
give 2 examples of suitable moderator materials
water or carbon-12 (graphite)
91
why are graphite and water good moderator materials?
they have a "low cross-section for neutrons," meaning they don't absorb the neutrons.
92
are big or small molecules better for a moderator material?
smaller molecules are better momentum has to be conserved in each collision so if moderator molecules are small, more energy is transferred from the neutron per collision, so fewer collisions are required.
93
what is meant by the critical mass of uranium?
for a chain reaction to occur, the mass of the fissile material must be greater them the minimum/critical mass.
94
explain why there is a critical mass for fissile material.
if mass of material is small, it will have a high surface area to mass ration. the neutrons produced in a fission reaction are more likely to escape from the material without triggering another fission event the number of neutrons that escape is proportional to surface area smaller fraction of neutrons escape with a larger mass.
95
what should be done if the rate of chain reaction is too high?
push the control rods in further
96
what is a control rod
controls the rate of chain reaction they absorb some of the neutrons produced so they cannot go on to trigger more reactions.
97
why are cadmium and boron good materials for control rods?
they have a high cross section for neutrons. (they can absorb a lot of neutrons without undergoing fission.) they retain their shapes at high temperatures.
98
what is the coolant in a nuclear reactor?
absorbs the heat released in fission reactions in the core of the reactor. this energy is used to heat water to turn turbines in the generator.
99
give 2 examples of coolants in nuclear reactors.
water and carbon dioxide
100
why do water/carbon dioxide make good coolants for nuclear reactors?
water/CO2 are used as they have a high specific heat capacity, meaning they can transfer large amounts of energy without getting too hot/pressure doesn't get too high.
101
why are multiple fuel rods used, not a single one?
neutrons need to pass through a moderator to slow them. neutrons that have left one fuel rod and pass through the moderator are unlikely yo re-enter the same fuel rod. makes it easier to replace spent fuel in stages.
102
explain 3 safety mechanisms in a nuclear reactor.
1. fuel rods are handled remotely to limit the workers exposure to radiation 2. the nuclear reactor has very thick concreate shielding that blocks radiation from escaping the reactor and affecting workers in the power station. 3. in an emergency, the control rods are dropped all the way into the reactor core to entirely stop fission reactions from occurring as quickly as possible by absorbing all the free neutrons in the core (called an emergency shut down)
103
name some sources of radioactive waste from a nuclear reactor.
spent fuel rods, lab equipment protective clothing.
104
give advantages of the development of nuclear power compared to fossil fuels.
no polluting gases reliable for production of power uses far less fuel as more energy is released per kilogram of fuel less harm to the environment
105
give disadvantages of developing nuclear power compared to fossil fuels.
produces radioactive waste potential for nuclear meltdown long term responsibility (as waste needs to be stored for a long time.)
106
what does "vitrified" mean?
when radioactive material is turned into Pyrex glass
107
explain how nuclear waste is treated after leaving the reactor.
spent fuel rods are handled remotely to limit exposure to fuel rods are placed in a cooling lake for up to 10 years plutonium/uranium is separated to be recycled liquid waste is vitrified places in lead/steel/concrete containers DEEP underground
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