6.4 Nuclear & Particle Physics Flashcards
what was the alpha particle scattering experiment?
Ernest Rutherford fired a narrow beam of fast moving alpha particles at a thing gold foil to see how the alpha particles were deflected (or a a scattered), the detector was a zinc sulphide screen that would produce a tiny spec of light whenever an alpha particle hit it
What are the two significant observation, which could not support Thomson’s plum-pudding model of the atom
- most of the alpha particles passed straight through the thin gold foil with very little scattering. About 1 in every 2000 alpha particles were scattered.
- very few of the alpha particles - about 1 in every 10, 000 - were deflected through angles of more than 90•
what were the deductions from the alpha particle scattering experiment?
- the vast majority of the mass of the atom is contained within a small volume called the nucleus
- the nucleus has a positive charge
- the nuclear distance is considerably smaller than the diameter of the atom, meaning that the atom is mostly empty space
what was the evidence in the experiment that showed there was a very dense nucleus?
most of the mass must be in the nucleus since the fast alpha particles (with high momentum) are deflected by the nucleus
what was the evidence in the experiment that showed the nucleus was positive?
some of the alpha particles were deflected through large angles, so the centre of the atom must have a large, positive charge to repel them, Rutherford named this the nucleus
what was the evidence in the experiment that showed that the atom is mostly made up of empty space?
most of the alpha particles went straight through the foil, so the atom is mainly empty space
what is the charge and mass of a proton (in terms of e for charge and u for mass)?
+1 charge
mass = 1
what is the charge and mass of a neutron (in terms of e for charge and u for mass)?
0 charge
mass = 1
what is the charge and mass of a electron (in terms of e for charge and u for mass)?
-1 charge
mass = 1 / 1836 (negligible)
what is the defintion of nucleon number?
no. of protons and neutrons inside the nucleus (also known as the mass number)
what are isotopes?
isotopes contain the same no. of protons and electrons but different no. of neutrons
what is mass number equal to?
no. of protons and neutrons
what is atomic number equal to?
just no. of protons
what is the strong nuclear force?
the strong nuclear force acts between nucleons and holds the nucleus together against the electrostatic attraction repulsion of the protons
So why do protons not fly apart in a nucleus
For example, In a helium-4 nucleus, the two protons are separated by a distance of about 10^-15m and exert a large repulsive electrostatic force given by equation
F= Qq/kr^2
This is an extremely large repulsive force. Now the attractive gravitational force between the proton is far too small to keep them together, so there must be another, much stronger force acting on the proton. This force is the strong nuclear force
is the strong nuclear force attractive or repulsive?
(trick question) it is BOTH, it is attractive to about 3 x 10^-15 and 0.5 x 10^-15m, it is a repulsive force between nucleons for distances of separation up to around 0.5x10^-15m, it is a SHORT RANGE force - it doesn’t act over large distances
State the approximate radii of the atom and the nucleus
atom ~ 10^-10 m and nucleus ~ 10^-15 m
how much bigger is the diameter of the whole atom compared to the diameter of the nucleus?
roughly 10,000 times greater 10^5
what will a graph of radius of nucleus against nucleon number look like (think about the relationship)?
exponential, starts steep then decreasing gradient until it plateaus
what will a graph of radius of nucleus against cube root of nucleon number look like (think about the relationship)?
constant gradient, straight line upwards
what is the equation that links radius of a nucleus to nucleon number?
R = Ro x A^1/3
where Ro = a constant (1.4 x 10^-15)
R = nuclear radius
A = nucleon number
what is greater atomic density or nuclear density?
nuclear density is much greater than atomic density
what are the two equations needed to calculate nuclear density? to get to ρ = 3Mn / (4πro^3)?
ρ = m / v
and assuming v = 4/3πr^3,
subbing in R^3 = Ro^3A from R = Ro x A^1/3 gets you to a simplified formula to work out the mean density of the nucleus: ρ = 3Mn / (4πro^3)
P = Amnucleon/ 4/3πr^3A
what are fundamental particles?
fundamental particles are particles that cannot be broken down into smaller components (e.g electrons, quarks)
what are hadrons?
hadrons are particles and antiparticles consisting of a combination of quarks to give a net zero or whole number change. Examples include protons, neutrons and mesons are hadrons - hadrons are particles that feel the strong force, weak force and electromagnetic force (if charged)
what are leptons?
lepton are fundamental particles and antiparticles, electrons, neutrinos and muons are leptons - leptons are particles that feel the weak force and electromagnetic force - not the strong force
what are quarks?
quarks are components of hadrons and have a fractional electric charge, to date, they are believed to be fundamental particles, there are different types of quark, for example, up, down and strange quarks
what are neutrinos?
a neutrino is a fundamental particle (lepton) with almost no mass and zero charge, each neutrino has an antimatter partner called an antineutrino
what is the weak nuclear force?
the weak nuclear force is felt by both quarks and leptons, it can change quarks from one type to another or leptons from one type to another or leptons from one type to another and is responsible for beta decay
what is an antiparticle?
an antiparticle is a particle of antimatter that has the same rest mass (mass at zero speed) but, if charged, the equal and opposite charge to its corresponding particle, for example the positron (e+) is the antiparticle of the electron (e-) - every known particle has its own ‘opposite particle’ of antimatter particle called an antiparticle
what are the two classifications of particles?
hadrons and leptons
are hadrons fundamental?
no - they can be broken down into quarks
what charge does a down quark have?
-1/3
what charge does an up quark have?
+2/3
what quarks are in a proton?
uud - two up and one down making the charge +1, 2/3 +2/3 - 1/3 = 1
what quarks are in a neutron?
udd, one up and two down making the charge 0, 2/3 - 1/3 - 1/3 = 0
what does each lepton also have?
a neutrino which is also fundamental particle, and which is produced in beta decay
what is special about neutrinos?
they have ZERO MASS and ZERO CHARGE
sub classification of hadrons & there respective quark combination
baryons - 3 quarks
mesons - 2 quarks
strangeness number of up, down and strange respectively
0,0,-1
Baryon number of up, down and strange
+1/3, +1/3, +1/3
what does the squiggly line above a particle denote?
an antiparticle
what is the charge and name of a proton’s antiparticle?
antiproton, charge = -1
what is the charge and name of a neutron’s antiparticle?
antineutron, charge = 0
what is the charge and name of an electron’s antiparticle?
positron, charge = +1
what is the charge and name of a neutrino’s antiparticle?
antineutrino, charge = 0
State two fundamental forces with an infinite range
State two fundamental forces with an short range
- electromagnetic and gravitational force
- strong (~10^-15m) and weak nuclear force (~10^-18m)
what emits radiation
unstable nuclei. Basically there is a change in the nuclei of the atoms. For example, for beta decay; the changes occur to the neutrons or protons
what is radioactive decay?
radioactive decay is the spontaneous and random decay of an unstable nucleus into a more stable nucleus by the emission of an α, β or γ radiation
what is the force responsible for beta decay
weak nuclear force it acts to change quark types over very small distances
what is the nature, mass, charge and symbol of a beta-minus particle?
- electron
- mass = 1/1840 (negligible)
- charge = -e
- symbol = β-
what is the nature, mass, charge and symbol of a beta-plus particle?
- positron
- mass = 1/1840 (negligible)
- charge = +e
- symbol = β+
what is the nature, mass, charge and symbol of a gamma ray?
- high freq. electromagnetic radiation
- mass = zero
- charge = zero
- symbol = γ
what happens in beta minus decay in terms of numbers of protons and neutrons?
a neutron is converted to a proton which produces an electron and an electron anti-neutrino
what happens in beta plus decay in terms of numbers of protons and neutrons?
a proton is converted to a neutron which produces a positron and an electron neutrino
which radiation is the most penetrating out of alpha, beta and gamma?
gamma, beta then alpha
which radiation is the most ionising out of alpha, beta and gamma?
alpha, beta then gamma
what is beta minus decay?
in beta minus decay a neutron in the nucleus breaks down into a proton under the influence of the weak nuclear force, and a beta minus particle (an electron) are emitted to conserve change, and an electron anti-neutrino are emitted to conserve lepton number,
what is beta plus decay?
in beta plus decay a proton in the nucleus breaks down into a neutron under the influence of the weak nuclear force, and a beta plus particle (a positron) and an electron neutrino are emitted
what is the quark change when a nuclei undergoes beta minus decay?
udd changes into uud
neutron into proton
what is the quark change when a nuclei undergoes beta plus decay?
uud changes into udd
proton into neutron
what is the equation in terms of the quark model for beta minus decay?
d —> u + e + ̅ν
what is the equation in terms of the quark model for beta plus decay?
u —> d + e + v
what are the names of the nuclei after it has undergone alpha decay?
daughter nuclei from parent nuclei
can you get pure gamma decay?
no - gamma radiation frequently accompanies either α or β decay, but never occurs as purely a gamma decay
Gamma decay is caused when a nucleus has surplus energy following alpha or beta emission. There is no change to nucleon composition, but energy is released in the form of a gamma photon.
Describe radioactive decay
it is random because:
- we cannot predict when a particular nucleus in a sample will decay or which one will decay next
- each nucleus within a sample has the same chance of decaying per unit time
It is spontaneous because of the decay of nuclei is not affected by
- the presence of other nuclei in the sample
- external factors such as pressure
how many half life formula
use the half life exponential graph
counts per min/sec divided by time (days)
How long will it take for the activity of an isotope with a decay constant of 4.22 x 10^-5 s/1 to fall to 1% of its initial value
t = 1.09 x 10^5 s
A beta-emitting source has an activity of 4.0kBq
Calculate the number of beta particles emitted in a period of 1.0 minutes. State any assumptions made
number of decay: 4000 x 60 = 2.4 x 10^5
Assumptions: activity remains constant over the 1.0min period
what happens to the rate of activity of a radioactive sample over time (the rate at which unstable nuclei decay)?
the rate decreases over time because there are fewer unstable nuclei
Define the background count rate and corrected count rate
It is the count rate without the radioactive source present
The corrected count rate is the background count rate subtracted from the measured count rate
what is activity A and state it’s SI units
The activity A of a source is the rate at which nuclei decay or disintegrate
You can also think of the activity as the number of alpha, beta, or gamma photons emitted from the source per unit time
Bq
What is 1 becquerel (BQ)
An activity of one decay per second
what is the equation that links activity to no. of undecayed nuclei?
A = λN
where A = activity
λ = decay constant
N = no. of undecayed nuclei
what is the decay constant?
λ, the decay constant can be definite as the probability of decay of an individual nucleus per unit time, given by λ = A / N and has units (s^-1)
what is the definition for half life?
the half-life (t1/2) is defined as the average time it takes for half the number of active nuclei in the sample to decay
Radioactivity is 2000-250 Bq in 6 days. What is it’s half life
2000>1000>500>250
3 half lives = 6 days
1 half life = 2 days
Half-life is 10 hours. What would activity be after 40 hours if started at 145 c.p.s. (25cps background)
40/10 = 4 half lifes
120>60>30>15>7.5
7.5Bq
what is the equation for the half life of an isotope? And derive it
t = In2/ λ
N = No x e^-λt
After a time t = t(1/2), N = No/2
No/2= No x e^-λt(1/2)
1/2 = e^-λt(1/2)
Now take the logs of both side
In(1/2) = -λt
In1 - 1n2 = - λt
In1 = 0
what are the two exponential equations, one for activity and no. of undecayed nuclei?
A = Ao x e^-λt
N = No x e^-λt
where A = activity, Ao = initial activity
N = no. of undecayed nuclei, No = initial no. of undecayed nuclei, λ = decay constant, t = time spent decaying
A freshly prepared sample of thorium-227 has 4.0 x 10^12 nuclei. The isotope of thorium - 227 has a half-life of 18 days. Calculate its activity after 22 days
7.6 x 10^5 Bq
outline an experiment to investigate the absorption of α-particles, β and γ radiation
- you can investigate the penetrating power of alpha, beta and gamma radiation using a Geiger-Muller tube and counter to detect the radiation passing through sheets of different materials placed between the tube and the radioactive source
- the background radiation count rate (the number of ionising events detected per second or per minute when no radioactive source is nearby) must be first measured and then subtracted from all recorded count rates. Use paper and aluminium foils and pieces of lead of different thickness to investigate what thickness of each material stops each type of radiation
GM tube must not touch the bottle. Why?
Inorder to avoid contamination
how does carbon dating work?
In the atmosphere there are three main isotopes: C-12, C-13 and C-14. The ratio between these isotopes is a known constant, approximately 99% is C- 12, 1% C-13 and C-14 exists in trace amounts. Living organisms absorbs Carbon isotopes during their lifetimes either through photosynthesis of CO2 or through the consumption of other organisms containing carbon. Therefore, the ratio of C-14 to C-12 in the organism will match the atmospheric ratio yet at the point of death the ratio of C-14 to non-radioactive C-12 will decrease over time. This is because the living matter is not consuming the carbon isotopes anymore. The C-14 will therefore decays via beta emission with a half-life of ~5700 years. Hence, by measuring the ratio of C-14 to C-12 in the dead tissue and comparing this to the atmospheric composition or in a living sample can be used for dating
what are the limitations of carbon dating?
- Because the quantity of carbon-14 is very small count rates are correspondingly small and after a few half lives may be indistinguishable from the background count rate
- the ratio of carbon-14 to carbon-12 in the air and in all living things is assumed to be constant, that is there has been no variation over time, this may not be true if there has been changes in the rate at which carbon-14 is produced in the atmosphere
- Radiocarbon dating is not a perfect technique however, firstly it assumes the ratio of C- 12 to C-14 has remained constant throughout history. Increases emission of carbon dioxide due to burning of fossil fuels may have reduced this ratio, as would natural events such as volcanic eruption. Ratio may also be affected by solar flares from the sun and by the testing nuclear bombs.
- Also, for small samples the amount of C-14 in the sample can be unnoticeable in comparison to the background radiation.
- Finally for samples much older than 5700 years, the amount of C-14 becomes immeasurably small so this technique cannot be used. Instead, rubidium is often used due to its longer half-life of about 49 billion years
How does rubidium-87 work
Geologist use the decay of rubidium-87 to date ancient rocks. Nuclei of rubidium-87 emir beta-minus particles and transform into stable nuclei of strontium-87
State how atmospheric C-14 is produced in the earths atmosphere
C-14 nuclei are formed when neutrons (produced from collisions of cosmic particles) collide with nitrogen-14 nuclei
neutron + N-14»_space;» C-14 + Proton
Explain why dating an ancient wooden axe by measuring its activity may be problematic
The count rate from a sample is comparable to the background count rate
what is Einstein’s mass energy equation?
E = mc^2
what is annihilation?
annihilation is the process in which a particle and its antiparticle interact and their combined mass is converted to energy via E = mc^2 (photons are released)
what is pair production?
pair production is the process of creating a particle-antiparticle pair from a high-energy photon (like from annihilation)
what has greater mass, the total mass of the separated nucleons or the total mass of the nucleons?
total mass of the separated nucleons has MORE ENERGY than the total mass of the nucleons
why is there a difference between the mass of the separate nucleons and the mass of the whole nucleus?
this arises from the fact that all the nucleons are bound together by the strong nuclear force, this means work has to be done to separate the nucleons, so the separated nucleons gain potential energy and by Einstein’s mass-energy equation they then have more mass
in radioactive decay, how is energy released? in what form?
energy is released in the form of the kinetic energy of the fast moving alpha or beta particles, and may also be released as gamma ray photons
what is the opposite of pair production?
annihilation (and vice versa)
what can the instability of a nucleus be caused by?
- too many neutrons
- too few neutrons
- too much energy in the nucleus
- too many nucleons in total (too heavy)
can you ever get a free quark on its own?
no - you cannot get a free quark on its own, you cannot separate quarks, if you tried put energy in you would just convert more quarks and antiquarks, you just get pair production
when energy is converted to mass what is the ratio between matter and antimatter produced?
equal amounts of matter and antimatter
outline an experiment to determine the half-life of an isotope such a protactinium
(see page 495 for diagram)
- measure background count rate in the absence of the proctanium generator -use a protactinium generator, a bottle containing an organic solvent and a solution of uranyl[VI] nitrate in water is used to separate the proctanium from thorium. ( Now this works because the proctanium daughter isotope is soluble in the organic solvent, whereas the parent thorium compound is not.)
- shake the bottle to allow the proctanium to dissolve in the organic solvent for about 15s
- wait for the liquids to separate, the protactinium-234 will be in solution in the top layer, and the uranium salt will stay in the bottom layer
- then position the Geiger-Muller tube at the top layer using a clamp stand to measure the activity of the protactinium-234
- now as soon as the liquids separate, record the count rate (how many counts in 10 secs), re-measure the count rate at sensible intervals (e.g every 30 seconds)
- once you’ve collected your data, leave the bottle to stand for at least ten mins
- then take the background count rate again to double check, this is the background count rate corresponding to background radiation
- subtract this value from your measured count rates
- the corrected count rate is directly proportional to the activity of the source
- therefore half life can be determined by plotting a graph of count rate against time
- (it should look like an exponential downwards on decreasing gradient, you can use your graph to work out the half-life by looking at how long it takes for the count rate (or activity) to halve)
what does alpha decay occur in?
very heavy nuclei like uranium and radium, the nuclei of these atoms are too massive to be stable
what doe beta minus decay occur in?
neutron-rich nuclei, where they have many more neutrons than protons in their nucleus
what is the mass defect?
the difference in mass between the mass of a nucleus and the total mass of its separate nucleons (extra mass)
what is the mass defect in a decay reaction equal to?
this extra mass (mass defect) is equivalent to the binding energy of a nucleus and is released as energy
what is the definition of binding energy?
binding energy of a nucleus is the minimum energy required to separate the nucleus (protons and neutrons) into its constituent parts
what does the graph of binding energy per nucleon against nucleon number look like? what does it mean?
you get a curve, a high binding energy per nucleon means more energy is needed to remove nucleus from the nucleus so in other words the most stable nuclei occur around the maximum point on the graph at around iron (Fe 56)
what does fusion do in terms of binding energy per nucleon?
increases the binding energy per nucleon which means lots of energy is released during fusion
what does fission do in terms of binding energy per nucleon?
fission is when large nuclei split in two, the nucleon numbers of the two new nuclei are smaller than the original nucleus , which means there is an increase in the binding energy per nucleon, and the difference in the binding energy is the energy released during nuclear fission
what is alpha absorbed by?
absorbed by paper of a few cm of air
what is beta-minus absorbed by?
absorbed by 3mm of aluminium
what is beta-plus absorbed by?
annihilated by electron - so virtually no range
what is gamma absorbed by?
absorbed by many cm of lead, or several m of concrete
when balancing decay equations, what must be conserved?
charge and nucleon number
gamma radiation is emitted from what type of nuclei?
a nuclei with an excess of energy, an ‘excited’ nuclei
what can the binding energy per nucleon graph be used to estimate?
the energy released from nuclear reactions
what is induced nuclear fission?
induced nuclear fission occurs when a nucleus (such as uranium-234) absorbs slow-moving neutrons and the resulting unstable nucleus undergoes a fission reaction to split into two smaller nuclei and a small number of neutrons, releasing energy
Explain the splitting of uranium-235, uranium-238 and uranium-236
Both uranium-235 and uranium-238 nuclei can split spontaneously without absorbing neutrons, but this is very rare. However, uranium-236 nuclei have a much greater chance of splitting spontaneously. Which is why we induce a fission reaction of uranium-235 to produce the unstable uranium-236
The uranium-235 nucleus’s captures a thermal neutron and becomes a highly unstable nucleus of uranium-236. In less than a microsecond, the uranium-236 nucleus splits. The daughter nuclei produced in this example are barium-141 and krypton-92, and there many other possible variants. Three fast moving neutrons are also produced too.
In terms of binding energy explain the nuclear fission reaction: before and after
The total mass of the particles after the fission reaction is always less than the total mass of the particles before the reaction. The difference in mass corresponds to the energy released in the reaction. Put another way, the total binding energy of the particles after fission is greater than the total binding energy before it. The difference in the binding energies is equal to the energy released.
what is a chain reaction?
a chain reaction is the sequence of nuclear reactions produced when an induced nuclear fission reaction triggers more than one further fission reaction
what is a control rod?
a control rod is a rod that can be lowered up and down into the core of a nuclear reactor, absorb neutrons and slow down the chain reaction, they are usually made of boron or cadmium (to control rate of fission)
what is a moderator and how does it work?
a moderator is a substance used in nuclear reactor which slows down neutrons so that they have a greater chance of being absorbed by the nuclear fuel rods (uranium-234 nuclei),
What happens is that the fast-moving electrons just bounce off the uranium nuclei with negligible loss of kinetic energy. However, when they collide elastically with protons (or deuterium) in water or with carbon nuclei in graphite, they transfer significant kinetic energy and thus slow down
they are usually made of graphite but can be water too
What is a coolant
It is used to remove the thermal energy produced from the fission reactions within the fissile fuel
how is an ideal chain reaction brought about
the position of the control rods is adjusted to ensure that exactly one slow neutron survive per fission reaction.
what does the diagram look like for a fission reactor?
see page 512 textbook
what are the three types of nuclear waste?
high level, intermediate level, low level
why are thermal neutrons absorbed by the nuclear fuel (such as uranium-234)
because their mean kinetic energy is similar to the thermal energy of particles in the reactor core
What are slow moving neutrons known as
thermal neutrons
what is high level nuclear waste?
It is waste that produces large amounts of radiation, includes fuel rods removed from the core and waste resulting from the reprocessing of this fuel
what is intermediate level nuclear waste?
It is waste material which has become radioactive because it has been in a nuclear reactor, for example the reactor’s metal cladding
For example, when a neutron is absorbed by uranium-238, it decays into a highly radioactive plutonium-239
what is low level nuclear waste?
It waste that are slightly radioactive due to becoming contaminated with small amounts of radioactivity, such as cleaning materials and protective clothing
why does high level nuclear waste need shielding?
because they produce heat as a result of the rapid decay of some of the short lived isotopes, so it needs cooling for a few years, as well as shielding to block radioactive emissions over many thousands of years while the longer half-life isotopes decay
what are two main advantages of nuclear fusion?
- no radioactive waste products are formed by the fusion process (unlike with fission)
- there is a virtually unlimited supply of the raw materials, about 1% of seawater molecules have a deuterium atom in them (deuterium and tritium are the two isotopes used in fusion)
what is nuclear fusion?
when two light nuclei can combine to create a larger nucleus and in the process release a huge quantity of energy
why is a high temp and high pressure required for nuclear fusion?
The only way to make nuclei fuse is to bring them close together, to within a few 10^-15 m, so that the short-range strong nuclear force can attract them into a larger nucleus. All nuclei have a positive change, so they repel each other. The repulsive electrostatic force between nuclei is enormous at small separations. At low temperatures, the nuclei does not have enough kinetic energy to get close together to trigger fusion. However, at higher temperatures, they enough kinetic energy to overcome to electrostatic repulsion and be close enough to fuse with with the nuclear strong force
what are the problems associated with building and decommissioning nuclear power plants?
because of all the necessary safety precautions, building and decomissioning is very time-consuming and expensive
also it is very difficult to confine the extremely hot fuel within a fusion reactor
what does the word fission actually mean?
splitting up or breaking into parts/components
why is a lot of energy released in nuclear fusion?
a lot of energy is released during nuclear fusion because the new, heavier nucleus has a much higher binding energy per nucleon (and so a lower total mass), the energy released helps to maintain the high temperatures needed for further fusion reactions
why is nuclear fusion not currently a source of energy?
not efficient, any apparatus that can induce fusion requires much more electrical energy than it could produce, too expensive
what is a control rod made up of
Boron and cadmium
where does nuclear fusion often occur in naturally?
stars like our Sun
Explain why fussion cannot occur at low temperature
at low temp, the nuclei will have low kinetic energy and so will be moving very slow, and won’t be able to get close enough to over come the strong electrostatic force of repulsion, and for the strong nuclear force to bring about fusion
Does a neutron on its own have binding energy
no because it is not bounded to anything
what is the difference in binding energy
It is the energy released
How do we get energy from a fission reaction
Take a large nuclei and split it. This gives you two smaller nuclei compared to the original nuclei which both have an increased binding energy. The difference in the binding energy is the energy released
