6 Radioactivity Flashcards
alpha radiation
consists of positively charged particles.
each particle comprises 2 protons and 2 neutrons (a helium nucleus), and has charge +2e, where e is the elementary charge
beta radiation
consists of fast moving electrons (B-) or fast moving positrons (B+)
a beta-minus particle has charge -e and a beta-plus particle has charge +e
gamma radiation
(or rays)
consists of high-energy photons with wavelengths less than about 10^-13 m
they travel at the speed of light and carry no charge
all are emitted from the nuclei of atoms as a result of changes within unstable nuclei
radioactive decay
the spontaneous breakdown of an atomic nucleus resulting in the release of energy and matter from the nucleus. It is a random process meaning that it is impossible to predict which of a number of identical nuclei will decay next. Yet each decay follows a defined pattern and given a large enough number of nuclei this yields predictable results.
effect of electric and magnetic field on radioactive decay
uniform electric field provided by two oppositely charged parallel plates can distinguish between different types of radiation
the negative B- particles (electrons) are deflected towards the positive plate, whilst he positive alpha and B+ (positron) particles are deflected towards the neg plate
alpha particles are deflected less than beta particles because of their greater mass
the paths of the B- and B+ particles are mirror images
gamma rays are not deflected, bc they are uncharged
flemings left hand rule
Hold your thumb, forefinger and second finger at right angles to each other: the forefinger is lined up with magnetic field lines pointing from north to south. the second finger is lined up with the current pointing from positive to negative.
Thumb is direction of the force
absorption of alpha radiation
the large mass of alpha particles mean they interact with surrounding particles to produce strong ionisation, and therefore they have a very short range in air
it only takes a few cm of air to absorb most alpha particles
a thin sheet of paper completely absorbs them
absorption of beta radiation
the small mass and charge of B particles make them less ionising than alpha particles
this means that they have a much longer range in air, about a metre
it takes about 1-3mm of aluminium to stop most B particles
absorption of gamma radiation
gamma rays have no charge and this makes them even less ionising than B particles
the count rate decays exponentially with the thickness of a lead absorber
you need a few cm of lead to absorb a significant proportion of gamma rays
dangers of radioactivity
all radiation causes ionisation, which means they can damage living cells
handling radioactive substances
stored in lead lined containers
use pair of tongs with long handles when handling
transmutation
the changing of one element into another by radioactive decay
parent nucleus- nucleus before decay
daughter nucleus- nucleus after decay
alpha radiation charge
+2e
beta minus radiation charge
-e
beta plus radiation charge
+e
gamma radiation charge
0
alpha decay
loss of an a particle removes 2 protons and 2 neutrons from a parent nucleus, so the nucleon number drops by 4
daughter has diff proton number so is diff element
energy is released in the decay
beta minus decay
RA nuclei that emit B- radiation are characterized as having too many neutrons for stability
the weak nuclear force is responsible for one of the neutrons decaying into a proton
an electron and electron anti neutrino are emitted
beta plus decay
RA nuclei that emit B+ radiation are characterized as having too many protons for stability
the weak nuclear force is responsible for one of the protons decaying into a neutron
a positron and electron neutrino are emitted
gamma decay
photons are emitted if a nucleus has surplus energy following an alpha or beta emission
composition of the nucleus remains the same
patterns for stability
graph of no of neutrons N against proton no Z
all stable nuclei lie on a very narrow band known as the stability band
the ration of neutrons to protons ins table nuclei gradually increases as the no of protons in the nuclei increases
only nuclei with Z less than about 20 are stable with an equal no of protons and neutrons
most nuclei have more neutrons than protons
-nuclei with more than 82 protons are likely to decay by emitting a particles
-nuclei to the right of the band have too many protons and will likely decay by B+
-nuclei to the left of the band have too many neutrons and will likely undergo B- decay
how is RA decay random?
- 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
how is RA decay spontaneous?
-decay of nuclei not affected by the presence of other nuclei or external factors such as pressure
half life
half life of an isotope is the average time it takes for half the number of active nuclei in the sample to decay
the number N must therefore decay exponentially with time
activity
A
the rate at which nuclei decay or disintegrate
no of a, B or gamma photons emitted from the source per unit time
measured in decays per second
an activity of one decay per second is one becquerel
decay constant
lambda
number of nuclei decaying is directly proportional to N and change in t
therefore
change in N/ change in time= -N
A=lambda x N
exponential decay
number of undecayed nuclei decreases exponentially with time
activity A of the source is directly proportional to N
therefore the activity also decreases exponentially with time
carbon dating
atmospheric carbon is mainly stable isotope C-12 but also a tiny amount of radioactive isotope C-14
C-14 has half life of 5700 yrs
ratio of C14 to C12 nuclei in atmospheric carbon is constant
the ratio is the same in all living things
once organism dies it stops taking in carbon whilst the total amount of C14 it contains ontinues to decay, so this ratio decreases over time
the time since the organism died can be determined by comparing the activities or ratios C14 to C12 nuclei of the dead material and similar living material
limitations to carbon dating
carbon dating is not a perfect technique as it assumes the ratio of C12 to C-14 has remained constant throughout history. increased emission of CO2 due to burning fossil fuels may have reduced this ratio
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.
dating rocks
decay of rubidium-87 to date ancient rocks
nuclei of R87 emit B- particles and transform into stable nuclei of strontium-87
half life of R87 is 49 billion yrs
rutherfords alpha scattering experiment
narrow beam of alpha particles, all of same KE, from a radioactive source were targeted at a thin piece of gold foil which was only a few atomic layers thick
a particles were scattered by the foil and detected on a zinc sulfide screen mounted in front of a microscope
each a particle hitting this fluorescent screen produced a tiny speck of light
the microscope was moved around in order to count the number of a particles scattered through different values of the angle per minute for zero to almost 180 degrees
observations and conc for rutherfords a scattering experiment
- most of a particles passed straight through the thin gold foil with very little scattering. 1 in every 2000 a particles was scattered
- very few of the a particles- 1 in every 10000- were deflected through angles of more than 90 degrees
most of atom is empty space with most of the mass concentrated in a small region - nucleus
nucleus has positive charge as it repelled the few positive a particles that came near it
what is the nucleus of an atom for a particular element represented as?
A
X
Z
X is chemical symbol for the element
A is nucleon number
Z is proton number
isotopes
nuclei of the same element that have the same number of protons but diff no of neutrons
atomic mass units
masses of atoms and nuclear particles are often expressed in atomic mass units (u)
one u is 1/12 the mass of a neutral C12 atom
radius of nuclei equation
R=r0A1/3
r0 is 1.2 fm (1.2 x 10-15 m)
A is nucleon number
nucleus radius
10^-15 m
radius of atoms
10^-10 m
how many times larger is the atom than the nucleus?
10^5 times larger
strong nuclear force
acts between all nucleons
is a very short range force
force is attractive to about 3fm and repulsive below 0.5fm
what keeps 2 protons from flying apart?
strong nuclear force
nuclei density
10^17 kgm-3
density of matter made up of atoms
10^3 kgm-3
positron
anti particle of electron
same mass - 9.11 x 10-31 kg
opposite charge- +1.6 x 10-19 C
fundamental forces
- strong nuclear
- electromagnetic
- weak nuclear
- gravitational
electromagnetic force
experienced by static and moving charged particles
infinite range
weak nuclear force
responsible for beta decay
range 10^-18m
gravitational force
experienced by all particles with mass
infinite range
fundamental particles
particles with no internal structure
quarks
electrons (lepton)
neutrinos (lepton)
subatomic particles are divided into which two categories?
hadrons and leptons
hadrons
particles and anti particles affected the strong nuclear force. e.g. protons, neutrons and mesons. hadrons, if charged, also experience the electromagnetic force.
they decay by the weak nuclear force
leptons
particles and antiparticles that are not affected by the strong nuclear force. e.g. electrons, neutrinos and muons
leptons, if charged, experience the electromagnetic force
6 types of quarks
up down strange charm top bottom
proton
up up down
Q= +2/3e + +2/3e + -1/3e = 1e
neutron
down down up
Q= -1/3e + -1/3e + +2/3e = 0e
baryons
are any hadrons made with a combination of three quarks
e.g. protons and neutrons (and their anti particles)
mesons
are the hadrons made with a combination of a quark an an anti quark
neutrinos
fundamental particles that carry no charge and may have a tiny mass
3 types- electron neutrino Ve, muon neutrino V(mu) and tau neutrino Vt (half pi)
what force is responsible for beta decay?
weak nuclear force
B- decay
a neutron in an unstable nucleus decays into a proton, an electron and an electron antineutrino
1/0n –> 1/1p + 0/-1e + antiVe
d –> u + 0/-1e + antiVe
B+ decay
a proton decays into a neutron, a positron, and an electron neutrino
1/1p –> 1/0n + 0/+1e + Ve
u –> d + 0/+1e + Ve
balancing quark transformation equations
the total charge on both sides of the equation are equal
charge has been conserved
two interpretations of E=mc2
- mass is a form of energy
- energy has mass
conservation of mass-energy
unstable nuclei decay by emitting either particles or photons. in alpha decay, the parent nucleus emits an alpha particle, creating a daughter nucleus, which recoils in the opposite direction. both have KE.
the total amount of mass and energy in a system is conserved.
since energy is released in radioactive decay, there must be an accompanying decrease in mass
so the mass of the alpha particle and daughter nucleus must be less than the mass of the parent nucleus
same with B decay
annihilation and creation
when positrons and electrons meet, they annihilate each other, and their entire mass is transformed into energy in the form of two identical gamma photons
opposite is pair production
pair prodcution
a single photon vanishes and its energy creates a particle and a corresponding antiparticle
e.g. an electron-positron pair
mass defect
the difference between the mass of the completely separated nucleons and the mass of the nucleus
binding energy
binding energy of a nucleus is the minimum energy required to completely separate a nucleus into its constituent protons and neutrons
binding energy of nucleus= mass defect of nucleus x c^2
binding energy per nucleon
the greater the binding energy per nucleon, the more tightly bound are the nucleons within the nucleus
a nucleus is more stable if it has a greater BE per nucleon
binding energy per nucleon against nucleon number curve
- The most stable isotope is iron-56 as it has the maximum binding energy per nucleon.
- For low nucleon numbers, A<56 , the binding energy per nucleon increases as A increases.
- He-4 and O-16 nuclei have abnormally greater BE per nucleon than their immediate neighbors
- in a fusion process, two low A number nuclei join together to produce a higher A number nucleus. the newly formed nucleus has much greater binding energy than the initial nuclei and therefore energy is released
- For high nucleon numbers, A>56, the binding energy per nucleon decreases as A increases
- in a fission process, a high A number nucleus splits into two lower A number nuclei. energy is released because the two nuclei produced have higher binding energy than the parent nucleus.
induced fission
an uranium-235 nucleus captures a slow (thermal) neutron and becomes a highly unstable nucleus of uranium-236. in less than a microsecond, the U-236 nucleus splits
the daughter nuclei produced for example are barium-141 and krypton-92 (many other possible variants)
three fast neutrons are also produced
1/0n + 235U –> 236U –> 141Ba + 92Kr + 3 1/0n
fission energy
total mass of particles after the fission reaction is always less than the total mass of particles before the reaction. the difference in mass corresponds to the energy released in the reaction
so the total binding energy of the particles after the fission is greater than the total BE before it
difference in BE is equal to the energy released
the energy released in a single fission reaction is a combination of KE of the particles produced and the energy of photons and neutrinos emitted
chain reaction
fission of U-235 nucleus is more likely with slow neutrons than fast. if the three fast neutrons produced in a fission reaction can be slowed, they could instigate further fission reactions in other U-235 nuclei.
this makes a chain reaction possible, with these three neutrons starting three more reactions, which each produce another three neutrons and so on
after n generations of fission events, the number of neutrons would be 3^n - the growth in neutron numbers will be exponential
this isn’t wanted in a nuclear reactor
inside a fission reactor
fuel rods are spaced evenly within a steel-concrete vessel known as the reactor core
a coolant is used to remove the thermal energy produced from the fission reactions within the fissile fuel
the fuel rods are surrounded by the moderator, and control rods can be moved in and out of the core
fuel rods
contain enriched uranium, which consists mainly of U-238 with 2-3% U-235
moderator
slows down the fast neutrons produced in fission reactions. slowed neutrons can carry on fission reactions
material for moderator must be cheap and readily available, and must not absorb the neutrons in the reactor
fast moving neutrons collide elastically with protons in water or with carbon nuclei, they transfer significant KE and slow down, so water and carbon are good moderators
in many reactors, the moderator is also the coolant
control rods
made of a material whose nuclei readily absorb neutrons, most commonly boron or cadmium.
the position of the control rods is automatically adjusted to ensure that exactly one slow neutron survived per fission reaction, this prevents a chain reaction
to slow down or completely stop the fission, the rods are pushed further into the reactor core
environmental impact of nuclear waste
neutrons of intermediate kinetic energies are readily absorbed by U-238 nuclei within the fuel rods
these U-238 nuclei quickly decay into plutonium-239 nuclei
238/92U + 1/0n –> 239/92U –> 239/93Np –> 239/94Pu
B- decay
Pu-239 is one of the most hazardous material produced in nuclear reactors
radioactive waste from nuclear reactors cannot be disposed of as normal waste.
high level radioactive waste has to be buried deep underground for many centuries because isotopes with long half lives must not enter our water and food supplies
These burial locations must be safe from attack and designed to be protected against earthquakes
plutonium-239
Pu-239 is one of the most hazardous material produced in nuclear reactors
it is extremely toxic as well as radioactive, and it has a half life of 24000 years
the daughter nuclei produced from its numerous fission reactions are also radioactive
fusion
the only way to make nuclei fuse is to brig them close together, to within a few 10^-15m, so that the short range strong nuclear force can attract them into a large nucleus
all nuclei have a positive charge, so they will repel each other
the repulsive electrostatic force between nuclei is enormous at small separations. at low temps, the nuclei cannot get close enough to trigger fusion
however, at higher temps, they move faster and can get close enough to absorb each other through the strong nuclear force
conditions for fusion
like the core of a star, e.g. the sun
-high temp
-large density
high temp to allow the nuclei to overcome the repulsive electrostatic force between them and fuse together
large density ensures a high number fusion reactions per second
proton-proton cycle
-2 protons fuse together to produce deuterium nucleus, a positron and neutrino and energy is released
1/1p + 1/1p –> 2/1H + 0/+1e + V
-deuterium nucleus fuses with a proton. a helium-3 nucleus is formed and energy is released
2/1H + 1/1p –> 3/2He
-the He-3 nucleus combines with another nucleus. a He-4 nucleus and two protons are formed. energy is released
3/2He + 3/2He –> 4/2He + 2 1/1p
fusion on earth
there are no power stations using fusion yet
the main problems are maintaining high temps for long enough to sustain fusion and on confining the extremely hot fuel within a reactor
