Nuclear Physics Flashcards
history of atom
-Democritus proposed all matter was made up of lumps called atomos
-jj thompson created plum pudding model
Rutherford experiment
1909, stream of alpha particles from radioactive source hit thin sheet of gold foil
and strike a fluorescent screen surrounding it
rutherford experiment conclusions
-atoms must be mostly empty space
-nucleus must have large positive charge
-most of the mass is concerntrated at the nucleus
-nucleus is very tiny
initial Ek equation
Qnucleus x q alpha / 4pi εo r
how to estimate radius of nucleus
rutherford scattering experiment, some particles are deflected 180
the alpha particle does this when Ep = initial Ke
electron diffraction
electrons are leptons that dont interact with the SNF
more accurate for measuring the radius
lambda = hc / E
E = energy of atom
radius of atom
0.05nm ( 5 x 10^-11)
radius of smallest nucleus
1fm (1 x 10^-15
sin theta
1.22 lambda / 2R
nuclear radius equation
R = Ro A^1/3
Ro
1.4fm (1.4 x 10^-15)
nuclear density equation
A m-nucleon/(4/3)pi (Ro A^1/3)^3
alpha radiation
strongly positive
can easily pull electrons off atoms
why alpha particles have ashort range
ionising atoms requires energy from the alpha partcile
the alpha particle quicly ionises many atoms ( about 10000 ionisations per mm of air for each alpha particle) and loses all their energy very quickly
alpha radiation uses
smoke detectors
beta radiation
lower mass and charge than alpha particle but higher speed
means it can still knock electrons off atoms
each beta particle will ionise around 100 atoms per mm of air
beta particle uses
can be used to measure the thickness of materials, like aluminium wood, etc.
gamma radiation uses
used to diagnose patients without the need for surgery
a radioactive source with short half life is eaten or injected and PET scanner is used to detect emitted gamma rays
background radiation
weak level of nuclear radiation found everywhere
how to measure background radiation
take 3 readings of the count rate using at greiger counter without a radioactive source present and find mean
sources of background radiation
-air(alpha radiation gas released from rocks)
-grounds and buildings (rocks)
-cosmic radiation
-living things
-man-made radiation
decay constang
constant of proportionality
the probability of a specific nucleus decaying per unit time and is a measure of how quickly an isotope will decay
activity equationS (3)
A = lambda x N
A = -N / t
A = Ao e^-lambda t
equation for N (2)
N = n Na
N = No e^-lambda x t
half life
average time it takes for the number of unstable nuclei to halve
or activity to halve ( how to measure as you cant accuratly measure number of unstable nuclei too small )
half life equations (3)
0.5No = No e^-lambda T0.5
ln0.5 = -lambda x T0.5
T0.5 = ln2 / lambda
radioactive dating
carbon-14 is used
living plants take in CO2 from the atmosphere during photosynthesis along with carbon-14
when they die the activity of carbon-14 starts to fall
half life is roughly 5730 years
medical diagosis
Technetium-99m widely used in medical tracers
tracer swallowed / injected and moves through body to point of interest
radiation emitted is recorded and an image is produced
gamma radiation with half life of 6 hours is suitable
a nucleus is unstable if
-too many neutrons
-too few neutrons
-too many nucleons - heavy
-too much energy
above line of stability
nuclei have too many neutrons
beta minus decay
below line of stability
nuclei have too many protons
beta plus dceay
very heavy nuclei
too many nucleons
alpha decay
beta minus decay
neutron rich nucleus
beta plus decay
proton rich nucleus
gamma emission
after beta and alpha decay they have excess energy and are in an excited state therefore it must undergo gamma emission
mass defect
difference of the mass of the nucleus and the added up mass of the individual nucleon constituents
einstein equation
e = mc^2
lost mass
as nucleons join together mass decreases
the lost mass is converted into energy and released
the energy released is equal to the mass defect
binding energy
energy needed to separate all of the nucleons in a nucleus
equal to mass defect
1u is roughly equal to…
931.5 MeV
average binding energy per nucleon
binding energy / nucleon number
fission sources
uranium 235 and plutonium 239
fission
large nuclei are unstable and some can randomly split into two smaller nuclei
energy is released and the new smaller energy have a higher average binding energy per nucleon
fission reaction
neutron fired at unstable nucleus
the unstable nucleus fissions into two smaller nuclei and a few neutrons and also releases energy
fusion
two light nuclei join together to create a larger nucleus
a lot of energy is released as new heavier nuclei have much higher average binding energy per nucleon
fusion energy required
all nuclei are positive
therefore electrostatic force of repulsion between them
nuclei can only fuse together if if they overcome this force and get close enough for the attractive SNFto hold them together
roughly 1MeV of Ke needed
how to calculate energy released for fission or fusion
e = change m c^2
where m is total diffrence in mass between initial and final nuclei
chain reaction
nuclear reactors use rods of uranium as fuel for fission reactors
these are placed into the reactors
this fission reaction produce more neutrons which induce other nuclei to fission
moderator
fuel rods need to be placed in a moderator (e.g. water) to slow down / and/or absorb neutrons
you need a moderator that will slow down some neutrons enough so they can cause further fission, keeping the reaction going at a steady rate
the moderator slows down neutrons through elastic collisions with the nuclei of the moderator material
when nuetrons collide with particles of a similar mass, theyre are slowed down more efficiently
why is water commonly used as moderators
wtaer is used as it contains hydrogen which has a similar mass to a neutron
moderator by elastic collision
assuming the collison is perfectly elastic both Ke and momentum is conserved
assuming the moderator particle is stationary at first then conservation of momentum = mn x vn = mn vn’ + mm vm’
conservation of kinetic energy moderator
0.5mn vn^2 = 0.5mn vn’^2 + 0.5mm vm’^2
final velocity of neutron (vn’)
(mn - mm)/(mn + mm) X vn
final velocity of moderator
vm’ = 2mn / (mn + mm) X vn
if mass of moderator particle = mass of neutron
final mass of neutron would be 0 and all of Ke and momentum woukd be transferred to moderator particle
critical mass
the amount of fuel needed for a fission chain reaction to continue at a steady rate on its own
if less than critical mass
sub-critical mass and reaction will just peter out
supercritical mass
nuclear reactors use a supercritical mass of fuel and control the rate of fission using control rods
control rods
control the chain reaction by limiting the number of neutrons in a reactor
they absorb neutrons so that the rate of fission is controlled
control rods are made up of boron that can absorb neutrons and can be inserted by varying amounts to control the reaction rate
coolant
sent around the reactor to remove heat produced by fission
material used should be a liquid or gass at room temperature and be efficient and transferring heat
often the coolent is the same water that is being used as the moderator
the heat from the reactor can be used to make steam which can be used to power the letrcity generating turbines
reactor shielding
nuclear reactor usually surrounded by a thick concrete case which acts as shielding preventing the radiation from reaching any of the workers
emergency shut down
in an emergency the reactor can be shut down remotely by the release of control rods
control rods are lowered fully into the reactor which slows down the reactions as quickly as possible
handling and storing waste products
unused uranium fuel rods only emit alpha radiation so its easy to contain
spent duel rods are more dangerous as since fission waste products usually have a larger proportion of neutrons than nuclei
fission waste usually emits beta and gamma which is strongly penetrating
when material is removed its very hot and placed into cooler ponds until it colls down and placed in a sealed container until its activity drops
how waste products can be used
can be used in practical applications like tracers in medical diagnosis
