7.3 Nuclear physics - IB

nuclear fission

nuclear fission

the splitting of a heavy nucleus into lighter nuclei. releases energy

nuclear fission - chain reaction

nucleus is struck by neutron - splits into nuclei and more neutrons are ejected, colliding with other nuclei and continuing fission, lasting until all material is used up

critical mass

a certain minimum mass of an element (uranium-235) that must be present, otherwise neutrons escape without causing further reaction

nuclear fusion

joining of two lighter nuclei to produce a heavier nucleus. energy is released due to mass defect

energy of nuclei undergoing fusion

kinetic energy of the nuclei has to be very high to overcome electrostatic repulsion

rutherford geiger marsden particle scattering experiment

alpha particles were directed at thin gold foil in vacuum chamber. the number of particles deflected at different angles were recorded

why does fusion occur at low values of A

because attractive nuclear forces between nucleons dominate over repulsive electrostatic forces between protons

why is energy released in fusion

the mass of nucleus created is slightly less than total combined mass of original nuclei. the mass defect is equal to binding energy released since the nucleus formed is more stable

why does fission occur at high values of A

repulsive electrostatic forces between protons begin to dominate, and these forces tend to break apart the nucelus

why is energy released in fission

an unstable nucleus is converted into stable nuclei with lower total mass - the mass defect is equal to binding energy released

fusion vs fission

fusion releases more energy per kg. the gradient of binding energy per nucleus much stepper at lower A values (when fusion occurs), meaning larger binding energy per nucleon is released

majority of alpha particles went through foil suggesting that:

the atom is mainly empty space

some alpha aprticles deflected at small angles suggesting:

there is a pos nucleus at the centre, repelling the positively charged alpha particle

a very small number of alpha particles deflected at large angles suggesting:

that the nucleus is extremely small and chrage of atom is concentrated

elementary/fundamental particle

particles not made up of any smaller components

quarks

fundamental particles that make up other subatomic particles, eg neutron and proton

hadrons

particle made up of quarks

flavours of quark

up u, charm c, top t, down d, strange s, bottom b

uct quark charge

+2/3

dsb quark charge

-1/3

anti particles

same mass but all properties are opposite, eg charge, baryon number, strangeness…

proton

uud

neutron

ddu

baryon

particle made of three quarks (no anti quarks)

meson

pair of quark and anti quark

baryon number for quarks

+1/3 (antiquarks have -1/3)

strangeness

strange quark has -1, anti strange has 1, everything else 0

when is strangeness not always conserved

weak interaction

conservation

charge, baryon number, lepton family number always, strangeness sometimes

leptons

fundamental particle, not made up of quarks

types of leptons

electron, muon, tau, electron neutrino, muon neutrino, tau neutrino

mass of leptons

tau heaviest, then muon, then electron. neutrinos have negligible amost zero mass

what force do leptons interact with

all weak nuclear force/interaction, those that have charge also with electromagnetic interaction

family lepton number

all leptons have lepton number of +1, just belongs to different families

what force do quarks interact with

strong, electromagnetic, weak

protons as baryons

the most stable baryon, has the longest half life and is lightest - cannot decay into lighter particle

beta minus decay

neutron turns into proton - the down quark turns into an up quark, antineutrino and electron also emmitted

beta plus decay

proton turns into neutron - up quark turns into down quark + positron + neutrino

pions

pos (u and anti d), neg (anti u and d) and neutral (u and anti u or d and anti d). lightest mesons, most stable. exchange particle of strong force

kaons

pos (u and anti s), neg (anti u and s) and neutral (anti d and s or d and anti s). heavy and unstable, decay into pions. long lifetimes (characteristic of particles w strange quarks). decay through weak interaction

strange particles

contain strange quark. produced through strong interaction, decay through weak

strong nuclear force

holds the nucleus together against electromagnetic repulsion. very short range. keeps quarks bound within nucleus

range of strong force

repulsive until 0.5fm

attractive from 0.5 to 3fm

max attractiveness at 1fm

zero after 3fm

exchange particles of strong interaction

pion (between nucleons) and gluon (between quarks) - hadrons subject to interaction since made up of quarks

leptons and strong force

leptons cannot interact w strong force since not made of quarks

weak nuclear force

responsible for radioactive decay of atoms

exchange particles of weak force

w-, w+, z0 bosons - depends on type of interaction

boson used in beta minus decay

w- boson

exchange particle used in beta plus decay

w+ boson

exchange particles of strong force

gluons and mesons

exchange particle of electromagnetic interaction

photon

gravitational exchange particle

graviton

quark confinement

it is impossible to have isolated quarks - they have never been found on their own, only in mesons or baryons

higgs boson

a particle responsible through its interactions for the mass od the particles od the standard model, in particular for masses of w and z bosons

electron capture

an electron is absorbed by a proton in the nucleus, resulting in release of a neutron and electron neutrino - mediated by w+ boson

electron proton collision

an electron collides with a proton, and a neutron and electron neutrino is emmitted - mediated by w- boson

properties of photon

no mass, no charge, its own antiparticle

exchange particles

virtual particles/bosons that mediate/carry/transmit the fundamental force between interacting particles - when two particles exert a force on each other, a virtual particle is created

graviton

only theoretical, no mass, no charge, its own anti particle