Particles and Radiation

nuclide notation

a nucleon is a proton or a neutron (a particle inside the nucleus)

nucleon number, A, is the combined number of protons and neutrons inside that nucleus

proton number, Z, the number of protons in the nucleus. this defines the element

nuclide and isotope

nuclide — a specific type of nucleus defined by its number of protons and neutrons

isotopes — atoms of the same element (same proton number, Z) but with different neutron numbers

changing neutron number doesn’t affect the atom’s chemical properties but rather it affects the stability of the nucleus

(in general, the greater the number of neutrons compared to protons, the more unstable the nucleus)

specific charge

the ratio of a particle’s charge (C) to its mass (kg)

charge / mass

electromagnetic force in the nucleus

causes the positively charged protons

to repel each other

extends over an infinite range.

gravitational force in nucleus

causes all the nucleons on the nucleus to attract each other due to their mass

this is much smaller in magnitude (due to the very small masses of nucleons) than the electromagnetic force — negligible

strong nuclear force in nucelus

an attractive force that is stronger than the electromagnetic force

keeps nuclei stable by overcoming the electrostatic force of repulsion between protons

acts between all nucleons

very short range of only a few fm — can’t hold together very large, unstable nuclei

repulsive for very small nucleon separations of less than 0.5fm

0 at 0.5fm

as separation increases to between 0.5-3fm, snf is attractive and reaches a maximum attractive value

rapidly falls towards 0 above 3fm

nuclear decay

the process of unstable nuclei emitting particles to become more stable

alpha decay

usually happens only in heavy nuclides with proton number greater than 82

emit an alpha particle from their nucleus

energy is released, with most transferred to the alpha particle as kinetic energy. a small amount goes to the decayed nucleus.

nucleon number decreases by 4

proton number decreases by 2

alpha particles have a very short range of only a few cm in air and can be stopped by a sheet of paper

beta decay

beta minus

• neutron is changed into proton

• high speed electron ejected from the nucleus along with an antineutrino particle which carries away some energy and momentum

• happens in neutron rich nuclei

• nucleon number doesn’t change

• proton number increases by 1

beta plus

• a proton is changed into a neutron

• high speed positron ejected from the nucleus along with a neutrino

• nucleon number doesn’t change

• proton number decreases by 1

both types of beta decay have a range of a few metres in air and can be stopped by an aluminium sheet

hypothesis of neutrinos

when the nucleus decays, it releases a fixed amount of energy according to which nuclide it was.

observations showed that the energy of the particles after beta decay was less than before — the kinetic energies varied up to a maximum value, depending on the proportion of energy it received) — this didn’t fit the principle of conservation of energy 

so another particle must be emitted alongside the beta particles and thus should be neutral ( to conserve charge) and have zero or negligible mass (as it had never been detected) 

this particle shares the energy released from decay with the beta particle

antiparticles

• every particle had corresponding antiparticle

• equal in mass and rest energy but opposite in charge (if charged)

electron volt eV

1eV = 1.6 x 10^-19 C

the energy transferred when an electron is moved through a potential difference of 1V

rest energy

the rest energy of a particle is the amount of energy that would be produced if all of its rest mass was converted into energy

pair production

when energy is converted into mass, equal amounts of matter and antimatter are produced. 

only occurs if there is enough energy to produce the rest masses of the particles

it must always produce a particle and its corresponding antiparticle because certain quantities must be conserved.

E_min = 2E₀

minimum energy for pair production = the combined rest energy of the two particles produced

2E because each particle has the same rest energy as the other as they are a particle and antiparticle pair

If the photon has greater than the required minimum energy, then the remaining energy is transferred to the particle and antiparticle pair as kinetic energy

annihilation

when a particle comes into contact with its corresponding antiparticle, the combined rest mass of the particle and the antiparticle is converted into energy in the form of two gamma ray photons which move in opposite directions to conserve momentum

electromagnetic spectrum

a continuous spectrum of all the possible frequencies of electromagnetic radiation.

• frequency - number of waves passing a point per second

• wavelength - distance between adjacent crests of a wave

• the higher the frequency of EM radiation, the greater its energy.

photon model of EM radiation

photon - a discrete packet, quanta, of EM energy

the four fundamental forces

• strong - gauge boson is pion. affects hadrons only

• electromagnetic - gauge boson is the virtual photon. affects charged particles only

• weak - gauge boson is the W+ and W- bosons. affects all types of particles

• gravitational is incredibly weak in comparison to all of these. affects all particles with mass.

exchange particles

• how forces act between two particles

• virtual particles, existing only for a very short time - long enough to transfer energy, momentum and other properties during an interaction

• cannot be directly detected

• gauge bosons are exchange particles

• the size of the exchange particle determines the range of the force - heavier have shorter range - force itself has shorter range

electron capture

• proton in a proton rich nucleus can capture an electron from inside the atom and change into a neutron.

• a neutrino is them emitted

• the proton is acting on the electron so the W+ boson comes from the  proton.

electron proton collision

• electron collides with high speed with a proton.

• proton becomes a neutron and a neutrino is emitted.

• the electron acts on the proton so the W- boson comes from the electron.