P2-Particles and radiations

Constituents of the atom:

Relative charge charge Relative weight Mass/kg
proton 1 +1.6×1019 1 1.67×10-27
neutron 0 0 1 1.67×10-27
electron -1 -1.6×1019 1/2000 9.11×10-31



Isotopes are the atoms with the same number of protons and different number of neutrons. An isotope has the same atomic number but different mass number. The number of neutrons can be calculated by A-Z. The greater number of neutrons the more unstable the nucleus.

Specific charge:

Specific charge= charge of particle(C)/mass of particle (kg)

Stable and unstable nucleus (due to nucleus masses)

Gravitational forces: A very weak but attractive force pushing the nucleus together.

Electrostatic force (due to proton changes)

Strong repulsive forces between positive neutrons pushing them away from each other.

Strong nuclear force:

Strong attractive force which overcomes electrostatic force to keep the nucleus together.

Radioactive decay:

Naturally occurring radioactive isotopes release 3 types of radiations.

Alpha radiation:

Consists of alpha particles are made up of 2 neutrons and 2 protons 2α4


Beta radiations:

– Consists of fast moving electron.

– Beta radiations occur when the a neutron in the nucleus changes into a proton. The nucleus then has to create a negative charge to make up for it which is released as beta particle.

– in addition an anti particle with  no charge is emitted. This consumes energy.

Gamma radiations:

Gamma radiation is an electromagnetic radiation emitted by an unstable nucleus. It can pass through thick metal plates. It has no mass and no charge. It is emitted by a nucleus with too much energy after an alpha or beta radiation.

Particles and antiparticles:

Each particle has a matching antiparticle with the same mass and rest energy but with opposite charge.

Relative charge Mass/kg Rest energy
proton 1  


Antiproton -1
neutron 0  


Antineutron 0
electron -1  




antielectron +1



To explain the photo electric effect Einstien came up with a theory that electromagnetic waves can be emitted m short bursts, each packet of electromagnetic waves called a photon. This meant that light and other electromagnetic waves can act as particles as

well as waves. These photons have different frequencies and different energies can calculated using


For wave length


this can be paired with another equation p=c/v to find the wavelength of the waves


Annihilation occurs when a particle and its antiparticle collide and their masses are converted into energy.

Used in PET scanners

patients adminstrered with postron emitting isotops emits position that annihilates with electron releasing gamma which is absorbed and comes up on scan -> showing tumors

Pair production:

When a photon splits into a particle-antiparticle pair.

Create rest mass energy of two particles. Excess energy turns into kinetic.

How particles interact

Particles interact in different ways due to different forces that act upon them.

Force Particles it      effects Exchange particles Range Relative strength
Electromagnetic Anything with charge Virtual photons Infinite 10-2
Weak Fundamental particles W+,W-,Zo 10-18m 10-5
Strong nuclear Quarks Gluon 10-15m 1
Gravity Anything with mass Graviton Infinite 10-39


Electromagnetic force:

When two protons approach each other they repel each other and move away. The electromagnetic force between two charged particles is due to the exchange of virtual photons. we can draw this interaction on feynman diagram

Weak nuclear force:

Beta decay shows us there must be another force at work, one weaker than the strong nuclear force and yet that interacts with fundamental particles. This is the weak nuclear force.


Weak interactions are due to exchange of particles called W bosons unlike photons, these exchange particles have:

-A non-zero rest mass

-A very short range.

-Are positively or negatively charged.

There are 4 weak interaction between particles

  1. Neutron interactions:

Neutrons rarely interact with other particles. However a neutron can interact with a neutron making it a proton by means of the weak interaction.



  1. Antineutron interactions

In the opposite way an antineutron can interact with a proton to turn it into a neutron emitting a beta particle as a result.



  1. Beta decay

A neutron decays into a proton as a result releases a fast moving electron. A proton turns into a neutron releasing a beta particle.


a proton turns into neutron releasing a position





  1. Electron Capture


In a proton rich nucleus the highly positive nucleus pulls in an electron from the inner shell. The proton turns into a neutron as a result of interacting through weak interactions with the electron.



Feynman Diagrams


  1. In coming particles start at the bottom and move upwards.
  2. The baynons and laptons stay on either sides.
  3. The W bosons carry charge from one side to the other.
  4. A W- particle moving toward to the left has the same effects as W+ going towards the right.
  5. The W bosons cannot be drawn flat.


Classification of particles

Strange particles are the particles produced through the strong interaction and decay through the weak interaction. Strangeness is always conserved in a strong interaction where as it can by 0, +1, -1 in weak interactions.

Quarks and antiquarks:

Hadrons are made of quarks.



  Charge Baryon no strange
U +2/3 +1/3 0
D 1/3 +1/3 0
S 1/3 +1/3 1




  Charge Baryon no strange
U 2/3 -1/3 0
D +1/3 -1/3 0
S +1/3 -1/3 +1


Mesons are hactons each consisting of quark and antiquark.

Quarks and beta decay:

Conservation laws:


Symbol e µ Ve
Charge 0 -1 -1 0
Lepton number +1 +1 +1 +1




Symbol Vµ’ e+ µ+ Ve’
Charge 0 +1 +1 0
Lepton number -1 -1 -1 -1


Particles and antiparticles obey certain conversation rules when they interact.

ENERGY is always conserved and MOMENTUM.

CHARGE is always conserved.

LEPTON NUMBERS are conserved in a particle and antiparticle interaction + decays

BARYON NUMBERS are consumed in any reaction.

STRANGENESS is consumed only in a STRONG interaction. However then it is always consumed.

The photoelectric effect:

A metal contains delocalized electrons which move around the inside of the metal. If these electrons are given enough energy they are released from the surface of the metal. This can be done by shinning a light on the metal, this is known as the photoelectric effect.

The photo electric effect refers to the emission of electrons from the surface of a metal in response to incident light.

Work function: The minimum energy needed by an electron to escape the metal surface.

When a light is incident on a metals surface an electron at the surface absorbs the energy of a single photon. If the energy gained by this single photon exceeds the work function of the metal it will leave the metal surface any excess energy gained then becomes kinetic energy.


Therefore the maximum kinetic energy of an emitted electron will be: hf -ǿ



Threshold frequency minimum frequency of light that can cause photo electric emission. fmin=ǿ/hf as hf>ǿ

Therefore to increase the kinetic energy of the released photoelectrons you must

– increase the frequency of the wave

– use a different metal with smaller work function

Increasing the intensity of the light does NOT increase the resulting kinetic energy of the photoelectrons. A higher intensity of light means there are more photons. Meaning more electrons will be released from the metal surface.

Collisions of electrons with atoms

Ionisation: is adding or removing electrons, creating in charged atom.


Excitation is a process in which an atom absorbs energy without becoming ionized as a result an electron inside an atom moves up from an inner shell to an outer shell.



Deexitation Is a process in which an atom losses energy by photon emission. As a result of an electron inside an atom moving from an outer shell to an inner shell or in which an excited nucleus emits a alpha photon.

– when high voltage is applied to a florescent tube electrons collide with the mercury atoms in the tube


florescent tubing 

– the collision cause ionization and excitation of the mercury droplets. These will then de excite releasing UV photons.

– this UV light gets absorbed by the phosphorous coating. These phosphorus atoms then excite. When they de excite they release photons in the room of visible light,

Energy levels and photon emission:

1 electron volt= 1.6×10-19 Joules

Electrons can only exist in atoms in certain energy levels. To move from one level to the next require set amounts of energy.

The lowest energy state of an atom is called its ground state.

When an atom de excites an electron moves down an energy level and a photon is released. When an electron moves from energy level E1 to lower energy level E2




Energy of emitted photon= E1-E2

As each shell energy levels are unique to that element by measuring the wavelength of the photons emitted you can work out the atoms element.

Wave-particle duality:

Dual nature of light:

– Light can act like a wave. This is shown as it can be refracted through mediums.

– But light can also act like a particle. This is seen by the photoelectric effect. Waves and particles do not interact so far the light to release an electron from the surface of the metal it must act like a particle hence Einstiens photon theory.

Dual nature of particles:

– Known particles are shown to act like particles. For example electrons can deflected by a magnetic field. They can have mass charge etc.

– however electrons are also seen to act like waves. Dr Broglie proved this when he refracted a narrow beam of electrons through a thin sheet of foil.

We call this the wave-particle duality and we can see the relationship between wave and particle using wavelength.