IN ☢️ Particles and Radiation

What is at the centre of the atom

Small nucleus contain protons and neutrons

• Overall positive charge

What is the charge of a proton?

$1.6\times10^{-19}$C

What is the charge of a neutron?

0

What is the charge of an electron?

$-1.6\times10^{-19}$C

What is the mass of a proton?

$1.673\times10^{-27}$ Kg

What is the mass of a neutron?

$1.675\times10^{-27}$ Kg

What is the mass of an electron?

$9.11\times10^{-31}$ Kg

What is a nucleon number?

Number of protons and neutrons: atomic mass

What is an isotopes?

Same number of protons but different number of neutrons

How can isotopes be used?

• Carbon dating

• Medicine

What is carbon dating?

Carbon Dating:

• All living things contain the same ratio Carbon 14 and Carbon 12

• Carbon 14 is radioactive (created by cosmic rays the strike the atmosphere

• After death Carbon 14 begins to decay

• Physicists can use the percentage of Carbon 14 left and compare with isotopic data to calculate age

What is specific charge?

Specific charge is ratio of charge to mass

$$specific.charge=\frac{charge}{\operatorname{}mass}$$

How does number of neutrons affect the stability of an atom?

The more number of neutrons compared to protons the more unstable the nucleus. So the nuclei is more likely to be radioactive. (therefore decay to become more stable)

Why do radioactive nuclei decay?

They decay to become more stable

What forces act on a nucleus?

• Electromagnetic force

• Gravity (very small compared to other forces)

• Strong nuclear force

How do the 3 forces that act on nucleus influence it?

• If strong nuclear force (SNF) didn’t exist:

The nucleus is act on by gravity which pulls the atoms together.

However there is also the electromagnetic force which causes repulsion between the protons in nucleus

It is much stronger than gravity so the nucleus would just fly apart

• However due to SNF the nucleus stays together:

The strong nuclear force holds the nucleus together

What is strong nuclear force?

Attractive force which holds nucleus together

Much stronger than electrostatic force

What is the range of strong nuclear force?

0.5fm to 3-4fm

What is a femtometer? (fm)

$1\times10^{-15}$ m

What happens to strong nuclear force when larger than 3-4 fm?

It becomes 0/negligable

What happens to strong nuclear force when less than 0.5 fm?

It becomes repulsive at very small distances

Why does the SN force become repulsive after 0.5 fm?

Otherwise nucleus’ would be crushed to a point

Does the SN force act the same between all nucleons?

Yes irrelevant if proton-proton, proton-neutron, neutron-neutron

What is relationship between Electrostatic force and SN force?

Refer to image

What is the range of electromagnetic force?

Infinity (but becomes weaker as distance increases)

When does alpha decay occur?

It happens in very big atoms (with more than 82 protons)

Why does alpha decay occur?

Because the nuclei of these atom are two big for the strong nuclear force to keep them stable so they decay and emit alpha particles

What is an alpha decay particle ?

$$\alpha_2^4$$

What is an alpha particles decay equation?

$$X_{Z}^{A}=Y_{Z-2}^{A-4}+\alpha_2^4$$

What is range of alpha particles?

Very short only a few cm in air

How to check for radiation?

• Geiger miller counter

• Spark counter

They measure ionising radiation

How to check if radiation source is alpha?

Using an ionising radiation detector (e.g, Geiger counter)

If it is held to an alpha source and moved away slowly the count rate will drop within a few cm

What is beta minus decay occur?

The emission of an electron(beta particle) from the nucleus along with an antineutrino

When does beta minus decay occur?

Occurs in particles that are ‘neutron rich’ so have to many neutrons compared to electrons

What happens during beta minus decay?

1. A beta particle is emitted from the nucleus.

2. When this happens a neutron is turned into a proton I the nucleus

The antineutrino particle released carries the same energy and momentum

What is beta minus decay equation?

$$X_{Z}^{A}=W_{Z+1}^{A}+\beta_{-1}^0+\overline{\nu}_{e}$$

How did scientists hypothesis neutrino?

Scientific originally believed only electrons were emitted during beta minus decay, but they then realised energy before decay was different to the energy after.

Which didn’t fit with conservation of energy.

So a particle (now called neutrino) was hypothesised which would have the missing energy, be neutral (so charge was conserved) and negligible/0 mass (as was not detected then)

What is beta particle?

$$\beta_{-1}^0$$

What is a neutrino?

$$\nu_{e}$$

What is an antinuetrino?

$$\overline{\nu}_{e}$$

What is different/similar between antiparticles and particles?

They have the same mass

But opposite charge

What is mass and charge of neutrino?

Negligible mass and 0 charge

How to covert from Joules to Electron Volts?

1eV = $1.6\times10^{-19}$ J

What is EM radiation made of?

Made of particles of light called photons

What is annihilation?

When a particle meets its antiparticle

All of the mass gets converted to energy

What happens to the mass of particles on annihilation?

All of it gets converted to energy in form of gamma rays (2 photons)

Why does annihilation not happen in ordinary particles?

Because antiparticles only exist for a fraction of a second

What is the equation for energy of a photon?

$E=\frac{hc}{\lambda}$In joules

How to work out the minimum energy of a photon?

The rest energy of particle (MeV)= minimum energy of the photon produced

What is produced in annihilation?

All mass of particle is turned into energy in form of 2 gamma ray photons (

How do PET scanners work?

• A positron-emitting isotope is place in then blood stream

• The gamma rays are then detected by the electron-positron annihilation that occurs

• The gamma rays are detected by a scintillator (they are easily differentiated from other gamma rays as they travel I opposite directions)

Why are gamma ray photons from annihilation easily dectected?

When annihilation occurs the gamma ray photons produced travel in opposite direction so they can easily be differentiated other gamma rays

What is pair production?

A photon turns into particle-antiparticle pair [energy is converted into mass]

The minimum energy of a photon is at least twice the rest energy of 1 particle produced

How does minimum energy of photon relate to the energy of a particle-antiparticle pair in pair production?

The minimum energy of a photon= total rest energy of particles produced (MeV)

$$E_{\min}=2E_0$$

How does minimum energy of photon relate to the energy of a particle in annihilation ?

The minimum energy of 1 photon produced = Rest energy of 1 particle (MeV)

$E_{\min}=E_{0_{}}$

When can an electron-positron pair be produced in pair production?

Only from a gamma ray photon when they are near a nucleus

What are photons?

Packets of EM radiation

When particles are produced in pair production in a detector what do they do?

They are in an applied magnetic field and have opposite charges so they curve away from each other in opposite directions

What is frequency?

The number of complete waves passing a point per second

What is wavelength?

The distance between two adjacent crests of a wave

What is the order of the EM spectrum?

⬆Increasing wavelength

• Radio Waves

• Microwaves

• Infrared

• Visible Light

• Ultra Violet

• X-Rays

• Gamma Rays

⬇Increase frequency

What happens to energy of a photon $E=hf$ as frequency increases?

As frequency increases so does energy

$$E=hf$$

What are the 4 fundamental force?

• Electromagnetic force

• Weak nuclear force- Responsible for nuclear decay

• Strong nuclear force- holds the nucleus together

• Gravity- significantly weaker than other forces

What is Weak nuclear force?

It is a fundamental force responsible for nuclear decay

What is Strong nuclear force?

A fundamental force which holds nucleus together working against electromagnetic force (which repels protons)

What is an exchange particles also known as?

Gauge particles

What is the exchange particle and which particles will the electromagnetic force affect?

Virtual photon and all charged particles

What is the exchange particle and which particles will the weak nuclear force affect?

$w^{+},w^{-}$ Bosons and all particles

What is the exchange particle and which particles will the strong nuclear force affect?

Pions $\pi^{+},\pi^{-},\pi^0$ and hadrons

What is the exchange particle and which particles will the gravity affect?

The graviton(????) and all particles

What are virtual particles?

Real particles that exist for a very short time and transfer energy and momentum

What is y-axis in Feynman diagrams?

Time

What is equation and Feynman diagram for Beta plus decay?

$$p\rightarrow n+e^{+}+\nu_{e}$$

W+ boson exchanged

What is equation and Feynman diagram for Beta minus decay?

$$n\rightarrow p+e^{-}+\overline{\nu}_{e}$$

W- boson exchanged

What happens in beta plus decay?

A proton turns into a neutron, positron and neutrino

W+ boson exchanged

What happens in beta minus decay?

A neutron turns into a proton, electron and antineutrino

W- boson exchanged

What is equation and Feynman diagram for Electron capture decay?

$p+e^{-}\rightarrow n+\nu_{e}$

W+ boson exchanged

What happens in electron capture?

A proton rich nuclei captures an electron from inside atom and it is turned into a neutron and neutrino

W+ boson exchanged

How is the range of force determined form exchange particles?

The heavier the particle the shorter the range of force

As W boson is heavy the weak interaction has a very short range

However as photon has negligible mass the range of electromagnetic force is infinite

How to draw Feynman diagram?

• Baryons on one side leptons on other side

• The charges must be balanced

• The y axis is time

• A W+ going left is same as W- going right

What is electron proton collision?

It is very similar to electron capture except that electron collide at high speed with proton so W- (travel left from electron to conserve charge)

What electromagnetic repulsion?

When two particles of equal charge get closer to each other they repel and a virtual photon is exchanged

What are particles are affected by strong nuclear interaction?

Hadrons

What are the two types of hadrons?

• Baryon: have 3 quarks

Proton, Neuton

• Meson: two quark (typical particle & antiparticle pair)

Pion, Kaons

What is baryon number of Hadrons?

• Baryons: B=1

• Mesons: B=0

What do Baryons always eventually decay into?

They are unstable so decay into proton (only stable baryon)

What are leptons?

Particles which don’t interact with the strong nuclear force

Electrons, muons, electron neutrino, muon neutrino

$$e^{-},m^{-},\nu_{e},\nu_{m}$$

What values are conserved?

Charge

Lepton Number

Baryon Number

Strangeness

What will muon eventually decay into?

It will eventually decay into an electron

Are quarks fundamental particle?

Yes hadrons are made of them

What is quark structure of proton?

uud

What is quark structure of a neutron?

ddu

What particles have strangeness?

A strange quark, they have strangeness of -1

What happens to quarks when neutron decays into protons?

As neutron (ddu) turns into a proton (uud) and electron and neutrino

A down quark must turn into an up quark via the weak nuclear interaction

What are hadrons?