11 | Atomic Physics
IB Physics Content Guide
• Atomic nuclei decay to form more stable configurations and produce radiation in the process
• The rate of decay can be predicted for different materials and used to determine age based on isotope count
• Mass and energy are different manifestations of the same thing
• More energy efficient configurations mean that fission and fusion reactions release energy
• It is believed that all matter is made up of fundamental particles called quarks and leptons
• There is a symmetry between all matter with particles and their corresponding anti-particles
• The standard model has helped spur discovers of new particles but it may not yet be complete
11.1 – Radiation
- I can define nucleon as any subatomic particle inside the nucleus (protons and neutrons)
- I can identify isotopes of an element
- I can describe a nuclide using isotope notation
- I can interpret isotope notation to determine the number of protons and neutrons
- I can describe why the nucleus of an atom stays together despite the electrostatic repulsion
- I can describe why large nuclei have more neutrons than protons
- I can compare the radiation at different locations and list the sources of background radiation
11.2 – Radioactive Decay
- I can describe the process of alpha decay
- I can describe the process of beta decay and the differences between beta-negative and positive
- I can write the notation for the decay particles (alpha particle, beta-negative/positive, gamma)
- I can predict the products of alpha and beta decay
- I can describe the impact of ionizing radiation and the ionizing effect of different types of decay
- I can predict the penetration of the radiation byproducts of nuclear decay
- I can describe the deflection of the radiation byproducts moving through a magnetic field
- I can describe the deflection of the radiation byproducts moving through an electric field
11.3 – Half-Life
- I can describe the meaning of the half-life of a substance
- I can predict the percentage of an isotope remaining after a given number of half-lives
- I can predict the calculate the age of a sample when given the percentage of an isotope remaining
- I can describe the process of radiocarbon dating
- I can describe the role of medical tracers
- I can describe the danger implications of long and short half-lives for radioactive materials
11.4 – Energy and Mass Defects
- I can relate units of mass between kilograms (kg) and atomic mass units (u)
- I can use the mass/energy equivalence to mathematically relate mass and energy
- I can convert between joules (J) and electron-volts (eV)
- I can describe how MeV c-2 is a valid unit for mass
- I can define mass defect and explain how it is related to energy
- I can calculate the mass defect of a nuclide
- I can calculate binding energy from mass defect
- I can interpret a chart showing binding energy per nucleon to locate stable nuclei
11.5 – Fission and Fusion
- I can use mass defect to calculate the energy released in a nuclear reaction
- I can compare the processes of fission and fusion
- I can predict the products of a fission reaction and the conditions requires for a chain reaction
- I can describe the conditions required for fusion to take place
- I can identify if an element will undergo fission or fusion based on its binding energy per nucleon
11.6 – The Particle Adventure
- I can describe the quest to discover the fundamental building blocks of matter
- I can identify the general categories of particles in the standard model
- I can interpret the IB Physics data booklet tables to identify the properties of all 24 quarks/leptons
11.7 – Hadrons, Baryons, and Mesons
- I can classify particle categories into an organized family tree with examples of each
- I can describe how quarks can be combined to create whole number charges
- I can identify the quarks required to form protons and neutrons
- I can calculate the charge of a given baryon or meson
11.8 – The Standard Model
- I can describe the phenomenon of Quark Confinement
- I can analyze a reaction based on conservation of Baryon #, Lepton #, Charge, and Strangeness
- I can describe forces in terms of exchange particles
- I can rank the fundamental forces based on strength
- I can describe the role of the Standard Model in the discovery of new particles
11.9 – Feynman Diagrams and the Higgs Boson
- I can describe key features of the Large Hadron Collider and its role in the Higgs Boson discovery
- I can follow the general rules for creating a Feynman Diagram
- I can describe a particle interaction using Feynman Diagram
11 | Atomic Physics
Types of Decay
Beta (β++ or β-)
+1 or -1
2.5 × 108 m s-1
3.0 × 108 m s-1
# of Half-Lives
|Half Live Remaining|
Data Booklet Equation:
Speed of Light
Unified Atomic Mass Unit
1.661 × 10-27 kg
931.5 MeV c-2
Electron Rest Mass
9.110 × 10-31 kg
0.511 MeV c-2
Proton Rest Mass
1.673 × 10-27 kg
938 MeV c-2
Neutron Rest Mass
1.675 × 10-27 kg
940 MeV c-2
Converting between Joules and Electron-Volts
Process for Calculating Binding Energy
1. Add up the “before and after” masses
2. Find the mass defect by taking the difference
3. Convert atomic mass units (u) into MeV c-2 by using the conversion factor 1 u = 931.5 MeV c-2
4. The c-2 cancels out when converting to energy using E = mc2 so this is your binding energy
Lighter elements are created by splitting heavier elements
Proper amounts of fissionable elements required to maintain chain reaction
Heavier elements are created by combining lighter elements
Requires high heat and high pressure
The following two tables are provided in the IB Physics Data Booklet
All quarks have a strangeness number of 0 except the strange quark that has a strangeness number of –1
All leptons have a lepton number of 1 and antileptons have a lepton number of –1
Explain the phenomenon of Quark Confinement:
Quarks have never been observed on their own. The amount of energy required to overcome the strong nuclear force holding the quarks together gets converted into mass and forms a new quark pair.
You can only draw two kinds of lines
You can only connect these lines if you have two lines with arrows meeting a single wiggly line
The x-axis represents time and is read from left to right. Everything left of the vertex is the “before” condition.
SOME IMPORTANT FACTS ABOUT ATOMIC MASS, SIZE AND COMPOSITION OF NUCLEUS
- Proton was discovered by Goldstein
- Atomic mass unit
1.a.m.u = 1/12th of mass of C-12 isotope,
1 a.m.u = 1.660565 × 10–27 kg
- Mass of a proton, mp = 1.0073 a.m.u = 1.6726 × 10–27 kg
- Chadwick’s experiment
Neutrons were detected.
- Mass of neutron, mn = 1.00866 a.m.u = 1.6749 × 10–27 kg
- Mass of electron = 9.1 ×10–31 kg
- Mass number, A = total number of nucleons (neutrons + protons present in the nucleus of an atom)
- Atomic number, Z = number of protons = number of electrons
- Types of nuclei :
- Isotopes : The atoms of the element which have the same atomic number but different atomic mass numbers. e.g., 1H1, 1H2, 1H3 ; 8O16, 8O17, 8O18
- Isobars : The atoms of differents element which have the same atomic mass number but different atomic numbers. e.g., 6C14, 7N14, 18Ar40, 20Ca 40 etc.
- Isotones : The nuclides which contain the same number of neutrons e.g., 2H23, 2He24, 2Be59, 5Be510 etc.
- Isomers : having same mass number, same atomic number but different radioactive properties.
- Rest mass of nucleus is less than sum of rest masses of constituent nucleons, the difference is called mass defect.
- Size of the nucleus : Radius of nucleus, R = R0 A1/ 3 where R0 = 1.1 × 10–15 m.
- Nuclear density of all elements ~ 1017 kg m–3.
MASS ENERGY AND NUCLEAR BINDING ENERGY
- A nuclide is a specific nucleus of an atom characterised as ZXA where A = mass number and Z = atomic number.
- Binding energy per nucleon is nearly 8.4 MeV for nuclei in the range of mass number 40 to 120.
- Binding energy is highest in Fe56.( 8.8 MeV)
- Binding energy curve predicts :
- Fission : Breaking up of a heavy nucleus (A > 200) into two nuclei of approximately equal size, and release of energy.
- Fusion : Lighter nuclei ( A < 20) combine together to form heavier nucleus and release of energy.
- BE/ A varies by less than 10% above A = 10 suggests that each nucleon interacts with its neighbouring nucleon only.
- For A > 56, BE/A decreases because of the destabilising effect of long-range coulombic force.
CHARACTERISTICS OF NUCLEAR FORCE
- It is a short range force effective only in range 10–15 m
- It is charge independent. It acts between proton-proton, proton-neutron and neutron – neutron.
- It is not a central force.
- It is spin dependent.
- It is 1038 times stronger than gravitational force and 102 times stronger than electric force.
- The main cause of nuclear force is the exchange of π− mesons between nucleus
- α-rays (i.e., Helium nuclei or α – particles)
- β-rays (i.e., electron or positron or β – particles)
- γ-rays (photons or gamma radiations)
- chemical combination
- changing physical environment other than nuclear bombardment
FEATURES OF RADIOACTIVITY
- It is a statistical process.
- When a nucleus undergoes alpha or beta decay, its atomic number and mass number changes (in β-decay only atomic number changes) & it transforms into a new element.
- (α-particle), it means that by emission of alpha particle (α-particle), it loses 2 units of charge and 4 units of mass.
- (positron). It means that by emission of beta particle (β+-particle), nucleus loses one unit of charge. It is surprising to note that a nucleus does not contain β+ then how is it emitted. Reason : During a β+ particle(i.e., positron) decay, a protron converts into a neutron
- When a nucleus emits a gamma ray, neither the mass nor the charge of the nucleus changes
PROPERTIES OF α, β & γ-RAYS
- It is a positively charged particle & contains a charge of 3.2 × 10–19 coulomb (exactly double the charge of electron).
- The mass of α-particles is 6.645 × 10–27kg (It is equal to mass of a helium nucleus). Actually α-particle is nucleus of helium, hence it is called doubly ionised helium.
- They (α-particles) get deflected in both electric & magnetic fields.
- The velocity of α-particle is very less than the velocity of light i.e., , where c is velocity of light.
- The range of α-particle in air depends on radioactive substance.
- The ionisation power of α-particle is higher than both β (100 times of β & 10,000 times of γ) and γ particle.
- The penetrating power of α particle is lowest (in comparison to β & γ particles). It is 1/100 times of β-particles & 1/10,000 times of γ-rays.
- The α-particles can produce fluorescence in barium platinocyanide and zinc sulphide.
- They show little effect on photographic plate.
- They show heating effect on stopping.
- The beta particles (i.e., β– or β+) may be positive & negative particle & contain of charge. Actually β– is electron & β+ is positron.
- They get deflected in both electric & magnetic field.
- The velocity of β-particle varies between 0.01c to .99c, where c is velocity of light.
- The mass of β particle is relativistic, because its velocity is comparable to velocity of light
- They have both ionisation & penetration power. Ionisation power less than α-particle and penetration power more than α-particle.
- They produce fluorescence on barium platinocyanide & zinc sulphide.
- They are electromagnetic waves as x-rays.
- They are not deflected in electric & magnetic field, it means that they are chargeless.
- The velocity of γ-particle is equal to velocity of light.
- The ionisation power of gamma rays is less than β & α rays but penetration power more than β and α-rays.
- The γ-particles are emitted from the nucleus, while X-rays are obtained, when electron goes from one state to another in an atom.
- When γ-rays photon strikes nucleus in a substance, then it gives rise to a phenomenon of pair production i.e.,
RUTHERFORD AND SODDY LAW FOR RADIOACTIVE DECAY
HALF-LIFE OF A RADIOACTIVE SUBSTANCE
then by eq. (2)
MEAN LIFE OF A RADIOACTIVE SUBSTANCE
- Specific activity is the activity of 1 gram of material.
- Geiger Muller Counter is used for detecting α and β particles.
- Cloud chamber is used for detecting radioactive radiations and for determining their paths, range and energy.
- Baryon number
B = 1, for a neutron and a proton.
- Lepton number (L)
L = 1 for electron, and neutrino
- Radioactive isotope
- Iodine-131 For detecting the activity of thyroid gland
- Chromium-51 To locate the exact position of haemorrhage
- Phosphorus-32 In agriculture
- C–14 Carbon dating, Photosynthesis in plants
- Co60 Cancer treatment
- Na24 For circulation of blood
- Charge conservation
- Conservation of linear momentum
- Conservation of angular momentum
- Conservation of energy (Rest mass energy + K.E.)
NUCLEAR FISSION (BY OTTO HAHN AND STRASSMANN)
- Nuclear fuel : U233, U235, Pu239 etc.
- Moderator : Graphite, heavy water (D2O). To slow down the neutrons (or slow down the nuclear reaction).
- Control rods : (Cadmium, boron). To absorb excess neutrons. It controls the chain reaction.
- Coolant : (water etc). To remove the heat produced in the core to heat exchanger for production of electricity.