Nuclear Physics

ionising radiation

makes ions - alpha & beta particles, and high energy e-m rays, like gamma rays

nuclide

species of atom classified according to number of protons and neutrons as well as energy state

isotopes

same atomic number

radioactive atoms

emit either alpha or beta particles along with gamma rays

radioactive decay

• random, spontaneous and uncontrollable

• changes from parent nuclide into daughter nuclide of either different element (a or b decay) or same element at lower energy state (gamma decay)

• decay process also called nuclear transformation, disintegration or transmutaition

Alpha particle

it is a helium nucleus so it has atomic mass 4 and atomic number 2

alpha decay

element, arrow, new element when 4 is subtracted from mass and 2 subtracted from protons and + 4He2 (helium nucleus)

Beta - decay

• happens when electron emitted from nucleus not electron cloud

• happens in atoms with too many neutrons

• neutron decays into proton and emits electron (b particle) and uncharged massless particle: antineutrino (curvy v with line on top)

• add 1 to atomic number

• if question does not specify type of decay assume its B-

beta + decay

• happens in atoms with too many protons

• proton decays into neutron and emits positron (b+) and neutrino

• minus 1 from atomic number

gamma decay

• when element in excited state

• represented in equation by asterisk

• written as + ⁰₀ y

half-life

• if large number of atoms can predict half-life

• how long it takes for half of radioactive atoms in given mass to decay

• in general for sample of N₀ particles, the number N remaining after n half-lives is given by: N = N₀(1/2)ⁿ

Decay series

when radionuclide decays, daughter nucleus may also be unstable so will undergo further decay until stable isotope is reached and sequence ends

Decay series still operating

• U-238 to Pb-206

• Ac-235 to Pb-207

• Th-232 to Pb-208

all naturally occurring

• Np-237 to Tl-205 (artificially occurring)

artificial transmutation

when normal nucleus takes in neutron, it becomes less stable so often becomes a beta emitter but uranium splits into 2 nuclei of intermediate mass fission or becomes transuranic element (higher than uranium)

Transuranic Elements

• all elements above uranium (Z >92) are transuranic

• don’t exist naturally and are all radioactive

ionising power: alpha particles

slow-moving, positively charged, attract electrons from atoms and ionise them, losing energy as they do so (strongest)

ionising power: B- particles

repelled by atom’s electron clouds, so particles can get bounced between atoms, causing electrons to be ejected and thus ionising

ionising power: B+ particles

interact with electrons in atoms, as electrons antiparticle, if positron meets electron two will annihilate each other turning their combined mass into energy according to E=mc²

ionising power: gamma rays

can transfer enough energy to electron that will leave atom or molecule, leaving positive ion behind

ionising power: neutrinos and antineutrinos

so weak they don’t ionise atoms

Penetrating power

• larger the mass, lower the penetrating power because particle more easily interacted with and stopped

• a: stopped by paper, skin or few cm of air

• B: stopped by ~5mm Al or few m of air

• Y: not absorbed by air, stopped by 30cm of steel, intensity halved by 1cm lead

• neutrons: highly penetrating in most materials, absorbed strongly by materials containing lots of hydrogen e.g. water or concrete

Effect of electric fields

• electric field lines point from positive towards negative potential

• positively charged a and B+ are deflected in same direction as field (towards negative), a have more mass and thus inertia so don’t deflect as sharply

• B- get deflected sharply (due to low mass) opposite to field (towards positive potential)

• Y rays have no charge so pass through unaffected

Effects of magnetic fields

• also deflect charged particles

• can identify radiation type by comparing effects

• a and B+ deflected in same direction but B+ deflects sharper

• B- deflect as sharp as B+ but opposite direction

• Y rays still unaffected

• Radiation can be deflected through cloud chambers and Geiger-Muller Tubes (Geiger counters)

Nuclear medicine

• radiopharmaceuticals used for treatment and diagnosis

• diagnosis: external detector records where a radioactive nuclide goes or accumulates in body

• treatment: radioactive nuclide destroys cells or promotes healing

How radiopharmaceuticals act

• isotope exchange: radioactive nuclide replaces some non-radioactive nuclides normally present in body

• foreign label nuclides are attached to chemicals that follow a well-known pathway through body

• biosynthesis: radionuclides introduced into body, metabolised, then removed by excretion or intervention

• choice of radiopharmaceutical guided by effective half-life, combo of biological and physical half-life

Treatment radionuclides

• choice depends on problem and location of problem

• nuclide should concentrate in location where it’s most effective, stay for sufficient time and leave body in reasonable time (biological half-life)

• considerations also include type and energy level of nuclide’s radiation emissions

energy from nucleus

• strong nuclear force holds nucleons together

• 4 fundamental forces: gravity, strong nuclear force, weak nuclear force, and electromagnetic force

• weak force acts within nucleons and governs radioactive decay

Nuclide stability

• neutrons help reduce effect of electrostatic repulsion between protons

• when effect of strong nuclear force is sufficient, nuclide is stable, otherwise will decay

• nuclides with <40 nucleons stable if = number of protons and neutrons (N=Z)

• heavier nuclides need more neutrons

Binding energy

• energy required to split nucleus up into all individual protons and neutrons

• mass of individual nucleons added together is greater than mass of nucleus - mass defect

• binding energy calculated using change in E = (change in m)c²

• energies on atomic scale are small so units of eV used

• unified mass units (u) also used

Binding energy per nucleon (BEPN)

• binding energy divided by number of nucleons gives better measure of nuclide stability and BEPN

• greater BEPN = harder to pull nucleus apart

• in fusion 2 small (Z<56) nuclei are combine to form larger nucleus with larger BEPN (more stable)

• in fission 1 large nucleus (Z>56) splits into smaller nuclei (fission fragments) each with greater BEPN

• in both cases, moving to nuclei with greater BEPN results in release of energy

Nuclear Fission

• triggered by nucleus absorbing neutron making it unstable and causing it to split into two fragments of varying size

• additional neutron and energy stored as binding energy are released

Fission fragments

• U-235 is one of only 2 readily fissile nuclei and most common fuel in nuclear reactors

• can split in around 40 different ways

Mass defect in fission

• total mass beforehand greater than total mass after event because mass converted into energy

• most of energy carried by fragments as kinetic energy

• nucleon numbers still conserved in nuclear reactions, mass defect doesn’t change number of protons or neutrons involved but is tied to binding energy of parent and daughter nuclei

Nuclear reactors

device that uses controlled fission to produce new substances and release energy

Reactor components

• moderator: nuclides slightly heavier than neutrons, fission neutrons share energy with nuclides through collision, slowing them down so U-235 can capture them

• Reactor vessel: design with right surface area to volume ratio for U-235 fuel to be close enough, reflect any neutrons lost from fuel rods back to sample to cause more fission

• control rods: contain neutron poison to control rate of chain reactions, remove rod = faster, insert = slower

• coolant

• radiation shield

Thermal nuclear reactors

• produce heat to make steam to drive generator of turbine to produce electricity

• 2 types: advanced gas-cooled (AGRs) and pressurised water reactors (PWRs)

• for both, energy transformations: mass energy of fuel to heat energy of coolant to KE of steam to KE of turbines to electric energy

nuclear fission risks

• energy production gets concentrated into few locations: can become target for militants and organisation for attack

• can take thousands or millions of year for waste to become safe

• nuclear weapons

Nuclear waste systems

• some buried deep at sea or in underground storage bunkers

• container can decompose or damage or stolen

• groups of people exposed to radioactive uranium during entire process

• hot water escaping from plant

• escaping neutrons could emit radiation when interacting and be damaging

Nuclear fusion

• 2 small nuclei (<Fe-56) form larger nucleus with greater binding energy per nuclean, large nucleus more stable so energy released

Nucleosynthesis

• main fusion process in sun

• proton - proton cycle

• 2 protons form deuterium H-2, by fusion, one of protons undergoes positron decay and converts into neutron

• deuterium nucleus fuses with another proton to form He-3 (new proton doesn’t decay)

• 2 He-3 nuclei fuse, form He-4 nucleus and release 2 protons which return to the cycle

Controlling fusion for energy

• happens at very high temperatures (100 million C) (sun only 15 million degrees but has more pressure)

• PPC 1st step too rare to use in reactor so use deuterium + tritium (D-T reaction)

• ²₁H (deuterium) + ³₁H — ⁴₂He + ¹₀n

Effect of radiation on humans

• when ionising radiations interact with electrons, form ions or free radicals - atoms with unpaired valence electrons

• in living matter, these can start unwanted chemical reactions which damage or kill cells, cell division can also be affected, leading to cancer

• effect on body depends on quantity of radiation (dose) and how much gets absorbed (absorbed dose)

Radiation dose

• dose = energy E carried from source to a mass m

• absorbed dose is energy arriving at body per unit of mass: D = E/m, measured in J/Kg or Gy (gray)

• different types of radiation have different effects so given radiation weighting factor Wr to compare: B = 1 Y = 1 a = 20 slow neutrons = 3

• used in equivalent dose (H = D x Wr) which is measure of biological effect of different radiations, measured in Sv

Biological effects of radiation

• categorised as somatic (body effects like cancers/tumors) and genetic (gene mutations in reproductive cells leading to birth defects)

• somatic egs: anaemia, severe nausea, vomiting, diarrhoea, loss of vision, seizures, coma, fever, shock, death (>2Sv)

Radiation sickness

• acute irradiation is secs, mins, hrs and chronic is days, weeks, months

• delayed effects may not appear for many years after exposure

• effects can be immediate and continuous, but lower doses usually have acute phase, then calm, then more severe symptoms return