physics - radioactivity (6.1 - 6.46)

6.1 atom

positively charged nucleus

• consists of protons & neutrons

• contains almost all mass

• radius much smaller than atom radius

surrounded by negatively charged electrons

6.2 typical size of atoms & small molecules

atom = 1 × 10^-10m

molecule = 1 × 10^-9m

6.3 structure of nuclei of isotopes

isotope: 2 atoms of same element with diff. numbers of neutrons

e.g. carbon-13:

• proton number = 6: 6 protons

• mass number = 13: 6 protons, 7 neutrons

• number in name (13) = mass number

6.4 nucleus charge

characteristic positive charge

6.4 isotopes mass

isotopes of element differ in mass - diff. numbers of neutrons

6.5 protons, neutrons, electrons, positrons - relative masses & relative electric charges

6.6 number of protons & electrons, charge of atom

number of protons = number of electrons - charge is neutral

6.7 electrons in atom

electrons orbit nucleus at diff. set distances from nucleus

6.8 what do electrons do when there’s absorption/emission of EM radiation?

electrons change orbit

atom absorbs EM radiation: electrons → higher orbit

atom emits EM radiation (can be visible light): electrons → lower orbit

6.9 how do atoms form positive ions?

atom gains energy - loses outer electrons

loses electron = protons > electrons = + charge = + ion

6.10 α, β^-, β⁺, γ rays, neutron radiation - emitted from where & in what process? (general)

from unstable nuclei

in random process

6.11 α, β^-, β⁺, γ rays - type of radiation (general)

ionising radiation

6.12 background radiation

weak, ionising radiation - constantly exposed to from space & naturally radioactive substances in environment

6.13 origins of background radiation - earth

radon:

• main source

• radioactive gas

• produced by rocks containing small amount of uranium

• diffuses into air from rocks & soil

• can build up in houses - esp. where poor ventilation

• amount in air depends on type of rock & its uranium content

some food:

• naturally contain small amounts of radioactive substances

hospital treatments:

• e.g. X-rays, gamma-ray scans, cancer treatments

6.13 origins of background radiation - space

sun & other stars:

• high-energy, charged particles (cosmic rays) stream out of them - form of radiation

• many cosmic rays stopped in atmosphere

• some still reach earth’s surface

6.14 measuring & detecting radioactivity - photographic film

more radiation reaches it - becomes darker

film must be developed to measure amount of radiation (dose)

(some dosimeters (film badges) change colour without needing to be developed)

6.14 measuring & detecting radioactivity - Geiger-Muller tube

radiation passes through tube - ionises gas inside, lets short pulse of current flow

connected to counter - counts pulses of current

clicks each time radiation detected

count rate = number clicks/sec or min

6.15 what is an α particle equivalent to?

helium nucleus

• 2 protons, 2 neutrons

• relative mass = 4

• charge = +2

6.15 what is β^- particle?

electron

• relative mass = 0

• charge = -1

what is a β⁺ particle?

proton

• relative mass = 0

• charge = +1

6.15 where is β particle emitted from?

nucleus

6.15 what is gamma ray?

EM radiation

(high frequency)

6.16 α, β, γ - ability to penetrate & ionise

6.17 Thompson’s plum pudding model

atoms contain electrons (- charge, very small mass)

atom = + ‘pudding’

- electrons = ‘plums’ scattered through ‘pudding’

6.17 Rutherford α particle scattering

what happens when + α particles passed through gold foil

most α particles passed through gold foil, few bounced back - plum pudding model can’t explain

atoms mostly empty space, most mass in tiny central nucleus (+, electrons moving around it)

6.17 Bohr model

amended Rutherford’s atomic model to explain atoms absorbing & emitting light

electrons in certain fixed orbits (shells) around nucleus

cannot be part way between 2 orbits

explains lines in emission & absorption spectra

6.18 β^- decay

neutron → proton + electron

electron (β particle) ejected from atom

atomic number: +1

mass number: no change

6.19 β⁺ decay

proton → neutron + positron

positron (β particle) ejected from atom

atomic number: -1

mass number: no change

6.20 α decay - effect on atomic number & mass number

atomic number: -2

mass number: -4

6.20 β^- decay - effect on atomic number & mass number

atomic number: +1

mass number: no change

6.20 β⁺ decay - effect on atomic number & mass number

atomic number: -1

mass number: no change

6.20 γ decay - effect on atomic number & mass number

atomic number: no change

mass number: no change

neutron emission

neutron emitted

6.20 neutron emission - effect on atomic number & mass number

atomic number: no change

mass number: -1

6.21 what happens to nuclei that have undergone radioactive decay?

nuclear rearrangement

lose energy as gamma radiation

α, β^-, β⁺, neutron - symbols

6.22 use given data to balance nuclear equations - mass & charge

6.23 how does activity of radioactive source decrease over time?

nucleus decays - becomes more stable

sample of substance contains more stable nuclei = lower activity

activity definition

number of nuclear decays/second

6.24 unit of activity of radioactive isotope

Becquerel (Bq) (= 1 nuclear decay/second)

6.25 half-life of radioactive isotope

time taken for half undecayed nuclei to decay

or time taken for activity of source to decay by half

6.26 predicting decay & half-life

cannot predict when particular nucleus will decay - decay is random process

half-life lets us predict activity of large number of nuclei during decay process

6.27 calculations on decay of radioactive isotope using half-life

6.28 uses of radioactivity - household fire (smoke) alarms

smoke alarm contains source of α particles

detector has electrical circuit with air gap between 2 electrically charged plates

α particles released - ionise molecules in air

ions attracted to plates with opposite charge - let small electrical current flow

current flowing - alarm won’t sound

smoke gets into air gap → smoke particles slow down ions → current flowing across gap decreases

current drops below certain level - alarm sounds

6.28 uses of radioactivity - irradiating food

foods contain microorganisms - cause decomposition

irradiated with gamma rays - kill bacteria

food: safer to eat; stored longer

doesn’t make food radioactive

6.28 uses of radioactivity - sterilising equipment

some instruments (e.g. plastic syringes) can’t be heated to kill microorganisms

sealed in bags, irradiated with gamma rays - penetrate bag & equipment

6.28 uses of radioactivity - tracing

radioactive isotopes used as tracers

e.g. gamma source added to water - detects leaks in water pipes underground

at leak water flows into surrounding earth

GM tube follows path of pipe - detects more radiation at leak

6.28 uses of radioactivity - gauging thicknesses

2 rollers squeeze wood pulp with force - produces diff. paper thicknesses

detector counts rate β particles get through paper from source on one side

paper too thin: more β particles penetrate paper → detector records higher count rate → computer senses higher count rate → reduces force on rollers → paper thicker

paper too thick: less β particles penetrate paper → detector records lower count rate → computer senses lower count rate → increases force on rollers → paper thinner

6.28 uses of radioactivity - diagnosing & treating cancer

diagnoses cancer: tracers in body

treats cancer

6.29 dangers of ionising radiation (body)

large amount: tissue damage

small amounts over long period of time: mutations (damages DNA in cell)

• make cell malfunction

• may cause cancer

• can be passed onto next gen.

• not all mutations harmful

• cells often repair damage if dose is low

6.29 dangers of ionising radiation - precautions

sources handled with tongs - distance from source increases = intensity of radiation decreases

sources stored in lead-lined containers

6.30 how do dangers of ionising radiation depend on half-life?

contaminated with radioactive materials with longer half-lives poses greater hazard

effects last longer than for materials with shorter half-lives

6.31 precautions to ensure safety of people exposed to radiation - liming dose for patients

only exposed to radiation when benefits are greater than possible harm radiation could cause

minimum possible dose used

sources with short half-lives used - minimises time patient is exposed

6.31 precautions to ensure safety of people exposed to radiation - risks to medical personnel

increase distance from source

shield source

minimise time spent in presence of sources

exposure monitored using dosimeter badges

6.32 contamination

radioactive material particles on skin/in body

exposed to radiation as unstable isotopes in material decay

until material all decayed/source of contamination removed (not always possible)

water & soils contaminated - spreads into food chain

6.32 irradiation

exposed to α/β/γ radiation from nearby radioactive materials

person moves away - radiation stops

could expose cells to damage & mutation

6.33 treating tumours - radiation applied internally

internal radiotherapy:

• β emitter (e.g. iodine-131) placed in/close to tumour

• doesn’t always require surgery - patient stays in room alone while source in place

6.33 treating tumours - radiation applied externally

external radiotherapy:

• beams of γ rays/x-rays/protons directed at tumour from outside body

• several weaker beams directed at tumour from diff. directions - only tumour absorbs lots of energy & surrounding tissues harmed as little as possible

6.34 uses of radioactive substances in diagnosing medical conditions - PET scanners

tracer emits positron

positron meets electron - both destroyed, 2 gamma rays emitted in opposite directions

detector in PET scanner moves around patient - produces images showing where diff. amounts of γ radiation come from

6.34 uses of radioactive substances in diagnosing medical conditions - tracers

tracers contain radioactive isotope attached to molecules - taken up by particular organs

radioactive tracer emits γ rays

γ cameras detect location of tracer in body

find source of internal bleeding:

• tracer injected into blood

• γ cameras detect area of highest γ radiation - location of bleeding

detect tumour:

• tracer made using radioactive glucose molecules

• tracer injected into blood

• active cells (cancer cells) take glucose up more quickly than other cells

• γ cameras detect area of highest γ radiation - location of tumour

6.35 isotopes used in PET scanners - produced nearby

radioactive isotopes used in medical tracers need short half-life - other parts of body affected as little as possible

lose their radioactivity quickly - must be made close to hospital

6.36 nuclear power generating electricity - advantages

don’t produce CO₂: nuclear fuels don’t burn; don’t contribute to climate change

6.36 nuclear power generating electricity - disadvantages

major accidents could occur: very serious consequences for many people

negative public view: many don’t think benefits of nuclear energy are worth risks

expensive waste disposal: produce waste that will stay radioactive for millions of years; needs to be sealed into concrete/glass & buried safely

expensive decommission: parts become radioactive during use

6.37 what are nuclear reactions source of?

source of energy (fission, fusion & radioactive decay)

6.38 fission of U-235

U-235 nucleus absorbs neutron

splits into 2 smaller daughter nuclei (also radioactive)

emits 2/more neutrons

energy transferred by heating

6.39 controlled nuclear chain reaction

neutrons released absorbed by other nuclei (of same isotope)

nuclei become unstable & split - release more neutrons

chain reaction controlled if other materials absorb some neutrons

6.40 chain reaction controlled in nuclear reactor - moderator

fission reactions occur - neutrons leave fuel rods at high speed

fuel rods in holes in moderator

slows down neutrons - increases chance of absorption by another U-235 nucleus

6.40 chain reaction controlled in nuclear reactor - control rods

contain elements that absorb neutrons

placed between fuel rods in reactor core

to increase rate of fission: control rods moved out of core → fewer neutrons absorbed

to decrease rate of fission: control rods moved into core → more neutrons absorbed

6.41 how is thermal energy from chain reaction used to generate electricity in nuclear power station?

energy released from core transferred to coolant

coolant: pumped through reactor; water/gas/liquid metal

hot coolant pumped to heat exchanger - used to make steam

steam drives turbine → turns generator → produces electricity

6.42 products of nuclear fission (general quality)

radioactive

6.43 nuclear fusion

2 small nuclei combine to form larger nucleus

mass of new nucleus < total masses of 2 smaller nuclei

lost mass converted to energy - released

energy source for stars (hydrogen nuclei combine to form helium nucleus)

6.44 nuclear fusion vs nuclear fission

energy: fusion = more; fission = less

products: fusion = not radioactive; fission = radioactive

• materials that contain fusion reactions become radioactive

disposing radioactive waste: fusion reactors - fewer problems; fission reactors - more problems

6.45 nuclear fusion - high temp.

nuclei more likely to collide at higher temps. - moving faster

nuclei fast enough - overcome electrostatic repulsion & fuse

6.45 nuclear fusion - high pressure

for nuclei to fuse, must be close together

+ protons in nuclei repel (electrostatic repulsion of protons)

nuclei close enough - overcome electrostatic repulsion & fuse

6.46 conditions for fusion & making power station

fusion requires extreme temps. & pressures - hard to sustain

difficult to be practical & economic power station