RADIOACTIVITY
Radioactivity: spontaneous random emission (emitted in irregular bursts in any direction w/ no set pattern) of particles or radiation from within the unstable nucleus of an atom to become more stable (after emission of particles: atom > atom of another substance)
Radioactive decay: process when a group of unstable nuclei disintegrates in order to become more stable
Rate of decay is independent of temp, pressure or chemical combination, thus occurs randomly over space and time

Detection of radioactivity
Photographic plates: affected by radiation despite wrapped in black paper
Gold leaf electroscope: radioactivity causes ionisation > ions neutralise charged electroscope > gold leaf collapses
Cloud chamber: path of ionising radiation (radiation causes condensation of alcohol vapour on the ions formed)
a: straight, thick tracks, strongly ionising (particles move slowly- more chance, 2 protons- more ions)
b: twisted, thin tracks: particles easily deviated by collisions w/ vapour molecules, thin tracks: less ioninsing
g : short, thin, irregular tracks, least ionising
Geiger-Muller tube/ G-M tube: a metal tube w/ a thin wire down the centre: filled w/ argon gas at low pressure: argon atoms ionised into e^- and argon ion pairs > e^-s: > anode > collide w/ argon atoms > ionising > +ve ions: > cathode > collection of e^-s & +ve ions at electrodes > current pulse > amplified > counted by ratemeter
Background radiation- always present in air: due to cosmic radiation form outer space, radiation from: Sun, rocks in Earth, naturally occurring radio-isotopes- radon, thoron gas, medical instruments: corrected count-rate = (measured - background) count-rate

Ionising effect of radiation
Radiation emitted by radioactive substance knock on electron out of the air molecule giving positive ions and negative electrons (ionising properties of radiation are used in the detecting and counting of the radiation)

Types of radiation

Properties	α-particles	β-particles	γ-radiation
nature	positive particle: helium nucleus, a stream of He nuclei (~7000 times mass of an electron)	stream of high energy e-s, formed from nucleus decay	electromagnetic waves, v.short wavelength
affected by electric and magnetic fields	Yes	yes, bent strongly	no
penetration	stopped by paper or skin or 6cm of air	stopped by 3mm of Al	reduced but no stopped by lead (50%)
causes ionisation	Strongly	weakly	very weakly
dangerous	yes	yes	yes
speed	10% speed of light	50% speed of light	speed of light
detection	Photographic film, cloud chamber, G-M tube, spark counter, gold-leaf electroscope	photographic film, cloud chamber, G-M tube	photographic film, cloud chamber, G-M tube

since radioactive decay is a random process, rate of decay # of unstable nuclei present
Half-life of a substance: differ in length
-time taken for ½ of the unstable nuclei to decay
or
-time taken for # of radioactive particles emitted per second to drop to ½ original value
to measure half-life: measure rate of decay (aka activity) at diff times

Uses of radioactive materials:
tracers: find leaks (leak: high radiation count), find amt of wear & tear, monitor internal organs, investigate how plants absorb subs
penetrating radiation: to photograph objects and find faults, cracks, control thickness of sheets, to kill bact in food
power sources: uranium-235 in nuclear power stations, a-emitting subs in fire alarms: air slightly ionised > smoke disrupts air > alarm
medical treatment: kill cancer, tumours (gammatrons used in radiotherapy emit g radiation)
dating archaeological specimens: activity of carbon-14 measured, since half-life known, age of specimen can be found

Hazards of radiation
-overexposure to radioactive radiation: radiation burns > sores & blisters
-extreme exposure: sickness, death
-lead to delayed conditions: eye cataracts, leukaemia
-mutation of genes: offsping > physiological & other abnormalities
-large scale destruction if leaks occur

Precautions against radiation hazards
-prevent overexposure to radiation
-workers working w/ g-radiation carry pocket dosimeters to monitor accumulated amt of radiation they are exposed to
-radioactive subs kept in lead-lined boxes and labelled "Radioactive Material"
radiation symbol must be displayed whenever exp w/ radioactive source conducted
-wear special protective suits, lead lined suits
-use apparatus to move radioactive substances
-food and drinks prohibited during radioactivity exps, prevent contamination

Geiger-Marsden Experiment
A beam of α particles fired at v.thin sheet of gold foil (μm thin), particles detected by zinc sulphide screen mounted on a rotatable microscope (done in dark: can see v.small flash of light when α-particle strikes zinc sulphide screen)

Observations	Conclusion
most α particles passed straight through the foil	Atoms are mostly made of space
some particles were deflected slightly through small angles	Atoms have a +ve charge nucleus
v.few particles were scattered backwards	The nucleus is v.small & heavy

Atomic model
An atoms is made of protons, neutrons, electrons
Protons + neutrons = nucleons from in nucleus, electrons: in space around nucleus

	proton	neutron	electron
charge	+1	0	-1
mass	1 unit	1 unit	1/2000 units (0)
symbol

An atom is electrically neutral: # of protons = # of electrons


Atoms represented by nuclide: X: chemical symbol of the element, A: mass/ nucleon # = + of neutrons & protons in the nuclues, Z: atomic # = # of protons (= # of electrons)
(A - Z) = # of neutrons
Isotopes: atoms w/ same atomic #, but diff mass #'s
Some isotopes are radioactive, uses: Cobolt-60: cancer treatment, carbon-14: carbon dating, phosphorus-32, iodine-131: tracers in bodies for medical diagnosis, detecting wear & tear of materials or check thickness of sheets

Radioactive decay
Alpha decay: when a α-particle emitted by nucleus, mass # decreases by 4, atomic # decreases by 2

Beta decay: when a β-particle emitted by nucleus, mass # remains the same, atomic # increases by 1

Gamma decay: when a γ-particle emitted by nucleus, mass & atomic # remains the same (energy content of nucleus only decreases)

Radioisotopes
Nuclear reactors can artificially produce radioactive isotopes by bombarding lighter nuclides w/ protons, neutrons, or a-particles

Nuclear energy
Einstein: mass & energy are equivalent: E = mc² [E = energy, m = mass, c = speed of light] > Δm = αE/c²

Nuclear fission
-process where heavy unstable nuclides break up to produce energy
-uranium-235 is useful in nuclear energy prod: bombarded by neutrons to form uranium-236 + energy
-²³⁶U: unstable > breaks down into 2 nearly radioactive nuclei: often barium + krypton + 2/3 neutrons
-products much less mass > energy released in ^ KE's of product particles
-2 fast moving fission fragments collide w/ surrounding atoms > ^ KE > ^ temp > heat produced
-3 fast moving neutrons are slowed down to react w/ ²³⁵U > more neutrons produce > chain reaction (not controlled > explode)

Nuclear fusion (can't be controlled)
-process where lighter nuclides fuse together to form a heavier nucleus w/ release of energy (due to loss of mass- products lighter)
-temp raised: nuclei brought together at high speed to overcome repulsion > attractive forces in nuclei attract > nuclei combine

Measurements In Physics
Forces
Energy
Pressure
Optics
Magnetism
Electrostatic Charging
Electricity
Thermal Physics
Waves & Sound

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