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Radioactivity (Nuclear
Issues (Background Radiation
Geiger still detects…
Radioactivity
Nuclear
Issues
Background Radiation
Geiger still detects rad. w/o source
Sieverts (Sv) - Avr. dose per person per year
Compare effect of different sources on health
Radon - from RA substance underground (A part.)
A part. - danger to breath; Rn can seep into buildings
Pipe suction pumps underground can draw gas out first
Nuclear Waste
Used fuel rods - hot, RA; stored in tanks of water for 1 yr
Machines extracts (chem.) unused; stored for later use
RA waste stored in secure areas for many yrs
Chernobyl & Fukushima
1986 - Ukraine, reactor explodes; Fallout drifts over all EU
100k evacuated, 30 killed; more dev. leukaemia, other cancers
Operators ignored safety rules; Lessons used in Fukushima (3/11)
Population in 20km evacuated; Rad. levels, food, milk monitored
Health also monitored; 3 crippled reactors; nearby - much less effect
New Reactors
Most reactors need replace in ~20 yrs w/ 3rd gen
Standard design: reduce cost, construct time
Longer operate life (60 yrs); more eco-friendly
Safety features - convect. of outside air; cooling panels
Half-lives & Instability
RA isotopes - wide range of HL; seconds to yrs
Shorter HL - more RA; HL tells speed of CR decrease
Radioactive Risk
Bigger dose - more cancer risk
A - Much danger in body, affects all cells
A - Some danger out body, absorbed by skin, retina
B, G - Danger, reaches cells throughout body
Nuclear
Medicine
RA tracers
Trace substance flow through organ
RA Iodine, emits G rad. - detect outside system
Patient drinks RA water; Detector connect to chart recorder
Healthy Kidney - reading goes up, down
Blocked Kidney - reading stays up; RA decays in weeks
RA lasts long enough for test; Emits detectable radiation
Decays to stable product
Gamma Cameras
G, RA isotope inject; absorbed by organ, cameras detect
G pass holes in lead grid to detector (detects right in front)
Half-life: Long for good image, short for quick decay
Gamma cancer
Destroys cancerous tumours; rad. from RA cobalt
(HL 5yrs) G rad. penetrates deeper into body
Radioactive Permanent Implants
Destroy cancer tumour; B, G rad. in small seeds/tiny rods
HL long enough to irradiate tumour; still short for quick decay
Background Radiation
Radon - 50%
Food, Drink - 11.5%; Cosmic - 10%
Ground - 14%; Medical - 14%
Nuclear weapons - 0.2%; Air travel - 0.2%;
Nuclear reactors - 0.1%
Nuclear
Fission
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Fission
Nucleus of fiss. substance struck by n - splits in 2 equal
Several n released; spontaneous (w/o n) is possible
Chain Reaction
n released cause more fiss. nuclei split
In reactor, energy is needs release at steady rate
Fissionable Isotopes
Fuel contains fiss. isotopes - most common is enriched U
2-3% fiss. (U-235); Mostly U-238 (turns into Pu-239)
Pu-239 is fiss. (Not in U-235 reactor)
Inside Reactor
U fuel rods (spread); control rods; hi psi water (moderator)
Fiss. n slowed by H2O molecules (fast n don't cause fiss.)
Control rods absorb n, depth adjusted for steady reaction
Water is coolant; Intern. energy increase from n energy transfer
Pipe loop round heat exchanger outside; water transfers energy
Core in vessel of thick steel (hi psi, temp.) Vessel in thick concrete
Absorbs ionise rad. escaping steel vessel
Nucleus
Discovery
Alpha Scattering
Vacuum chamber apparatus, prevents air absorbing A
Detector - microscope focus on glass plate
Light from A particle collision, observed at diff. positions
Most A part. passed metal foil, 0.01% deflected >90°
Amount deflect decr. w/ greater angle (nucleus - 0.1 nm)
Rutherford astonished by results:
Most A part. pass through - nucleus much smaller than atom
Most mass must be nucleus; Result investigated by Geiger, Marsden
Agreed w/ measures, Explains RA of unstable nucleus
Predicts Neutron's existence
Plum Pudding Model
Ruth's discovery - model was rejected
1914 - Only knew of e-; PP = +ve matter w/ some e-
Bohr's model
Ruth. = +ve nucleus, e- around it; Niels Bohr proposed shells
e- absorb EM radiation to move away from nucleus;
Emits it to get closer; calc. agreed w/ light emit observe by atoms
Nuclear Puzzle
A scatter experiment:
H nucleus must have least charge amount
Charge equal between No. of nucleons (same charge amount)
Proton given to H nucleus - thought other nucleus contained it
Atom mass was more than atom No. - must be neutral part.
Neutrons then proven by James Chadwick (20 yrs later)
Nucleus
Change
Decay
A decay = p+ & n; B decay = p+
Atomic No. = Z; Mass No. = A
Isotopes = same element, different mass
A emission
Nucleus: Z - 2, A - 4; mass, charge reduced
2 p+, 2n; A = 4, 2+ charge (identical to He nucleus)
228/90 Thallium = 224/88 Radium + 4/2 A
Thallium decays = Radium isotope + A particle
B emission
B decay = e- from nucleus w/ too many n to p+
n changes to p+ & e-, B part. emitted (0/-1 B)
Nucleus: Z + 1; Only charge change
40/19 K = 40/20 Ca + 0/-1 B
Potassium decay = Calcium isotope + B particle
G emission
EM rad. from nucleus; no charge, mass
n emission
Emitted by A part. collide w/ (unstable) nucleus
Pass through substances easier than A, B
Atoms &
Radiation
Discovery
1896 - Henri Becquerel - FR physicist
Badge image found on photographic film
Uranium salt on metal on film - radiation
Marie Curie (MC) described it radioactivity
MC & Husband research, found new element
(Radioactive) Polonium - MC's home Poland
Inside atoms
Ernest Rutherford tests what blocked radiation
Found Alpha, Beta radiation Alpha < Beta
A stopped by paper, B went through
A made of +ve particles (could probe atoms)
Student Hans Geiger made Counter (radiation)
They direct A beams at thin metal - some rebound
Proved due to +ve nucleus - protons, neutrons
Radioactive (RA) puzzle
All atoms have p+ e- n w/ same structure
RA substance - unstable nucleus, emits radiation
(to regain stability) Nucleus decays when emitting
Radiation is random, not bound to phys. conditions
Alpha, Beta,
Gamma
Penetration test
Measure background count rate of RA
Measure count rate w/ source - find difference
Test range and materials; A can't pass paper
Can add more layers/distance, until rate is zero
RA danger
Rad. can knock out e-; atoms ionised = irradiated
RA contamination = unwanted RA mat. on other mat.
Hazard due to decay; type of rad. = level of hazard
X-rays (p+, n) - ionising live cells kills/damages genes
A - most ionise power; protect through distance, short time
Protect through thick concrete/lead
Peer review
Published findings are checked by other scientists
Radiation Use
Smoke alarms: A part. ionise air in circuit gap (current)
B, G not ionising enough; Smoke absorbs A, current drops
Thickness monitor: B rad. through foil; measure amount passed
Signal to increase roller pressure, if foil too thick, readings drop
A - can't penetrate;G - all would penetrate
Nuclear
Fusion
Reactions
2 small nuclei fuse into one - release energy
Some mass = energy (some is nuclear rad.)
New nucleus can't be heavier than Fe (55)
Sun - 75% H, 25% He; core is nucleus w/o e- (plasma)
2 p+ = "heavy hydrogen" (other part. created, emitted)
2x H = He nucleus (energy - KE of product, emission)
Fusion Reactors
Plasma heated by big current
Hi temp, psi, light nuclei plasma needed (nuclei repel)
Contained by magnet field - to not touch walls, go cold
Start construct - 1950; now - experimentals work (few mins)
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Activity &
Half-life
Activity
No. of atoms decaying in RA /s
Unit - Becquerel (Bq) 1 decay /s
Parent atom - nucleus of unstable atom
Geiger monitors activity; Count rate prop. to Bq
CR decr. w/ time; Avr. time of CR to half is same
Half-life = Time taken for CR, mass of parent to half
Random Nature
RA decay is random; individ. atom decay is unpredictable
Can predict how many atoms decay in given time
1000 atoms, 10% decay /h = 900, 810, 729, 656.1 etc