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Radioactivity
■ Emission of subatomic particles or high-energy
electromagnetic radiation by nuclei
Its discovery
■ Discovered in 1896 by Becquerel
■ Called strange, new emission uranic rays
■ Cuz emitted from uranium
■ Marie Curie & hubby discovered two new
elements, both of which emitted uranic rays
Types of radioactivity
■ Rutherford and Curie found that emissions produced
by nuclei
■ Different types:
■ Alpha decay ■ Beta decay
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Isotopic symbolism
■ Let’s briefly go over it ■ Proton = 1
1p
■ Neutron = 1
0n
■ Electron = 0
Types of decay: alpha decay
■ Alpha (α) particle: helium-4 bereft of 2e -■ = 4
2He
■ Parent nuclide ⇒ daughter nuclide + He-4
238
92U ⇒
234
90Th + 4
2He
■ Daughter nuclide = parent nuclide atomic # minus 2
■ Sum of atomic #’s & mass #’s must be = on both
Alpha decay
■ Has largest ionizing power
■ = ability to ionize molecules & atoms due to
largeness of α-particle
■ But has lowest penetrating power
■ = ability to penetrate matter
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Beta decay
■ Beta (β) particle = e
-■ How does nucleus emit an e-?
■ ⇒ neutron changes into proton & emits e -■ ∴ 1
0n ⇒ 1
1p + 0
-1e
■ Daughter nuclide = parent nuclide atomic number plus 1
137
55Cs ⇒
137
56Ba + 0
-Beta decay
■ Lower ionizing power than alpha particle
■ But higher penetration power
■ Requires sheet of metal or thick piece of wood
to arrest penetration
■ ∴ more damage outside of body, but less in
Gamma ray emission
■ Electromagnetic radiation ■ High-energy photons
■ 0
0γ
■ No charge, no mass
■ Usually emitted in conjunction with other radiation
types
■ Lowest ionizing power, highest penetrating power ⇒
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Problems
■ Write a nuclear equation for each of the
following:
Cont.
■ In determining nuclear
stability, ratio of neutrons to protons (N/Z)
important
■ Notice lower part of
valley (N/Z = 1)
■ Bi last stable
(non-radioactive) isotopes
■ N/Z too high: above
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Magic numbers
■ Actual # of n & p affects nuclear
stability
■ Even #’s of both n & p give
stability
■ Similar to noble gas electron
configurations: 2, 10, 18, 36, etc.
■ Since nucleons (= n+p) occupy
energy levels within nucleus
■ N or Z = 2, 8, 20, 28, 50, 82, and
N = 126
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Detecting radioactivity
■ Particles detected through interactions w/atoms
or molecules
■ Simplest ⇒ film-badge dosimeter
■ Photographic film in small case, pinned to
clothing
■ Monitors exposure
■ Greater exposure of film ⇒ greater exposure
Geiger counter
■ Emitted particles pass
through Ar-filled chamber
■ Create trail of ionized Ar
atoms
■ Induced electric signal
detected on meter and then clicks
■ Each click = particle passing
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Radioactive decay kinetics
■ Half-life = time taken for ½ of parent nuclides
Radiometric dating: radiocarbon
dating
■ Devised in 1949 by Libby at U of
Chicago
■ Age of artifacts, etc., revealed by
presence of C-14
■ C-14 formed in upper atmosphere via: ■ 14
7N + 1
0n ⇒ 14
6C + 1
1H
■ C-14 then decays back to N by
β-emission:
■ 14
6C ⇒ 14
7N + 0
-1e; t1/2 = 5730 years
■ Approximately constant supply of
C-14
■ Taken up by plants via 14CO
2 & later
incorporated in animals
■ Living organisms have same ratio of
C-14:C-12
■ Once dead, no longer incorporating
C-14 ⇒ ratio decreases
■ 5% deviation due to variance of
atmospheric C-14
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Radiometric dating: uranium/lead
dating
■ Relies on ratio of U-238:Pb-206 w/in igneous
rocks (rocks of volcanic origin)
■ Measures time that has passed since rock
solidified
■ t
1/2 = 4.5 x 10
9 years
■ For ex, if rock contains equal amts of isotopes
Fission
■ Meitner, Strassmann, and Hahn discovered fission:
splitting of uranium-235
■ Instead of making heavier elements, created a Ba and
Kr isotope plus 3 neutrons and a lot of energy ■ Sample rich in U-235 could create a chain rxn ■ To make a bomb, however, need critical mass =
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Nuclear power
■ In America, about 20% electricity generated by
nuclear fission
■ Imagine:
■ Nuclear-powered car
■ Fuel = pencil-sized U-cylinder
Nuclear power plant
■ Controlled fission through U
fuel rods (3.5% U-235)
■ Rods absorb neutrons ■ Retractable
■ Heat boils water, making
Comparing
■ Typical nuclear power plant makes enough E
for city of 1,000,000 ppl and uses about 50 kg of fuel/day
■ No air pollution/greenhouses gases
■ But, nuclear meltdown (overheating of nuclear
core)
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Comparing
■ OTOH, coal-burning power plant uses about
2,000,000 kg of fuel to make same amt of E
■ But, releases huge amts of SO
Fusion
■ H-bonds utilize fusion
(but needs high-temps to react cuz both
positively charged)
■ As does the sun: 2
1H + 3
1H ⇒ 4
2He + 1
0n
■ 10 x more energy/gram
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Transmutation
■ Transforming one element into another
■ In 1919, Rutherford bombarded N-17 to make O-17 ■ The Joliot-Curie’s bombarded Al-27 to form P-30 ■ In ’30’s, devices needed that could accelerate
particles to high velocities:
Linear accelerator
■ Charged-particle accelerated
in evacuated tube
■ Alternating current causes
particle to be pulled into next tube
■ Continues, allowing velocity
= 90% speed of light!
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Cyclotron
■ Similar alternating
voltage used
■ But applied btwn two
semicircular halves of cyclotron
■ Particle spirals due to
magnets
Radiation on life
■ 3 divisions
■ 1. acute radiation
■ 2. Increased cancer risk
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The first
■ Quickly dividing cell at greatest risk: ■ Intestinal lining
■ Immune response cells
■ Likelihood of death
■ Depends on dose/
2
nd■ Cancer = uncontrolled cell growth leading to
tumors
■ Dose? Unknown
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3
rd38
Good site
■ http://www.deq.idaho.gov/inl_oversight/radiati
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More facts
■ 20 rem ⇒ decreased white blood
cell count after instantaneous exposure
■ 100-400 rem ⇒ vomiting,
diarrhea, lesions, cancer-risk increase
Diagnostic and therapeutic radiation
■ Radiotracer = radioactive nuclide in brew to
track movement of brew in body
■ Tc-99 ⇒ bones
■ I-131 ⇒ thyroid
■ Tl-201 ⇒ heart
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PET
■ Positron emission tomography
■ Shows both rate of glucose metabolism and
structural features of imaged organ
■ F-18 emits positrons
■ Positron and e- produce two gamma rays
■ Rays detected
Radiotherapy
■ Using radiation to treat cancer
■ Develop symptoms of radiation sickness:
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Other applications
■ Irradiating foods
■ Nuking bugs like fruit flies and screw-worm
