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Nuclear Chemistry. Chapter 23. Radioactivity. Emission of subatomic particles or high-energy electromagnetic radiation by nuclei Such atoms/isotopes said to be radioactive. Its discovery. Discovered in 1896 by Becquerel Called strange, new emission uranic rays

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radioactivity
Radioactivity
  • Emission of subatomic particles or high-energy electromagnetic radiation by nuclei
  • Such atoms/isotopes said to be radioactive
its discovery
Its discovery
  • Discovered in 1896 by Becquerel
    • Called strange, new emission uranic rays
      • Because emitted from uranium
  • Marie Curie discovered two new elements both of which emitted uranic rays
    • Po & Ra
  • Uranic rays became radioactivity
types of radioactivity
Types of radioactivity
  • Rutherford and Curie found that emissions produced by nuclei
  • Different types:
    • Alpha decay
    • Beta decay
    • Gamma ray emission
    • Positron emission
    • Electron capture
isotopic symbolism
Isotopic symbolism
  • Remember from 139/141?
  • Let’s briefly go over it
  • Nuclide = isotope of an element
  • Proton = 11p
  • Neutron = 10n
  • Electron = 0-1e
types of decay alpha decay
Types of decay: alpha decay
  • Alpha () decay
  • Alpha () particle: helium-4 bereft of 2e-
  • = 42He (don’t write He+2)
  • Parent nuclide  daughter nuclide + He-4
  • 23892U  23490Th + 42He
  • Daughter nuclide = parent nuclide atomic # minus 2
  • Sum of atomic #’s & mass #’s must be = on both sides of nuclear equation!
alpha decay
Alpha decay
  • Has largest ionizing power
    • Ability to ionize molecules & atoms due to largeness of -particle
  • But has lowest penetrating power
    • Ability to penetrate matter
  • Skin, even air, protect against -particle radiation
beta decay
Beta decay
  • Beta () decay
  • Beta () particle = e-
  • How does nucleus emit an e-?
  •  neutron changes into proton & emits e-
  •  10n  11p + 0-1e
  • Daughter nuclide = parent nuclide atomic number plus 1
beta decay1
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 (alpha particle is opposite)
gamma ray emission
Gamma ray emission
  • Gamma () ray emission
  • Electromagnetic radiation
  • High-energy photons
  • 00
  • No charge, no mass
  • Usually emitted in conjunction with other radiation types
  • Lowest ionizing power, highest penetrating power
    • Requires several inches lead shielding
positron emission
Positron emission
  • Positron = antiparticle of e-
  •  same mass, opposite charge
  • (Collision with e- causes -ray emission)
  • Proton converted into neutron, emitting positron
  • 0+1e
  • 11p 10n + 0+1e
  • 3015P  3014Si + 0+1e
  • Atomic # of parent nuclide decreases by 1
  • Positrons have same ionizing/penetrating power as e-
electron capture
Electron capture
  • Particle absorbed by, instead of ejected from, an unstable nucleus
  • Nucleus assimilates e- from an inner orbital of its e- cloud
  • Net result = conversion of proton into neutron
  • 11p+ 0-1e  10n
  • 9244Ru + 0-1e 9243Tc
  • Atomic # of parent nuclide decreases by 1
problems
Problems
  • Write a nuclear equation for each of the following:
  • 1. beta decay in Bk-249
  • 2. electron capture in I-111
  • 3. positron emission in K-40
  • 4. alpha decay of Ra-224
the valley of stability
The valley of stability
  • Predicting radioactivity type
    • Tough to answer why one radioactive type as opposed to another
      • However, we can get a basic idea
  • Neutrons occupy energy levels
    • Too many lead to instability
valley of stability
Valley of stability
  • 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 valley: too many n, convert n to p
      • Beta-decay
  • N/Z too low
    • Below valley, too many p, convert p to n
      • Positron emission/e—capture and, to lesser extent, alpha-decay
predict type of radioactive decay
Predict type of radioactive decay
  • 1. Mg-28
  • 2. Mg-22
  • 3. Mo-102
magic numbers
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
    • Magic numbers
      • N or Z = 2, 8, 20, 28, 50, 82, and N = 126
detecting radioactivity
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 to radioactivity
geiger counter
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 through gas chamber
scintillation counter
Scintillation counter
  • Particles pass through material (NaI or CsI) that emits UV or visible light due to excitation
    • Atoms excited to higher E state
    • E releases as light, measured on meter
radioactive decay kinetics
Radioactive decay kinetics
  • All radioactive nuclei decay via 1st-order kinetics
    •  rate of decay  to # of nuclei present
      • Rate = kN
  • Half-life = time taken for ½ of parent nuclides to decay to daughter nuclides
problem
Problem
  • Pu-236 is an -emitter w/half-life = 2.86 years. If sample initially contains 1.35 mg, what mass remains after 5.00 years?
  • How long would it take for 1.35 mg sample of Pu-236 above to decay to 0.100 mg?
    • Assume 1.35 mg/1 L air
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:

147N + 10n  146C + 11H

C-14 then decays back to N by -emission:

146C  147N + 0-1e; t1/2 = 5730 years

There is an approximately constant supply of C-14

Taken up by plants via 14CO2 & 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

Bristlecone pine used to calibrate data

Carbon-dating good for 50,000 years

Radiometric dating: radiocarbon dating
problem1
Problem
  • Artifact is found to have C-14 decay rate of 4.50 disintegration/min  g of carbon.
  • If living organisms have a decay rate of 15.3, how old is the artifact?
    • Given decay rate is  to amount of C-14 present.
radiometric dating uranium lead dating
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
    • t1/2 = 4.5 x 109 years
example
Example
  • A meteor contains 0.556 g Pb-206 (to every 1.00 g U-238). Determine its age.
problem2
Problem
  • A rock from Australia was found to contain 0.438 g of Pb-206 to every 1.00g of U-238. Assuming that the rock did not contain any Pb-206 at the time of its formation, how old is the rock?
fission
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 = enough mass of U-235 to produce a self-sustaining rxn
nuclear power
Nuclear power
  • In America, about 20% electricity generated by nuclear fission
  • Imagine:
  • Nuclear-powered car
  • Fuel = pencil-sized U-cylinder
  • Energy = 1000 20-gallon tanks of gasoline
  • Refuel every 1000 weeks (about 20 years)
  • !
nuclear power plant
Nuclear power plant
  • Controlled fission through U fuel rods (3.5% U-235)
  • Rods absorb neutrons
  • Retractable
  • Heat boils water, making steam, turning turbine on generator to make electricity
comparing
Comparing
  • Typical nuclear power plant makes enough energy for city of 1,000,000 people and uses about 50 kg of fuel/day
    • No air pollution/greenhouses gases
      • But, nuclear meltdown (overheating of nuclear core) is a potential threat
        • No problem!
      • Also, waste disposal
        • Location, containment problems
comparing1
Comparing
  • Coal-burning power plant uses about 2,000,000 kg of fuel to make same amount of energy
    • But, releases huge amounts of SO2, NO2, CO2
mass to energy
Mass to energy
  • E = mc2
  • Explains relationship between energy formation and matter loss
    • Amount of energy released in U-235 fission per atom of U-235 is 2.8 x 10-11 J
      • BUT, amount of energy released in U-235 fission per 1 mole of U-235 is 1.7 x 1013 J!
        • More than a million times more energy per mole than a chemical rxn!
mass defect
Mass defect
  • Mass products < mass reactants
  • Difference in mass due to conversion of mass into energy
    • Called mass defect
  • Nuclear binding energy is energy corresponding to mass defect
    • Amount of energy required to break apart nucleus into nucleons (n + p)
some more stuff
Some more stuff
  • Nuclear physicists use eV or MeV (mega eV) instead of joules
  • 1 MeV = 1.602 x 10-13 J
  • 1 amu = 931.5 MeV
    • Energy per nucleus and not per mole
  • To compare energy of 1 nucleus to another, calculate binding energy per nucleon
    • = nuclear binding energy of nuclide per #of nucleons (n + p) in nuclide
  • As binding energy per nucleon increases so does stability of species
example1
Example
  • Calculate the mass defect and nuclear binding energy per nucleon (in MeV and in J) for:
  • 42He
    • Made from: 211H + 210n
  • 11H = 2 x 1.00783 amu
  • 210n = 2 x 1.00866 amu
    • Net mass = 4.03298 amu
problem3
Problem
  • Calculate the mass defect and nuclear binding energy per nucleon (in MeV) for C-16
    • Consider C-16 being made from 6 11H & 10 10n
      • C-16 mass = 16.014701 amu
curve of binding energy
Curve of binding energy
  • Measure of stability of nucleus (binding energy/nucleon)
    • Reaches max at Fe-56
fusion
Fusion
  • 21H + 31H 42He + 10n
    • Ten times more energy per gram than fission
transmutation
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 1930’s, devices needed that could accelerate particles to high velocities:
    • Linear accelerator
    • Cyclotron
linear accelerator
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!
  • 2 miles long 
cyclotron
Cyclotron
  • Similar alternating voltage used
  • But applied between two semicircular halves of cyclotron
  • Particle spirals due to magnets
    • Hits target
radiation on life
Radiation on life
  • 3 divisions
  • Acute radiation
  • Increased cancer risk
  • Genetic effects
the first
The first
  • Quickly dividing cells at greatest risk:
    • Intestinal lining
    • Immune response cells
  • Likelihood of death depends on dose & duration
the second
The second
  • Cancer = uncontrolled cell growth leading to tumors
    • Dose?
      • Unknown
  • Cancer is a murky illness
the third
The third
  • Causes genetic defects  teratogenic
how to measure radiation exposure
How to measure radiation exposure?
  • Decay events exposure
    • Curie (Ci)
      • 3.7 x 1010 decay events/second
        • Based on one gram of Ra-226 decay events/second
  • Or, amount of energy absorbed by body tissue
    • Gray (Gy)
      • 1 joule of energy absorbed/kg body tissue
    • Rad (radiation absorbed dose) = 0.01 Gy
      • 1 rad = 0.01 J/kg body tissue
  • SI unit
    • Becquerel (Bq)
      • 1 Bq = 1 decay/second
        • 1 Ci = 3.7 × 1010 Bq
  • All measure radiation but none account for amount of damage to life
ah ha
Ah ha!
  • Biological effectiveness factor (RBE = relative biological effectiveness)
    • Dose in rads x RBE = dose in rems
  • Rems = (roentgen equivalent man)
    • Roentgen = amt of radiation producing 2.58 x 10-4 C of charge/kg air
good site let s take a look
Good site: let’s take a look
  • http://www.deq.idaho.gov/inl_oversight/radiation/radiation_guide.cfm
more facts
More facts
  • 20 rem
    • Decreased white blood cell count after instantaneous exposure
  • 100-400 rem
    • Vomiting, diarrhea, lesions, cancer-risk increase
  • 500-1000
    • Death w/in 2 months
  • 1000-2000
    • Death w/in 2 weeks
  • Above 2000
    • Death w/in hours
diagnostic and therapeutic radiation
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
  • F-18  heart, brain
  • P-31  tumors
slide66
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
        • Imaged
radiotherapy
Radiotherapy
  • Using radiation to treat cancer
  • Depending on duration/dose can develop symptoms of radiation sickness
    • Vomiting, diarrhea, skin burns, hair loss
other applications
Other applications
  • Irradiating foods
  • Nuking bugs like fruit flies and screw-worm flies
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