RADIOCHEMISTRY

1. Spontaneous disintegration

   Annotations:

   • nothing initiates; nothing stops energy range: 15 keV < x < 20 MeV
2. Artificial mutation

   Annotations:

   • interaction of two nuclei producing other nuclei
3. Nuclear fission

   Annotations:

   • heaviest nuclei split into 2 or more lighter nuclei
4. Nuclear fusion

   Annotations:

   • light nuclei combined to form heavier nuclei

Annotations:

• characterisedby the type and energy of the radiation they emit

1. Easily detected and quantified
2. Generally have low background
3. Measure concentrations as low as 10^-14 M

1. Measuring Naturally Occuring Radiation

   Annotations:

   • e.g. Measuring radon in homes Dating artifacts or sediments
2. Tracer Methods

   Annotations:

   • Radioactivity is physically introduced to the sample by adding a measured amt of radioactive sp. (tracer) most important - ISOTOPE DILUTION METHOD (a weighed quantity of radioactive tagged analyte having a known activity is added to a measured amt of the sample. mixed- a fraction of the component of interest is isolated & purified.
3. Activation Analysis

1. Units

   Annotations:

   • Define these as measure of the activityof a substance In general activities in the range of 0.1 –20 kBq(3-500 nCi) are sufficient for analytical applications.
   1. Becquerel (Bq)= 1 decay per second
   2. Curie (Ci)= the activity of 1 g of 226Ra = 3.70 x 10^10Bq
   3. Electron Volt = 1 MeV = 1.6 ×10–13J per particle≈105 MJ/mol
2. Half Life

   Annotations:

   • Time required for one half of the number of radioactive atoms in a sample to undergo decay
3. Positron ẞ+

   Annotations:

   • - forms when the number of protons in nucleus reduced by 1 - has mass of electron- transitory existence –reacts with electron to form γrays
4. Negatron ẞ-

   Annotations:

   • high energy electron that is formed when a neutron is converted to a proton
5. Electron capture

   Annotations:

   • electron is captured by the nucleus and combines with proton to form a neutron. Neutrino is emitted.
6. Decay Processes
   1. α decay

      Annotations:

      • in isotopes mass >150 particles either monoenergetic or distributed among few discrete energies lose energy as result collisions as pass through matter low penetrating power easily measured
   2. β decay

      Annotations:

      • - atomic number changes but mass number does not - lose energy by interacting with electrons in orbitals - particles have continuos spectrum of energies  - penetrating power greater because smaller particles
      1. negatron formation
      2. positron formation
      3. electron capture
   3. γ-ray emission - (produced by nuclear relaxation)

      Annotations:

      • - occurs when nucleus left in excited state by α or β emission process  - returns to ground state in one or more quantized steps with release of monoenergetic γ-rays  - X-rays result of electronic relations                          = γ-ray wavelength generally 1/100thX-ray            = γ-rays produced by nuclear relaxations - γ-ray emission spectrum characteristic for each nucleus highly energetic = highly penetrating
      1. Lose E via:
         1. photoelectric effect (γ-ray photon disappears)

            Annotations:

            • electrons released on exposure of (metal) surface by overcoming electron binding energy (low E)
         2. Compton effect (photon recoils, repeat the rxn)

            Annotations:

            • electrons ejected from atoms but with only part of the photon’s energy; photon recoils with reduced energy to act again (Relatively energetic)
         3. pair production (creates positron & electron)

            Annotations:

            • photon totally absorbed in creating a positron and an electron in the field near the nucleus (High Energy)

1. Spontaneous disintegration
   1. Stability of nucleus

      Annotations:

      • - complicated function of atomic number and atomic mass - trend for stable nuclei to have more neutrons than protons
   2. Probability

      Annotations:

      • - cannot tell when an individual atom will disintegrate but the average behaviour of a large number of nuclei can be predicted precisely
   3. Characteristic spectra

      Annotations:

      • each type of nucleus has a characteristic pattern of disintegration i.e. emit specific kinds of radiation with individual energy patterns
   4. Disintegration rates

      Annotations:

      • rates are always first order (straight line - easy to predict)
2. Radioactive decay rates
   1. –dN/dt = λN

      Annotations:

      • N - # radioactive nuclei t- time  λ - decay constant
      1. If we look at an interval from t0 to t then: –( ln N –ln N0) = λdt ln (N / N0) = –λt N = N0e –λt
   2. Activity (A) : A = A0e –λt (A: disintegration rate)

      Annotations:

      • A = c N c = detection coefficient of detector This means:–dA/dt = –dN/dt and hence also  ln (A / A0) = –λt
   3. Half Life t(1/2) = (0.693/ λ)

      Annotations:

      • half life = time taken for N = N0/2
3. Radioactive equilibrium

   Annotations:

   • - Steady state in which all radioactive ‘child’ products are decaying at the same rate as they are being formed by decay of their radioactive ‘parents’. - useful for determining the half life of a very long-lived radioactive element such as uranium.
   1. λ1N1= λ2N2

1. depends on the ionization of matter caused by radioactivity
2. Photographic emulsions

   Annotations:

   • Photoelectrons knocked off halide ions cause reduction of Ag+ Ag atom then act as a focus for further reduction Produce black spot on the photographic film
3. Cloud chambers

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   • Air supersaturated with water (or other vapour like ethanol) Produces ‘tracks’ along the path of radiation Droplets condense around ions / electrons produced by radiation.
4. Gas-filled detectors

   Annotations:

   • QUANTITATIVE
   1. Inert gas filled - ionized (Ar+) and electron = (ion pairs)
   2. e- move towards anode and ions move twrds cathode. - e- movement - charged electric current twrds the meter - reading
   3. detector behaviour depends on the voltage applied

      Annotations:

      • too low, the ion pairs simply recombine and are not detected. high enough, collisions between accelerated electrons and gas molecules cause secondary ion pair production, i.e. amplification.
      1. Ionisation chamber ==> (# electrons reach anode = total # produced by radiation)

         Annotations:

         • e.g. smoke detector
      2. Proportional ==> (# electrons increases with voltage because increase ions)
      3. Geiger ==> (enormous amplification but current limited by tube design)

         Annotations:

         • * most sensitive
   4. 'dead time' = Ionizations in the chamber take a finite time to dissipate, during which no further response can occur.
5. Scintillation counters

   Annotations:

   • QUANTITATIVE
   1. Based on radiation-induced luminescence

      Annotations:

      • i.e. electronic excitation rather than ionization
   2. Excited atom relaxes, emitting a flash of light (at longer wavelength than γ)
   3. Structure

      Annotations:

      • - scintillation crystal - doped with organic substance (usually w benzene ring) to absorb the gamma radiation - use crystal (more dense) and gamma radiation tend to react w that
6. Semiconductor detectors

   Annotations:

   • QUANTITATIVE Lithium-drifted silicon detectors Si(Li) Lithium-drifted germanium detectors Ge(Li)
   1. Diode-like devices based on lithium-doped germanium and silicon
   2. gamma / X-rays

      Annotations:

      • gamma rays react with LI- pop the electron from Li. the electron migrate to form the actual compound
   3. structure

      Annotations:

      • Li vapour-deposited onto p-type Ge/Si crystal and heated Li atoms diffuse into the crystal where they act like ionizable atoms in a gas-filled detector
   4. Better than a proportional gas-filled counter because there is no secondary ionization so dead times are small.
   5. disadvantage- must be cooled by liquid N2

      Annotations:

      • - prevent Li diffusion out of the Ge/Si - decrease noise to an acceptable level  - modern examples only need cooling when in use
      1. not portable
   6. Ge used instead of Si when wavelength < 0.3 Angstroms must be cooled at all times
7. Counting Corrections
   1. Background radiation

      Annotations:

      • cosmic effects surroundings laboratory contamination
   2. Counting geometry

      Annotations:

      • radiation is emitted in all directions orientation and distance of counter is important
   3. Back-scattering

      Annotations:

      • - radiation directed away from the counter can be reflected back into it by objects behind sample  - use reproducible sample holders made of absorbing material
   4. Self-scattering

      Annotations:

      • - radiation is deflected and absorbed by the sample itself so counts = f (sample thickness) - make samples as thin and as reproducible as possible
   5. Decay

      Annotations:

      • samples with short half-lives decay during counting need to correct if counting period &gt; 10% of t½
   6. Dead-time corrections

      Annotations:

      • - all detectors take finite time to recover after sensing the arrival of a radioactive particle - other particles arriving in this time interval are not detected- also called ‘co-incidence corrections’
      1. Geiger tube50 –200 μs Scintillation0.25 μs Know dead time R* = R/(1-Rτ)

         Annotations:

         • R* = true count rate R = obs count rate τ = dead time
   7. Determining Dead Time

1. Chemical tagging

   Annotations:

   • Introduce a ‘hot’ (radioactively visible) isotope into a process containing a ‘cold’ isotope, to follow the latter
   1. Analytical procedures
      1. Used to determine % efficiencies (errors from co-ppt, occlusion, etc. are thus eliminated)
   2. Industrial processes
      1. Trace disposed waste / transport of pollutants
   3. Biological processes
      1. Trace biochemical pathways e.g. in citric acid cycle
   4. Determining solubilities
      1. Can measure the activity of a saturated solution without evaporation or weighing.
   5. Advantages
      1. -Selectivity (Independent of quantitative isolation) -Simplicity of equipment
   6. Disadvantages
      1. Exchangeability

         Annotations:

         • If isotope occurs in a compound it can sometimes ‘exchange’ e.g. tritium+ and H+
      2. Differences due to atomic mass

         Annotations:

         • Isotopes do not always mimic each other exactly e.g. diffusion of deuterium and tritium is much slower than that of hydrogen
2. Isotope Dilution

   Annotations:

   • Rm= activity of isolated mass wm= weight of isolated mass wx= weight of unknown wt= weight of tracer Rt= count of tracer
   1. Specific activity of a radionuclide is reduced when it is mixed with its stable counterpart.
   2. The extent of reduction in activity provides a measure of the amount of stable isotope with which the radio-isotope is mixed.
3. Activation Analysis

   Annotations:

   • making sample radioactive by irradiating with neutrons/charged particles
   1. Sources : reactors radionuclides particle accelerators
   2. thermal neutrons

      Annotations:

      • Energetic neutron passed through moderating material to dissipate energy and create
   3. fast neutrons

      Annotations:

      • For light elements e.g. F, O, N use fast neutrons(14 eV)
   4. detection limit

      Annotations:

      • Detection limits for many elements are &lt; 1 μg; for some elements as low as picogram level
   5. Neutron captured by analyte nucleus causing excited state and γray emmission
   6. Capture cross-sections

      Annotations:

      • measures of probability that interaction will occur. The values are obtained from tables. Units are in barns (1 b = 10-24cm2)
   7. Number of nuclei that are produced during irradiation (N* = N φσS)

      Annotations:

      • S = saturation factor
   8. activity is directly proportional to number of radionuclides
   9. Advantages
      1. high sensitivity, minimal sample preparation, easy to calibrate, often non-destructive,
   10. Disadvantages
       1. Expensive equipment and special facilities required OHS considerations when dealing with radionuclides Time for analysis when using long-lived radionuclides

1. Dosimetry

   Annotations:

   • Special steps need to be taken when working with radioactive sources e.g. personal radiation monitors for monitoring dose
   1. Thermoluminescent dosemeter (TLD)

      Annotations:

      • Electrons in the crystal structure of the TLD material excited to higher energy levels as a result of irradiation and are trapped in the crystal structure. Response of TLD dependent on the energy and type of radiation
2. Units

   Annotations:

   • Different units are used because the effects depend on the type of radiation, type of material and other circumstances
   1. Roentgen (ion dose)

      Annotations:

      • unit of exposure defined in terms of amount of charge generated when radiation is stopped in air Now, 1 R = 2.58 ×10-4Ci / kg For a standard human, I R ≈10-5J
   2. RAD (energy dose)

      Annotations:

      • unit of absorbed dose defined as amount of radiation that liberates 10-2J/kg of irradiated material independent of the type of radiation but function of material
   3. Gray (Gy)

      Annotations:

      • SI unit for energy dose 1 Gy = 1 J/kg = 100 rad
   4. REM (equivalent dose)

      Annotations:

      • measure of biological damage, defined in terms of a factor called the Relative Biological Effect(RBE)which takes account of different circumstances 1 REM = dose in RADS ×RBE RBE= dose of γrays that produces the same effect as 1 RAD of the particular type of radiation (so standardising REM)
      1. RBE
   5. Sievert (Sv)

      Annotations:

      • dose in Gy multiplied by an effectiveness factor 1 mGy dose of αrays = 20 mSv of equivalent dose 1 mGy dose of βrays = 1 mSv equivalent dose In most cases the effectiveness factor is unity and the dose in grays is equal to the dose in sieverts