RADIOCHEMISTRY

RADIOCHEMISTRY

1. RADIATION : The energy radiated or transmitted in the form of rays, waves, or particles. A stream of particles or electromagnetic waves that is emitted by the atoms and molecules of a radioactive substance as a result of nuclear decay.

2. NUCLEAR REACTIONS

   • Spontaneous disintegration
     • nothing initiates; nothing stops energy range: 15 keV < x < 20 MeV
   • Artificial mutation
     • interaction of two nuclei producing other nuclei
   • Nuclear fission
     • heaviest nuclei split into 2 or more lighter nuclei
   • Nuclear fusion
     • light nuclei combined to form heavier nuclei

3. Why is Radiation a Useful Analytical Tool?

   • characterisedby the type and energy of the radiation they emit
   • Easily detected and quantified
   • Generally have low background
   • Measure concentrations as low as 10^-14 M

4. Main Radioanalytical Methods

   • Measuring Naturally Occuring Radiation
     • e.g. Measuring radon in homes Dating artifacts or sediments
   • Tracer Methods
     • 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.
   • Activation Analysis

5. Language

   • Units
     • 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.
     • Becquerel (Bq)= 1 decay per second
     • Curie (Ci)= the activity of 1 g of 226Ra = 3.70 x 10^10Bq
     • Electron Volt = 1 MeV = 1.6 ×10–13J per particle≈105 MJ/mol
   • Half Life
     • Time required for one half of the number of radioactive atoms in a sample to undergo decay
   • Positron ẞ+
     • - forms when the number of protons in nucleus reduced by 1 - has mass of electron- transitory existence –reacts with electron to form γrays
   • Negatron ẞ-
     • high energy electron that is formed when a neutron is converted to a proton
   • Electron capture
     • electron is captured by the nucleus and combines with proton to form a neutron. Neutrino is emitted.
   • Decay Processes
     • α decay
       • 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
     • β decay
       • - 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
       • negatron formation
       • positron formation
       • electron capture
     • γ-ray emission - (produced by nuclear relaxation)
       • - 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
       • Lose E via:
         • photoelectric effect (γ-ray photon disappears)
           • electrons released on exposure of (metal) surface by overcoming electron binding energy (low E)
         • Compton effect (photon recoils, repeat the rxn)
           • electrons ejected from atoms but with only part of the photon’s energy; photon recoils with reduced energy to act again (Relatively energetic)
         • pair production (creates positron & electron)
           • photon totally absorbed in creating a positron and an electron in the field near the nucleus (High Energy)

6. Measuring Radioactivity

   • Spontaneous disintegration
     • Stability of nucleus
       • - complicated function of atomic number and atomic mass - trend for stable nuclei to have more neutrons than protons
     • Probability
       • - cannot tell when an individual atom will disintegrate but the average behaviour of a large number of nuclei can be predicted precisely
     • Characteristic spectra
       • each type of nucleus has a characteristic pattern of disintegration i.e. emit specific kinds of radiation with individual energy patterns
     • Disintegration rates
       • rates are always first order (straight line - easy to predict)
   • Radioactive decay rates
     • –dN/dt = λN
       • N - # radioactive nuclei t- time  λ - decay constant
       • If we look at an interval from t0 to t then: –( ln N –ln N0) = λdt ln (N / N0) = –λt N = N0e –λt
     • Activity (A) : A = A0e –λt (A: disintegration rate)
       • A = c N c = detection coefficient of detector This means:–dA/dt = –dN/dt and hence also  ln (A / A0) = –λt
     • Half Life t(1/2) = (0.693/ λ)
       • half life = time taken for N = N0/2
   • Radioactive equilibrium
     • - 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.
     • λ1N1= λ2N2

7. DETECTORS

   • depends on the ionization of matter caused by radioactivity
   • Photographic emulsions
     • 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
   • Cloud chambers
     • Air supersaturated with water (or other vapour like ethanol) Produces ‘tracks’ along the path of radiation Droplets condense around ions / electrons produced by radiation.
   • Gas-filled detectors
     • QUANTITATIVE
     • Inert gas filled - ionized (Ar+) and electron = (ion pairs)
     • e- move towards anode and ions move twrds cathode. - e- movement - charged electric current twrds the meter - reading
     • detector behaviour depends on the voltage applied
       • 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.
       • Ionisation chamber ==> (# electrons reach anode = total # produced by radiation)
         • e.g. smoke detector
       • Proportional ==> (# electrons increases with voltage because increase ions)
       • Geiger ==> (enormous amplification but current limited by tube design)
         • * most sensitive
     • 'dead time' = Ionizations in the chamber take a finite time to dissipate, during which no further response can occur.
   • Scintillation counters
     • QUANTITATIVE
     • Based on radiation-induced luminescence
       • i.e. electronic excitation rather than ionization
     • Excited atom relaxes, emitting a flash of light (at longer wavelength than γ)
     • Structure
       • - 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
   • Semiconductor detectors
     • QUANTITATIVE Lithium-drifted silicon detectors Si(Li) Lithium-drifted germanium detectors Ge(Li)
     • Diode-like devices based on lithium-doped germanium and silicon
     • gamma / X-rays
       • gamma rays react with LI- pop the electron from Li. the electron migrate to form the actual compound
     • structure
       • 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
     • Better than a proportional gas-filled counter because there is no secondary ionization so dead times are small.
     • disadvantage- must be cooled by liquid N2
       • - prevent Li diffusion out of the Ge/Si - decrease noise to an acceptable level  - modern examples only need cooling when in use
       • not portable
     • Ge used instead of Si when wavelength < 0.3 Angstroms must be cooled at all times
   • Counting Corrections
     • Background radiation
       • cosmic effects surroundings laboratory contamination
     • Counting geometry
       • radiation is emitted in all directions orientation and distance of counter is important
     • Back-scattering
       • - radiation directed away from the counter can be reflected back into it by objects behind sample &nbsp;- use reproducible sample holders made of absorbing material
     • Self-scattering
       • - radiation is deflected and absorbed by the sample itself so counts = f (sample thickness) - make samples as thin and as reproducible as possible
     • Decay
       • samples with short half-lives decay during counting need to correct if counting period &gt; 10% of t½
     • Dead-time corrections
       • - 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’
       • Geiger tube50 –200 μs Scintillation0.25 μs Know dead time R* = R/(1-Rτ)
         • R* = true count rate R = obs count rate τ = dead time
     • Determining Dead Time

8. RADIOANALYTICAL METHODS

   • Chemical tagging
     • Introduce a ‘hot’ (radioactively visible) isotope into a process containing a ‘cold’ isotope, to follow the latter
     • Analytical procedures
       • Used to determine % efficiencies (errors from co-ppt, occlusion, etc. are thus eliminated)
     • Industrial processes
       • Trace disposed waste / transport of pollutants
     • Biological processes
       • Trace biochemical pathways e.g. in citric acid cycle
     • Determining solubilities
       • Can measure the activity of a saturated solution without evaporation or weighing.
     • Advantages
       • -Selectivity (Independent of quantitative isolation) -Simplicity of equipment
     • Disadvantages
       • Exchangeability
         • If isotope occurs in a compound it can sometimes ‘exchange’ e.g. tritium+ and H+
       • Differences due to atomic mass
         • Isotopes do not always mimic each other exactly e.g. diffusion of deuterium and tritium is much slower than that of hydrogen
   • Isotope Dilution
     • Rm= activity of isolated mass wm= weight of isolated mass wx= weight of unknown wt= weight of tracer Rt= count of tracer
     • Specific activity of a radionuclide is reduced when it is mixed with its stable counterpart.
     • The extent of reduction in activity provides a measure of the amount of stable isotope with which the radio-isotope is mixed.
   • Activation Analysis
     • making sample radioactive by irradiating with neutrons/charged particles
     • Sources : reactors radionuclides particle accelerators
     • thermal neutrons
       • Energetic neutron passed through moderating material to dissipate energy and create
     • fast neutrons
       • For light elements e.g. F, O, N use fast neutrons(14 eV)
     • detection limit
       • Detection limits for many elements are &lt; 1 μg; for some elements as low as picogram level
     • Neutron captured by analyte nucleus causing excited state and γray emmission
     • Capture cross-sections
       • measures of probability that interaction will occur. The values are obtained from tables. Units are in barns (1 b = 10-24cm2)
     • Number of nuclei that are produced during irradiation (N* = N φσS)
       • S = saturation factor
     • activity is directly proportional to number of radionuclides
     • Advantages
       • high sensitivity, minimal sample preparation, easy to calibrate, often non-destructive,
     • Disadvantages
       • Expensive equipment and special facilities required OHS considerations when dealing with radionuclides Time for analysis when using long-lived radionuclides

9. OHS and Radioanalytical Chemistry

   • Dosimetry
     • Special steps need to be taken when working with radioactive sources e.g. personal radiation monitors for monitoring dose
     • Thermoluminescent dosemeter (TLD)
       • 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
   • Units
     • Different units are used because the effects depend on the type of radiation, type of material and other circumstances
     • Roentgen (ion dose)
       • 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
     • RAD (energy dose)
       • 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
     • Gray (Gy)
       • SI unit for energy dose 1 Gy = 1 J/kg = 100 rad
     • REM (equivalent dose)
       • 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)
       • RBE
     • Sievert (Sv)
       • 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

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