Zusammenfassung der Ressource
RADIOCHEMISTRY
- 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.
- NUCLEAR REACTIONS
- Spontaneous disintegration
Anmerkungen:
- nothing initiates; nothing stops
energy range: 15 keV < x < 20 MeV
- Artificial mutation
Anmerkungen:
- interaction of two nuclei producing other nuclei
- Nuclear fission
Anmerkungen:
- heaviest nuclei split into 2 or more lighter nuclei
- Nuclear fusion
Anmerkungen:
- light nuclei combined to form heavier nuclei
- Why is Radiation a Useful Analytical Tool?
Anmerkungen:
- 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
- Main Radioanalytical Methods
- Measuring Naturally Occuring Radiation
Anmerkungen:
- e.g. Measuring radon in homes
Dating artifacts or sediments
- Tracer Methods
Anmerkungen:
- 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
- Language
- Units
Anmerkungen:
- 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
Anmerkungen:
- Time required for one half of the number of radioactive atoms in a sample to undergo decay
- Positron ẞ+
Anmerkungen:
- - 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-
Anmerkungen:
- high energy electron that is formed when a neutron is converted to a proton
- Electron capture
Anmerkungen:
- electron is captured by the nucleus and combines with proton to form a neutron. Neutrino is emitted.
- Decay Processes
- α decay
Anmerkungen:
- 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
Anmerkungen:
- - 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)
Anmerkungen:
- - 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)
Anmerkungen:
- electrons released on exposure of (metal) surface by overcoming electron binding energy
(low E)
- Compton effect
(photon recoils, repeat
the rxn)
Anmerkungen:
- 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)
Anmerkungen:
- photon totally absorbed in creating a positron and an electron in the field near the nucleus
(High Energy)
- Measuring Radioactivity
- Spontaneous disintegration
- Stability of nucleus
Anmerkungen:
- - complicated function of atomic number and atomic mass
- trend for stable nuclei to have more neutrons than protons
- Probability
Anmerkungen:
- - cannot tell when an individual atom will disintegrate but the average behaviour of a large number of nuclei can be predicted precisely
- Characteristic spectra
Anmerkungen:
- each type of nucleus has a characteristic pattern of disintegration
i.e. emit specific kinds of radiation with individual energy patterns
- Disintegration rates
Anmerkungen:
- rates are always first order
(straight line - easy to predict)
- Radioactive decay rates
- –dN/dt = λN
Anmerkungen:
- 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)
Anmerkungen:
- 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)
Anmerkungen:
- half life = time taken for N = N0/2
- Radioactive equilibrium
Anmerkungen:
- - 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
- DETECTORS
- depends on the
ionization of
matter caused
by radioactivity
- Photographic emulsions
Anmerkungen:
- 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
Anmerkungen:
- 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
Anmerkungen:
- 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
Anmerkungen:
- 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)
Anmerkungen:
- Proportional ==>
(# electrons
increases with
voltage because
increase ions)
- Geiger ==> (enormous
amplification but current
limited by tube design)
Anmerkungen:
- 'dead time' = Ionizations in the
chamber take a finite time to
dissipate, during which no further
response can occur.
- Scintillation counters
Anmerkungen:
- Based on radiation-induced
luminescence
Anmerkungen:
- i.e. electronic excitation rather than ionization
- Excited atom relaxes,
emitting a flash of light (at
longer wavelength than γ)
- Structure
Anmerkungen:
- - 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
Anmerkungen:
- 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
Anmerkungen:
- gamma rays react with LI- pop the electron from Li. the electron migrate to form the actual compound
- structure
Anmerkungen:
- 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
Anmerkungen:
- - 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
Anmerkungen:
- cosmic effects
surroundings
laboratory contamination
- Counting geometry
Anmerkungen:
- radiation is emitted in all directions
orientation and distance of counter is important
- Back-scattering
Anmerkungen:
- - radiation directed away from the counter can be reflected back into it by objects behind sample
- use reproducible sample holders made of absorbing material
- Self-scattering
Anmerkungen:
- - radiation is deflected and absorbed by the sample itself
so counts = f (sample thickness)
- make samples as thin and as reproducible as possible
- Decay
Anmerkungen:
- samples with short half-lives decay during counting
need to correct if counting period > 10% of t½
- Dead-time corrections
Anmerkungen:
- - 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)
Anmerkungen:
- R* = true count rate
R = obs count rate
τ = dead time
- Determining Dead Time
- RADIOANALYTICAL METHODS
- Chemical tagging
Anmerkungen:
- 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
Anmerkungen:
- If isotope occurs in a compound it can sometimes ‘exchange’
e.g. tritium+ and H+
- Differences due to atomic mass
Anmerkungen:
- Isotopes do not always mimic each other exactly
e.g. diffusion of deuterium and tritium is much slower than that of hydrogen
- Isotope Dilution
Anmerkungen:
- 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
Anmerkungen:
- making sample radioactive by irradiating with neutrons/charged particles
- Sources : reactors
radionuclides particle
accelerators
- thermal neutrons
Anmerkungen:
- Energetic neutron passed through moderating material to dissipate energy and create
- fast neutrons
Anmerkungen:
- For light elements e.g. F, O, N use fast neutrons(14 eV)
- detection limit
Anmerkungen:
- Detection limits for many elements are < 1 μg; for some elements as low as picogram level
- Neutron captured by
analyte nucleus causing
excited state and γray
emmission
- Capture cross-sections
Anmerkungen:
- 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)
Anmerkungen:
- 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
- OHS and Radioanalytical Chemistry
- Dosimetry
Anmerkungen:
- Special steps need to be taken when working with radioactive sources
e.g. personal radiation monitors for monitoring dose
- Thermoluminescent dosemeter (TLD)
Anmerkungen:
- 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
Anmerkungen:
- Different units are used because the effects depend on the type of radiation, type of material and other circumstances
- Roentgen (ion dose)
Anmerkungen:
- 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)
Anmerkungen:
- 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)
Anmerkungen:
- SI unit for energy dose
1 Gy = 1 J/kg = 100 rad
- REM (equivalent dose)
Anmerkungen:
- 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)
Anmerkungen:
- 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