Gamma spectroscopy
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Gamma spectroscopy is a radiochemical measurement method. While a Geiger counter determines only the count rate, a gamma spectrometer will determine the energy and the count rate of gamma rays emitted by radioactive substances.
Gamma spectroscopy is an extremely important method. In investigating a radioactive source, one generally finds gamma lines of various energies and intensities; the result is called a gamma energy spectrum. A detailed analysis of this spectrum is used to determine the identity and quantity of gamma emitters present in the source. The gamma spectrum is characteristic of the gamma emitting nuclides contained in the source, just as in optical spectroscopy, the optical spectrum is characteristic of the atoms and molecules contained in the probe.
The equipment used in gamma spectroscopy includes an energy sensitive particle detector, a pulse sorter (multichannel analyzer), and associated amplifiers and data readout devices. The detector is often a sodium iodide (NaI) scintillation counter or a high purity germanium detector.
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[edit] The Sodium Iodide Spectrometer
In the scintillator, a transparent sodium iodide single crystal, the gamma quantum produces a photo electron via photoelectric effect, or a Compton electron via Compton effect, or an electron-positron pair via pair creation. The electron energy deposited within the scintillator produces a light flash whose intensity is proportional to the deposited energy. The photomultiplier on which the scintillator is mounted then produces a measurable voltage pulse whose size is again proportional to the energy of the original electron. A multichannel analyzer then measures the height of every pulse and classifies all the pulses into "channels" according to their height. The resulting graph of the number of pulses versus channel number is then called the gamma spectrum. As an example, the gamma spectrum of the isotope 137Cs is shown, which emits a single gamma line of 662 keV.
The spectrum was measured using a NaI-crystal on a photomultiplier, an amplifier and a multichannel analyzer and plotted on an x-y-plotter. The figure shows the number of counts (within the measuring period) versus channel number. The spectrum shows the following peaks (from left to right):
- low energy x radiation (due to Internal conversion of the gamma ray)
- a backscatter peak which sits on top of the Compton distribution
- a photopeak (Full Energy Peak) at an energy of 662 keV
The Compton distribution is a continuous distribution which goes up to channel 150 in this figure. It is due to primary gamma rays undergoing Compton effect within the crystal: depending on the scattering angle, the Compton electrons have different energies and hence produce pulses of different heights.
If many gamma rays are present in a spectrum, Compton distributions are a disturbing nuisance. In order to reduce them, one can use an anticoincidence shield (see Compton suppression). This is especially useful for small Ge(Li) detectors (see below).
The next figure shows another example: the gamma spectrum of the isotope 60Co with two gamma transitions with 1.17 and 1.33 MeV, respectively, again measured by a NaI counter. (For the decay scheme of 60Co, see Decay scheme). The two gamma lines can be seen well separated; the rise to the left of channel 200 probably indicates a strong background that has not been subtracted. At channel 150, one can see a backscatter peak (like in the figure above). The multichannel spectrum was plotted by means of an x-y plotter.
The distance between the two gamma lines, or the width of one line, is a measure of the energy resolution of the NaI spectrometer. Depending on this resolution (which is analogous to Resolving power in optical spectroscopy, one will be able to separate two gamma lines which are close to each other.
[edit] The Ge(Li) Spectrometer
A much better energy resolution can be obtained by using a Ge(Li)-Detektor. This consists of a single crystal of Germanium, into which lithium ions have been made to drift, in order to produce an intrinsic region which functions like an ionization chamber. The figure shows a narrow region of the 60Co spectrum: in the upper part, the two peaks (1.17 and 1.33 MeV) of a NaI spectrum, in the lower part the same two lines, as measured by a Ge(Li) spectrometer with an obviously much higher resolution. The lower spectrum also shows the peak of an electronic pulser used for calibration purposes.
A limitation of the Ge(Li) spectrometer lies in the fact that Ge crystals cannot normally be grown as large as NaI crystals. Also, the Ge(Li) crystals must be permanently kept at the temperature of liquid nitrogen, to prevent the lithium from drifting back out.
[edit] Calibration, background
If the spectrometer is used to investigate an unknown sample, its energy scale must be calibrated first. This is done using the peaks of a known source (like 137Cs or 60Co shown above). Since the channel number is proportional to energy, the channel scale can then be converted to an energy scale. If one knows the size of the detector crystal, one can also perform an intensity calibration, so that not only the energies but also the intensities of an unknown source (or the amount of a certain isotope in the source) can be determined.
Because some radioactivity is present everywhere, one must also determine the "background", i.e., the spectrum when no source is present. The background must then be subtracted from the actual measurement. By using lead absorbers around the apparatus, one can reduce the background.
[edit] Use in the Litvinenko case?
In the Alexander Litvinenko case several different different isotopes could have been suspected. In the graph the gamma spectra (photon energy in keV against intensity) for three isotopes (thallium-201, thallium-202 and polonium-210) are shown, assuming an ideally good resolution.
Note: since 210Po is essentially an alpha emitter and emits only very few gamma rays, it is more likely that alpha spectroscopy should have been used to identify this isotope, see Polonium.
[edit] See also
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