Gamma spectroscopy

Gamma spectroscopy is a radionuclidic 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. Most radioactive sources produce gamma rays of various energies and intensities. When these emissions are collected and analyzed with a gamma spectroscopy system, a gamma energy spectrum can be produced. A detailed analysis of this spectrum is typically 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 radiation detector, a pulse sorter (multichannel analyzer), and associated amplifiers and data readout devices. The most common detectors include sodium iodide (NaI) scintillation counter and high purity germanium detectors.

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System Components

A gamma spectroscopy system consists of a detector, electronics to collect and process the signals produced by the detector, and a computer with processing software to generate the spectrum and display and store it for analysis.

Gamma spectroscopy detectors are passive materials that wait for a gamma interaction to occur in the detector volume. The most important interaction mechanisms are the photoelectric effect, the Compton effect, or pair production. The photoelectric effect is preferred, as it absorbs all of the energy of the incident gamma ray. Full energy absorption is also possible when a series of these interaction mechanisms take place within the detector volume. When a gamma ray undergoes a Compton interaction or Pair Production, and a portion of the energy escapes from the detector volume without being absorbed, the background rate in the spectrum is increased by one count. This count will appear in a channel below the channel that corresponds to the full energy of the gamma ray. Larger detector volumes reduce this effect.

The voltage pulse produced by the detector (or by the photomultiplier in a scintillation detector) is shaped by a multichannel analyzer (MCA). The multichannel analyzer takes the very small voltage signal produced by the detector, reshapes it into a Gaussian or trapezoidal shape, and converts it into a digital signal. In some systems, the analog to digital conversion is performed before the peak is reshaped. The analog to digital converter (ADC) also sorts the pulses by their height. ADCs have specific numbers of "bins" to sort the pulses into; these are the "channels" in the spectrum. The number of channels can be changed in most modern gamma spectroscopy system by changing a software or hardware setting. The number of bins is a power of two. Common values include 512, 1024, 2048, 4096, 8192, or 16384 channels. The choice of number of channels depends on the resolution of the system and the energy range being studied.

The MCA output is sent to a computer which stores, displays, and analyzes the data. A variety of software packages are available from several manufacturers, and generally include spectrum analysis tools such as energy calibration, peak area and net area calculation, and resolution calculation.

Other components, such as rate meters and peak position stabilizers, may also be included.

Detector Performance

Gamma spectroscopy systems are selected to take advantage of several performance characteristics. Two of the most important include detector resolution and detector efficiency.

Detector Resolution

Gamma rays detected in a spectroscopic system produce peaks in the spectrum. These peaks can also be called lines, by analogy to optical spectroscopy. The width of the peaks is determined by the resolution of the detector, a very important characteristic of gamma spectroscopic detectors. Resolution is analogous to Resolving power in optical spectroscopy. High resolution enables the spectroscopist to separate two gamma lines which are close to each other. Gamma spectroscopy systems are designed and adjusted to produce symmetrical peaks of the best possible resolution. The peak shape is usually a Gaussian distribution. In most spectra, the horizontal position of the peak is determined by the gamma ray's energy, and the area of the peak is determined by the intensity of the gamma ray and the efficiency of the detector.

The most common figure used to express detector resolution is Full Width at Half Maximum (FWHM). This is the width of the gamma ray peak at half of the highest point on the peak distribution. Resolution figures should be given with reference to specified gamma ray energies. Resolution can be expressed in absolute terms (eV or keV), or relative terms. For example, a NaI detector may have a FWHM of 9.15 keV at 122 keV, and 82.75 keV at 662 keV. These are resolution values expressed in absolute terms. To express the resolution in relative terms, the FWHM in eV or keV are divided by the energy of the gamma ray and multiplied by 100. In these examples, the resolution is 7.5% at 122 keV, and 12.5% at 662 keV. A Germanium detector might give resolution of 560 eV at 122 keV, a relative resolution of 0.46%.

Detector Efficiency

Not all gamma rays that are emitted by the source and pass through the detector will produce a count in the system. The probability that an emitted gamma ray will interact with the detector and produce a count is the efficiency of the detector. High efficiency detectors produce spectra in less time than low efficiency detectors. In general, larger detectors have higher efficiency than smaller detectors, although the shielding properties of the detector material are also an important factor. Detector efficiency is measured by taking a spectrum from a source of known activity, and comparing the count rates in each peak to the count rates expected from the known intensities of each gamma ray.

Efficiency, like resolution, can also be expressed in absolute or relative terms. The same units are used, percentages, so the spectroscopist must take care to determine which kind of efficiency is being given for the detector. Absolute efficiency values give the probability that a gamma ray of a specified energy passing through the detector will interact with the crystal and be detected. Relative efficiency values are often used for Germanium detectors, and compare the efficiency of the detector at 1332 keV to that of a 3" by 3" NaI detector (1.2E-3 cps/Bq). Relative efficiency values greater than 100% can therefore be encountered when working with very large Germanium detectors.

The energy of the gamma rays being detected is an important factor in the efficiency of the detector. By plotting the efficiency at various energies, an efficiency curve can be obtained. This curve can then be used to determine the efficiency of the detector at energies different from those used to obtain the curve.

Scintillation Detectors

Scintillation detectors use crystals that emit light when gamma rays interact with the atoms in the crystals. The intensity of the light produced is proportional to the energy deposited in the crystal by the gamma ray. The mechanism is similar to that in a Thermoluminescent Dosimeter. The detectors are joined to photomultipliers that convert the light into electrons and amplify the electrical signal provided by the electrons. Because the photomultipliers are also sensitive to ambient light, scintillators are packaged in light-tight coverings. Common scintillators include Thallium-doped Sodium Iodide (NaI(Tl)), often simply called Sodium Iodide (NaI) detectors, and Bismuth germanate (BGO) detectors.

Scintillation detectors also have other uses, such as alpha- and beta-detectors.

Sodium Iodide {NaI(Tl)} Detectors

Thallium-doped Sodium iodide has two principal advantages; it can be produced in large crystals, giving good efficiency, and it produces intense bursts of light compared to other spectroscopic scintillators. It is also convenient to use, making it popular for field work such as identification of unknown materials for law enforcement purposes. An example of a NaI spectrum is the gamma spectrum of the isotope ¹³⁷Cs shown in the illustration. ¹³⁷Cs emits a single gamma line of 662 keV. It should be noted that the 662 keV line shown is actually produced by ¹³⁷Ba_m, the decay product of ¹³⁷Cs, which is in secular equilibrium with ¹³⁷Cs.

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 at the low energy end 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 rays 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.

Sodium Iodide systems, like all scintillator systems, are sensitive to temperature changes. Changes in the operating temperature caused by changes in environmental temperature will shift the spectrum on the horizontal axis. Peak shifts of tens of channels or more are commonly observed. Spectrum stabilizers are used to prevent such shifts.

Because of their poor resolution, NaI detectors are not suitable for the identification of complicated mixtures of gamma ray - producing materials. Such analyses require higher resolution.

Semiconductor Detectors

Semiconductor detectors are also called solid-state detectors. These detectors are fundamentally different from Scintillation detectors. In semiconductor detectors, an electric field is applied to the detector volume. An electron in the semiconductor is fixed in its Valence band in the crystal until a gamma-ray interaction gives the electron enough energy to move to the Conduction band. Electrons in the conduction band can respond to the electric field in the detector, and therefore move to the positive contact that is creating the electrical field. The gap where the electron used to be is called a "hole" and is filled by an adjacent electron. This shuffling of holes effectively moves a positive charge to the negative contact. The arrival of the electron at the positive contact and the hole at the negative contact produces the electrical signal that is sent to the preamplifier, the MCA, and on through the system for analysis. The movement of electrons and holes in a solid state detector is very similar to the movement of ions within the sensitive volume of gas-filled detectors such as ionization chambers.

Common semiconductor detectors include Germanium, Cadmium Telluride, and Cadmium Zinc Telluride.

Germanium Detectors produce much higher energy resolution than NaI, as shown in the discussion of resolution above. Germanium detectors produce the highest resolution commonly available today.

Calibration, background

If the spectrometer is used to identify a sample of unknown composition, 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 the size of the detector crystal is known, 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.

See also

• Alpha-particle spectroscopy
• Liquid scintillation counting
• Mass spectrometry
• X-ray spectroscopy

References

• Gilmore G, Hemingway J. Practical Gamma-Ray Spectrometry. John Wiley & Sons, Chichester: 1995, ISBN 0-471-95150-1.
• Knoll G, Radiation Detection and Measurement. John Wiley & Sons, Inc. NY:2000, ISBN 0-471-07338-5.