These resolution values are expressed in absolute terms. To express the resolution in relative terms, the FWHM in eV or MeV is divided by the energy of the gamma ray and usually shown as percentage. Using the preceding example, the resolution of the detector is 7. Not all gamma rays emitted by the source that 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 important factors. Detector efficiency is measured by comparing a spectrum from a source of known activity to the count rates in each peak to the count rates expected from the known intensities of each gamma ray.
Efficiency, like resolution, can be expressed in absolute or relative terms. The same units are used i. Absolute efficiency values represent the probability that a gamma ray of a specified energy passing through the detector will interact and be detected. Relative efficiency values greater than one hundred percent 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.
An efficiency curve can be obtained by plotting the efficiency at various energies. This curve can then be used to determine the efficiency of the detector at energies different from those used to obtain the curve.
High-purity germanium HPGe detectors typically have higher sensitivity. Scintillation detectors use crystals that emit light when gamma rays interact with the atoms in the crystals. The mechanism is similar to that of a thermoluminescent dosimeter. The detectors are joined to photomultipliers ; a photocathode converts the light into electrons; and then by using dynodes to generate electron cascades through delta ray production, the signal is amplified. Because photomultipliers are also sensitive to ambient light, scintillators are encased in light-tight coverings.
Scintillation detectors can also be used to detect alpha - and beta -radiation. NaI Tl is also convenient to use, making it popular for field applications such as the identification of unknown materials for law enforcement purposes. Electron hole recombination will emit light that can re-excite pure scintillation crystals; however, the thallium dopant in NaI Tl provides energy states within the band gap between the conduction and valence bands.
Following excitation in doped scintillation crystals, some electrons in the conduction band will migrate to the activator states; the downward transitions from the activator states will not re-excite the doped crystal, so the crystal is transparent to this radiation. An example of a NaI spectrum is the gamma spectrum of the caesium isotope Cs — see Figure 1.
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The keV line shown is actually produced by m Ba , the decay product of Cs , which is in secular equilibrium with Cs. The spectrum in Figure 1 was measured using a NaI-crystal on a photomultiplier, an amplifier, and a multichannel analyzer.
The figure shows the number of counts within the measuring period versus channel number. The spectrum indicates the following peaks from left to right :. The Compton distribution is a continuous distribution that is present up to channel in Figure 1. The distribution arises because of primary gamma rays undergoing Compton scattering within the crystal: Depending on the scattering angle, the Compton electrons have different energies and hence produce pulses in different energy channels. If many gamma rays are present in a spectrum, Compton distributions can present analysis challenges.
To reduce gamma rays, an anticoincidence shield can be used— see Compton suppression. Gamma ray reduction techniques are especially useful for small lithium -doped germanium Ge Li detectors.
The gamma spectrum shown in Figure 2 is of the cobalt isotope 60 Co , with two gamma rays with 1. See the decay scheme article for the decay scheme of cobalt The two gamma lines can be seen well-separated; the peak to the left of channel most likely indicates a strong background radiation source that has not been subtracted. A backscatter peak can be seen at channel , similar to the second peak in Figure 1.
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Sodium iodide systems, as with all scintillator systems, are sensitive to changes in temperature. 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. Such shifts can be prevented by using spectrum stabilizers.
Because of the poor resolution of NaI-based detectors, they are not suitable for the identification of complicated mixtures of gamma ray-producing materials.
Practical Gamma-ray Spectroscopy (2nd ed.)
Scenarios requiring such analyses require detectors with higher resolution. Semiconductor detectors , also called solid-state detectors, are fundamentally different from scintillation detectors: They rely on detection of the charge carriers electrons and holes generated in semiconductors by energy deposited by gamma ray photons. 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 provides 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 created by the moving electron 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-based detectors include germanium , cadmium telluride , and cadmium zinc telluride. Germanium detectors provide significantly improved energy resolution in comparison to sodium iodide detectors, as explained in the preceding discussion of resolution.
Germanium detectors produce the highest resolution commonly available today. However, a disadvantage is the requirement of cryogenic temperatures for the operation of germanium detectors, typically by cooling with liquid nitrogen. In a real detector setup, some photons can and will undergo one or potentially more Compton scattering processes e. This leads to a peak structure that can be seen in the above shown energy spectrum of Cs Figure 1, the first peak left of the Compton edge , the so-called backscatter peak.
The detailed shape of backscatter peak structure is influenced by many factors, such as the geometry of the experiment source geometry, relative position of source, shielding and detector or the type of the surrounding material giving rise to different ratios of the cross sections of Photo- and Compton-effect. For incident photon energies E larger than two times the rest mass of the electron 1. In a real detector i. Search WorldCat Find items in libraries near you.
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Practical Gamma-ray Spectroscopy, 2nd Edition
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