Nuclear Radiation Detectors
Radiochemists detect the presence of radionuclides in a sample by identifying either their emitted radiation or atomic mass. Radiation detection exploits the different interaction of different radiation with matter to develop specific radiation detectors. The energy of the radiation emitted by every radionuclide is characteristic of the nuclide itself, so it can be used for its identification. This is the basic principle of gamma and alpha spectrometry, the most used techniques for radionuclide identification. For example, gamma spectrometry can be used even if the samples contain tens of different radionuclides, provided their emissions do not overlap. Another characteristic property of each radionuclide that supports its identification is the half-life.
Quantitative analysis can be carried out by determining the count rate R, the number of decays detected per unit time, obtained with some detection system. By knowing the detection efficiency Ɛ of the detection system, which is a measure of how many of all decay events in the sample are detected, the activity A can be calculated by division of the count rate with detection efficiency:
Another method of quantitative analysis is by the comparison of the count rate of a standard sample with a known activity with that of the unknown sample, both containing the same radionuclides, in the same conditions (sample characteristics, geometry, …).
Until the ‘60s, it was necessary to separate each radionuclide before measuring them. The progress of gamma spectroscopy and the introduction of new detectors made it possible to simultaneously measure different gamma-emitting radionuclides. Beta- and alpha-emitting radionuclides still require
separation.
Beta nuclides emit particles (electrons or positrons) with a continuous energy spectrum, so they must be separated from other beta- or alpha- emitting nuclides to avoid interference and overlap during detection. The measurement technique used is either Liquid Scintillation Counting (Figure 1) or Proportional counting.
Figure 1 - Liquid Scintillation Counting system.
Some substances such as phosphors, when irradiated, can emit some of the absorbed energy in the form of light by a process called fluorescence. These are named scintillating agents: when they are mixed with an organic solvent, they make up a liquid scintillation cocktail. Nowadays, most common scintillating agents are aromatic molecules: their electronic states are excited by the radiation energy transferred from beta or alpha particles; when they return to their ground state they emit visible photons that can be then detected. To perform LSC, a liquid sample containing the radionuclide is put inside a vial and mixed with a liquid scintillation cocktail. Beta radiation emitted by nuclides dissolved in the cocktail induce the emission of light that is turned into electric pulses by dedicated electronics. The magnitude of the electric pulse obtained is proportional to the initial energy of the emitted particle: each keV of the particle energy creates 5–7 light photons. The
light signal is detected and multiplied by the system in such a way that the final electric pulse amplitude is proportional to the initial energy of the beta particles.
Proportional counting is another technique that can be used to determine beta emitters, such as Sr-90, Pb-210 or gross beta activity. Generally, detectors used in proportional counting have a cylindrical geometry. During operation, a voltage is applied between the inner cylindrical surface, which acts as the cathode, and an axial wire, the anode. One face of the cylinder is closed with a thin Mylar foil to enable entry of beta or alpha particles into the active volume of the detector, which is filled with a gas mixture of 90% argon and 10% methane, known as P10. This particular gas composition has been optimized in order to guarantee optimal performance and stability of the detector. Once particles enter the detector, they ionize the gas: electrons are stripped from their molecules and they travel towards the anode, while resulting positive charges do the opposite, and start moving towards the cathode. A voltage pulse is recorded due to this charge movement; most importantly, its amplitude is proportional to the energy of the primary impinging particle. Such events are counted to obtain the count rate.
Proportional counters have usually lower background, while they suffer more severely on self-absorption issues, which is loss of energy of particles in the sample: this drawback does not exist in LSC, since the radionuclide is intimately mixed with the scintillating molecules. Consequently, LSC is preferred for low-energy beta emitters, such as H-3 and C-14. In addition, LSC can provide additional information on radionuclides in the sample as it can display their energy spectrum, while proportional counter can discriminate only between alpha and beta events.
Radionuclides that decay by Electron Capture do not emit easily observable radiation: their decay can be measured thanks to the emissions of characteristic X-rays and Auger electrons as a consequence of the electronic rearrangement of the daughter nuclide. Since the energy of characteristic X-rays is, at maximum, up to tens of keV, it is necessary to separate them from the matrix to prevent self-absorption and to improve their measurement.
Also alpha-emitting radionuclides need the separation from the matrix, since it causes self-absorption of the alpha radiation and thus a broadening of the alpha peak. Separation from other alpha-emitting radionuclides is also required, in order to prevent the overlapping of their respective spectra. They can be measured using Silicon Semiconductor detectors or LSC. Modern LSC enables the discrimination of alpha from beta emitters since they have very different pulse shape/length, but, due to a low resolution, it is used only to measure a total activity (gross-alpha and gross-beta determination).
Figure 2 - Alpha spectrometer (left) and silicon detector.
As previously stated, gamma emitting radionuclides usually do not require separation, and it is not uncommon to perform multiple identifications simultaneously with gamma spectrometry. Some gamma radionuclides however, especially if present in high concentrations, may still require separation. This is the case for example of Cs-137 , originating from spent nuclear fuel: it emits gamma rays of high energy, which could prevent the simultaneous discrimination of gamma emitters with lower energies by effectively covering their emission.
One of the detectors used in gamma spectrometry is the solid scintillation counter (Figure 3). This is an inorganic crystal that, similarly to scintillating cocktails, after absorbing the energy of gamma radiation produces light emission when its excitation states are relaxed. One of the most common detectors is thallium-doped sodium iodide NaI(Tl) counter. This consists in NaI crystals containing a small fraction of thallium ions, that act as luminescent centers. The major advantages of scintillation crystals are their good detection efficiency and the capability of shaping them with different geometries: for example, crystals with a hole in the middle to accommodate the sample allow a very high detection efficiency. However, if a high energy resolution is required, it is necessary to use germanium-based detectors, such as the high-purity germanium (HPGe) detector (Figure 3). Germanium detectors are semiconductor diodes having a biased p-i-n structure, in which the intrinsic (i) region is sensitive to ionizing radiation. When photons interact within the sensitive volume of the detector, charge carriers (holes and electrons) are produced and are swept by the electric field to the p and n electrodes. These charges, which are proportional to the energy deposited in the detector by the incoming photon, are converted into a measurable voltage pulse by an integral charge-sensitive preamplifier. Since germanium has a relatively low band gap, these detectors must be cooled by liquid nitrogen, otherwise leakage-current-induced noise destroys the energy resolution of the detector.
Figure 3 - Solid scintillation counter (left) and HPGe semiconductor (right).
Long-lived radionuclides can be measured by mass spectrometry (Figure 4), and they need to
be separated from other elements that could interfere, like neighbor isotopes with higher abundance or isobaric interferences.
Figure 3 - Solid scintillation counter (left) and HPGe semiconductor (right).
Inorganic mass spectrometry is broadly used, also outside the field of radiochemistry, to determine isotopic concentration of elements when their concentration is extremely low, in the trace and ultra-trace ranges, lower than ppm (parts per million). It allows to measure the number of atoms with a specific mass number, thus differentiating different isotopes or, equivalently, radioisotopes. In this case, the instrument measures the number of atoms N with a specific atomic mass and, by knowing the corresponding decay constant λ, the activity A can be calculated.
Mass spectrometry becomes more sensitive than radiometric methods when the half-life of radionuclides of interest is very long, which means that there are not so many radioactive decays occurring for the same number of atoms (or mass) compared to radionuclides with shorter half-lives.
Radiometric and mass spectrometric detection of radionuclides are important tools for radiochemists. These enable them to determine which specific radionuclide and how much of it is in the sample. This information is crucial in the assessment of risk to population and environment due to radioactivity originating from anthropogenic activities.