Thallium-sodium iodide, scintillation

With a resolution of 7-10% the sodium iodide scintillation detector offers inferior discrimination, but detectors are available with much higher detection efficiencies than are possible with high-resolution detectors. The detection of 7-rays is based on scintillations produced in the thallium-activated sodium iodide crystal and observed by a photomultiplier tube, operating efficiently at ambient temperature. 

Scintillation counters are constructed by coupling a suitable scintillation phosphor to a light-sensitive photomultiplier tube. Figure 25 illustrates an example of a scintillation counter using a thallium-activated sodium iodide crystal. 

As discussed above, the measurement of characteristic y rays is very similar to the methods used in EDXRF. Early studies used a scintillation counter, typically a crystal of sodium iodide containing a small amount of thallium (Tite 1972). y ray absorption by these counters produces visible light, which is converted into an electrical pulse using a photosensitive detector. More recently semiconductor detectors have been used, either a lithium drifted germanium crystal, or, more typically, a pure ( intrinsic )... 

To present a quantitative example of the energy conversions involved in scintillation detection, we trace the results of the interaction of a single 1.17-MeV 7 ray from 60Co with a thallium-activated sodium iodide crystal [Nal(TI)] ... 

The most versatile method for measuring radiation in the laboratory is the scintillation counter, in which a substance called a phosphor, often a sodium iodide crystal containing a small amount of thallium iodide, emits a flash of light when struck by radiation. The number of flashes is counted electronically and converted into an electrical signal. 

Alpha-particle detector Beta-particle detector Gamma-ray detector proportional counters silicon (Si) diode with spectrometer proportional counters Geiger-Muller counters liquid scintillation (LS) counters thallium-activated sodium iodide (Nal(Tl) detector with spectrometer germanium (Ge) detector with spectrometer... 

Scintillation counters usually consist of a sodium iodide crystal doped with 1% thallium. The incident X-ray photons cause the crystal to fluoresce producing a flash of light for every photon absorbed. The size of the light pulse is proportional to the energy of the photon and is measured by a photomultiplier. A deficiency associated with scintillation counters is that they do not provide as good energy resolution as proportional or solid state detectors. 

Gamma (y) Sodium iodide doped with thallium iodide are used pSlal(Tl)] thallium, which is present at 0.1-0.4% of sodium emits 420 nm scintillations. This phosphor is very efficient in absorbing y-radiation because of high atomic number of iodine and high density of sodium iodide. Sodium iodide is hermetically sealed. It is normally used in well counters. A typical well counter used in laboratory and that used in cameras in nuclear medicine are show in Figs. 3 and 4. 

Rutherford and his students used a screen coated with zinc sulfide to detect the arrival of alpha particles by the pinpoint scintillations of light they produce. That simple device has been developed into the modern scintillation counter. Instead of a ZnS screen, the modern scintillation counter uses a crystal of sodium iodide, in which a small fraction of the Na ions have been replaced by thallium (TH) ions. The crystal emits a pulse of light when it absorbs a beta particle or a gamma ray, and a photomultiplier tube detects and counts the light pulses. 

These are used on single counter four-circle diffractometers. The detector is often sodium iodide. The scintillator is used in conjunction with a photomultiplier tube. Thallium activated sodium iodide has an energy resolution of about 40% at lOkeV. Hence, the attractiveness of such a detector lies not only with its counting of individual photons but also its... 

For simple gamma counting, thallium-activated sodium iodide [Nal(TI)] scintillation detectors, which became commercially available in the early 1950s, continue to render excellent sen/ices. In spite of the introduction of many other scintillation materials, they remained preeminent. Nal(TI) detectors can be manufactured in various sizes and shapes... 

ZnS(Ag) is a traditional phosphor for a-detection while anthracene and stilbene can be used for /3-particle detection. For y-rays, sodium iodide with a small amount of thallium impurity, Nal(Tl), is the most common phosphor. CsI(Tl) is another often used scintillator because it can be formed to special shapes, e.g. thin sheets, much easier than Nal(Tl). Plastics with incorporated organic scintillators are often used in nuclear physics experiments because they produce short light pulses and can be made in various shapes. 

The most commonly used commercial scintillation detector has a thallium-doped sodium iodide crystal, Nal(Tl), as the scintillating material. A single crystal of Nal containing a small amount of T1 in the crystal lattice is coupled to a PMT, shown in Fig. 8.25. When an X-ray photon enters the crystal, it causes the interaction... 

The employment of solid scintillation detector materials like thallium-activated sodium iodide detectors for the measurement of gamma radiation... 

The most widely used modern scintillation detector consists of a transparent crystal of sodium iodide that has been activated by the introduction of 0.2% thallium iodide. Often, the crystal is shaped as a cylinder that is 3 to 4 in. in each dimension one of the plane surfaces then faces the cathode of a photomultiplier tube. As the incoming radiation passes through the crystal, its energy is first lost to the scintillator, this energy is subsequently released in the form of photons of fluorescence radiation. Several thousand photons with a wavelength of about 400 nm are produced by each primary particle or photon over a period of about 0.25 ps, which is the dead time. The dead time of a scintillation counter is thus significantly smaller than the dead time of a gas-filled detector. 

As it happens, the band gap of sodium iodide is large and photons emitted by de-excitation of electrons directly from the conduction band would be far outside of the visible range. This makes detection of the light difficult. Not only that, the bulk of the material absorbs the emitted photons before they reach the photomultiplier. Both problems are solved by using an activator. In the case of Nal, this would be thallium and for Csl it is thallium or sodium. The shorthand descriptions for these activated scintillators are Nal(Tl), CsI(Tl) and CsI(Na). 

SODIUM IODIDE (Nal) The most common material used in a scintillation detector for measuring gamma-rays. Often shown with its usual added trace of thallium as Nal (Tl). 

The efficiency of the GM counter for the detection of y radiation is low (about 1 %), owing to the indirect method of detection. A more efficient device is the scintillation counter, where the ionization and excitation due to the electrons produced by one or more of the processes outlined above is converted into fluorescent radiation from a suitable scintillating material. The commonest scintillator is a single crystal of sodium iodide with a small amount of thallium added to it. The scintillator is optically coupled to a... 

Scintillation methods offer the possibility of high-efficiency detectors with a more rapid time response than the BF3 counter. As mentioned in the previous section, the basis of the scintillation detector is the conversion, in a suitable crystal, such as thallium-activated sodium iodide, Nal(Tl), of the kinetic energy of the charged particle to light, which can be amplified by a photomultiplier tube to provide an electrical pulse. Again, the neutron has to interact to produce either a charged particle or a 7 ray, the latter of which may in turn interact to produce ionizing particles. 

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