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NaI(pure),
NaI(Tl) and POLYSCIN® NaI(Tl)
Properties of NaI(Tl) and POLYSCIN® NaI(Tl)
single crystal NaI(Tl) Of all available scintillators, NaI(Tl) is the most extensively used material. NaI(Tl) has a very high luminescence (scintillation) efficiency and is available in single crystal or polycrystalline forms in a wide variety of sizes and geometries. The material exhibits no significant self absorption of the scintillation light. Fig. 1 shows the emission spectrum of NaI(Tl). The emission maximum is well matched to the sensitivity curve of photomultiplier tubes (PMTs) with bialkali photocathodes. NaI(Tl) produces the highest signal per amount of radiation absorbed in the crystal of all presently known scintillators. Under optimum conditions, an average of 1 x 104 photoelectrons are produced per MeV γ-rays. The relation between the scintillation intensity and the temperature can be found in Fig.2 for several alkall halide scintillators. The shape of the curves for CsI (TI) and CsI (Na) are so close that they are represented by the same curve. NaI(Tl) exhibits several decay time constants. The primary single exponential decay constant is 0.25µs at room temperature. The relation between the effective decay time and the temperature is presented in Fig. 3. (Data from IEEE NSS NS-30, 380 (1983)) NaI(Tl) is susceptible to radiation damage, i.e. prolonged exposure to intense radiation degrades the scintillation performance. Radiation damage has been observed above levels of 1 Gray (102 rad). The crystal should not be exposed to ultraviolet radiation from flourescent lamps or sunlight. Scintillation crystals of NaI(Tl) may be grown with a potassium content of less than 0.5 ppm for low background applications. NaI(Tl) crystals are widely used for radiation detection: in nuclear medicine, for environmental monitoring, in nuclear and high energy physics, and in many other applications.
POLYSCIN® NaI(Tl) POLYSCIN® NaI(|Tl) crystals are widely recognized as suitable alternatives to single crystal scintillators in many applications where thermal and mechanical shock are encountered. This crystal offers ruggedness combined with a scintillation performance identical to single crystal NaI(Tl). Current applications include aerospace research, oil well logging, geophysical survey and radiation environmental monitoring. The polycrystalline structure of POLYSCIN® NaI(Tl) is derived from a unique manufacturing process in which single crystal ingots are recrystallized under heat and pressure. The resulting material may be characterized as a polycrystalline material with a randomly oriented mosaic lattice structure. The characteristic improves mechanical strength but has no effect on the scintillation performance since the material is optically equivalent to single crystal NaI(Tl). Any fractures produced by thermal or mechanical shock in POLYSCIN® NaI(Tl) are normally blocked or confined to the small local volumes called grains. Because the cleavage planes of the grains are randomly oriented, it is unlikely that a small fracture would propagate across the grain boundaries. This makes POLYSCIN® the material of choice where ruggedness is important, such as well logging, MWD and aerospace applications. In contrast, single crystals will cleave along <100> planes under similar shock conditions. In a detector assembly fabricated from single crystal material, even a small crack may propagate along the entire crystal, interfering with the light collection and degrading the pulse height resolution. Another feature of POLYSCIN® NaI(Tl) is the possibility to manufacture complicated detector geometries directly without extensive machining. Special geometries currently offered include hexagonal detectors, square and rectangular assemblies and large diameter slabs. POLYSCIN® NaI(Tl) has been used to construct crystals of 10 x 10 x 100 cm3 (4" x 4" x 40") and large flat crystals of 60cm x 90cm area (24" x 36"). The use of long continuous bars eliminates the discontinuity encountered by interfacing two or more shorter crystals, improving the scintillation characteristics. Long crystals may be used as position sensitive detectors while at the same time providing spectroscopic data. Detectors of this kind can be fabricated in steel or low background aluminum housings.
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