Sodium Iodide crystals have a very high luminescence (scintillation) efficiency and are available in a wide variety of sizes and geometries that makes it the most widely used scintillator. Thallium-doped Sodium Iodide produces one of the highest signals in a PMT per amount of radiation absorbed. It is second only to our Lanthanum Bromide scintillator. Under optimum conditions, an average of 1x 104 photoelectrons are produced per MeV gamma rays.
The emission maximum is well matched to the sensitivity curve of photomultiplier tubes (PMTs) with bialkali photocathodes.
NaI(Tl) exhibits several decay time constant components. The primary single exponential decay constant is 250ns at room temperature. As the temperature increases, the longer time constant components decrease in intensity and the 1 ms and 12 ms response curves become identical.
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 fluorescent lamps or sunlight.
Scintillation crystals of NaI(Tl) are routinely grown with a potassium content of less than 0.5 ppm, and are appropriate for low background applications. NaI(Tl) crystals are widely used for radiation detection: in nuclear medicine, for environmental monitoring, in nuclear physics, aerial survey, well logging and in many other applications.
Polyscin® NaI(Tl) scintillation crystals are widely recognized as suitable alternatives to single-crystal scintillators in many applications where thermal and mechanical shock are encountered, such as oil and gas exploration. This crystal offers ruggedness combined with a scintillation performance identical to single-crystal NaI(Tl).
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 randomly oriented crystal grains in a mosaic structure. The density of NaI is not changed in the process. 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® NaI(Tl) the material of choice where ruggedness is important, such as well logging, MWD and aerospace applications.
In contrast, single crystals can cleave along 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.
|Melting point [K]||924|
|Thermal expansion coefficient [C-1]||47.7 x 10-6|
|Wavelength of emission max. [nm]||415|
|Refractive index @ emission max||1.85|
|Primary decay time [ns]||250|
|Light yield [photons/keVγ||38|
|Temperature coefficient of light yield||0.3%C-1|
NaI(Tl) is hygroscopic and must be hermetically packaged. The proper packaging and integration of scintillation crystals is a science combining advanced design and engineering skills with proven assembly techniques and materials to produce stable, high-resolution radiation detectors. Saint-Gobain Crystals offers a variety of detector designs to fulfill most radiation counting applications: a packaged scintillator, a scintillator integrated (permanent or demountable) with a light-sensing device [such as a photomultiplier tube (PMT) or silicon photomultiplier (SiPM)], and a detector with low energy entrance window. Each of those standard detector configurations can be designed with one or more of the various options listed plus integrated electronics.
- Cylindrical: 1" diameter x 1" thick, 2" diameter x 2" thick, 3" diameter x 3" thick, 5" diameter X 5" thick
- Rectangular: 2"x4"x16", 4"x4"x16
- Camera plates: 24" x 18" x 0.375" thick
- Pixellated arrays for use with PMTs or PSPMTs