Detector sodium iodide, resolution

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. 

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... 

Bismuth germanate detectors have even better efficiency than sodium iodide detectors but their energy resolution is poor. Therefore, this type of detector material is used for special purposes but not for spectrometry. 

For analytical purposes particularly in the analysis of complex gamma spectra having many closely spaced peaks semiconductor detectors are used. They have excellent resolution but lower efficiency because of their lower atomic number. In addition, the volume of available semiconductor detectors is usually much smaller compared to sodium iodide detectors. Both of these limits result in much lower detection efficiency. 

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. 

One gamma-ray of 661.67keV energy completely absorbed in germanium can be expected to create 3.9 million electron-hole pairs. The same gamma-ray absorbed in sodium iodide will only give rise to 3.9 thousand photoelectrons. This thousand-fold difference in the number of charge carriers created is the most important reason for the poor resolution of sodium iodide detectors. 

The reason hes in the fact that the counts in a germanium spectrum, even though fewer in number, are concentrated within a few channels, whereas the counts in the sodium iodide spectrum are spread over many channels. This means that peaks are easier to detect in the germanium spectrum and ultimately the limit of measurement is lower. The huge difference in resolution between the two detectors cannot be compensated by the increase in the count rate from the sodium iodide detector. That being said, there are low count rate situations where the need for high-resolution spectrometry is not paramount and the higher cost of a semiconductor detector is not justified. 

Too many times the methodology that is devised for a particular analytical problem is just good enough . The analysis can be performed but the method is barely adequate in terms of resolution of peaks or in terms of sensitivity. And because it is just barely adequate, the method is not rugged. Often these issues can be solved by understanding how detectors operate, how ions are detected and choosing a better detector. For example, detection of iodide or nitrate in the presence of a salt (sodium chloride) matrix can be accompUshed with conductivity detection. But UV detection would be much better because iodide will absorb UV light and chloride will not. Because the detection is selective for iodide, the separation conditions can be optimized for rapid interference-free elution. 

---