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Scintillators as X-ray Detectors

Scintillators work by converting X-ray energy into visible light. We distinguish scintillators from phosphors, at least in X-ray astronomy, by defining bulk crystalline materials such as NaI and CsI as scintillators, and thin granular layers of rare earth oxysulphides as phosphors.

The alkali halides NaI and CsI, activated by a small amount of either thallium or sodium impurity, have been the scintillators of choice so far in X-ray astronomy. This has been true because these materials can be made into large area crystals, have good X-ray stopping power, and are efficient light producers. NaI(Tl) was first produced in the early 1950s, while CsI(Na) came along in the mid-1960s. Of the two materials, CsI(Na) is mechanically more robust and more immune to the ravages of moisture.

Other materials, such as plastics and the higher-Z bismuth germanate (BGO), have well defined roles in X-ray scintillators. BGO is still too difficult to make with large areas, and so, is used for small area detectors only and sometimes for anti-coincidence shielding. Plastics, thanks to their low efficiency in detecting low energy X-rays, are used almost exclusively as anti-coincidence shields.

As for scintillating gases, light production by activated alkali halides results from a complex sequence of excitations and de-excitations. The role of the impurity is to produce luminescent centers energetically between the valence and conduction bands of the host crystal. Below 100 keV, X-ray photon interactions for both NaI and CsI are predominately through the photoelectric effect. The energy conversion efficiency (or fraction of the X-ray energy which appears as scintillation light) for NaI(Tl) is 0.12, for CsI(Na) is 0.10, and for CsI(Tl) is 0.05. These values are true at 20 degrees C, and are all highly temperature dependent.

The decay constant for the optical emission lies in the microsecond range for most inorganic scintillators, and in the nanosecond range for plastics. Thus, by surrounding an alkali halide crystal with a plastic shield, and observing the "phoswich" these two create with a single photomultiplier tube, scientists can use pulse shape discrimination to determine whether the energy loss occurred in the shield or the main detector. This is an excellent method of background discrimination.

For material thicknesses of 5 mm, for both NaI and CsI, the detection efficiency between 20-100 keV is essentially unity.

The energy resolution of a scintillation counter is determined primarily by photoelectron statistics, i.e. the variation in the number of electrons liberated from the PMT photocathode. If one assumes this variation is Poissonian, limits to the FWHM energy resolution of NaI(Tl) can be estimated as (Delta E)/E ~ 1.67/sqrt(E). In other words, not very good. It is clear that in today's world, scintillators such as NaI and CsI are useful only for their large collecting areas and their high quantum efficiency above 20 keV.

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