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
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
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
lies in the microsecond range for most inorganic scintillators, and in the
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.
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
areas and their high quantum efficiency above 20 keV.