Page 541 - Instrumentation Reference Book 3E

524 Nuclear instrumentation technology

vary enormously, from the small NaI (Tl) (this is
the way of writing "sodium iodide with thallium
additive") crystal used in medical probes up to
tanks of liquid scintillator of thousands of cubic
meters used in cosmic ray research.
The selection of a suitable material-some of
whose characteristics are given in Tables 22.5 and
22.64epends on a number of competing factors.
No material is perfect so comproinises must be
made in their selection for particular purposes.

22.2.2.1 Inorgaizicscintillators
NaI (TI) is still after many years the best gamma-
and X-ray detector actually available, yet it is
very hygroscopic and must be completely sealed
from moisture. It is sensitive to shock, except in
0.1 0.3 0.5 0.9 2 4 6 8 10
the new form of extruded NaI (Tl) called Poly-
Maximum beta energy (MeV) scin, developed by Harshaw, and it is expensive,
Figure 22.3 Transmission of thin windows. especially in large sizes. Its light output is, in
general, proportional to the energy of the photon
converter (generally a photomultiplier) and the absorbed, so that the pulse height is a measure of
electronic amplifier; we consider the wide variety the energy of the incident photon and. when cali-
of each of these items which are available today. brated with photons of known energy, the instru-
The scintillator can consist of a single crystal of ment can be used as a spectrometer. The decay
organic or inorganic material; a plastic fluor, an lifetime of NaI (Tl) is relatively slow, being about
activated glass, or a liquid. Shapes and sizes can 230 ns, although NaI without any thallium when

Table 22.4 Transmission of thin windows
Nuclide Max. energy Percentage transinission fov window thickness of
E,,,, (MeV)
30 mg/cm2 20 mgtcm' 7 mg/cm2 3 mdcm'
'4c 0.15 0.01 0.24 12 40
32P 1.69 72 80.3 92 96

Table 22.5 Inorganic scintillator materials
Material' Density Refractive Light Decuji Wuwlength Operuting Hygroscopic
(g/cm3) index outpui constunt ofn~u.~i~~~i~n~ teinyerutirre
(n) (% anthracene) (s) emission (nm) ("C)

NaI (Tl) 3.67 1.775 230 0.23 x 413 Room Yes
NaI (pure) 3.67 1.775 440 0.06 x 303 ~ Yes
CsI (TI) 4.51 1.788 95 1.1 x 10-6 580 Room No
CsI (TI) 4.51 1.787 150-190 0.65 x 420 Room Yes
CsI (pure) 4.51 1.788 500 0.6 x 400 ~ No
CaFz (EU) 3.17 1.443 110 1 x 10-6 435 Room No
LiI (EU) 4.06 1.955 75 1.2 x 10-6 475 Room Yes
CaWO, 6.1 1.92 36 6 x 430 Room No
ZnS (Ad 4.09 2.356 300 0.2 x 10-6 450 Room No
ZnO (Ga) 5.61 2.02 90 0.4 x 10-9 385 Room No
CdWO4 7.90 0.9-20 x 530 Room No
Bi4Ge;Olz 7.13 2.15 0.3 x 480 Room No
CsF 4.64 1.48 40 5 x lo-" 390 Room No
1. The deliberately added impurity is given in parentheses.
2. Light output is expressed as a percentage of that of a standard crystal of anthracene used iii the same geometry.