a design study for a cobra upgrade to - Institut für Kern- und ...

40 4 Scintillation detectors
Figure 4.9: Emission spectrum of CsI:Tl, NaI:Tl, BGO and CWO [36]
and large atomic number. It has very little self-absorption and its mechanical
and chemical properties make it easy to handle. However its
low light yield of just 10-20 % compared to NaI:Tl leads to a resolution
of about a factor two worse than NaI:Tl. It also has a high refraction
index of 2.15, making the light collection more difficult. BGO is suitable
if high gamma ray detection efficiency is more important than a
good energy resolution. The primary decay component is 300 ns and
an additional fast component (60 ns) has a fraction of 10 %. The emission
peak wavelength lies at 505 nm. The light yield increases at low
temperatures, so it has a limited use at temperatures higher than room
temperature. BGO is two to three times more expensive than NaI:Tl
[26, 27, 28, 29].
CADMIUM TUNGSTATE (CdWO4 or CWO) has been known as a scintillator
since about 1950, but has been produced only since 1990 with
interesting crystal size and good optical properties. The light yield is
40 % of NaI:Tl and the emission peak wavelength is at 495 nm. The
density and the effective atomic number are even higher than these of
BGO. A pulse shape discrimination is also possible. The use of CWO is,
due to its long decay time of 1.1 µs (40 %) and 14.5 µs (60 %) limited for
low count rates. With an refraction index of 2.3 the light collection and
coupling to readout devices are also difficult. CWO is more expensive
than BGO [26, 27, 28, 29].