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

50 4 Scintillation detectors
photocathode, fabricated by Electron Tubes Ltd.. The energy resolution
at 662 keV was around 4.2 % (RMS) and the resolution curve was
parametrised by σ = 0.58 · � E (keV ) + 0.0191 · E (keV ). The relative
standard derivation was evaluated with radioactive sources like 55 Fe,
241 Am, 57 Co, 137 Cs, 22 Na and 60 Co positioned in the centre of the crystal
and a 60 Co source with collimator positioned in 5 cm distances. In
this way the energy dependent and also position depending resolution
due to doping nonuniformity and different collection efficiency was regarded
[43].
The announced standard derivation σ determines the FWHM of any
Gaussian through the relation FWHM= 2 √ 2ln2σ ≈ 2.35σ.
The requirements for the COBRA upgrade are, as is discussed in the
previous chapter 3, a low radioactive contamination, a high energy resolution,
a moderate price and a high detection efficiency at compact
size.
The four scintillators NaI:Tl, CsI:Tl, BGO and CWO can provide some
of these properties.
All four scintillators can have low radioactive contamination, depending
on the used scintillator powder and crystal production process.
CWO contains the intrinsic 113 Cd, which has a low Q-value, BGO can
contain 207 Bi, which has a problematically high Q-value and CsI:Tl contains
the anthropogenic radioisotope 137 Cs. CWO and BGO have high
density and high Z, but a lower light yield than NaI:Tl and CsI:Tl. Especially
NaI:Tl has low density and Z, whereas the level of radioactive
contamination can be very small. The drawback of NaI:Tl is also the
emission spectrum, matching well with bialkali PMTs. CWO, BGO and
CsI:Tl can be read out with semiconductor readouts. The reached energy
resolution depends, beside the light yield of the scintillator and
the matching of the scintillation emission spectrum with the quantum
efficiency of the readout, also on the crystal and readout dimensions.
It is not possible to deduce the reachable energy resolution of a scintillator
of several hundred cm 3 from the resolution obtained with a
small scintillator. The required compact size of the planned COBRA upgrade
favours the scintillator readout with semiconductors, providing,
the detection efficiency and energy resolution are high enough and the
background contamination is small enough. The consideration, which
of these scintillators is most suitable, requires studies, which are presented
in the following chapter.