Detectors with photomultiplier tubes exist
basically in two versions: a demountable version called A-style and an integrated one
called B-style.
Assemblies
with integrated photomultiplier tubes: B-styles
FEATURES
Scintillation detectors with integrated photomultiplier tube(s) consist of
a scintillation crystal, coupled directly to a photomultiplier tube with a slightly
flexible, high refractive index optical coupling medium. The crystal and PMT are
hermetically sealed (gas tight) in a lighttight housing with an aluminium or beryllium
entrance window. Detectors have an internal m-magnetic shielding or a solid m-metal
housing (special option) around the PMT and can be supplied as plug-in units to connect to
a Voltage Divider (VD) or with a built-in VD (see section 8).
The advantages of this construction are :
- Permanent light sealing
- Improved energy resolution due to direct coupling to the PMT
- Guaranteed energy resolution
- Simplification of detector design by eliminating support hardware to maintain contact
between crystal and PMT
- No problems with degradation of optical contact between crystal and PMT.
APPLICATION
One of the most widely used scintillation detectors world-wide is the 76 x 76 mm
NaI(Tl) detector (SCIONIX type 76 B 76 / 3) which is the scintillation detector standard
for general gamma spectroscopy having excellent efficiency and energy resolution.
For low background (LB) applications, B-style assemblies can be supplied in low
background steel or electrolytic copper housings. All detector components can be selected
on their lowest possible radioactive background. In LB units, quartz or undoped NaI light
guides are used to reduce the background from the PMT (see
section 5.7).
On the next pages some standard B-style detectors are presented as well as an example
of an assembly equipped with an axial well (BP-style).
Beside the above shown, many other PMT diameters are possible. Standard PMT diameters
are : 13, 19, 25, 28, 38, 51,76, 90, and 127 mm.. |