In this section some standard scintillation detector assemblies are presented. Sometimes the drawings are such that one can fill in the desired diameter and height of the scintillator to calculate the actual dimensions of the assembly. Only some examples are given; many other options are possible.
w/o Photomultiplier Tubes
C-styles and CP-styles
SCIONIX C-style detector assemblies basically consist of a scintillation crystal which is machined and mounted in a housing of aluminum, stainless steel or copper. In case of hygroscopic crystals the assembly is sealed by a glass or quartz window for optical coupling to a photomultiplier tube or other light detection device. C-style assemblies can be manufactured from low background materials (LB steel or copper).
The standard housing thickness is 0.8 mm. C-style assemblies can be provided in a ruggedized version (CR-styles) or designed for use at elevated temperatures.
The optical window can be extended in height or tapered to form an optical light guide to a different diameter than that of the crystal. For optimum energy resolution, the diameter of the crystal is chosen equal or slightly smaller than the diameter of the PMT.
C-style assemblies are intended for use in combination with customer’s photomultiplier tube in demountable assemblies. The optical contact to the PMT is accomplished by using optical coupling compound or customer’s own optical glue.
Housings can be equipped with flanges or grooves for O-rings in order to facilitate a demountable coupling to the PMT and its housing.
C-style assemblies can be provided with an axial well and are then called CP-styles, or with thin (30 mm thick) Al entrance windows at the flat side of the crystal, then called CA-styles.
Standard inner well dimensions for CP-style assemblies are
51 CP 51 : 25.4 mm diameter, 39.3 mm deep
16.6 mm diameter, 39.3 mm deep
76 CP 76 : 25.4 mm diameter, 52 mm deep
The standard well thickness is 0.3 mm (Aluminium).
Standard C-style assemblies are used for detection of gamma-rays and X-rays between 30 keV and 2 MeV. For higher energies, BGO can be chosen as the scintillator of choice in which case the optical window is omitted. CP-styles are used in well counters for e.g. medical applications or wipe test detectors
Thin window crystal assemblies: CA-styles, CD-styles
For the detection of low energy X-ray radiation, a detector can be made intrinsically insensitive to higher energy radiation by choosing the scintillation crystal only a few mm thick. For optimum energy resolution, also at very low (several keV) energies, NaI(Tl) scintillation crystals are often used.
For transmission of the radiation of interest, 30 mm thin Aluminum (Al) windows or 0.2 – 0.3 mm thick Beryllium (Be) windows are used. Al windows are suitable for energies down to 10 keV whereas Be windows can be used down to 3 keV (see Fig. 5.3). The advantage of Al windows is their low cost but Be windows are less easily damaged. For non-hygroscopic crystals, aluminized mylar entrance windows can also be used.
For fast X-ray spectroscopy, thin (0.5 – 1 mm thick) YAP:Ce CA- or CD-styles can be used. Thanks to the decay time of YAP:Ce of only 27 ns (see Table 3.1), count rates of several MHz can be achieved
|w/ Photomultiplier Tubes
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
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 :
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..
Thin window B-style assemblies : BA-, BDand BM-styles
For the detection of low energy radiation, thin scintillation crystals are combined with PMTs in a thinwindow assembly. For the detection of X-rays > 10 keV, 30 mm thick Aluminum windows (BA-styles) are used. For lower energies, the BD-style with a 0.2 or 0.3 mm thick Be window is the detector of choice. BM is the notation for a thin mylar entrance window for the detection of heavy ions or low energy b-particles with non-hygroscopic crystals. The thickness of the above materials is not fixed and can be modified to your application.
Thin window B-style assemblies are often constructed with light guides to compensate for local inhomogeneities in the response of the photocathode of the PMT
Assemblies with demountable photomultiplier tubes: A-styles
Detectors with more than one PMT are often constructed as demountable assemblies. The advantage is that in case of break-down of one PMT, it can be replaced easily by the user without having to take apart the entire crystal assembly. Especially for complicated detectors such as Anti-Compton shields or large whole body counters, this is the normal approach. A-styles are also available with axial well (AP-style) or thin entrance window (AA-styles).
The advantages and limitations of photodiode detectors were discussed in section 4.2 . In general, photodiode scintillation detectors consist of a small PIN photodiode, integrally coupled to a scintillation crystal, often CsI(Tl). As a standard rule, a charge sensitive preamplifier is incorporated in the assembly.
The intrinsic noise of the photodiode/preamplifier combination prohibits the use of large scintillation crystals for detection of low energy (< 1 MeV) gamma-rays. This noise determines the lowest energy that can be detected with the device. CsI(Tl) crystals of 1 cm3 coupled to 10×10 mm2 PIN photodiodes can be used down to 40 keV; for larger crystals (e.g. for 2x2x2.5 cm3coupled to 18×18 mm2 diodes), this energy is about 70 keV.
Photodiode scintillation detectors can be used e.g. in applications where:
CsI(Tl) crystals do not crack or cleave and photodiodes are shock resistant. Many configurations are possible. The noise level and energy resolution of the detector depend very much on the crystal/diode configuration. Contact SCIONIX for your specific requirement. The noise of photodiode scintillation detectors increases with temperature. Above 50 o C these instruments are not advised.
An important application of photodiode detectors is in physics research for the detection of charged particles. A thin silicon detector is placed in front of a CsI(Tl) crystal read out with a photodiode to perform an E / D E measurement.
To avoid the necessity for extra pulse shaping electronics, the SPD2000 was developed. This special photodiode detector includes a shaping amplifier with a maximal output of 10 V and line driving capabilities of 40 m. The output from the SPD2000 can be directly fed into an MCA or counter. For more information we refer to the special technical information leaflet of this instrument.
For the detection of low energy X-rays, a bare photodiode without a scintillation crystal can be used in an assembly essentially the same as above but without the crystal. These assemblies are only useful below 50 keV since the standard thickness of PIN photodiodes is only 0.2 – 0.3 mm.
In fact, many SCIONIX scintillation detectors are custom-made with detector configuration and materials adapted to the specific requirements of the user. Regarding the special applications we would like to mention here anti-Compton shields, scintillation detectors with miniature PMTs and phoswiches.
Anti-Compton (AC) systems are scintillation detector assemblies mounted around a High Purity Ge-detector (HPGe) that detect the gamma rays Compton scattered by the Ge crystal and generate a veto signalwhen such a Compton event occurs. This improves the peak-to-total ratio in the pulse height spectrum of the Ge detector significantly.
Crucial parameters are a large solid angle coverage around the HPGe detector and high stopping power. The use of segmented well-type BGO detectors is the generally accepted optimum approach, except for low background systems where the intrinsic high background of BGO is prohibitive and NaI(Tl) is normally used. Below some examples are shown.
AC-shields can be provided with special backcatcher detectors around the cryostat arm for optimum solid angle coverage. To save cost, BGO AC-shields are often equipped with a NaI(Tl) “nose” since at these angles scattered energies are low and a high stopping power is not required.
Many options are possible and we advise to contact SCIONIX to discuss the optimum AC shield configuration for your Ge detector assembly.
The recent development of miniature mesh-type dynode photomultiplier tubes has opened up some possibilities to construct short small diameter detector assemblies. The active area of these devices is around 1 – 2 cm which limits the diameter of scintillation crystals that can be used. The detectors are equipped with built-in voltage dividers. Miniature scintillation detectors are useful in applications where space is limited. Miniature PMTs are rugged and quite insensitive to magnetic fields. Below an example is presented. As an option, assemblies with built-in high voltage generator are available.
Phoswiches are detectors employing a combination of two different scintillation materials. The application is low intensity detection of X-rays and a– or b-particles in the presence of a g -ray background. The (thin) primary scintillation crystal detects the radiation of interest and the secondary (guard) crystal detects background absorptions in the guard crystal and radiation that is scattered by the primary crystal and is absorbed in the guard crystal. This last aspect implies that by setting a veto in case of a time coincidence in signals between primary and guard crystal, the background contribution from the primary crystal can be reduced.
Both primary and guard crystal are read out with the same PMT. Whether a signal in the PMT originated from the primary or the guard crystal is determined by choosing scintillation materials having different decay times.
The most frequent combination is a NaI(Tl) and CsI(Tl) phoswhich (effective decay time 0.25 ms and 1 ms) for low energy X-ray detection or a CaF2(Eu)/ NaI(Tl) phoswich for low background a– and b-particle detection.
Signal separation is done by pulse shape analysis for which it is important that the decay times of the two scintillators are sufficiently different.
Large well detector (10cm diameter x 20cm deep well)
76 and 127mm diameter spherical NaI(Tl) detectors for optimal uniform sensitivity