Custom scintillators and probes

Sodium Iodide Detectors

NaI(Tl) scintillation crystals are used in most standard applications for detection of Gamma radiation because of their unequalled high light output and the excellent match of the emission spectrum to the sensitivity of photomultiplier tubes, resulting in a good energy resolution.

Physical Properties:

Density
-
3.67 g/cm3
Emission Max
-
415 nm
Decay Constant
-
0.23 µs
Refractive index
-
1.85
Conversion Effency
-
100
Hygroscopic
-
Yes

Cesium Iodide Detectors (Tl)(Na)(undoped)

CsI(Tl) has the advantage that it is non-hygroscopic, does not cleave and can be read out using silicon photodiodes instead of photomultiplier tubes. These so-called Scintillator Photodiode Detectors are compact, very stable, do not require any high voltage, are rugged, and can be operated in high magnetic fields. These detectors are frequently used in arrays or matrices in particle physics research.
Physical Properties:

Density
-
4.51 g/cm3
Emission Max
-
550 nm
Decay Constant
-
0.6/3.4 µs
Refractive index
-
1.79
Conversion Effency
-
45
Hygroscopic
-
No

CsI(Na) is a non-hygroscopic, high light output scintillator mainly used for applications where mechanical stability and good energy resolution are required. Below 120 o C it is an alternative to NaI(Tl).
Physical Properties:

Density
-
4.51 g/cm3
Emission Max
-
420 nm
Decay Constant
-
0.63 µs
Refractive index
-
1.84
Conversion Effency
-
85
Hygroscopic
-
Slightly

CsI(Undoped) Fast, non-hygroscopic, radiation hard, low light output. Applications - Physics (calorimetry).

Physical Properties:

Density
-
4.51 g/cm3
Emission Max
-
315 nm
Decay Constant
-
16 ns
Refractive index
-
1.47
Conversion Effency
-
4-6
Hygroscopic
-
No

BGO Detectors

BGO has the extreme high density of 7.13 g /cm 3 and has a high Z value which makes these crystals very suited for the detection of natural radioactivity (U, Th, K), for high energy physics applications (high photofraction) or in compact Compton suppression spectrometers.

Physical Properties:

Density
-
7.13 g/cm3
Emission Max
-
480 nm
Decay Constant
-
0.3 µs
Refractive index
-
2.15
Conversion Effency
-
15-20
Hygroscopic
-
No

CaF2 Detectors

CaF 2 (Eu), Europium doped calcium fluoride is a low density scintillation crystal with a high light output. Thanks to its low Z value it is well suited for the detection of electrons (beta particles) with a high efficiency (low backscatter fraction). CaF 2 (Eu) is a crystal that is also used in phoswich scintillation detectors in combination with NaI(Tl).

Physical Properties:

Density
-
3.18 g/cm3
Emission Max
-
435 nm
Decay Constant
-
0.84 µs
Refractive index
-
1.47
Conversion Effency
-
50
Hygroscopic
-
No

6Lil(Eu) Detectors

High neutron cross-section, high light output. Applications - Thermal neutron detection and spectroscopy.

  • 3 mm LiI(Eu) stops 90% of thermal neutrons
  • Rugged crystal (> 20 degrees C gradient / hour allowed)
  • Excellent neutron / Gamma discrimination (neutron peak > 3 MeV)
  • Neutron peak resolution < 7% FWHM (PMT readout)

Physical Properties:

Density
-
4.1 g/cm3
Effective decay time
-
1.4 s
High Neutron Peak Position
-
> 3 MeV
Photoelectron yield
-
30-35 % of NaI(tl)
6-Li enrichment
-
96%

Neutron Detection

Neutrons do not produce ionization directly in scintillation crystals, but can be detected through their interaction with the nuclei of a suitable element. In a 6 LiI(Eu) scintillation crystal for example, neutrons interact with 6 Li nuclei to produce an alpha particle and a triton (tritium nucleus), which both produce scintillation light that can be detected. For this purpose, also enriched 6 Li containing glasses can be used, doped with Ce as activator.

6Li - Glass Detectors
Glass scintillator with enriched Li-6 content of 96%.
Applications - Thermal neutron detection.
Physical Properties:

Composition
-
SiO2, MgO, Al2O3, Ce2O3, 6-Li2O (18 %)
Li-6 content by weight
-
7.9 %
Density
-
2.5 - 2.7 g / cm3
Primary decay time
-
60 ns
Thermal Neutron peak loc
-
1.7 – 1.9 MeV
Photoelectron yield
-
30-35 % of NaI(tl)4 - 6 %

relative to NaI(Tl) for Gamma rays

CdWO4

This material has a very high density, low afterglow and is radiation hard.

Applications - low afterglow CT applications, DC measurement of X-rays (high intensity), readout with photodiodes, Computerized Tomography (CT).
Physical Properties:

Density
-
7.9
Emission Max
-
470 / 540 nm
Decay Constant
-
20 / 5 µs
Refractive index
-
2.3
Conversion Effency
-
25-30
Hygroscopic
-
no

CeBr3

CeBr3 offers an alternative to NaI(Tl) crystals for high resolution gamma
spectrometry. Above an energy of 200 keV, the resolution is superior to NaI(Tl). CeBr3
scintillation detectors do not suffer from the intrinsic La-138 background typical for Lahalide
scintillators.
Physical Properties:

Density
-
5.23 g/cm3
Emission Max
-
370 nm
Decay Constant
-
16 ns
Refractive index
-
1.9
Conversion Effency
-
130
Hygroscopic
-
Yes

YAP:Ce

YAP:Ce is a high density (5.5 g/cm 3) oxide crystal with a decay time about 10 times shorter than NaI(Tl). It is used in detectors for high count rate (up to several MHz) X-ray spectrometry. The non-hygroscopic nature of this material allows the use of thin mylar entrance windows and guarantees a long lifetime of the detector.
Physical Properties:

Density
-
5.55 g/cm3
Emission Max
-
350 nm
Decay Constant
-
27 µs
Refractive index
-
1.94
Conversion Effency
-
35-40
Hygroscopic
-
No

Plastic Detectors

Organic (plastic) scintillators consist of a transparent host material (a plastic) doped with a scintillating organic molecule (e.g. POPOP : p-bis [2-(5-phenyloxazolyl)] benzene). Radiation is absorbed by the host material, mostly via Compton effect because of the low density and Z- value of organic materials. Therefore, plastic scintillators are mostly used for the detection of b - and other particles. Furthermore plastic scintillators are mainly used when large detector volumes are required e.g. in security or health physics applications. The cost of large plastic scintillation detectors (per volume) is much lower than that of equivalent size NaI(Tl) detectors; plastic scintillators can be manufactured in meter long slabs.

Physical Properties:

Density
-
1.03 g/cm3
Emission Max
-
375-600 nm
Decay Constant
-
1-3 µs
Refractive index
-
1.58
Conversion Effency
-
25-30
Hygroscopic
-
No

Dynode Chains & Electronics

Scintillation detectors usually employ a Voltage Divider (VD) network to operate a Photomultiplier Tube (PMT). This network (sometimes referred to as a "bleeder network") defines the potential (voltage) differences between the cathode, dynodes and anode of the PMT. The exact design of this network is significant for optimization of the scintillation detector. Some details of voltage divider networks are discussed below. The descriptions below are not exhaustive; for more details we refer to the photomultiplier manufacturer's literature.

Positive or Negative High Voltage?

It is possible to operate a photomultiplier tube in two ways:

  • A. anode at positive potential (cathode at ground).
  • B. anode at ground (cathode at negative potential).

For measurements of DC anode current such as in some X-ray applications, option B is the only choice since in the first option the anode must be separated from the follow-up electronics by means of a high voltage capacitor.

On the other hand, option A is used for most standard applications since the m -metal shield should be preferably at cathode potential. Option A implies that cathode, detector mass (ground) and shield are all connected together. In option B, the shield must be very well insulated from the detector mass and special construction requirements apply.

Negative high voltage is required for some fast timing applications where the possibility of discharges between the cathode of the PMT and the m -shield are to be avoided. These PMTs are operated at more than 2 kV for fast response.

Voltage dividers for detectors operated at positive high voltage can be wired with a single connector for signal and HV. At the electronic side, these can be separated using a simple splitter, as illustrated in fig. 8.2.

Design of Voltage Dividers

The design of the voltage divider influences the performance of a detector. At high count rates, the voltage across dynodes may drop and the average bleeder current should always be defined as at least 10 times larger than the average anode current in the detector. A standard resistor value between dynodes is 470 k W . This is a compromise between bleeder current and gain stability which is sufficient for count rates up to approximately 50.000 counts per second.

Voltage dividers may be linear (most common), tapered or specially stabilized with Zener dynodes or transistors.

The number of possibilities is large. A very important aspect is the potential (electric field) between the cathode and the first dynode of the PMT. In any case, this potential should be sufficient to ensure a good photoelectron collection efficiency. Usually, this voltage is prescribed by the PMT manufacturer.

The gain of a scintillation detector varies with each PMT and is also strongly influenced by the exact design of the voltage divider. If the absolute detector gain is of importance, it can be defined as: the output voltage (in e.g. 1 M W ) at a specific operating voltage of the PMT for a certain energy absorbed in the detector.

PMTs can be selected on gain but adjustment of the gain of the detector by varying the voltage in the VD by means of a precision potentiometer is much more convenient. Extra options on voltage dividers are e.g. a gain potentiometer, an extra dynode output or a focus potentiometer.

Berkeley Nucleonics can design the voltage divider best suited for your application without any additional cost.

Plug-on or Integrated?

Voltage Dividers and other electronics can be incorporated into the scintillation detector. In this case, the resistor network is directly soldered onto the pins of the PMT which implies a minimal length of the assembly. For low background applications this is the preferable option. The connector(s) for high voltage and signal are located at the back of the assembly. Also, flying leads are an option.

When it is expected that detectors have to be interchanged often it may be preferable to use a so-called "plug-on" option in which case the voltage divider and associated electronics are mounted in a small housing with the same diameter as the detector which is plugged on the pins of the base of the PMT. Most frequently used PMT bases in this respect are the 12 pin JEDEC B12-43 base for 38 mm diameter PMTs and the 14 pin JEDEC B14-38 base for 51, 76 and 127 mm diameter PMTs. These are also the standard bases for scintillation detectors supplied without voltage divider. Below some examples are presented.

Voltage Dividers and Preamplifiers

A PMT signal will be attenuated in a long cable and when signals have to be transported over more than say 10 m of cable this effect cannot be neglected. Signals even may become deformed and signal differences between a set of detectors having different cable lengths can be a problem.

Furthermore, the signal that is to be fed into a main amplifier (also called shaping amplifier or spectroscopic amplifier) needs to have a certain pulse fall time (typically 50 m s) in order to allow proper pole-zero and base-line correction. This effect is especially important at high count rates.

To solve the above problems, scintillation detectors can be supplied with a built-in (or plug-on) voltage divider/ preamplifier. This amplifier has an output impedance of 50 W for proper matching to the most frequently used cable impedance (reflections). Usually, the end stage of this amplifier is based on the principle of an emitter follower.

The standard Berkeley Nucleonics voltage divider/ preamplifier the VD (12) 14 / E2 is an example of this suited for a wide variety of PMTs. This amplifier operates with a wide variety of voltages, is very fast (rise time < 50 ns) and can drive cable lengths of 100 meters or more. Varieties for ultra low power consumption exist. The amplifier is very small so that it will fit in almost every scintillation detector.

Please consult Berkeley Nucleonics for your specific requirements regarding signal shape, power consumption etc.

Connectors

Often, high voltage, signal and preamplifier power are fed in via separate connectors. The Berkeley Nucleonics standard connector for high voltage is the SHV(Super High Voltage) connector, the most frequently used standard in nuclear electronics. For signals, BNC connectors are the standard and for preamplifier power signals, the dual LEMO type 0 and the 9 pin sub-D connector are normally used.

Other possibilities are e.g. flying leads options, water tight connectors, MHV, TNC, PET-100 and different types of LEMO or FISHER connectors.

Power HV and signal can also be fed in (out) via a single large multipole connector.
Built-in High Voltage Generators and Other Electronics

Recent developments in hybrid circuitry have allowed to incorporate a number of other electronic components into the scintillation detector assembly which eliminates in some applications the necessity of NIM based electronics.

An example of the above is the scintillation detector with a built-in High \/oltage Generator( - HV option, see section 6). This is a small Cockroft - Walton generator which produces the high voltage required to operate a PMT. This unit only requires a DC voltage of + 5 V or + 12 V and uses only 100 mW of power. The unit is fully integrated with the PMT so there are no high voltage leads anywhere in the assembly. The gain of the PMT is maintained even at high anode currents (up to 100 m A) and the unit adds only 50 mm to the length of the PMT. The high voltage can be factory set, precision potentiometer adjustable or set by a 0 - 1 V regulating voltage. Below the advantages are summarized.

Advantages built-in high voltage generators:

  • Compact
  • Low power consumption
  • Sealed
  • High gain stability versus count rate

Besides the above mentioned preamplifiers it is also possible to incorporate e.g. shaping amplifiers (spectroscopic amplifiers) or Single Channel Analyzers (SCAs) into a detector assembly. All these components, constructed as small SMD or hybrid circuits, are very small in dimension. Specific parameters of these devices can be defined by the user since the standard models can be easily adapted. Please consult Berkeley Nucleonics for more details.