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Scintillation detectors usually employ a
Voltage Divider (VD) network to operate the PMT. This sometimes called "bleeder
network" defines a potential (voltage) difference between the cathode, dynodes and
anode of the PMT. The exact design of this network is of influence for proper working 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. |
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Positive or
Negative High Voltage?
Design of Voltage Detectors
Plug-on or Integrated?
Voltage Dividers and Preamplifiers
Connectors
Built-in High Voltage Generators and Other
Electronics |
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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's side, these can be separated using a simple splitter, as illustrated
in fig. 8.2. |
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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 kW. This is a compromise between bleeder current
and gain stability which is sufficient for count rates up to approximately 50.000 c/s.
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 MW) 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.
SCIONIX can design the voltage
divider best suited for your application without any additional cost. |
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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 |
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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 ms)
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 50W 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 SCIONIX
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 m 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 SCIONIX for your
specific requirements regarding signal shape, power consumption etc. |
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Connectors
Often, high voltage, signal and
preamplifier power are fed in via separate connectors. The SCIONIX 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 |
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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 mA) 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 SCIONIX for more details. |
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BGO / NaI(Tl) anti-Compton shield |
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