HPGe Radiation Detector Electronics

With the exception of the front-end electronics, both the sealed HV filter and charge-sensitive hybridized preamplifier circuitry are assemblies located behind the radiation detector within the PopTop capsule. They are outside the vacuum chamber and are at room temperature.

The very long time constant of the HV filter ensures that the voltage is applied gradually to the radiation detector. This protects the FET from voltage spikes. When cold, any modern HPGe radiation detector from ORTEC may have the full bias voltage switched on without risking damage to the FET.

Some additional observations about preamplifiers used with radiation detectors: all are of the “charge sensitive” type, and all have the “front-end” cryogenically cooled for lowest electronic noise and energy resolution. Three different feedback mechanisms are used:
 

1. The simplest and most common is “resistive feedback" (see Front-End Electronics). In this case the feedback circuit contains a resistor and a capacitor in parallel.
2. For coaxial radiation detectors used at high count rates (>75 kcps) the feedback circuit replaces the feedback resistor with a junction transistor, switched on when the voltage at the gate of the input FET exceeds a certain threshold voltage. This resets the input FET gate voltage to zero. This type of preamplifier is called a transistor-reset preamplifier (TRP).
3. For small radiation detectors, electronically equipped for optimum resolution below 50 keV, the feedback circuit contains an LED which rezeros the voltage at the gate of the input FET via a mechanism similar to that of the TRP, but using light instead of an electronic switch. Ultra-low noise performance is achieved with this type of preamplifier [special order Pulsed Optical Feedback (POF)] because the feedback resistance is infinite (in contrast to resistive feedback), and the capacitance on the gate of the input FET is not increased by the addition of the collector-to-ground capacitance of a junction transistor (as with the TRP). Very high feedback resistance and very low capacitance are the keys to low noise performance.

Model A250 Series ORTEC resistive-feedback preamplifiers require only 1 watt of power. This makes the preamplifiers ideal for demanding portable applications such as Safeguards. The transistor-reset preamplifier (“Plus” option) requires ~1.8 watts of power.

If the system is warmed up with bias on, the automatic HV shutdown feature protects the input FET and the critical radiation detector surfaces. ORTEC uses a thermistor on the mount, not the radiation detector element itself, to sense the onset of warm up.

Unless otherwise specified at time of order, each ORTEC radiation detector is supplied with a single, captive cable bundle 3 ft in length and terminating into individual in-line connectors:

• Signal out 93-Ω BNC
• Timing out 93-Ω BNC
• Test in 93-Ω BNC
• HV in 75-Ω Male
• Preamp power 9-pin D female

A mating cable pack of individual conductors, 10 ft in length is provided for connections to supporting electronic systems.

SMART-1 is an option on GEM, GMX, GLP, SGD and Profile series radiation detectors. The option integrates a microprocessor controlled module with the radiation detector assembly. The module, controlled via an RS-232 link from the MCA, provides the HV bias for the radiation detector element and provides state of health information on the following radiation detector functions:

• Preamplifier ±12, ±24 V
• Detector element temperature
• Detector high voltage on/off/set/read
• Detector overload status
• Detector shutdown status
• Detector serial number (read only)
• Detector authentication code (read/write)