HPGe Radiation Detector Electronics

In addition to the front-end electronics, the sealed HV filter and charge-sensitive hybrid preamplifier circuit are located behind the radiation detector assembly in the PopTop package. They are located outside the vacuum chamber at room temperature.



The long time constant of the HV filter ensures that the voltage is gradually applied to the radiation detector. This protects the FET from voltage spikes. When cold, any of ORTEC's new HPGe radiation detectors can be switched on to full bias voltage without risk of damaging the FET.

Some additional notes about radiation detector preamplifiers: All preamplifiers are of the "charge sensitive" type, and all have cryogenically cooled "front ends" for minimal electronic noise and excellent energy resolution. Three different feedback mechanisms are used:br />

1. and see "resistive feedback" (see front-end electronics). In this case, the feedback circuit contains a resistor and capacitor in parallel.
2. For coaxial radiation detectors at high count rates (>75 kcps), the feedback circuit replaces the feedback resistor with a junction transistor that turns on when the input FET gate voltage 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, where the electronics have resolution below 50 keV, the feedback circuit contains an LED that uses light (non-electronic switching) to reset the voltage input to the FET gate to zero through a TRP-like mechanism. This preamplifier [pulsed optical feedback (POF) available by special order] achieves ultra-low noise performance because the feedback resistance is infinite (as opposed to resistive feedback) and the capacitance on the input FET gate is not increased by the collector-to-ground capacitance of the junction transistor (as with TRP). High feedback resistance and low capacitance are key to reducing noise performance.

The A250 Series ORTEC resistive feedback preamplifier requires only 1 watt of power. This makes the preamplifier ideal for demanding portable applications such as security. The transistor reset preamp (the "add-on" option) requires about 1.8 watts of power.

An automatic HV shutdown protects the input FETs and critical radiation detector surfaces if the system warms up with bias on. ORTEC uses a thermistor on the mount rather than the radiation detector element itself to sense the onset of preheating.

Unless otherwise specified when ordering, each ORTEC radiation detector comes with a 3-foot cable bundle with individual push-in connectors:

• Signal output 93-Ω BNC
• Timing output 93-ΩBNC
• Test input 93-ΩBNC
• High voltage input 75-Ω male connector
• Preamplifier power supply 9-pin D female connector

A matching 10-foot cable pack for each conductor is provided for connection to supporting electronic systems.

SMART-1 is an option for the GEM, GMX, GLP, SGD and Profile series of radiation detectors. This option integrates a microprocessor-controlled module with a radiation detector assembly. This module, controlled via the MCA's RS-232 link, provides HV bias to the radiation detector elements and provides status information for the following radiation detector functions:

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