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:
- 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.
- 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).
- 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.
|Energy Rate of Preamplifiers Supplied with Radiation Detectors.
||Maximum Energy Rate (MeV Sec-1)
GLP (25, 32, 36 mm)
GLP (6, 10, 16 mm)
|Transistor Reset (-PL option)
|Pulsed Optical Feedback (special order POF option)
||GLP (6, 10, 16 mm)