|
Photon Detector Assembly Preamplifiers Used with
Germanium (HPGe) and Si(Li) Detectors
With the exception of the front-end electronics, both the sealed HV
filter and charge-sensitive hybridized preamplifier circuitry are assemblies located
behind the detector within the PopTop capsule (Fig. 10). 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 detector. This protects the FET from voltage spikes. When cold,
any modern Germanium detector from ORTEC may have the full bias voltage switched on without
risking damage to the FET.
Some additional observations about preamplifiers used with Photon [HPGe
or Si(Li)] Detectors (See Fig. 13): 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." In
this case the feedback circuit contains a resistor and a capacitor in parallel.
2) For coaxial detectors used at high count rates (>75 kcps) the
feedback circuit contains 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 detectors (IGLET, IGLET-X, SLP, GLP), electronically
equipped for optimum resolution below 50 keV, the feedback circuit contains an LED
(light-emitting diode) 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 [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.
Table 2. Energy
Rate of Preamplifiers Supplied with Photon Detectors. |
| Preamplifier Type |
Detector Type |
Maximum Energy Rate
(MeV Sec–1) |
|
Resistive Feedback |
GEM, GMX,
LO-AX, GLP (25, 32, 36 mm)
GLP (6, 10, 16 mm) |
145,000
4,000
2,200 |
Transistor Reset
(Order "Plus") |
GEM, GMX |
1,000,000 |
Pulsed Optical Feedback
(Order "POF") |
IGLET, IGLET-X, SLP, GLP, SLB
(6, 10, 16 mm) |
4,000* |
Modified For HCR
(Resistive Feedback) |
All GLPs |
10,000 |
|
*The POF does not "lock up" or
saturate at high count rates, unlike resistor-feedback designs. At ultra-high count rates
with the POF, throughput is limited by reset pulse rates. 4000 MeV/s is an estimate of
maximum "useable" energy rate. |
All new ORTEC resistive-feedback preamplifiers (Model A250 Series)
require only 1 watt of power, which is 50% less than previous ORTEC preamps. This makes
the new preamplifiers ideal for demanding portable applications such as Safeguards. The
transistor-reset preamplifier ("Plus" option) requires ~1.8 watts of power. |
