Silicon Drift Detectors
SDD technology manifests itself in three significant advantages to the user:
- Liquid Nitrogen (LN) cooling is eliminated
- the detector can handle input rates up to 600,000 cps, allowing a throughput rate of 100,000 cps
- Improved resolution at low countrates, which is substantially maintained up to 100,000 cps
The technical parameters giving rise to these advantages are
- Low capacitance
- Low leakage current
The primary innovation of SDD devices is the implementation of concentric rings of progressively higher bias voltage, which guide the photo-electrons generated when an X-Ray is absorbed to a "point" anode. Si(Li)s, on the other hand, use the entire back contact surface area as the anode, creating much higher capacitance and correspondingly higher leakage current. It is the high leakage current that requires the Si(Li) to be cooled to LN temperatures; conversely, it is the low leakage current of the SDD that allows it to operate at much higher temperatures achieved by simple Peltier cooling.
Another innovation in SDDs is the use of a lithographically built FET, that is monolithically created during the semiconductor process of producing the sensor. This technology further reduces the capacitance of the Sensor + FET device, eliminating the need for wires between the sensor and FET and allowing the production of a FET with minimum capacitance.
Devices are also currently being made with external FETs, which can provide nearly as good performance at low and moderate count rates, while facilitating the interfacing of conventional Si(Li) preamplifiers.
PulseTor currently uses integrated-FET devices, having designed a pulse reset preamplifier specifically for the device manufactured by PNSensor at the Max Plank Institute in Munich.