MAXIM SDDs

Silicon Drift Detector (SDD) concepts were first formally described by Gatti while at Politecnico di Milano in a 1983 paper (Gatti and Rehak1). A description of the early implementation of SDDs can be found in the paper by Struder et al 2 (1998).

SDD technology manifests itself in three significant advantages to the user:

  1. Liquid Nitrogen (LN) cooling is eliminated
  2. The detector can handle much higher input countrates, allowing throughput countrates of up to 100,000 cps
  3. Improved resolution at low countrates, which is substantially maintained up to 100,000 cps

The technical parameters giving rise to these advantages are

  1. Low capacitance
  2. 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. Initially, the anode was placed at the center of the device; however, such placement resulted in a dead zone that was later eliminated by moving the FET to one side of the device (see the figure below, courtesy of Ketek, GmbH.

Devices are also currently being made with external FETs, which can provide nearly as good performance, 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. Our large area detector (with nominal active area of 40 square millimeters) uses an external-FET device manufactured by Ketek GmbH, also located in Munich.

TORRENT

Advanced Digital Processor

PulseTor designed the first DPP optimized for high rate analysis of spectra with a substantial low energy X-Ray component. Most X-ray detectors for SEM applications are sold with light-element windows. Although SDDs are capable of very high rate count rates, currently available pulse processors, whether analog or digital, cannot maintain spectral accuracy beyond a few tens of thousands of counts per second when line intensities below 1 keV become significant. X-Ray spectrometers function under the assumption that each pulse can be discretely analyzed before the next one arrives. If a following pulse arrives before its predecessor has been analyzed, both pulses are rejected through a process called pile-up rejection. If pile-up detection fails, the resulting spectrum is distorted. The worst case occurs when low energy X-Rays (C, N, and O, or L lines from the first group of transition metals, for example) closely follow pulses generated from higher energy X-rays. The result is that counts are lost from both the low and higher energy peaks and reappear as spurious "sum peaks"; further, spectral background is increased, as piled-up pulses add to the normally predictable Bremsstrahlung above all major peaks in the spectrum.

At M&M 2007, Rick Mott delivered a paper discussing TORRENT pile-up performance relative to other pulse processors currently in use. See the figure below, from that paper:

[1] E. Gatti, P. Rehak, Semiconductor Drift Chamber - An Application of a Novel Charge Transport Scheme, Nucl. Instr. and Meth. A 225, 1984, pp. 608-614

[2] Struder, L., Fiorini, C., Gatti, E., Hartmann, R., Holl, P., Krause, N., Lechner, P., Longoni, A., Lutz, G., Kemmer, J., Meidinger, N., Popp, M., Soltau, H., and van Zanthier, C., High resolution non dispersive x-ray spectroscopy with state of the art silicon detectors, Mikrochim. Acta, Suppl, 15, 11-19 (1998)