Si (Li) detector

A Si (Li) detector (pronounced "Silly detector" of Engl. Lithium-drifted silicon detector , dt. Lithium gedrifteter silicon detector ) is a special radiation detector based on a with lithium (Li) doped crystalline silicon (Si). The semiconductor detector is used in X-ray spectrometers for energy-dispersive (EDX) and wavelength-dispersive X-ray spectroscopy (WDX).

In addition to Si (Li) detectors, there are also Ge (Li) detectors with a similar structure that use a lithium-doped germanium crystal .

Construction and manufacture

Scheme of a Si (Li) detector

The doping profile corresponds to that of a pin diode , that is, the p- and n-doped areas after the contact electrodes are separated by a significantly larger quasi- intrinsic conductive area. Such a Si (Li) crystal is produced by doping a p-doped silicon crystal a few millimeters thick with lithium after production. A large p-doped volume is compensated for by the diffused lithium atoms, so that a quasi-intrinsically conductive zone is created, which is typically 3 mm to 6 mm (max. 15 mm) thick. The lithium concentration is increased on the rear side and a highly doped n-conductive area (n ⁺ area) is formed, which is usually coated with gold (the rear side contact). The non-compensated p-doped side is contacted either with a metal or a thin, highly doped p-conductive layer.

functionality

The X-ray photons are absorbed in the charge-free central area of the crystal and the resulting electron-hole pairs are separated using an externally applied electrical voltage of 300 to 1000 V. The charge carriers drift through the detector material due to the potential difference, are picked up at the contact electrodes and passed to an amplifier circuit (preamplifier directly on the crystal and a main amplifier). Since the number of electron-hole pairs is proportional to the absorbed ionizing radiation and an average of 3.6 eV is required to generate each pair, the charge measured on the electrodes is proportional to the incident energy.

Advantages and disadvantages

Si (Li) detectors are generally characterized by a higher quantum yield and better proportionality. The measurable spectral range extends from a few hundred electron volts (eV) to 40 keV. The spectral resolution of commercial devices in the 6 keV range is around 135 eV. In order to keep the noise of the detector low and to avoid the diffusion of the lithium atoms due to the applied electric field, it is necessary to cool the Si (Li) crystal and the preamplifier, usually with the help of liquid nitrogen . The used therefor nitrogen - cryostat is provided with a thin beam entrance window, which separates the sensitive detector area from the ambient atmosphere and ensures a good transmission for radiation of interest. A 7 µm thick piece of beryllium used to be the window material . Since such beryllium windows absorb the radiation noticeably below 2 keV and almost completely below 1 keV, the associated weakening of the intensity of the incident radiation in this spectral range led to a lower signal-to-noise ratio as well as to a restricted usable spectral range or limitations in the detectable elements (for example, the K edge of boron is 192 eV). For this reason, research was conducted into alternative materials that can both withstand atmospheric pressure and enable measurements in the range below 1 keV. Among other things, detectors with windows made of diamond , boron nitride or silicon nitride as well as thin polymer film , for example made of Mylar, have been developed. Since the layer thicknesses are only 300 nm and smaller, these detectors are also referred to as UTW or SUTW detectors (from English (super) ultra thin window , dt. Very thin window ).

Windowless and UTF-Si (Li) detectors absorb X-rays in the range 2 to 20 keV approximately 100%, which means that they are very sensitive in this spectral range. At energies higher than 20 keV, however, Si (Li) detectors show a significant drop in efficiency, that is, in the ratio of detected radiation to incident radiation. X-ray photons of this energy can increasingly traverse the detector material without generating electron-hole pairs. An alternative for this spectral range are intrinsic, high-purity germanium detectors ( HP-Ge detector , from English high-purity germanium detector ).

Individual evidence

↑ ^a ^b ^c Burkhard Beckhoff, Birgit Kanngießer, Norbert Langhoff, R. Wedell, Helmut H. Wolff: Handbook of Practical X-ray Fluorescence Analysis . Springer, 2006, ISBN 978-3-540-36722-2 , pp. 220 .
^ William R. Leo: Techniques for Nuclear and Particle Physics Experiments: A How-to Approach . Springer, 1994, ISBN 978-3-540-57280-0 , pp. 235 .
↑ PA Mando: PIXE (Particle-induced X-ray emission) . In: Encyclopedia of Analytical Chemistry: Applications, Theory and Instrumentation . Wiley, 2000, ISBN 0-471-97670-9 , pp. 17 .
↑ ^a ^b Claus Grupen, Irène Buvat: Handbook of Particle Detection and Imaging . Springer, 2012, ISBN 978-3-642-13271-1 ( limited preview in Google book search).
^ ^A ^b ^c David Bernard Williams, C. Barry Carter: Transmission Electron Microscopy: A Textbook for Materials Science . 2nd Edition. Springer, 2009, ISBN 978-0-387-76500-6 , pp. 587-588 .

[Beckhoff-1] Burkhard Beckhoff, Birgit Kanngießer, Norbert Langhoff, R. Wedell, Helmut H. Wolff: Handbook of Practical X-ray Fluorescence Analysis . Springer, 2006, ISBN 978-3-540-36722-2 , pp. 220 .

[2] William R. Leo: Techniques for Nuclear and Particle Physics Experiments: A How-to Approach . Springer, 1994, ISBN 978-3-540-57280-0 , pp. 235 .

[3] PA Mando: PIXE (Particle-induced X-ray emission) . In: Encyclopedia of Analytical Chemistry: Applications, Theory and Instrumentation . Wiley, 2000, ISBN 0-471-97670-9 , pp. 17 .

[Grupen-4] Claus Grupen, Irène Buvat: Handbook of Particle Detection and Imaging . Springer, 2012, ISBN 978-3-642-13271-1 ( limited preview in Google book search).

[Williams-5] A ^b ^c David Bernard Williams, C. Barry Carter: Transmission Electron Microscopy: A Textbook for Materials Science . 2nd Edition. Springer, 2009, ISBN 978-0-387-76500-6 , pp. 587-588 .