P. Lechner

Silicon drift detectors for high count rate X-ray spectroscopy at room temperature

Abstract Silicon Drift Detectors (SDDs) combine a large sensitive area with a small value of the output capacitance and are therefore well suited for high resolution, high count rate X-ray spectroscopy. The low leakage current level obtained by the elaborated processing technology makes it possible to operate them at room temperature or with moderate cooling. A brief description of the device principle is followed by the presentation of first results of a new production of large area SDDs with external electronics. Performance and applications of the already established SDDs with on-chip amplification are summarised. Various shapes of Multichannel Drift Detectors are introduced as well as their use in new experiments like X-ray holography and in new systems like an Anger camera for γ-ray imaging.

Gamma ray spectroscopy with CsI(Tl) scintillator coupled to silicon drift chamber

A /spl gamma/-ray detector, designed around a silicon drift chamber having a diameter of 3 mm and coupled to a CsI(Tl) scintillator of the same diameter and 10 mm thick, has been realized and extensively tested. This detector may operate over a wide energy band with a minimum energy threshold that may be set around 10 keV at room temperature or at around 4 keV at 0/spl deg/C. At the typical energy of 662 keV used to characterize a /spl gamma/-ray spectrometer, an energy resolution of 4.4% full-width at half-maximum has been measured at room temperature. This detector shows high energy resolution over the full energy range explored, 6 keV-1.27 MeV, and to our knowledge the results reported are the best ever obtained with a scintillator.

Ultrathin entrance windows for silicon drift detectors

Abstract Detectors with ultrathin entrance windows have been fabricated, which show an overall improvement of the detector performance in the optical and X-ray region as well as for heavy ions. The quantum efficiency was higher than 60% within the entire wavelength range between 200 nm and 800 nm. In the soft X-ray region the spectroscopic resolution could be improved significantly. For the MnKα line a peak to valley ratio of 5700:1 was achieved. Measurements with241Am α-particles revealed an effective “dead” layer width of less than 150A. The compatibility of the technology to produce thin entrance windows with the planar process allows its application on various pn-junction detector designs. A new silicon drift detector with a total area of 21 mm2 was successfully tested and operated at count rates up to 3 × 107s−1cm−2. At room temperature, the devices have shown an energy resolution for the MnKα line of 227 eV (FWHM) with shaping times of 250–500 ns, decreasing to 152 eV at −20°C. The fast readout in combination with a large detector area, a homogeneous entrance window and an exceptionally low noise without the need of an extensive cooling system makes them especially suited for spectroscopic applications in non-laboratory environments.

Gamma-ray spectroscopy with LaBr/sub 3/:Ce scintillator readout by a silicon drift detector

In this paper, the authors propose a gamma-ray spectrometer based on a LaBr3 :Ce scintillator coupled to a silicon drift detector (SDD). The SDD is a photodetector characterized by a very low noise thanks to the low value of output capacitance independent from the active area. With respect to a PMT, the SDD offers a higher quantum efficiency which reduces the spread associated to the statistic of photoelectrons generation. Also with respect to an APD, the SDD offers a lower photoelectrons statistic contribution, which, in the APD, is worsened by the excess noise factor with respect to pure Poisson statistics. Moreover, the SDD has a stable behavior, less sensitive to temperature and bias drift. In the past years, good energy resolutions were measured using a SDD coupled to a CsI:Tl crystal. However, the long shaping time, to be used with this scintillator to prevent ballistic deficit, was too far to exploit the best noise performances achievable with a SDD obtained at shaping times in the order of 1 mus. On the contrary, this optimum shaping time is fully compatible with the short decay time of the LaBr3 :Ce crystal (about 25 ns). The results of the experimental characterization of the LaBr3 :Ce-SDD gamma-ray spectrometer are presented in this work and are compared with the performances achieved with the same crystal coupled to a PMT and to a CsI(Tl) crystal coupled to the same SDD. The SDD has an active area of 30 mm2. Antireflective coatings have been implemented. Good energy resolutions were measured at room temperature, thanks to the low leakage current of the detector: 2.7% at the137 Cs 661.7 KeV line and 6.1% at the 57Co 122 KeV line. A resolution of 5.7% at 122 KeV line was measured at 0 degCIn this work we propose a gamma-ray spectrometer based on a LaBr/sub 3/:Ce scintillator coupled to a silicon drift detector (SDD). The SDD is a photodetector characterized by a very low noise thanks to the low value of output capacitance independent from the active area. With respect to a PMT, the SDD offers a higher quantum efficiency which reduces the spread associated to the statistic of photoelectrons generation. Also with respect to an APD, the SDD offers a lower photoelectrons statistic contribution, which, in the APD, is worsened by the excess noise factor with respect to pure Poisson statistics. Moreover, the SDD has a stable behaviour, less sensitive to temperature and bias drift. In the past years, good energy resolutions were measured using a SDD coupled to a CsI:Tl crystal. However, the long shaping time, to be used with this scintillator to prevent ballistic deficit, was far to exploit the best noise performances achievable with a SDD obtained at shaping times in the order of 1 /spl mu/s. On the contrary, this optimum shaping time is fully compatible with the short decay time of the LaBr/sub 3/:Ce crystal (about 25 ns). The results of the experimental characterization of the LaBr/sub 3/:Ce-SDD gamma-ray spectrometer are presented in the work and are compared with the performances achieved with other photodetectors, coupled to the same scintillator crystal. The SDD has an active area of 30 mm/sup 2/. Antireflective coatings have been implemented. Good energy resolutions were measured also at room temperature, thanks to the low leakage current of the detector: a resolution of 2.7% was measured at the /sup 137/Cs 661.7 KeV line and a resolution of 6.1% at the /sup 57/Co 122 KeV line. A discussion of the noise performances of the SDD is carried out in the last part of the paper.

Low energy response of silicon pn-junction detector

Abstract The response function of implanted silicon detectors in the soft X-ray region (150 eV-6 keV) has been measured. To reduce signal charge loss in the highly doped p + -region just beneath the detector surface, different techniques of producing shallow doping profiles and enhancing the electric field at the pn-junction are presented. The spectroscopic resolution could be improved significantly. On 〈100〉 detector material, a peak to valley ratio of 5700: 1 for the mangan K α line was achieved. The measured pulse-height distributions were fitted by a detector model, taking the doping profile of the entrance window into account. The results of the fit were in excellent agreement with the measurement data over the entire energy range.

A new XRF spectrometer based on a ring-shaped multi-element Silicon Drift Detector and on X-ray capillary optics

This paper describes an innovative X-ray Fluorescence spectrometer specifically designed to reach high performances in terms of energy resolution, position resolution and detection rate in elemental mapping applications. The spectrometer is characterized by several novelty contents. In particular, it is based on a ring-shaped monolithic array of Silicon Drift Detectors with a hole cut in its center. In this way a coaxial X-ray excitation beam can reach the sample, allowing to obtain a particular geometry that optimizes the solid angle for the collection of the X-ray fluorescence. Moreover, the X-ray beam is collimated on the sample by means of capillary optics in order to obtain high photon density in a small excitation spot. Detector, optics and generator are assembled in a compact vacuum tightened unit. The paper describes the structure of this spectrometer and presents the first experimental results obtained with the XRF spectrometer.

PAIR CREATION ENERGY AND FANO FACTOR OF SILICON IN THE ENERGY RANGE OF SOFT X-RAYS

Abstract The pair creation energy and the Fano factor of silicon are examined experimentally in the energy range of soft X-rays. Both quantities are shown to be a function of the energy of the absorbed radiation and of the detector temperature. For the pair creation energy our experimental data are in accordance with theory. The observed behaviour of the Fano factor cannot be explained by existing models.

Silicon drift detector – the key to new experiments

Originally designed as position-sensitive detectors for particle tracking, silicon drift detectors are nowadays used for high-count-rate X-ray spectroscopy, operating close to room temperature. Due to their low-capacitance read-node concept, they are among the fastest high-resolution detector systems. They have opened a new spectrum of experiments in the wide field of X-ray spectroscopy: fluorescent analysis, diffractometry, material analysis, synchrotron experiments and X-ray holography. In addition, the detection of visible light, near-infrared light and UV light is measured with high efficiency. The low-noise readout of the light of scintillation crystals extends the spectrum of possiblities to the hard X- and gamma-ray spectrum. The fact that the detector system can be used at room temperature with good spectroscopic performance, and at –10°C with excellent energy resolution, avoiding liquid nitrogen for cooling and high-quality vacuum, guarantees a large variety of new applications, independent of the laboratory environment. A brief description of the device principles is followed by basics on low-noise amplification. The performance figures of a complete detector system are presented along with some already realized dedicated applications, with emphasis on a recently operated “mini-spectrometer” for the analysis of works of art.

First results of DEPFET based Active Pixel Sensor prototypes for the XEUS Wide Field Imager

The concept of an Active Pixel Sensor (APS) based on the integrated detector/amplifier structure DEPFET (DEpleted P-channel Field Effect Transistor) has been developed to cope with the challenging requirements of the XEUS Wide Field Imager. The DEPFET-APS combines high energy resolution, fast readout, and random accessible pixels allowing the application of flexible readout modes. First prototypes of DEPFET-based Active Pixel Sensors with a 64 x 64 pixel format and 75 μm x 75 μm pixel area have been produced at the MPI semiconductor laboratory. The APS is read out row by row, i.e. the pixel signals of one row are processed in parallel by a 64 channel CMOS amplifier/multiplexer chip of the CAMEX type. The addressing of one row of pixels for readout and reset is done by two control chips of the SWITCHER type fabricated in a high-voltage CMOS technology. The processing time for one row is of the order of a few micro-seconds. APS operation, the control and data acquisition system are described, and first experimental results are presented.

XRF spectrometers based on monolithic arrays of silicon drift detectors: elemental mapping analyses and advanced detector structures

This paper describes a new X-ray fluorescence spectrometer based on a ring-shaped monolithic array of silicon drift detectors (SDD), presents several examples of the application of this instrument in elemental mapping analyses and introduces a new multi-element detector based on SDDs whose structure has been specifically optimized for very high energy-resolution and very fast sample scanning in elemental mapping. The existing spectrometer is based on a ring-shaped monolithic array of SDDs with a hole cut in its center. The coaxial X-ray excitation beam, focused by a polycapillary X-ray lens, reaches the sample going through the central hole. This geometry maximizes the solid angle for the collection of the fluorescence from the sample and minimizes the absorption of the fluorescence from the air. The X-ray optics maximizes the photon density in a small diameter excitation spot. These features, together with the very high detection rate of the SDDs allow to reach high scanning rate in elemental mapping. A few examples of the new spectrometer in archaeometry and biology are presented. The paper introduces also a new multi-element detector based on four SDDs monolithically integrated in a silicon chip and surrounding a hole cut in its center. The structure of the four SDDs has been specifically designed to obtain very high energy-resolution and peak-to-background ratio. The structure of the chip has been optimized for elemental mapping applications. The first experimental results obtained with this detector are here presented.