P. Seller

The OPAL silicon microvertex detector

A silicon strip microvertex detector has been designed, constructed and commissioned in the OPAL experiment at the LEP electron-positron collider. The microstrip devices incorporate a new FoxFET biassing scheme developed together with Micron Semiconductor Ltd., UK. The devices digitise with a precision close to 5 μm and have an exceptionally high signal-to-noise ratio. The associated microelectronics were all custom made for the OPAL project. The detector began operation in 1991 and has since continued to be part of the OPAL experiment, performing to a very high standard and opening up new areas of physics studies.

The OPAL silicon strip microvertex detector with two coordinate readout

The OPAL experiment at the CERN LEP collider recently upgraded its silicon strip microvertex detector from one coordinate readout (φ only) to two coordinate readout (φ and z). This allows three dimensional vertex reconstruction and should improve lifetime measurements as well as b quark jet identification. This paper describes the new microvertex detector system with emphasis on the novel techniques and new components used to obtain the second coordinate information. These include the use of back-to-back single-sided detectors with orthogonally oriented readout strips, a gold printed circuit on a thin glass substrate to route the z strip signals to the electronics at the end of the detector, and the use of MX7 readout chips. Results on the performance of the new detector are presented.

The DELPHI Microvertex detector

The DELPHI Microvertex detector, which has been in operation since the start of the 1990 LEP run, consists of three layers of silicon microstrip detectors at average radii of 6.3, 9.0 and 11.0 cm. The 73728 readout strips, oriented along the beam, have a total active area of 0.42 m2. The strip pitch is 25 μm and every other strip is read out by low power charge amplifiers, giving a signal to noise ratio of 15:1 for minimum ionizing particles. On-line zero suppression results in an average data size of 4 kbyte for Z0 events. After a mechanical survey and an alignment with tracks, the impact parameter uncertainty as determined from hadronic Z0 decays is well described by (69pt)2 + 242 μm, with pt in GeV/c. For the 45 GeV/c tracks from Z0 → μ− decays we find an uncertainty of 21 μm for the impact parameter, which corresponds to a precision of 8 μm per point. The stability during the run is monitored using light spots and capacitive probes. An analysis of tracks through sector overlaps provides an additional check of the stability. The same analysis also results in a value of 6 μm for the intrinsic precision of the detector.

FOXFET biassed microstrip detectors

Abstract A method has been developed for biassing the strips of a silicon microstrip detector with a tunable dynamic resistance. This allows the strip potentials to be tied to a fixed voltage, virtually independent of the strip leakage currents, whilst requiring no processing steps additional to those needed for a standard capacitively coupled detector. Results are presented for full sized detectors (3.3 cm × 6.0 cm) both measured on a probe station and equipped with VLSI readout (MX3) chips. Assemblies are currently undergoing beam tests at CERN with indications of very promising performance.

Pixellated Cd(Zn)Te high-energy X-ray instrument

We have developed a pixellated high energy X-ray detector instrument to be used in a variety of imaging applications. The instrument consists of either a Cadmium Zinc Telluride or Cadmium Telluride (Cd(Zn)Te) detector bump-bonded to a large area ASIC and packaged with a high performance data acquisition system. The 80 by 80 pixels each of 250 μm by 250 μm give better than 1 keV FWHM energy resolution at 59.5 keV and 1.5 keV FWHM at 141 keV, at the same time providing a high speed imaging performance. This system uses a relatively simple wire-bonded interconnection scheme but this is being upgraded to allow multiple modules to be used with very small dead space. The readout system and the novel interconnect technology is described and how the system is performing in several target applications.

Charge sharing in silicon pixel detectors

We used a pixellated hybrid silicon X-ray detector to study the effect of the sharing of generated charge between neighbouring pixels over a range of incident X-ray energies, 13–36 keV. The system is a room temperature, energy resolving detector with a Gaussian FWHM of 265 eV at 5.9 keV. Each pixel is 300 μm square, 300 μm deep and is bump bonded to matching read out electronics. The modelling packages MEDICI and MCNP were used to model the complete X-ray interaction and the subsequent charge transport. Using this software a model is developed which reproduces well the experimental results. The simulations are then altered to explore smaller pixel sizes and different X-ray energies. Charge sharing was observed experimentally to be 2% at 13 keV rising to 4.5% at 36 keV, for an energy threshold of 4 keV. The models predict that up to 50% of charge may be lost to the neighbouring pixels, for an X-ray energy of 36 keV, when the pixel size is reduced to 55 μm.

GaAs solid state detectors for particle physics

Abstract We report on progress with Schottky diode and p-i-n diode GaAs detectors for minimum ionising particles. The radiation hardness and potential speed of simple diodes is shown to be more than competitive with silicon detector. A discussion is given of the present understanding of the charge transport mechanism in the detectors since it influences their charge collection efficiency. Early results from microstrip detectors are also described.

Measurement of the spatial resolution of double-sided double-metal AC-coupled silicon microstrips detectors

Abstract The design and first results from double-sided silicon microstrip detectors designed for use in the DELPHI experiment at LEP are presented. The detectors are AC-coupled on both the n- and p-side. A novel readout scheme using a second metal layer has been implemented, allowing the readout of both coordinates on the same edge of the detector. The detectors have been tested in a high energy beam at the CERN SPS. Results on spatial resolution, pulse-height correlation and charge division are presented. The spatial resolution of the n-side has been measured as a function of the beam particle incident angle from 0 to 60°.

Investigation of the small pixel effect in CdZnTe detectors

The signal shapes produced by alpha and X-ray radiation in 2 mm thick CdZnTe detectors have been measured. The signals produced in a single large pad detector and a 300 mum pixilated detector have been compared. The influence of the small pixel effect and its variation with detector bias is visible. Synopsys Sentaurus TCAD is used to simulate the charge carrier motion in the detectors and is compared against the measured signals. A description of how the simulations will aid detector design with optimal pixel size, inter-pixel spacing and bias voltage is included.

An ASIC for the Study of Charge Sharing Effects in Small Pixel CdZnTe X-Ray Detectors

An Application Specific Integrated Circuit (ASIC) has been developed at the Rutherford Appleton Laboratory (RAL) to study the small pixel effect in spectroscopic CdTe and CdZnTe detectors. The PIXIE ASIC consists of four arrays of 3 × 3 channels flip chip bonded directly to the detector pixels. The active circuitry of each channel is a charge sensitive preamplifier and an output buffer which is multiplexed directly off chip. Each of the four detector arrays has a different anode geometry. The HEXITEC series of small pixel detectors developed at RAL have demonstrated energy resolutions of ~1 keV per pixel for both CdTe and CdZnTe, however, charge sharing events account for between 30-40% of the total count rate and can lead to degradation of the spectroscopy if not corrected for. The PIXIE ASIC will be used to study the effect of anode geometry on charge sharing and other aspects of the small pixel effect.