Progress Report of ongoing RD Project

SDD Production and Study : To date, two major batches of SDD wafers have been produced at BNL under the supervision of H. Kraner. The first batch of detectors (STAR1) are 4cm4cm300m in size and use uni-directional drift with an anode pitch of 250m and a `standard' sized guard region. The second batch of detectors (STAR2) features a larger area (6cm6cm), a smaller guard area and uses bi-directional drift from the center of the detector allowing operation at a reduced voltage. Here, we summarize the results of thorough tests conducted at OSU, LBL and WSU with the STAR1 detectors [[38]].

Drift time versus drift distance curves for a STAR1 detector are shown in fig. 16a. These curves show good large scale drift-time linearity along the whole length of the detector for several anodes. The growth in the width of the cloud as a function of drift distance is shown in fig. 16b and is found to be in good accord with the behavior expected from simple transport equations.

A measurement of detector precision in the drift direction was made by taking a series of single laser pulse measurements of the drift time at a fixed position (30 mm from the anode). Variations of the drift time were observed during measurements extending over several hours and were found to be correlated to small temperature variations as monitored with thermistors. As the drift velocity is a monotonic and simple function of temperature, the measurements of drift distance were easily compensated for these variations and are shown in fig. 17a. The RMS of the distribution is of the order of . It is likely that an improved temperature control could lead to even better accuracy.

Lateral position measurements (along the anodes, perpendicular to the drift direction) performed in a similar manner are shown in fig. 17b. For these measurements, the laser light was directed at a point located at the center of an anode and 20 mm from the detector edge. The charge sharing between anodes permits an excellent lateral RMS deviation of . Similar results were also obtained for shorter drift distances.

  
Figure: a.) Drift-time versus drift distance for detector #395 biased with external resistor chain.
b.) Measured signal width () as a function of drift distance.

  
Figure: a.) Precision in the measured drift time after correction for temperature effects.
b.) Precision in the measured lateral position

In addition to the good position resolution, total charge measurements performed at LBL indicate that little or no charge is lost while drifting the full length of the STAR1 detectors.

The STAR1 test results show that the chosen technology meets the basic design requirements of the STAR SVT detector. A detailed examination of the small scale response of these detectors in order to understand the source of small differential non-linearities is in progress. Pulse pair resolution studies are also underway. An in-beam test of three STAR1 drift detectors mounted in a tracking configuration was conducted successfully in the RHIC test beam at the AGS in July 94.

Contact with industrial vendors for wafer prototyping and fabrication has been established successfully.

Electronics: The BNL instrumentation group designed the first prototype of an SVT specific bipolar preamplifier/shaper chip (PASA). The LBL electrical engineering group is working on a modification of the CMOS based switch capacitor array (SCA) designed for the TPC, to meet the specific requirements of the SVT. Functional block diagrams, specifications and requirement documents were established for the complete readout chain. Commercially available FEE components (logic chips, ADC etc.) have been identified. First prototypes of Al-Kapton cables for signal read out were tested.

Mechanical Structure: The SVT support structure was designed by Chong Jer Liaw (BNL) in collaboration with the LBL mechanical engineering group. Comparative studies between air, water and evaporative cooling called for water cooling. The mechanical structure design features three super-layers mounted in a clamp-shell structure. The clamp-shell allows removal of the detector from the STAR system without dismounting the RHIC beam pipe. Ladder and end-cap prototypes made of Aluminum were successfully produced in the WSU machine shop. Contact with Be machining companies in the U.S. and Russia has been established. A finite element analysis of the current structure design showed that the Beryllium shell should have the necessary robustness and stability. Simulations including the water cooling channels indicated that the proposed under-pressure cooling scheme is adequate and does not overstress the mechanical structure. The SVT cooling system design is now integrated within the STAR cooling system. The feasibility of the detector mount requiring 4-side wire-bonding as well as the hybrid substrate mount was established.

SVT review in October 93: A panel of technical experts was appointed by RHIC and DoE in October 93 to review the SVT RD progress and examine the feasibility of the SVT for STAR. The panel, chaired by V. Polychronakos (BNL) and including renowned experts in the field (Vacchi (Trieste), Weilhammer (CERN), van den Berg (Penn State), Haber (LBL), Heijne (CERN)), stated in its report summary that the SVT project is well advanced, involves strong technical experts covering all SVT subsystems, and has solid support by strong university based research groups throughout the collaboration. The overall approach and design was approved and encouraged for the future. Specific suggestions and recommendations were made for wafer production, electrical engineering and for system integration in particular. The panel recommended a systems test by the end of the RD phase (end of FY95). The systems test will include a wafer ladder, based on the final design, assembled with SDDs, front end electronics hybrids and a water cooling system. The ladder will be read out by a 'mini' data acquisition based on parts of the final STAR DAQ.