F. Bieser

The STAR time projection chamber: a unique tool for studying high multiplicity events at RHIC

The STAR Time Projection Chamber (TPC) is used to record the collisions at the Relativistic Heavy Ion Collider (RHIC). The TPC is the central element in a suite of detectors that surrounds the interaction vertex. The TPC provides complete coverage around the beam-line, and provides complete tracking for charged particles within ± 1.8 units of pseudo-rapidity of the center-of-mass frame. Charged particles with momenta greater than

Radial flow in Au + Au collisions at E = (0.25-1.15)A GeV

A systematic study of energy spectra for light particles emitted at midrapidity from Au+Au collisions at {ital E}= (0.25--1.15){ital A} GeV reveals a significant nonthermal component consistent with a collective radial flow. This component is evaluated as a function of bombarding energy and event centrality. Comparisons to quantum molecular dynamics and Boltzmann-Uehling-Uhlenbeck models are made for different equations of state.

Novel integrated CMOS sensor circuits

Three novel integrated CMOS active pixel sensor circuits for vertex detector applications have been designed with the goal of increased signal-to-noise ratio and speed. First, a large-area native epitaxial silicon photogate sensor was designed to increase the charge collected per hit pixel and to reduce charge diffusion to neighboring pixels. High charge to voltage conversion is maintained by subsequent charge transfer to a low capacitance readout node. Second, a per-pixel correlated double sampling kT/C reset noise reduction circuit was tested. It requires only one read, as compared to two for typical double sampling in active pixel sensors, and no off-pixel storage or subtraction is needed. The technique reduced input-referred temporal noise by a factor of 2.5 to a measured 15.6 e/sup -/, rms. Finally, a column-level active reset technique was designed that suppresses kT/C reset noise. It reduced noise by up to a factor of 7.6, to an estimated 8.3 input-referred electrons, rms. The technique also dramatically reduces fixed pattern (pedestal) noise, by up to a factor of 21. This may reduce pixel-by-pixel pedestal differences enough to permit sparse data scan without per-pixel offset corrections.

The STAR trigger

Abstract We describe the trigger system that we designed and implemented for the STAR detector at RHIC. This is a 10 MHz pipelined system based on fast detector output that controls the event selection for the much slower tracking detectors. Results from the first run are presented and new detectors for the 2001 run are discussed.

The forward time projection chamber in STAR

Two cylindrical forward TPC detectors are described which were constructed to extend the phase space coverage of the STAR experiment to the region 2.5 < |�| < 4.0. For optimal use of the available space and in order to cope with the high track density of central Au+Au collisions at RHIC, a novel design was developed using radial drift in a low diffusion gas. From prototype measurements a 2-track resolution of 1 - 2 mm is expected.

A Readout system for the STAR time projection chamber

We describe the readout electronics for the STAR Time Projection Chamber. The system is made up of 136,608 channels of waveform digitizer, each sampling 512 time samples at 6–12 Mega-samples per second. The noise level is about 1000 electrons, and the dynamic range is 800:1, allowing for good energy loss (dE/dx) measurement for particles with energy losses up to 40 times minimum ionizing. The system is functioning well, with more than 99% of the channels working within specifications.

Front end electronics for the STAR TPC

The Solenoidal Tracker at RHIC (STAR) is a large acceptance detector now being built to study high energy heavy ion collisions. It detects charged particles with a large time projection chamber. The 136,600 TPC pads are instrumented with waveform digitizers, implemented in custom low noise preamplifier/shaper and switched capacitor array/ADCs ICs. The system is highly integrated with all analog functions mounted on small cards that plug into the TPC. Detector mounted readout boards multiplex data from 1152 channels onto a 1.5 Gbit/sec fiber optic link to the data acquisition system.

First use of a high-sensitivity active pixel sensor array as a detector for electron microscopy

There is an urgent need to replace film and CCD cameras as recording instruments for transmission electron microscopy (TEM). Film is too cumbersome to process and CCD cameras have low resolution, marginal to poor signal-to-noise ratio for single electron detection and high spatial distortion. To find a replacement device, we have tested a high sensitivity active pixel sensor (APS) array currently being developed for nuclear physics. The tests were done at 120 keV in a JEOL 1200 electron microscope. At this energy, each electron produced on average a signal-tonoise ratio about 20/1. The spatial resolution was also excellent with the full width at half maximum (FWHM) about 20 microns. Since it is very radiation tolerant and has almost no spatial distortion, the above tests showed that a high sensitivity CMOS APS array holds great promise as a direct detection device for electron microscopy.

Integrated x-ray and charged particle active pixel CMOS sensor arrays using an epitaxial silicon-sensitive region

Integrated CMOS Active Pixel Sensor (APS) arrays have been fabricated and tested using X-ray and electron sources. The 128 by 128 pixel arrays, designed in a standard 0.25 micron process, use a ~10 micron epitaxial silicon layer as a deep detection region. The epitaxial layer has a much greater thickness than the surface features used by standard CMOS APS, leading to stronger signals and potentially better signal-to-noise ratio (SNR). On the other hand, minority carriers confined within the epitaxial region may diffuse to neighboring pixels, blur images and reduce peak signal intensity. But for low-rate, sparse-event images, centroid analysis of this diffusion may be used to increase position resolution. Careful trade-offs involving pixel size and sense-node area verses capacitance must be made to optimize overall performance. The prototype sensor arrays, therefore, include a range of different pixel designs, including different APS circuits and a range of different epitaxial layer contact structures. The fabricated arrays were tested with 1.5 GeV electrons and Fe-55 X-ray sources, yielding a measured noise of 13 electrons RMS and an SNR for single Fe-55 X-rays of greater than 38.

Statistical signatures of critical behavior in small systems

The cluster distributions of three different systems are examined to search for signatures of a continuous phase transition. In a system known to possess such a phase transition, both sensitive and insensitive signatures are present; while in systems known not to possess such a phase transition, only insensitive signatures are present. It is shown that nuclear multifragmentation results in cluster distributions belonging to the former category, suggesting that the fragments are the result of a continuous phase transition.