A Time Projection Chamber (TPC) with Gas Electron Multiplier (GEM) technology is well suited for usage as central tracker at the International Linear Collider (ILC). To study the high potential of this detector type a small prototype of 25cm length was built in Karlsruhe and used in several experimental setups. In this publication the results of these measurements and of additional Monte Carlo simulations are presented. By introducing the so-called equivalent drift distance a combination of all results was possible leading to a recommended configuration of the multi-GEM tower for the ILC–TPC. It will be shown that for conditions considered in the TESLA–TDR the transverse spatial resolution will be able to reach 65μm for 10cm and 190μm for 200cm drift at the ILC. This as well as the expectations for longitudinal spatial resolution, for energy resolutions of the specific ionization, and for single pad row efficiency should be able to meet the requirements of a future ILC–TPC.

A vertex detector that can measure particles with charm or bottom quarks would dramatically expand the physics capability of the STAR detector at RHIC. To accomplish this, we are proposing to build the Heavy Flavor Tracker (HFT) using 2×2cm Active Pixels Sensors (APS). Ten of these APS chips will be arranged on a ladder (0.28% of a radiation length) at radii of 1.5 and at 5.0cm. We have examined several properties of APS chips, so that we can characterize the performance of this detector. Using 1.5GeV/c electrons, we have measured the charge collected and compared it to the expected charge. To achieve high efficiency, we have considered two different cluster finding algorithms and found that the choice of algorithm is dependent on noise level. We have demonstrated that a Scanning Electron Microscope can probe properties of an APS chip. In particular, we studied several position resolution algorithms. Finally, we studied the properties of pixel pitches from 5 to 30μm.

Research has shown that Active Pixel CMOS sensors can detect charged particles. We have been studying whether this process can be used in a collider environment. In particular, we studied the effect of radiation with 55 MeV protons. These results show that a fluence of about 2 x 10(12) protons/cm(2) reduces the signal by a factor of two while the noise increases by 25%. A measurement 6 months after exposure shows that the silicon lattice naturally repairs itself. Heating the silicon to 100 degrees C reduced the shot noise and increased the collected charge. CMOS sensors have a reduced signal-to-noise ratio per pixel because charge diffuses to neighboring pixels. We have constructed a photogate to see if this structure can collect more charge per pixel. Results show that a photogate does collect charge in fewer pixels, but it takes about 15 ms to collect all of the electrons produced by a pulse of light. (c) 2005 Elsevier B.V. All rights reserved.

Comparisons of particle production from high-energy ion collisions with next-to-leading order perturbative QCD calculations show good agreement down to moderate transverse momentum values. Distributions of azimuthal angle differences between coincident hadrons in these collisions support a partonic origin to the particle production, again down to moderate transverse momentum values. The rapidity dependence of inclusive and coincident particle production can therefore be used to probe parton distribution functions down to small momentum fractions where theory anticipates that parton saturation could be present. This paper describes how such experiments could be completed.

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-to-noise 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.

An introduction to the STAR detector and a brief overview of the physics goals of the experiment are presented.

The fragment yields from the multifragmentation of gold, lanthanum, and krypton nuclei obtained by the EOS Collaboration are examined in terms of Fisher's droplet formalism modified to account for Coulomb energy. The critical exponents sigma and tau and the surface energy coefficient c(0) are obtained. Estimates are made of the pressure-temperature and temperature-density coexistence curve of finite neutral nuclear matter as well as the location of the critical point.

Active Pixel Sensor (APS) technology has shown promise for next-generation vertex detectors. This paper discusses the design and testing of two generations of APS chips. Both are arrays of 128 by 128 pixels, each 20 by 20 mum. Each array is divided into sub-arrays in which different sensor structures (4 in the first version and 16 in the second) and/or readout circuits are employed. Measurements of several of these structures under Fe-55 exposure are reported. The sensors have also been irradiated by 55 MeV protons to test for radiation damage. The radiation increased the noise and reduced the signal. The noise can be explained by shot noise from the increased leakage current and the reduction in signal is due to charge being trapped in the epi layer. Nevertheless, the radiation effect is small for the expected exposures at RHIC and RHIC II. Finally, we describe our concept for mechanically supporting a thin silicon wafer in an actual detector.

Active pixel sensor (APS) is a promising technology for next-generation vertex detectors. This paper discusses the design and testing of two generations of APS chips. Both are arrays of 128 by 128 pixels, each 20 by 20 /spl mu/m. Each array is divided into subarrays in which different sensor structures (4 in the first version and 16 in the second) and/or readout circuits are employed. Measurements of several of these structures under Fe/sup 55/ exposure are reported. The sensors have also been irradiated by 55 MeV protons to test for radiation damage. The radiation increases the noise and reduces the signal. The noise can be explained by shot noise from the increased leakage current, and the reduction in signal is due to charge being trapped in the epi layer. Nevertheless, the radiation effect is small for the expected exposures at RHIC and RHIC II. Finally, we describe our conception for mechanically supporting a thin silicon wafer in an actual detector.

Novel CMOS active pixel structures for vertex detector applications have been designed and tested. The overriding goal of this work is to increase the signal to noise ratio of the sensors and readout circuits. A large-area native epitaxial silicon photogate was designed with the aim of increasing the charge collected per struck pixel and to reduce charge diffusion to neighboring pixels. The photogate then transfers the charge to a low capacitance readout node to maintain a high charge to voltage conversion gain. Two techniques for noise reduction are also presented. The first is a per-pixel kT/C noise reduction circuit that produces results similar to traditional correlated double sampling (CDS). It has the advantage of requiring only one read, as compared to two for CDS, and no external storage or subtraction is needed. The technique reduced input-referred temporal noise by a factor of 2.5, to 12.8 e(-). Finally, a column-level active reset technique is explored that suppresses kT/C noise during pixel reset. In tests, noise was reduced by a factor of 7.6 times, to an estimated 5.1 e- input-referred noise. The technique also dramatically reduces fixed pattern (pedestal) noise, by up to a factor of 21 in our tests. The latter feature may possibly reduce pixel-by-pixel pedestal differences to levels low enough to permit sparse data scan without per-pixel offset corrections.

The STAR detector sits in a high radiation area when operating normally; therefore it was necessary to develop a robust system to remotely control all hardware. The STAR hardware controls system monitors and controls approximately 14,000 parameters in the STAR detector. Voltages, currents, temperatures, and other parameters are monitored. Effort has been minimized by the adoption of experiment-wide standards and the use of pre-packaged software tools. The system is based on the Experimental Physics and Industrial Control System (EPICS) [1]. VME processors communicate with subsystem-based sensors over a variety of field busses, with High-level Data Link Control (HDLC) being the most prevalent. Other features of the system include interfaces to accelerator and magnet control systems, a web-based archiver, and C++-based communication between STAR online, run control and hardware controls and their associated databases. The system has been designed for easy expansion as new detector elements are installed in STAR.

The STAR Time Projection Chamber (TPC) is used to record the collisions at the Relativistic Heavy Ion Collider. 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 100MeV/c are recorded. Multiplicities in excess of 3000 tracks per event are routinely reconstructed in the software. The TPC measures 4m in diameter by 4.2m long, making it the largest TPC in the world.

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.

Details concerning the design, fabrication and performance of the STAR Barrel Electromagnetic Calorimeter are presented.

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<|η|<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 1–2mm is expected.

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.

We report the elliptic flow of charged and identified particles at mid-rapidity in Au+Au collisions at √ s NN = 130 GeV using the STAR TPC at RHIC. The integrated elliptic flow signal, v 2 , for charged particles reaches values of about 0.06, indicating a higher degree of thermalization than at lower energies. The differential elliptic flow signal, v 2 (p t) up to 1.5 GeV/c, shows a behavior expected from hydrodynamic model calculations. Above 1.5 GeV/c, the data deviate from the hydro predictions; however the v 2 (p t) is still large, suggesting finite asymmetry for the products of hard scattering. For the identified particles, elliptic flow as a function of p t and centrality differ significantly for particles of different masses. This dependence can be accounted for in hydrodynamic models, indicating that the system created shows a behavior consistent with collective hydrodynamical flow.

Multifragmentation MF results from 1AGeV Au on C have been compared with the Copenhagen statistical multifragmentation model (SMM). The complete charge, mass, and momentum reconstruction of the Au projectile was used to identify high momentum ejectiles leaving an excited remnant of mass A, charge Z, and excitation energy E* which subsequently multifragments. Measurement of the magnitude and multiplicity (energy) dependence of the initial free volume and the breakup volume determines the variable volume parametrization of SMM. Very good agreement is obtained using SMM with the standard values of the SMM parameters. A large number of observables, including the fragment charge yield distributions, fragment multiplicity distributions, caloric curve, critical exponents, and the critical scaling function are explored in this comparison. The two stage structure of SMM is used to determine the effect of cooling of the primary hot fragments. Average fragment yields with Z>~3 are essentially unaffected when the excitation energy is ⩽7 MeV/nucleon. SMM studies suggest that the experimental critical exponents are largely unaffected by cooling and event mixing. The nature of the phase transition in SMM is studied as a function of the remnant mass and charge using the microcanonical equation of state. For light remnants A<~100, backbending is observed indicating negative specific heat, while for A>~170 the effective latent heat approaches zero. Thus for heavier systems this transition can be identified as a continuous thermal phase transition where a large nucleus breaks up into a number of smaller nuclei with only a minimal release of constituent nucleons. Z<~2 particles are primarily emitted in the initial collision and after MF in the fragment deexcitation process.Received 7 June 2000DOI:https://doi.org/10.1103/PhysRevC.64.054602©2001 American Physical Society

The transverse momenta (p(x) ,p(y)) of projectile fragments produced by 1.0A GeV Au-197 nuclei incident on Au and C targets have been measured. The medium and heavy fragments have p(x) and p(y) distributions, which are wider than predicted by models. For the Au target the widths of the distributions are significantly larger than those for C, particularly for the heavy fragments. The C distributions show a different gross structure, which may be due to the target-projectile size difference.