Minutes of the E895 meeting Graduate Chemistry Rm 403, SUNY Stony Brook Monday 7/24/95 Attending: GR - Gulshan Rai RL - Roy Lacey JA - John Alexander RM - Bob McGrath MG - Mark Gilkes HL - Heng Liu PC - Paul Chung JL - Jerome Lauret EL - Erwan LeBras EC - Eric Colin TM - Tatiana Magda RS - Rulin Sun JB - Jacques Bouffety MS - Meena Srivastava GR - Heng Liu joins us today from Kent State. He'll be here throughout the summer. HL - I'll be working on the high voltage control for EOS. Among other things, I will be HV testing the anode wires this week, measuring the leakage currents. GR - Heng's work will really be a test of the EPICS system for STAR. We provide the real live experimental conditions for the test. HL - EPICS is still being used in STAR tests right now. We're working on the parallel connection to the VME. GR - The layout of the experiment now depends on the manner in which the TPC is to be loaded into the magnet. We're not ready to lay cable runs yet. This will be decided by the position of the boom arm, but not this week. GR - Tom Case should have arrived today. AGS personnel are still preparing the magnet so that it can be energized. Tom can then begin the magnet mapping. He'll need some help, and a PC. RL - He can use one of the Chemistry PCs. GR - Eshed Robotech representatives may be on hand to take PR photos of the devices in operation. This technique, dubbed the 'Hummingbird' by Jim Thomas, is of interest to others (PHENIX). Gulshan Rai described the events of the past week. TPC: The fiber optics tests were completed last week at Stony Brook. There were four or five bad cables that we already knew about. It appears that all the others are in good condition. The shortest cable length is 130 feet, which may be marginal for routing the cables. We may need to buy extensions. Tomorrow, when the replacement O-rings become available, we'll continue with the water tests. The water recirculator works. The TPC will be opened up later in the week, when there is time for the air conditioner to clean out the room. We need to flow more nitrogen than we are currently doing, to really dry the detector out. CONTROL ROOM: The electronics racks have been moved into the control room. This turned out to be a very difficult operation, though it was completed successfully. The racks were taller than the standard BNL racks (and also taller than the control room door) and they had to be moved without unloading the crates, to avoid disturbing the trigger electronics. A second transformer has to be installed in order to supply them with enough power. Until then, we can use extension cables. But the breakout box on the racks themselves has to be hooked up by the electricians. The computers will be booted tomorrow with their own IP addresses, separately from the BNL backbone. RL tested the NIM crate power supplies, finding and fixing one with a bad connection. BEAMLINE: Brian Cole plans to build a pulser system into the S2 assembly, and test the scintillators with a laser. The TPC laser could do the job, but it is on the critical path now, as we do not yet have the necessary permissions. The paper work may not be completed before Dan Cebra arrives for the STAR meetings. The final signature has to come from Director Samios' office, and we don't know how long this will take. RM - How long before the laser will actually be holding things up ? GR - Not until the TPC goes into the magnet. But we need to build a platform for the laser. If we could at least get the laser interlocks installed in room B6, it might be enough to get Dan going. MISC: GR - Carnegie-Mellon will not, after all, be taking responsibility for the trigger and scaler programs, but they will take charge of the TPC gain calibrations. They have an Alpha farm, and will be centrally involved in data processing. GR - I spoke to Dirk Rishke recently and have asked him to give a talk on hydrodynamics during an upcoming meeting. GR - Some people have expressed interest in the operation of the EOS TPC. Gulshan Rai gave a description of the TPC. The EOS TPC is a single pad plane device, with a rectangular geometry, unlike its ancestor, the PEP4 TPC. Like STAR, the PEP4 TPC was a cylindrical design with wire and pad planes at both endcaps. PEP4 was a 10 atm device, because of the amount of dE/dx necessary to provide enough PID resolution. The EOS TPC is a 1 atm device, but the major advance in its design over the earlier PEP4 TPC is the large-scale integration of a major portion of its electronics chain onto the detector itself. While the PEP4 TPC was read out by a wall of electronics racks that filled a large room, the off-detector readout for the EOS TPC occupies only four VME crates. A charged track passing through the active volume of the TPC leaves a trail of primary ionization in the P10 gas. The electrons drift downward under the influence of parallel electric and magnetic fields. The uniform electric field is produced by a field cage bounding the active volume, with conducting stripes connected to a resistor chain, that maintain uniformity of the field between the cathode plate at -10 kV and the anode wire plane at 1180 V. This field is parallel to the external B field produced by the magnet within which the TPC is positioned. The presence of these parallel E and B fields is essential to the operation of the TPC. Electrons drifting over large distances in an electric field will deviate from straight paths due to diffusion in the gas. The magnetic field causes the ionization electrons to spiral tightly around the B field lines, preserving the overall straight drift path and minimizing diffusion. This magnetic field is strong enough such that the electrons follow the B field lines, rather than the E field lines. The electrons drift downward towards a plane of anode wires just above the pad plane, composed of sense wires at 1180 V and field wires at ground. Due to the high field gradients in the vicinity of the sense wires, the electrons are accelerated and avalanching occurs - the source of the gas amplification of the detector. But the sense wires are capacitatively coupled to the cathode pads, and an image charge appears on the pads, mirroring the cluster of electron charge on the wires. Each such cluster defines two coordinates of a space point along the track of the passing particle. The third coordinate is reconstructed from the drift time (the drift velocity is known). The drift time is essentially the difference between the charge collection time and the event trigger time. This 3D reconstruction using the time information gives the device its name - Time Projection Chamber. Many such space points are reconstructed for each particle, allowing the eventual determination of the entire 3D trajectory of the particle in the TPC. Unlike in the case of the PEP4 TPC, the anode wires on the EOS TPC are not read out, because of the problem of multiple hits on a single wire, which are a serious difficulty in heavy ion collisions with a high multiplicity of tracks. Instead, the pads alone provide the 2D information which corresponds to the projection of the particle's position onto the pad plane. This, then, is how the electrons are collected and give rise to a signal that can be read out. However, both the primary and secondary ionization in the TPC gas volume give rise to positive ions as well as electrons. These positive ions move a thousand times more slowly than the electrons, and have a tendency to build up inside the volume. If this were allowed to continue unchecked, the positive ion cloud in the detector could destroy the uniformity of the electric field. To prevent this, a gating grid is interposed between the drift volume and the avalanche region, above the anode wire plane. The essential function of the gating grid is to periodically sweep charge out of the gas volume. While this cleanup occurs, the detector is effectively switched off. This is done in the following way: When the gating grid is open, and the TPC is taking data, the wires of the grid are held to the equipotential of the field cage at that vertical position. The grid is then invisible to the drifting charges in the gas volume. But when the grid closes, alternating wires are set to a voltage either +120 V or -120 V relative to the equipotential value. As a result, neither electrons nor positive ions can pass through the grid - electrons are collected on the positive wires, and positive ions on the negative wires. To maintain this shutter action throughout a data run, large amounts of power must be switched by the gating grid drivers. The last wire plane is a Frisch grid at ground potential between the gating grid and the anode wire plane, which screens the anodes from the effects of the charge distribution in the gas volume above, until the electrons pass through it. This ultimately allows for fast rise times for pulses on the charge sensitive preamps under the pad plane. These preamps are the first link in an electronic chain integrated onto specially designed cards (the sticks, as they are called) directly below the pad plane. The signals from the pads are processed and ultimately digitized on the sticks, each of which contains electronics for 128 pads. After the voltage pulse from the charge collected on a single pad leaves the preamp and shaper, it is gated into an analog memory, the Switched Capacitor Array (SCA). There is a 256-cell-deep SCA for each pad. Each cell of the SCA is designed to contain a time sample of the amplitude of the incoming pulses from the pad; i.e. the SCA clock establishes the quantization of the data in time. In principle, then, the TPC is divided into a maximum possible number of 256 slices in the vertical direction. With 128 pad rows of 120 pads each, this gives the TPC a maximum of 3.9 million pixels. In practice the occupation fraction of these pixels is relatively low, even in a high multiplicity Au+Au event. During the Bevalac experiment, only 140 of the available time slices - 'time buckets' - were used. The TPC drift velocity in the P10 gas (90% argon, 10% methane) is 5.5 cm per microsecond, so that the active time of the TPC for a given triggered event is no less than about 15 microseconds. The optimum rate of time sampling is determined by the Nyquist criterion. The SCA buffers are read out and digitized sequentially by an ADC, then stored briefly in a digital memory on board the sticks until finally being shipped off the detector over optical fiber links to four receiver crates, each responsible for reading out one rectangular quadrant of the TPC. Each of the receivers, also referred to as a Front End Processor, contains a digital signal processor (DSP). Pedestal subtraction, gain correction and data compression (zero suppression) are performed within the DSP. The pedestal events (32 of them) are actually taken ahead of time, before a data run. The mean and sigma of the baseline, and the list of bad pads, are written into the DSP. The gain correction is accomplished by pulsing the Frisch grid. Since it takes 8 milliseconds to read out the SCAs, and there are another 8 milliseconds elsewhere in the system, the TPC can read an event every 16 ms. But in practice it is the write cycle that is the limit on the data rate - the taper busy signal locks potentially interesting events out of the DAQ. Two issues limit the TPC itself to a data rate of 1000-2000 events per spill: 1. Primary positive ion buildup in the drift volume (before the gating grid closes). This reaches a steady state at a rate of 1000 beam particles in a one second spill that results in an ion cloud small with respect to field distortions. 2. Pileup. There is no provision for screening out post-pileup, though the pre-pileup is taken care of by timing beam triggers in a multihit TDC. Poisson statistics at a rate of 1000/spill limit pre-pileup to within 20 microseconds. A UV laser is used for drift velocity determination in the TPC, which can fluctuate depending on gas quality. At first, the scheme entailed calculating the velocity from the position of the laser tracks themselves, produced by an array of optical elements which split the incoming beam from the laser and distribute it into the volume, forming a grid. It was found, however, that the positions of these tracks were not stable enough for a really accurate determination of the drift velocity. But it was later discovered that the laser beam occasionally clips the top of the entrance port and scatters light onto the TPC cathode! The cathode then becomes a source of photoelectrons whose position is perfectly well known. This allows a very accurate drift velocity determination. The reconstructed position of particles hitting the cathode in an actual physics event has also been used for perhaps the best velocity determination of all. The laser's other important role, however, has not been usurped - the laser events, taken between spills in an ordinary data run, provide the information for magnetic field distortion corrections. Prepared by: Mark Gilkes