Minutes of the CALCOM Analysis meeting, July 11, 1997. ===================================================== Agenda: (1) Time-walk corrections for TOF counters L. Elouadrhiri (2) Gain matching for TOF counters S. Taylor (3) Cherenkov counter calibration A. Vlassov (no report) ( + comments on pion rejection, V. Burkert) (4) A first look at the large angle em calorimeter P. Rossi (5) Status and plans of drift chamber commissioning M. Mestayer (no report) (6) Priorities for CALCOM analysis V. Burkert -------------------------------------------------------------------------------- Summary of Cherekov counter studies (V.B.): ========================================== Alexander Vlassov reported on his analysis of the Cherencov counter calibration. Elastic electron-proton scattering events from the unfortunately very short 1.6GeV run were used to uniquely identify electrons in sector 6, and the response of the Cherenkov counter in this sector was determined. Using a calibration of the single photo-electron peak as 150 ADC channels, the average number of photo-electrons per Cherenkov counter element (basically a polar angle slice at fixed azimuthal angle) could be determined. The results are systematically lower than the simulations by about 20%, which is about the accuracy of the single photo-electron calibration. A better determination of this quantity is currently under study by John Price. Alex also looked into the 4 GeV run, however, the lack of accurate tracking in RECSIS makes it impossible to separate the elastic channel from inelastic events. Such an analysis has to await progress in the area of time-based tracking within RECSIS. V. Burkert reported briefly about a study using CED to quantify the Cherenkov counter response to charged hadrons. Using runs with "minimum bias " trigger, e.g. 3783, the response to positive charged particles (mostly pions, protons) was determined to be (3.8 +/- 1.7)%; with a possible small contamination of positrons. For negative charged tracks the response was (17.5 +/- 5.3)%, likely with a significant contribution from electrons. The study illustrated that the detector not only is efficient in detecting electrons, but also rejects most pions, as expected. Clearly, a more detailled, quantitative analysis is needed. Individual reports: ================== L. Elouadrhiri: --------------- EXTRACTION OF TIME-WALK CORRECTIONS PARAMETERS FOR TOF SYSTEM During the operation of the CLAS TOF system, the sizes of the pulses is not constant. Large pulses pass the leading edge discriminator threshold sooner than smaller pulses. This causes shifts ( time-walk) in the timing of the pulses. Corrections for time walk are then necessary. We used laser runs that deposit varying amounts of light at the center of each counter which then illuminate both the left and right PMTs of each scintillator equally. Several steps were required to obtain these corrections: 1- Conversion of counts values to real time values Tns = C0 + C1*TDC +T2*TDC**2 The converted time in ns was measured relative to the converted time from a TDC whose input was a signal generated by a diode measuring the the laser signal time. 2- Time-Walk corrections Tw = P1 + P2/(ADC)**P3 Using the above function we fitted TC vs ADC and the parameters resulting from this least-square fit are the time walk corrections. The resulting resolution using the same laser data is estimated to be around 130 ps. We performed this method on each channel in each sector. These parameters are now included into the Recsis data base. The next step will be to study the effect of these corrections on the beam data. S. Taylor: ---------- Preliminary gain-matching results for the forward angle time-of-flight counters were presented. The TOF gain-matching process involves a two-prong approach: the left PMT/right PMT gains are adjusted for each counter by exciting each scintillator at the center with a UV laser and measuring the mean ADC values, from which new high voltage settings can be estimated; and the counter-to-counter gains are adjusted using beam or cosmic ray data. The goal is to have the geometric mean of the ADC signals from each side of a counter for normally-incident minimum-ionizing particles show up at channel 600 +/- 5% after pedestal subtraction. Beam data from run 3693 suggests that we are within 20% of the goal for most counters, but the conditions for this run were not ideal for the purpose of gain-matching due to the presence of the main toroidal magnetic field. The laser data suggests that the left-right matching is better than +/-10% for most counters. P. Rossi: -------- Some preliminary results of June runs analysis have been reported. Run 3570, 3606, 3617, 3758-59 have been analysed for LAC sectors 2. To select minimum ionising particles (MIP) the istograms have been obtained filtering data with following cuts: 1)coincidence between left & right 2)exactly 1 signal for each side ( 8 signals). In this way one cross in upper part and one in lower are defined. 3)upper and lower cross position can be shifted at maximum by one pixel in each direction. The mean value of the energy deposition for the inner and outer part and left and right sides shows, on average, a peak at a channel lower than 500 (channel expected for MIP) for the runs 3750,3606,3617 (trigger: EC sectors 1,2,3,4,5,6). On the contrary no peaks were found for the same istograms for the runs 3758-59 (trigger: LAC sectors 2 ). An explanation for this last result could be found in the following reasons: 1) a too tight cut in the definition of the particle trajectory (upper and lower cross position can be shifted more than one pixel due to the track bending in the magnetic field) was applied 2) the trigger was only LAC sector 2 so all kind of particles could be reach the module with different energies 3) signals may not have been in time inside the gate. V. Burkert ---------- Priorities for CALCOM analysis: =============================== Priorities for the CALCOM analysis for the near term future (approximately two months) were discussed. Based on the data and preliminary results of the June commissioning run the following tasks have the highest priorities. (1) Background contributions in CLAS. The implementation of additional shielding around the downstream beam pipe has largely eliminated a major source of background in the downstream detectors (region3 DC, Cherenkov counters, TOF, EC). However, lower levels of background related to specific trigger configurations become more visible. These can, and must be reduced for the next electron run. Another source of background was presumably opened-up by our effort to increase the beam opening in the mini- torus, which in effect reduced the lead shielding for small angle scattered Moller electrons. Other sources of background could be related to the beam halo interacting with the target support structure or other massive structures around the beam line. To effectively combat these background contribtions a detailled study of the tracks origins is needed, using e.g. the zero-field, empty target runs. (2) Improvements in tracking accuracy. Many projects in the calibration and commissioning phase cannot be carried out, or are significantly limited due to the lack of accurate tracking in RECSIS. Hit-based tracking has helped in varies calibration projects (e.g. Cherenkov counter calibration at lower energies), however the kinematical resolution at 4 GeV is much too poor to be effective e.g. in the use of exclusive reactions for calibration purposes, such as elastic ep scattering, or single pion production. A significant effort is necessary to get the time-based tracking incorporated in the RECSIS frame work. An improvement of a factor of ten in position resolution would be of significant help in advancing the CLAS commissioning. (3) Calibration of the time-of-flight counters. Accurate calibration of the TOF counters, in conjuntion with particle tracking, is essential for particle identification. Progress on using exclusive reactions for calibration purposes depends largely on progress made in the TOF counter calibration. It is good to see that progress is made in this area, as evidenced by the two contributions presented here today (L.E., S.T.). (4) Detector alignment, vertex reconstruction, magnetic field distribution. With time-based tracking in place questions of drift chamber alignment, the correct reconstruction of the target position, and the magnetic field distribution may be adressed. These issues will be essential for the analysis of physics data later this year. (5) Understanding elastic and inclusive electron rates. One of the goals of the past run has been to get a more quantitative understanding of the electron scattering rates, especially the elastic ep rate, as well as the inclusive rates. Unfortunately, the Faraday cup information was not, or only sporadically read out. Therefore an absolute cross section measurement may not be possible. However, one could still use the elastic rate for normalization and determine the ratio inelastic/elastic rate. Again, this requires accurate tracking (TBT) and a much improved tracking efficiency. (6) Energy calibration of the EC and EC1 calorimeters. Significant amounts of data were accumulated for calibartion of the forward angle and the large angle calorimeters that should be used to obtain gain matching of pmts (using min dE/dx) as well as an absolute energy calibration using electrons or/and pi-zero identified in exclusive reactions. (7) Measurement of acceptance functions for charged particles in CLAS. Large amounts of data were taken during the June run with a "min dE/dx" trigger requirement in the forward calorimeter. This trigger proved extremely efficient in providing events with "minimum bias". They are typically quite clean and have usually one or more tracks (typically without an electron). As they are produced in the entire angular range, they are effective probes of the CLAS acceptances, and as such may be used to determine acceptance functions for single charged hadrons in CLAS. Single particle acceptance functions extracted from CLAS events may then be used as realistic input to GSIM. This will be essential for understanding acceptances for correlated particles.