PPAC Parallel Plate Avalanche Counter Edwin Norbeck University of Iowa For meeting at SLAC June 2, 2004

June 2, 2004PPAC E. Norbeck U. Iowa2 Typical low-pressure PPAC Two flat plates Separated by 2 mm Filled with 10 torr isobutane MIPs often leave no signal 700 V between plates Timing resolution better than 300 ps Used with 50 MeV/nucleon heavy ions

June 2, 2004PPAC E. Norbeck U. Iowa3 PPAC in Calorimeter Three flat plates, separated by 2 mm Middle plate at high voltage Outer plates hold atmospheric pressure Filled with torr of a suitable gas Gas flows in one side and out the other Timing resolution better than 300 ps Plate composition chosen to maximize signal, i.e. maximize conversion of soft photons to electrons

June 2, 2004PPAC E. Norbeck U. Iowa4 PPAC energy resolution Poor for single heavy ion Current per  m   is huge! Same size signal from shower should have good resolution. Measure resolution with double PPAC Look at ratio between two sides

June 2, 2004PPAC E. Norbeck U. Iowa5 Double PPAC for testing energy and time resolution. The PPAC detector concept can be developed as a candidate for the luminosity monitor. Iowa PPAC - a radiation hard detector

June 2, 2004PPAC E. Norbeck U. Iowa6 Tests with double PPAC Test with EM showers using 80 ps bunches of 7 GeV electrons from the Advanced Photon Source, at Argonne National Laboratory Planned test with low energy hadron showers using the 120 GeV proton test beam at Fermilab

June 2, 2004PPAC E. Norbeck U. Iowa7 PPAC Test at ANL IOWA double PPAC was tested for energy and time resolution with electron showers from the Advanced Photon Source (APS) at Argonne National Laboratory. The booster ring of the APS puts out 76 ps bunches of 7 GeV positrons at the rate of two per second, with 3.6 x positrons in each bunch. In normal operation the positrons are injected into the main storage ring where they are used to produce synchrotron radiation. There are maintenance and development periods during which the beam is directed into a beam dump. We set up our equipment next to the beam line just in front the beam dump. The entire beam bunch has an energy of 2.5 x eV, or 2.5 x 10 8 TeV, much more than we needed.

June 2, 2004PPAC E. Norbeck U. Iowa8 Results of PPAC Test at ANL To make use of this beam we placed the PPAC close to the beam line where it would be exposed to showers generated by the outer halo of the beam striking the beam pipe. Because of the small angle between the positrons and the wall of the beam pipe, the wall acted as an absorber with a thickness of several centimeters. The showers were developed in this absorber. We expected the time resolution between the front and back PPACs to be less than 300 ps. What we found was 3 ns (FWHM). This is still a fast signal even though it is an order of magnitude slower than expected. The poorer than expected time resolution was caused by noise that required the discriminator levels to be set high in order to eliminate spurious events. One source of noise was caused by the necessity, because of safety regulations, to have the power for the preamps near the beam line come from a wall plug in the beam tunnel while the rest of the electronics was powered from a wall plug in the floor above.

June 2, 2004PPAC E. Norbeck U. Iowa9 Timing resolution A lower limit on the expected timing resolution was measured by cross connecting timing signals from alpha particles. The results are shown in the next slide.

June 2, 2004PPAC E. Norbeck U. Iowa10 Timing resolution  = 95 ps

June 2, 2004PPAC E. Norbeck U. Iowa11 Energy Resolution Data of PPAC Test at ANL Ratio E front to E back is constant to within ± 2%

June 2, 2004PPAC E. Norbeck U. Iowa12 Electron and positive-ion currents The electrons are collected in 1 ns. It is the moving electrons that generate the signal that is measured. The current from the slow moving positive ions is smaller by a factor of a thousand. We have looked at the positive-ion signal using special electronics and find that it lasts for about a microsecond.

June 2, 2004PPAC E. Norbeck U. Iowa13 Background reduction The background can be reduced by subdividing the detector into small sectors. One such design has a single plate at high voltage and the grounded plate divided into small segments, of perhaps 1. cm 2. With such a small area the plate spacing would be reduced to 1 mm, which provides the additional benefit of a faster signal. Of course, the subdivision comes at the price of additional electronics to measure the signal size of each segment.

June 2, 2004PPAC E. Norbeck U. Iowa14 No “Texas tower effect” With above-atmosphere hydrocarbon gas occasional proton from n-p scattering gives huge signal. In PPAC, proton hits wall at almost full energy. PPAC signal mostly from low-energy electrons. We will test this with detailed simulations. (No multi-TeV hadron test beams.)

June 2, 2004PPAC E. Norbeck U. Iowa15 CONCLUSIONS PPACs for sampling calorimeters Can be made radiation hard. Have good energy resolution. Are fast—subnano-second time resolution. Can be made to reject background.