1. CALICE 03/14/05Ed Norbeck U. of Iowa1 PPACs in a Calorimeter Edwin Norbeck University of Iowa

2. CALICE 03/14/05Ed Norbeck U. of Iowa2 What is a PPAC? Two flat, conducting plates with a little gas between them Simple, low cost device Can be radiation hard Unaffected by heat, light No electronics or photodetectors attached (Parallel Plate Avalanche Counter)

3. CALICE 03/14/05Ed Norbeck U. of Iowa3 One plate can be divided into pixels to provide position resolution. We have studied a small PPAC as a single pixel of a detector for electromagnetic showers. What we learned from these studies have broad application. Results from small PPAC Use of PPACs in calorimeters Outline of talk:

4. CALICE 03/14/05Ed Norbeck U. of Iowa4 Single Pixel PPAC For Test With High- Energy Electrons Gap 0.6 mm 950 V across gap Cathode 7X 0 = 29 mm of tantalum Area of anode is 1.0 cm 2 Guard ring to simulate neighboring pixels Gas is isobutane at 120 torr

5. CALICE 03/14/05Ed Norbeck U. of Iowa5 Detail of gap and guard ring

6. CALICE 03/14/05Ed Norbeck U. of Iowa6 Test at home with a 7 mCi 137 Cs source Get up to 20 mV signals directly into 50  coax

7. CALICE 03/14/05Ed Norbeck U. of Iowa7 Signal into coax with no amplifier Signal observed directly with fast scope

8. CALICE 03/14/05Ed Norbeck U. of Iowa8 We did not have high-energy electrons so we made them in situ from protons interacting near the front end of our tantalum cylinder. The showers had amplitudes as much as 40 mV

9. CALICE 03/14/05Ed Norbeck U. of Iowa9 Signal shape from shower -30 mV 1.62 ns FWHM

10. CALICE 03/14/05Ed Norbeck U. of Iowa10 The signal comes from moving charges. In an avalanche, most of the electrons and ions are formed near the anode. The electron signal is fast but with a total area small compared with the ion signal. The ion signal is flat while the ions are moving and stops when the ions are collected. In the next slide, when the ions are collected at the cathode they liberate electrons.

11. CALICE 03/14/05Ed Norbeck U. of Iowa11 20 torr ethane 550 V 0.6 mm gap

12. CALICE 03/14/05Ed Norbeck U. of Iowa12 PPAC gasses Alkanes [methane (CH 4 ), ethane (C 2 H 6 ), butane, etc.] good Perfluoro analogs (CF 4, C 2 F 6, etc.) even better We have used: CF 4 (AKA carbon tetrafluoride and Freon 14) C 3 F 8 (Perfluoropropane) C 4 F 8 (Perfluorocyclobutane)

13. CALICE 03/14/05Ed Norbeck U. of Iowa13 Advantages of Perfluoro gasses Not toxic Does not burn Resists aging Easily recycled (A small chemical cartridge can be inserted in the gas line that will remove all impurities from the gas) High density (more primary electrons) Contains no chlorine so is allowed in national laboratories

14. CALICE 03/14/05Ed Norbeck U. of Iowa14 CF 4 at one atmosphere Required 2500 V with plate spacing of 0.6 mm Less than 1.5 ns electron signal Ion collection time 960 ns PPAC design much simplified at one atm.

15. CALICE 03/14/05Ed Norbeck U. of Iowa15 Beam In Typical Calorimeter The green is solid metal (W). Detectors that sample the shower are shown in blue. Detector near front end is for EM shower

16. CALICE 03/14/05Ed Norbeck U. of Iowa16 For electromagnetic showers in a high Z material the final deposition of most of the energy is by low energy electrons. If the plates of a PPAC are made of the same high Z material, the PPAC will provide a faithful sample of the energy deposition in the absorber. This does not work for thicker detectors because the lower energy electrons stop in the surface of the detector. The sampling fraction in a PPAC is small. The fractional error from sampling fluctuations is proportional to E -½. Fluctuations are not a problem if the showers have a large enough energy.

17. CALICE 03/14/05Ed Norbeck U. of Iowa17 High-energy showers with double PPAC Test with EM showers using 80 ps bunches of 7 GeV positrons from the Advanced Photon Source, at Argonne National Laboratory Each bunch contained 3.6 x 10 10 positrons The showers were made by the beam halo striking the beam pipe. The energy was a small fraction of the 2 x 10 20 eV in the bunch This is still a very large energy! (Shower passes through both PPACs)

18. CALICE 03/14/05Ed Norbeck U. of Iowa18 Double PPAC for testing energy resolution

19. CALICE 03/14/05Ed Norbeck U. of Iowa19 Energy Resolution Data of PPAC Test at ANL Ratio E front to E back is constant to within ± 2%

20. CALICE 03/14/05Ed Norbeck U. of Iowa20 CONCLUSIONS PPACs for a calorimeter Can be made radiation hard. Can provide position information. Have good energy resolution for high energy showers. Have sub nanosecond time resolution. Can connect PPACs directly into 50  coax Can test with  source on side Can operate at atmospheric pressure with CF 4 gas

21. CALICE 03/14/05Ed Norbeck U. of Iowa21

22. CALICE 03/14/05Ed Norbeck U. of Iowa22 At isobutane pressures less than 30 torr afterpulses sometimes occur during the first 20 ns. This is a worst case example. Total charge from the afterpulses can be much larger than primary signal. 10 torr 500 V