PPAC Jonathan Olson University of Iowa Thesis Defense 8 April 2005

2 What have we done? Investigate PPACs for use as a high energy particle detector It should be useful in a calorimeter looking for shower particles

3 Example of low-pressure PPAC (Parallel Plate Avalanche Counter) Two flat plates Separated by 1 mm Filled with 100 torr hydrocarbon 1000 V between plates

4 Non-homogenous Sampling Calorimeter Absorber sandwiched detectors

5 How it works Particle enters the calorimeter Particle develops a shower in the absorber material Shower particles ionize the active gas medium in the detector Avalanche results from ionization We collect the ionization charge and try to determine the energy of the initial particle

6 A Look at the Detectors “Alpha” PPAC Double PPAC Pixel PPAC (aka Electron PPAC) New Pixel PPAC (just built, not yet used)

7 Alpha PPAC  First detector built  intended to study gas mixtures

8 Double PPAC Intended to test energy and time resolution

9 Single Pixel PPAC 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 Detail of gap and guard ring

10 How we readout the signal

11 Signal Shape 37 mV in 1.6 ns ~1 mV in 500 ns ions electrons

12 Motion of the charges Anode Cathode

13 Afterpulses Delayed photo-ionization, Photoelectric emission, Electron detachment

14 20 torr ethane 550 V 0.6 mm gap

15 Electromagnetic Shower Test (APS at ANL) PPAC under beam line to beam dump

16 Conditions of the beam at APS 7 GeV positrons 76 ps bunches 2 Hz bunches 3.6 x positrons/bunch. The entire beam bunch has an energy of 2.5 x eV, or 2.5 x 10 8 TeV !!! Outer halo of the beam hit the beam pipe. Wall acted as an absorber

17 Energy Resolution at APS Ratio E front to E back is constant to within ± 2%

18 Hadronic Shower Test (MTBF at FNAL)

19 Fermilab’s Accelerator Chain

20 Shower from protons interacting near the front end of our tantalum cylinder. The showers had amplitudes as much as 40 mV MTBF with the Pixel PPAC

21 MTBF with the Pixel PPAC 37 mV in 1.6 ns ~1 mV in 500 ns

22 Gamma Source

23

24

25

26

27

28 Gases that have been used C 4 H 10 C 2 H 6 CH 4 Argon - CO 2 Alkanes work well as avalanche gases Can be operated at 1 atm pressure and low voltage (~1 kV) C 3 F 8 CF 4 C 4 F 8 Additional benefit with perfluoro-analog

29 Why Fluorocarbons? Have fast electron drift velocity to give even faster signal than C 4 H 10 Molecules have high cross-section No Hydrogen – so even less likely to have Texas Tower effect Shouldn’t polymerize or age No health or fire hazard Can be easily purified and reused

30 Radiation Resistant A PPAC can be entirely metal and ceramic so that it will not be damaged by radiation levels that would melt scintillators

31 For high speed, the RC time constant must be kept small. Only PPACs of small area are fast, ~1 ns R = 50 Ω (coax cable). C is the capacity between the plates Small PPAC ~1 ns C =.885 pF for 1 mm gap and area of 1 cm 2 Larger PPAC with C = 168 pF for 2 mm gap and area of 1 cm 2 rise time ~5 ns fall time ~7 ns Speed vs Size

32 Ion and electron signals with 2 mm spacing 168 pF 1.6  s 6.2ns 1.3  s Amplified signal using gamma source. Positive overshoot is from amplifier. Ion collection time is three times as long with the 1 mm spacing.

33 Individual PPAC to replace Scintillators Coax and gas lines extend out of radiation area No organic materials in high-radiation region Beam In

34 PPAC Readout - + Summing amplifier can be used to add PPAC signals, increasing the effective size of PPAC (without increasing the time width of the signals)

35 CONCLUSIONS PPACs Have good energy resolution. Have sub nanosecond time resolution. Can be made radiation hard. Can provide position information by making into pixels.