E. Norbeck U. IowaFast Gas Detector APR 05 Tampa1 Small Fast Gas Detector for High-Energy Electrons E. Norbeck, J.E. Olson, and Y. Onel University of Iowa

E. Norbeck U. IowaFast Gas Detector APR 05 Tampa2 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 Simple, low cost device Unaffected by heat, light Can be radiation hard

E. Norbeck U. IowaFast Gas Detector APR 05 Tampa3 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 Speed vs Size

E. Norbeck U. IowaFast Gas Detector APR 05 Tampa4 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 Detail of gap and guard ring

E. Norbeck U. IowaFast Gas Detector APR 05 Tampa5 New Detector with less inductance Same signal width of 1.5 ns for isobutane at 80 torr.

E. Norbeck U. IowaFast Gas Detector APR 05 Tampa6 Test at home with a 7 mCi 137 Cs source Get up to 20 mV signals directly into 50  coax

E. Norbeck U. IowaFast Gas Detector APR 05 Tampa7 Signal into coax with no amplifier Signal observed directly with fast scope C (out)

E. Norbeck U. IowaFast Gas Detector APR 05 Tampa8 The signal is divided between the detector capacitance and the output capacitance. The sum of the two provides energy for damaging sparks. For small detectors we use a ratio of 20. For large detectors we make them equal.

E. Norbeck U. IowaFast Gas Detector APR 05 Tampa9 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

E. Norbeck U. IowaFast Gas Detector APR 05 Tampa10 Signal Shape from Shower -30 mV 1.62 ns FWHM One third of amplitude of fast signal is lost in 20 m of standard RG-58 coax.

E. Norbeck U. IowaFast Gas Detector APR 05 Tampa11 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.

E. Norbeck U. IowaFast Gas Detector APR 05 Tampa12 20 torr ethane 550 V 0.6 mm gap

E. Norbeck U. IowaFast Gas Detector APR 05 Tampa13 Gases that have been used C 4 H 10 C 2 H 6 CH 4 Argon - CO 2 Alkanes work well as avalanche gases But only at 1 atm pressure CF 4 C 3 F 8 C 4 F 8 cyclo Additional benefit with perfluoro-analog

E. Norbeck U. IowaFast Gas Detector APR 05 Tampa14 Many advantages to operating at atmospheric pressure CF 4 gives nice signals at 2500 V 95% Ar + 5% CO 2 operates at 1100 V, but with a slow, 5 ns, fast signal. It also has a smaller useful operating range than CF 4 For 0.6 mm spacing

E. Norbeck U. IowaFast Gas Detector APR 05 Tampa15 Advantages of Perfluro 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 We found good results with 0.6 mm spacing, but not so good with 2.5 mm. More study on this is needed.

E. Norbeck U. IowaFast Gas Detector APR 05 Tampa16 One plate can be divided into pixels to provide position resolution.

E. Norbeck U. IowaFast Gas Detector APR 05 Tampa17 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)

E. Norbeck U. IowaFast Gas Detector APR 05 Tampa18 Cautions Simple scaling of plate spacing, pressure, voltage, etc. does not apply over a wide range of parameters. Look at wave form to make sure the signal is clean.

E. Norbeck U. IowaFast Gas Detector APR 05 Tampa19 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