A New Look at the PPAC

21st WW 02/11/05 Ed Norbeck U. of Iowa 1

A New Look at the PPAC

E. Norbeck, J.E. Olson, and Y. Onel

University of Iowa

For the 21st Winter Workshop On Nuclear Dynamics


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)


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


Single Pixel PPAC For Test With High-Energy Electrons

• Gap 0.6 mm 950 V across gap• Cathode 7X0 = 29 mm of tantalum• Area of anode is 1.0 cm2

• Guard ring to simulate neighboring pixels

• Gas is isobutane at 120 torr


Detail of gap and guard ring


Test at home with a 7 mCi 137Cs source

Get up to 20 mV signals directly into 50 coax


Signal into coax with no amplifier

Signal observed directly with fast scope


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


Signal shape from shower

-30 mV

1.62 ns FWHM


•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 the ions are collected at the cathode they liberate electrons.


20 torr ethane 550 V 0.6 mm gap


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


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


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.


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 1010 positrons

•The showers were made by the beam halo striking the beam pipe. The energy was a small fraction of the 2 x 1020 eV in the bunch

•This is still a very large energy!

(Shower passes through both PPACs)


Double PPAC for testing energy resolution


Energy Resolution Data of PPAC Test at ANL

Ratio Efront to Eback is constant to within ± 2%


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

•With isobutane need more than 30 torr for good energy resolution


Documents

A New Look at the PPAC