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PPAC in a ZDC

E. NorbeckUniversity of Iowa

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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)

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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.

•PPAC design for ZDC

•Results from small PPAC

•Use of PPACs in calorimeters

Outline of talk

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Individual PPACs for ZDC

Individual PPACs can be replaced if necessary

Rigid coax and gas lines extend to top of TAN

No organic materials in high-radiation region

Beam In

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PPAC Array for ZDC EM Front-end

Coax and gas lines extend out of radiation area

No organic materials in high-radiation region

Beam In

Gives excellent position and time resolution.

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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

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Detail of gap and guard ring

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Test at home with a 7 mCi 137Cs source at the side of the detector

Get up to 20 mV signals directly into 50 coax

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Signal into coax with no amplifier

Signal observed directly with fast scope

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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

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Signal shape from shower

-30 mV

1.62 ns FWHM

This signal is so fast that one third of its amplitude is lost in 60 feet of standard RG58 coax.

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Beam In

Typical Calorimeter

The green is solid metal (W). Detectors that sample the shower

are shown in blue.

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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.

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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)

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Double PPAC for testing energy resolution

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Energy Resolution Data of PPAC Test at ANL

Ratio Efront to Eback is constant to within ± 2%

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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

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