PPAC in a ZDC

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PPAC in a ZDC
E. Norbeck University of Iowa
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What is a PPAC?
(Parallel Plate Avalanche Counter)

• 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

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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.
Outline of talk

• PPAC design for ZDC
• Results from small PPAC
• Use of PPACs in calorimeters

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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
Gives excellent position and time resolution.
Beam In
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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 W 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
This signal is so fast that one third of its
amplitude is lost in 60 feet of standard RG58
coax.
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Typical Calorimeter
Beam In
The green is solid metal (W). Detectors that
sample the showerare 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
(Shower passes through both PPACs)

• 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!

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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

• Can connect PPACs directly into 50 W coax
• Can test with g source on side

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.

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