PPACs for HE upgrade

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PPACs for HE upgrade

• E. Norbeck, J. E. Olson, A. Moeller,
• F. D. Ingram, and Y. Onel
• University of Iowa
• For Eclipse 2006

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What is a PPAC?
(Parallel Plate Avalanche Counter)

• Two flat plates
• Separated by 0.5 to 4 mm
• Filled with 5 torr to 1 atm of suitable gas
• 500 V to 3500 V between plates
• Excellent timing resolution.
• Signals large, can go directly into 50 O cable

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PPACs at one atmosphere
For PPACs in vacuum, for low energy heavy ions,
plates must be as thin as possiblemust use low
pressure. For HE upgrade PPACs are in air, best
to operate at 1 atm Small gas leaks are
unimportant if gas is non-toxic and nonflammable.
Get useful signals from MIPs, a dozen primary
electrons Gas gain 104 To keep voltage under 4
kV, use plate spacing 0.5 mm. To fine tune gain
HV may need to be adjusted for temperature and
barometric pressure.
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Best gas we have found so far is R134A(after
testing dozens of gasses and gas mixtures)
Tetra-fluoro-ethane Used in automobile air
conditioners We found automobile grade as good as
CP grade Excellent for anti-aging (use gold
plated electrodes) Little hydrogen (big signals
from n-p) Large stopping power (MW 102) Self
quenching No gas mixing required
F3CCH2F
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Double PPAC for testing energy resolution
Plate spacing 0.6mm Diameter 10 cm
Requires 3500 V
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Tests with double PPAC

• Test with EM showers using 80 ps bunches of 7
  GeV positrons from the Advanced Photon Source,
  at Argonne National Laboratory
• Test with hadron showers using the 120 GeV
  proton test beam at Fermilab

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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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120 GeV proton test
Placed detector after 26 cm of iron to develop
shower Measured Ef vs Eb Compare with similar
measurements with pairs of scintillators and
Cerenkov detectors
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Double PPAC in FNAL Test Beam
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Double PPAC in FNAL Test Beam
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Double PPAC in FNAL Test Beam
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Ef vs Eb for the PPAC
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HE scintillator vs Quartz-plate Cerenkov detector
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Same PPAC data plotted with a different vertical
scale
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Signal Shape
Signal consists of a large, fast electron signal
followed by a small, slow ion signal. Speed of
electron signal limited by RC time constant R
50 O (coax cable). C is capacity between the
plates For a small area PPAC, FWHM 1.3 ns For
large area double PPAC electron signal gone in 10
nsStill fast enough for beam crossing time of 25
ns.
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PPAC ion signal

• For most gasses the fast electron signal is
  followed by a small, slow (500 ns) signal from
  ions moving between the plates.
• With R134A the ion signal is very small but
  lasts for 2500 ns.

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Tetra-fluoro-ethane
R134a
Less stable form
F3CCH2F
F2HCCHF2
Large electric dipole moment (about the same as
for water)
Symmetric No dipole moment
Fluorine atoms (blue) attract electrons and so
are negative
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Moving ion slowed by cloud of polar molecules
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-

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Symmetric C2F4H2 should have normal ion speed
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Work to be done

• Gasses
• Eliminate hydrogen entirely
• Reduce HV requirements but keep gas self
  quenching
• Detector design
• Subdivide into pixels, each with its own
  resistor to prevent damage in case of a spark
• Finally, use in HE will require a rad-hard design
  that meets all of the restrictions of the HE
  location.