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Small Fast Gas Detector for High-Energy Electrons

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

University of Iowa

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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

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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

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Single Pixel PPAC

• 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

Detail of gap and guard ring

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New Detector with less inductanceSame signal width of 1.5 ns for isobutane at 80 torr.

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Test at home with a 7 mCi 137Cs source

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

C(out)

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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.

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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

One third of amplitude of fast signal is lost in 20 m of standard RG-58 coax.

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•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.

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20 torr ethane 550 V 0.6 mm gap

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Gases that have been used

• C4H10

• C2H6

• CH4

• Argon -CO2

Alkanes work well as avalanche gases

But only at 1 atm pressure

• CF4

• C3F8

• C4F8 cyclo

Additional benefit with perfluoro-analog

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Many advantages to operating at atmospheric pressure

CF4 gives nice signals at 2500 V

95% Ar + 5% CO2 operates at 1100 V, but with a slow, 5 ns, fast signal. It also has a smaller useful operating range than CF4

For 0.6 mm spacing

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Advantages of Perfluro gasses

Not toxicDoes not burnResists agingEasily 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 laboratoriesWe found good results with 0.6 mm spacing, but not so good with 2.5 mm.

More study on this is needed.

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One plate can be divided into pixels to provide position resolution.

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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)

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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.

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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