small, fast, low-pressure gas detector

Small, fast, low-pressure gas detector

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

University of Iowa

For DNP04 at Chicago October 2004

E. Norbeck U. Iowa

DNP04 BB.014 Gas Detector 2

Typical low-pressure PPAC

• Two flat plates

• Separated by1-3 mm• Filled with 10-80 torr

isobutane• 500-1000 V between plates

(Parallel Plate Avalanche Counter)

E. Norbeck U. Iowa


Small PPAC for showers from high-energy (10-1000 GeV)

electrons

• The original object of this study was to determine the suitability of a PPAC as an inexpensive, very fast, rad-hard pixel detector to use in a calorimeter for electrons.

• Our measurements have broader application.

E. Norbeck U. Iowa


Single Pixel PPAC For Test With High-Energy Electrons

• Gap 1.0 mm

• Cathode 7X0 = 29 mm of tantalum

• Area of anode is 1.0 cm2

• Guard ring to simulate neighboring pixels• Gas is isobutane at 10 to 100 torr

E. Norbeck U. Iowa


Detail of 1 mm gap and guard ring

E. Norbeck U. Iowa


• A MIP will usually leave no ionization in the low pressure gas. With a high-energy electron shower there are 100s or 1000s of electrons contributing to the signal.

• To date we have not yet put a high-energy electron into the detector.

• Our measurements have all been with Compton electrons from a 137Cs gamma source. With the source to the side of the PPAC, a few of the electrons travel parallel to the face of the plates and produce a usable amount of ionization in the gas.

E. Norbeck U. Iowa


1.8 ns

50 torr 790 V7 mv

into

50

Electron signal

Single peak with considerable noise. The noise is large because of the small size of the signal using our 137Cs source. With the much larger signals from high-energy electrons, the noise will be negligible.

E. Norbeck U. Iowa


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 C = .885 pF for 1 mm gap and area of 1 cm2

For our larger PPAC with C = 168 pF rise time ~5 ns fall time ~7 nsFast enough for a Zero Degree Calorimeter at the LHC where minimum beam crossing time is 25 ns.

E. Norbeck U. Iowa


Ion collection time

.3 s0.5 s

50 torr 790 V

E. Norbeck U. Iowa


Signal out

Guard ring

Reflections are a problem with such fast signals.

Should be 50 Ω all the way to the anode.

View with covers removed

E. Norbeck U. Iowa


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

E. Norbeck U. Iowa


The afterpulses seen here are usually hidden inside of signals that are more than 20 ns wide. This may be the cause of the typically bad energy resolution of PPACs operated in the 5 to 20 torr range.

What causes the afterpulses?They are most likely caused by UV photons producing photoelectrons at the cathode. These electrons then initiate a new avalanche.

Changing the anode from stainless steel to graphite had no effect on the afterpulses. This shows that the photons do not come from the anode.

E. Norbeck U. Iowa


Perhaps the excited molecules emit photons with a lifetime long compared with 20 ns, with molecular collisions limiting the lifetime of the excitations. Collision time in isobutane gas is too long to account for the data.

Isobutane speed 350 m/sFragments are fasterIon speed > 2000 m/s (1 mm in 500 ns)

Note also that electrons acquire a larger energy between collisions at the lower gas pressures.

500 V at 10 torr but 1000V at 80 torr

E. Norbeck U. Iowa


Ion current from same event

Afterpulses are real avalanches

E. Norbeck U. Iowa


The area under the ion peak is clearly larger than the area under the electron peak.

The signal is caused by the motion of the charges in the 1 mm gap (not by the collection of the charges). Most of the charges generated by the avalanche are produced close to the anode so that electrons move only a short distance, while the ion move almost the entire millimeter.

Signal processing can easily remove the slow ion peak form the signal.

E. Norbeck U. Iowa


PPAC can be made resistant to radiation

damage • The walls and electrodes can be made of

durable metal in high-energy applications. • A single spark can make a sharp point on the

metallic surface of the cathode that will make the PPAC inoperable. The energy carried by a spark must be kept small, and provision must be made to keep sparking to a minimum.

• Aging (polymerizing of the gas) must be prevented. (Low pressures and short distances require special considerations.)

E. Norbeck U. Iowa


Conclusions

• Small area PPACs can be made to be radhard and fast ~ ns.

• PPACs have been used for 30 years, but more research is still needed maximize their potential.

small, fast, low-pressure gas detector

Documents