Drift chamber Lecture 7 Drift chamber Basic idea

Drift chamber Lecture 7

Drift chamber Basic idea: Obtain spatial information from drift time of the electrons from ionization. Natural extension of MWPC concept (Charpak 1969) Drift path: If v. D constant: Example: Typical drift cell dimension: Drift detector principle: TDC: Time to digital converter x

Drift chamber designs Planar: Cathode wires at linearly varying potentials -> uniform drift, also possible is cathode planes and field shaping wires. By clever design one can obtain linear time to distance relation == > good resolution eeee- Same E–field in all space points, larger distance from anode needs higher voltage because [E]=[V/m] particle Cylindrical: At collider experiments the chambers surround the interaction point. To measure the momentum a solenoidal B-field is applied. Cylindrical layers of drift cells, axial and stereo layers for z-coordinate. (see the example of the Belle CDC)

Belle CDC • 50 cylindrical layers, 23 of which stereo (40 -70 mrad) • Cathode read-out for innermost layers == > instant z-information (trigger) • 8400 square drift cells, 16 mmx 17 mm • Sense wires: gold plated tungsten, 30μm diameter • Field wires aluminum , 126μm • Spatial resolution, 130μm

Drift chamber derivatives LHCb OT Straw tubes: Thin cylindrical cathode, 1 anode wire, example LHCb outer tracker. Outer Tracker : 450 cm 5 mm diameter (polyamide, Kapton) 25 um gold-tungsten anode wires 5° stereo angle Gas Ar/CF 4/C 02 (75/15/10) 5 m long ! Two layers per station Inner Tracker 595 cm

Time projection chamber (TPC) The “ultimate tracking detector” (David Nygren, 1978), for collider experiments with solenoidal B-field. Full 3 -D information: typical resolution (Aleph) z: drift time measurement r: anode wires φ: cathode pads ALEPH TPC Drift over 1. 2 m what about diffusion? -> strongly reduced by magnetic field (by a factor ~10) But E-and B-field must be aligned ! Problem from space charge of avalanche ions drifting to high voltage plane, can distort E-field! Solution: Grounded grid close to anode wire plane captures ions (ion gate). cathode pads anode wires

MPGD (Micro Pattern Gas Detector) MPGD development has been started in 1988, Anton Oed, introduced a novel concept in detection, the MSGC (Micro Strip Gas Chamber). It consists of a set of tiny metal strips engraved on a thin insulating support, and alternatively connected as anodes and cathodes. Avalanche multiplication occurs in the high field region as for the proportional counter. It was indented to fill a gap in available detectors technologies, between solid state strip detectors, having excellent performances but high costs, and the cheap but rate-limited due to the slowly moving ions traditional gas devices. drift electrode 3. 5 k. V Fast evacuation of avalanche ions, they don’t have to drift back through drift volume! Resolution 30 -40μm

MPGD (Micro Pattern Gas Detector) Close view of a MSGC after long-term irradiation. An insulting, transparent film coats the irradiated areas. The cathode edges are damaged by micro discharges. Special gases (dimethylehter and carbon tetrafluoride) can reduce aging effects under radiation. drift electrode 3. 5 k. V The appearance of discharges during operation is a permanent problem with all gas micro-pattern detectors. When the total charge exceeds a value between 107 and 108 electron-ion pairs, an enhancement of the electric field in front and behind the primary avalanche induces the fast growth of a long , filament –like streamer.

GEM (Gas Electron Multiplier) The Gas Electron Multiplier (GEM) consists of a thin, metal-clad polymer foil, chemically pierced by a high density of holes (typically 50 to 100 per mm 2). On application of a difference of potential between the two electrodes, electrons released by radiation in the gas on one side of the structure drift into the holes, multiply and transfer to a collection region. Each hole acts as an individual proportional amplifier. The multiplier can be used as detector on its own, or as a preamplifier in a multiple structure; in this case, it permits to reach large overall gains in harsh radiation environment. Field lines and equipotentials in the GEM holes on application of a voltage between the two metal sides. A Drift (top) and Transfer field (bottom) transport ionization electrons into and out of the holes: Close view of a GEM electrode, etched on a metal-clad, 50 µm thick polymer foil. The hole's diameter and distance are 70 µm and 140 µm

GEM (Gas Electron Multiplier) Schematics of a single GEM detector. A triple-GEM detector: gain sharing between Electrons released by ionization in the top the foils improves the reliability of operation gas volume drift and multiply in the holes; at high gains the charge is collected on the anode, with 1 D or 2 -D projective strips, pads or other patterns Proportional gain in a single, double and triple. GEM detector as a function of voltage (equal on each foil)