Lecture 2.3: Wire-based Detectors

1. Lecture 2.3: Wire-based Detectors

2. Geiger-Muller Counter 1928 Tube filled with a low pressure inert gas and an organic vapor or halogen and contains electrodes between which there is a voltage of several hundred volts but no current. Anode is a wire passing through it. Cathode is the walls. Avalanches in a Geiger Discharge Ionising radiation passing through the tube ionizes the gas. The free electrons are accelerated by the field. The avalanche begins as these in turn ionise more. Cathode UV photon Anode wire Cathode

3. Multiwire Proportional Chamber Invented by Georges Charpak in 1968. Nobel Prize in 1992.

4. MWPC The particle ionizes the gas producing electrons and free ions. The liberated electrons move rapidly move towards the anode wire and the ions towards the cathode plans More electrons are liberated which in turn ionise the gas. An avalanche of charges is produced giving rise to and electric pulse on the anode wire.

5. MWPC Grid of parallel thin anode wires between two cathode planes. Electrons drift to the anodes and are amplified in avalanche. Drift of ions produced in the avalanche induces a negative charge on the wire and positive charges on surrounding electrodes. Positive induced charge on adjacent wires overcomes the negative charge due to the large capacitance between the wires

6. Two-Dimensional Readout An MWPC with the cathode strips perpendicular to the wires. Charge profile recorded on both anodes and cathodes. Centre of gravity provides X and Y projections: Xi;Yi: Strip coordinates Qi(X), Qi(Y): Charge on strips Q(X), Q(Y): Total Charge 2D readout essential for medical imaging applications.

7. Applications of MWPCs Low dose X-ray digital radiography scanner based on the MWPC Applications include crystal diffraction, beta chromatography and dual energy angiography Film of congenital hip dislocation in a 7-year old boy. Satisfactory visualization of femoral architecture and bone structure

8. Applications in Medical Imaging Activity in a vasopressine-labelled rat’s brain (from CERN-Geneva hospital). E. Tribollet et al, Proc. Natl. Acad. Sci. USA 88(1991) 1466 Regional uptake of deoxyglucose in a dog’s heart M.G. Trivelli et al, Cardiovasclar Res. 26(1992) 330

9. Drift Chambers D An alternating sequence of wires with different potentials, there is an electric field between the ‘sense’ and ‘field’ wires. The electrons move to the sense wires and produces an avalanche which induces a signal read out by the electronics. The time between the passage of the particle and the arrival of the electrons is measured measure of the particles position. Can increase the wire distance to save electronics channels.

10. Typical Geometries of Drift Chambers W. Klempt. Detection of Particles with Wire Chambers, Bari ‘04

11. Straws If a single wire breaks in an MWPC the entire detectors is impacted. A solution is to replace the volume, with arrays of individual single-wire counters, known as straws. Typically a wire is strung between two supports within a thin straw (either metallic or with the internal surface coated with a metal) Portion of the ATLAS TRT End Cap

12. MDTs The ATLAS barrel muon spectrometers uses Monitored Drift Tubes. These reconstruct tracks to 100 μm accuracy.

13. ATLAS MDTs MDTs can also be used for making music! MDT pipe organ made by HenkTieke from NIKHEF, Amsterdam.

14. Time Projection Chamber (TPC) A TPC is a gas-filled cylindrical chambers (with parallel E and B field) with MWPCs as endplates. • Drift fields of 100-400 V/cm • Drift times 10 -100 μs • Distance up to 2.5 m Gas volume drift B Event display of a Au-Au collision in STAR at RHIC. Typically ~200 tracks per events. E Wire chamber

15. Modern TPCs STAR TPC ALICE TPC

16. Gas for TPCs A common gas filling used is 90% Argon, 10% CH4, but this has saturated drift velocity at low fields and transverse diffusion is reduced with a magnetic field. Best choice is CF4 because it has low diffusion even without a magnetic field. Requires high drift fields. Computed with MAGBOLTZ S. Biagi, NIM A42(1999) 234

17. Cherenkov Radiation Photons are emitted by a charged particle moving faster than the speed of light in a medium at an angle which depends on the particle’s velocity: β=1/n cos(θ) θ These are reflected on a spherical mirror. The radius of the ring is R = rθ/2 Cerenkov Radiation in the core of a nuclear reactor

18. RICH Detectors ALICE HMPID LHCb RICH Detector Can be used for particle identification together with tracking detectors

19. COMPASS RICH Event Display Array of 8 MWPCs with CsIphotocathodes

20. Time Resolution Time Resolution of Wire Chambers It takes the electrons some time to move from their creation point to the wire. Fluctuations in this primary deposit and diffusion times to ~5ns If one uses a parallel plate geometry with a high field, the avalanche starts immediately so that time resolutions down to 50 ps can be achieved. These detectors can be used for triggering.

21. Resistive Plate Chambers Place resistive plates (Bakelite or window glass) in front of the metal electrodes Sparks cannot develop because the resistivity and capacitance will allows only a very localized discharge. CMS RPCs Large area detectors can be made Rate limit of kHz/cm2

22. ALICE TOF Detector Large Time-Of-Flight (TOF) system with 50 ps time resolution made from window glass and fishing line (high precision spacers) Before RPCs were available, very expensive photomultipliers were used with scintillators