Seminar talks. Overall description of CLAS12 (Jefferson Lab) MAPS. Talks on Feb. 6 th, (Contact JR) (Contact TS)

Transcription

1 Seminar talks Overall description of CLAS12 (Jefferson Lab) (Contact JR) MAPS (Contact TS) Talks on Feb. 6 th, 2015

2 Review old ionization detectors: Emulsion, Cloud chambers, Ionization chambers, Spark chambers Transport of charge in gas Primary ionization Diffusion Drift E and B fields Recombination Avalanche/Quenching Signal generation

3 Operation modes Ionization chamber Geiger-Müller counter Proportional counter

4 Ionization chamber An ionization chamber collects the generated charge (from ionization) without amplification I ~ radiation dose in the Ionization volume per time Impossible to measure a single ionization: electron charge (= - ion charge) = Coulomb (for 1 na 1000 ions / s) electronics needs at least Coulomb (fc) è If we want to see a single ionization we need an amplification of at least 10,000 times without amplification of the noise

5 Proportional Counter Gas amplification via secondary ionization: Amplification from 10 3 up to Small wire diameter necessary

6 Plateau Region Below a certain voltage part of the electrons recombine with ions before they are collected at the anode wire. Above this voltage the countrate is independent of the voltage (plateau). For even higher voltages the countrate rises again because of the creation of UV photons which can trigger secondary avalanches

7 Proportional Region Voltage region were the collected charge is proportional to the number of primary electron-ion pairs. This area ends with the creation of photo electrons.

8 Geiger-Müller Counter Proportional area is left behind. Collected charge is independent of the primary ionization. Typical amplification 10 8 Quenching gas is necessary to absorb photons created by recombination. (e.g. addition of multi atomic organic gases like alcohol)

9 Streamer Mode Thich anode wires 100µm High voltage (5 kv) High quenching gas content > 50% Gas amplification typ Avalanche stays at the transmission point Long dead time Discontinous transition between proportiona and streamer mode

10 Comparison Gas discharge (wire is vertical, particle trajectory is horizontal)

11 Space point measurement with MWPC Multi-Wire-Proportional-Chamber (MWPC): Plane layer of proportional chambers without intermediate walls Anlode wire, typ. gold plated tungsten (10-30 µm) Gas amplification up to 10 5 Typ. distance d = 2 mm Space resolution RMS = d / 12 ~ 600 µm Signal amplitude at the cathode depends from distance to avalanche

12 Multi-Wire-Proportional-Chamber Field lines of a MWPC Scetch of the cathode readout of a MWPC: Pros and Cons: Simple and robust Wire instability via electro static repulsion and pass through of particles è distortion of field If cathode readout is used: higher mechanical work load and more complicated signal processing

13 Drift chambers Space point = drift velocity x drift time Resolution: approx µm

14 Drift chamber Comparable to MWPC, but much less wires Prinziple: propagation time of avalanche x = v t typ. resolution 100 µm

15 Jet Drift Chamber r-φ point from drift time (σ~0.1mm) z-position from charge sharing (σ~1cm), runtime differences or stereo wires left-right ambiguity solved by displacement of anode wires Lorentz angle because of crossing E and B field Hier: K 0 S,K+,π +

16 Jet Drift Chamber

17 Time Projection Chamber (TPC) Setup: The detector volume has no anode or potential wires The generated charge carriers drift to the end cap detectors Typical: Electrical drift field and magnetic field (for track bending) are parallel (E~ V/cm/atm) Drift velocity ~ 5-7 cm/µs Space resolution: ( rϕ ) 200µ m σ ( z) = 1mm σ =

18 TPC Setup Electrons are collected at the gating grid until the gate is opened by a trigger. After the grid the e - are accelerated (avalanches). e - are collected at the wire and ions at the pads. Space resolution improved by chevron pads:

19 Solenoidal Tracker At RHIC STAR Transp ort

20 STAR in CAVE

21 Introduction Overview of detector systems Sources of radiation Radioactive decay Cosmic Radiation Accelerators Content Interaction of Radiation with Matter General principles Charged particles heavy charged particles electrons Neutral particles Photons Neutrons Neutrinos Definitions Detectors for Ionizing Particles Principles of ionizing detectors Semiconductor detectors Semiconductor basics Sensor concepts

22 Semiconductor detectors Different detector materials Readout electronics Gas detectors Principles Detector concepts Scintillation detectors General characteristics Organic materials Inorganic materials Light output response Content Velocity Determination in Dielectric Media Cerenkov detectors Cerenkov radiation Cerenkov detectors Transition Radiation detectors Phenomenology of Transition Radiation Detection of Transition Radiation Complex Detector Systems Particle Identification with Combined Detector Information Tracking

23 Lecture 7 Scintillating Detectors and Calorimeters

24 Scintillation photodetector Energy deposition by ionizing particle à production of scintillation light (luminescense) Scintillators are multi purpose detectors: calorimetry time of flight measurement tracking detector (fibers) trigger counter veto counter Two material types: Inorganic and organic scintillators

25 Organic materials sp 2 -hybridisation: 2p x and 2p y mix with s-orbital à σ-orbital p z remains unchanged à π-orbital

26 Pi electron energy levels Organic scintillators: Monocrystals or liquids or plastic solutions Monocrystals: naphtalene, anthracene, p-terphenyl. Liquid and plastic scintillators They consist normally of a solvent + secondary (and tertiary) fluors as wavelength shifters. Fluorescence sec peak ~ 320 nm ~10-11 sec non-radiative transition ~ 10-6 sec (Förster transf.) Phosphorescence 10-4 sec Fast energy transfer via non-radiative dipole-dipole interactions (Förster transfer). shift emission to longer wavelengths longer absorption length and efficient read-out device

27 Wavelength shifting no self-absorption also used for light re-direction