Lecture 18 New gas detectors Solid state trackers
Time projection Chamber Full 3-D track reconstruction x-y from wires and segmented cathode of MWPC z from drift time de/dx information (extra) Drift over long distances à very good gas quality+ uniformity of field required Space charge problem from positive ions, drifting back to central wall à timing gate
Microstrip gas chambers Higher speed and more precision -> smaller structure drift electrode (ca. -3.5 kv) semiconducting glass coating, r=10 16 Ω/cm gas volume 80 µm 10 µm 100 µm 3 mm Gold strips + Cr underlayer C (-700V) A substrate 300 µm backplane Gas: Ar-DME, Ne-DME (1:2), Lorentz angle 14º at 4T. Gain ~10 4 Passivation: non-conductive protection of cathode edges Resolution: ~ 30-40 mm Aging: Seems to be under control (expect a total charge ~ 100 mc/cm in 10 years at LHC)
Microstrip gas chambers Field geometry A ions C Fast ion evacuation à high rate capability ~ 10 6 /(mm 2 /s)
GEM Gas Electron Multiplier (R. Bouclier et al., NIM A 396 (1997) 50) 140 200 µm 50 120 µm 50 µm Kapton + 2 x 5-18 µm Copper Micro photo of a GEM foil
Single GEM + readout pads
Double GEM + readout pads Fast ion evacuation à high rate capability ~ 10 6 /(mm 2 /s) Ø Same gain at lower voltage Ø Less discharges
Solid State Detector A solid state detector is an ionization chamber Ionizing radiation creates electron/hole pairs Charge carriers move in applied E field Motion induces a current in an external circuit, which can be amplified and sensed. Free carriers must first be removed so the applied voltage doesn t simply result in a DC current this is usually accomplished with a reverse biased diode.
Signal properties Energy (E) to create e-h pair in silicon is ~3.6 ev ~10 times smaller than gas ionization ~30 ev increase charge good E resolution ΔE E 1 N 1 E /ε i ε i Specific density ρ = 2.33 g/cm 3 reduced range of secondary electrons > good spatial resolution average energy loss of MIP track ~390 ev/µm ~108 e-h pairs/µm no charge multiplication (charge proportional to thickness) Typical thickness 300 µm -> ~32,000 e-h pairs (strong signal) Charge mobility <20 ns (fast signal) as compared to 100 ns 10 µs in gas
Silicon detectors E conductance band e In a pure intrinsic (undoped) material the electron density n and hole density p are equal. n =p = n i E f h valence band For Silicon: n i ~ 1.45 10 10 cm -3 In this volume we have 4.5 10 8 free charge carriers, but only 3.2 ~10 4 e-h pairs produced by a M.I.P. 300 µm 1 cm 1 cm -> Reduce number of free charge carriers, i.e., deplete the detector Most detectors use reverse biased p-n junctions
Silicon detectors Solid state detectors for energy measurements (Si, Ge, Ge(Li)) Can be uses as precision trackers! Characteristic numbers for silicon G Band gap: E g =1.12 V. G E(e - -hole pair) = 3.6 ev, (~ 30 ev for gas detectors). G High specific density (2.33 g/cm 3 ) à ΔE/track length for M.I.P. s.: 390 ev/mm à 108 e-h/ mm (average) G High mobility: m e =1450 cm 2 /Vs, m h = 450 cm 2 /Vs G Microelectronic techniques à small dimensions à fast charge collection (<10 ns). G Rigidity of silicon allows thin self supporting structures. Typical thickness 300 mm à ~ 3.2 10 4 e-h (average) H Downside: No charge multiplication mechanism!
E Doping E E f C B e CB V B E f h VB n-type: Add elements from V th group, donors, e.g. As. Electrons are the majority carriers. p-type: Add elements from III rd group, acceptors, e.g. B. Holes are the majority carriers. doping concentration detector grade 10 12 cm -3 (n) - 10 15 cm -3 (p + ) electronics grade 10 17(18) cm -3 resistivity ~ 5 kω cm ~1 Ω cm E p CB n pn junction e. V E f VB There must be a single Fermi level! Deformation of band structure à potential difference.
Silicon detectors Application of a reverse bias voltage (about 100V) the thin depletion zone gets extended over the full junction fully depleted detector. Energy deposition in the depleted zone due to traversing charged particles or photons (X-rays) creates free e - -hole pairs. Under the influence of the E-field, the electrons drift towards the n-side, the holes towards the p-side detectable current. Spatial information by segmenting the p doped layer single sided microstrip detector. 50-150 mm readout capacitances SiO 2 passivation 300mm
Silicon detectors diffusion of e - into p-zone, h + into n- zone à potential difference stopping diffusion thin depletion zone no free charge carriers in depletion zone A. Peisert, Instrumentation In High Energy Physics
Silicon pixel detectors Segment silicon to diode matrix Readout electronic with same geometry Connection by bump bonding technique Flip-chip technique detector electronics bump bonds