Resistive Plate Chambers for Time-of-Flight P. Fonte LIP/ISEC - PowerPoint PPT Presentation

Resistive Plate Chambers for Time-of-Flight P. Fonte LIP/ISEC Coimbra, Portugal. Compressed Baryonic Matter May 13 – 16, 2002 GSI Darmstadt/Germany CBM2002@GSI 1

Plan of the Presentation • Main physical mechanisms • Description of several. interesting devices • High frequency front-end electronics • Small segmented counters • Bidimensional position-sensitive single-gap counters • Large area • Practical difficulties to be addressed for future applications • Crosstalk • Ageing • Tails • Background • Rate capability CBM2002@GSI 2

Timing RPCs – General features • Several (~4) thin (~ 0.3 mm) gas gaps Experimental • Atmospheric pressure operation • Both charge and time readout 1-2 ns • Offline correction for time-charge correlation. ∼ 100 ps Time-charge correlation Electronics Detector-related rise time (not yet fully understood) CBM2002@GSI 3

Basic physical mechanisms 2 main questions: How can we reach 50 ps σ resolution in a gas counter? How is it possible to reach efficiencies of 75% in a 0.3 mm gas gap? N= average number of visible primary clusters i − ε = N e cathode = − ε = N ln( ) 1.4 Detection level independent from the cluster position Av. cluster density ⇒ exact timing anode >> 1.4/0.3 = 4.6 i CBM2002@GSI 4

Time resolution-principles i cathode Monte Carlo i e α = vt i t 0 anode i i 0 [Mangiarotti and Gobbi, 2001] −τ − i0 ( ) ( ) − − τ −τ N ( e ) ( ) e v N e BesselI ( 1 2 , i0 N ) = N e BesselI ( 1 2 , e ) p i0 ( ) = p t ( ) e N − −τ N d ( 1 ) i0 N ( ) e N − ( 1 ) e τ τ = α v t n 0eff =i 0 d/(ev) CBM2002@GSI 5

Time resolution-principles 1.4 K(N) (time jitter in units of α v) Standard deviation 1.2 Sigma (gaussian fit) 1 0.8 0.6 0.4 τ = α vt 0.2 1 gap 4 gaps 0 0 2 4 6 8 10 12 Average number of primary clusters (N) K N ( ) α =first Townsend coefficient σ t = α v v = electron’s drift velocity [Mangiarotti and Gobbi, 2001] CBM2002@GSI 6

Signal properties 1-Signal shape cannot be changed by any linear system i e α v e α = = α vt vt � i t v t ≈ i Z s Linear system 0 0 Z(s) 2- α v can be easily measured ( ) = − Th 1 TDC ln( / ) Th Th s t t t 1 1 2 1 2 Th 2 t 2 10 y = 0.1967x - 9.6104 9 100 α v (GHz) Threshold 1 (mV) y = 20.69e 8.7043x 8 7 6 Experimental Experimental 5 10 70 80 90 100 -0.1 0 0.1 0.2 Applied field (kV/cm) Time difference (ns) CBM2002@GSI 7

Time resolution-theory vs. measurements K N K N ( ( ) ) σ t σ t = = α v α v Single 0.3mm gap 120 Time-charge corrected 110 Time resolution (ps σ ) Uncorrected 100 0.75/( α v) 90 80 70 60 50 40 2.4 2.6 2.8 3 3.2 3.4 Applied Voltage (kV) Good agreement CBM2002@GSI 8

The efficiency problem There should be at least one cluster i in the efficient region of the gap cathode Efficient region d d* G min ~10 6 Inefficient region 1 (too small avalanches) Isobutane (IB) anode Experimental 0.9 C2H2F4 (TFE) i TFE+IB+SF6 0.8 Methane Efficiency/gap 0.7 0.6 = − ε * ln(1 ) d L 0.5 ⇒ Lmin=5/mm ε = 0.4 efficiency 0.3 = clusters/mm L 0.2 ⇒ Lmin=1.6/mm > − ε L Lmin=ln(1 ) d 0.1 0 0 0.1 0.2 0.3 0.4 Gas gap (mm) CBM2002@GSI 9

The efficiency problem-cluster density data 0.1 mm gap in C 4 H 10 C 2 F 4 H 2 F.Sauli [Finck, RPC2001] CERN 77-09 Lmin C 4 H 10 [Fisher, NIMA238] Experimental CH 4 Lmin 50 60 70 80 L(cm -1 ) No contribution from cathode (would also provide an effect to explain the t-q correlation) Calculation (HEED) [Riegler, RPC2001] Use this data CBM2002@GSI 10

The efficiency problem (maybe solved) 1 i Isobutane (IB): L=9.5/mm cathode C2H2F4 (TFE): L=9/mm 0.9 TFE+IB+SF6: L=9/mm d d* 0.8 G 0 Methane: L=3/mm G min ~10 6 0.7 G 0 =4x10 14 d*/d anode 0.6 i 0.5 = − ε 0.4 d * ln(1 ) L ε = efficiency 0.3 = L clusters/mm 0.2 0 0.1 0.2 0.3 0.4 1 ( ) * G = G 1-d /d Gas gap (mm) 0 min Large values of G 0 must be assumed (much above the Raether limit of 10 8 ). Possible by an extremely strong avalanche saturation effect (space charge effect). CBM2002@GSI 11

Some hardware CBM2002@GSI 12

Timing RPCs – Small single counters (9 cm 2 ) 4x0.3 mm gaps [ Fonte 2000] Aluminum Glass -HV σ = − = 2 2 68 49 47 ps t Resolution of the reference counter ε = 99.5 % for MIPs (75%/gap) 3 σ (optimum operating point ⇒ 1% of discharges) CBM2002@GSI 13

Timing RPCs – Array of 32 small counters [ Spegel et al, 2000 ] σ = 88 ± 9 ps ε = 97 ± 0.5 % Crosstalk < 1% (not tested for multiple hits) CBM2002@GSI 14

Very high frequency front-end electronics Input signal based on commercial chips (experimental) i=i 0 e st s =8.7 GHz [ Blanco 2000] Pre-amplifier based on the INA-51063 chip (HP/Agilent) • 2.5 GHz bandwidth • 20 db power gain • 3 dB noise figure Realistic tests using chamber-generated signals 70 RPC at Threshold=12.5 fC 60 Time resolution per RPC at threshold=25 fC channel ( σ/√ 2) (ps) RPC at threshold=50 fC 50 Pulser at threshold=25 fC 40 30 20 Amplifier-discriminator-delay 10 module 10 ps σ resolution 0 10 100 1000 above 100 fC Signal fast charge per channel (fC) CBM2002@GSI 15

Double readout 4-gaps glass-metal counter Edge effects [Finck, Gobbi et al, RPC2001] CBM2002@GSI 16

Timing RPCs – Large counter Active area = 10 cm × 160 cm = 0.16 m 2 (400 cm 2 /electronic channel) 1,6 m HV 5 cm 4 timing channels Top view Cross section Ordinary 3 mm “window glass” Copper strips [Blanco 2001] CBM2002@GSI 17

Timing RPCs – Large counter Time distribution Charge distribution 3 3 10 10 300 σ = 63.4 ps Events/20 ps ± 1.5 σ fit 0.5 pC ε ≈ 98 % Events / 20 fC 200 2 10 2 10 100 0 1 10 0 2 4 6 1 “300 ps tails” 10 0 10 0 2 4 6 8 Fast charge (pC) 0 10 -1000 -500-300 0 300 500 1000 Time difference (ps) σ σ 64 2 - 35 2 = 54 ps 64 2 - 35 2 = 54 ps 64 2 - 35 2 = 54 ps 64 2 - 35 2 = 54 ps = = Time resolution essentially independent from electrode size CBM2002@GSI 18

Timing RPCs – Large counter 100% ε = 95 to 98 % 99% Time efficiency 98% 97% 96% Strip A 95% Strip B 94% Strips A+B 93% -80 -70 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 [Blanco 2001] Center of the trigger region along the strips (cm ) 100 Time resolution (ps σ ) σ = 50 to 75 ps 90 80 70 60 50 40 -80 -70 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 Center of the trigger region along the strips (cm ) No degradation when the area/channel was doubled to 800 cm 2 /channel CBM2002@GSI 19

Timing RPCs – Large counter Position resolution along the strips 5.0 cm 8 400 400 400 6 Events/bin Events/bin Events/bin y=0.0709 x + 0.0001 ⇔ v=14 .1 cm/ns 350 350 350 4 ∆ t/2 (ns) 300 300 300 2 250 250 250 0 200 200 200 -2 Strip A 150 150 150 -4 100 100 100 Strip B -6 50 50 50 -8 0 0 0 -350 -350 -350 -300 -300 -300 -250 -250 -250 -80 -70 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 TDC bins TDC bins TDC bins 0.3 Very good linearity ( ± 1.4 cm) σ X = 1.2 cm Fit residuals (ns) 0.2 (0.75% of detector length) 0.1 0 [Blanco 2001] -0.1 -0.2 -80 -70 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 Center of the trigger region along the strips (cm) CBM2002@GSI 20

Timing RPCs – single gap 2D-position sensitive readout (for small and accurate TOF systems) Precise construction Time signal HV 2 mm thick black glass lapped to ~1 µ m flatness metal box (no crosstalk) 300 µ m thick high ρ [Blanco 2002] glass disk (corners) X right X left 4 cm Well carved into the glass XY readout plane (avoid dark currents out left from the spacer) RC passive network 10 strips for each Y-strips coordinate (on PCB) at 4 mm pitch out right RC passive network X-strips (deposited on glass) CBM2002@GSI 21

Timing RPCs – single gap Charge distribution Time distribution 3000 Events=26186 10 4 ineficiency 3 10 ± 1.5 σ fit ε = 75 % 2500 σ = 66.6 ps (54 ps) 3 σ Tails=1.9 % Events/bin 2000 Events/10ps 10 2 300ps Tails=0.36 % 2 10 1500 1000 10 0 1 10 0 500 1000 3 σ 500 0 0 10 0 200 400 600 800 1000 -1000 -500 -300 0 300 500 1000 Fast charge (a.u.) Time difference (ps) σ σ 67 2 - 40 2 = 54 ps 2 = 54 ps 2 = 54 ps 2 = 54 ps 2 - 2 - 2 - = = Time resolution of single-gap and 4-gap counters may be similar! CBM2002@GSI 22

Timing RPCs – Single gap Efficiency and time resolution 100 85 Efficiency 90 80 Time resolution (ps σ ) Efficiency (%) 80 75 70 70 60 65 #2 (time only) 50 60 #3 (time + XY) Resolution 40 55 2.4 2.6 2.8 3 3.2 3.4 Applied Voltage (kV) σ = 50 to 60 ps ε = 75 to 80 % No influence from XY readout CBM2002@GSI 23

Timing RPCs – single gap Bidimensional position resolution 1500 Events/0.5mm Trigger edge Chamber 1000 (3 mm) edge Events/mm 2 500 Y(mm) 0 -25 -20 -15 -10 -5 0 5 10 15 20 25 Position along X (mm) 1500 Events/0.5mm 4 mm strip pitch 1000 500 X (mm) 0 -25 -20 -15 -10 -5 0 5 10 15 20 25 Position along Y (mm) edges ≤ 3 mm ⇓ resolution ≤ 3 mm FWHM (strips=4mm) CBM2002@GSI 24