Investigation of GEM space point resolution for a TPC tracker

Transcription

1 Investigation of GEM space point resolution for a TPC tracker Dean Karlen / Carleton University Carleton GEM group: Bob Carnegie, Madhu Dixit, Jacques Dubeau, Dean Karlen, Hans Mes, Morley O'Neill, Ernie Neuheimer, Anna Kristofferson, Jeff Mottershead

2 Outline! What is a GEM?! Why use GEMs in a TPC?! Point resolution studies at Carleton! experimental setup hexagonal pads! data and results Ar CO 2 and P1! simulation work! Future plans! Summary 26 October 2 Dean Karlen / Carleton University 2

3 Gas Electron Multiplier (GEM) E drift =.2 1. kv/cm E transfer 3 kv/cm E induction 3.5 kv/cm V 4 V V 4 V Readout pads 26 October 2 Dean Karlen / Carleton University 3

4 GEM foils and pads for this study fabricated at the CERN PCB workshop GEM foil GEM pads 5 µm 2.5 mm pitch hexagons 14 µm 26 October 2 Dean Karlen / Carleton University 4

5 Using a GEM in a TPC Conventional TPC readout GEM TPC readout 26 October 2 Dean Karlen / Carleton University 5

6 Potential advantages for GEM readout! Improved space point resolution! E B and track angle systematics suppressed! Improved two particle separation power! r -φ : signals distributed over smaller area! z : faster induction pulses (v e > v ion )! Natural ion feedback suppression! no gating required (non-triggered expt.)! Less mass in TPC endcap! no wires held under tension 26 October 2 Dean Karlen / Carleton University 6

7 Point resolution studies at Carleton x-ray tube pin hole GEM cell 2D micrometer stage 26 October 2 Dean Karlen / Carleton University 7

8 Details! x-ray mean energy: 4.5 kev! pinhole diameter: 5 µm! Gas: Ar CO 2 (~9:1) / P1 : Ar CH 4 (9:1)! pre-amps:! fast Lecroy HQV 81 with Ar CO 2! slower ALEPH TPC pre-amp with P1! readout:! two 4-channel digital scopes (9 bit ADC)! 5 MHz sampling for HQV 81! 125 MHz sampling for ALEPH preamps 26 October 2 Dean Karlen / Carleton University 8

9 An event x x Ar CO 2 (9:1) HQV81 preamps x x x x x October 2 Dean Karlen / Carleton University 9

10 Analysis Direct pulse Induced pulse direct charge deduced from tail peak value used 26 October 2 Dean Karlen / Carleton University 1

11 Localization using charge sharing PAD 2 PAD 8! assume electron cloud is 2D Gaussian y v x! charge fraction is given by integral over pad area! 1 to 1 mapping between v and (f 1,f 8 ) PAD 1! One free parameter cloud size 26 October 2 Dean Karlen / Carleton University 11

12 Determining cloud size Ar CO 2 (~9:1) P1: Ar CH 4 (9:1) charge fraction in central pad σ x = 37 ± 3 µm y coordinate of collimator (mm) σ x = 53 ± 3 µm 26 October 2 Dean Karlen / Carleton University 12

13 Position from pad fractions! figure: 1 to 1 mapping from (x,y) to (f 1,f 8 )! invert the mapping to determine (x,y) from (f 1,f 8 ) y coordinate (mm) Pad 1 fraction 9 Pad 8 fraction x coordinate (mm) 26 October 2 Dean Karlen / Carleton University 13

14 Charge sharing result Ar CO x =.95 mm σ x =.43 mm (x,y) col = (-.1,1.143) mm ID Entries Mean RMS E E / 58 Constant Mean E-1 Sigma.4284E y = mm σ y =.61 mm ID Entries Mean RMS E / 55 Constant Mean Sigma.6122E x coordinate y coordinate 26 October 2 Dean Karlen / Carleton University 14

15 Charge sharing result Ar CO 2 xmeas - xtrue (mm) First data from Aug 28 Last data from Aug ymeas - ytrue (mm) y collimator (mm) First data from Aug 28 Last data from Aug October 2 Dean Karlen / Carleton University 15

16 Charge sharing result P1! standard deviation again 5-6 µm not diffusion limited x meas x true (mm) y meas y true (mm) y collimator setting (mm) ! increase in σ y due to sensitivity of algorithm to cloud size 26 October 2 Dean Karlen / Carleton University 16

17 Localization from charge sharing! Method only works in regions where significant charge is deposited on 3 pads Ar CO 2 P1 Charge sharing regions Charge sharing region 26 October 2 Dean Karlen / Carleton University 17

18 Localization from induced pulses! The amplitude of the induced pulse from a charge cloud that falls on a neighbouring pad depends on the distance between the cloud and pad. distance to pad centre (mm) Pad 2 Ar CO 2 Pad Pad Pad 7 Pad 4 Pad P1 induced pulse / total charge pulse October 2 Dean Karlen / Carleton University 18 Pad 2 Pad 8 Pad 3 Pad 7 Pad 4 Pad 6

19 Example! collimator location: (-.1,1.143) x x x x October 2 Dean Karlen / Carleton University 19

20 Induction signal results Ar CO y meas y true (mm) x meas x true (mm) First data from Aug Last data from Aug 25 σ x 5 8 µm σ y 6 1 µm σ smallest near centre of central pad biases likely due to cross-talk y collimator position (mm) 26 October 2 Dean Karlen / Carleton University 2

21 Induction signals results P1.1.2 x meas x true (mm) y meas y true (mm) y collimator position (mm) σ x 5 8 µm σ y 9 1 µm 26 October 2 Dean Karlen / Carleton University 21

22 Simulation work! Java simulation of GEM is in development! arbitrary GEM structure can be defined interactively! keeps track of electron distributions on pads! Comparison with P1 data:! predicts average cloud width of.5 mm compared to observed value of.56 mm! confirms that diffusion contribution to measurement spread less than 25 µm! spread in measurements due primarily to x-ray spot size 26 October 2 Dean Karlen / Carleton University 22

23 Simulation program 26 October 2 Dean Karlen / Carleton University 23

24 Future plans! Further studies with hexagonal pads! begin studies of track resolution! optimize GEM pad geometry for tracking! rectangles stretched along r direction (?)! small (15 cm) TPC under construction! FADC readout in development! eventually a large scale TPC test in B field 26 October 2 Dean Karlen / Carleton University 24

25 Summary! Interesting new results:! relatively large pads (2.5 mm diameter) can give excellent space point resolution (of order 5 µm) by using charge sharing! large pads are necessary for TPC application to keep the electronics channel count to a reasonable level! induction pulses (previously neglected) can provide similar resolution, but is more challenging! recovers lost resolution for clusters without charge sharing! GEM readout for TPC looks promising! 26 October 2 Dean Karlen / Carleton University 25