Development of the GridPix Detector for Dual Phase Noble Gas Time Projection Chambers

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1 Development of the GridPix Detector for Dual Phase Noble Gas Time Projection Chambers M. Alfonsi, N. van Bakel, M. P. Decowski, H. van der Graaf, G. Hemink, R. Schön

2 The GridPix detector The GridPix is a Micro-Pattern Gaseous Detector made of an aluminum mesh (grid) built over a Timepix, a pixelated CMOS chip. Gas amplification is produced in the 50 µm gap between the grid and the chip. The Timepix chip provides readout with a matrix of 256 x 256 pixels, 55 µm pitch. A thin resistive layer protects the chip against discharges. 24/10/2011 M. Alfonsi (Nikhef) - IEEE NSS 2011 Valencia 2

3 GridPix features Single electron detection efficiency > 98 % < 20 µm spatial resolution Time coordinate (µtpc) Low noise electronics: threshold ~1000 electrons See W. Koppert s poster [NP5.S-216] 24/10/2011 M. Alfonsi (Nikhef) - IEEE NSS 2011 Valencia 3

4 Dual phases noble gas TPC PMT PMT S1/S2 ratio: large discrimination power between electronic and nuclear recoils Charge is converted to light in the gas phase by proportional scintillation Within the DARWIN Consortium [arxiv: ], alternative charge readouts are under investigation: GridPix technology is one candidate 24/10/2011 M. Alfonsi (Nikhef) - IEEE NSS 2011 Valencia 4

5 GridPix in Dual Phase TPC Main challenges for the technology: Cryogenic environment Stable operation in pure noble gas Low outgassing properties Material radiopurity 24/10/2011 M. Alfonsi (Nikhef) - IEEE NSS 2011 Valencia 5

6 Cryogenic robustness The border has been reinforced with glue Thermal stress along the border due to materials with different thermal expansion properties 24/10/2011 M. Alfonsi (Nikhef) - IEEE NSS 2011 Valencia 6

7 Room temperature Nikhef The signal on the grid has been read out and used to trigger the pixel readout. 24/10/2011 M. Alfonsi (Nikhef) - IEEE NSS 2011 Valencia 7

8 Reference mixture: Ar/isobutane 90/10 The gain is measured from the signal collected on the grid, comparing it with a calibration pulse Single electron efficiency is measured comparing the number of hit pixels with the primary ionization as reported in literature 24/10/2011 M. Alfonsi (Nikhef) - IEEE NSS 2011 Valencia 8

9 Argon without quenchers Without quenchers sparking begins already at moderate voltages. The maximum achievable gain can be extrapolated to be around 100. In very pure (1 ppm) argon the signal on the grid is too low to trigger: events are shown using long integration window 24/10/2011 M. Alfonsi (Nikhef) - IEEE NSS 2011 Valencia 9

10 CERN In collaboration with the ETH Zurich, measurements in a dual phase argon TPC were performed. 24/10/2011 M. Alfonsi (Nikhef) - IEEE NSS 2011 Valencia 10

11 Room temperature argon Results confirm the Nikhef, but the use of the PMT allows to trigger and select single events, as shown below. 24/10/2011 M. Alfonsi (Nikhef) - IEEE NSS 2011 Valencia 11

12 Cold argon gas The amplification of the GridPix can be verified in the light detected by PMT. As the grid voltage is increased 24/10/2011 M. Alfonsi (Nikhef) - IEEE NSS 2011 Valencia 12

13 Cold argon gas 24/10/2011 M. Alfonsi (Nikhef) - IEEE NSS 2011 Valencia 13

14 Towards dual phase argon.. In a cold gas environment the higher gas density (almost 3 times) requires higher grid voltages to achieve the same gain. Before going to dual phase argon, we pushed the detector beyond specification. Unfortunately a spark damaged the threshold circuitry of the chip. Hereafter we filled the TPC with liquid argon: the detector still sustained the high voltage, but when we warmed up and opened the vessel 24/10/2011 M. Alfonsi (Nikhef) - IEEE NSS 2011 Valencia 14

15 Damaged grid the grid showed wrinkles and ruptures in several spots The moment of the damage cannot easily be inferred (either during the warm up, the liquid argon filling or even before) 24/10/2011 M. Alfonsi (Nikhef) - IEEE NSS 2011 Valencia 15

16 Conclusion and prospects Still a long way to go, but there are no stopping points related to the technology itself. The materials composing the device can be chosen and should have similar thermal expansion properties. Higher stable gains can be achieved by different geometries (e.g. closed amplification cells like in a GEMGrid ) or resistive layers (saturation operation). The operation in xenon is less demanding in the cryogenics aspects and it can be favorable for achieving higher gains 24/10/2011 M. Alfonsi (Nikhef) - IEEE NSS 2011 Valencia 16

17 Acknowledgements André Rubbia s ArDM group, especially Filippo Resnati and Devis Lussi Bas vd Heijden and Henk Boterenbrood Joop Rövekamp and Berend Munneke Peter Thobe and Herman Boer Rookhuizen.. and thanks for your attention! 24/10/2011 M. Alfonsi (Nikhef) - IEEE NSS 2011 Valencia 17

18 Backup 24/10/2011 M. Alfonsi (Nikhef) - IEEE NSS 2011 Valencia 18

19 Argon/Isobutane event 24/10/2011 M. Alfonsi (Nikhef) - IEEE NSS 2011 Valencia 19

20 Argon % event 24/10/2011 M. Alfonsi (Nikhef) - IEEE NSS 2011 Valencia 20