1. Development and first tests of a microdot detector with resistive spiral anodes R. Oliveira, S. Franchino, V. Cairo, V. Peskov, F. Pietropaolo, P. Picchi

2. Motivation In one of previous meetings we reported development of microdot detector as readout element for a special design of noble liquid TPC

3. Usual noble liquid TPC

4. Double phase noble liquid dark matter detectors Two parallel meshes where the secondary scintillation light is produced Primary scintillation light From the ratio of primary/secondary lights one can conclude about the nature of the interaction

5. Several groups are trying to develop designs with reduced number of PMs See: E. Aprile XENON: a 1-ton Liquid Xenon Experiment for Dark MatterXENON: a 1-ton Liquid Xenon Experiment for Dark Matter http://xenon.astro.columbia.edu/presentations.html and A. Aprile et al., NIM A338,1994,328; NIM A343,1994,129 Large amount of PMs in the case of the large-volume detector significantly increase its cost Another option for the LXe TPC, which is currently under the study in our group, is to use LXe doped with low ionization potential substances (TMPD and cetera). One large low cost “PM”

6. The purpose of our efforts was to exploit CsI photocathode immersed inside the liquid

7. Experimental setup (a dual phasce LAr detector) Ar gas, 1 atm LAr+ gas phase V. Peskov, P. Pietropaolo, P. Pchhi, H. Schindler ICARUS group Performance of dual phase XeTPC with CsI photocathode and PMTs readout for the scintillation light PMTs readout for the scintillation light Aprile, E.; Giboni, K.L.; Kamat, S.; Majewski, P.; Ni, K.; Singh, B.Ketal Dielectric Liquids, 2005. ICDL 2005. 2005 IEEE International Conference Publication Year: 2005, Page(s): 345 - 348 Dielectric Liquids, 2005. ICDL 2005. 2005 IEEE International Conference

8. Using a dedicated analysis program we calculated the area under each peak in order to obtain a numerical evaluation of the feedback effect. From this data and also taking into account the geometry of the test set-up, we calculated the quantum efficiency of the CsI photocathode to be about 14% for a photon wavelength of 128 nm. Stability with time

9. Event Charge hv R-Microdot- microhole CsI photocathode Shielding RETGEMs with HV gating capability LAr Photodetectors (optional) AnodesResistive cathodesMultiplication region One of the ways to suppress the feedback In hybrid R-MSGC, the amplification region will be geometrically shielded from the CsI photocathode (or from the doped LXe) and accordingly the feedback will be reduced

10. Why microdot-microhole? The main advantages of this detector is a high reachable gain and geometrical shielding with respect to the CsI photocathode

11. Our previous design

14. EII Feeding the anode dot always was a problem (see early Biagi works) Since it created azimuthally field line nonouniformuty and electrical weak points

16. Old microdot (at a gain of~10000)

17. A new state of art design An original idea belongs to Rui

18. Main feature-resistive spiral anode to make electric field more azimuthally symmetric

19. Production steps (1) Standard PCB with Cu backplane and readout lines; thickness 2.4 mm, 35 µm Cu Pressing over readout lines a fiber-glass epoxy glue (75 µm) and Copper (18 µm) Photolithography deposition of Resistive spirals: – Complementary image in the copper of resistive spirals, Cu etching – Filling the Cu image with resistive paste (1MOhm/sq) – Cooking of R paste in order to polymerize and harden it – Polishing of R paste up to reaching the Cu image – Etch remaining Cu  Resistive spiral image PCB readout Resistive spiral S. Franchino

20. Readout strips layout Lines pitch 1mm

21. Spiral design 150μm

22. Some photos Resistive paste: 1Mohm/sq, photolithography technique Measured R: 4-7 GOhm

23. Dielectric over resistive strips: – photoimageable coverlay, 50 µm thickness – holes of 100 µm done with photolithography technique – Cooking in order to harden it Cu cathode: – Laminated 17 µm Copper + 25 µm no-flow glue – Mechanical drilled holes of 500 µm in both of them – Glued at the top of the circuit with the press Production steps (2) Dielectric Cu cathode Encountered problems in first prototypes: Misalignment of ~ 40 µm between drilled cathode and anode during the pressing It happened in one of the two produced prototypes (pressed at the same time) Already tested a new production technique to overcome this problem; this is being used in the next prototype (in production ) S. Franchino

24. Magnified photograph

25. Photograph of the resistive spiral detector 25

26. Preliminary Simulation Program used: COMSOL multiphysics Goal: quick check of good collection of all electric field lines with the used geometry 150 um 35 um 100 um 200 um 75 um 50 um Cu CATHODE: 0V Cu readout: 0V Res Anode: 600V S. Franchino, V. Cairo

27. Electric potential S. Franchino, V. Cairo

28. Electric field S. Franchino, V. Cairo

29. E field on lined parallel to surfaces Active area All E peaks are hidden in the material a part the two at the edges of anode and the ones at the edges of cathode Cathodes edges S. Franchino, V. Cairo

30. A comment: This design is still not the perfect one concerning all field lines collection and because there are some peaks of E field on the edges of the cathode (the improved version of the design is in progress)

31. Setup

32. VdVd VcVc Anode dots Gas chamber Window Cathode strips Charge-sensitive amplifier Radioactive source Drift mesh X-ray gun Collimators 5-20mm R-Microdot Cryostat Removable 55 Fe Readout strips

33. First promising measurements

34. Anode voltage (V) Gain Ne Ar Streamers -Alpahs-55Fe Gain curves Symbols: and

35. Energy resolution Gain FWHM(%)

36. Spectrum transformation at high gains At high gain (10 5 ), before to streamers transition-Geiger mode

37. Rate characteristics Hz/cm 2 Signal amplitude …they are close to the previous design

38. Conclusions Preliminary it looks that with the spiral design we increased the maximum achievable gain, improved stability with time and the pulse-height spectrum becomes symmetrical More developments and tests are in progress which will probably end up with new interesting results

39. Backups