Development and first tests of a microdot detector with resistive spiral anodes

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 From the ratio of primary/secondary lights one can conclude about the nature of the interaction Primary scintillation light

5. Several groups are trying to develop designs with reduced number of PMs One large low cost “PM” Large amount of PMs in the case of the large-volume detector significantly increase its cost See: E. Aprile XENON: 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 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).

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

7. Ar gas, 1 atm Experimental setup (a dual phasceLAr detector) 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 lightAprile, E.; Giboni, K.L.; Kamat, S.; Majewski, P.; Ni, K.; Singh, B.KetalDielectric Liquids, 2005. ICDL 2005. 2005 IEEE International Conference Publication Year: 2005 , Page(s): 345 - 348

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 CsIphotocathode to be about 14% for a photon wavelength of 128 nm. Stability with time

9. One of the ways to suppress the feedback Photodetectors (optional) R-Microdot- microhole Anodes Multiplication region Resistive cathodes Shielding RETGEMs with HV gating capability Charge Event LAr hv hv CsI photocathode 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) Resistive spiral PCB readout • 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 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. Production steps (2) Cu cathode Cu cathode Dielectric 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 • Dielectric over resistive strips: • photoimageablecoverlay, 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

24. Magnified photograph

25. Photograph of the resistive spiral detector 25

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

27. Electric potential S. Franchino, V. Cairo

28. Electric field S. Franchino, V. Cairo

29. E field on lined parallel to surfaces S. Franchino, V. Cairo Cathodes edges 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

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

33. First promising measurements

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

35. Energy resolution FWHM(%) Gain

36. Spectrum transformation at high gains At high gain (105), before to streamers transition-Geiger mode

37. Rate characteristics Signal amplitude Hz/cm2 …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