Development and first tests of a microdot detector with resistive

spiral anodes

R. Oliveira, S. Franchino, V. Cairo , V. Peskov, F. Pietropaolo, P. Picchi

Motivation

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

Usual noble liquid TPC

Double phase noble liquid dark matter detectors

Two parallel mesheswhere the secondary scintillation light is produced

Primary scintillation light

From the ratio ofprimary/secondarylights one canconclude about thenature of the interaction

Several groups are trying to develop designs with reduced number of PMs

See: E. Aprile XENON: a 1-ton Liquid Xenon Experiment for Dark Matterhttp://xenon.astro.columbia.edu/presentations.htmland A. Aprile et al., NIM A338,1994,328; NIM A343,1994,129

Large amount of PMs in thecase of the large-volume detectorsignificantly 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 lowcost “PM”

The purpose of our efforts was to exploit CsI photocathode

immersed inside the liquid

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 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

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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

EventCharge

hvhv

R-Microdot-microhole

CsI photocathode

Shielding RETGEMswith HV gatingcapability

LAr

Photodetectors (optional)

Anodes Resistive 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

Why microdot-microhole?

The main advantages of this detector is a high reachable gain and

geometrical shielding with respect to the CsI photocathode

Our previous design

EII

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

Old microdot

(at a gain of~10000)

A new state of art design

An original idea belongs to Rui

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

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

Readout strips layout

Lines pitch 1mm

Spiral design

150μm

Some photosResistive paste: • 1Mohm/sq,• photolithography technique• Measured R: 4-7 GOhm

• 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 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

Magnified photograph

Photograph of the resistive spiral detector

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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 um200 um

75 um

50 um

Cu CATHODE: 0V Cu CATHODE: 0V

Cu readout: 0V

Res Anode: 600V

S. Franchino, V. Cairo

Electric potentialS. Franchino, V. Cairo

Electric fieldS. Franchino, V. Cairo

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

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)

Setup

Vd

Vc

Anode dots

Gas chamber

Window

Cathode strips

Charge-sensitive amplifier

Radioactivesource

Driftmesh

X-ray gun

Collimators

5-20mm

R-Microdot

Cryostat

Removable 55Fe

Readout strips

First promising measurements

Anode voltage (V)

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n

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NeAr

Streamers

-Alpahs -55Fe

Gain curves

Symbols: and and

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Energy resolution

Gain

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Spectrum transformation at high gains

At high gain (105), before to streamers transition-Geiger mode

Rate characteristics

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…they are close to the previous design

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

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