aida-2020 milestone - cern · 90/10 and read-out by apv hybrid cards [8] handled by an srs system....

AIDA-2020 Consortium, 2017

Grant Agreement 654168 PUBLIC 1 / 12

Grant Agreement No: 654168

AIDA-2020 Advanced European Infrastructures for Detectors at Accelerators

Hor izon 2020 Research In f rast ructures pro ject AIDA -2020

MILESTONE REPORT

SMALL-SIZE PROTOTYPE OF THE

µ-RWELL BUILT AND QUALIFIED

MILESTONE: MS53

Document identifier: AIDA-2020-MS53

Due date of milestone: End of Month 24 (April 2017)

Report release date: 19/04/2017

Work package: WP13: Innovative gas detectors

Lead beneficiary: INFN

Document status: Final

Abstract:

The µ-RWELL is a compact spark-protected single amplification stage Micro-Pattern-Gaseous-

Detector (MPGD). The detector amplification stage, realized with a polyimide structure micro-

patterned with a blind-hole matrix, is embedded, through a thin DLC sputtered resistive layer, in the

readout PCB. The introduction of the resistive layer deposition, strongly suppressing the transition

from streamer to spark, gives the possibility to achieve large gains (> 104), while partially reducing

the capability to stand high particle fluxes.

Several prototypes with different surface resistivity of the resistive layer have been built and fully

qualified. The characterization has been performed either with collimated 5.9 keV X-rays or with a

muon beam at the H4-SPS CERN facility.

SMALL-SIZE PROTOTYPE OF THE μ-RWELL

BUILT AND QUALIFIED

Milestone: MS53

Date: 19/04/2017


AIDA-2020 Consortium, 2017

For more information on AIDA-2020, its partners and contributors please see www.cern.ch/AIDA2020

The Advanced European Infrastructures for Detectors at Accelerators (AIDA-2020) project has received funding from

the European Union’s Horizon 2020 Research and Innovation programme under Grant Agreement no. 654168. AIDA-

2020 began in May 2015 and will run for 4 years.

Delivery Slip

Name Partner Date

Authored by G. Bencivenni INFN 14/04/2017

Edited by G.Bencivenni INFN 14/04/2017

Reviewed by

G.Bencivenni [Task coordinator]

S.Dalla Torre, I. Laktineh [WP Coordinator]

P. Giacomelli [Deputy Scientific Coordinator]

INFN

INFN, CNRS

INFN

19/04/2017

Approved by Scientific Coordinator 19/04/2017

http://aida2020.web.cern.ch/


BUILT AND QUALIFIED

Milestone: MS53

Date: 19/04/2017


TABLE OF CONTENTS

1. INTRODUCTION.................................................................................................................................................... 4

2. DETECTORS FEATURES ..................................................................................................................................... 5

2.1 DLC SPUTTERING TECHNOLOGY ..................................................................................................................... 6

3. LABORATORY TEST RESULTS ......................................................................................................................... 7

4. BEAM TEST RESULTS ......................................................................................................................................... 9

5. CONCLUSIONS .................................................................................................................................................... 11

6. REFERENCES ....................................................................................................................................................... 12


BUILT AND QUALIFIED

Milestone: MS53

Date: 19/04/2017


1. INTRODUCTION

The µ-RWELL has been introduced as a thin, simple and robust detector for very large area

applications requiring the operation in harsh environments [1]. The detector, as sketched in fig. 1, is

composed of two elements: the cathode and the µ-RWELL-PCB, the core of the detector. The µ-

RWELL-PCB, realized as a multi-layer circuit by means of standard photolithography technology, is

composed of three different elements: a suitable WELL-patterned kapton foil that acts as

amplification stage of the detector; a resistive layer acting as discharge limitation stage; a standard

segmented PCB for readout purposes. Two different schemes have been studied for the resistive stage:

the simplest layout, based on a single-resistive layer with edge grounding (2D - current evacuation),

has been designed for low-rate applications (fig. 2); a more sophisticated one, based on a double-

resistive layer with a suitable density of through-vias between the two layers, with the grounding

done by means of the readout electrodes (3D - current evacuation) is under study for high-rate

purposes.

Fig.1: Sketch of the micro-Resistive WELL

In this report, we focus on the single-resistive layer scheme. The implementation of a resistive layer

between the amplification stage and the readout PCB is strictly correlated with the aim of suppressing

the spark processes. MPGDs, due to their typical micrometric distance of the electrodes, generally

suffer from spark occurrence that can eventually damage or destroy the detector as well as the

associated front-end electronics.

In the µ-RWELL this problem has been solved with the introduction of a resistive layer deposition

created between the amplification stage and the readout PCB (similarly to resistive-Micromegas [2]).

The spark suppression mechanism is the same of the resistive electrode used in the RPCs [3, 4, 5]):

the streamer generated in the gas gap inside the amplification volume, inducing a large current

through the resistive layer, generates a local drop of the amplifying voltage with an effective

quenching of the multiplication process in the gas. This mechanism, strongly suppressing the

discharge amplitude (down to few tens of nano-ampere), gives the possibility to achieve large gains

with only one amplification stage. A drawback, strictly correlated with the Ohmic behaviour of the

detector, is the reduced capability to stand high particle fluxes [1]. Finally, the implementation of the

resistive layer affects the charge spread on the readout electrodes (fig. 3): the charge induced on the

resistive stage is dispersed with a time constant [6]: 𝝉 = 𝝆 × 𝒄 = 𝝆 × 𝜺 𝒕⁄ , being ρ the surface

resistivity and c the capacitance per unit area and t the distance between the resistive layer and the

readout plane.


BUILT AND QUALIFIED

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Date: 19/04/2017


Fig. 2 : Single resistive layer scheme for low rate operation.

Fig. 3: Sketch of the electronic avalanche in a µ-RWELL: the charge dispersion above the resistive layer capacitively

induces the signal on the readout electrodes.

2. DETECTORS FEATURES

The three prototypes used to study the dependence of the performance as a function of the resistivity

are single-resistive layer detectors with resistivity of 12/80/880 MΩ/ .


BUILT AND QUALIFIED

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Date: 19/04/2017


The resistive layer of the prototypes has been realized by means of DLC (Diamond Like Carbon)

sputtering, the highest chemical and mechanical stable coating. All prototypes were realized on a

readout-PCB patterned with a 400 µm pitch strips.

In table 1, the characteristics of each single prototype are reported.

Table 1: prototypes description

Detector characteristics detector n.1 detector n.2 detector n.3

Resistivity (MΩ/) 12 80 880

Hole pitch (µm) 140 100 100

External/internal hole diameter (µm) 70/50 60/50 70/30

Active area (mm2) 100 × 100 50 × 50 50 × 50

Readout strip pitch (mm) 0.4 0.4 0.4

Fig. 4: Resistivity as a function of the sputtered DLC thickness. The full dots refer to the MM calibration curve reported

in [7]; the full squares refer to the results obtained for the 2016 µ-RWELL test production; while the two open square

markers refer to the 2017 test production obtained with a pre-drying cycle of the polyimide.

2.1 DLC SPUTTERING TECHNOLOGY

Sputtering technology is commonly used to deposit thin protective films on a substrate. For the

production of our prototypes, a large industrial sputtering chamber in Be-Sputter Co., Ltd. has been

used. The carbon is sputtered on the kapton foil as an amorphous diamond like carbon (a-DLC) using

a pure graphite target. The resistivity dependence as a function of the DLC thickness for different

sputtering batches is shown in fig. 4. The different trend for the two curves seems to be related with

the humidity trapped by the Kapton before the sputtering process. The MicroMegas (MM) calibration

curve has been obtained drying the polyimide film, before the sputtering, in the oven at a temperature

of 200°C for about 2 hours.

The two open square markers, referring to the µ-RWELL 2017 test production, obtained with the pre-

drying cycle of the base material, match the MM calibration curve (performed with humidity

controlled Kapton samples).


BUILT AND QUALIFIED

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Date: 19/04/2017


Fig.5: Measured gain for the three detectors.

Fig.6: Normalized gain for 12 MΩ/ (blue squares), 80 MΩ/ (black full circles) and 880 MΩ/ (red empty circles).

3. LABORATORY TEST RESULTS

The basic characterization of the detectors is performed with a collimated flux of 5.9 keV photons

generated by a Philips PW2217/20 X-ray gun. The gas gain of the detectors has been measured in

current mode, correlating the current drawn through the resistive layer as a function of the potential

applied to the amplification stage.


BUILT AND QUALIFIED

Milestone: MS53

Date: 19/04/2017


As shown in fig. 5, the gas gain of the various detectors, measured for the Ar/i-C4H10 = 90/10 gas

mixture, and parametrized as 𝐺 = 𝑒(𝛼∆𝑉− 𝛽), is typically ≥ 10000, being stopped when current

instabilities (not discharges) are observed: the largest gain is generally reached with the most resistive

detector.

Since the detectors reach the full efficiency at a gain of ~3000 (see next section), the rate capability

has been measured at a gain of about 4000. The X-ray gun can provide photon-converted rates ranging

from ~1 kHz up to ~300 kHz, while the radiation flux is obtained dividing the rate by the collimator

surface (we performed the local irradiation of the detector with a 2.5 mm diameter brass collimator)

and for each flux we have plotted the ratio of the measured gain to the nominal one (fig. 6). The

points are fitted with the function introduced and derived in [1]:

𝐺

𝐺0=

−1 + √4𝑝0𝛷

2𝑝0𝛷

Using this function we can obtain the value of the flux at a given normalized gain drop, say 3%,

standing for our definition of the detector rate capability.

The results are summarized in the following tab. 2.

Detector resistivity (MΩ/) Photon flux for a 3% gain drop

880 77 kHz/cm2

80 850 kHz/cm2

12 3.4 MHz/ cm2

As expected, the gain decrease is correlated with the voltage drop due to the resistive layer: larger is

the resistivity lower is the rate capability, ranging, for local irradiation, from few tens of kHz/cm2 up

to few MHz/cm2, even though in case of global irradiation and a large area detector the rate capability

is clearly smaller than the values reported in tab. 2.

A more trustworthy measurement of the rate capability has been obtained irradiating a detector with

a pion beam, with a spot area of ~3x3 cm2 (FWHM), two different sectors of a single-resistive layer

detector with a ~70MΩ/ resistivity. The results reported in fig. 7 show that the detectors can be

operated up to 30 kHz/cm2 without appreciable gain losses.


BUILT AND QUALIFIED

Milestone: MS53

Date: 19/04/2017


Fig.7: Rate capability as a function of the beam flux for two different sectors of a single layer µ-RWELL.

4. BEAM TEST RESULTS

The tracking performances of the three detectors have been investigated with a muon beam (see tab.

3) of the H4 line of the SPS test area at CERN. The setup has been composed of four trigger

scintillators (two read-out with SiPM and two with PMT) and two tracking stations consisting of

triple-GEM detectors equipped with XY-readout. All the devices have been operated in Ar/i-C4H10 =

90/10 and read-out by APV hybrid cards [8] handled by an SRS system. The APV chip, supplying

analog output signals, allows the study of the detector tracking performance based on the charge

centroid method.

Fig.8: Tracking efficiency as a function of the gain for the three detectors.


BUILT AND QUALIFIED

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Date: 19/04/2017


In fig. 8 the tracking efficiency of the detectors is reported as a function of the gain: all detectors

reach an efficiency above 98%. The shift of the efficiency curve of the 12 MΩ/ prototype with

respect to the other curves is correlated with the large charge spread occurring at low DLC resistivity

(fig. 9): the spread of the charge on the readout strips increases, the signal collected by each pre-

amplifier channel decreases thus requiring a higher gain to reach the full detector efficiency.

Fig. 9: Strip cluster size (average number of contiguous hit strips per track) as a function of the gain for the three

detectors.

Moreover, the charge spread affects the space resolution of the detector. Indeed, as reported in fig.10,

the sigma of the residuals (defined as xfit - xmeas, where xfit is the intersection of the reconstructed

track with the detector plane and xmeas is the cluster centroid coordinate) depends on the resistivity of

the DLC, showing a minimum around a resistivity of about 100-200 MΩ/.

Fig. 10: Sigma of the residuals (black points/curve) and strip cluster size (red points/curve) as a function of the DLC

resistivity


BUILT AND QUALIFIED

Milestone: MS53

Date: 19/04/2017


At very low resistivity the charge distribution loses the typical Gaussian shape and consequently the

σ becomes larger, while at high resistivity the charge spread is so negligible (the strip cluster size

being close to 1) that the charge centroid method becomes ineffective and the σ approaches the limit

of pitch/12.

5. CONCLUSIONS

The µ-RWELL R&D is matching the schedule with very good results, and the first milestone has

been successfully fulfilled. The most important performances of the detector have been measured as

a function of the DLC resistivity.

All the detectors, realized with the single resistive layer scheme, exhibit a gas gain up to and above

104.

The rate capability, measured with a collimated X-ray beam (local irradiation), strongly depends on

the DLC resistivity ranging on two orders of magnitude as the resistivity goes from 12 to 880 MΩ/,

while in conditions closer to experimental environment the detector stands fluxes up to 30 kHz/cm2.

As a direct consequence of the charge spread the spatial resolution shows a minimum between 100

and 200 MΩ/.


BUILT AND QUALIFIED

Milestone: MS53

Date: 19/04/2017


6. REFERENCES

[1] G. Bencivenni at al., The micro-Resistive WELL detector: a compact spark-protected single

amplification-stage MPGD, JINST 10 (2015) P02008.

[2] P. Fonte et al., Advances in the Development of Micropattern Gaseous Detectors with Resisitve

Electrodes, Nucl. Instr. & Meth. A 661 (2012) 153.

[3] Y. Pestov et al., A spark counter with large area, Nucl. Instr. & Meth. 93 (1971) 269.

[4] R. Santonico, R. Cardarelli, Development of Resistive Plate Counters, Nucl. Instr. \& Meth. A

377 (1981) 187.

[5] M. Anelli et al., Glass electrode spark counters, Nucl. Instr. & Meth. A 300 (1991) 572.

[6] M.S. Dixit et al., Simulating the charge dispersion phenomena in Micro Pattern Gas Detectors

with a resistive anode, Nucl. Instr. & Meth. A 566 (2006) 281.

[7] A. Ochi et al., Carbon sputtering Technology for MPDG detectors, Proceeding of Science

(TIPP2014) 351.

[8] M. Raymond et al., The APV25 0.25 m CMOS readout chip for the CMS tracker, IEEE Nucl.

Sci. Symp. Conf. Rec. 2 (2000) 9/113.

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