GridPix application to dual phase TPC

. GridPix is a gas-ﬁlled detector with an aluminium mesh stretched 50 µ m above the Timepix CMOS pixel chip. This deﬁnes a high electric ﬁeld where gas ampliﬁcation occurs. A feasibility study is ongoing at Nikhef for the application of the GridPix technology as a charge sensitive device in a dual phase noble gas Time Projection Chamber (TPC), within the framework of the DARWIN design study for next generation dark matter experiments. The smallness of the device and well deﬁned materials allow for high radio-purity and low outgassing. The high granularity of a pixel readout and the high detection eﬃciency of single electrons of GridPix bring beneﬁts especially in terms of energy resolution for small energy deposits. This feature is interesting also for the measurement of the scintillation yield and the ionisation yield of noble liquids. The accurate measurements of such quantities have a direct impact on the data interpretation of dark matter experiments. The application in dual phase argon or xenon TPCs implies several technological challenges, such as the survival of the device at cryogenic temperature as well as the operation in a pure noble gas atmosphere without discharges. We describe here the recent developments of the project.


Introduction
GridPix [1] is a gas-filled detector where an aluminium mesh, the grid, is stretched 50 ñm above the pixelated Timepix CMOS chip. It is realised by means of wafer post processing technology.
The Timepix chip is characterised by a matrix of 256×256 pixels and a pitch of 55 ñm. The picture of the microscopic structure in Fig. 1 shows the pillars, made of an insulating material 2 and supporting the grid, as well as the pixels pattern on the chip surface. A thin resistive layer 3 over the chip surface protects the pixel circuitry in the event of sparks in the gas.
We investigate the application of GridPix as a charge sensitive device in a dual phase Time Projection Chamber (TPC). The development is part of the alternative charge readout working package of the DARWIN project [2], a design study for the next generation dark matter search experiments with dual phase argon or xenon TPCs.
These detectors are already playing a leading role in dark matter searches, as demonstrated by the latest results of XENON100 experiment [3]. The key concept is the simultaneous measurement of the ionisation and the scintillation light produced by a particle interaction in the liquid phase, which is the sensitive volume of the apparatus. The principle of operation is depicted in Fig. 2. The prompt scintillation light, the so-called S1 signal, is detected by photodetectors arrays, typically placed at the bottom of the liquid volume and in the vapour phase above the liquid. The electrons from ionisation are drifted to the liquid surface and extracted to the vapour phase by the applied electric field. Electrons, subject to a higher electric field in this region, produce electroluminescence as they collide with the gas atoms along the path to the anode electrode. This delayed scintillation light, the so-called S2 signal, is detected again by the photodetectors arrays.  The delay between the S1 and the S2 signals gives the Z-coordinate information, leading to a tri-dimensional position reconstruction. The ratio between the light and ionisation production depends on the density of the energy deposition and can be exploited for particle identification. Electronic recoils, i.e. electromagnetic interactions that transfers the energy to an atomic electron, are characterised by a higher charge to light ratio with respect to nuclear recoils, produced by neutral particles that transfer the energy to a nucleus.

GridPix as direct charge readout
An array of GridPix devices can instrument the TPC in place of the top photodetectors array. In this case the electroluminescence method is not required, but the electrons are collected through the grid holes in the GridPix amplification gap.
The high granularity of a pixel readout can improve the spatial resolution and the discrimination of double scatters. However it is especially effective in order to improve the energy resolution of the detector. The pixels' low noise and the high efficiency in detecting single electrons (close to 100% in Ar/iC 4 H 10 gas mixtures) allow for a digital readout approach. The electron cloud size becomes broader along the drift (in particular in the gas phase) because of diffusion and each electron is very likely collected in a different GridPix hole and detected by a different pixel: therefore the number of ionisation electrons can be inferred from the number of hit pixels. These features are also interesting for fundamental measurements of the noble liquid scintillation and ionisation yields. The accurate knowledge of such quantities is crucial for dark matter search experiments that adopt noble liquids as target. As an example, the deposited energy is inferred from the amount of scintillation light or ionisation electrons produced. The measurements are performed in dedicated small size setups. As depicted in Fig. 3a, Compton scattering or neutron elastic scattering is exploited in order to deduce the energy deposition from the energy and the direction of the scattered photon or neutron, respectively. Compton scattering is used to obtain the scintillation and ionisation production as a function of the deposited energy for electronic recoils, while neutron scattering allows to understand the response of the medium to nuclear recoils. Systematic errors in these measurements are coming from the uncertainty on the exact spot of the interaction within the active volume (Fig. 3b) and double scatters (Fig. 3c). GridPix can provide an accurate position reconstruction reducing such systematics. High radiopurity of all the components of a rare event search experiment is imperative to reduce the background. GridPix is a small device and the mass of the material introduced in the apparatus is therefore small. It is composed mainly by silicon, characterised by very low natural radioactivity. Similar arguments are valid for the outgassing properties of GridPix. The outgassing of materials is also an important property for dual phase TPC because electronegative impurities that dilute in the noble liquid capture the electrons during their drift path, hence reducing the S2 signal.

GridPix operation at low temperature and development of optimised devices
The first measurements of GridPix operated in gaseous argon environment at room and low temperature as well as the test in a dual phase argon TPC are described in [7]. The test in dual phase argon revealed a mechanical weakness of the structure at cold temperature (87 K), due to the different thermal expansion coefficients of materials. As described in [8], a systematic study was performed in a temperature-controlled environment, continuously purged with nitrogen to avoid ice formation from moist air: the problem was identified in the walls, the so-called dykes, that support the grid along the perimeter of the device. Each side of the perimeter is divided in three sections. The thermal stress produces a loss of adhesion of a section from the underlying protection layer, which is then ripped off by the tension of the shrinking grid, as shown in Fig. 4. Figure 4. The dykes that support the grid along the perimeter are the weak point of the structure at cold temperature (∼ 140 K in this test). Under thermal stress they lose adhesion to the underlying protection layer and the tension of the grid rips them off.

Figure 5.
Engineering calculation (Finite Elements Method) of the thermal stress on the GridPix elements at a temperature of 87 K. The maximum stress is found (red region) on the edges of the sections in which the dykes are divided, in correspondence of the discontinuity between two sections. In this simulation the grid and the pillars are not included.
A similar indication is obtained by engineering calculations (using Finite Elements Methods) of the thermal stress. In Fig. 5 this is shown for a simplified structure without pillars and grid. The red regions emphasise that the discontinuity between two sections is subject to the largest stress.
We investigated with the same engineering tools modifications of the geometry that lead to more robust structures. As shown in Fig. 6, possible solutions are a finer segmentation of the dykes and the introduction of extended structures in addition to pillars for the grid support. Several prototypes with different combinations of these geometries are under production on dummy wafers, i.e. on a bare conductive substrate instead of a Timepix wafer. We will study the robustness at cold temperature of these new geometries. The same devices will allow to study the operation in noble gases and to compare with the standard device properties as the maximum achievable gain and gain uniformity.
At the same time, an R&D on the manufacturing process is ongoing for the realisation of a "fully ceramic GridPix", i.e. a device where pillars and dykes are made of silicon oxide (an insulator material) or a high resistivity silicon nitride. A successful development in this direction is a breakthrough for the low temperature operation because all the materials of the device can have matching thermal expansion properties. Low outgassing properties and high radiopurity are also expected. Furthermore the realisation of a resistive grid made of Si-enriched silicon nitride with an internal conductive aluminium network can strongly suppress the discharge development.

Future prospects
The GridPix production recently moved to 8" inch wafers. This important achievement demonstrates the possibility of an industrialisation and therefore the reduction of the costs. The maximum achievable gain and the detection efficiency of single electrons will be measured with these new devices. A pressurised vessel (Fig. 7) allows to simulate at room temperature the higher gas density of a cold environment with a higher pressure. The study will be extended to pure xenon and Xe/CH 4 gas mixtures. Furthermore we are building at Nikhef a dual phase xenon TPC (Fig. 8). The apparatus will be equipped with a gas recirculation system with hot getters to purify the xenon. We will test the GridPix operation in dual phase xenon and eventually the performance of the device as direct charge readout.  The detector will be built in the bottom vessel while the top vessels will house the cooling system and the heat exchanger of the gas recirculation.