1.1Principle of the Gridpix TPC and Gossip

1Introduction

1.1Principle of the GridPix TPC and Gossip

Gaseous detectors are widely used in particle physics in the form of wire chambers since the beginning of the seventies in the last century. While the primary ionisation is mostly too low to get sufficient signal to noise ratio, amplification by avalanche is generally used to overcome this problem. Advantages of gaseous wire chambers are the low costs per detecting surface and the low material budget. However the limited position resolution, rate capability, ageing performance and channel density makes them basically unsuited for the ATLAS inner detector at the sLHC.

A first breakthrough of this situation occurred around 1990 by the development of the MicroStrip Gas Chamber (MSGC). Here the alternating anode and cathode wires were replaced by metal strips on an insulating surface, thus solving the problem of electrostatic instability that prevents wire chambers to have a pitch much smaller than 2 mm. The gas amplification occurred in the narrow region between anode and cathode strip. The MSGC was a competitor of the planar silicon strip detector offering a comparable performance for a lower price. However, the MSGC suffered fromserious ageingproblems which depended strongly on the cleanliness of the chamber gas. The counting rate was limited by ion drift towards the drift cathode.

Fig. 1. Principle of the Gossip detector as a GridPix TPC with a narrow drift volume. **** we should make a drawing with a drafting program rather than Word (in 3D?)****

A second breakthrough came with the invention of Micromegas (Charpak & Giomataris 1995) and GEM (Sauli 1996), enabling a pixeled anode geometry.This line is continued in the GridPix detector where the fine granularity is given by the front-end pixel chip which serves as active anode. As such the GridPix concept can be used for all kind of TPC applications.

From GridPix we devised a speciality with a drift gap in the millimetre region: Gossip(Gas On Slimmed Silicon Pixels), to get a geometry comparable to planar silicon pixel or strip detectors. Its gap width is tuned to be just wide enough to detect minimum-ionizing particles at 99.5% of the hits, and thin enough to have a maximum drift time smaller than 25 ns, to collect all primary ionization within the (supposed) SLHC bunch distance. In Gossip a pixel is formed by the assembly of the detection volume:a shortgas column in the drift gap, the associated hole in the Micromegas, and the under laying pad and preamp circuit of the pixel chip. As such an individual stand-alone detector is created with a pitch in X-Y direction that may be as small as 60 m. For this detector we expect at a particle rate of 0.4 GHz/cm2 an occupancy of the order of 0.15% and a position resolution for a traversing MIP track of 50 µm, values that match well the requirements of the next generation experiments at CLIC, ILC and Super LHC.

The functioning of Gossip is illustrated in fig.1. In the gas filled drift volume, clusters of electron-ion pairs are created along the track of a traversing particle. Due to the electric field, the electrons drift towards the Micromegas grid and are subsequently focused into the holes of the grid. The grid is put at about -400 V with respect to the (grounded) anode pixel chip, creating a strong field in the 50 µm thick avalanche gap. As a result, each single electron entering a grid hole causes an electron avalanche of sufficient charge to be detected by the pixel front-end preamp. By recording the arrival time of the avalanche, the original position of the primary electron can be deduced. As such Gossip is a single-electron sensitive Time Projection Chamber (TPC).

Fig.2. The picture shows an opened GridPix detector. The chamber, of which the guard electrode and chip is visible, includes a TimePix chip which has been covered by a 20 µm thick layer of amorphous silicon. On top of this, an InGrid structure was deposited.The drift gap was 30 mm high.

Fig.3. The plot shows a measurement made by this detector of twotracks from a 90Sr source. The fiducial readout surface was 14 mm x 14 mm (256 x 256 pixels, with square pixel pitch of 55 µm). The detector was placed in a magnetic field of 0.2 T with the field lines running vertical.

The Gossip detector has been made possible by two recent technical innovations: InGrid [Appendix 1] and WaProt [Appendix 2]. InGrid is a Micromegas foil that is produced on the surface of the pixel chip by means of photolithographic wafer post processing. With this technology the pillars can be made much narrower such that they fit in the space between two pixels, eliminating the insensitive zone that occurs with the broader pillars of Micromegas. In addition this method, that is well suited for mass production, enables a much better geometric accuracy of the amplification area.

WaProt is a high resistivity layer that is deposited on the surface of the pixel chip as a protection against microdischarges. This protection is essential since the functioning of gas avalanche detectors is always accompanied by certain degree of microdischarges. Starting with a 20 µm thick layer of amorphous Silicon, we recently got good results with 7 µm of Si3N4. The resistivity of this layer can be tuned with a Si dopant from 1014 to 108 Ωcm.

Equipped with WaProt and InGrid, the pixel anode chip forms the monolithic active readout anode of the gas-filled drift volume.

1.2Properties of Gossip

Basically, the detection medium in Gossip is gas. Compared to solid state detectors, gas has the advantage of a negligible mass of the detecting medium (only the housing has some mass) but the disadvantage oftoo low primary ionisation for direct detection. Therefore, the charge signal has to be enhanced by an avalanche process, making operation more critical. Compared to solid state detectors,these properties result for Gossipin the followingadvantages and disadvantages:

Advantages of Gossip

• Gas can be exchanged or refreshed: therefore there is no radiation damage of sensor material.
• The (charge) signalcan be made sufficiently large by applying gas amplification.
• There is no bias current.
• In gas ɛr=1: therefore, and for geometrical reasons,the signal source capacity is as low as ~10 fF, allowing low-power, low noise preamps.
• Gossip measures, in three dimensions, the positions of all single electrons of a track, left in the gas, by a passing fast charged particle. Atrack segment is thus measured instead of track point position, and dE/dX information is obtained, and δ-rays can be recognized and omitted.
• The technology to produce Gossip detectors is cheap, no bump bonding is required.
• Gossip is little sensitive for neutrons and X-rays.
• Gossip can operate in a wide temperature range between -30 deg C and room temperature.
• The low electronics power dissipation and the higher operating temperature greatly reduces the demands on the cooling system. As a result the mass of the cooling system may be significantly diminished.

Disadvantages & limitations of Gossip:

• Deterioration of the position resolution by diffusion of the drifting electrons.
• Most hits liberate only a few primary electrons, so s are not easily rejected.
• Discharges are possible between grid and pixel chipthat may damage or destroy the pixel chip.This problem has been solved using WaProt (see Appendix WaProt).
• Possible ageing by depositon the anode, i.e the pixel chip leading to a rate dependent decrease of the gas gain.
• More services: two high voltage lines are needed instead of one (grid + drift anode) + two thin gas lines.
• The grid voltage is critical and depends on parameters like the gas composition and the detector geometry. An increase of the grid voltage by -30V gives an increase of the gas gain by a factor of two.

Fig.4. First working prototype of a Gossip detector(left). As FE readout chip the CMS Pixel FE chip (PSI-46) was applied. On top of the chip, which was protected by a layer of amorphous silicon, a Micromegas foil was placed. This unit was covered by a gas/cathode seal foil.

Fig.5. Example of a β-event from a 90Sr source. While most events activate a small cluster of one or more pixels, this event displays a track which must have been running rather parallel to the chip surface.

1.3Other possible applications of Gossip/GridPix in ATLAS

1. Gossip as replacement of the SCT Si strip detectors. The essential element of this detector would be a large CMOS chip (30 mm x 30 mm) containing 512 ‘strixels’ (thus 512 pixels with, as input pad, a strip with a length equal to the full width (30 mm) of the chip.

Lower costs are the main advantage of a Gossip Strixel detector with respect to a Si strip detector: compare the price of CMOS and Si sensor material per cm2, and the fact that the functionality and the costs of FE chips is included in the strixel CMOS chips.

The power dissipation of this detector would be a factor 4 less compared to equivalent Si strip detectors. *** we have to work this further out (numbers) or we should omit this statement****

1. GridPix as Transition Radiation Detector.Using a 17 mm wide drift gap and a Xe-filling, Gridpix detects the conversions from soft X-rays that are emitted by transition radiators in front of the detector. These conversions appear as clusters of tens of electrons that are superimposed onto the MIP track,. Several TRT layers could be installed at the outer radius inside the solenoid.*** also here some numbers and documentation (Anatoli?)***.

We plan to submit a separate R&D proposal on this subject.

1. Using GridPix as a LVL1 trigger. In a GridPix detector with a drift gap of ~20 mm, tracks appear as projection on the XY pixel plane. The projected length depends on the angle of incidence, and hence on the momentum of the track. The projected track length could be collected within one µs, and could thus provide LVL 1 trigger.

We plan to submit a separate R&D proposal on this subject.

1. Preshower detector: track sampling/photon discrimination, dE/dX measurement[more description required]
2. Hadron Calorimeter: digital (track counting) hadron calorimeter. The energy resolution and granularity of such a calorimeter would be very competitive, provided that there is sufficient correlation between tracks at both sides of the absorbers. With GridPix, dE/dX is measured precisely, and low-cost CMOS wafers can be applied as readout. *** I believe that in the past gaseous dE/dx detectors were not very successful since they require a very long path to get sufficient primary statistics. Would this be better in this case??***

2Participating Institutions

****To be completed and confirmed. I suggest that we only mention here senior staff members to make the list not too long ****

• NikhefAmsterdam
• Harry van der Graaf
• Fred Hartjes
• Nigel Hessey
• Els Koffeman
• University of Nijmegen
• Adriaan König
• Thei Wijnen
• Technical University Twente/MESA+
• Jurriaan Schmitz
• Physics Institute of the University of Bonn
• Klaus Desch
• Norbert Wermes
• Moskow Physical Engineering Inst. (MePhI)
• Anatoli Romaniouk
• Serguei Morozov
• Seguei Konovalov
• Saclay/CEA: Paul Colas, Yannis Giomataris
• ATLAS: Claude Guyot
• University of Freiburg
• Andreas Bamberger*** still in function??***
• Liverpool?
• LBL?
• Harvard?
• John Oliver
• MIT?
• Ulrich Becker
• Joe Paradiso
• Collaboration with CMS: PSI
• Roland Horisberger
• Tilman Rohe

3Topics, goalsand subprojects

3.1Topics

1. Investigation of the optimal operational parameters. This involves both simulation and experimental verification
2. Optimization of the counter gas
3. Required gas gain
4. Nature and resistivity of the chip protection layer
5. Thickness of the drift gap
6. Measurement of the performance of Gossip for MIP tracks
7. Efficiency
8. Time resolution
9. X-Y resolution
10. Double track separation
11. δ-ray identification and suppression
12. Background dependence for neutrons and X-rays
13. Dependence of the gas gain on temperature and pressure
14. Development of the robustness of operation in a harsh environment
15. Spark protection
16. Radiation tolerance for MIPs until 1016/cm2
17. Rate dependence up to 1 GHz/cm2
18. Design of a generic Gossip tracker
19. Mechanical support structure
20. Services
21. Cooling (CO2 based)
22. Gossip hybridization (MCM)
23. Dedicated Gossip Read Out Chip (ROC) optimized for low input capacity and negligible input current (possibly TimePix-3)

3.2Final goal of the Gossip R&D

1. The development of a Gossip tracker as replacement for the present pixel inner tracker of ATLAS.
2. Developinga Gossip tracker for the B-layer replacement: GOAT-1 (GOssip-ATlas),prior to the full upgrade of the ATLAS ID provided that the technology is available in time. GOAT-1 has been simulated (see appendix). ****how far are we with this??***

3.3Subprojects

Based on the R&D topics that are formulated above, we aim to pursue the following subprojects.

3.3.1Gossip demo based on the PSI-46 ROC

A Gossip made from a single PSI-46 chip equipped with a HV-protection layer and InGrid is at present being investigated. A test beam experiment is foreseen where a stack of these Gossip detectors will be placed in a silicon telescope. This will give us experimental numbers for the efficiency, position resolution and double track separation. However, this chip is made by an older technology and is optimized for silicon sensors. As a result, both the noise level and the granularity are worse than aimed for at Gossip and there is practically no drift time measurement.

3.3.2Gossip demo based on the Gossipo chip

Basically, the existing Gossipo-2 chip, designed in 130 nm technology, has completely the aimed performance in terms of pixel size (55 x 55 µm), time resolution (1.8 ns) and input noise **NEC. However, the minor dimensions of the readout pad matrix (16 x 16 pixels => a square of 0.88 mm) make it hard, but not impossible, to measure the position resolution in theXY plane. For Gossipo-2 the variations in the pixel settings vary such that at best 85% of the pixels are operational. Therefore we will use in a later stage the anticipated Gossipo-3 chip that will have more uniform pixel settings.

Fig.6. A Gossipo-2 chip equipped with an Ingrid structure. The SU-8 insulator carrying the (black) aluminium mesh is visible as a transparent square.

3.3.3Gossip demo based on the TimePix-2 ROC

The planned TimePix-2, to be designed in 130 nm technology, will also have a 1.8 ns resolution in the arrival time measurement of the hit, enabling track reconstruction in the 1.2 mm wide drift gap. Since the chip is also designed for use with moderately irradiated silicon, the front-end performance will not be optimal for Gossip.

3.3.4Upgrade of Gossipo-2 to Gossipo-3 and Gossipo-4

We plan to design an improved pixel front-end to overcome the range problems that were experienced with Gossipo-2. To facilitate tracking, Gossipo-3 will possibly have more pixels than Gossipo-2. For Gossipo-4 we will try a readout architecture comparable to Timepix-2.

3.3.5Development of TimePix-2

As a follow-up of the existing TimePix (in 250 nm technology) we plan the following modifications:

1. Convert the design to 130 nm technology
2. Implement the Gossipo frontend
3. Adapt the digitizing parameters to the Gossipo design
4. 55-60 µm pixel size
5. 560 MHz TDC clock

To enable a larger scale testing, the chip will be produced in an engineering run

3.3.6Development of the Gossip Multi-Chip Module (MCM)

We will design a structure supporting a number of ROCs (16?) that includes the cooling circuitry, HV and LV distribution structure, data communication lines, and the gas circuitry. To minimise the dead inter-chip zone, we will use the seamless technology that is developed by the ReLAXD collaboration. Prototyping will include thermo-mechanical simulations and testing.