Study of Ion Back Flow suppression with thick COBRA GEM

Ion Back Flow (IBF) suppression is essential to avoid a space-charge distortion of the electric field under a high rate condition in the Time Projection Chamber (TPC). A GEM technology is one possible solution to achieve a small IBF and to keep a good performance in terms of particle tracking and particle identification at high rates in TPC. We developed Thick COBRA GEMs to investigate the capability of further IBF suppression. It was found that the COBRA GEM can suppress IBF more effectively compared to a standard GEM. IBF reaches about 0.1–0.5% with a stack configuration consisting of one standard GEM facing to the drift field and two COBRA GEMs. In this paper, the current status of development of COBRA GEM is described.


Introduction
ALICE [1] is the dedicated experiment to study the Quark Gluon Plasma (QGP), the hot and dense QCD medium, via heavy ion collisions at LHC. The ALICE Time Projection Chamber (ALICE TPC) [2], which is the main device in the central barrel for tracking and particle identification of charged particles, consists of a 90 m 3 cylinder filled with Ne/CO 2 /N 2 (90/10/5). Multi Wire Proportional Chambers (MWPCs) are adopted with gating grids as readout chambers. The gating grid prevents ions from going back to the drift space (Ion Back Flow; IBF). Suppression of IBF is essential to avoid the worsening of the TPC performance, in particular under high rate and high multiplicity conditions.
A continuous readout of the ALICE TPC is planned with 50 kHz Pb-Pb collisions after 2019. The current ALICE TPC has a limitation of the data taking with the maximum rate of 3.5 kHz due to the gating grid operation. The required IBF is 0.5-1.0% at gain ∼ 2000 to achieve a continuous readout with the current TPC performance. One possible solution to fulfill the requirement is to replace the MWPCs with the Gas Electron Multipliers (GEMs) [3], which is a type of micro pattern gaseous detectors, because GEM has a good IBF blocking capability.
The structure of a standard GEM is a 50 µm-thick insulator sandwiched by 5 µm-thick copper foils with 70 µmφ holes in 140 µm pitch. When high voltage is applied between GEM electrodes, an electron avalanche is induced by the high electric field inside holes. Ions going to the drift region can be blocked by the GEM electrodes. Furthermore, a stack configuration of GEMs can make the ion-blocking more efficient. The development of readout chambers with standard GEMs is therefore a main approach of the ALICE TPC upgrade. The performance evaluation of standard GEMs with triple and quadruple GEM stacks is being investigated. In addition, we are exploring the basic performance of the COBRA GEM [4] as another optional approach. The COBRA GEM is specially designed to achieve better IBF suppression compared to a standard GEM by a double electrode pattern on a surface. This characteristic electrode pattern enhances to make electric force lines converge.
-1 -  Its performance is being studied for application to the GEM-based TPC. In this article, we present the results of our recent studies of COBRA GEMs in Ne/CO 2 (90/10) and Ar/CO 2 (70/30) at atmospheric pressure.

Thick COBRA GEM
In collaboration with SciEnergy Co., Ltd. [6], we have developed two prototype COBRA GEMs; 200 and 400 µm-thick. Table 1 summarizes the geometry of these GEMs. The glass epoxy laminate (FR5) is employed as an insulator, and 6 µm-thick copper layers cover an active area of 3 × 3 cm 2 . The holes were pierced by a drill. The patterns on the COBRA electrodes and the rims around the holes were produced by a wet etching technique. Our COBRA GEM is characterized by a double electrode pattern on both sides. A microphotograph of the COBRA GEM surface and a schematic view of a cross section are given in figure 1. By creating a voltage difference between these two COBRA electrodes, ∆V AC , ions can be efficiently absorbed. We define   3 Measurement setup Figure 3 shows a schematic view of the measurement setup. The chamber is filled up with Ar/CO 2 (70/30) or Ne/CO 2 (90/10) at atmospheric pressure and the X-ray beam is injected perpendicular to the surface of the COBRA GEM. The distance between the readout pad (3×3 cm 2 ) and the lower GEM electrode is 2 mm. The electric fields in the drift and induction regions, E d,i are 0.4 kV/cm and 3 kV/cm, respectively. In this measurement, the distance between the mesh plane and the upper GEM (=drift space) is 3 mm. The measurement with the stack configuration (figure 4) were also carried out. Two 200 µm-COBRA GEMs were placed below a standard GEM (50 µm thick; holes diameter: 70 µmφ ; pitch: 140 µm). The electric field in the transfer region between the standard GEM and the upper COBRA GEM, E t1 is 0.4 kV/cm, and that between the COBRA GEMs, E t2 is 0.4 kV/cm.    Figure 5 shows the gain with the single COBRA configuration as a function of ∆V GEM . We calculate the gain as I a /(I 0 c /2), where I a and I 0 c are the current at the pad plane and the mesh current without electron multiplication, respectively. The factor 2 in the expression of the gain comes from the fact that only half mesh current contributes to electron multiplication because the attenuation coefficient for the X-ray energy we used in this measurement is longer than 3 cm and the -4 - number of seed electrons in mesh-shield and shield-COBRA is almost the same. Open and closed squares represent the results of the gain for the 200 µm-thick COBRA GEM in Ne/CO 2 (90/10) and Ar/CO 2 (70/30), respectively. Closed circles show the gain for the 400 µm-thick COBRA GEM in Ar/CO 2 (70/30). The gain of single COBRA GEM increases exponentially as ∆V GEM increases. It can reach more than 10 3 as expected from the result for a 400 µm-thick GEM [7]. Figure 6 shows the gain and IBF with the single 200 µm-thick COBRA configuration in Ne/CO 2 (90/10) as a function of ∆V AC . We define (IBF) ≡ (I c − I 0 c )/I a , where I c is the current at the mesh plane. ∆V GEM of the COBRA GEM is 550 V. The X-ray rate is tuned to match the current density at the readout pad to the expected one in 50 kHz Pb-Pb collisions. A better IBF is achieved at a higher ∆V AC since the ion absorption at the COBRA electrodes is improved. A higher ∆V AC also brings a higher gain because of a better collection of the electrons and a higher electric field in the hole. The gain is decreasing above ∆V AC ∼ 240 V because the field switched to the reverse direction and seed electrons cannot reach the GEM. Figure 7 shows the gain and IBF with the stack configuration in Ne/CO 2 (90/10) as a function of ∆V AC , where ∆V AC is applied simultaneously on both 200 µm-thick COBRA GEMs. ∆V GEM of the standard GEM is kept at 200 V during this measurement. We changed ∆V GEM of each COBRA GEM; ∆V GEM2,3 = 430, 430 V (black square), 390, 470 V (blue cross), 350, 510 V (green triangle), 310, 550 V (yellow plus), and 260, 590 V (red circle). IBF can be more suppressed with higher ∆V AC . IBF achieved 0.1-0.5% at ∆V AC = 250 V at gain ∼ 1000. A clear difference among different settings of ∆V GEM2,3 is seen at ∆V AC = 250 V. To better understand the ∆V AC dependence of the gain and IBF, further measurements with differenct ∆V GEM , ∆V AC , and E t1,2 are needed.

Summary and outlooks
We have developed COBRA GEMs with two different geometries. The gain that a COBRA GEM can reach is larger than 10 3 . The COBRA GEM achieves 0.1-0.5% IBF at a gain around 1000 -5 - with the stack configuration. Further IBF suppression is possible by optimizing the ∆V GEM , ∆V AC , and E t1,2 . We are going to evaluate the gain stability, energy resolution and efficiency through both measurement and simulation. In addition, we are developing a 100 µm-thick COBRA GEM without rim to reduce the influence of charging-up of the insulator.