Development of New MicroStrip Gas Chambers for X-ray Applications

Transcription

1 Joint International Workshop: Nuclear Technology and Society Needs for Next Generation Development of New MicroStrip Gas Chambers for X-ray Applications H.Niko and H.Takahashi Nuclear Engineering and Management, Faculty of Engineering, The University of Tokyo, 7-3-1, Bunkyo Tokyo, , Japan ABSTRACT To cope with the high intensities of new generation beams such as the X-ray Free Electron Laser, a novel detector design ensuring a steady performance is needed. One of candidate detectors is a Mi crostrip Gas Chamber which is the first MicroPattern Gas Detector (MPGD). Although original MSGCs suffered from discharges and could not realize a high gas gain, we are developing Multi- Grid-Type MSGCs, which have grid electrodes between the anode and the cathode electrodes in order to obtain a stable electric field. M-MSGCs are fabricated in house by using recent Electron Beam lithography techniques. While studying these M-MSGCs, we have developed the concept of a fine-pitch MSGC for high counting rate applications. A very fine-pitch MSGC or a NanoStrip Gas Chamber (NSGC) is promising for very high counting rate applications, as well as for fine resolution with high pressure heavy gas. The narrowest pitch between anodes has reached 30 μm with the anode width of 800 nm. Our first trial for 50 μm pitch plate showed a gas gain sufficient to detect 8 kev X-rays. We are currently studying the characteristics of these new fine-pitch MSGC with X-rays and seek the possibilities of MSGCs as a next generation detector. Key Words: MSGC, NanoStrip Gas Chamber (NSGC), High counting rate and Fine pitch 1. INTRODUCTION MicroStrip Gas Counters (MSGCs) are the oldest type of gas counters among micropattern detector developed by photolithography technique (Fig.1). With its narrow spacing electrodes, MSGCs overcome the limits of MultiWire Proportional counters such as the position resolution limit and the rate capability due to their finite wire-spacing. MSGCs are expected to be a good candidate for high intensity beam experiments performed for example at with synchrotron radiation and high energy facility. However the breakdown of electrodes by discharges was considered as one serious problem, which causes critical damage to the detectors. For this problem, we proposed a multi-grid-type MSGC (M-MSGC) [1-3]. M-MSGCs are equipped with many intermediate strips between the anode and the cathode strip. These strips are connected individually to a high voltage source to maintain intermediate potentials between the anode and the cathode. As a result, the electric field is stabilized by this grid potential, and therefore a very high gas gain can be achieved without discharges.

2 H. Niko and H. Takahashi In addition to operating the M-MSGC in pulse mode, we are exploring the possibility of the charge integration mode, where pile-up pulses can also provide the intensity information of incident radiation as well. Although we lose the benefit of pulse counting, we can obtain a wide dynamic range in this operating mode. While studying these MSGCs, we have developed the concept of a fine-pitch MSGCs under high counting rate (Section 2). Based on our studies so far, the very fine-pitch MSGC or NanoStrip Gas Chamber (NSGC) appears very promising for very high counting rate applications, as well as for fine resolution with high pressure high-z gas. We fabricated a first trial 50 μm pitch plate and tested its characteristics. Fig.1 The cross section of the MSGC mounted inside a chamber (left). The conventional plate design is shown in the right figure Charge-Integrating-type M-MSGC 2. DYNAMIC RANGE MEASUREMENT For very high counting rate applications, pulse counting mode is not so suitable since the typical pulse width of MSGCs is about several hundred ns. Therefore we adopted the use of charge integrating readout of M-MSGC [4] where the performance at a high counting rate is only limited by a space charge effect. We designed an M-MSGC plate for charge integrating readout from cathode strips. Anode pitch was 400 μm and 4 grids were inserted between the anode and the cathode. The widths of anode, grid1, grid2, grid3, and grid4 were 10, 20, 25, 35, and 40 μm, respectively. Gaps between neighboring strips were set to 10 μm. Cathode strips were separated so that we can read current signals from individual cathode strips. We connected 32 cathode strips to input channels of integrating amplifier ASICs. For readout ASICs, we utilized two HX2 [5] chips for 32 cathode strips. HX2 contains an array of 16 integrating amplifiers, each with a 10pF feedback capacitor. The integration period can be varied over the range around 5 μs to around 100 ms by an external clock frequency, which provides enough dynamic range. The integration period is followed by the readout period where the 16 channels are multiplexed onto an analog output bus. 2/5

3 Development of New MicroStrip Gas Chambers for X-ray and Neutron Application 2.2. X-ray Irradiation from Top of the Plate As we already reported in our previous paper [4], the charge integrating mode can assure the measured linearity of output current from the cathode as much as 10 9 cps/mm 2 at the a gas gain of 100, although the curve shows that saturations, called space charge effect start at around 10 8 cps/mm 2. Since detailed investigation into the saturation region of such a high counting rate operation is not well reported in the literature, we carefully analyzed the characteristics of the detector. When we increased the gas gain, saturation occurred at a lower counting rate, which shows that the effect is due to the space charge X-rays Irradiation Parallel to the Strips To overcome the saturation by the space charge, we are trying to enlarge an avalanche region per unit area. First we have tested with another geometry, where we irradiated 8 kev X-rays from the side of the plate (parallel to the strips, see Fig.1) so that we can take advantage of the full depth of the anode electrode effectively. We performed the experiment at Super Photon ring-8 GeV (SPring-8) [6] with 8 kev X-rays in the flowing gas mixture of Ar (70%) + CH4 (30%). The result is shown in Fig.2. We observed linearity of the detector response up to cps/mm 2 in the charge integrating mode. Fig.2(a) Fig.2(b) Fig.2. The direction of the X-rays is shown with simplified schematic of the experimental set up. (a) Top view of the set up (The chamber is shown in dot rectangle. (b) Side view. Fig.3. Counting rate characteristics of a charge integrating operation. Fig.4. The comparison of the charge collection p rocess for a conventional MSGC (left) and a fine r pitch MSGC (right). Either plate is described without grids from the cross section view. Joint International Workshop: Nuclear Technology and Society Needs for Next Generation 3/5

4 H. Niko and H. Takahashi 2.4. Towards Fine-Pitch M-MSGC We have confirmed our idea to increase the dynamic range by enlarging avalanche area by the result at SPring-8. Linear region of the same plate was 3 decades wider by enhancing the avalanche region by changing the direction of X-rays irradiation as shown in Fig.3. We consider that a plate which has a smaller pitch length than electron range, about 400 μm in the gas mixture of Ar (70%) + CH 4 (30%) at 1 atm, can work under much higher counting rate. We consider that narrow pitch of a finer pitch MSGC enhances the counting rate characteristics by dispersing charges of the cloud into several electrodes. The idea is shown in Fig.4 with comparing the process of conventional MSGC Plate Design 3. NanoStrip Gas Chamber We have decided to develop a fine-pich MSGC that can increase the area of the avalanche region and suppress the space charge effect. As a first trial, we have fabricated a 50 μm pitch M-MSGC. Its anode width is chosen as small as 800 nm. In the following we call this implementation a NanoStrip Gas Chamber (NSGC) in this aspect. Our first prototype 50 μm pitch of NSGC has 2 grids. Their widths are 3 μm. 40 sets of strips are aligned and each strip is 20 mm long ( Effective area is 2 mm x 20 mm) X-ray Test The NSGC was tested with 8 kev X-rays at KEK, Photon Factory, BL-14A. A Gas Chamber is filled with an Ar (70%) + CH4 (30%) gas for all experiments. The detector is successfully operated. The gas gain of 280 is achieved. Fig. 5 shows waveforms obtained from the anode and the cathode at this condition. The rise times of signals are around 1 μsec. This rise time is not so fast as what we expected but it is because of the high resistance of narrow anode electrode. We measure its pulse height spectrum as well (shown in Fig.5). A relative energy resolution of 22% (FWHM) was achieved by employing a summing amplifier, we able to improve the relative energy resolution to 15% (FWHM) Uniformity We scanned the plate with collimated (100 μm in diameter) beam irradiation with 10 kev X-rays at KEK Photon Factory, BL-14A. We scanned the beam across the strips at the middle of the anode length in 100 μm pitch. The detector was operated at a gas gain of 220. We could observe signals at the effective area. We show the map of the scan in Fig.6. At the edge of the plate, the signal becomes larger because of the electrical field getting higher in this configuration. 4/5

5 Development of New MicroStrip Gas Chambers for X-ray and Neutron Application Fig. 5. The energy spectrum achieved 22 % (FWHM) of energy resolution with 50 μm NSGC at 8 kev X-rays. Fig. 6. The result of uniformity. Collimated beam were scanned across the electrodes with 10 kev X-rays at gas gain CONCLUSIONS We have made a very fine pitch M-MSGC as the first test of NanoStrip Gas Chamber (NSGC). The fabricated 50 μm pitch NSGC was successfully operated at a gas gain of 280. The energy resolution was 22 % (FWHM) for Fe-55 X-rays. We found that our NSGC design operates as conventional M-MSGCs. While our approach of a NSGC could have a significant impact highintensity beam experiments, more detailed studies with refined and high-rate X-ray measurements are needed to optimize and demonstrate the full capabilities. ACKNOWLEDGMENTS The Author thanks Mr. Kubota and Mr. Imai, who maintain the EB writer at VDEC to keep its best condition and helping the Author to realize these fine-pitch masks. REFERENCES 1. H.Takahashi, K.Yokoi, K.Yano, D.Fukuda, M. Nakazawa, S. Kishimoto, K.Hasegawa, IEEE Trans.Nucl.Sci.48;(6) (2001) H. Takahashi, K. Mori, et al., Nucl. Insr. And Meth. A 477;(2002) H. Takahashi, C. Hagai, et al., Nucl. Insr. And Meth. A 513;(2003) H. Takahashi, T. Ishitsu, et al., Nucl. Insr. And Meth. A 513;(2003) S.L. Thomas et al., IEEE Trans on Nucl.Sci.42;(4)(1995) Spring-8 Joint International Workshop: Nuclear Technology and Society Needs for Next Generation 5/5