Bachmann, S.

Kappler, Steffen

Ketzer, B.

Mueller, Thomas

Ropelewski, L.

Sauli, F.

Schulte, E.

English

KITopen

Proceedings of the Vienna Conference on Instrumentation, Feb. 2001, submitted to ELSEVIER PREPRINT. 1High rate X-ray imaging using multi-GEM detectors with a novel readoutdesignS. Bachmanna, S. Kapplerab, B. Ketzera, Th. Mullerb, L. Ropelewskia, F. Saulia, and E. SchultecaCERN, EP division, CH-1211 Geneva 23, SwitzerlandbInstitut fur Experimentelle Kernphysik, Karlsruhe University, GermanycInstitute of Physics, Helsinki University, FinlandModern micropattern gas counters are able to detect soft X-rays with high spatial resolution. Based on theprinciple of gas amplication, this detector type has a wide dynamic range and provides energy-selection duringimage reconstruction. One of the most recent developments is the Multi-GEM, an eÆcient, high-rate capablemicropattern gas detector. Owing to the separation of gas amplication and readout stage, this detector allowshigh exibility in the geometry of the readout structure.With a two-dimensional projective readout, GEM detectors have shown good capability in providing X-rayabsorption radiographies of small mammals. We present a detector with a novel design where the readout boardconsists of micropads connected to three layers of strips in such a way that two-dimensional spatial reconstructionand an unambiguous resolution of events with multiple hits is possible.Keywords: X-ray imaging; Gas Electron Multiplier; GEM; high rate radiation detector; micropad readout; mul-tiple hits; unambiguous resolutionPACs: 29.40.Cs ; 77.22.Jp ; 51.70.+f1. IntroductionInitially developed for High Energy Physics,gas detectors are used in a wide range of appli-cations. Recent developments showed that mod-ern micropattern gas counters are able to detectsoft X-rays with high spatial resolution [1{3]. Areview of the application of gas detectors in radi-ology can be found in [4].Exploiting the gas amplication of this detec-tor type provides wide dynamic range and, if op-erated in a single photon counting mode, energy-selection during image reconstruction.The Gas Electron Multiplier (GEM) [5], whichwas initially invented as a preamplication stagefor micropattern gas detectors, is a two-sidecopper-clad Kapton foil, perforated with a highdensity of holes. These holes are etched using aphotolithographic process with a pitch of usually140m and diameters of about 70m (so-calledCorresponding author: Steen.Kappler@cern.chstandard geometry). Application of a potentialdierence between upper and lower electrode cre-ates a high electric eld inside the holes. Insertedin the drift gap of a gas detector, nearly all drift-ing electrons are guided into these holes wherethey undergo proportional gas amplication.Stacking several GEMs in a cascade leads tothe multi-GEM detector (g. 1). This eÆcient,high-rate capable detector is one of the most re-cent developments in the family of micropatterngas detectors. The eective gain obtained with aGEM detector can be adapted to the needs ofthe application choosing the number of GEMsand the voltage across each one. The big ad-vantage of this detector type is the separation ofgas amplication and readout stage, resulting inthe fact, that the readout signal is only due tomotion and collection of the electrons. This sep-aration provides not only a margin of safety incase of discharges occurring in the GEM foils, italso allows high exibility in the geometry of the2 Proceedings of the Vienna Conference on Instrumentation, Feb. 2001, submitted to ELSEVIER PREPRINT.ionizingparticlesconversion& drift gaptransfergapinductiongapreadoutelectronicsreadoutelectronicsenergytriggerreadoutboardGEMcathodeGEMFigure 1. Schematic view of a multi-GEM detec-tor.readout structure. Experience from High EnergyPhysics showed, that in a single GEM operated inAr:CO2 (70:30) eective gains well above 103 canbe reached. The rate capability of these detec-tors was veried in a set of studies up to uxesexceeding 105Hz/mm2. Using double-GEM de-tectors for X-ray detection, discharge probabilityand aging-eects can be neglected [6]. With goodresolution in time and space and the possibilityto read the lower electrode of the lower GEM totrigger the readout electronics, the multi-GEMdetector meets the basic requirements for fast X-ray imaging devices.2. A double-GEM detector for X-ray ab-sorption radiographyA double-GEM detector with an active areaof 10  10 cm2 was built to provide X-ray ab-sorption radiographies of small mammals. Thetwo-dimensional projective microstrip readout ofthis detector (g. 2) was equipped with electron-ics based on the PreMux128 chip [7]. This chargexy200 m80 m150 mFigure 2. Microscope photograph of the two-dimensional projective microstrip readout.sensitive electronics, developed for High EnergyPhysics applications, is not optimized for imag-ing purposes.The detector was operated with a gas mix-ture of Ar:CO2 (70:30) at atmospheric pressure.The sensitive gap between drift cathode and rstGEM was chosen to be 3mm, the transfer gapbetween the GEMs and the induction gap to thereadout board was 2mm. Fields of 3:3kV/cm inthe drift gap, 4:0kV/cm in the transfer gap, and4:3kV/cm in the induction gap were selected. TheGEMs (standard geometry) were operated at avoltage dierence of 430V each.Figure 2 shows a photograph of the microstripreadout structure used. It consists of two layersof strips, each at a pitch of 200m, and separatedby 25m Kapton. The strip width is 80m in theupper, and 150m in the lower layer, resulting ina well correlated charge sharing, essential for aneÆcient detection on both coordinates [8].With an X-ray tube providing a beam of 8keVphotons, this detector was used to generate e.g.the X-ray absorption radiography of a bat, shownProceedings of the Vienna Conference on Instrumentation, Feb. 2001, submitted to ELSEVIER PREPRINT. 310mmFigure 3. X-ray absorption image of a small mammal. The color scale indicates the number of photonscounted in the pixels.in gure 3. This 60  35mm2 image with pixelsizes of 100100m2 consists of 36.5 million pho-ton counts. In this image, the skeleton includingspine, vertebrae, and even the small ribs can berecognized.Despite the good imaging properties of thisreadout structure, at higher rates (exceeding afew hundred kHz over the detector area) the re-construction eÆciency is limited by ambiguitiesdue to multiple hits within one readout cycle, re-sulting from the intrinsic time resolution of thereadout electronics (around hundred ns). Witha conventional two-dimensional projective read-out structure, no information is obtained whichwould allow the resolution of those multiple hits.(We remind the reader that for monoenergetic X-rays the correlation in charge normally can notbe used.)3. Resolution of multiple hits - a novelreadout designIn order to obtain the missing information, athird projection can be added. Therefore, a novelreadout board was designed, consisting of mi-cropads, which are alternately connected to threedierent layers of strips, crossing at an angle of120o (g. 4). The strips are embedded in layersof insulating photo-polymer and have a pitch of520m. The uppermost layer contains the mi-cropads, which have an outer diameter of 300m.By choosing the coordinate system indicated ingure 4 (with the axes u, v, and w perpendicularto the corresponding strips) each point fullls thefollowing plane equation:u+ v + w = 0 (1)For clusters covering three projections, this equa-tion can be used for the resolution of events withmultiple hits. An example for a three-hit event isshown in gure 5. There, the charge correlationhelps to couple the three coordinates of one hit,but for the remaining two hits a dierent proce-dure has to be chosen. One possible approach isthe following: In a run with N hits, an algorithmfor cluster recognition provides the coordinatesu1 : : : uN , v1 : : : vN , and w1 : : : wN . A combinato-rial check,jui + vj + wkj < Æ (2)which is derived from equation (1) and includespossible avalanche dispersions (represented by4 Proceedings of the Vienna Conference on Instrumentation, Feb. 2001, submitted to ELSEVIER PREPRINT.uvw520 mFigure 4. Microscope photograph of the novelmicropad readout. The micropads are alternatelyconnected to three dierent layers of strips.Æ), gives the valid combinations (ui; vj ; wk). Torecord the resolved hits in an image with Carte-sian coordinates (x; y), a coordinate transforma-tion is necessary.The present prototype of this readout boardwas produced at CERN by R. de Oliveira andconsists of seven layers of conductive patterns,embedded in an insulator and carried by a -breglass plate. The production process consistsof the following steps: The rst set of readoutstrips (coordinate u) is chemically etched into acopper layer on the support plate. In the sec-ond step, a photo-polymer layer is laminated ontothe strips. Covered by a photolithographic mask,it is exposed to UV light. The uncovered re-gions polymerize and the remaining material isremoved afterwards. Then, the resulting holes inthe insulator are lled with a conductive silver-epoxy, and after hardening the surface is polished.The following layers (coordinates v and w, pads,and connectors) are added the same way, startingfrom the second step.Figure 5. Typical event with three hits over thedetector area.To check properties and imaging performanceof this readout circuit, it was integrated into adouble-GEM detector with a geometry similarto the one described above. Connected to thesame electronics and operated in the conditionsmentioned, an energy resolution of Efwhm=E =20:1% could be achieved, and the charge sharingwas well correlated with ratios of 1 : 1 (v to w)and 1 : 1:2 (u to v and u to w), as shown in gure6.0.0 0.5 1.0 1.5 2.0 2.5 3.0v : wv : uEventsCharge Sharing RatiosFigure 6. Charge sharing ratios v : w and v : u.Proceedings of the Vienna Conference on Instrumentation, Feb. 2001, submitted to ELSEVIER PREPRINT. 5Figure 7. X-ray absorption images of slit mask and LED, taken with the micropad readout structure.The grey scales indicate the number of photons counted in the pixels.With the 8keV X-rays, the radiography of aslit mask and a LED were taken (g. 7). These1010mm2 images with pixel sizes of 5050m2demonstrate the imaging performance of thisreadout structure. Even the small slits of 300mwidth, which are 210m apart, can be separated.The multi-hit capability is mainly limited byclusters overlapping in one projection. In Ar:CO2(70:30, 1 atm), a mean cluster size of 770m(fwhm) was measured, and thus the probabilityof overlapping clusters is not negligible. To quan-tify the multi-hit capability, a set of 105 single-hitevents was recorded at low rate. In order to sim-ulate pure N -hit runs, N record sets of this runwere superimposed. Afterwards, an algorithmwith a relatively primitive cluster recognition wasused to resolve the N hits according to the recon-struction principle mentioned above. For an ac-tive area of 10 10mm2, this algorithm was ableto nd 66% of the hits in a 2-hit run, and 50% ina 3-hit run. Optimizing the detector for smallercluster sizes and developing better reconstructionalgorithms, would even improve these numbers.4. ConclusionsThe studies carried out showed that multi-GEM detectors are able to detect soft X-rays athigh uxes with good spatial resolution. Thenovel readout design provides suÆcient resolutionand reconstruction of multi-hit events, and thepresent prototype with the hexagonal pad designshows reasonable imaging qualities.High rate X-ray imaging, Cerenkov ring imag-ing, and the readout of Time Projection Cham-bers oer promising perspectives for the applica-tion of projective pad readout designs.REFERENCES1. E.A. Babichev et al., NIM A310 (1991) 449-454.2. H.J. Besch, NIM A360 (1995) 277-282.3. M. Dixit et al., IEEE Trans. 47 (1998) 809.4. A. Sharma, Gaseous Micropattern Detec-tors In Astrophysics, Radiology, And PlasmaPhysics, CERN-OPEN/2000-313, October31, 2000.5. F. Sauli, NIM A386 (1997) 531-534.6. S. Bachmann et al., Progress in developmentof GEM detectors, Proceedings of the ViennaConference on Instrumentation, February 19-24, 2001.7. L.L. Jones, PreMux128 specication, version2.3, RD20 internal document, January 1995.8. S. Bachmann et al., Performance of GEMdetectors in high intensity particle beams,CERN-EP/2000-116, August 2000.

High rate X-ray imaging using multi-GEM detectors with a novel readout design

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High rate X-ray imaging using multi-GEM detectors with a novel readout design

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