Sanghyo Han

GEM: performance and aging tests

Performance and aging tests have been done to characterize gas electron multipliers (GEMs), including further design improvements such as a thicker GEM and a closed GEM. Since the effective GEM gain is typically smaller than the absolute GEM gain, due to trapping of avalanche electrons at the bottom GEM electrode, we performed field simulations and measurements for better understanding, and discuss methods to eliminate this effect. Other performance parameters of the GEMs are also presented, including absolute GEM gain, short-term and long-term gain stabilities.

Application of the LIGA process for fabrication of gas avalanche devices

Arrays of holes having steep wall sides have been successfully prepared by using a deep X-ray lithography technique, or LIGA process, on various thicknesses (50-1000 /spl mu/m) polymethylmethacrylate (PMMA) plastic sheets. Electrical contact layers onto the top and bottom sides were deposited by metal evaporation in a vacuum. The completed LIGA devices were studied as an alternative design of the gas electron multiplier (GEM). The first measurements of performance were very promising: a lower limit to the avalanche gain of /spl sim/3,000 was obtained, and the actual gain is probably much larger. Detailed experimental results and field simulations will be described in this study. In addition, an application of a LIGA device to serve as a drift plane electrode, avoiding the angle dependency, will be discussed.

Initial Test of the PEFP 20 MeV DTL

A conventional 20 MeV drift tube linac (DTL) for the Proton Engineering Frontier Project (PEFP) has been developed as a low energy section of 100 MeV accelerator. The machine consists of four tanks with 152 cells supplied with 900 kW RF power from 350 MHz klystron through the ridge-loaded waveguide coupler. We assembled the fabricated accelerator components and aligned each part with care. We have also prepared the subsystems for the test of the DTL such as RF power delivery system, high voltage DC power supply, vacuum system, cooling system, measurements and control system and so on. The detailed description of the initial test setup and preliminary test results will be given in this paper.

Status of Beam Diagnostic Systems for the PEFP

A proton linear accelerator is currently the construction at the KAERI (Korea Atomic Research Institute) to the PEFP (Proton Engineering Frontier Project) in Korea. We are accomplished the technique development of beam diagnostic system to be currently the construction. We treat beam diagnostics for the high power proton linear accelerator. Prototype beam position & phase monitor (BPPM) electronics was made and tested successfully in one of the beam diagnostic systems. The beam position monitor pickup electrode is a capacitive type (electrostatic type) which has a button form. Button form electrode, in common use around electron synchrotrons and storage rings, are a variant of the electrode with small button form (e.g., sub mm diameter). However, we are designed button form electrode to measure beam position of proton beam. The BCM (Beam Current Monitor) is developed Tuned CT (Current Transformer) for collaborate with Bergoz Instruments. This paper describes the status of beam diagnostic systems for the PEFP.

Columnar cesium iodide (CsI) and LIGA micro-hole arrays for use in gas avalanche detectors

The characteristics of a columnar CsI layer with gas multiplication was investigated by using beta rays from a /sup 90/Sr source. This layer is intended for use as the primary electron source in any gas avalanche microdetector to avoid severe performance loss by the oblique angle of the incident charged particle. It was initially thought that the columnar structure might provide a larger detection efficiency than a planar CsI layer, because of the many surface crossings of incident particle for the columnar geometry. The columnar CsI study was performed in a thin parallel-plane structure, which has a 50-500 /spl mu/m gas gap between a columnar CsI cathode and metal anode. We discuss the secondary emission and electric fields in the columnar structure, and explain why, based upon field simulation and experimental results, this approach does not succeed in this case. New ideas are presented for driftless gas avalanche detectors insensitive to the angle of incidence.

Fabrication of GEM Detector and X-ray Image Acquisition Based upon its Scintillation Lights

The gas electron multiplier (GEM), placed in the drift volume of a conventional gas detector, is a conceptually simple device for producing a large gas gain by concentrating the drift electric field over a very short distance to the point that electron avalanching occurs. This device consists of a thin insulating foil of several tens of jum in thickness, covered on each side with a thin metal layer, with tiny holes, usually 100 |um or less in diameter, and with a spacing of 100-200 Jim through the entire foil, perforated by using chemical etching or high-powered laser beam technique. In this study, we have investigated its operating properties with various experimental conditions and demonstrated the possibility of using this device as a digital X-ray imaging sensor, by acquiring X-ray images based upon the scintillation lights of the GEM with a standard CCD camera.

Study of Radiation Hardness of Silicon Pixel Deieciors after Neuiron and Proton Irradiations

A new design of silicon pixel detectors with n+/n/p+ (pixel array/bulk/pad) configuration has been developed for more radiation-tolerant CMS forward pixel sensors. In this design, a single (640 jim wide) n+ implant is placed on the n+ side, and guard rings on the p+ side are always kept active before and after type inversion. The whole n+ side is grounded and connected to readout chips. All tested devices were fabricated from both standard (normal) and oxygen-enriched silicon wafers, and radiation-hardness effects after neutron (1 MeV equivalent) and proton (24 GeV) irradiations of these devices were investigated. Other electrical characteristics such as the leakage current and the potential distribution over guard rings were tested using standard measurement techniques.

Insensitive zones and space resolution of microdot (MDOT) detectors of 50, 100 and 200 micron pitch

Insensitive regions and spatial resolution of the microdot (MDOT) detectors having 200 /spl mu/m, 100 /spl mu/m, and 50 /spl mu/m pitches were studied using a collimated X-ray as a probe. The insensitive zones measured by the probe scan were compared with those estimated previously from the drift voltage dependence of the count rate variation. The X-ray probe scan was done along an isolated strip of a ganged linear array of MDOT anodes, and this showed the existence of insensitive zones in the 200 /spl mu/m pitch detector. However, we did not observe these in the in the 100 /spl mu/m and 50 /spl mu/m pitch detectors. The spatial resolution, indicated by the standard deviation of the fitted Gaussian distribution of scanning signals across the anode strip, was determined to be 80 /spl mu/m, 55 /spl mu/m, and 51 /spl mu/m for the 200 Gun, 100 /spl mu/m, and 50 /spl mu/m pitch detectors, respectively. Therefore, the 100 /spl mu/m was determined to be the optimum pitch for absence of the insensitive zones, good spatial resolution, and good avalanche gain.

Performance of a gas avalanche pixel detector of 50 /spl mu/m pitch

We describe the performance of a square micro-dot (MDOT) detector of 50 /spl mu/m pitch, in terms of the defocussing effect and gas gain. Both the count rate variation measurement and the computer simulation showed that the defocussing effect disappeared in the 50 /spl mu/m pitch MDOT. Initial charge build-up on the detector surface was also minimized when the small lateral dimension was combined with a surface coating. The maximum gas gain was limited to /spl sim/1200 due to the small distance available for avalanche multiplication. We also developed a two-level microdot, namely a closed-end gas electron multiplication (CEGEM) chamber. A CEGEM detector having a pitch of 200 /spl mu/m, holes of 40 /spl mu/m width and a spacer layer of 18.5 /spl mu/m thickness showed a maximum gain of /spl sim/400. This gain is expected to increase by increasing the spacer layer thickness and adjusting the hole size.

Performance of a gas avalanche pixel detector of 50 μm pitch

We describe the performance of a square micro-dot (MDOT) detector of 50 /spl mu/m pitch, in terms of the defocussing effect and gas gain. Both the count rate variation measurement and the computer simulation showed that the defocussing effect disappeared in the 50 /spl mu/m pitch MDOT. Initial charge build-up on the detector surface was also minimized when the small lateral dimension was combined with a surface coating. The maximum gas gain was limited to /spl sim/1200 due to the small distance available for avalanche multiplication. We also developed a two-level microdot, namely a closed-end gas electron multiplication (CEGEM) chamber. A CEGEM detector having a pitch of 200 /spl mu/m, holes of 40 /spl mu/m width and a spacer layer of 18.5 /spl mu/m thickness showed a maximum gain of /spl sim/400. This gain is expected to increase by increasing the spacer layer thickness and adjusting the hole size.