W.S. Hong

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.

Use of tungsten anodes in microgap gas chambers

In a continuation of our earlier tests, tungsten has been used for anode strips in the fabrication of microgap gas chambers (MGCs) in an attempt to find a metallization suitable for gas avalanche microdetectors (MSGCs or MGCs) that will be both highly robust against sparking and yet have sufficiently low resistivity to permit use of detectors of relatively larger sizes. MGCs having about 5500 and 7500 /spl Aring/ thick tungsten anodes were fabricated using a sputtering technique, and the sheet resistances for these layers were measured as 0.42 and 0.27 /spl Omega///spl square/, respectively. The detectors were subjected to sparks having a range of energies, and the damage to the anode strips was assessed using optical and scanning electron microscope (SEM) photographs, and by measuring leakage current before and after sparking. In this paper, the spark damage test results of tungsten anodes in MGCs are reported, and an interpretation is made regarding the viability of use of this metal for larger sizes of detectors.

Amorphous silicon/crystalline silicon heterojunctions for nuclear radiation detector applications

Results on the characterization of the electrical properties of amorphous silicon films for the three different growth methods, RF sputtering, PECVD, and LPCVD are reported. The performance of these a-Si films as heterojunctions on high resistivity p-type and n-type crystalline silicon is examined by measuring the noise, leakage current and the alpha particle response of 5 mm diameter detector structures. It is demonstrated that heterojunction detectors formed by RF sputtered films and PECVD films are comparable in performance with conventional surface barrier detectors. The results indicate that the a-Si/c-Si heterojunctions have the potential to greatly simplify detector fabrication. Directions for future avenues of nuclear particle detector development are indicated.

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.

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.

Heavy ion response of amorphous silicon transmission detectors for particle identification

In an effort to determine the applicability of a-Si:H transmission detectors to integrated /spl Delta/E-E detector telescopes, we have measured the response of discrete a-Si:H detectors to ions with atomic numbers between 7 and 36 and ranging in energy from 0.5 to 4.5 MeV/nucleon. Measured pulse height spectra show substantial pulse height deficits which depend on detector bias. Additionally an unexpected result is obtained in that the amplitudes of the detector signals correlate better with the total ion energy than with the ion energy loss in the transmission detector.