P.M. Sarro

Thin photodiodes for a neutron scintillator-silicon well detector

In the development of new neutron imaging applications, it is crucial to achieve a detector combining high spatial resolution, fast response, and high detection efficiency. To achieve such features, we have proposed a new design for position sensitive radiation sensors, which we called the micromachined Si-well scintillator pixel detector. It consists of an array of scintillator crystals encapsulated in silicon wells with photodiodes at the bottom. In the following, we describe such a detector, which makes use of a powder of /sup 6/Li/sub 6//sup 158/Gd(BO/sub 3/)/sub 3/(Ce/sup 3+/). The first experiments obtained with a prototype detector using a thermal neutron beam show the presence of a signal above the detector noise tail. In addition, to improve the characteristics of the well-type silicon sensor, we have investigated the deep reactive ion etching on silicon-on-insulator wafers. The process to etch 700-/spl mu/m-wide vertical wells into a 500-/spl mu/m-thick silicon wafer has been optimized. Test detectors with 10-/spl mu/m-thick photodiodes at the bottom have been fabricated by means of this process.

Diminished electron cloud broadening in a silicon drift detector by sawtooth p/sup +/ strips

Already in 1993, sawtooth-shaped p/sup +/ strips were proposed to diminish lateral diffusion in linear multi-anode silicon drift detectors. The sawtooth structure generates small electric fields directed parallel to the detector surface and perpendicular to the drift direction. These fields confine the drifting electrons within a sawtooth period, In this paper the authors present for the first time experimental proof of the applicability of the concept. For a sawtooth period of 500 /spl mu/m, we have tested the confinement of electron clouds as a function of injected charge up to 5/spl times/10/sup 6/ electrons. The maximum number of electrons for which full confinement is achieved has been measured as a function of the potential gutter depth generated by different sawtooth angles.

Two-dimensional microgap gas chambers on silicon

Abstract We are manufacturing microgap chambers (MGC) on silicon with two-dimensional read-out. They have an anode pitch of 100 μm and a cathode pitch of 400 μm. We obtained a maximum gas gain of 2500 and the avalanche charge collection takes place within 15 ns. The stability of the gas gain is very good. We have tested the position resolution for the detection of X-rays. It appears that the gas composition and the strength of the drift field are important parameters. A high concentration of dimethylether and a rather small drift field provide the smallest size of the total avalanche.

A LaF3 : Nd(10%) scintillation detector with microgap gas chamber read-out for the detection of γ-rays

Abstract A LaF 3 : Nd(10%) scintillator crystal has been placed in a microgap gas chamber to obtain a position-sensitive detector for γ-rays. Directly on the crystal a thin layer of nickel is evaporated, and on top of that a thin semi-transparant CsI photocathode. γ-rays absorbed in the scintillator crystal produce a UV-light flash, which liberates electrons in the photocathode. These electrons are multiplied and detected in the microgap chamber. By comparing the spectrum measured when this detector is irradiated with 511 keV γ-rays with a spectrum that is computed in a Monte Carlo simulation, it is concluded that the probability that a UV-light photon created in the scintillator produces a photo-electron in the photocathode, is about 2.5%.

Scintillation light read-out by low-gain thin avalanche photodiodes in silicon wells

We have proposed a new type of /spl gamma/-ray camera, which takes advantage of micromachining technology. It consists of an array of scintillator crystals encapsulated in well-type silicon sensors. The light created by the interaction of an X-ray or a gamma ray with the crystal material is confined by vertical silicon sidewalls and collected onto the avalanche photodiode at the bottom of the well. Several parameters of the photodiode need to be optimised: uniformity and efficiency of the light detection, gain, electronic noise and breakdown voltage. In order to evaluate these parameters we have processed 3*3 arrays of 1.8 mm/sup 2/, /spl sim/10 /spl mu/m thick photodiodes using [100] wafers etched in a potassium hydroxide (KOH) solution. Their optical response at 675 nm is comparable to that of a 500 /spl mu/m thick silicon PIN diode. The low light detection efficiency is compensated by internal amplification. Several scintillator materials have been positioned in the wells on top of the thin photodiodes, i.e. a layer of structured CsI(Tl) and single crystals of CsI(Tl) and Lu/sub 2/S/sub 3/(Ce/sup 3+/). First experiments on /spl gamma/-ray detection have been performed.

Thin photodiodes for a scintillator-silicon well detector

In developing position sensitive radiation sensors, e.g. for medical imaging, low-gain silicon well sensors were made for the detection of scintillation light. The 3/spl times/3 arrays include N/sup ++/NP diodes, processed in the /spl sim/12 /spl mu/m thick membranes that remain after thinning of 530 /spl mu/m thick [100] silicon wafers by means of a potassium hydroxide (KOH) solution. A comparison is made for the light detection efficiency of these diodes with that of a 500 /spl mu/m thick PIN photodiode.

Design and characteristics of MicroGap Counters on silicon

Abstract The microgap couter (MGC) is a promising development in the field of small gas-filled position sensitive detectors. In the MGC the anode strips are carried by small insulating ridges of SiO2 or polyimide, that are mounted on top of the cathode strips. The exact widths of the anode and the insulator strips can play an important role in the maximum attainable gas gain. We present results of a study of the gas gain and the maximum anode–cathode potential for anode widths running from 4 to 15 μ m and SiO2 strip widths between 6 and 30 μ m. For the realisation of these micro-structures in standard IC technology, we use a wafer stepper machine with a step size of 1×1 cm 2 . Despite this surface limitation, we have managed to enlarge the MGC to an active area of 5×5 cm 2 . Details about this project will be highlighted.

Silicon drift detector with reduced lateral diffusion: experimental results

Abstract In a standard multi-anode silicon drift detector electron cloud broadening during the drifting towards the anode pixels deteriorates the energy and position resolution. This makes the detector less applicable for detection of low-energy X-rays. The signal charge sharing between several anodes can be eliminated by introducing sawtooth-shaped p+ field strips. The sawtooth structure results in small electric fields directed parallel to the sensor surface and perpendicular to the drift direction which produce gutters. The drifting electrons are confined in these gutters of one saw tooth period wide. For a detector with a sawtooth period of 500xa0μm, we have measured the maximum number of fully confined electrons as a function of the potential gutter depth induced by different sawtooth angles.

X-ray spectroscopy with a multi-anode sawtooth silicon drift detector: the diffusion process

Abstract The position sensitive detection of low-energy X-rays can be realized by means of a multi-anode linear silicon drift detector (SDD). However, a severe worsening of the spectroscopic quality of the detector is observed due to the lateral broadening of the X-ray generated electron cloud during drift. Recently, we have proved experimentally that electron confinement can be achieved by means of sawtooth-shaped p + strips; the sawtooth concept. This paper will present room temperature X-ray spectroscopy measurements clearly demonstrating the improvement of spectroscopic quality of the sawtooth SDD as compared with a traditional linear SDD. Using a sawtooth SDD we have measured an energy resolution of 1.4 keV FWHM at the 13.9 keV peak of 241 Am at room temperature and a substantial reduction of the number of split events is also observed. The calculation of the influence of diffusion on the quality of the pulse height spectrum will also be given.

Scintillation light read-out by thin photodiodes in silicon wells

Several applications of X-ray and gamma ray imaging detectors, e.g. in medical diagnostics, require millimeter or sub-millimeter spatial resolution and good energy resolution. In order to achieve such features we have proposed a new type of camera, which takes advantage of micromachining technology. It consists of an array of scintillator crystals encapsulated in silicon wells with photodiodes at the bottom. Several parameters of the photodiode need to be optimised: uniformity and e