C.P. Allier

Comparative study of silicon detectors

We studied three different types of silicon sensors: PIN diodes, circular drift detectors, both made at the Delft University of Technology (DUT), and Hamamatsu S5345 avalanche photodiodes. Measurements have been carried out in the same optimized experimental setup, both at room temperature and at low temperatures. Comparison is made for direct X-ray detection and CsI(Tl) scintillation light readout.

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.

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.

Readout of a LaCl/sub 3/(Ce/sup 3+/) scintillation crystal with a large area avalanche photodiode

Large area avalanche photodiodes (APD) with high quantum efficiency and low noise figures have recently become available. In the observation of scintillation light they may replace the photo-multiplier tubes in several applications. In this paper we describe a very high resolution gamma-ray detector consisting of an 8 mm diameter and 5 mm thick LaCl/sub 3/(Ce/sup 3+/) scintillation crystal coupled to a 16 mm diameter APD from Advanced Photonix Inc. We have measured an energy resolution of /spl sim/3.65% full-width at half maximum at the 662 keV peak of /sup 137/Cs at room temperature. This is to our knowledge the best result obtained with a scintillation crystal coupled to a silicon detector. This was achieved taking advantage of the very low intrinsic resolution of the crystal which is assumed to be around 2%. Also the high light yield of the crystal (about 46000 photons/MeV) and the high quantum efficiency of the diode are very important.

Micromachined Si-well scintillator pixel sensors for thermal neutron detection

With the introduction of new high-intensity neutron sources new sensors have to be developed for thermal neutrons combining high spatial resolution, fast response and high detection efficiency. The sensors are intended for energy selective imaging with pulsed neutron sources. This requires time resolved detection on a microsecond scale, ruling out CCD’s for scintillation light readout. In order to achieve these characteristics we have proposed a new type of position sensitive sensor for thermal neutrons, which consists of a matrix of well-type silicon sensors filled with 6Li6 158Gd(BO3)3 (Ce3+) scintillation powder [1], [2]. The neutron detection is based on the 6Li (n,α) reaction, which produces in the scintillation material ∼35000 photons. The emitted light is confined by the vertical silicon sidewalls, is reflected at the top by several layers of Teflon tape and observed by means of silicon photodiodes in the bottom of the wells. In this paper we present a matrix of 700 µm wide well sensors at pitches of 0.9 and 1 mm, which allows millimeter spatial resolution.