A. Mireshghi

High efficiency neutron sensitive amorphous silicon pixel detectors

A multi-layer a-Si:H based thermal neutron detector was designed, fabricated and simulated by Monte Carlo method. The detector consists of two a-Si:H pin detectors prepared by plasma enhanced chemical vapor deposition (PECVD) and interfaced with coated layers of Gd, as a thermal neutron converter. Simulation results indicate that a detector consisting of 2 Gd films with thicknesses of 2 and 4 /spl mu/m, sandwiched properly with two layers of sufficiently thick (/spl sim/30 /spl mu/m) amorphous silicon diodes, has the optimum parameters. The detectors have an intrinsic efficiency of about 42% at a threshold setting of 7000 electrons, with an expected average signal size of /spl sim/12000 electrons which is well above the noise. This efficiency will be further increased to nearly 63%, if we use Gd with 50% enrichment in /sup 157/Gd. We can fabricate position sensitive detectors with spatial resolution of 300 /spl mu/m with gamma sensitivity of /spl sim/1/spl times/10/sup -5/. These detectors are highly radiation resistant and are good candidates for use in various application, where high efficiency, high resolution, gamma insensitive position sensitive neutron detectors are needed. >

Amorphous silicon pixel layers with cesium iodide converters for medical radiography

We describe the properties of evaporated layers of cesium iodide (thallium activated) deposited on substrates that enable easy coupling to amorphous silicon pixel arrays. The CsI(Tl) layers range in thickness from 65 to 220 /spl mu/m. We used the two-boat evaporator system to deposit CsI(Tl) layers. This system ensures the formation of the scintillator film with homogenous thallium concentration which is essential for optimizing the scintillation light emission efficiency. The Tl concentration was kept to 0.1-0.2 mole percent for the highest light output. Temperature annealing can affect the microstructure as well as light output of the CsI(Tl) film. 200-360/spl deg/C temperature annealing can increase the light output by a factor of two. The amorphous silicon pixel arrays are p-i-n diodes approximately 1 /spl mu/m thick with transparent electrodes to enable them to detect the scintillation light produced by X-rays incident on the CsI(Tl). Digital radiography requires a good spatial resolution. This is accomplished by making the detector pixel size less than 50 /spl mu/m. The light emission from the CsI(Tl) is collimated by techniques involving the deposition process on patterned substrates. We have measured MTF of greater than 12 line pairs per mm at the 10% level. >

Amorphous silicon position sensitive neutron detector

An investigation of the possibility of using a-Si:H diode coated with an appropriate converter as a position-sensitive neutron detector is presented. Monte Carlo simulation predicts that, using a Gd film approximately 2- mu m thick, coated on a sufficiently thick amorphous silicon n-i-p diode, one can achieve a neutron detection efficiency of 25%. The experimental results presented give an average signal size of about 12000 e/sup -/ per neutron interaction, which is well above the noise and is in good agreement with the expected values. One can also fabricate pixel detectors with an element size as small as 300 mu m and still register a count rate of 2200 events/sec in a typical neutron flux of about 10/sup 7/ n/cm/sup 2/ per second. The fact that these detectors are not sensitive to gamma rays and show excellent radiation hardness makes them good candidates for use in applications such as neutron imaging, neutron crystallography, and neutron scattering.<<ETX>>

Improved electrical and transport characteristics of amorphous silicon by enriching with microcrystalline silicon

The authors have deposited n-i-p diodes with microcrystalline intrinsic layers for radiation detection applications. The diodes show interesting electrical characteristics which have not been reported before. From TOF measurement for their best samples, the authors obtained {mu}{sub e} values which are about 3 times larger than their standard a-Si:H. for {mu}{tau} values approximately a factor of 2 improvement was observed. The N*{sub D} values derived from hole-onset measurements show lower ionized dangling bond density than normal a-Si:H material. The authors propose a simple model which can very well explain the experimental results.

Utilization of a-Si:H Switching Diodes for Signal Readout from a-Si:H Pixel Detectors

Two-dimensional arrays of amorphous silicon photodiodes can be used as position-sensitive radiation detectors when they are coupled to an appropriate phosphor. We have developed signal readout schemes from amorphous silicon photodiode arrays utilizing one or two switching diodes attached to each pixel photodiode. Individual cells and prototype arrays of amorphous silicon photodiodes with single- and double-diode switching readout were fabricated and tested. A charge storage time and a readout time were measured. The measurement results were analyzed by simple circuit theory.

Thick (∼ 50 μm) amorphous silicon p-i-n diodes for direct detection of minimum ionizing particles

Thick (∼ 50 μm) amorphous silicon (a-Si:H) p-i-n diodes of device quality are made by helium dilution of the process gas and heat treatment for application to minimum ionizing particle detection. Dilution of SiH4 with He decreased the dangling bond density and increased the deposition rate. The internal stress, which causes substrate bending and delamination, was reduced by a factor of 4 to ∼ 90 MPa when deposited at low (150°C) temperature. The electronic quality of the a-Si:H film was somewhat degraded when grown at a low temperature, but could be mostly recovered by subsequent annealing at 160°C. By this technique 50 μm thick n-i-p diodes were made without significant substrate bending, and the electronic properties, such as electron mobility and ionized dangling bond density, were suitable for detecting minimum ionizing particles. Diode readouts and integrated amplifiers for pixel arrays are also described.

Improvement of Electronic Transport Characteristics of Amorphous Silicon by Hydrogen Dilution of Silane

We have investigated the electrical and material properties of intrinsic amorphous silicon deposited with hydrogen dilution of silane. The hydrogenated material was used as intrinsic layers of n-i-p diodes, which showed interesting electrical characteristics. From time of flight (TOF) measurement for our best samples produced at hydrogen to silane ratio of 15, we obtained mobility (µ) values about 3-4 times larger than our standard amorphous silicon (a-Si:H). Approximately a factor of 2 improvement was observed for µτ values. The N D* values of the hydrogen diluted a-Si:H were measured for the first time and show lower ionized dangling bond density than the normal a-Si:H material. At a hydrogen to silane gas flow ratio of 20, some microcrystalline formation was observed in the deposited material. We propose a simple macroscopic model to assess the effect of microcrystals and grain boundaries on the electronic properties of mixed amorphous and microcrystalline material.

Hydrogenated Amorphous Silicon (a-Si:H) Based Gamma Camera - Monte Carlo Simulations

A new gamma camera using a-Si:H photodetectors has been designed for the imaging of heart and other small organs. In this new design the photomultiplier tubes and the position sensing circuitry are replaced by 2D array of a-Si:H p-i-n pixel photodetectors and readout circuitry which are built on a substrate. Without the photomultiplier tubes this camera is light weight, hence can be made portable. To predict the characteristics and the performance of this new gamma camera we did Monte Carlo simulations. In the simulations 128 X 128 imaging array of various pixel sizes were used. 99mTc (140 keV) and 201Tl (70 keV) were used as radiation sources. From the simulations we could obtain the resolution of the camera and the overall system, and the blurring effects due to scattering in the phantom. Using the Wiener filter for image processing, restoration of the blurred image could be achieved. Simulation results of a-Si:H based gamma camera were compared with those of a conventional gamma camera.

Signal readout in a-Si:H pixel detectors

Summary form only. Amorphous or poly-silicon thin-film technology can be used to make readout electronics for a-Si:H pixel detectors. A switch consisting of two a-Si:H p-i-n diodes was studied to read out signals from pixels for imaging of X-rays or gamma rays. A charge storage time of approximately 20 ms and a readout time of 0.7 mu s were achieved. In detection of single ionizing particles, a poly-silicon thin-film amplifier can be integrated to amplify the small signal at pixel level before readout. Prototype poly-silicon TFT (thin-film transistor) amplifiers were designed and fabricated. The measured gain-bandwidth product was approximately 300 MHz and the input equivalent noise charge was approximately 1000 electrons for a 1 mu s shaping time. >

Noise in a-Si:H p-i-n detector diodes

Noise from a-Si:H p-i-n diodes (5 approximately 50- mu m thick) under reverse bias was investigated. The current-dependent 1/f-type noise was found to be the main noise composition at high bias. At low bias, the thermal noise from a series resistance of the p-layer and from metallic contacts is the dominant noise source, which is unrelated to the reverse current through the diode. The noise associated with the p-layer resistance decreased significantly on annealing under reverse bias, reducing the total zero bias noise by approximately a factor of 2. The noise recovered to the original value on subsequent annealing without bias. In addition to the resistive noise there seems to be a shaping time independent noise component for zero-biased diodes.<<ETX>>