D. Wildermuth

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. >

X-ray and charged particle detection with CsI(Tl) layer coupled to a Si:H photodiode layers

A compact real-time X-ray and charged-particle imager with digitized position output can be built either by coupling a fast scintillator to a photodiode array or by forming one on a photodiode array directly. CsI(Tl) layers 100-1000- mu m thick were evaporated on glass substrates from a crystal CsI(Tl). When coupled to a crystalline Si or amorphous silicon (a-Si:H) photodiode and exposed to calibrated X-ray pulses, their light yields and speed were found to be comparable to those of a crystal CsI(Tl). Single beta particle detection was demonstrated with this combination. The light spread inside evaporated CsI(Tl) was suppressed by its columnar structure. Scintillation detection gives much larger signals than direct X-ray detection due to the increased energy deposition in the detector material. Fabrication of monolithic-type X-ray sensors consisting of CsI+a-Si:H photodiodes is discussed. >

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>>

Measurements of 1/f noise in A-Si:H pin diodes and thin-film-transistors

We measured the equivalent noise charge of a-Si:H pin diodes (5 {approximately} 45{mu}m i-layer) with a pulse shaping time of 2.5 {mu}sec under reverse biases up to 30 V/{mu}m and analyzed it as a four component noise source. The frequency spectra of 1/f noise on the soft-breakdown region and of the Nyquist noise from contact resistance of diodes were measured. Using the conversion equations for a CR-RC shaper, we identified the contact resistance noise and the 1/f noise as the main noise sources in the low bias and high bias regions respectively. The 1/f noise of a-Si:H TFTs with channel length of 15 {mu}m was measured to be the dominant component up to {approximately}100kHz for both saturation and linear regions. 15 refs., 7 figs.

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.

Amorphous silicon pixel arrays

Abstract Amorphous silicon pin diodes can detect minimum ionizing particles either by direct electron-hole pair generation or when used as photodiodes in conjunction with a scintillator material. We summarize recent results on such detectors and describe progress towards fabrication of pixel arrays.

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>>

New a-Si:H Photo-Detectors for Long-Term Charge Storage

Using the high light absorption properties of amorphous silicon, we developed a new device configuration that can detect photons and store the induced charges for relatively long time. This device, coupled to a scintillator such as CsI(Tl) in an array form, could be used as a scintillation camera, or for long-term photo-detection such as radionuclide labeled chromatography. The detector has a simple sandwich structure consisting of a scintillator followed by a top metal layer, p-i-n layers of hydrogenated amorphous silicon (a-Si:H), a second metal layer, a thin insulating layer and a bottom metal layer. The electron-hole pairs generated in the i-layer by the interaction with the incident light will be separated by the imposed electric field and be stored in the central metal-insulator interface. Readout will be done by switching the external bias to ground after the storage time, which depends on the needs for the specific application. Prototype devices were fabricated and tested. The performances of the devices were analyzed in connection with the storage time and the background signal produced by the thermally generated charges.

Improvement of thick a-Si radiation detectors by field profile tailoring

Application of thick ({approximately}50 {mu}m) a-Si p-i-n diodes as a direct radiation detector for minimum ionizing particles is hampered by the need to apply large bias voltages in order fully to deplete the detecting intrinsic layer, which typically contains 5 {minus} 10 {times} 10{sup 14} ionizable dangling bonds per CM{sup 3}. By insertion of thin p-type layers at intervals within the intrinsic layer, the required depletion voltage can be reduced by a factor of at least 1/(n+l) where n is the number of layers inserted. This principle is demonstrated for devices approximately 12{mu}m in thickness. It is shown that electron losses within the p type layer can be kept to minimum by choice of a low doping concentration for the introduced players.