T. Jing

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. >

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>>

Detection of charged particles and X-rays by scintillator layers coupled to amorphous silicon photodiode arrays

Hydrogenated amorphous silicon (a-Si:H) p-i-n diodes with transparent metallic contacts are shown to be suitable for detecting charged particles, electrons, and X-rays. When coupled to a suitable scintillator using CsI(Tl) as the scintillator we show a capability to detect minimum ionizing particles with S/N {approximately}20. We demonstrate such an arrangement by operating a p-i-n diode in photovoltaic mode (reverse bias). Moreover, we show that a p-i-n diode can also work as a photoconductor under forward bias and produces a gain yield of 3-8 higher light sensitivity for shaping times of 1 {mu}s. n-i-n devices have similar optical gain as the p-i-n photoconductor for short integrating times ( < 10{mu}s). However, n-i-n devices exhibit much higher gain for a long term integration (10ms) than the p-i-n ones. High sensitivity photosensors are very desirable for X-ray medical imaging because radiation exposure dose can be reduced significantly. The scintillator CsI layers we made have higher spatial resolution than the Kodak commercial scintillator screens due to their internal columnar structure which can collimate the scintillation light. Evaporated CsI layers are shown to be more resistant to radiation damage than the crystalline bulk CsI(Tl).

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.

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.

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.

Thick Amorphous Silicon Layers Suitable for the Realization of Radiation Detectors

Thick silicon films with good electronic quality have been prepared by glow discharge of He-diluted SiH{sub 4} at a substrate temperature {approximately} 150{degree}C and subsequent annealing at 160{degree}C for about 100 hours. The stress in the films obtained this way decreased to {approximately} 100 MPa compared to the 350 MPa in conventional a-Si:H. The post-annealing helped to reduce the ionized dangling bond density from 2.5 {times} 10{sup 15} cm{sup {minus}3} to 7 {times} 10{sup 14} cm{sup {minus}3} without changing the internal stress. IR spectroscopy and hydrogen effusion measurements implied the existence of microvoids and tiny crystallites in the material showing satisfactory electronic properties. P-I-N diodes for radiation detection applications have been realized out of the new material.

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. >