Selig N. Kaplan

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. >

MULTIWIRE PROPORTIONAL CHAMBERS FOR BIOMEDICAL APPLICATION

Abstract Multiwire proportional chambers with delay-line readouts, currently used for particle trajectory measurements in high-energy physics, are being adapted for a variety of biomedical applications including X-radiography, neutron radiography, and radioisotope imaging. This paper describes chamber-design features and gives calculated and measured efficiency and resolution data. Sample radiographic images taken with prototype chambers are shown.

Nuclear fission induced by radiationless transitions in the MU-mesonic atoms Th232, U235 and U238

Abstract The time distribution of fissions in Th232, U235, and U238 indueed by μ− mesons was measured with a multiplate gas-scintillation fission chamber. A significant number of prompt fissions not associated with μ− nuclear capture was observed. The results are: Nucleus Ratio of Promp fissions to fissions from nuclear capture Th232 0.064±0.022 U238 0.072±0.014 U235 0.111± The work of Mukhin et al. shows that the intensities of μ-mesic K x-rays for these elements relative to Pb are 0.85±0.0 (Th), 0.77±0.04 (U238), and 0.71±0.05 (U235). This intensity reduction is qualitatively consistent with earlier predictions that, for these elements, a direct excitation of the nucleus competes with electromagnetic radiation in the transition to the ground state of the mesic atom. Our results indicate such direct nuclear excitation. The number of fissions observed may be consistent with the results of Mukhin et al. and with photofission data, if allowance is made for the effect on the fission barrier of the μ meson in the 1S state of the mesic atom.

Detection of charged particles in amorphous silicon layers

Abstract The succesful development of radiation detectors made from amorphous silicon coul offer the possibility for relatively easy construction of large area position-sensitive detectors. We have conducted a series of measurements with prototype detectors, on signals derived from alpha particle. The measurement results are compared with simple model calculations, and projections are made of potential applications in high-energy and nuclear physics.

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>>

Enhanced columnar structure in CsI layer by substrate patterning

Columnar structure in evaporated can be controlled by patterning substrates as well as varying evaporation conditions. Mesh-patterned substrates with various dimensions were created by spin-coating polyimide on glass or amorphous silicon substrates and defining patterns with a standard photolithography technique. CsI(Tl) layers 200-1000 mu m were evaporated. Scintillation properties of these evaporated layers, such as light yield and speed, were equivalent to those of the source materials. The spatial resolution of X-ray detectors consisting of these layers and a linear array of Si photodiodes was evaluated by exposing them to a 25- mu m narrow beam of X-rays. The results obtained with 200- mu m-thick CsI layers coupled to a linear photodiode array with 20-dots/mm resolution showed that the spatial resolution of CsI(Tl) evaporated on patterned substrates was about 75 mu m full width at half maximum (FWHM) whereas that of CsI(T) on flat substrates was about 220 mu m FWHM. Micrographs taken by scanning electron microscopy revealed that these layers retained the well-defined columnar structure originating from substrate patterns.<<ETX>>

Amorphous silicon based radiation detectors

We describe the characteristics of thin(1μm) and thick (>30μm) hydrogenated amorphous silicon p-i-n diodes which are optimized for detecting and recording the spatial distribution of charged particles, x-rays and γ rays. For x-ray, γ ray, and charged particle detection we can use thin p-i-n photosensitive diode arrays coupled to evaporated layers of suitable scintillators. For direct detection of charged particles with high resistance to radiation damage, we use the thick p-i-n diode arrays.

Signal generation in a hydrogenerated amorphous silicon detector

The signals produced in thick hydrogenated amorphous silicon p-i-n detectors were measured using incident light pulses with different mean free paths. The signal shapes as a function of bias potential were analyzed in terms of the mobilities and mean free paths of the electrons and holes. The latter were measured by transient photoconductivity methods using a pulsed nitrogen-dye laser system. Measurements on relatively thick samples of a-Si:H show ionized dangling bond densities in the range of 6-70*10/sup 14/ cm/sup -3/. While the electron mobility increases by approximately 20% at high field, hole mobility increases by approximately 40% from the low-field values. This increase in mobility is attributed to dispersion of the transport, but there may be a Poole-Frenkel effect involving the nondispersive electrons. >