In order to check possible novel neutron detectors based on composite semiconductor detectors containing nuclides with large cross sections for neutron, we tested their response to alpha particles. In the present paper we describe results obtained with composite samples made of hexagonal Boron Nitride particles bound with Polystyrene or Nylon-6. The samples were tested under 5.5 MeV alpha particle radiation emitted from 241Am source and 4.8MeV alpha particle of 226Ra source. Some of the responses of these composite detectors to thermal neutrons were already reported and here we shall show some newer results obtained with thermal neutrons, from a low intensity 241Am - 9Be and also from a medium intensity 252Cf source, which were thermalized using 10 cm thick paraffin. The Alpha detection experiments show that all the tested samples, regardless of the binder, show a well-defined peak around the 270 energy channel. There was very little polarization of the alpha radiation, since the amplitude of the alpha peak is reduced after ~ 2min from start of the irradiation, from 100% to 95% and it stayed stable at this level for another 10 minutes. The alpha spectrum detected from a PbI2 single crystal is also shown for comparison. The neutron spectrum obtained by the composite BN samples showed an apparent peak around the 150 energy channel. The Signal to noise ratio for neutron detection from radionuclide shown here is about 2 only, whereas recent results to be published later, obtained with our composite BN detectors from a neutron beam of about 107 sec-1cm-2 is ~2 5. The 1.4 and 1.7 MeV alpha peaks resulting from the nuclear reaction of thermal neutrons with 10B of the boron nitride detector are not buried in the noise range. The capacitance noise requires small contact areas, therefore for large area detectors it is necessary to produce an electronic read-out device which can add up a multitude of small (less than 10sq.mm) pixilated contacts.
Polycrystalline PbI2 films were deposited by Physical Vapor Deposition on ITO and gold coated glass substrates. The structural characterization of the samples was studied by using X-ray diffraction and Scanning Electron Microscopy. Two different preferred orientations - [00l] and [110] directions were found as a function of substrate temperatures, and type of substrate (ITO or Gold). The [110] direction is perpendicular to the c-axis in the hexagonal system. Four types of crystalline preferred orientations were found and named as A, B, C, and D type. The different samples were also annealed at different temperatures to improve their densities. The nuclear spectroscopic data for single crystal and polycrystalline samples were taken with 5.64 MeV Alpha particles emitted from 241Am source. The samples were also measured for their response to a blue light LED irradiation. Both polarities were studied for all samples. In most measurements it was confirmed that holes are the major carriers. The correlation between microstructure and electrical properties as affected by annealing and crystalline orientation is discussed.
Transient charge transport (TCT) measurements were used to evaluate the electrical conduction properties of HgI2 single crystals. Some comparative preliminary results for polycrystalline mercuric iodide (poly-HgI2) thick-film X-ray detectors are also reported. The latter were prepared by physical vapor deposition (PVD). The mobility , trapping time 2, and surface recombination velocity s of electrons or holes were determined by analyses of transient voltages developed across the sample in response to a drift of the corresponding charge carriers created by alpha particle absorption near one of the electrodes. Electron-, and hole mobilities of single crystal HgI2 were n = 80 cm2/V•s and p = 4.8 cm2/V•s, respectively. Trapping times were 2n ≅ 22 V and 2p ≅ 8 V, and surface recombination velocities sn ≅ 1.1 ×105 cm/s and sp ≅ 3.6 ×103 cm/s . Those of the polycrystalline material depend on the deposition technology, and vary between 65 and 88 cm2/V•s for electrons, and between 4.3 and 4.1 cm2/V•s for holes. Bulk trapping-times and surface recombination velocities appear of the same order of magnitude as in the single crystal. An effect of carriers being first generated in near-surface traps and then gradually released is observed for both the single crystal and the polycrystalline material. It is stronger for electrons as compared to holes, and stronger in the polycrystalline material as compared to the single crystal.
Photoconductive polycrystalline mercuric iodide coated on amorphous silicon flat panel thin film transistor (TFT) arrays is the best candidate for direct digital X-ray detectors for radiographic and fluoroscopic applications in medical imaging.
The mercuric iodide is vacuum deposited by Physical Vapor Deposition (PVD). This coating technology is capable of being scaled up to sizes required in common medical imaging applications. Coatings were deposited on 2”×2” and 4”×4” TFT arrays for imaging performance evaluation and also on conductive-coated glass substrates for measurements of X-ray sensitivity and dark current. TFT arrays used included pixel pitch dimensions of 100, 127 and 139 microns. Coating thickness between 150 microns and 250 microns were tested with beam energy between 25 kVP and 100kVP utilizing exposure ranges typical for both fluoroscopic, and radiographic imaging.
X-ray sensitivities measured for the mercuric iodide samples and coated TFT detectors were superior to any published results for competitive materials (up to 7100 ke/mR/pixel for 100 micron pixels). It is believed that this higher sensitivity can result in fluoroscopic imaging signal levels high enough to overshadow electronic noise. Diagnostic quality of radiographic and fluoroscopic images of up to 15 pulses per second were demonstrated. Image lag characteristics appear adequate for fluoroscopic rates. Resolution tests on resolution target phantoms showed that resolution is limited to the TFT array Nyquist frequency including detectors with pixel size of 139 microns resolution ~3.6 lp/mm) and 127 microns (resolution~3.9 lp/mm). The ability to operate at low voltages (~0.5 volt/micron) gives adequate dark currents for most applications and allows low voltage electronics designs.
Polycrystalline Mercuric Iodide coated TFT arrays are now the best candidates for direct digital radiographic detectors for both static and dynamic (fluoroscopic) applications. Their high X-ray sensitivity, high resolution, low dark current, low voltage operation, and good lag characteristics meet the required imaging performance parameters. Small area (2'x2') mercuric iodide x-ray detectors were already reported to have good resolution and high x-ray sensitivity. The present paper reports results obtained with good quality large active area (4'x4') HgI2 imagers deposited on an a-Si TFT matrix containing 768 x 768 pixels with pitch dimensions of 139 micrometers . Coating thickness of the HgI2 is between 150 micrometers and 250 micrometers and the imagers were tested in the 25 kVp-100 kVp x-ray energy range utilizing exposures typical for both fluoroscopic and radiographic imaging. The resolution of the imagers is practically limited only by the pixel size (139micrometers ). The imager works at 15 Hz fluoro rate and the image lag is very small. The imager can use exceptionally low dose-rate x-ray illumination because of the very high x-ray sensitivity, which exceeds any other known X-ray imager materials. The paper also discusses potential application fields of this new unique imager.
The x-ray response of polycrystalline HgI2 for direct detection x-ray imagers, is studied using test arrays with 512 X 512 pixels of size 100 micron. We quantify the contributions to the x-ray sensitivity from electron and hole charge collection, x-ray absorption, effective fill factor and image lag, for x-ray energies from 25-100 kVp. The data analysis compares the measured sensitivity to the theoretical limit and identifies the contributions from various loss mechanisms. The sensitivity is explained by the ionization energy of approximately 5 eV, coupled with small corrections arising from incomplete x-ray absorption, incomplete charge collection, and image lag. Hence, imagers with HgI2 approach the theoretical maximum response for semiconductor detectors, with external array sensitivity demonstrated to within 50 percent of the limit.
Single crystals of mercuric iodide have been studied for many years for nuclear detectors. We have investigated the use of x-ray photoconductive polycrystalline mercuric iodide coatings on amorphous silicon flat panel thin film transistor (TFT) arrays as x-ray detectors for radiographic and fluoroscopic applications in medical imaging. The mercuric iodide coatings were vacuum deposited by Physical Vapor Deposition (PVD). This coating technology is capable of being scaled up to sizes required in common medical imaging applications. Coatings were deposited on 4 inches X 4 inches TFT arrays for imaging performance evaluation and also on conductive-coated glass substrates for measurements of x-ray sensitivity, dark current and image lag. The TFT arrays used included pixel pitch dimensions of both 100 and 139 microns. Coating thickness between 150 microns and 250 microns were tested in the 25 kVp-100 kVp x-ray energy range utilizing exposures typical for both fluoroscopic, and radiographic imaging. X-ray sensitivities measured for the mercuric iodide samples and coated TFT detectors were superior to any published results for competitive materials (up to 7100 ke/mR/pixel for 100 micron pixels). It is believed that this higher sensitivity, can result in fluoroscopic imaging signal levels high enough to overshadow electronic noise. Image lag characteristics appear adequate for fluoroscopic rates. Resolution tests on resolution target phantoms showed that resolution is limited to the Nyquist frequency for the 139 micron pixel detectors. The ability to operate at low voltages gives adequate dark currents for most applications and allows low voltage electronics designs. Mercuric Iodide coated TFT arrays were found to be outstanding candidates for direct digital radiographic detectors for both static and dynamic (fluoroscopic) applications. Their high x-ray sensitivity, high resolution, low dark current, low voltage operation, and good lag characteristics provide a unique combination of desirable imaging performance parameters.
New results for polycrystalline HgI2 detectors are reported here. Due to its decent electrical properties and high stopping power for X-rays and gamma rays, HgI2 is a good candidate for many medical imaging applications. HgI2 were deposited by a hot wall Physical Vapor Deposition (PVD) method, and the electrical properties of the films, including X-ray response and dark current data are reported. Results of imaging capabilities and spatial resolution obtained by polycrystalline HgI2 deposited onto a 2'x2' TFT imaging array on an amorphous silicon substrate are also given. These tests were carried out at Xerox-PARC Research Center.
Polycrystalline mercuric iodide (HgI2) photoconductor material was directly deposited on flat panel amorphous silicon (a-Si) thin film transistor (TFT) pixel arrays in order to test their application as direct x-ray conversion detectors. The 4' x 4' and 2' x 2' detector plates were fabricated either by Physical Vapor Deposition (PVD) or the Screen-Print (SP) method. Although developed for medical radiological imaging, they can also be used for nondestructive test imaging. The present HgI2 arrays have 100 μm x 100 μm pixels on the 2' x 2' detector and 139μm x 139μm on the 4' x 4' imager. The initial results are very promising and show high x-ray sensitivity and low leakage current. The advantage of these detectors is that they can be directly deposited on the pixellated arrays containing the TFTs and other electronic read out circuits and can be fabricated in large sizes. These polycrystalline PVD-HgI2 thick film detectors have now been fabricated up to 1,800μm thick, which makes them also useful for higher-energy X-ray applications. Imaging results obtained by both PVD- and SP-HgI2 will be shown. The effect of the crystallite size on the imaging properties will be demonstrated and the difference in sensitivity applying positive or negative bias on the top electrode will be discussed. Comparison of x-ray sensitivity to other photoconductor materials such a-Se and PbI2 will also be presented.
Robert Street, Marcelo Mulato, Michael Schieber, Haim Hermon, Kanai Shah, Paul Bennett, Yuri Dmitryev, Jackson Ho, Rachel Lau, Evgenie Meerson, Steve Ready, Benjamin Reisman, Y. Sado, Koenraad Van Schuylenbergh, Alexander Vilensky, Asaf Zuck
X-ray imaging properties are reported for HgI2 and PbI2, as candidate materials for future direct detection x- ray image sensors, including the first results from screen- printed HgI2 arrays. The leakage current of PbI2 is reduced by using new deposition conditions, but is still larger than HgI2. Both HgI2 and PbI2 have high spatial resolution but new data shows that the residual image spreading of PbI2 is not due to k-edge fluorescence and its possible origin is discussed. HgI2 has substantially higher sensitivity than PbI2 at comparable bias voltages, and we discuss the various loss mechanisms. Unlike PbI2, HgI2 shows a substantial spatially non-uniform response that is believed to originate from the large grain size, which is comparable to the pixel size. We obtain zero spatial frequency DQE values of 0.7 - 0.8 with PbI(subscript 24/ under low energy exposure conditions. A model for signal generation in terms of the semiconducting properties of the materials is presented.
We report x-ray imaging results on polycrystalline HgI2 detector used for direct x-ray imaging. Due to its good electrical properties and high stopping power for x-rays and gamma rays, the material is a good candidate for many applications in medical imaging. The deposition of the HgI2 thick films is made by hot wall physical vapor deposition, (PVD) method and some of the structural features are described here. The x-ray response and some dark current data measured on some recently prepared detectors are reported. Some results obtained with poly-HgI2 thin film deposited on an amorphous-Si TFT's imaging array 4' X 4' performed at the Ginzton Technology Center is reported for the first time. Also phantom images received with a similar deposited poly-HgI2 thin film deposited on an amorphous- Si TFT's imaging array 2' X 2' performed at Xerox-PARC Research Center is also given here. The status of HgI2 technology will be discussed.
For the first time polycrystalline HgI2 photoconductor material directly evaporated on a-Si array for direct conversion of x-rays for imaging purposes, were successfully imaged at Xerox-Palo Alto Research Center. The initial results are very promising and show a high x-ray sensitivity and low leakage current. Since Ti-W alloys are used as pixel electrodes, an intermediate passivation layer must be used to prevent a chemical reaction with the detector plate. The thickness that these Polycrystalline HgI2 thick film detectors have been fabricated until now is up to 1,800 micrometers , which makes them useful also for high energy applications. The characterization of the Polycrystalline HgI2 thick films deposited with or without the passivation layers by measuring their dark currents, sensitivity to 65 and 85 kVp x-rays and residual signals after 1 minute of biasing, will be shown for several detectors. Some preliminary results will be shown for some novel screen-printed HgI2 detectors.
Robert Street, Marcelo Mulato, Steve Ready, Rachel Lau, Jackson Ho, Koenraad Van Schuylenbergh, Michael Schieber, Haim Hermon, Asaf Zuck, Alexander Vilensky
Measurements of polycrystalline HgI2 films on active matrix direct detection image sensors are described, for possible application to high sensitivity room temperature x- ray detection. The arrays exhibit low leakage current and very high sensitivity - roughly an order of magnitude better than has been demonstrated with other designs. The uniformity of the response varies randomly from pixel to pixel, for reasons that are not yet understood, but are probably related to the large grain size.
A first image of some tiny screws were obtained for the first time with polycrystalline HgI2 acting as the photoconductor material deposited on a-Si direct conversion X- ray image sensors, produced by Xerox -- Palo Alto Research Center. The initial results are very promising and show a high X-ray sensitivity and low leakage current. The response of these detectors to a radiological X-ray generator of 65 kVp has been studied using the current integration mode. Already its sensitivity expressed in (mu) C/R*cm2, is very high, values of 20 (mu) C/R*cm2 have been measured for films of 100 - 250 microns thickness and bias of 50 - 200 volts respectively, which is superior to the published data for competing materials such as polycrystalline PbI2 and a-Se detectors. The fabrication and characterization measurements of the Polycrystalline HgI2 thick film detectors will be given. The characterization data which will be reported here consists of: (a) sensitivity, (b) dark currents, (c) stability of sensitivity dependence on the number of exposure, (d) X-ray response dependence on dose energy and (e) signal decay dependence on the number of exposures.
A theological model is presented which analyses the sensitivity of composite detectors to a flux of x-rays emerging form a radiological x-ray generator. The model describes the many factor which influenced the x-ray response, for the case where the detector is composed of several layers of crystallites separated by a polymeric glue as is the case of composite HgI2 detectors fabricated by the screen print method. The model also describes the variation of the sensitivity with grain size and dielectric constant, taking into account the dielectric constant of the binder showing also the experimental result. Finally, the experimental result of the sensitivity vs. the voltage is shown for single crystal and composite HgI2 detectors and these results are compared with polycrystalline PbI2 and a-Se, which are the main material candidates for medical digital radiology.
The fabrication of polycrystalline HgI2 thick film detectors using the hot wall physical vapor deposition, method is described. The X-ray response of these detectors to a radiological X-ray generator of 60 kVp has been studied using the current integration mode. The response expressed in (mu) A, the dark current expressed in pA/cm2 and sensitivity expressed in (mu) C/R(DOT)cm2 are given for these detectors for several thickness and grain sizes. The optimal sensitivity is compared with published data on the response to X-rays by polycrystalline PbI2 and A-Se detectors.
Recently polycrystalline mercuric iodide have become available, for room temperature radiation detectors over large areas at low cost. Though the quality of this material is still under improvement, ceramic detectors have been already been successfully tested with dedicated low-noise, low-power mixed signal VLSI electronics which can be used for compact, imaging solutions. The detectors used are of different kinds: microstrips and pixels; of different sizes, up to about 1 square inch; and of different thickness, up to 600 microns. The properties of this first-generation detectors are quite uniform from one detector to another. Also for each single detector the response is quite uniform and no charge loss in the inter-electrode space have been detected. Because of the low cost and of the polycrystallinity, detectors can be potentially fabricated in any size and shape, using standard ceramic technology equipment, which is an attractive feature where low cost and large area applications are needed.
Michael Schieber, Asaf Zuck, M. Braiman, Leonid Melekhov, J. Nissenbaum, Renato Turchetta, Wojtek Dulinski, D. Husson, J. Riester, S. Sanguinetti, M. Montalti, M. Guzzi
The direct deposition of polycrystalline semiconductor HgI2 detectors on pre-deposited specially designed pixel electrodes is described, using two methods, the hot wall vapor deposition, HWVD, and thick film screen print (SP) methods. Some characterization results of the HgI2 material used to facilitate the detectors are described. The pre-deposited substrate is made by standard hybrid technology. The electrode pattern is a 16*16 pixel square pattern each with a size of 1.48 mm and with 0.1 mm spacing; the total area covered by the pixels is (25.28 mm)2 equals 639.078 mm2. In order to fan out the pixels to read-out electronics, holes were made through the ceramic thickness and connecting lines were drawn on the opposite side of the ceramic alumina substrate, where complicated patterns can be produced. The pixel detector is tested with beta particles, and data showing the leakage current vs. bias, are given showing a resistivity of about 2*1012 ohm cm. The current and the average charge signal are reported for three different HgI2 pixel detectors. The signal for one of the detectors is about 1100 electrons at 800 V bias voltage and for the second detector, the resistivity is in the same order of magnitude and the charge collection is somewhat better, reaching 1600 electrons at 700 V. One of the detectors was connected to a second hybrid designed for mounting of 8 castor 1.0 chips. CASTOR 1.0 is a VLSI circuit designed for imaging and the results are being evaluated.
Polycrystalline mercuric iodide nuclear radiation detectors have been prepared using different ceramic fabrication methods, such as hot pressing, hot wall vapor phase deposition and screen printing. Areas varying between 0.01 to 100 cm2 and thicknesses varying between 30 to 600 microns, have been fabricated. Gold or carbon electrodes were deposited having the shape of single continuous, linear strip or square pixel contacts and tested for their response to lower and higher gamma energy and beta particles. The (mu) (tau) value is of the order of 10-7 cm2/V for both holes and electrons and therefore can act as particle counter without energy resolution. THe low production cost for potential large detector area make these compounds interesting for certain imaging applications.
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