Donald Earl Castleberry

Evaluation of a prototype small-animal PET detector with depth-of-interaction encoding

We report on the evaluation of a prototype small-animal positron emission tomography (PET) detector with depth-of-interaction (DOI) encoding. The detector consisted of an 8/spl times/8 array of scintillator crystals that were read out on the top and bottom by position sensitive avalanche photodiodes (PSAPD). Each scintillator crystal had dimensions of 1.65 mm/spl times/1.65 mm/spl times/22.00 mm, and the array had a pitch of 1.75 mm. The 14/spl times/4 mm PSAPDs were coupled to compact, discrete transimpedance preamplifiers. The DOI response was measured by illuminating the detector module from the side with an electronically collimated fan beam of 511 keV gamma rays. The signals from the corner contacts of the PSAPDs were used to identify the crystal of interaction, and the ratio of the total signals from the two PSAPDs was used to calculate the depth z at which the interaction took place. By stepping the crystal array in the z direction (perpendicular to the fan beam), we were able to determine the DOI resolution for each individual crystal. For measurements made at 10/spl deg/C, the average DOI resolution was better than 3 mm. Energy resolution and timing resolution (versus a fast plastic scintillator coupled to a photomultiplier tube) were made under the same operating conditions. The average energy resolution across the array was 16.6% at 511 keV, and the average timing resolution was 3.9 ns. Importantly, the detector performance was maintained all the way out to the crystals at the edges of the PSAPDs.

ZnO varistors for liquid crystal displays

ZnO varistors are excellent materials for multiplexing dichroic liquid crystal displays. We describe here the development of varistor materials tailored for this application. The varistors are large (∼25 cm diameter), uniform, and have relatively low dielectric constant.

Digital Breast Tomosynthesis: Potentially a New Method for Breast Cancer Screening

Despite recent advances in mammography imaging, it has been shown that many cancers are missed by mammography [1–4]. One of the main reasons cancers are missed is that they are masked by radiographically dense fibroglandular breast tissue which may be overlying or encompassing the cancer [5–11]. Standard mammography techniques, either analog (film) or digital, suffer from the limitation that despite breast compression, three-dimensional anatomical information is projected onto a two-dimensional detector. Tomosynthesis is a technique that allows the radiologist to view individual planes of the breast, potentially reducing the problem of superimposed structures which may limit conventional mammography techniques.

4 pi direction sensitive gamma imager with RENA-3 readout ASIC

A 4π direction-sensitive gamma imager is presented, using a 1 cm3 3D CZT detector from Yinnel Tech and the RENA-3 readout ASIC from NOVA R&D. The measured readout system electronic noise is around 4-5 keV FWHM for all anode channels. The measured timing resolution between two channels within a single ASIC is around 10 ns and the resolution is 30 ns between two separate ASIC chips. After 3D material non-uniformity and charge trapping corrections, the measured single-pixel-event energy resolution is around 1% for Cs-137 at 662 keV using a 1 cm3 CZT detector from Yinnel Tech with an 8 x 8 anode pixel array at 1.15 mm pitch. The energy resolution for two pixel events is 2.9%. A 10 uCi Cs-137 point source was moved around the detector to test the image reconstruction algorithms and demonstrate the source direction detection capability. Accurate source locations were reconstructed with around 200 two-pixel events within a total energy window ±10 keV around the 662 keV full energy peak. The angular resolution FWHM at four of the five positions tested was between 0.05-0.07 steradians.

The Demands On The a-Si FET As A Pixel Switch For Liquid Crystal Displays

The matrix-addressed liquid crystal display (LCD) has attracted considerable attention in the past few years as an alternative to the cathode ray tube (CRT). This type of display has been demonstrated to have stable performance over a long life as well as the attractive features of small volume, light weight, low power, good brightness and full color. Amor-phous silicon (a-Si) is currently the preferred material for switch devices and plasma enhanced chemical vapor deposition (PECVD)is the preferred deposition technology since the process can be carried out at low temperature, yields material with a low density of defect states and affords good step coverage. The electrical requirements for an a-Si field effect transistor (FET) used as a pixel switch for a LCD include switching time, on current and off current. These parameters depend on the instrinsic characteristics of the amorphous materi-als used, the overall display structure and the device geometry. Present and potential material and geometry limitations will be discussed for different display systems of current interest.

Evaluation of a prototype PET detector with depth-of-interaction encoding

We report on the evaluation of a prototype PET detector with depth of interaction (DOI) encoding. The detector consisted of an 8/spl times/8 array of mixed lutetium oxyorthosilicate (MLS) crystals that were read out on the top and bottom by position sensitive avalanche photodiodes (PSAPD). Each crystal had dimensions of 1.65 mm /spl times/ 1.65 mm /spl times/ 22.00 mm, and the array had a pitch of 1.75 mm. The PSAPDs were coupled to compact, discrete transimpedance preamplifiers. The DOI response was measured by illuminating the detector module from the side with an electronically collimated fan beam of 511 keV gamma rays. The signals from the corner contacts of the PSAPDs were used to identify the crystal of interaction, while the summed signals from the two PSAPDs were used to calculate the depth z at which the interaction took place. By stepping the crystal array in the z direction (perpendicular to the fan beam), we were able to determine the DOI resolution for each individual crystal. For measurements made at 10/spl deg/C, the average DOI resolution was 2.9 mm. Energy resolution and timing resolution (vs. a fast plastic scintillator coupled to a photomultiplier tube) were made under the same operating conditions. The average energy resolution across the array was 16.6% at 511 keV, and the average timing resolution was 3.9 ns. Importantly, the detector performance was maintained all the way out to the crystals at the edge of the PSAPDs.

Amorphous Silicon FET Addressed Liquid Crystal Spatial Light Modulator

Advances in the technology of fabrication of large arrays of thin-film field-effect transistors for liquid crystal display applications is making available an important new component for many other applications such as spatial light modulators for optical signal processors. Hydrogenated amorphous silicon has a number of advantages over other semiconductors for such arrays relating primarily to cost and yield due to the simple, low temperature processing, and options offered by a transmissive device. There are also disadvantages stemming from the low electron mobility which is roughly 0.1% that of crystalline silicon. Still, sub-micro-second switching times are possible which is not only adequate for liquid crystal control within very large arrays but suitable for much of the row and column driving circuitry as well. In this paper we will describe the characteristics of a-Si devices, circuits, and LC cells controlled by them. We will also discuss the technology in the context of projecting what the capabilities of such devices could be for spatial light modulators and displays.

Photon-Counting Monolithic Avalanche Photodiode Arrays for the Super Collider

In fiber tracking, calorimetry, and other high energy and nuclear physics experiments, the need arises to detect an optical signal consisting of a few photons (in some cases a single photoelectron) with a detector insensitive to magnetic fields. Previous attempts to detect a single photoelectron have involved avalanche photodiodes (APDs) operated in the Geiger mode [1], the visible light photon counter [2], and a photomultiplier tube with an APD as the anode [3]. In this paper it is demonstrated that silicon APDs, biased below the breakdown voltage, can be used to detect a signal of a few photons with conventional pulse counting circuitry at room temperature. Moderate cooling, it is further argued, could make it possible to detect a single photoelectron.