High-Resolution X-ray Spectroscopy Close to Room Temperature.

: Originally designed as position-sensitive detectors for particle tracking, silicon drift detectors (SDDs) are now used for high-count rate X-ray spectroscopy, operating close to room temperature. Their low-capacitance read-node concept places them among the fastest high-resolution detector systems. They have been used in a new spectrum of experiments in the wide field of X-ray spectroscopy: fluorescent analysis, diffractometry, materials analysis, and synchrotron experiments such as X-ray holography and element imaging in scanning electron microscopes. The fact that the detector system can be used at room temperature with good spectroscopic performance and at -10 degrees C with excellent energy resolution, avoiding liquid nitrogen for cooling and high-quality vacuum, guarantees a large variety of new applications, independent of the laboratory environment. A brief description of the device principles is followed by basics on low noise amplification. The performance results of a complete detector system are presented as well as some dedicated applications already realized, including use in a surface mapping instrument and use of a "mini-spectrometer" for the analysis of works of art. Fully depleted pn-charge-coupled devices (pn-CCDs) have been fabricated for the European X-ray Multi-Mirror mission (XMM) and the German X-ray satellite ABRIXAS, enabling high-speed, low-noise, position-resolving X-ray spectroscopy. The detector was designed and fabricated with a homogeneously sensitive area of 36 cm2. At -70 degrees C it has a noise of 4 e- rms, with a readout time of the total focal plane array of 4 msec. The maximum count rate for single photon counting was 10(5) cps under flat field conditions. In the integration mode, more than 10(9) cps can be detected at 6 keV. Its position resolution is on the order of 100 µm. The quantum efficiency is higher than 90%, ranging from carbon K X-rays (277 eV) up to 10 keV.

A computer program to aid in visual concept development in dentistry.

Beginning dental students normally receive their first exposure to the study of tooth forms (morphology) through a dental anatomy laboratory course in which they are required to reproduce tooth morphology, usually with wax. The fabrication of a tooth in wax requires proper visual recognition skills and fine eye-hand coordination. Many students struggle with one or both of these. A computer program, designed to teach recognition concepts, was delivered to three groups of beginning freshman dental students in conjunction with their dental anatomy laboratory course while a group of their classmates served as the controls. This study investigated (1) instructional design and interface improvement and (2) the best method to implement the computer program. Experimental and control groups all received normal daily critiques of their course project work. After completion of the computer program, all groups were tested with a recognition-based examination as well as with a practical examination, requiring the reproduction of a tooth in wax. All experimental groups scored better than the control group on both examinations. Results indicated that computer-based instruction may be a useful means to foster visual concept development. An expanded program, using better graphics, animation and movies is currently under development.

Effects of etching and rinsing times and salivary contamination on etched glass-ionomer cement bonded to resin composites.

This study investigated the effect of salivary contamination on glass-ionomer/resin-composite bond strength after establishment of an appropriate etching and rinsing time for the glass-ionomer cement. Three hundred and thirty samples of glass-ionomer cement (Ketac-Fil) were injected into cavities prepared in acrylic molds. Groups containing 15 samples each were etched for 0, 15, 30, or 60 sec, after which Scotchbond and either Silux or P-30 were applied and polymerized. The samples were stored under water for 24 hr at 37 degrees C and were then tested in shear. A 30-second etching time was found to be optimal when Silux and P-30 were considered together. In a similar manner, a 20-second optimal rinsing time was determined. Samples of glass-ionomer cement were then contaminated with saliva, dried, and bonded to composite resins and tested in shear as before. Significantly lower bond strengths resulted than with the uncontaminated controls. Efforts to clean contaminated samples by rinsing or re-etching failed to increase the bond strength obtained with the contaminated samples.

Marginal ridge strength of restored teeth with modified Class II cavity preparations.

This study compared marginal ridge fracture resistance in two groups of teeth restored with modified Class II cavity preparations, with control groups. The results indicated that the composite resin and dental amalgam restored teeth fractured at compressive loads that were not statistically different from those of the control (unprepared, intact) teeth. The marginal ridges of the unrestored, prepared teeth were significantly weaker.

Bond strength and hardness of light-activated composite resins.

The bond strength of visible light-activated composites to etched enamel was measured as a function of exposure time. Studies have shown that the bottom surface may not be as hard as the top after customary cure times. Therefore this study also measured hardness and related it to enamel bonding. Plexiglas molds were used to simulate extracoronal applications where light is not attenuated by enamel. Bond strength and hardness of the bottom surfaces reached maximum values after the same exposure times with two of the three composites. More exposure time than normally recommended by manufacturers was needed to obtain maximum bond strength to enamel.

Teaching dental anatomy with light-activated resins.

A method has been described in which light-activated resins are incorporated into the dental anatomy laboratory. This procedure is a valuable addition to the anatomy course because students (1) work with a restorative material appropriate for anterior teeth, (2) learn the unique properties of the light-activated resins, and (3) apply the principles of dental anatomy to a clinically relevant task.