Geometric model for nuclear absorption from microscopic theory

A parameter-free geometric model for nuclear absorption is derived from microscopic theory. The expression for the absorption cross section in the eikonal approximation taken in integral form is separated into a geometric contribution, described by an energy-dependent effective radius, and two surface terms which are shown to cancel in an asymptotic series expansion. For collisions of light nuclei, an expression for the effective radius is derived using harmonic-oscillator nuclear density functions. A direct extension to heavy nuclei with Woods-Saxon densities is made by identifying the equivalent half density radius for the harmonic-oscillator functions. Coulomb corrections are incorporated and a simplified geometric form of the Bradt-Peters type obtained. Results spanning the energy range of 1 MeV/nucleon to 1 GeV/nucleon are presented. Good agreement with experimental results are obtained.     

RBS/channelling study of ion-implanted and proton-exchanged LiNbO3:Nd:MgO planar waveguides

We present a structural study of two Nd³⁺/MgO codoped LiNbO₃ waveguides fabricated by two different methods: ion-implantation and proton-exchange. Rutherford backscattering spectrometry in combination with channelling was used to study the influence of the waveguide fabrication method on the possible modification of the LiNbO₃ crystal structure within the waveguiding layer. The results show that the ion-implanted waveguide mainly maintains the same properties as the virgin crystal, apart from a deterioration of a thin surface layer, whilst the proton-exchanged waveguide exhibits a quite different behaviour, although its surface is less damaged.     

Novel expression and regulation of gastrin gene in human ovarian cancer cell line, SW626

Gastrin-secreting tumors have been identified in ectopic locations including the ovary; the mechanisms regulating gastrin gene expression, its distribution, and signaling pathways in these ectopic tissues are not known. The purpose of our present study was to determine: (1) whether the gastrin gene and peptide could be detected in ovarian cancer cell lines, (2) if functional gastrin releasing peptide receptors (GRP-R) are present, and (3) whether gastrin gene expression is altered by GRP. Five ovarian cancer cell lines (SW626, OVCA 420, OVCA 429, OVCA 432, and OVCA 433) were analyzed. We identified gastrin gene and peptide expression in the SW626 cell line but not in the OVCA lines. SW626 cells express a functional GRP-R that is correctly coupled to the Ca2+ signaling pathway. Treatment of SW626 cells with bombesin, the amphibian equivalent of GRP, inhibited expression of the gastrin gene in a time- and dose-dependent fashion. The SW626 ovarian cancer cell line will provide a useful model to further define regulation and expression of both the gastrin gene and peptide in ectopic (nongastrointestinal) tissues.     

Use of highly overcoupled couplers to detect shifts in Braggwavelength [strain sensors]

The authors present a technique for detecting Bragg wavelength shifts using highly overcoupled couplers (HOCC). An HOCC of 60%/nm was fabricated with 626 cycles. With our HOCC, a 5 μW LED and a 95% reflective Bragg grating we were able to detect 10 μstrain (Δλ=0.01 nm)     

An algorithm for three-dimensional reconstruction incorporating cross-plane rays

Conventional multislice positron cameras reconstruct a three-dimensional distribution of a positron-emitting radioscope as a set of two-dimensional transverse sections. Consequently, annihilation photons which cross two or more planes are eliminated from the data. Such an approach makes efficient use of the emitted photon flux. A method is proposed which makes more efficient use of the available photons by including both oblique and transverse section in the reconstruction. The implementation of the method consists of centering a scaled convolution filter on each detected coincidence event line and backprojecting the filter values through the three-dimensional reconstruction volume. The final image is normalized to allow for the different number of oblique and transverse sections that contribute to each point in the imaging volume. The method has been evaluated using both simulated data and measured data obtained with a routing area detector positron camera.     

Assessment of response function in two PET scanners with and without interplane septa

The authors have assessed the response function both experimentally and theoretically for two commercial tomographs: CTI 931/08-12 and CTI 953B with and without interplane septa. Monte Carlo simulations were undertaken using the GEANT package from CERN. Spatial resolution (tomographic and axial) was calculated for line sources at various positions in the field of view. Sensitivity and scatter fraction (SF) were calculated for various source geometries as a function of energy discrimination. A very realistic response function in positron emission tomography (PET) is obtained by Monte Carlo methods, using global parameters to account for unsimulated phenomena such as scintillation light transport inside a detector block and its sharing among the various phototubes. Minor discrepancies remain for sensitivity and SF at high energy thresholds and may probably be explained by introducing the observed dispersion in the energy response for the various crystals within a detector block.     

Noise equivalent count measurements in a neuro-PET scanner with retractable septa

The noise-equivalent count-rate (NEC) performance of a neuro-positron emission tomography (PET) scanner has been determined with and without interplane septa on uniform cylindrical phantoms of differing radii and in human studies to assess the optimum count rate conditions that realize the maximum gain. In the brain, the effective gain in NEC performance for three-dimensions (3-D) ranges from >5 at low count rates to approximately 3.3 at 200 kcps (equivalent to 37 kcps in 2-D). The gains of the 3-D method assessed by this analysis are significant, and are shown to be highly dependent on count rate and object dimensions.     

Photon counting with passively quenched germanium avalanche

We demonstrate photon counting in germanium avalanche photodiodes biased beyond breakdown and quenched with a simple series resistance circuit. The devices show moderate (> 7%) quantum efficiency with limited afterpulsing and dark counts and subnanosecond jitter.