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The Silicon Vertex Detector (SVD) is the main diagnostic tool in the ALPHA-experiment. It provides precise spatial and timing information of antiproton (antihydrogen) annihilation events (vertices), and most importantly, the SVD is capable of directly identifying and analysing single annihilation events, thereby forming the basis of ALPHA's analysis. This paper describes the ALPHA SVD and its upgrade, installed in the ALPHA's new neutral atom trap.

The growth of GaAs on MnSb by molecular beam epitaxy (MBE) has been investigated. MnSb epilayers on GaAs(111), GaAs(001) and In0.5Ga0.5As(111) of thickness ∼ 50 nm were used as substrates for GaAs films. The MBE growth of GaAs was monitored using synchrotron X-ray diffraction and reflection high energy electron diffraction. The strain relaxation and growth mode of GaAs on MnSb are presented as well as the epitaxial relationship between GaAs and MnSb.

Laser excitation of the 4s4p(3) P-2-4s3d(1) D-2 transition in atomic calcium has been observed and the wavelength determined to 1530.5298(6) nm. The metastable 4s4p(3) P-2 atoms were magnetically trapped in the quadrupole magnetic field of a magneto-optical trap. This state represents the only "loss" channel for the calcium atoms when laser cooled on the 4s(2) S-1(0)-4s4p(1) P-1 transition. A rate equation model shows that an order of magnitude more atoms are trapped in this state compared with those taking part in the main cooling cycle. Excitation of the P-3(2) atoms back up to the 4s3d(1) D-2 state provides a means of accessing these atoms. Efficient repumping is achieved if the 1530-nm laser is used in conjunction with a 672-nm laser driving the 4s3d(1) D-2-4s5p(1) P-1 transition. In the present experiment, we detected about 4.5 x 10(4) trapped P-3(2) atoms, a relatively low atom density, and measured a lifetime of approximately 1 s, which is limited by background collisions.

With the growth of online businesses, it is necessary for consumers to have easy access to the desired product. This access is usually achieved through search features which associate lists of keywords to the available products or by browsing through the different categories. Using Information Retrieval techniques like indexing and searching, this paper shows how to create wordlists from the collections of documents sold by an online publisher and compare the lists of associated keywords with the indexes so as to evaluate their completion, and if new keywords are obtained, a proposition will be made to be added to the existing lists. This will be particularly useful for the consumers whose access to the documents will be simplified, and to the business itself who will obtain customer satisfaction.

Using the Hubble Space Telescope (HST) and Wide Field Planetary Camera 2, we have imaged the central 20 pc of the giant H II region 30 Doradus Nebula in three different emission lines. The images allow us to study the nebula with a physical resolution that is within a factor of 2 of that of typical ground-based observations of Galactic H II regions. We present a gallery of interesting objects within the region studied. These include a tube blown by the wind of a high-velocity star and a discrete H II region around an isolated B star. This small isolated H II region appears to be in the midst of the champagne flow phase of its evolution. Most of the emission within 30 Dor is confined to a thin zone located between the hot interior of the nebula and surrounding dense molecular material. This zone appears to be directly analogous to the photoionized photoevaporative flows that dominate emission from small, nearby H II regions. For example, a column of material protruding from the cavity wall to the south of the main cluster is found to be a direct analog to elephant trunks in M16. Surface brightness profiles across this structure are very similar to surface brightness profiles taken at ground-based resolution across the head of the largest column in M16. The dynamical effects of the photoevaporative flow can be seen as well. An arcuate feature located above this column and a similar feature surrounding a second nearby column are interpreted as shocks in which the photoevaporative flow stagnates against the high-temperature gas that fills the majority of the nebula. The ram pressure in the photoevaporative flow, derived from thermal pressure at the surface of the column, is found to balance with the pressure in the interior of the nebula derived from previous X-ray observations. By analogy with the comparison of ground-based and HST images of M16, we infer that the same sharply stratified structure seen in HST images of M16 almost certainly underlies the observed structure in 30 Doradus, which is a crucial case because it allows us to bridge the gap between nearby H II regions and the giant H II regions seen in distant galaxies. The real significance of this result is that it demonstrates that the physical understanding gained from detailed study of photoevaporative interfaces in nearby H II regions can be applied directly to interpretation of giant H II regions. Stated another way, interpretation of observations of giant H II regions must account for the fact that this emission arises not from expansive volumes of ionized gas but instead from highly localized and extremely sharply stratified physical structures.

The proto-planetary nebula, CRL 2688, has been imaged through a wideband filter centered at 606 nm (F606W) with the Wide Field Planetary Camera 2 onboard the Hubble Space Telescope. CRL 2688 is the prototypical bipolar reflection nebula in which a star is surrounded by a dense, flattened cocoon of dust seen nearly edge-on and starlight escapes preferentially along the polar directions producing a pair of bright nebulosities, one above and one below the equatorial plane. We find a pair of radial "searchlight beams" emerging from within the dusty cocoon which intersect at the position expected for the central star when extrapolated inside the cocoon. The beams are crisscrossed by a large number of roughly round arcs with their center of curvature in the vicinity of the central star. The arcs are not systematically elongated along the polar axis of the nebula, as would be expected in the current model of CRL 2688 where the nebular density decreases with latitude. Our image directly shows the last ~ 13,000 yr history of mass ejection from the central star while it was on the tip of the asymptotic giant branch (AGE). We find that the average surface brightness varies as r^(-3.7), implying that the average mass-loss rate or the scattering opacity of the dust grains varies as r^(-0.7), i.e., one or both of these parameters have steadily increased with time. The temporal resolution of ≈25 yr in our images has provided direct evidence for episodic increases in the mass-loss rate by factors of ~2 or more, occurring every 150-450 yr, and lasting over periods of 75-200 yr. These irregularities in the mass-loss rate are roughly spherically symmetric. We have resolved the edges of the searchlight beams, as well as the peculiar structure of the inner region of the nebula (1".5 < radius < 6"). Our data require a new model for CRL 2688 in which the beams result from starlight escaping through a pair of nonuniform annular holes in the dust cocoon which are coaxial with the polar axis of the nebula. The holes have probably been generated by a young (less than 200 yr) high-velocity outflow which streams out through these holes and interacts with the surrounding dense AGE wind to produce the peculiar structure of the inner nebula. The cocoon contains dust grains of size about 0.6 µm, significantly larger than those in the extended nebula.

The color-magnitude (CM) diagram of the Draco dwarf spheroidal was first studied by Baade & Swope (1961). In addition to finding a general similarity to the CM-diagrams of metal-poor globular clusters, they also detected the anomalous Cepheids and broad giant branch which are characteristic of dwarf spheroidals. The best ground-based CCD CM diagrams of the Draco dwarf spheroidal to date have been presented by Stetson, Vandenberg, & McClure (1985) and Carney & Seitzer (1986, hereafter CS86). Here we describe the first results of an HST program to investigate the stellar population of this galaxy.