Walther Bothe's Graphite: Physics,Impurities, and Blame in the German Nuclear Program

The physics, experimental setup, and analysis of results involved in Walther Bothe and Peter Jensen's mistaken 1941 measurement of the capture cross section for thermal neutrons by carbon are examined. Their experiment, while well-conceived and executed, was corrupted by the presence of unappreciated impurities. This erroneous measurement was crucial as it prompted a decision by German military administrators to abandon graphite as a possible moderator for a nuclear pile in favor of heavy water, a decision which set their program back irreparably.     

On Planar Perfectly Contractile Graphs

An even pair in a graph is a pair of vertices such that every chordless path between them has even length. A graph is called perfectly contractile when every induced subgraph can be transformed into a clique through a sequence of even-pair contractions. In this paper we characterize the planar graphs that are perfectly contractile by determining all the minimal forbidden subgraphs. We give a polynomial algorithm for the recognition of perfectly contractile planar graphs.     

Packing directed circuits

We prove a conjecture of Younger, that for every integern0 there exists an integert0 such that for every digraphG, eitherG hasn vertex-disjoint directed circuits, orG can be made acyclic by deleting at mostt vertices.Research partially supported by DONET ECHM contract CHRXCT930090.Research partially supported by DIMACS, by NSF grant DMS-9401981 and by ONR grant N00014-92-J-1965, and partially performed under a consulting agreement with Bellcore.Research partially supported by DIMACS, by Université de Paris VI, by NSF grant DMS-9303761 and by ONR grant N00014-93-1-0325, and partially performed under a consulting agreement with Bellcore.     

Naphthalene-1,2,3-dithiazolyl and its selenium-containing variants

Synthetic routes to salts of the 3H-naphtho[1,2-d][1,2,3]dithiazolylium cation and its three selenium-containing variants (SSeN, SeSN, and SeSeN) are described. The most efficient and general method involves the intermediacy of bis-acetylated aminothiolates and aminoselenolates. These reagents react smoothly with sulfur and selenium halides to afford the desired ring closure products. Electrochemical reduction of the four cations indicates that corresponding radicals (SSN, SSeN, SeSN, and SeSeN) are stable in solution. The EPR spectra of all four have been recorded, and experimental spin distributions have been cross-matched with those obtained from DFT calculations. The selenium-containing radicals are thermally unstable at or slightly above room temperature, but the all-sulfur species has been isolated and characterized crystallographically. In the solid state, the radicals are associated into cofacial dimers which are closely linked to other dimers by intermolecular S---S, S---N, and C-H---aromatic ring interactions.     

Carborane acids. New "strong yet gentle" acids for organic and inorganic chemistry

Icosahedral carborane anions such as CHB11Cl11- are amongst the least coordinating, most chemically inert anions known. They are also amongst the least basic, so their conjugate acids, H(carborane), are superacids (i.e. stronger than 100% H2SO4). Acidity scale measurements indicate that H(CHB11Cl11) is the strongest pure Br?nsted acid presently known, surpassing triflic and fluorosulfuric acid. Nevertheless, it is also an extremely gentle acid--because its conjugate base engages in so little chemistry. Carborane acids separate protic acidity from anion nucleophilicity and destructive oxidative capacity in the conjugate base, to a degree not previously achieved. As a result, many long-sought, highly acidic, reactive cations such as protonated benzene (C6H7+), protonated C60(HC60+), tertiary carbocations (R3C+), vinyl cations (R2C=C(+)-R), silylium ions (R3Si+) and discrete hydronium ions (H3O+, H5O2+ etc.) can be readily isolated as carborane salts and characterized at room temperature by X-ray crystallography.     

The effect of selenium incorporation on the bandwidth and conductivity of neutral radical conductors

The first example of an undimerized pi-stacked bis-1,2,3-thiaselenazolyl radical displays improved bandwidth and conductivity relative to an isostructural bis-1,2,3-dithiazolyl.     

Pseudorapidity and centrality dependence of the collective flow of charged particles in Au+Au collisions at sqrt[s(NN)]=130 GeV

This paper describes the measurement of collective flow for charged particles in Au+Au collisions at sqrt[s(NN)]=130 GeV using the PHOBOS detector at the Relativistic Heavy Ion Collider (RHIC). The measured azimuthal hit anisotropy is presented over a wide range of pseudorapidity (-5.0相似文献   

Indium oxide nanostructures

In this report we review the growth of indium oxide (In₂O₃) nanostructures, including octahedral nanocrystals (NCs), nanobelts (NBs), nanosheets (NSs), and nanowires (NWs), by hot-wall chemical vapor deposition (HW-CVD). This system is highly controllable, allowing the user to easily access different growth regimes – each corresponding to the growth of a different nanostructure – by changing growth variables of the HW-CVD system. Hot-wall CVD produces crystalline nanostructures; here we present a survey of microstructural characterizations of the four types of In₂O₃ nanostructures using transmission- and scanning-electron microscopy. Interestingly, the In₂O₃ nanostructures have different preferred growth directions: NCs have (111) faces, NBs are predominantly (200), and NWs are predominantly (110). We end the review by discussing the current shortcomings of HW-CVD growth of In₂O₃ nanostructures. PACS 61.46.-w; 61.82.Rx; 73.31.Hb; 81.02.-b     

Crystal and molecular structure of the smectogenic compound di-n-propyl-p-terphenyl-4,4″-carboxylate

An X-ray crystallographic study of di-n-propyl-p-terphenyl-4,4-carboxylate (DPTC) showed that the crystals belong to the triclinic system with space groupP¯1,a=22.191(14),b=5.960(1),c=10.951(4) Å, =129.651(16), =102.560(51), and =83.103(39) ° with two molecules per unit cell. The crystal structure was solved by Patterson synthesis, Fourier, and difference Fourier calculations. It was refined by block-diagonal and full-matrix least-squares methods to a finalR factor of 0.17. The molecules form a herringbone type of packing in tilted layers. Each of the two molecules is nonplanar in itself and shows highly anisotropic behavior of the end groups.