Design and construction of a 9 MeV γ-ray source based on capture of moderated plutonium-beryllium neutrons in nickel.

We have designed and constructed a high-energy γ-ray source for detector characterisation and calibration. The source is a composite type based on a plutonium-beryllium neutron emitter embedded in a paraffin moderator, which is homogeneously mixed with nickel powder. The 9 MeV γ-ray source produces approximately 450 photons per second in 4π when 2.2×105 neutrons per second are emitted, corresponding to a surface flux of 9 MeV γ-rays of approximately 2.5×10-6 cm-2 per emitted neutron. Here we discuss the properties and design of this source, including the characterisation of homogeneity and high-energy γ-ray emission spectra.

Simultaneous and coordinated rotational switching of all molecular rotors in a network.

A range of artificial molecular systems has been created that can exhibit controlled linear and rotational motion. In the further development of such systems, a key step is the addition of communication between molecules in a network. Here, we show that a two-dimensional array of dipolar molecular rotors can undergo simultaneous rotational switching when applying an electric field from the tip of a scanning tunnelling microscope. Several hundred rotors made from porphyrin-based double-decker complexes can be simultaneously rotated when in a hexagonal rotor network on a Cu(111) surface by applying biases above 1 V at 80 K. The phenomenon is observed only in a hexagonal rotor network due to the degeneracy of the ground-state dipole rotational energy barrier of the system. Defects are essential to increase electric torque on the rotor network and to stabilize the switched rotor domains. At low biases and low initial rotator angles, slight reorientations of individual rotors can occur, resulting in the rotator arms pointing in different directions. Analysis reveals that the rotator arm directions are not random, but are coordinated to minimize energy via crosstalk among the rotors through dipolar interactions.

Probing Magnetism in 2D Molecular Networks after in Situ Metalation by Transition Metal Atoms.

Metalated molecules are the ideal building blocks for the bottom-up fabrication of, e.g., two-dimensional arrays of magnetic particles for spintronics applications. Compared to chemical synthesis, metalation after network formation by an atom beam can yield a higher degree of control and flexibility and allows for mixing of different types of magnetic atoms. We report on successful metalation of tetrapyridyl-porphyrins (TPyP) by Co and Cr atoms, as demonstrated by scanning tunneling microscopy experiments. For the metalation, large periodic networks formed by the TPyP molecules on a Ag(111) substrate are exposed in situ to an atom beam. Voltage-induced dehydrogenation experiments support the conclusion that the porphyrin macrocycle of the TPyP molecule incorporates one transition metal atom. The newly synthesized Co-TPyP and Cr-TPyP complexes exhibit striking differences in their electronic behavior, leading to a magnetic character for Cr-TPyP only as evidenced by Kondo resonance measurements.

Self-doping of ultrathin insulating films by transition metal atoms.

Single magnetic Co atoms are deposited on atomically thin NaCl films on Au(111). Two different adsorption sites are revealed by high-resolution scanning tunneling microscopy (STM), i.e., at Na and at Cl locations. Using density functional based simulations of the STM images, we show that the Co atoms substitute with either a Na or Cl atom of the NaCl surface, resulting in cationic and anionic Co dopants with a high thermal stability. The dependence of the magnetic coupling between neighboring Co atoms on their separation is investigated via spatially resolved measurement of the local density of states.

Structure and growth of quasi-one-dimensional YSi2 nanophases on Si(100).

Quasi-one-dimensional YSi(2) nanostructures are formed via self-assembly on the Si(100) surface. These epitaxial nanowires are metastable and their formation strongly depends on the growth parameters. Here, we explore the various stages of yttrium silicide formation over a range of metal coverages and growth temperatures, and establish a rudimentary phase diagram for these novel and often coexisting nanophases. We identify, in addition to previously identified stoichiometric wires, several new nanowire systems. These nanowires exhibit a variety of surface reconstructions, which sometimes coexist on a single wire. From a comparison of scanning tunneling microscopy images, tunneling spectra, and first-principles density functional theory calculations, we determine that these surface reconstructions arise from local orderings of yttrium vacancies. Nanowires often agglomerate into nanowire bundles, the thinnest of which are formed from single wire pairs. The calculations show that such bundles are energetically favored compared to well-separated single wires. Thicker bundles are formed at slightly higher temperature. They extend over several microns, forming a robust network of conducting wires that could possibly be employed in nanodevice applications.

Spatially extended Kondo state in magnetic molecules induced by interfacial charge transfer.

An extensive redistribution of spin density in TBrPP-Co molecules adsorbed on a Cu(111) surface is investigated by monitoring Kondo resonances at different locations on single molecules. Remarkably, the width of the Kondo resonance is found to be much larger on the organic ligands than on the central cobalt atom-reflecting enhanced spin-electron interactions on molecular orbitals. This unusual effect is explained by means of first-principles and numerical renormalization-group calculations highlighting the possibility to engineer spin polarization by exploiting interfacial charge transfer.

Structure of YSi(2) nanowires from scanning tunneling spectroscopy and first principles.

Exceptionally long and uniform YSi(2) nanowires are formed via self-assembly on Si(001). The in-plane width of the thinnest wires is known to be quantized in odd multiples of the silicon lattice constant. Here, we identify a class of nanowires that violates the "odd multiple" rule. The structure of the thinnest wire in this category is determined by comparing scanning tunneling spectroscopy measurements with the calculated surface density of states of candidate models by means of the Pendry R-factor analysis. The relative stability of the odd and even wire systems is analyzed via first-principles calculations.

Investigating molecular charge transfer complexes with a low temperature scanning tunneling microscope.

Electron donor-acceptor molecular charge transfer complexes (CTCs) formed by alpha-sexithiophene (6T) and tetrafluoro-tetracyano-quinodimethane (F4TCNQ) on a Au(111) surface are investigated by scanning tunneling microscopy, spectroscopy, and spectroscopic imaging at 6 K. New hybrid molecular orbitals are formed in the CTCs, and the highest occupied molecular orbital of the CTC is mainly located on the electron accepting F4TCNQ while the lowest unoccupied molecular orbital is predominantly positioned on the electron donating 6T. We observed the conductance switching of F4TCNQ inside CTCs, which may find potential applications in novel molecular device operations.

Decompositional incommensurate growth of ferrocene molecules on a Au(111) surface.

Deviating from the common growth mode of molecular films of organic molecules where the adsorbates remain intact, we observe an essentially different growth behavior for metallocenes with a low temperature scanning tunneling microscope. Ferrocene molecules adsorb dissociatively and form a two layer structure. The top layer unit cell is composed of two tilted cyclopentadienyl (cp) rings, while the first layer consists of ferrocene molecules and cp-Fe complexes. Surprisingly a fourfold symmetry is observed for the top layer while the first layer displays threefold symmetry elements. It is this symmetry mismatch which induces an incommensurability between these layers in all except one surface direction. The top layer is weakly bonded and has an antiferromagnetic ground state as calculated by local spin density functional approximation.

[Studies of dysenteric infection in a closed children's community before and after anti-dysentery vaccination. II. Data on the response to antibiotics and chemotherapy]. / Cercetari privind infectia dizenterica într-o colectivitate închisa de copii, înainte si dupa vaccinarea antidizenterica. Nota II. Date privind comportamentul la antibiotice si chimioterapice.

Study of the behaviour to 7 antibiotics and chemotherapeutics of the Shigella and E. coli strains, isolated from the feces samples of children and adults in a closed community, revealed the high incidence of multiple resistance, especially to the currently used drugs tested. The existence of the phenomenon of plasmid transferable resistance in commensal or pathogenic E. coli strains is an important factor in the appearance of epidemic foci caused by Shigella and T. coli strains, multiply resistant to antibiotics.

[Studies concerning dysenteric infections in a closed children's community before and after antidysenteric vaccination. I. Epidemiological considerations on acute intestinal infections]. / Cercetari privind infectia dizenterica într-o colectivitate de copii închisa înainte si dupa vaccinarea antidizenterica.

In the closed children's community studied between 1 Jan. 1976 and 13 June 1977, a high proportion (54%) of the total number of acute intestinal infections was of dysenteric etiology, i.e. 46,9% in the 0--1 year age-group and 21,1% in the 1--3 years age-group, the dominant Shigella subtype being represented by Shigella flexneri 2a (25.1%). As only 37.2% of the total number of dysentery cases were manifested by enterocolitis and a high proportion (33.3%) ran a chronic course, the disease was not immediately diagnosed, an inadequate treatment was applied and a great number of carriers appeared. This, together with the high receptivity of such communities, accounts for the endemoepidemic character of the infection. Antidysenteric vaccination with the VADIZEN Dr. Istrati live bacillus vaccine, followed by a period of postvaccinal protection, with diminution in the number of dysentery, carriage and enteritis cases, both among the vaccinated and the non-vaccinated children, proves the utility of this vaccine in closed children's communities.

[Response of the body to smallpox vaccination in persons with repeated negative vaccinal antecedents]. / Raspunsul organismului dupa vaccinarea antivariolica, la persoanele cu repetate antecedente vaccinale negative

A study was carried out on the humoral response and vaccinal reaction after repeated administration of the smallpox vaccine (1 to 3 times), at ten days interval, to 97 subjects with a past history of repeated vaccination failures. Revaccination ended in 52.6% failures. After revaccination the antibody titer increased in 97.8% of the cases of successful vaccination and in 43.5% of the negative cases. In 29.5% of the latter cases a decrease in the antibody titer by 1--3 binary dilutions was found. The results suggest that the repeated application at short intervals of an antigenic stimulus perturbs the immune response.