Atomic layer deposition α-Al2O3 diffusion barriers to eliminate the memory effect in beta-gamma radioxenon detectors

Well designed scintillator detectors, including such examples as ARSA, SAUNA, and XIA’s “PhosWatch”, can readily achieve the state of the art radioxenon detection limits required for nuclear explosion monitoring. They are also reliable, robust detectors that do not require cryogenic cooling for operation. All three employ the principle of beta-gamma coincidence detection to reduce background counting rates, using a BC-404 plastic scintillator to detect the betas and a CsI or NaI scintillator to detect the gamma-rays. As a consequence of this commonality of design, all three also display a “memory effect” arising from the diffusion of Xe into BC-404. Thus, when one sample is pumped out of the detector, a fraction remains behind, embedded in the BC-404, where it artificially raises the signal counting rate for the next sample. While this is not a fatal flaw in scintillator detectors, developing a method to eliminate the memory effect would significantly enhance their utility. This paper reports efforts to develop thin, amorphous Al₂O₃ films, deposited by atomic layer deposition (ALD) to act as diffusion barriers on the BC-404 surfaces exposed to radioxenon. Using radon as a convenient substitute for Xe, film thicknesses between 2 and 10 nm were originally investigated and found to show a memory effect to varying degrees. A second set of 20 and 30 nm films was then produced, which appeared to completely eliminate the radon memory effect, but, when consequentially tested with radioxenon, were found to exhibit xenon memory effects that were approximately half of the effect found on uncoated BC-404. We draw two conclusions from this result. The first is that it will be necessary to develop an improved method for depositing thicker ALD Al₂O₃ films at lower temperatures while still retaining high film quality. The second is that, since xenon is required to test for the xenon memory effect, we need a test method that does not require xenon radio-isotopes in order to facilitate screening large numbers of samples.     

Subcentimeter depth resolution using a single-photon counting time-of-flight laser ranging system at 1550 nm wavelength

We demonstrate subcentimeter depth profiling at a stand off distance of 330 m using a time-of-flight approach based on time-correlated single-photon counting. For the first time to our knowledge, the photon-counting time-of-flight technique was demonstrated at a wavelength of 1550 nm using a superconducting nanowire single-photon detector. The performance achieved suggests that a system using superconducting detectors has the potential for low-light-level and eye-safe operation. The system's instrumental response was 70 ps full width at half-maximum, which meant that 1 cm surface-to-surface resolution could be achieved by locating the centroids of each return signal. A depth resolution of 4 mm was achieved by employing an optimized signal-processing algorithm based on a reversible jump Markov chain Monte Carlo method.     

Improved measurement of Vub with inclusive semileptonic B decays

We report a new measurement of the Cabibbo-Kobayashi-Maskawa parameter Vub made with a sample of 9.7 x 10(6) BB- events collected with the CLEO II detector. Using heavy quark theory, we combine the observed yield of leptons from semileptonic B decay in the end-point momentum interval 2.2-2.6 GeV/c with recent CLEO II data on B-->X(s)gamma to find Vub = (4.08+/-0.34+/-0.44+/-0.16+/-0.24)x10(-3), where the first two uncertainties are experimental and the last two are from theory.     

Three-dimensional blow-up solutions of the Navier-Stokes equations

In this paper we extend the plane blow-up results of Grundy& McLaughlin (1997) to the three-dimensional Navier-Stokes equations.Using a solution structure originally due to Lin we first providenumerical evidence for the existence of blow-up solutions on- < x, z < , 0 y 1 with boundary conditions on y = 0and y = 1 involving derivatives of the velocity components.The formulation enables us to consider plane and radial flowas special cases. Various features of the computations are isolatedand are used to construct a formal asymptotic solution closeto blow-up. We show that the numerical and asymptotic analysesprovide a mutually consistent global picture which supportsthe conclusion that, for the family of problems we considerhere, blow-up in fact can take place in three dimensions butat an inverse linear rate rather than the faster inverse squareof the plane case.     

Long-term retention of ice-nucleating active Pseudomonas fluorescens by overwintering colorado potato beetles

Ice nucleating-active Pseudomonas fluorescens F264C was fed to Colorado potato beetles to determine bacterial retentioin in the beetle gut and its effect on the cold hardiness of this insect pest. The bacrterium was present in beetles recovered after overwintering in the field, seven months after their exposure to P. fluorescens. Retention was evident not only in the detection of the P. fluorescens ice nucleating gene, inaW, in bacterial cultures from beetle guts but also in the elevated supercooling points of some treated beetles.     

A study of high spin states in the transitional nucleus90Mo

The reaction⁵⁸Ni(³⁶Ar, 4p)⁹⁰Mo has been studied at a beam energy of 149 MeV. A detector array consisting of the OSIRIS spectrometer (12 Compton-suppressed Ge detectors), four charged-particleδE detectors and seven NE-213 neutron detectors at the VICKSI accelerator in Berlin has been used to measure the gamma radiation inγγ- and particle-γγ-coincidence mode. An additional Ge detector was placed at 162? to the beam direction to provide information on DCO ratios. The level scheme of⁹⁰Mo has been extended up to an excitation energy of about 12 MeV and probable spin 23?. Some 70 transitions and 40 levels have been newly identified. Spin assignments have been proposed on the basis of measured DCO ratios. Shell model calculations in a model space consisting of the proton 1f _5/2, 2p _3/2, 2p _1/2, and 1g _9/2 orbits and the neutron 2p _1/2, 1g _9/2, 1g _7/2, 2d _5/2, 2d _3/2, and 3s _1/2 orbits with some truncation were made for states above 9? and the predicted structure of these states is discussed.     

Optically probing the fine structure of a single Mn atom in an InAs quantum dot

We report on the optical spectroscopy of a single InAs/GaAs quantum dot doped with a single Mn atom in a longitudinal magnetic field of a few Tesla. Our findings show that the Mn impurity is a neutral acceptor state A0 whose effective spin J=1 is significantly perturbed by the quantum dot potential and its associated strain field. The spin interaction with photocarriers injected in the quantum dot is shown to be ferromagnetic for holes, with an effective coupling constant of a few hundreds of mueV, but vanishingly small for electrons.     

Rheological properties of thin gelatin films

Summary The stability of oil-in-water emulsions depends ultimately on the physical nature of the oil-water interface. If a hydrocolloid or protein is used as an emulsifying agent an interfacial film is formed at the oil-water interface and this may be liquid or viscoelastic solid in properties. The film is formed by the two processes of diffusion from the bulk to the interface and the formation of secondary intermolecular cross-links.Gelatine solutions of up to 2% w/v were used to form an interface with light liquid paraffin over a restricted range of p _h . For the 2% w/v gelatin solutions, the p _h range was confined to the acid region p _h 2.5–3.2 owing to the onset of gelation of the whole aqueous phase, at room temperature, if the p _h was increased numerically above this range. At lower concentrations of gelatin, e. g., 0.5% w/v, the useful p _h range could be extended to p _h 4.The interfacial rheological properties were examined using the surface rheometer described in a previous paper. Preliminary results showed that, in the case of the 1% w/v gelatin solution at p _h 3, the interfacial film was liquid up to the end of the first half-hour after formation. Solid properties then developed. After 4 hs it was possible to carry out a creep experiment on the film which exhibited a longest retardation time of several minutes. Further creep and recovery experiments conducted up to 200 h after the formation of the film clearly showed that the growth of solid properties of the film was continuing.The gelatin interfacial films were considerably more compliant than similar films of acacia Senegal, over a comparative period of time, and at the same bulk concentration. This difference in behaviour can be attributed to differences in the basic molecular structure of the two polymers.

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Zusammenfassung Die Stabilität von Öl-in-Wasser-Emulsionen hängt letztlich von der physikalischen Eigenschaft der Öl/Wasser-Grenzfläche ab. Wenn ein Hydrokolloid oder ein Protein als Emulgator verwendet wird dann bildet sich in der Öl/Wasser-Grenzschicht ein Film, der Flüssigkeitsoder viskoelastische Eigenschaften aufweisen kann.Dieser Film wird durch zwei Diffusionsvorgänge von der Gesamtmasse zur Grenzschicht und durch zwischenmolekulare Vernetzung gebildet.Gelatinelösungen bis 2% (Gewicht/Volumen) wurden verwendet, um mit dünnflüssigem Paraffin über einen beschränkten p _h -Bereich eine Grenzschicht zu bilden. Für die 2% Gelatinelösung wurde der p _h -Wert im sauren Bereich zwischen p _h 2,5–3,2 gehalten, weil bei Erhöhung des p _h die gesamte wässerige Lösung bei Raumtemperatur zu gelieren begann. Bei geringeren Gelatinekonzentrationen, wie z. B. 0,5% (g/ml), konnte der brauchbare p _h -Bereich bis zu P _h 4 ausgedehnt werden.Die rheologischen Eigenschaften der Grenzschicht wurden mit dem Oberflächenrheometer untersucht, das in einem vorhergehenden Vortrag beschrieben wurde.Vorläufige Ergebnisse haben gezeigt, daß im Falle einer 1% Gelatinelösung mit p _h 3 der Grenzfilm bis gegen Ende der ersten halben Stunde nach seiner Bildung flüssig war. Dann zeigte dieser Festkörper-Eigenschaften. Nach 4 Std. konnte der Film in einem Kriechtest untersucht werden; wobei maximale Retardationszeiten von mehreren Minuten beobachtet wurden. Weitere Kriech- und Kriecherholungsversuche, die bis zu 200 Std. nach der Filmbildung durchgeführt wurden, zeigten deutlich, daß die Festkörper-Eigenschaften des Filmes weiterhin zunahmen.Gelatinegrenzflächenfilme waren für einen vergleichbaren Zeitraum und eine ähnliche Gesamtkonzentration beträchtlich nachgiebiger als ähnliche Filme mit Acacia Senegal. Dieses unterschiedliche Verhalten kann von den verschiedenen molekularen Grundstrukturen der beiden Polymeren herrühren.

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Paper presented to the British Society of Rheology Conference on Rheology in Medicine and Pharmacy, London, April 14–15, 1970.