Search

Sterile neutrinos are natural extensions to the standard model of particle physics and provide a possible portal to the dark sector. We report a new search for the existence of sub-MeV sterile neutrinos using the decay-momentum reconstruction technique in the decay of ^{7}Be. The experiment measures the total energy of the ^{7}Li daughter atom from the electron capture decay of ^{7}Be implanted into sensitive superconducting tunnel junction (STJ) quantum sensors. This first experiment presents data from a single STJ operated at a low count rate for a net total of 28 days, and provides exclusion limits on sterile neutrinos in the mass range from 100 to 850 keV that improve upon previous work by up to an order of magnitude.

The XIA DXP-4C, a 4 channel, CAMAC based X-ray spectrometer, is based on digitally processing directly digitized preamplifier signals. Designed for instrumenting multi-detector arrays for synchrotron radiation applications, the DXP-4C was optimized for very high count rates at a low cost per detector channel. These design constraints coincidentally lead to an instrument which is very compact and relatively low power (3.4 W/channel), considering its count rate and MCA capabilities, and which therefore offers interesting possibilities for effective extension to portable applications. Further, because all functions (gain, filter parameters, pileup inspection criteria and internal calibrations) are digitally controlled, the design can be readily adapted to a large variety of user interfaces, including remote access interfaces. Here we present the basics of the design and examine approaches to lowering the power to less than 300 mW/channel while retaining count rate capabilities in excess of 50,000 cps. We then consider the engineering issues associated with portable and remote spectrometry applications, examining in detail the three cases of a lead paint detector, a remote contamination monitor, and a space mission spectrometer.

In order to confirm that a ultra-low noise power supply was working as designed, we needed to measure its noise spectrum at 15 V at the 1 nV/√Hz level to frequencies below 10 Hz. We accomplished this using the “cross-spectrum” technique. Since this method is not widely known, we describe it briefly, present the preamplifier we developed to make the required measurements, and show spectra from our power supply, whose noise levels approach those of battery supplies.

The large number of constituent elements and high symmetry of the ordered Fe–Co−V alloys make a complete solution of its structure impossible from a single diffraction experiment. We have carried out a Rietveld refinement of the site occupancies for a partially ordered Fe–Co–V alloy using both neutron and anomalous dispersion x-ray powder diffraction data in order to obtain a complete crystallographic solution to its structure. Contrary to previous assumptions that V occupies both sites randomly or magnetic and Mössbauer studies that suggest that V has a site preference for the Fe sublattice, we find that the V dopant preferentially occupies the Co sites.

The nuclear line region of the gamma ray spectrum remains one of the most challenging and elusive goals of high energy astrophysics. The scientific objectives are well defined, but require well over a factor of 10 increase in sensitivity compared to present day instruments to be achieved. The most promising approach to achieve this sensitivity and a broad range of scientific objectives is offered by an Advanced Compton Telescope (ACT) that would function in the 0.5 to 30 MeV energy range. The ACT builds on the successful COMPTEL instrument on NASA’s Gamma Ray Observatory by substituting modern detectors with over an order of magnitude better energy and spatial resolution. Germanium detectors are a natural choice, as they are available in large volumes, provide the best possible energy resolution, and are capable of fine spatial resolution. These improvements alone provide the required gain in sensitivity. Further optimization and the use of more sophisticated techniques promise even greater improvements. ...

In this paper we examine a recently proposed concept for obtaining sub-pixel spatial resolution in compound semiconductors where hole transport properties are relatively poor. [1] This approach uses weighted sums and differences of local pixel signals to extract both accurate x-ray energy estimates and interpolate location at the sub-pixel level. A simple analysis, including noise estimates, suggests the possibility of obtaining locations at the 50–100 micron level using 1–2 mm wide stripe electrodes while obtaining 1–2% energy resolution for x-rays up to 100 keV. Following this examination, we will present the most recent experimental results from our program to develop electronics to implement this scheme.

There are important areas of synchrotron radiation research which would strongly benefit from the availability of energy dispersive detector arrays with high energy resolution and good spatial resolution. The goal of this developmental project is to produce a several element mercuric iodide (HgI2) submodule, which can later be multiplied and grouped into larger arrays of 100–400 elements. A prototype five‐element HgI2 array detector was constructed and tested using a synchrotron radiation beam at SSRL. An energy resolution of 280 eV (FWHM) has been obtained for the MnKα line at 5.9 keV. Miniaturized processing electronics were developed in parallel with array advancement. To date, we have completed the hybridized preamplifier and finished breadboarding a computer controlled amplifier. The system integration aspects of large array detectors were studied, producing a conceptual design for both the detector array and its computerized data‐acquisition and analysis system.

The pertinent experimental information now available on the chemisorption of nitrogen in the β phase on tungsten is reviewed. The concept of two distinct, unrelated types of absorption site being responsible for the β1‐ and β2‐ desorption peaks is considered to be inadequate to explain many recently published experimental observations. A new model for the chemisorbed state is proposed based on the existence of a single type of site for β‐phase adsorption and with emphasis on the role of surface diffusion on different types of planes. The model is then applied to the interpretation of a range of published observations on nitrogen adsorption on tungsten including desorption effects, isotopic mixing, contact potential changes, and field electron emission microscopy observations.

α-Carboline-1-oxide (I) with acetic anhydride gives 2-acetoxy-9-acetyl-α-carboline; with dimethyl sulphate it gives the salt (II; R = H). The latter salt is converted by alkali into the anhydronium base (III; R = OMe). The salt (II; R = H) and the base (III; R = OMe) are converted by sufficiently basic nucleophilic reagents into 2-substituted α-carbolines; the salt (II; R = Me) reacts even more readily with basic nucleophilic compounds.Diethyl malonate and ethyl acetoacetate with (III; R = OMe) give the ester (V; R = CH2CO2Et). The acid (V; R = CH2CO2H) is readily decarboxylated to 2-methyl-α-carboline.The benzotriazole (IV; R1= NO2, R2= H) in hot phosphoric acid does not give 3-nitro-α-carboline, but gives instead 2-nitropyrido[1,2-a]benzimidazole (VIII); the latter compound is also obtained by treating the amine (VII) with hydrochloric acid. However the benzotriazole (IV; R1= NH2, R2= H) is converted by phosphoric acid into 3-amino-α-carboline; the benzotriazole (IV; R1= H, R2= CO2H) is similarly converted into α-carboline-4-carboxylic acid. The preparation and proof of orientation of some 6-substituted α-carbolines are described.The 1H n.m.r. spectra of 2-, 3-, and 4-substituted α-carbolines are discussed.

When benzoyldicyandiamide is treated with hydroxylamine hydrochloride, the product is a mixture of 3-phenyl-5-ureido-1,2,4-oxadiazole (I)(major component) and 5-phenyl-3-ureido-1,2,4-oxadiazole (II). The infrared spectrum of the urea (I) is discussed. The urea (II) is degraded by alkali to 3-amino-5-phenyl-1,2,4-oxadiazole; further degradation gives benzoic acid. 5-Phenyl-3-ureido-1,2,4-oxadiazole has been prepared in good yield by the Curtius reaction from 3-ethoxycarbonyl-5-phenyl-1,2,4-oxadiazole.

The differential anomalous scattering technique is outlined and compared to other techniques for studying short-range order in amorphous systems, such as EXAFS. The differential distribution functions obtained for liquid GeBr4 were found to support a model for the liquid state based on the structure of the h.c.p. crystal. Application of the technique to aqueous ZnBr2 also allowed discr imination between structural models.

We first present a brief tutorial on Mercuric Iodide (HgI2) detectors and the intimately related topic of near-room temperature ultralow noise preamplifiers. This provides both a physical basis and technological perspective for the topics to follow. We next describe recent advances in HgI2 applications to x-ray microanalysis, including a space probe Scanning Electron Microscope (SEM), Synchrotron x-ray detectors, and energy dispersive detector arrays. As a result of this work, individual detectors can now operate stably for long periods in vacuum, detect soft x-rays to the oxygen K edge at 523 eV, or count at rates exceeding 2x10(5)/sec. The detector packages are small, lightweight, and use low power. Preliminary HgI2 detector arrays of 10 elements with 500eV resolution have also been constructed and operate stably. Finally, we discuss expected advances in HgI2 array technology, including improved resolution, vacuum operation, and the development of soft x-ray transparent encapsulants. Array capabilities include: large active areas, high (parallel) count rate capability and spatial sensitivity. We then consider areas of x-ray microanalysis where the application of such arrays would be advantageous, particularly including elemental microanalysis, via x-ray fluorescence spectroscopy, in both SEMs and in scanning x-ray microscopes. The necessity of high count rate capability as spatial resolution increases is given particular attention in this connection. Finally, we consider the possibility of Extended X-ray Absorption Fine Structure (EXAFS) studies on square micron sized areas, using detector arrays.

X-ray refraction through a prism was recently revived as a technique for measuring the anomalous x-ray scattering factor f', but deviations between theory and experiment were seen which were accentuated above absorption edges. We show these to arise from absorption effects on the angle of refraction and derive an expression demonstrating that they can increase dramatically as the angle of incidence approaches the critical angle. This refinement markedly improves agreement between theory and experiment. Further, we derive an expression for the profile of the refracted x-ray beam, finding a Cauchy distribution (Lorentzian) and suggesting thereby a possible means to obtain f' accurately from the profile parameters.

The advent of Synchrotron Radiation (SR) sources, which are capable of producing intense photon beams with energies ranging from ultraviolet to hard x ray, has created a need for new optical elements capable at once of focussing, collimating, or passing a restricted energy band of these photons and yet being durable enough to withstand extremely high power loads. Layered Synthetic Microstructures (LSM’s) are currently being intensively evaluated in this context [17.1–4]. The range of applications considered has included normal incidence imaging systems [17.5–7] and monochromators [17.8] in the soft x-ray regime, and wideband focussing monochromators or premonochromators in the hard x-ray regime [17.9,10]. In all of these applications, whether acting either as focussing or condensing elements, the large separations between source and image plane create rigorous demands for optical quality. Usually surface slope accuracies in the sub-arcsec region are required.

Recent developments in high resolution x‐ray measurements are reviewed. Among the more interesting developments are sub‐1000A x‐ray images, multilayer x‐ray interfernce coatings, free standing transmission gratings, 20 psec soft x‐ray streak cameras, normal incidence x‐ray mirrors, and tabulated atomic scattering factors.

No Beckmann rearrangement occurs when ω-chlorophenylglyoxime is treated with phosphorus pentachloride, and the steam-volatile product is 3-chloro-4-phenylfurazan (II; R = Cl), not, as previously reported, 3-chloro-5-phenyl-1,2,4-oxadiazole (I; R = Cl).The reported conversion of dialkyl benzoyliminothiocarbonates (III) into 3-phenyl-5-alkoxy-1,2,4-oxadiazoles by hydroxylamine has been confirmed, and the oxadiazolone (V) has been prepared by cyclodehydration of the urea (VI; R = Me). 3-Hydroxy-5-phenyl-1,2,4-oxadiazole (I; R = OH) has been prepared similarly from the urea (VI; R = H) and also from the amine (I; R = NH2).Dialkyl benzoyliminodithiocarbonates (VII) react with hydroxylamine to give 3-alkylthio-5-phenyl-1,2,4-oxadiazoles. The sulphide (I; R = SEt) is converted by hydrogen peroxide into the sulphoxide (I; R = SOEt) and by permanganate into the sulphone (I; R = SO2Et). The sulphone (I; R = SO2Et) reacts with sodium hydroxide to give 3-hydroxy-5-phenyl-1,2,4-oxadiazole. The thiourea (VIII) is converted by sulphuric acid into the disulphide (IX).

When certain aromatic nitriles containing electron-withdrawing substituents are treated with hydroxylamine in anhydrous methanol, substantial amounts of amides are formed, as well as the expected amidoximes. These amides probably result from initial attack by the oxygen atom of hydroxylamine or carbon and subsequent reaction of the intermediate (IV) with more hydroxylamine. The pure amidoximes can often be obtained in high overall yield by converting the nitriles into thioamides and treating these with hydroxylamine. p-Nitrobenzonitrile reacts slowly with methanol under neutral conditions, and with aqueous methanolic ammonia, to give methyl p-nitrobenzimidate.