3D silicon detectors for neutron imaging applications

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3D silicon detectors for neutron imaging applications

Neutron detection is of great importance in many fields spanning from scientific research, to nuclear science, and to medical application. The development of silicon-based neutron detectors with enhanced neutron detection efficiency can offer several advantages such as spatial resolution, enhanced dynamic range and background discrimination. In this work, increased detection efficiency is pursued by fabricating high aspect ratio 3D micro-structures filled with neutron converting materials (B4C) on planar silicon detectors. An in-depth feasibility study was carried out in all aspects of the sensor fabrication technology. Passivation of the etched structures was studied in detail, to ensure good electrical performance. The conformal deposition of B4C with a newly developed process showed excellent results. Preliminary electrical characterisation of the completed devices is promising, and detectors have been mounted on dedicated boards in view of the upcoming tests with neutrons.publishedVersio

Characterization of boron-coated silicon sensors for thermal neutron detection

Silicon neutron detectors can operate at low voltage and come with ease of fabrication and the possibility of integration of readout electronics and thus are attractive from an application point of view. In this paper, we have studied thermal neutron capture by silicon diodes coated with boron carbide (B4C). One of the surfaces of the diodes was covered with either natural B4C (B4C) or with enriched B4C (B4C). We have investigated: (a) the effect of increase in the sensitive area of the surface of the diode covered with B4C on the neutron detection efficiency and (b) the effect of enrichment of 10B in B4C. The difference in 10B in B4C (16 at.% in the deposited film) and B4C ( 79 at.% in the deposited film) leads to about three times increase in detection efficiency of the same detector. For the given experimental conditions, we do not observe a direct relationship between increase in the surface area and the detection efficiency. Energy spectra obtained by Geant4 simulations support the experimental observation of finding no direct relation between increase in the surface area and the detection efficiency.publishedVersio

Destabilization of NaBH4 by Transition Metal Fluorides

With the goal of improving performance of a hydrogen-rich storage medium, the influence of a collection of first and second period transition metal fluorides on the destabilization of NaBH4 is studied on samples produced by ball milling NaBH4 with 2 mol% of a metal fluoride additive. The effects obtained by increasing additive amount and changing oxidation state are also evaluated for NbF5, CeF3, and CeF4. The as-milled products are characterized by in-house power X-ray diffraction, while the hydrogen release and decomposition are monitored by temperature programmed desorption with residual gas analysis, differential scanning calorimetry, and thermogravimetry. The screening of samples containing 2 mol% of additive shows that distinctive groups of transition metal fluorides affect the ball milling process differently depending on their enthalpy of formation, melting point, or their ability to react at the temperatures achieved during ball milling. This leads to the formation of NaBF4 in the case of TiF4, MnF3, VF4, CdF2, NbF5, AgF, and CeF3 and the presence of the metal in CrF3, CuF2, and AgF. There is no linear correlation between the position of the transition metal in the periodic table and the observed behavior. The thermal behavior of the products after milling is given by the remaining NaBH4, fluoride, and the formation of intermediate metastable compounds. A noticeable decrease of the decomposition temperature is seen for the majority of the products, with the exceptions of the samples containing YF3, AgF, and CeF3. The largest decrease of the decomposition temperature is observed for NbF5. When comparing increasing amounts of the same additive, the largest decrease of the decomposition temperature is observed for 10 mol% of NbF5. Higher amounts of additive result in the loss of the NaBH4 thermal signal and ultimately the loss of the crystalline borohydride. When comparing additives with the same transition metal and different oxidation states, the most efficient additive is found to be the one with a higher oxidation state. Furthermore, among all the samples studied, higher oxidation state metal fluorides are found to be the most destabilizing agents for NaBH4. Overall, the present study shows that there is no single parameter affecting the destabilization of NaBH4 by transition metal fluorides. Instead, parameters such as the transition metal electronegativity and oxidation state or the enthalpy of formation of the fluoride and its melting point are competing to influence the destabilization. In particular, it is found that the combination of a high metal oxidation state and a low fluoride melting point will enhance destabilization. This is observed for MnF3, NbF5, NiF2, and CuF2, which lead to high gas releases from the decomposition of NaBH4 at the lowest decomposition temperatures

Effects of Ball Milling and TiF3 Addition on the Dehydrogenation Temperature of Ca(BH4)2 Polymorphs

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Destabilization of NaBH4 by Transition Metal Fluorides

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The Role of Ca(BH4)(2) Polymorphs

This study compares the structure and decomposition behaviors of theα,β, and γ polymorphs of Ca(BH 4) 2 for hydrogen storage. Samples with different polymorphic contents are characterized using powder X-ray diffraction and vibrational spectroscopy. Decomposition paths and formation of decomposition products are monitored by differential scanning calorimetry and temperature programed desorption as well as in situ synchrotron radiation powder diffraction. Vibrational spectroscopy in the <1000 cm -1 range shows different sharp librational bands for α- andγ-, which are not seen inβ-Ca(BH 4) 2. In the 1000-2700 cm -1 range, all three polymorphs show the vibrational features of the C 2 local structure corresponding to the internal vibrations of BH4 -. Shifts of these vibrational bands toward larger wavenumbers are observed forγ andβ-Ca(BH 4) 2. The increase in wavenumber coincides with an increase of the decomposition temperatures that can be up to 15°C between α- andγ-Ca(BH 4) 2 depending on the polymorphic content. The decomposition temperature of pureβ-Ca(BH 4) 2 is found to be about 6°C lower than the decomposition of the high-temperature modification obtained via the polymorphic transformation of α-Ca(BH 4) 2. This confirms that the pure Ca(BH 4) 2 polymorphs have slightly different kinetic barriers and that the polymorphic content determines the decomposition kinetics of the samples. In addition, simultaneous thermogravimetric and differential scanning calorimetry analyses show increasing mass losses from approximately 7 to 10 mass% depending on the polymorph and the heating rate. The largest hydrogen release occurs for the purest α-Ca(BH 4) 2 at a heating rate of 10°C/min. Calculated activation energies lead to 184 (14), 192 (3) and 230 (1) kJ/mol forγ-, α- andβ-Ca(BH 4) 2 samples, respectively. This is in agreement with the observed decomposition behavior. The results illustrate the complexity of the decomposition of Ca(BH 4) 2 and how the polymorphic content and the formation of intermediates can affect or not affect the decomposition reaction pathways. In particular, the origins of CaB 2Hx and the borohydride borate Ca 3(BH 4) 3(BO 3) seem to be unrelated to the nature of the polymorph. © 2012 American Chemical Society