Gallium arsenide 55Fe X-ray-photovoltaic battery

The effects of temperature on the key parameters of a prototype GaAs 55Fe radioisotope X-ray microbattery were studied over the temperature range -20 °C to 70 °C. A p-i-n GaAs structure was used to collect the photons from a 254 Bq 55Fe radioisotope X-ray source. Experimental results showed that the open circuit voltage and the short circuit current decreased with increased temperature. The maximum output power and the conversion efficiency of the device decreased at higher temperatures. For the reported microbattery, the highest maximum output power (1 pW, corresponding to 0.4 μW/Ci) was observed at -20 °C. A conversion efficiency of 9% was measured at -20 °C

Comparative Direct Analysis of Type Ia Supernova Spectra. IV. Postmaximum

A comparative study of optical spectra of Type Ia supernovae (SNe Ia) obtained near 1 week, 3 weeks, and 3 months after maximum light is presented. Most members of the four groups that were defined on the basis of maximum light spectra in Paper II (core normal, broad line, cool, and shallow silicon) develop highly homogeneous postmaximum spectra, although there are interesting exceptions. Comparisons with SYNOW synthetic spectra show that most of the spectral features can be accounted for in a plausible way. The fits show that 3 months after maximum light, when SN Ia spectra are often said to be in the nebular phase and to consist of forbidden emission lines, the spectra actually remain dominated by resonance scattering features of permitted lines, primarily those of Fe II. Even in SN 1991bg, which is said to have made a very early transition to the nebular phase, there is no need to appeal to forbidden lines at 3 weeks postmaximum, and at 3 months postmaximum the only clear identification of a forbidden line is [Ca II] 7291, 7324. Recent studies of SN Ia rates indicate that most of the SNe Ia that have ever occurred have been "prompt" SNe Ia, produced by young (100,000,000 yr) stellar populations, while most of the SNe Ia that occur at low redshift today are "tardy", produced by an older (several Gyrs) population. We suggest that the shallow silicon SNe Ia tend to be the prompt ones.Comment: Accepted by PAS

Al0.52In0.48P 55Fe x-ray-photovoltaic battery

An Al0.52In0.48P 55Fe radioisotope microbattery is demonstrated over the temperature range −20 °C to 160 °C. Al0.52In0.48P p+-i-n+ mesa structures were used to collect the photons from a 238 MBq 55Fe radioisotope x-ray source. The effects of temperature on the key microbattery parameters were studied. Increasing the temperature, the saturation current increased; whilst the open circuit voltage, the maximum power and the conversion efficiency decreased. An open circuit voltage of 0.97V and a conversion efficiency of 22% were measured in a single p+-i-n+ mesa structure at −20 °C. The highest total microbattery maximum output power using two mesa structures was 1.2 pW at −20 °C

Investigation of a temperature tolerant InGaP (GaInP) converter layer for a 63Ni betavoltaic cell

A prototype InGaP p+–i–n+ mesa photodiode was studied for its potential as the energy conversion device in a 63Ni betavoltaic cell; its electrical performance was analysed across the temperature range −20 °C to 100 °C. The results show that the InGaP detector when illuminated with a laboratory 63Ni radioisotope beta particle source had a maximum output power of 0.92 pW at −20 °C, this value decreased at higher temperatures. A decrease in the open circuit voltage and in the cell internal conversion ef ciency were also observed when the temperature was increased: at −20 °C, the open circuit voltage and the cell internal conversion ef ciency had values of 0.69 V and 4%, respectively. A short circuit current of 4.5 pA was measured at −20 °C

Characterization of Local pH Changes in Brain Using Fast-Scan Cyclic Voltammetry with Carbon Microelectrodes

Transient local pH changes in the brain are important markers of neural activity that can be used to follow metabolic processes that underlie the biological basis of behavior, learning and memory. There are few methods that can measure pH fluctuations with sufficient time resolution in freely moving animals. Previously, fast-scan cyclic voltammetry at carbon-fiber microelectrodes was used for the measurement of such pH transients. However, the origin of the potential dependent current in the cyclic voltammograms for pH changes recorded in vivo was unclear. The current work explored the nature of these peaks and established the origin for some of them. A peak relating to the capacitive nature of the pH CV was identified. Adsorption of electrochemically inert species, such as aromatic amines and calcium could suppress this peak, and is the origin for inconsistencies regarding in vivo and in vitro data. Also, we identified an extra peak in the in vivo pH CV relating to the presence of 3,4-dihydroxyacetic acid (DOPAC) in the brain extracellular fluid. To evaluate the in vivo performance of the carbon-fiber sensor, carbon dioxide inhalation by an anesthetized rat was used to induce brain acidosis induced by hypercapnia. Hypercapnia is demonstrated to be a useful tool to induce robust in vivo pH changes, allowing confirmation of the pH signal observed with FSCV

Carbon Microelectrodes with a Renewable Surface

Electrode fouling decreases sensitivity and can be a substantial limitation in electrochemical experiments. In this work we describe an electrochemical procedure that constantly renews the surface of a carbon microelectrode using periodic triangle voltage excursions to an extended anodic potential at a scan rate of 400 Vs−1. This methodology allows for the regeneration of an electrochemically active surface and restores electrode sensitivity degraded by irreversible adsorption of chemical species. We show that repeated voltammetric sweeps to moderate potentials in aqueous solution causes oxidative etching of carbon thereby constantly renewing the electrochemically active surface. Oxidative etching was established by tracking surface-localized fluorine atoms with XPS, by monitoring changes in carbon surface morphology with AFM on pyrolyzed photoresist films, and also by optical and electron microscopy. The use of waveforms with extended anodic potentials showed substantial increases in sensitivity towards the detection of catechols. This enhancement arose from the adsorption of the catechol moiety that could be maintained with a constant regeneration of the electrode surface. We also demonstrate that application of the extended waveform could restore the sensitivity of carbon microelectrodes diminished by irreversible adsorption (electrode fouling) of byproducts resulting from the electrooxidation and polymerization of tyramine. Overall, this work brings new insight into the factors that affect electrochemical processes at carbon electrodes and provides a simple method to remove or reduce fouling problems associated with many electrochemical experiments

Higher Sensitivity Dopamine Measurements with Faster-Scan Cyclic Voltammetry

Fast-scan cyclic voltammetry with carbon-fiber microelectrodes has been successfully used to detect catecholamine release in vivo. Generally, waveforms with anodic voltage limits of 1.0 V or 1.3 V (vs. Ag/AgCl) are used for detection. The 1.0 V excursion provides good temporal resolution, but suffers from a lack of sensitivity. The 1.3 V excursion increases sensitivity, but also increases response time which can blur the detection of neurochemical events. Here, the scan rate was increased to improve the sensitivity of the 1.0 V excursion while maintaining the rapid temporal response. However, increasing scan rate increases both the desired faradaic current response and the already large charging current associated with the voltage sweep. Analog background subtraction was used to prevent the analog-to-digital converter from saturating from the high currents generated with increasing scan rate by neutralizing some of the charging current. In vitro results with the 1.0 V waveform showed approximately a four-fold increase in signal to noise ratio with maintenance of the desired faster response time by increasing scan rate up to 2400 V/s. In vivo, stable stimulated release was detected with an approximate four-fold increase in peak current. The scan rate of the 1.3 V waveform was also increased, but the signal was unstable with time in vitro and in vivo. Adapting the 1.3 V triangular wave into a sawhorse design prevented signal decay and increased the faradaic response. The use of the 1.3 V sawhorse waveform decreased the detection limit of dopamine with FSCV to 0.96 ± 0.08 nM in vitro and showed improved performance in vivo without affecting the neuronal environment. Electron microscopy showed dopamine sensitivity is in a quasi-steady state with carbon-fiber microelectrodes scanned to potentials above 1.0 V

Characterization of large area photomultipliers and its application to dark matter search with noble liquid detectors

There is growing interest in the use of noble liquid detectors to study particle properties and search for new phenomena. In particular, they are extremely suitable for performing direct searches for dark matter. In this kind of experiments, the light produced after an interaction within the sensitive volume is usually read-out by photomultipliers. The need to go to masses in the tonne scale to explore deeper regions of the parameter space, calls for the use of large area photomultipliers. In this paper we address the need to perform laboratory calibration measurements of these large photomultipliers, in particular to characterize its behaviour at cryogenic temperatures where no reference from the manufacturer is available. We present comparative tests of phototubes from two companies. The tests are performed in conditions similar to those of operation in a real experiment. Measurements of the most relevant phototube parameters (quantum efficiency, gain, linearity, etc.) both at room and liquid Argon temperatures are reported. The results show that the studied phototubes comply with the stringent requirements posed by current dark matter searches performed with noble-liquid detectors.This work has been supported by CICYT Grants FPA-2006-00684, FPA-2002-01835 and FPA- 2005-07605-C02-01