335 research outputs found
Study of the 12C+12C fusion reactions near the Gamow energy
The fusion reactions 12C(12C,a)20Ne and 12C(12C,p)23Na have been studied from
E = 2.10 to 4.75 MeV by gamma-ray spectroscopy using a C target with ultra-low
hydrogen contamination. The deduced astrophysical S(E)* factor exhibits new
resonances at E <= 3.0 MeV, in particular a strong resonance at E = 2.14 MeV,
which lies at the high-energy tail of the Gamow peak. The resonance increases
the present non-resonant reaction rate of the alpha channel by a factor of 5
near T = 8x10^8 K. Due to the resonance structure, extrapolation to the Gamow
energy E_G = 1.5 MeV is quite uncertain. An experimental approach based on an
underground accelerator placed in a salt mine in combination with a high
efficiency detection setup could provide data over the full E_G energy range.Comment: 4 Pages, 4 figures, accepted for publication in Phys. Rev. Let
Measurement of the 187Re({\alpha},n)190Ir reaction cross section at sub-Coulomb energies using the Cologne Clover Counting Setup
Uncertainties in adopted models of particle+nucleus optical-model potentials
directly influence the accuracy in the theoretical predictions of reaction
rates as they are needed for reaction-network calculations in, for instance,
{\gamma}-process nucleosynthesis. The improvement of the {\alpha}+nucleus
optical-model potential is hampered by the lack of experimental data at
astrophysically relevant energies especially for heavier nuclei. Measuring the
Re187({\alpha},n)Ir190 reaction cross section at sub-Coulomb energies extends
the scarce experimental data available in this mass region and helps
understanding the energy dependence of the imaginary part of the
{\alpha}+nucleus optical-model potential at low energies. Applying the
activation method, after the irradiation of natural rhenium targets with
{\alpha}-particle energies of 12.4 to 14.1 MeV, the reaction yield and thus the
reaction cross section were determined via {\gamma}-ray spectroscopy by using
the Cologne Clover Counting Setup and the method of {\gamma}{\gamma}
coincidences. Cross-section values at five energies close to the
astrophysically relevant energy region were measured. Statistical model
calculations revealed discrepancies between the experimental values and
predictions based on widely used {\alpha}+nucleus optical-model potentials.
However, an excellent reproduction of the measured cross-section values could
be achieved from calculations based on the so-called Sauerwein-Rauscher
{\alpha}+nucleus optical-model potential. The results obtained indicate that
the energy dependence of the imaginary part of the {\alpha}+nucleus
optical-model potential can be described by an exponential decrease. Successful
reproductions of measured cross sections at low energies for {\alpha}-induced
reactions in the mass range 141{\leq}A{\leq}187 confirm the global character of
the Sauerwein-Rauscher potential
Stark shift and field ionization of arsenic donors in Si-SOI structures
We develop an efficient back gate for silicon-on-insulator (SOI) devices
operating at cryogenic temperatures, and measure the quadratic hyperfine Stark
shift parameter of arsenic donors in isotopically purified Si-SOI layers
using such structures. The back gate is implemented using MeV ion implantation
through the SOI layer forming a metallic electrode in the handle wafer,
enabling large and uniform electric fields up to 2 V/m to be
applied across the SOI layer. Utilizing this structure we measure the Stark
shift parameters of arsenic donors embedded in the Si SOI layer and find
a contact hyperfine Stark parameter of m/V. We also demonstrate electric-field driven dopant ionization in
the SOI device layer, measured by electron spin resonance.Comment: 5 pages, 3 figure
Superconducting Quantum Interference Device based on MgB2 nanobridges
The recently discovered superconductor MgB2, with a transition temperature of
39K, has significant potential for future electronics. An essential step is the
achievement of Josephson circuits, of which the superconducting quantum
interference device (SQUID) is the most important. Here, we report Josephson
quantum interference in superconducting MgB2 thin films. Modulation voltages of
up to 30 microvolt are observed in an all-MgB2 SQUID, based on focused ion beam
patterned nanobridges. These bridges, with a length scale < 100 nm, have
outstanding critical current densities of 7 x 10^6 A/cm2 at 4.2 K.Comment: submitted to Appl. Phys. Let
Dual Magnetic Separator for TRIP
The TRIP facility, under construction at KVI, requires the production
and separation of short-lived and rare isotopes. Direct reactions,
fragmentation and fusion-evaporation reactions in normal and inverse kinematics
are foreseen to produce nuclides of interest with a variety of heavy-ion beams
from the superconducting cyclotron AGOR. For this purpose, we have designed,
constructed and commissioned a versatile magnetic separator that allows
efficient injection into an ion catcher, i.e., gas-filled stopper/cooler or
thermal ionizer, from which a low energy radioactive beam will be extracted.
The separator performance was tested with the production and clean separation
of Na ions, where a beam purity of 99.5% could be achieved. For
fusion-evaporation products, some of the features of its operation as a
gas-filled recoil separator were tested.Comment: accepted by Nucl.Instr. Meth., final versio
Nearâmillimeterâwave response of high Tc rampâtype Josephson junctions
We have studied the response of a YBCO/PBCO/YBCO rampâtype junction to coherent radiation at 176 and 270 GHz. The IâV characteristic of the junction closely resembles the prediction of the RSJ model. The I cR n product of the junction is 0.25 mV at 5 K. The millimeterâwave radiation is coupled to the junction via a quasioptical structure that focuses the radiation onto the junction through a yttriumâstabilized ZrO2 substrate. At 176 GHz, we have observed as many as six Shapiro steps at the maximum power level of our Gunn oscillatorâpumped frequency doubler. Shapiro steps are still clearly seen up to 65 K. The amplitudes of the zeroth, first, and second Shapiro steps, as functions of the square root of the radiation power, agree remarkably well with a Bessel function fit, indicating the junction is voltageâbiased at the radiation frequency (rf). At 270 GHz, due to a combination of the heavy rf loss in the ZrO2 substrate and the lack of radiation power, we have observed only the first Shapiro step
Development of Cryogenic Current Comparators with DC Squid Readout for the Calibration of Electrical Standards
For the realization of the electrical quantum metrology triangle (V-A-Ω) a device to amplify very small currents with high precision is needed. The cryogenic current comparator (CCC) is by far the best instrument to do this. In order to make a very current sensitive CCC for calibration of electrical standards, we have developed optimum dc Superconducting QUantum Interference Devices (SQUIDs). The design, fabrication and characterisation of these devices is presented. The measurements concern the flux-to-voltage transfer and the noise properties, especially the input current noise. The optimisation of the flux transformer circuit that links the CCC with the SQUID will be treated. In addition, typical fabrication aspects of the CCC as the wires and tube assembly, the shields and the support system will be addressed
Influence of the aqueous matrix on the degradation of cyanotoxins by CWPO: a study on the Iberian Peninsula freshwaters
The increasing occurrence of toxic cyanobacterial blooms worldwide represents a critical health and environmental risk. Catalytic wet peroxide oxidation (CWPO) has emerged as an efficient and environmentally friendly technology for the removal of cyanotoxins in water. Nevertheless, its effectiveness has just been demonstrated in deionized water or simple synthetic aqueous matrices. In this work, the effect of the different components of the aqueous matrix on the CWPO of cyanotoxins was deeply evaluated considering the widespread properties of the Iberian Peninsula freshwaters. The presence of Cl-, HCO3- and SO42- ions reduced the oxidation rate of cylindrospermopsin (CYN) up to 70â80% at the highest concentrations tested (2000, 250 and 500 mg Lâ1 for Cl-, HCO3- and SO42-, respectively) due to their hydroxyl radical scavenging capacity. The presence of natural organic matter (NOM) resulted in a similar outcome (oxidation rate reduction up to 90% at the highest concentration tested, 20 mg Lâ1), but in this case due to the consumption of hydroxyl radicals in competition with CYN oxidation. The presence of NO3- and H2PO4- did not show any significant effect on CYN oxidation. Similarly, the presence of cyanobacteria (Chrysosporum ovalisporum, 50 ”g chlorophyll-a Lâ1) did not appreciably affect the CYN oxidation rate. These results were consistent with those obtained by evaluating the impact of real aqueous matrices from drinking water treatment plants (DWTPs) located in Castilla y LeĂłn and Extremadura regions (Spain) on CWPO performance. This knowledge is key to the implementation of the technology for the treatment of surface waters affected by toxic cyanobacterial bloomsThis research has been supported by the CYTED Ibero-American
Science and Technology Program for Development (CYTED-2019)
through the project TALGENTOX (PCI2020â112013) and by Spanish
Ministry for Science and Innovation (MICINN) through the project
PID2019â105079RB-I00. D. Ortiz thanks the Spanish Ministry of Universities (MIU) for the FPU predoctoral grant (FPU19/04816). M.
Munoz thanks the Spanish MINECO for the Ramon ÂŽ y Cajal postdoctoral
contract (RYC-2016â20648). We especially thank Aqualia for providing
the water samples from DWTPs employed in this study, which was
performed in the framework of Rewaise project (http://rewaise.eu
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