115 research outputs found
Comparison of measured and predicted performance of a SIS waveguide mixer at 345 GHz
The measured gain and noise of a SIS waveguide mixer at 345 GHz have been compared with theoretical values, calculated from the quantum mixer theory using a three port model. As a mixing element, we use a series array of two Nb-Al2O3-Nb SIS junctions. The area of each junction is 0.8 sq microns and the normal state resistance is 52 omega. The embedding impedance of the mixer has been determined from the pumped DC-IV curves of the junction and is compared to results from scale model measurements (105 x). Good agreement was obtained. The measured mixer gain, however, is a factor of 0.45 plus or minus 0.5 lower than the theoretical predicted gain. The measured mixer noise temperature is a factor of 4-5 higher than the calculated one. These discrepancies are independent on pump power and are valid for a broad range of tuning conditions
A low noise 410-495 heterodyne two tuner mixer, using submicron Nb/Al2O3/Nb tunneljunctions
A 410-495 GHz heterodyne receiver, with an array of two Nb/Al2O3/Nb tunneljunctions as mixing element is described. The noise temperature of this receiver is below 230 K (DSB) over the whole frequency range, and has lowest values of 160 K in the 435-460 GHz range. The calculated DSB mixergain over the whole frequency range varies from -11.9 plus or minus 0.6 dB to -12.6 plus or minus 0.6 dB and the mixer noise is 90 plus or minus 30 K
Terahertz hot electron bolometer waveguide mixers for GREAT
Supplementing the publications based on the first-light observations with the
German Receiver for Astronomy at Terahertz frequencies (GREAT) on SOFIA, we
present background information on the underlying heterodyne detector
technology. We describe the superconducting hot electron bolometer (HEB)
detectors that are used as frequency mixers in the L1 (1400 GHz), L2 (1900
GHz), and M (2500 GHz) channels of GREAT. Measured performance of the detectors
is presented and background information on their operation in GREAT is given.
Our mixer units are waveguide-based and couple to free-space radiation via a
feedhorn antenna. The HEB mixers are designed, fabricated, characterized, and
flight-qualified in-house. We are able to use the full intermediate frequency
bandwidth of the mixers using silicon-germanium multi-octave cryogenic
low-noise amplifiers with very low input return loss. Superconducting HEB
mixers have proven to be practical and sensitive detectors for high-resolution
THz frequency spectroscopy on SOFIA. We show that our niobium-titanium-nitride
(NbTiN) material HEBs on silicon nitride (SiN) membrane substrates have an
intermediate frequency (IF) noise roll-off frequency above 2.8 GHz, which does
not limit the current receiver IF bandwidth. Our mixer technology development
efforts culminate in the first successful operation of a waveguide-based HEB
mixer at 2.5 THz and deployment for radioastronomy. A significant contribution
to the success of GREAT is made by technological development, thorough
characterization and performance optimization of the mixer and its IF interface
for receiver operation on SOFIA. In particular, the development of an optimized
mixer IF interface contributes to the low passband ripple and excellent
stability, which GREAT demonstrated during its initial successful astronomical
observation runs.Comment: Accepted for publication in A&A (SOFIA/GREAT special issue
Co-production in primary schools: a systematic literature review
Co-production is the involvement of citizens in the design and delivery of services. In primary schools, this involves parents working with teachers to improve the educational development of their children. In this contribution, we present the results of a systematic literature review on co-production in primary schools to establish what research has been conducted and to what extent there is evidence on the effectiveness of co-production in this context. After three subsequent steps of literature selection, an initial database of 3121 articles was reduced to 122 articles which were then carefully analysed. Generally, co-production in education tends to be aimed at specific groups, which makes it hard to generalize, but some findings appear more generally applicable. Co-production does appear to improve students’ knowledge acquisition. Parent–teacher relationships can be difficult and ambiguous, but teacher training appears to be an effective tool for improving co-production.The politics and administration of institutional chang
Observation of Andreev Reflection Enhanced Shot Noise
We have experimentally investigated the quasiparticle shot noise in
NbN/MgO/NbN superconductor - insulator - superconductor tunnel junctions. The
observed shot noise is significantly larger than theoretically expected. We
attribute this to the occurrence of multiple Andreev reflection processes in
pinholes present in the MgO barrier. This mechanism causes the current to flow
in large charge quanta (Andreev clusters), with a voltage dependent average
value of m = 1+ 2 Delta/eV times the electron charge. Because of this charge
enhancement effect, the shot noise is increased by the factor m.Comment: 4 pages, 5 figures include
First observations with CONDOR, a 1.5 THz heterodyne receiver
The THz atmospheric windows centered at roughly 1.3 and 1.5~THz, contain
numerous spectral lines of astronomical importance, including three high-J CO
lines, the N+ line at 205 microns, and the ground transition of para-H2D+. The
CO lines are tracers of hot (several 100K), dense gas; N+ is a cooling line of
diffuse, ionized gas; the H2D+ line is a non-depleting tracer of cold (~20K),
dense gas. As the THz lines benefit the study of diverse phenomena (from
high-mass star-forming regions to the WIM to cold prestellar cores), we have
built the CO N+ Deuterium Observations Receiver (CONDOR) to further explore the
THz windows by ground-based observations. CONDOR was designed to be used at the
Atacama Pathfinder EXperiment (APEX) and Stratospheric Observatory For Infrared
Astronomy (SOFIA). CONDOR was installed at the APEX telescope and test
observations were made to characterize the instrument. The combination of
CONDOR on APEX successfully detected THz radiation from astronomical sources.
CONDOR operated with typical Trec=1600K and spectral Allan variance times of
30s. CONDOR's first light observations of CO 13-12 emission from the hot core
Orion FIR4 (= OMC1 South) revealed a narrow line with T(MB) = 210K and
delta(V)=5.4km/s. A search for N+ emission from the ionization front of the
Orion Bar resulted in a non-detection. The successful deployment of CONDOR at
APEX demonstrates the potential for making observations at THz frequencies from
ground-based facilities.Comment: 4 pages + list of objects, 3 figures, to be published in A&A special
APEX issu
Photon Dominated Regions in NGC 3603
Aims: We aim at deriving the excitation conditions of the interstellar gas as
well as the local FUV intensities in the molecular cloud surrounding NGC 3603
to get a coherent picture of how the gas is energized by the central stars.
Methods: The NANTEN2-4m submillimeter antenna is used to map the [CI] 1-0, 2-1
and CO 4-3, 7-6 lines in a 2' x 2' region around the young OB cluster NGC 3603
YC. These data are combined with C18O 2-1 data, HIRES-processed IRAS 60 and 100
micron maps of the FIR continuum, and Spitzer/IRAC maps. Results: The NANTEN2
observations show the presence of two molecular clumps located south-east and
south-west of the cluster and confirm the overall structure already found by
previous CS and C18O observations. We find a slight position offset of the peak
intensity of CO and [CI], and the atomic carbon appears to be further extended
compared to the molecular material. We used the HIRES far-infrared dust data to
derive a map of the FUV field heating the dust. We constrain the FUV field to
values of \chi = 3 - 6 \times 10^3 in units of the Draine field across the
clouds. Approximately 0.2 to 0.3 % of the total FUV energy is re-emitted in the
[CII] 158 {\mu}m cooling line observed by ISO. Applying LTE and escape
probability calculations, we derive temperatures (TMM1 = 43 K, TMM2 = 47 K),
column densities (N(MM1) = 0.9 \times 10^22 cm^-2, N(MM2) = 2.5 \times 10^22
cm^-2) and densities (n(MM1) = 3 \times 10^3 cm^-3, n(MM2) = 10^3 -10^4 cm^-3)
for the two observed molecular clumps MM1 and MM2. Conclusions: The cluster is
strongly interacting with the ambient molecular cloud, governing its structure
and physical conditions. A stability analysis shows the existence of
gravitationally collapsing gas clumps which should lead to star formation.
Embedded IR sources have already been observed in the outskirts of the
molecular cloud and seem to support our conclusions.Comment: 13 pages, 10 figures, accepted for publication by A&
Excitation and Abundance of C3 in star forming cores:Herschel/HIFI observations of the sight-lines to W31C and W49N
We present spectrally resolved observations of triatomic carbon (C3) in
several ro-vibrational transitions between the vibrational ground state and the
low-energy nu2 bending mode at frequencies between 1654-1897 GHz along the
sight-lines to the submillimeter continuum sources W31C and W49N, using
Herschel's HIFI instrument. We detect C3 in absorption arising from the warm
envelope surrounding the hot core, as indicated by the velocity peak position
and shape of the line profile. The sensitivity does not allow to detect C3
absorption due to diffuse foreground clouds. From the column densities of the
rotational levels in the vibrational ground state probed by the absorption we
derive a rotation temperature (T_rot) of ~50--70 K, which is a good measure of
the kinetic temperature of the absorbing gas, as radiative transitions within
the vibrational ground state are forbidden. It is also in good agreement with
the dust temperatures for W31C and W49N. Applying the partition function
correction based on the derived T_rot, we get column densities N(C3)
~7-9x10^{14} cm^{-2} and abundance x(C3)~10^{-8} with respect to H2. For W31C,
using a radiative transfer model including far-infrared pumping by the dust
continuum and a temperature gradient within the source along the line of sight
we find that a model with x(C3)=10^{-8}, T_kin=30-50 K, N(C3)=1.5 10^{15}
cm^{-2} fits the observations reasonably well and provides parameters in very
good agreement with the simple excitation analysis.Comment: Accepted for publication in Astronomy and Astrophysics (HIFI first
results issue
SOFIA FEEDBACK Survey: The Pillars of Creation in [C II] and Molecular Lines
We investigate the physical structure and conditions of photodissociation
regions (PDRs) and molecular gas within the Pillars of Creation in the Eagle
Nebula using SOFIA FEEDBACK observations of the [C II] 158 micron line. These
observations are velocity resolved to 0.5 km s and are analyzed
alongside a collection of complimentary data with similar spatial and spectral
resolution: the [O I] 63 micron line, also observed with SOFIA, and rotational
lines of CO, HCN, HCO, CS, and NH. Using the superb spectral
resolution of SOFIA, APEX, CARMA, and BIMA, we reveal the relationships between
the warm PDR and cool molecular gas layers in context of the Pillars' kinematic
structure. We assemble a geometric picture of the Pillars and their
surroundings informed by illumination patterns and kinematic relationships and
derive physical conditions in the PDRs associated with the Pillars. We estimate
an average molecular gas density cm
and an average atomic gas density cm
and infer that the ionized, atomic, and molecular phases are in pressure
equilibrium if the atomic gas is magnetically supported. We find pillar masses
of 103, 78, 103, and 18 solar masses for P1a, P1b, P2, and P3 respectively, and
evaporation times of 1-2 Myr. The dense clumps at the tops of the pillars
are currently supported by the magnetic field. Our analysis suggests that
ambipolar diffusion is rapid and these clumps are likely to collapse within
their photoevaporation timescales.Comment: 42 pages, 16 figures. Accepted for publication in The Astronomical
Journa
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