66 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
Atomic Carbon in M82: Physical conditions derived from simultaneous observations of the [CI] fine structure submillimeter wave transitions
We report the first extragalactic detection of the neutral carbon [CI]
3P2-3P1 fine structure line at 809 GHz. The line was observed towards M82
simultaneously with the 3P1-3P0 line at 492 GHz, providing a precise
measurement of the J=2-1/J=1-0 integrated line ratio of 0.96 (on a [K km s^-1]
-scale). This ratio constrains the [CI] emitting gas to have a temperature of
at least 50 K and a density of at least 10^4 cm^-3. Already at this minimum
temperature and density, the beam averaged CI-column density is large, 2.1
10^18 cm^-2, confirming the high CI/CO abundance ratio of approximately 0.5
estimated earlier from the 492 GHz line alone. We argue that the [CI] emission
from M82 most likely arises in clouds of linear size around a few pc with a
density of about 10^4 cm^-3 or slightly higher and temperatures of 50 K up to
about 100 K.Comment: 4 pages, 2 figures, ApJL in press, postscript also available at
ftp://apollo.ph1.uni-koeln.de/pub/stutzki/m82_pap.ps.gz
e-mail-contact:[email protected]
[12CII] and [13CII] 158 mum emission from NGC 2024: Large column densities of ionized carbon
Context: We analyze the NGC 2024 HII region and molecular cloud interface
using [12CII] and [13CII] observations. Aims: We attempt to gain insight into
the physical structure of the interface layer between the molecular cloud and
the HII region. Methods. Observations of [12CII] and [13CII] emission at 158
{\mu}m with high spatial and spectral resolution allow us to study the detailed
structure of the ionization front and estimate the column densities and
temperatures of the ionized carbon layer in the PDR. Results: The [12CII]
emission closely follows the distribution of the 8 mum continuum. Across most
of the source, the spectral lines have two velocity peaks similar to lines of
rare CO isotopes. The [13CII] emission is detected near the edge-on ionization
front. It has only a single velocity component, which implies that the [12CII]
line shape is caused by self-absorption. An anomalous hyperfine line-intensity
ratio observed in [13CII] cannot yet be explained. Conclusions: Our analysis of
the two isotopes results in a total column density of N(H)~1.6\times10^23 cm^-2
in the gas emitting the [CII] line. A large fraction of this gas has to be at a
temperature of several hundred K. The self-absorption is caused by a cooler
(T<=100 K) foreground component containing a column density of N(H)~10^22
cm^-2
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
Gas morphology and energetics at the surface of PDRs: new insights with Herschel observations of NGC 7023
We investigate the physics and chemistry of the gas and dust in dense
photon-dominated regions (PDRs), along with their dependence on the
illuminating UV field. Using Herschel-HIFI observations, we study the gas
energetics in NGC 7023 in relation to the morphology of this nebula. NGC 7023
is the prototype of a PDR illuminated by a B2V star and is one of the key
targets of Herschel. Our approach consists in determining the energetics of the
region by combining the information carried by the mid-IR spectrum (extinction
by classical grains, emission from very small dust particles) with that of the
main gas coolant lines. In this letter, we discuss more specifically the
intensity and line profile of the 158 micron (1901 GHz) [CII] line measured by
HIFI and provide information on the emitting gas. We show that both the [CII]
emission and the mid-IR emission from polycyclic aromatic hydrocarbons (PAHs)
arise from the regions located in the transition zone between atomic and
molecular gas. Using the Meudon PDR code and a simple transfer model, we find
good agreement between the calculated and observed [CII] intensities. HIFI
observations of NGC 7023 provide the opportunity to constrain the energetics at
the surface of PDRs. Future work will include analysis of the main coolant line
[OI] and use of a new PDR model that includes PAH-related species.Comment: Accepted for publication in Astronomy and Astrophysics Letters
(Herschel HIFI special issue), 5 pages, 5 figure
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
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