79 research outputs found
Angular and Polarization Response of Multimode Sensors with Resistive-Grid Absorbers
High sensitivity receiver systems with near ideal polarization sensitivity
are highly desirable for development of millimeter and sub-millimeter radio
astronomy. Multimoded bolometers provide a unique solution to achieve such
sensitivity, for which hundreds of single-mode sensors would otherwise be
required. The primary concern in employing such multimoded sensors for
polarimetery is the control of the polarization systematics. In this paper, we
examine the angular- and polarization- dependent absorption pattern of a thin
resistive grid or membrane, which models an absorber used for a multimoded
bolometer. The result shows that a freestanding thin resistive absorber with a
surface resistivity of \eta/2, where \eta\ is the impedance of free space,
attains a beam pattern with equal E- and H-plane responses, leading to zero
cross polarization. For a resistive-grid absorber, the condition is met when a
pair of grids is positioned orthogonal to each other and both have a
resistivity of \eta/2. When a reflective backshort termination is employed to
improve absorption efficiency, the cross-polar level can be suppressed below
-30 dB if acceptance angle of the sensor is limited to <60degrees. The small
cross-polar systematics have even-parity patterns and do not contaminate the
measurements of odd-parity polarization patterns, for which many of recent
instruments for cosmic microwave background are designed. Underlying symmetry
that suppresses these cross-polar systematics is discussed in detail. The
estimates and formalism provided in this paper offer key tools in the design
consideration of the instruments using the multimoded polarimeters.Comment: 22 pages, 15 figure
Photon noise correlations in millimeter-wave telescopes
Many modern millimeter and submillimeter (``mm-wave'') telescopes for
astronomy are deploying more detectors by increasing detector pixel density,
and with the rise of lithographed detector architectures and high-throughput
readout techniques, it is becoming increasingly practical to overfill the focal
plane. However, when the pixel pitch is small compared to the
product of the wavelength and the focal ratio , or
, the Bose term of the photon noise
correlates between neighboring detector pixels due to the Hanbury Brown & Twiss
(HBT) effect. When this HBT effect is non-negligible, the array-averaged
sensitivity scales with detector count less favorably than
the uncorrelated limit of . In this paper, we present
a general prescription to calculate this HBT correlation based on a quantum
optics formalism and extend it to polarization-sensitive detectors. We then
estimate the impact of HBT correlations on the sensitivity of a model mm-wave
telescope and discuss the implications for focal-plane design.Comment: 24 pages, 16 figure
Precipitable Water Vapor Measurement using GNSS Data in the Atacama Desert for Millimeter and Submillimeter Astronomical Observations
Precipitable water vapor (PWV) strongly affects the quality of data obtained
from millimeter- and submillimeter-wave astronomical observations, such as
those for cosmic microwave background (CMB) measurements. Some of these
observatories have used radiometers to monitor PWV. In this study, PWV was
measured from April 2021 to April 2022 using Global Navigation Satellite System
(GNSS) instruments in the Atacama Desert, Chile, where several millimeter and
submillimeter-wave telescopes are located. We evaluated the accuracy of these
measurements by comparing them to radiometer measurements. We calculated the
PWV from GNSS data using Canadian Spatial Reference System Precise Point
Positioning (CSRS-PPP), an online software package. When using GNSS data alone,
the estimated PWV showed a systematic offset of +1.08 mm. When combining GNSS
data with data from a barometer which was co-located with the GNSS receiver,
the estimated PWV showed a lower systematic offset of -0.14 mm. The GNSS PWV
showed a statistical error of 0.52 mm with an averaging time of an hour.
Compared to other PWV measurement methods, GNSS instruments are robust in bad
weather conditions, have sufficient time resolution, and are less expensive. By
demonstrating good accuracy and precision in low PWV conditions, this paper
shows that GNSS instruments are valuable tools for PWV measurements for
observing site evaluation and data analysis for ground-based telescopes
Broadband multi-layer anti-reflection coatings with mullite and duroid for half-wave plates and alumina filters for CMB polarimetry
A broadband two-layer anti-reflection (AR) coating was developed for use on a
sapphire half-wave plate (HWP) and an alumina infrared (IR) filter for cosmic
microwave background (CMB) polarimetry. Measuring tiny CMB B-mode signals
requires maximizing the number of photons reaching the detectors and minimizing
spurious polarization due to reflection with an off-axis incident angle.
However, a sapphire HWP and an alumina IR filter have high refractive indices
of about 3.1, and an AR coating must be applied to them. Thermally sprayed
mullite and Duroid 5880LZ were selected in terms of index and coefficient of
thermal expansion for use at cryogenic temperatures. With these materials, the
reflectivity was reduced to about 2% at 90/150 GHz and <1% at 220/280 GHz. The
design, fabrication, and optical performance evaluation of the AR coatings are
described. The coatings were used in a current ground-based CMB experiment
called the Simons Array. They could also be applied to next-generation CMB
experiments, such as the Simons Observatory.Comment: 7 pages, 4 figure
The Simons Observatory: A fully remote controlled calibration system with a sparse wire grid for cosmic microwave background telescopes
For cosmic microwave background (CMB) polarization observations, calibration
of detector polarization angles is essential. We have developed a fully remote
controlled calibration system with a sparse wire grid that reflects linearly
polarized light along the wire direction. The new feature is a
remote-controlled system for regular calibration, which has not been possible
in sparse wire grid calibrators in past experiments. The remote control can be
achieved by two electric linear actuators that load or unload the sparse wire
grid into a position centered on the optical axis of a telescope between the
calibration time and CMB observation. Furthermore, the sparse wire grid can be
rotated by a motor. A rotary encoder and a gravity sensor are installed on the
sparse wire grid to monitor the wire direction. They allow us to achieve
detector angle calibration with expected systematic error of .
The calibration system will be installed in small-aperture telescopes at Simons
Observatory
Identification of 45 New Neutron-Rich Isotopes Produced by In-Flight Fission of a 238U Beam at 345 MeV/nucleon
A search for new isotopes using in-flight fission of a 345 MeV/nucleon 238U
beam has been carried out at the RI Beam Factory at the RIKEN Nishina Center.
Fission fragments were analyzed and identified by using the superconducting
in-flight separator BigRIPS. We observed 45 new neutron-rich isotopes: 71Mn,
73,74Fe, 76Co, 79Ni, 81,82Cu, 84,85Zn, 87Ga, 90Ge, 95Se, 98Br, 101Kr, 103Rb,
106,107Sr, 108,109Y, 111,112Zr, 114,115Nb, 115,116,117Mo, 119,120Tc,
121,122,123,124Ru, 123,124,125,126Rh, 127,128Pd, 133Cd, 138Sn, 140Sb, 143Te,
145I, 148Xe, and 152Ba
- …