SPIRE Design Description - Research Services

Draft SPIRE Design Description Document
from the telescope. This corresponds to a background photon rate of ~ 10 10 photons per second. Note that
this is enormously larger than in the case of optical observations. The accurate subtraction of this thermal
background is essential if the much fainter astronomical signals are to be measurable. Furthermore,
statistical fluctuation in the arrival rate of these background photons creates a fluctuating noise power which
represents a fundamental thermodynamic limitation to the sensitivity. For SPIRE, the associated photonnoise
limited NEP, NEPph, is a few x 10 -17 W Hz -1/2 . In order to achieve photon noise-limited performance,
the inherent NEP of the detector system must be comparable to or lower than this. The noise of the
bolometer itself is a combination of Johnson noise and phonon noise (due to the quantised flow of thermal
energy from the bolometer to the heat sink).
The instrument performance models described in §6 contain detailed calculations of the background power
levels and sensitivity for SPIRE. In addition, the operating modes require time constants of 30 ms for the
photometer and 16 ms for the FTS. With current bolometer technology, these sensitivity and time constant
requirements can be met by detectors operating at a temperature of around 0.3 K, cooled by a 3 He
refrigerator.
4.4.3 SPIRE bolometer design and specifications
SPIRE will use arrays of semiconductor bolometers developed at Caltech/JPL. Figure 4-17 illustrates the
basic detector design. The absorber is a "spider-web" of metallised silicon nitride. This appears as a filled
planar absorber to the submillimetre radiation which has a wavelength much longer than the grid spacing.
The spider-web structure has high mechanical strength and a low filling, factor providing low heat capacity
and immunity to glitches that can be caused by ionising radiation. The diameter of the spider-web is
typically a few times the wavelength to be detected. The thermometer is a small (20 x 100 x 300 µm) crystal
of Neutron Transmutation Doped (NTD) germanium. This material is highly suited for use in low
temperature bolometers as its thermal behaviour closely approaches that of an ideal thermal device, the
material displays very low 1/f noise, and the manufacturing processes are highly repeatable and reliable. A
wide range of NTD materials is available to tailor the impedance of the device to the desired range for the
chosen operating temperature. Large arrays of bolometers can now be made, as illustrated by the array wafer
shown in Figure 4-17.
Figure 4-17 - Left: individual spider-web bolometer. Right: large-format array wafer used as a 350-µm SPIRE
prototype array. The absorbers have a 0.725 mm diameter with a grid spacing of 72.5 µm. The filling factor is 8%. The
absorber is suspended by five 5-µm wide, 240-µm long support legs. The thermistors are placed to one side of the
absorber and read out with two leads deposited on a single, 18-µm wide support member.
4.4.4 Bolometer readout electronics
The resistance of the detector at the operating point is typically 5 MΩ. Lower values make it difficult to
avoid being preamplifier noise limited while higher values can pose problems with electromagnetic pick-up
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