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SPIRE Design Description - Research Services

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Draft <strong>SPIRE</strong> <strong>Design</strong> <strong>Description</strong> Document<br />

from the telescope. This corresponds to a background photon rate of ~ 10 10 photons per second. Note that<br />

this is enormously larger than in the case of optical observations. The accurate subtraction of this thermal<br />

background is essential if the much fainter astronomical signals are to be measurable. Furthermore,<br />

statistical fluctuation in the arrival rate of these background photons creates a fluctuating noise power which<br />

represents a fundamental thermodynamic limitation to the sensitivity. For <strong>SPIRE</strong>, the associated photonnoise<br />

limited NEP, NEPph, is a few x 10 -17 W Hz -1/2 . In order to achieve photon noise-limited performance,<br />

the inherent NEP of the detector system must be comparable to or lower than this. The noise of the<br />

bolometer itself is a combination of Johnson noise and phonon noise (due to the quantised flow of thermal<br />

energy from the bolometer to the heat sink).<br />

The instrument performance models described in §6 contain detailed calculations of the background power<br />

levels and sensitivity for <strong>SPIRE</strong>. In addition, the operating modes require time constants of 30 ms for the<br />

photometer and 16 ms for the FTS. With current bolometer technology, these sensitivity and time constant<br />

requirements can be met by detectors operating at a temperature of around 0.3 K, cooled by a 3 He<br />

refrigerator.<br />

4.4.3 <strong>SPIRE</strong> bolometer design and specifications<br />

<strong>SPIRE</strong> will use arrays of semiconductor bolometers developed at Caltech/JPL. Figure 4-17 illustrates the<br />

basic detector design. The absorber is a "spider-web" of metallised silicon nitride. This appears as a filled<br />

planar absorber to the submillimetre radiation which has a wavelength much longer than the grid spacing.<br />

The spider-web structure has high mechanical strength and a low filling, factor providing low heat capacity<br />

and immunity to glitches that can be caused by ionising radiation. The diameter of the spider-web is<br />

typically a few times the wavelength to be detected. The thermometer is a small (20 x 100 x 300 µm) crystal<br />

of Neutron Transmutation Doped (NTD) germanium. This material is highly suited for use in low<br />

temperature bolometers as its thermal behaviour closely approaches that of an ideal thermal device, the<br />

material displays very low 1/f noise, and the manufacturing processes are highly repeatable and reliable. A<br />

wide range of NTD materials is available to tailor the impedance of the device to the desired range for the<br />

chosen operating temperature. Large arrays of bolometers can now be made, as illustrated by the array wafer<br />

shown in Figure 4-17.<br />

Figure 4-17 - Left: individual spider-web bolometer. Right: large-format array wafer used as a 350-µm <strong>SPIRE</strong><br />

prototype array. The absorbers have a 0.725 mm diameter with a grid spacing of 72.5 µm. The filling factor is 8%. The<br />

absorber is suspended by five 5-µm wide, 240-µm long support legs. The thermistors are placed to one side of the<br />

absorber and read out with two leads deposited on a single, 18-µm wide support member.<br />

4.4.4 Bolometer readout electronics<br />

The resistance of the detector at the operating point is typically 5 MΩ. Lower values make it difficult to<br />

avoid being preamplifier noise limited while higher values can pose problems with electromagnetic pick-up<br />

73

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