News from the Cryogenic Qualification Model (CQM) tests in Liège
A bolometer is a thermal detector: the incident radiation induce an increase of the temperature of an absorber that is connected to the thermal bath by a thermal conductance. This increase of the absorber temperature is measured with a thermistor and is proportionnal to the incident radiation power.
The two major requirements on the bolometers are the sensitivity and the time response. Scanning a 5 arcmin beam at 6 deg/s produces frequencies up to 100Hz and thus times constants less than 2ms are required for a proper measurement. In the same time, the intrinsic noise of the bolometer has to be less than the photon noise, which requires typically a Noise Equivalent Powers (NEPs) of the order of 10^-17 W.Hz^-1/2. These requirements could possibly met only by the spider web 0.1K bolometers developed in Pasadena by the California Institut of Technology (Caltech) and Jet Propulsion Laboratory (JPL).
Together with the development of zero gravity compatible dilution coolers, the existence of such detectors was one of the triggers of the Planck-HFI conception. The special point in spiderwed bolometers is that the radiation absorber is made of a grid with impedance matched with that of vacuum. Among other advantages these detectors are much less sensitive to ionizing radiation that conventional bolometers.
The main characteristics of the spiderweb bolometers are given in the following table:
| Frequency (GHZ) | Optical load (pW) | Required NEP (10^-17 W.Hz^-1/2) | Goal time constant (ms) | Required time constant (ms) |
|---|---|---|---|---|
| 100 | 1.0 | 1.2 | 3.9 | 7.8 |
| 143 | 1.1 | 1.5 | 2.9 | 5.7 |
| 217 | 1.1 | 1.8 | 2.2 | 4.4 |
| 353 | 1.0 | 2.2 | 2.2 | 4.4 |
| 545 | 5.0 | 6.0 | 2.2 | 4.4 |
| 857 | 16.0 | 13.5 | 2.2 | 4.4 |
The bolometers of the qualification model have been fabricated and proven to meet or exceed the required performances.
