Radiative Shields

Some of them use two reservoirs (liquid N2 and 4He, see below), but most metallic dewars do no longer use LN2 (which produces vibrations in boiling) to cool radiative shields. Instead superinsulation is used. 

Figure 2.49 shows a schematic presentation of a photodetector surrounded by a radiative shield. A hght pipe is shown on the right side as a cone-shaped opening with a sohd angle 9. Similar to the radiative shield itself, its walls are made of highly reflective material. The detector radiates as a blackbody source (temperamre Ti) and at the same time its radiation must be in equilibrium with its surroundings. 

If the emissivity of the radiative shield 2 is much lower than unity, the level of equilibrium radiation emitted by the detector must drop (suppression of spontaneous radiation) and thus its BLIP detectivity will increase. The ideal result would be obtained for an aperture of the hght pipe close to zero, but in that case diffractive effect completely change the behavior of the whole system. 

Putley proposed to use the so-called superisolators as the material of the radiative shield. These materials were developed for thermal isolation in cryogenic technique and their thermal impedance is larger than that of vacuum. These are actually concentric layers of isolators and highly reflective metal layers. 

In our consideration we assume that the detector and the radiative shield are at the same temperature T = T2 = T), which is near or equal to the room temperature. The detectivity of a photoconductive BLIP detector is given by [5]... 

The following expression is vahd for the radiation flux of a detector fuUy surrounded by a radiative shield... 

Fig. 2.51 Spectral dependence of specific detectivity of photoconductor for 1 conventional detector (no radiative shield) 2 radiative shield with a light pipe incident angle of 0 = 15° 3 the case of ideal total radiative shield, 0 = 0°. T = 297 K, Ei = 0.9, 82 = 0.01, A,oo = 10.6 pm...

Fig. 2.52 Specific detectivity of a photoconductor versus wavelength for a total enclosure within a radiative shield for various shield emissivities. T = 297 K, 8] = 0.9...

The influence of a PBG structure to the detectivity of a photonic detector may be considered in a manner analogous to that presented in Sect. 2.12. Actually a photonic crystal may be considered the ideal case of a radiative shields, describing... 

The second equihbrium group encompasses strucmres for increase in the optical path of the beam which already entered the active area of the detector, the so-called light trapping structures. These structures simultaneously increase radiative lifetime through the mechanism of reabsorption—photon recycling. They include different surface rehef stmctures for the increase in total internal reflection, from diflractive to macroscopic ones. Reflective detector surfaces also belong to this group, both the back-side ones and fuU resonant cavities (RCE—resonant cavity enhancement) with reflective surfaces both of the front and the back side of the detector. The most advanced stmctures for optical path and radiative time increase are radiative shields and photonic crystal enhancement stmcmres, which represent a fuU cavity enhancement and may support the existence of multiple modes. 

Radiation-Density Gauges Gamma radiation may be used to measure the density of material inside a pipe or process vessel. The equipment is basically the same as for level measurement, except that here the pipe or vessel must be filled over the effective, irradiated sample volume. The source is mounted on one side of the pipe or vessel and the detector on the other side with appropriate safety radiation shielding surrounding the installation. Cesium 137 is used as the radi-... 

The low-temperature working space was surrounded by a copper thermal shield at the temperature of the mixing chamber, hence, the radiative power was always negligible. For electrical connections, RF filters both at room temperature and at 4K were used the total spurious power on the sample was estimated to be below 10-11 W. 

Thermal shields connected to heat exchangers cooled by the evaporating gas are used to drastically reduce the radiative input. 

The conical coal injector was replaced with a blunt cyclin-der with a single axial jet so the flame could be stabilized at lower swirl numbers, thereby reducing the centrifugal deposition on the furnace walls. The radiation shield between the combustor and heat exchanger was removed to reduce particle losses further. The increased radiative transfer decreased the wall temperature substantially. The later experiments were also carried out at lower fuel-air equivalence ratios, i.e., (J> = 0.57. The combination of increased heat losses and increased dilution with excess air reduced the maximum wall temperature to 990°C for the experiments reported below. 

Protective radiation shields are used to reduce the radiative exchange between walls at different temperatures thin foils or sheets made of good reflecting materials are placed between the walls, Fig. 5.66. The spaces between the protective shields are normally evacuated so that heat transfer by convection is prevented. This multi-layer arrangement is used predominantly in cryogenic applications for the insulation of containers for very cold liquified gases. 

In order to reduce the radiative exchange between two large, parallel plates, a thin, flat radiation protection shield is introduced between the plates. However, the emissivities of the two surfaces of the shield are different one surface has emissivity s < 0.4, the other an emissivity of 2.5>s ... 

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