Temperature bolometer

Low-temperature thermometers are obtained by cutting a metallized wafer of NTD Ge into chips of small size (typically few mm3) and bonding the electrical contacts onto the metallized sides of the chip. These chips are seldom used as calibrated thermometers for temperatures below 30 mK, but find precious application as sensors for low-temperature bolometers [42,56], When the chips are used as thermometers, i.e. in quasi-steady applications, their heat capacity does not represent a problem. In dynamic applications and at very low temperatures T < 30 mK, the chip heat capacity, together with the carrier-to-phonon thermal conductance gc d, (Section 15.2.1.3), determines the rise time of the pulses of the bolometer. 

A bolometer has a conducting element whose electrical resistance changes as a function of temperature. Bolometers are fabricated from thin strips of metals, such as nickel or platinum, or from semiconductors consisting of oxides of nickel or cobalt the latter are called thermistors. 

Low temperature bolometers have also been used for detecting neutral particles in high resolution scattering experiments (Cavallini et al, 1971c). Especially the detection of H-atoms proved to be very efficient due to recombination effects on the surface (Marenco et al, 1972). 

In addition to radiation noise and temperature noise associated with the thermal impedance of the element, Johnson noise associated with the resistance r is one of the most important noise sources. With some types of bolometer low frequency current noise is important and is the principal factor limiting i. With room temperature bolometers amplifier noise should not be important but with cyrogenic devices it is usually the dominant noise source, especially when operating with cooled filters to limit the radiation noise to the sub-mm band. 

Boyle, W. S. Rodgers, K. F. (1959). Performance characteristic of a new low-temperature bolometer. Journal of the Optical Society of America, 49,66-9. 

In practice, the NEP of a room-temperature THz spectrometer is usually limited by fluctuations (shot-noise) in the ambient blackbody radiation. Usmg an optical bandwidth Av = 3 THz (limited by, for example, a polyethylene/diamond dust window), a field of view (at nomial incidence) 0 = 9 and a detecting diameter (using a so-called Winston cone, which condenses the incident radiation onto the detecting element) laboratory applications, the background-limited NEP of a bolometer is given by... 

From bolometer theory (1) the change in film temperature is proportional to the absorbed power and thermal resistance (inverse of the thermal conductance) and is given by the following ... 

For the bolometer detector having the above characteristics and operating withf 1 optics (/ = 1) the peak to peak temperature change of the a-Si film is approximately 3 mK per one degree kelvin change in scene temperature. 

A bolometer is essentially a thin blackened platinum strip in an evacuated glass vessel with a window transparent to the infrared rays it is connected as one arm of a Wheatstone bridge, and any radiation absorbed raises the temperature of the strip and changes its resistance. Two identical elements are usually placed in the opposite arms of a bridge one of the elements is in the path of the infrared beam and the other compensates for variations in ambient temperature. Both the above receptors give a very small direct current, which may be amplified by special methods to drive a recorder. 

Up to now, in the formulation of a bolometer model, only the heat capacity of itinerant carriers was considered [57], However, our measurements show that, even at 24 mK, the presence of a spurious heat capacity in the thermometer increases the expected value of the pulse rise time. We expect that the spurious contribution in Fig. 12.17 increases down to the temperature of the Schottky peak at T = k.E/khT about 10 mK. Since gc decreases at low temperatures, the total effect on pulse rise time and pulse amplitude can be dramatic at lowest temperatures. In reality, the measured rise time of CUORICINO pulses is about three times longer than that obtained from a model which neglects the spurious heat capacity of the thermistor. For the same reason, also the pulse amplitude is by a factor two smaller than the expected value (see Section 15.3.2). 

In a cryogenic experiment, one or several detectors are used for a definite goal for which they have been optimized. For example, in CUORE experiment described in Section 16.5, the sensors are the Ge thermistors, i.e. thermometers used in a small temperature range (around 10 mK). One detector is a bolometer made up of an absorber and a Ge sensor. The experiment is the array of 1000 bolometers arranged in anticoincidence circuits for the detection of the neutrinoless double-beta decay. Note that the sensors, if calibrated, could be used, as well, as very low-temperature thermometers. Also the array of bolometers can be considered a single large detector and used for different purposes as the detection of solar axions or dark matter. 

Because of their high heat capacity, only few of the thermometers described in Chapter 9 can be used as sensors for detectors. Resistance (carbon) sensors were used for the first time in a cryogenic detector by Boyle and Rogers [12] in 1959. The carbon bolometer had a lot of advantages over the existing infrared detectors [13]. It was easy to build, inexpensive and of moderate heat capacity due to the low operating temperature. 

As in the case of calorimeters, a bolometer consists of an absorbing element with heat capacity C, which converts the impinging electromagnetic radiation to heat, and which is linked to a heat sink at temperature Ts via a thermal conductance G. The temperature TA of the absorber is measured by a thermometer in thermal contact with the absorber. 

Early bolometers used, as thermometers, thermopiles, based on the thermoelectric effect (see Section 9.4) or Golay cells in which the heat absorbed in a thin metal film is transferred to a small volume of gas the resulting pressure increase moves a mirror in an optical amplifier. A historical review of the development of radiation detectors until 1994 can be found in ref. [59,60], The modern history of infrared bolometers starts with the introduction of the carbon resistor, as both bolometer sensor and absorber, by Boyle and Rogers [12], The device had a number of advantages over the Golay cell such as low cost, simplicity and relatively low heat capacity at low temperatures. 

The bolometer loses a power G (rbol — Ts) to the heat sink at temperature Ts, through the thermal conductance G. Hence, the thermal budget is given by ... 

The heat capacities are evaluated at the biased operating temperature of the bolometer (0.33 K). The heat capacity of the components that thermally connect the bolometer to the heat sink is supposed to contribute by 1/3. Hence, the total heat capacity at 330 mK should be 3.1 x l(r12J/K. 

Note about infrared radiation (IR) filters In the bolometer just described, the optimum conductance to the heat sink is G 2 x 10-10 W/K. This means that an absorbed power of the order of 1(T10 W saturates the bolometer. Since the bolometer is a broad-band detector, it would receive, e.g., a power of the order of 10 7 W from a 30 K black body. Of course, optical filtering is needed to reduce the bandwidth of the impinging radiation. Filtering takes usually place in several steps a room temperature filter eliminates visible light an intermediate temperature filter (at about 77 K) rejects the micron wavelengths, whereas the submillimetre or millimetre filter is made up of a low-pass and an interference band-pass filter. 

Infrared radiation has a very low energy and cannot eject electrons from most common photoemissive surfaces. The initial infrared sensors were temperature-sensing devices. Thermocouples and thermistors are forms of bolometers used for detecting infrared radiation. 

Detection of the middle and far range of infrared radiation requires thermal detectors, the simplest of which is a thermocouple, in which the change in temperature at one junction of the thermocouple results in a small voltage being produced. Although simple in design, thermocouples lack sensitivity. Bolometers are more sensitive and are based on the fact that as the temperature of a conductor... 

Techniques and procedures of such thermoeleastic measurements under unidirectional or uniform (hydrostatic) deformation of solid and rubberlike polymers are described in 1 64 66). Similar methods have been used more often for recording the temperature changes resulting from the plastic deformation of solid polymers. Besides thermocouples, fluorescent substances, liquid crystals and IR-bolometers are used for such measurements. 

The traditional source in IR absorption spectroscopy is a glowing rod or wire heated by the passage of an electric current the hot body emits radiation over a continuous frequency range. The radiation is dispersed using a prism NaCl, which is transparent over much of the IR region, is commonly used for IR prisms and windows. The sample may be a solid, liquid, or gas. Various detectors are used the most common are thermocouples, photoconductive materials such as PbS, bolometers (which are temperature-dependent resistors), and the Golay cell (which uses the thermal expansion of a gas contained in a chamber). 

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