A bolometer consists
of an absorptive element, such as a thin layer of metal, connected to a thermal
reservoir (a body of constant temperature) through a thermal link. The result
is that any radiation impinging on the absorptive element raises its
temperature above that of the reservoir — the greater the absorbed power, the
higher the temperature. The intrinsic thermal time constant, which sets the
speed of the detector, is equal to the ratio of the heat capacity of the absorptive element to the thermal conductance between the absorptive element and the
reservoir.[2] The temperature
change can be measured directly with an attached resistive thermometer, or the resistance of the
absorptive element itself can be used as a thermometer. Metal bolometers
usually work without cooling. They are produced from thin foils or metal films.
Today, most bolometers use semiconductor or superconductor absorptive elements rather than
metals. These devices can be operated at cryogenic temperatures, enabling significantly
greater sensitivity.
Bolometers are directly
sensitive to the energy left inside the absorber. For this reason they can be
used not only for ionizing particles andphotons, but also for non-ionizing particles, any sort
of radiation, and even to search for unknown
forms of mass or energy (like dark matter); this lack of discrimination
can also be a shortcoming. The most sensitive bolometers are very slow to reset
(i.e., return to thermal equilibrium with the environment). On the other hand,
compared to more conventional particle detectors, they are extremely efficient
in energy resolution and in sensitivity. They are also known as thermal
detectors
Conceptual schematic of a bolometer. Power P from an incident signal is absorbed by
the bolometer and heats up a thermal mass with heat capacity C and temperature T. The thermal mass is connected to a reservoir of constant
temperature through a link with thermal conductanceG. The temperature increase
is ΔT =P/G.
The change in temperature is read out with a resistive thermometer. The
intrinsic thermal time constant is τ =C/G.
The term bolometer is also used in particle physics to designate an
unconventional particle detector. They use the same
principle described above. The bolometers are sensitive not only to light but
to every form of energy. The operating principle is similar to that of a calorimeter in thermodynamics. However, the approximations, ultra low temperature,
and the different purpose of the device make the operational use rather
different. In the jargon of
high energy physics, these devices are not called calorimeters since this term
is already used for a different type of detector (see Calorimeter
(particle physics)). Their use as particle
detectors is still at the developmental stage. Their use as particle detectors
was proposed from the beginning of the 20th
century, but the first regular, though pioneering, use was only in the 1980s
because of the difficulty associated with cooling and operating a system at cryogenic temperature.
source : http://en.wikipedia.org/wiki/Bolometer
source : http://en.wikipedia.org/wiki/Bolometer
