A microcalorimeter is a sensitive thermal device used to measure the energy of single particles or photons, elementary particles of light. It is a type of calorimeter — an instrument that measures the heat released by physical or chemical reactions in a sample. Microcalorimeters are used in astrophysics to measure the energy of X-ray photons from space. A related device, the isothermal microcalorimeter, is used in biochemistry and related fields to detect tiny energy changes at low temperatures.
The law of conservation of energy, a basic law of physics, states that energy cannot be created or destroyed — it can be only converted into other forms. Microcalorimeters work based on this principle. Energy from a physical interaction or chemical reaction is transformed to heat inside the system, and by measuring the change in heat that results, the energy of the interaction can be deduced.
The type of microcalorimeter used in astrophysics consists of three main components: an absorber, a heat sink, and a thermistor. When an X-ray photon strikes the absorber, energy is transferred to an electron in an atom of the absorber material. This energy causes the electron to become excited — it jumps further from the atomic nucleus and breaks free of orbit. Other electrons in the absorber may become excited to lesser degrees by this loose electron, rising to higher energy orbits around their respective atoms.
The excited electrons release energy when they return to their ground state, or lowest energy state — a stable orbit around the atoms. The energy released in this process is conserved and converted to heat, causing the temperature in the absorber to rise by a tiny amount. A thermometer device in the absorber known as a thermistor detects this temperature change. The heat then flows into the heat sink, causing the absorber to return to its original temperature. By measuring the temperature change caused by the impact of the X-ray, the original energy of the X-ray can be calculated.
The isothermal microcalorimeter works in much the same way, although it is used to measure chemical interactions rather than photon energy. This device consists of a heat sink and a closed reaction vessel in which the chemical reaction takes place. The heat sink ensures that the reaction vessel is kept at a constant temperature, allowing for exact measurements. When the chemical reaction occurs, a certain amount of energy is either released as heat or absorbed, causing a change in temperature that is registered by the microcalorimeter. Isothermal microcalorimeters have applications in physical chemistry, biochemistry, and the pharmaceutical industry because they provide a highly sensitive way to analyze heat flow in a reaction.
Microcalorimeters must operate at low temperatures so that the minute changes in heat that they measure can register. For example, the devices used in astrophysics are maintained at close to absolute zero. At this temperature, even the small change in thermal energy caused by the impact of a single photon can be detected. Isothermal microcalorimeters are not as extreme, but are still maintained at much lower temperatures than macro-scale calorimeters.