Microcalorimetry Microcalorimetry is the calorimetry(is the science of measuring the heat of chemical reactions or physical changes) of small samples, specifically microgram samples (or thereabouts). In other word, Microcalorimetry is a versatile technique for studying these thermal activities in terms of heat, heat flow and heat capacity. Microcalorimetry can be completely nondestructive and non-invasive to the sample. It seldom requires any prior sample treatment nor does it limit analysis to a physical state of the sample. Solids, liquids and gases can all be investigated. Microcalorimetry does not require a sample that has a particular characteristic to enable measurement like FTIR, UV-VIS, NMR, etc. Inventor • Valerian Plotnikov, the father of microcalorimetry technology. • Albert Tian on his 91st birthday (in 1971), 50 years after his invention of heat-flow microcalorimetry Parts of Microcalorimeter A microcalorimeter is basically a thermal device made of an absorber, a thermistor, and a heat sink. The absorber must do 3 things: absorb X-rays from space efficiently, quickly, completely convert the absorbed energy into heat (thermalize the energy), and have a low heat capacity. There is no material known which excels at all 3 of these properties, so choosing the absorber material involves deciding on the best combination of them. A thermistor is a device that changes its electrical resistance dramatically with a small change in temperature. Since a thermometer is any device that measures temperature, a thermistor is a kind of thermometer. The combination of the absorber and the thermistor is called "X-ray detector". The heat sink is what absorbs heat from the detector, keeping it cool. In the case of a recently designed XRS, the heat sink used to keep the detector cool enough to work was a refridgeration unit called the Adiabatic Demagnetization Refrigerator (ADR). An ADR uses the magnetic properties of molecules in the "salt pill" to cool the detector to 60 milliKelvin (or 0.06 degrees above absolute zero). Mechanism of Microcalorimeter An X-ray photon hits the absorber and knocks an electron loose from an atom of the absorber material. This photoelectron (so-called because a photon of light knocked it loose) rattles around in the absorber, ultimately raising the temperature of the absorber by a few milliKelvin (that is, a few thousandths of one degree Kelvin). The temperature-sensitive thermistor is partially isolated from the absorber, to give the absorber time to come into equilibrium before the thermistor begins to see the temperature rise. After a few milliseconds, the thermistor comes to the same temperature as the absorber, a few milliKelvin warmer than the heat sink, which is at 65 milliKelvin. We know it's a little strange to be talking about 'heat' when something is near absolute zero! Next, the thermistor begins to cool as the heat flows out the weak link (the "legs" of the detector) to the heat sink. After a few tens of milliseconds, the thermistor has returned to its normal operating temperature. The temperature rise (delta T) measured by the thermistor is approximately proportional to the energy of the X-ray photon: delta T ~ E/C Where, delta T is the change in temperature, E is the energy of the X-ray and C is the heat capacity of the absorber. So by measuring how much the temperature changes, we can determine the energy of the X-ray. Use Microcalorimetry uses a suite of techniques to directly measure enthalpy and heat capacity changes that arise when chemical reactions occur. In aqueous solutions, heat flux into or out of the sample almost always happens on reaction. These reactions may involve a wide variety of situations, e.g., the interaction of two molecules (such as a cyclodextrin and its guest), changes in the conformation of complex macromolecules (such as proteins or DNA), or even in the structure of very complex multimolecular colloidal drug delivery systems (such as liposomes). Microcalorimeter is the a Better Way to Detect X-ray Photons In proportional counters and CCDs, the energy of the X-ray photon is shared among many electrons. Each of these electrons end up carrying a typical amount of energy, 3.65 eV in the case of the silicon-based CCDs. These electrons are then collected and counted by the electronics, and it's the number of the detected electrons that indicates the energy of the X-ray photon in a CCD detector system. An 3.65 keV X-ray photon, for example, will produce 1,000 electrons --- give or take. There is an uncertainty in the number, because the details of the X-ray - matter interaction is different each time, giving a slightly different amount of energy to each electron. The uncertainty can be estimated by taking the square root of the number of electrons --- 30 or so in this case, so the X-ray energy can be determined to an accuracy of 30/1000 ~ 3%. This is a fundamental limit of X-ray detectors that use conversion to electric charges. If you want a higher spectral resolution --- and astrophysicists always do --- you have to choose a detector that relies on a completely different principle, such as a microcalorimeter. As a result of its different approach, the microcalorimeter provides 10x better spectral resolution for detecting emission lines of iron.