Cryogenic Microcalorimeters

For Starters

Your opponent's serve was almost perfect, but you vigorously returned it beyond his outstretched raquet to win the point. Now the tennis ball sits wedged in the chain-link fence around the court. What happened to the ball's kinetic energy? It has gone to heat the fence, and you realize that if the fence were quite a bit colder, you might be able to measure that heat and determine just how energetic your swing really was.
Calorimetry has been a standard measurement technique since James Joule and Julius von Mayer independently concluded, about 150 years ago, that heat is a form of energy. But only in the past 15 years or so has calorimetry been applied, at millikelvin temperatures, to the measurement of the energy of the individual photons and particles with exquisite sensitivity. 

The picture here depicts the basic components of a microcalorimeter. 

An X-ray microcalorimeter is composed of three parts, an absorber that converts the energy of the incident x-rays into heat, a thermometer that detects the temperature variations of the absorber and a weak thermal link between the detector and a heat sink. The operating principle is simple. When an x-ray hits the absorber its energy is thermalized, that is, is distributed among thermal phonons, and the temperature of the detector first rises and then returns to its original value due to the weak thermal link to the heat sink. The temperature change is proportional to the energy of the incident x-ray and is detected by the thermometer. The thermometer is generally a resistor whose resistance has a strong dependence on the temperature at the working point.

 The Old Way of Doing Things

In the precalorimeter era, the choice between wavelength dispersive devices (such as Bragg crystals or grazing incidence diffraction gratings) and nondispersive spectrometers (solid state detectors or proportional counters) presented a dilemma. Dispersive spectrometers offer a very good energy resolution, but at low throughput. Nondispersive spectrometers, on the other hand, have very high efficiency, but relatively poor resolution.

The Advantages of the New

Microcalorimeters provide a number of advantages over these methods. Detectors can be relatively large and still be sensitive to small amounts of deposited energy. This energy is sensed after it has been converted to heat, so that even interactions that produce little or no ionization can be detected. Another useful property is that thermal detectors do not depend on the charge transport properties of the absorber. Only a very few materials can be used to make ionization detectors, while a calorimeter can incorporate a wide variety of materials. 

The Basics of X-ray Calorimetry:

  • The calorimeter should operate at a low temperature so that the energy deposited is large relative to the thermodynamically unavoidable random transfer of heat across the weak link. 
  • The absorber should be opaque to X-rays, and yet have a low heat capacity so that a small deposition of energy is translated into a measurable temperature change. 
  • The absorber must thermalize well -that is, it must reproducibly and efficiently distribute the energy of the initial photon across a thermal distribution of phonons (or electrons depending on the thermometer). 
    • <>The thermometer must be highly sensitive to temperature changes.
    And the thermal link should be weak enough such that the time for the base temperature to be restored is the slowest time constant in the system, yet not too slow otherwise the device cannot handle the incident x-ray flux. 

Microcalorimeters Put to Use

With the use of arrays of microcalorimeters it is possible to extend the limits of a singe microcalorimeter, and create a better detector. A typical array for the XQC experiment is composed of 36 elements, each with a collecting area of 1 mm2, for a total collecting area of 0.36 cm2. Each pixel is composed of a silicon thermistor implanted in a micro-machined silicon chip, thermally connected to a HgTe absorber. The absorber is 0.75 micrometers and has more than 99% quantum efficiency of the detector below 1 keV. The detector is installed in an adiabatic demagnitization refrigerator at a base temperature of 60 mK. 

Where is this Used and Where is it Going?

X-ray astrophysics is one of the fields where most of the efforts in the development of cryogenic microcalorimeters are spent. The first microcalorimeters for X-ray astronomy were developed by the University of Wisconsin / NASA Goddard Space Flight Center collaboration. They were first employed on a sounding-rocket experiment (the X-ray quantum calorimeter - XQC) that had an array of microcalorimeters for the study of X-rays from the interstellar medium in the energy range 30-1000 eV. 
Much effort now goes into creating second generation detectors capable of improving this performance. A major project is aimed at the development of large detectors arrays (more than 1000 elements) with Transition Edge Sensor (TES) thermometers.