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Magnetic calorimeter arrays for x-ray astronomy

A new class of detectors has great potential as x-ray cameras for future astrophysical observatories, with unprecedented energy resolution and array sizes greater than one megapixel.
4 February 2010, SPIE Newsroom. DOI: 10.1117/2.1201002.002620

Ever since the early 1980s, microcalorimeters were thought to have the potential to determine the energy of an x-ray photon with an accuracy of 1eV, a goal that promises to greatly enhance our understanding of astrophysics.1 A microcalorimeter detector at the focus of a high-angular-resolution x-ray telescope is a unique instrument because of its ability to simultaneously determine the energy of each x-ray photon extremely precisely while also producing x-ray images of various astronomical sources with very high detection efficiency. By studying the precisely measured distribution of x-ray photon energies emitted by various astronomical objects, we learn important information about them such as their composition, temperature, density, and dynamics. We can study a broad spectrum of astrophysical sources, such as the matter-accretion process near black holes, supernovae remnants, and the growth and evolution of galaxies and galaxy clusters. Since the invention of microcalorimeters, scientists around the world have developed ever more sensitive and innovative techniques to detect photons.2 At present, microcalorimeters with superconducting transition-edge sensors (TESs) hold the record for the best energy resolution (1.8eV) for detecting a 6keV x-ray.3 Metallic magnetic microcalorimeters (MMCs) are now on the verge of demonstrating the first ever ‘sub-eV’ energy-resolution x-ray detector.

MMCs use magnetism to produce a high-precision temperature sensor. The MMCs that have been developed use the paramagnetic susceptibility of gold doped with a low concentration of erbium ions (Au:Er) when placed in a dc magnetic field.4 In a paramagnet, the magnetization is inversely proportional to temperature, making it very sensitive to small changes at low temperatures. The paramagnet is attached to an x-ray absorber as depicted in Figure 1. When an incoming x-ray hits the microcalorimeter's absorber, its energy is converted into heat, which a thermometer then measures. The temperature rise is directly proportional to the x-ray's energy and is approximately 0.0005 Kelvin for a 6keV photon in a calorimeter operated at a temperature of 0.04 Kelvin.

Figure 1. Schematic representation of a magnetic calorimeter. SQUID: Superconducting quantum interference device.

During the development of MMCs it became clear that the full potential of high-resolution detectors would only be realized by microfabrication techniques. Groups at Heidelberg University (Germany) and my group at NASA's Goddard Space Flight Center (GSFC) in collaboration with George Seidel at Brown University have led efforts to develop fully microfabricated detectors and detector arrays. Techniques have been devised to make the best geometries for sensing the change in magnetic susceptibility using a meander-shaped pickup coil connected to a current sensor that is a specially designed state-of-the-art superconducting quantum interference device (SQUID).5 On top of these coils we sputter-deposit 1μm-thick films of Au:Er to produce the MMC sensor, and then fabricate cantilevered, high-quantum-efficiency x-ray absorbers consisting of a bismuth-gold bilayer (see Figure 2).6

Figure 2. (left) Scanning-electron-microscope image showing how the absorbers of a 5×5 array of pixels are cantilevered above the substrate. (right) Photograph of an individual pixel. The pattern of the underlying meander-shaped pickup is visible through the Au:Er film and the absorber. The central pickup is also the region on a silicon nitride membrane that supports the cantilevered absorber.

We have tested the thermodynamic properties of the MMC, in particular the magnetic susceptibility and the heat capacity, and these properties are now approaching those that have been determined for an MMC made using bulk Au:Er. The presently best-achieved energy resolution at 6keV is ΔEFWHM (full width at half maximum) =2.8eV in Heidelberg and ΔEFWHM=3.3eV at GSFC7 (as shown in Figure 3). These arrays of MMCs have some flaws in their fabrication (GSFC) and testing (Heidelberg) that are now well characterized and understood at both institutions, and can be overcome. In the next generation, implementing a modified fabrication technique, we anticipate that the energy resolution will break the 1eV barrier.

Figure 3. Spectrum of manganese Kα x-rays from an iron-55 source. The light blue line shows the intrinsic line shape, and the broadening of this shape is consistent with a metallic magnetic microcalorimeter (MMC) energy resolution of 3.28eV. FWHM: Full width at half maximum.

Figure 4. (top left) Schematic drawing showing the layout of a position-sensitive MMC. (top right) Photograph of such a device. (bottom) Measured average pulse shapes for x-ray events at the different absorbers. After the initial equilibration signal, the pulses decay with the same exponential time constant. The differences in the rise times and pulse shapes allow us to determine which element the x-ray was absorbed by.

To make some of the more advanced future concepts for astrophysics possible, such as Generation-X,8 it is desirable to have both sub-eV energy resolution and a dramatic increase in the size of arrays. Ideally, we will develop megapixel arrays of tiny pixels just as in everyday digital cameras. One of the key advantages of using MMC-based microcalorimeters is that no heat is dissipated within the pixels associated with the process to measure the change in magnetic susceptibility. The significance of this is that MMCs can be more easily scaled up to megapixel-sized arrays.

To build such arrays, one of the techniques being pursued is the use of position-sensitive MMCs. In this device multiple absorbers, each with a different thermal conductance, are coupled to one magnetic sensor. This results in different pulse shapes (see Figure 4), which enables position discrimination. We have fabricated and tested a position-sensitive magnetic calorimeter in which a single sensor was used to read out four absorbers. An energy resolution of less than 4.7eV was observed for 6keV x-rays in each of the absorbers, which is the present record for any position-sensitive microcalorimeter at this energy.9 Improved energy resolution is also expected for the next generation of designs for these devices.

In summary, MMC-based microcalorimeter arrays hold great potential for meeting the requirements of advanced mission concepts such as Generation-X, which has a goal of a focal-plane detector with less than 1eV energy resolution over a million pixels. The energy resolution that has been achieved in small arrays of single pixels and position-sensitive detectors is already competitive with other state-of-the-art technologies, and further progress is likely in the very near future from microfabricating arrays of pixels that have the same magnetic properties as has been seen in previous detectors with bulk magnetic samples. Our next steps are to build and test MMC arrays that will attempt to demonstrate the further energy-resolution potential, before moving on to increase the size of arrays to over a thousand pixels.

The X-ray Microcalorimeter Group at Goddard is led by Richard Kelley, Caroline Kilbourne, and F. Scott Porter, and includes Joseph Adams, Catherine Bailey, Simon Bandler, Meng Chiao, Megan Eckart, Fred Finkbeiner, Nikhil Jetvala, Jan-Patrick Porst, Jack Sadleir, and Stephen Smith. Wen-Ting Hsieh was the lead engineer responsible for fabricating the devices described.

Simon Bandler
Greenbelt, MD

Simon Bandler is an associate research scientist at the University of Maryland who works in the X-ray Astrophysics Laboratory at NASA's GSFC. He received his PhD in physics from Brown University in May 1995. He has also worked at Heidelberg University (Germany) and the Smithsonian Astrophysical Observatory.