Large Area Microcalorimeters of the Diffuse X-ray Background Sarah Bank Towson University August 5, 2004

Historically  The first astrophysical x-rays were found using Geiger counters (1962) - detect event only, not energy  Subsequent use of proportional counters allowed for rough spectra to be taken  Calorimeters are efficient, and can make use of many different absorber materials  They allow for fine energy divisions, even at low energies - particularly with absorption edge filters

Absorption Edge Filter Response Astrophysical-Journal. vol. 576, no.1, pt.1; 10 Sept. 2002, pp

What is a Microcalorimeter? Absorber Absorber Sensitive Thermometer - thermistor/TES Sensitive Thermometer - thermistor/TES Heat Sink connected by a weak thermal link Heat Sink connected by a weak thermal link X-rays are detected as a heat pulse whose height is dependant on the energy of the event X-rays are detected as a heat pulse whose height is dependant on the energy of the event

The Refrigerator  An ADR is used to keep the absorber at ~60mK  The low temperature makes the small temp increase of the x-ray comparatively large  Magnetic field is applied to a salt pill, aligning the spins  A pressurized liquid helium heat sink cools it to 1.4 K  Gradual ramp down of the magnet releases the spins from alignment, thus taking in energy and further cooling the detector

How to Make an Absorber Evaporative deposition:  The desired absorber material is heated to evaporation temperature within a vacuum chamber  The Si substrate is suspended above it  The cooler substrate allows the vapor to “condense” onto it  Unfortunately, Pb tends to form “islands” leaving a rough surface

Lead Islands

Solution - Substrate Heater  The substrate heater fits inside of the vacuum chamber and consists of:  a copper block  heater  thermistor - for regulating the temperature  The deposited lead is reheated within the vacuum to anneal the surface (like shaking a box full of blocks)  Smooth surface = better absorber

What to absorb?  Non-thermal emission - synchrotron radiation from relativistic electrons, and inverse Compton scattering

What to absorb?  Thermal emission -bremsstrahlung, and line emission  Charge Transfer (a special case)

What to Absorb?  Charge Exchange occurs between metallic ions in the solar wind, such as C VI, and neutral ISM flowing into the solar system.

The Local Bubble  At 1/4 keV, contributions from the SXRB are modeled after an irregular local bubble of hot, low density gas  Also irregular contributions from a halo component  Possible contribution from charge exchange between ions in the solar wind and neutral ISM material - focus of next rocket flight

The Next Generation  Past absorbers were 1 mm 2 arranged in a 2 by 16 array  The next flight will be equipped with an array of 6 by 6 absorbers that are 2mm 2  More area = more counts, high spatial resolution isn’t particularly necessary for measurements of the diffuse X-ray background  Higher spectral resolution may distinguish between CVI from thermal emission vs from charge CVI from thermal emission vs from charge exchange due to solar wind exchange due to solar wind Astrophysical-Journal. vol. 576, no.1, pt.1; 10 Sept. 2002, pp

Rocket Data  ~100 counts total for last rocket flight  Next rocket flight expected to quadruple the total number of counts collected Astrophysical-Journal. vol. 576, no.1, pt.1; 10 Sept. 2002, pp

Acknowledgements  Dr. Dan McCammon  Lindsay Rocks  Emily Barrentine And everyone in the Space Physics Group

Astrophysical-Journal. vol. 576, no.1, pt.1; 10 Sept. 2002, pp