Large Area Microcalorimeters of the Diffuse Large Area Microcalorimeters of the Diffuse X-ray BackgroundX-ray Background

Sarah BankSarah Bank

Towson UniversityTowson University

August 5, 2004August 5, 2004

HistoricallyHistorically

The first astrophysical x-rays were found The first astrophysical x-rays were found using Geiger counters (1962) - detect using Geiger counters (1962) - detect event only, not energyevent only, not energy

Subsequent use of proportional counters Subsequent use of proportional counters allowed for rough spectra to be takenallowed for rough spectra to be taken

Calorimeters are efficient, and can make Calorimeters are efficient, and can make use of many different absorber materialsuse of many different absorber materials They allow for fine energy divisions, even at They allow for fine energy divisions, even at

low energies - particularly with absorption low energies - particularly with absorption edge filtersedge filters

Absorption Edge Filter Absorption Edge Filter ResponseResponse

Astrophysical-Journal. vol. 576, no.1, pt.1; 10 Sept. 2002, pp. 188-203

What is a Microcalorimeter?What is a Microcalorimeter?

• AbsorberAbsorber

• Sensitive Thermometer - Sensitive Thermometer - thermistor/TESthermistor/TES

• Heat Sink connected by a Heat Sink connected by a weak thermal linkweak thermal link

• X-rays are detected as a X-rays are detected as a heat pulse whose height is heat pulse whose height is dependant on the energy of dependant on the energy of the eventthe event

http://phonon.gsfc.nasa.gov/intro/intro.html

The RefrigeratorThe Refrigerator An ADR is used to keep the absorber at An ADR is used to keep the absorber at

~60mK ~60mK The low temperature makes the small temp The low temperature makes the small temp

increase of the x-ray comparatively largeincrease of the x-ray comparatively large Magnetic field is applied to a salt pill, aligning Magnetic field is applied to a salt pill, aligning

the spinsthe spins A pressurized liquid helium heat sink cools it A pressurized liquid helium heat sink cools it

to 1.4 Kto 1.4 K Gradual ramp down of the magnet releases Gradual ramp down of the magnet releases

the spins from alignment, thus taking in the spins from alignment, thus taking in energy and further cooling the detectorenergy and further cooling the detector

How to Make an AbsorberHow to Make an AbsorberEvaporative deposition:Evaporative deposition: The desired absorber material is The desired absorber material is

heated to evaporation heated to evaporation temperature within a vacuum temperature within a vacuum chamberchamber

The Si substrate is suspended The Si substrate is suspended above itabove it

The cooler substrate allows the The cooler substrate allows the vapor to “condense” onto itvapor to “condense” onto it

Unfortunately, Pb tends to form Unfortunately, Pb tends to form “islands” leaving a rough surface“islands” leaving a rough surface

Lead IslandsLead Islands

Solution - Substrate HeaterSolution - Substrate Heater

The substrate heater fits inside of the The substrate heater fits inside of the vacuum chamber and consists of:vacuum chamber and consists of: a copper block a copper block heaterheater thermistor - for regulating the temperaturethermistor - for regulating the temperature

The deposited lead is reheated within the The deposited lead is reheated within the vacuum to anneal the surface (like shaking vacuum to anneal the surface (like shaking a box full of blocks)a box full of blocks)

Smooth surface = better absorberSmooth surface = better absorber

What to absorb?What to absorb? Non-thermal emission - synchrotron radiation Non-thermal emission - synchrotron radiation

from relativistic electrons, and inverse Compton from relativistic electrons, and inverse Compton scatteringscattering

What to absorb?What to absorb? Thermal emission -bremsstrahlung, and Thermal emission -bremsstrahlung, and

line emissionline emission Charge Transfer Charge Transfer

(a special case)(a special case)

What to Absorb?What to Absorb? Charge Exchange occurs between metallic Charge Exchange occurs between metallic

ions in the solar wind, such as C VI, and ions in the solar wind, such as C VI, and neutral ISM flowing into the solar system.neutral ISM flowing into the solar system.

http://pluto.space.swri.edu/IMAGE/glossary/charge_exchange.html

The Local BubbleThe Local Bubble

At 1/4 keV, contributions from the At 1/4 keV, contributions from the SXRB are modeled after an irregular SXRB are modeled after an irregular local bubble of hot, low density gas local bubble of hot, low density gas

Also irregular contributions from a halo Also irregular contributions from a halo componentcomponent

Possible contribution from charge Possible contribution from charge exchange between ions in the solar exchange between ions in the solar wind and neutral ISM material - focus wind and neutral ISM material - focus of next rocket flightof next rocket flight

The Next GenerationThe Next Generation Past absorbers were 1 mmPast absorbers were 1 mm22

arranged in a 2 by 16 arrayarranged in a 2 by 16 array The next flight will be equipped The next flight will be equipped

with an array of 6 by 6 absorbers with an array of 6 by 6 absorbers that are 2mmthat are 2mm22

More area = more counts, high More area = more counts, high spatial resolution isn’t particularly spatial resolution isn’t particularly necessary for measurements of necessary for measurements of the diffuse X-ray backgroundthe diffuse X-ray background

Higher spectral resolution may distinguish betweenHigher spectral resolution may distinguish between CVI from thermal emission vs from chargeCVI from thermal emission vs from charge exchange due to solar windexchange due to solar wind

Astrophysical-Journal. vol. 576, no.1, pt.1; 10 Sept. 2002, pp. 188-203

Rocket DataRocket Data

~100 counts total for last rocket flight~100 counts total for last rocket flight Next rocket flight expected to quadruple the total Next rocket flight expected to quadruple the total

number of counts collectednumber of counts collected Astrophysical-Journal. vol. 576, no.1, pt.1; 10 Sept. 2002, pp. 188-203

AcknowledgementsAcknowledgements Dr. Dan McCammonDr. Dan McCammon

Lindsay RocksLindsay Rocks Emily BarrentineEmily Barrentine

And everyone in the Space Physics GroupAnd everyone in the Space Physics Group

Astrophysical-Journal. vol. 576, no.1, pt.1; 10 Sept. 2002, pp. 188-203