Instrumentation in X-Ray Astrophysics
Advisers:
Dr. Mark Lindeman, Prof. Dan McCammon
Instrumentation is a multidisciplinary field, and in the summer of 2007 I was able to experience all of its different aspects, ranging from operation of lab equipment, assembling parts, to testing devices and analyzing the data. While working at University of Wisconsin-Madison, with Prof. Dan McCammon’s X-Ray Astrophysics group, Transition Edge Sensor (TES) calorimeters were the focus of my research.
Detecting X-Rays
X-Ray Astrophysics proves challenging. The atmosphere obscures majority of the X-Rays, and hence detectors have to be put into space, either via satellites (Chandra) or sounding rockets. The rocket provides a 5 minute observation time, which allows testing the detectors and giving enough time to collect valuable data.
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TES Calorimeter
TES basics 1
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A calorimeter contains an absorbing material, weakly coupled to a heat sink. When a photon hits the detector, the absorber heats up and the change in temperature is recorded. This allows for a precise and fast measurement of photon's energy.
A Transition Edge Sensor (TES) is a calorimeter which utilizes superconductivity. A superconducting material becomes resistanceless below a certain critical temperature, usually on the order of 0.1 K. In the transition zone between the superconducting and normal state the resistance changes very rapidly relatively to temperature, and hence the device is sensitive to extremely small changes in temperature. This property makes the TES an excellent detector, capable of measuring energy from one photon, as opposed to average power from radiation.
TES transition zone2
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SQUID's
In order to read out the small resistance in the TES, one can measure the voltage or current across the circuit. Extremely small currents can be read out by SQUIDs – Superconducting Quantum Interference Devices. Understanding how SQUIDs work requires knowledge of Quantum Mechanics, and Solid State physics.
SQUID3
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In the most basic terms, the SQUID detects a minute magnetic field created by the small currents one is trying to measure. In order to counteract the applied magnetic field, supercurrents flow through the SQUID’s coils. These currents can then be detected by the change in voltage across the device. However, the process of detection involves Josephson junctions, flux quantization and Cooper pairs, and the full explanation would go beyond the focus of my research experience.
A more detailed description can be found in
this presentation which I gave during the summer.
In the lab experiments, I used both a single-stage (containing one SQUID), and two-stage (with one SQUID and a SQUID array for greater amplification) systems. In the two-stage system the response of the single SQUID to input signal travels to inductance coils for detection by the array. Then the array’s response, send back to the single SQUID as feedback signal, cancels out the input signal. Hence the single SQUID is locked, which linearizes the recorded output. The feedback signal, amplified by the array, is then recorded by external devices.
Two-stage SQUID setup5
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Keeping it Cool with ADR Fridges
Superconducting devices, TES’s and SQUID’s , require very low temperatures in order to operate. In the lab, we use a couple of fridges which bring the temperatures down to 40mK. As part of my research, I learned how to maintain, operate, and cool down such fridges.
Pulse tube ADR4
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I used two fridges in the lab, which bring temperatures down to roughly 3K. One of them, the “dry fridge” uses a pulse-tube and mechanical compression. More information about this system can be found
here. The other fridge uses cryogens – both liquid Nitrogen and liquid Helium, but operates quietly as opposed to the pulse-tube fridge.
Once below 4K, both of the fridges utilize Adiabatic Demagnetization, from which they get their name (ADR for Adiabatic Demagnetization Refrigerator). This process uses quantum properties (spin) of paramagnetic salts inside the fridge, and brings the bath temperature down to 40mK. For more information on ADR see
this website.
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Hands-on Experience
As opposed to other types of research in Astrophysics, instrumentation requires hands-on work. One has to prepare experimentation tools, or improve on the existing ones.
ESD, heat sink
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As a student assistant, I was introduced to the machine shop, and learned how to use some basic tools, such as power drills, cutters, and hole-punchers. I was also able to solder-in connections and resistors, along with building an ESD (Electrostatic Discharge) box, and gold-plating several heat-sinks. The ESD box is crucial for Ohming-out (measuring the resistance across connections to see if everything is connected and working right) more delicate devices, such as SQUID’s. These are essential skills to any experimentalist.
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Running Experiments
The main goal of running the experiments in the lab is to test out the devices, and characterize them. Having a good characterization allows for better data analysis once real data is collected.
This summer I was able to learn how to run a whole experiment on the pulse-tube ADR, including installation of devices, preparing the ADR for a run (using a vacuum pump), testing connections (and re-connecting them as needed), and handling of delicate parts. I also learned how to operate the two-stage SQUID system through a specialized software together with an oscilloscope, which permitted me to characterize the SQUID output.
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TES Characterization
My main project this summer was to characterize and find thermal conductance G of a TES device. For this experiment, Lab ADR was used, containing a single SQUID for read-out. In the representative circuit below, I put a bias voltage, and measured the SQUID response as the temperature varied between 90mK and 200mK. This in turn let me determine the resistance of the TES, the power through it, and eventually its conductance. All the calculations and plots were done using IDL.
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X-Ray Astrophysics
Single detection timescale6
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Each time an X-Ray photon strikes the absorber on a detector, a pulse output results, similar to the one picture on the left. Even thought the sounding rocket allows only 5 minutes time of observation, numerous detections can be made since the absorber, thermally coupled to a heat sink, reaches its equilibrium temperature quickly, and is ready for another detection. TES detectors are also normally set up in arrays.
The UW-Madison X-Ray Astrophysics group is preparing for another rocket launch scheduled for the upcoming September. Data shall be taken in soft X-Rays, and the main focus of the launch is to observe the diffuse X-Ray background, unknown in origin. The group is also interested in solar X-Rays, and whether the emission from the sky changes in time.
In the future, Constellation-X and HIMS project should shed some light on the large-scale structure of the universe, and possibly the missing baryons. Astro-E2 and Constellation-X will also study black holes and other high-energy phenomena. More information on the past, current and future projects in which UW-Madison was involved can be found on the X-Ray Astrophysics group website -
http://wisp11.physics.wisc.edu/xray/xr_experiments.htm.
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References
Images:
1. http://constellationx.nasa.gov/public/instruments/xms.html
2. http://web.mit.edu/figueroagroup/ucal/ucal_tes/index.html
3. squid
4. http://www.janis.com/news0701.html
5. STAR Cryoelectronics, Manual
6. single pulse
Agnieszka Czeszumska 