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# Fall 2007 - X-rays and detectors lecture

## on Nov 29, 2010

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## Fall 2007 - X-rays and detectors lecturePresentation Transcript

• X-Ray Astronomy Lab
• X-rays
• Why look for X-rays?
• High temperatures
• Atomic lines
• Non-thermal processes
• X-ray detectors
• X-ray telescopes
• The Lab
• X-rays
• Measure X-ray energies in energy units (eV or keV) or wavelength units (Angstroms)
• Soft X-rays = 0.1-2 keV
• Medium (“standard”) X-rays = 2-10 keV
• Hard X-rays 20-200 keV
• Photons
• Energy of photon is set by frequency/wavelength
Unit is electon-volt (eV or keV) 1 eV = 1.6  10 -19 J = 1.6  10 -12 erg
• Thermal Radiation Thermal spectrum peaks at 2.7 kT, falls off sharply at higher and lower energies. Wien’s Law: Peak of radiation = 2.9  10 7 Å / T(K) = (0.43 keV)  (T/10 6 K)
• Black holes make X-rays
• BH of 10 solar masses can have a luminosity of 100,000 times the Sun’s emitted from a region ~ 200 km in radius
• Use Stefan-Boltzman law to find temperature, L = 4  R 2  T 4
T A = 1000  5700 K ~ 6,000,000 K Peak at 4.8 Å = 2.6 keV
• Atomic lines Link to tables of line energies Photons emitted from transitions to inner electron shells are in the X-ray band
• Non-thermal processes
• Particle acceleration in magnetic fields
• Supernova remnants
• Corona of black hole accretion disks
• Jet acceleration by black holes
• X-Ray Detectors
• Usually detect each individual photon
• Wish to measure photon properties
• Energy
• Number
• Time of arrival
• Position
• Polarization
• Solid State X-ray Detectors X-ray interacts in material to produce photoelectrons which are collected by applying a drift field
• Energy Resolution Number of initial photoelectrons N = E / w , where E = energy of X-ray, w = average ionization energy (3.62 eV for Si) Creation of photoelectrons is a random process, number fluctuates Variance of N:  N 2 = FN , where F is the “Fano” factor, fluctuations are lower than expected from Poisson statistics ( F = 0.17 for Ar, Xe) Energy resolution (FWHM) is For silicon, F = 0.115, w = 3.62 eV. Energy resolution is often degraded by electronic noise.
• Quantum Efficiency To be detected, X-ray must pass through window without being absorbed and then be absorbed in gas T w is geometric open fraction of window, t is window thickness, d is gas depth,  ’s are absorption length for window/gas (energy dependent)
• Charge Coupled Devices
• Pixelated Detectors CCDs have small pixel sizes, good energy resolution, and a single readout electronics channel, but are slow, thin (< 300 microns), and only made in Si. Pixelated detectors have larger pixel sizes, require many electronics channels, but are fast and can be made thick and of various materials – therefore can be efficient up to higher energies
• X-Ray Reflectivity
• Grazing Incidence Optics
• The Lab
• Shine X-rays on sample
• Measure energies of fluorescent X-rays
• Determine elements in sample
• Silicon X-Ray Detector X-Ray Generator
• Setup Preamp Multichannel analyzer X-ray source Target Si X X e - 1. Calibrate MCA eV/channel: Measure spectra of known targets 2. Determine composition of unknown target: Measure spectrum and identify lines.