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Fall 2007 - X-rays and detectors lecture






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Fall 2007 - X-rays and detectors lecture Fall 2007 - X-rays and detectors lecture Presentation 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
    • Radiation from pulsars
    • 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.