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Fall 2007 - X-rays and detectors lecture

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Fall 2007 - X-rays and detectors lecture

1. 1. 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
2. 2. 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
3. 3. Photons Energy of photon is set by frequency/wavelength λ ν hc hE == )Angstroms( 4.12 )keV( λ =E Unit is electon-volt (eV or keV) 1 eV = 1.6×10-19 J = 1.6×10-12 erg
4. 4. Thermal Radiation Thermal spectrum peaks at 2.7 kT, falls off sharply at higher and lower energies. Wien’s Law: Peak of radiation = 2.9×107 Å/ T(K) = (0.43 keV) ×(T/106 K)
5. 5. 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 = 4πR2 σT4 1000 1 000,100 000,700 100 4/12/14/12/1 =            =            = −− B A B A B A L L R R T T TA = 1000 × 5700 K ~ 6,000,000 K Peak at 4.8 Å = 2.6 keV
6. 6. Atomic lines Link to tables of line energies Photons emitted from transitions to inner electron shells are in the X-ray band
7. 7. Non-thermal processes Particle acceleration in magnetic fields • Supernova remnants • Corona of black hole accretion disks • Radiation from pulsars • Jet acceleration by black holes
8. 8. X-Ray Detectors • Usually detect each individual photon • Wish to measure photon properties – Energy – Number – Time of arrival – Position – Polarization
9. 9. Solid State X-ray Detectors X-ray interacts in material to produce photoelectrons which are collected by applying a drift field
10. 10. 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: σN 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 E wF NE E N 35.235.2 == ∆ σ For silicon, F = 0.115, w = 3.62 eV. Energy resolution is often degraded by electronic noise.
11. 11. Quantum Efficiency To be detected, X-ray must pass through window without being absorbed and then be absorbed in gas                 −−      −= gw w dt TQ λλ exp1exp Tw is geometric open fraction of window, t is window thickness, d is gas depth, λ’s are absorption length for window/gas (energy dependent)
12. 12. Charge Coupled Devices
13. 13. 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
14. 14. X-Ray Reflectivity
15. 15. Grazing Incidence Optics
16. 16. The Lab 1. Shine X-rays on sample 2. Measure energies of fluorescent X-rays 3. Determine elements in sample
17. 17. Silicon X-Ray Detector X-Ray Generator
18. 18. 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.