XRay Astronomy Lab Xrays Why look for Xrays
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 (e. V or ke. V) or wavelength units (Angstroms) • Soft X-rays = 0. 1 -2 ke. V • Medium (“standard”) X-rays = 2 -10 ke. V • Hard X-rays 20 -200 ke. V
Photons Energy of photon is set by frequency/wavelength Unit is electon-volt (e. V or ke. V) 1 e. V = 1. 6 10 -19 J = 1. 6 10 -12 erg
Thermal Radiation Thermal spectrum peaks at 2. 7 k. T, falls off sharply at higher and lower energies. Wien’s Law: Peak of radiation = 2. 9 107 Å/ T(K) = (0. 43 ke. V) (T/106 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 TA = 1000 5700 K ~ 6, 000 K Peak at 4. 8 Å = 2. 6 ke. V
Atomic lines Photons emitted from transitions to inner electron shells are in the X-ray band Link to tables of line energies
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 Xray, w = average ionization energy (3. 62 e. V 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 e. V. 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 Tw 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 1. Shine X-rays on sample 2. Measure energies of fluorescent X-rays 3. Determine elements in sample
X-Ray Generator Silicon X-Ray Detector
Setup X-ray source X Target X e- Si Preamp 1. Calibrate MCA e. V/channel: Measure spectra of known targets 2. Determine composition of unknown target: Measure spectrum and identify lines. Multichannel analyzer
- Slides: 18