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
Grazing Incidence Optics
The Lab 1.Shine X-rays on sample 2.Measure energies of fluorescent X-rays 3.Determine elements in sample
Silicon X-Ray Detector X-Ray Generator
Setup Preamp Multichannel analyzer X-ray source Target Si X Xe-e- 1. Calibrate MCA eV/channel: Measure spectra of known targets 2. Determine composition of unknown target: Measure spectrum and identify lines.