Transcript Detection of X-Rays

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
E  h 
hc

Unit is electon-volt (eV or keV)
1 eV = 1.610-19 J = 1.610-12 erg
12.4
E (keV) 
(Angstroms)
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)
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R2T4
TA  RA 
  
TB  RB 
1/ 2
1/ 4
 LA 
 
 LB 
 100 


 700,000
1/ 2
 100,000


1 

TA = 1000  5700 K ~ 6,000,000 K
Peak at 4.8 Å = 2.6 keV
1/ 4
 1000
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 eV for Si)
Creation of photoelectrons is a random process, number fluctuates
Variance of N: N2 = 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
N
E
wF
 2.35
 2.35
E
N
E
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
 d
 t 
1  exp 
Q  Tw exp 



w 
g







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
e-
X
Si
Preamp
1. Calibrate MCA eV/channel:
Measure spectra of known targets
2. Determine composition of
unknown target:
Measure spectrum and identify
lines.
Multichannel
analyzer