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

PhotonsEnergy of photon is set by frequency/wavelength

hchE

)Angstroms(

4.12)keV(

E

Unit is electon-volt (eV or keV)

1 eV = 1.610-19 J = 1.610-12 erg

Thermal Radiation

Thermal spectrum peaks at 2.7 kT, falls off sharply at higher and lower energies.

Wien’s Law:

Peak of radiation

= 2.9107 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 = 4R2T4

10001

000,100

000,700

1004/12/14/12/1

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

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 ResolutionNumber 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: 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

E

wF

NE

E N 35.235.2

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

gww

dtTQ

exp1exp

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

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.