CH 12 - Atomic X-ray Spectrometry

X-rays are generated in four ways:

1. by bombardment of a metal target with a beam of high-energy electrons

2. by exposure of a substance to a primary beam of X-rays to generate a secondary beam of X-ray fluorescence (XRF)

3. by use of a radioactive source whose decay process results in X-ray emission

4. from a synchrotron radiation source

X-ray sources often produce both __________ and _______ spectra.

continuum; line

Line Spectra from Electron Beam Sources

Consist of two series of lines; the shorter-wavelength group is called the K series and the other the L series.

Atomic # > 23 =

2-line series (K and L, EK >> EL)

Atomic # < 23 =

K only

Requirements for X-ray diffraction:

1. the spacing between layers of atoms must be roughly the same as the wavelength of the radiation

2. the scattering centers must be spatially distributed in a highly regular way

Three types of sources used in X-ray instruments:

1. tubes

2. radioisotopes

3. secondary fluorescence sources

X-ray Tube Sources

Highly evacuated tube in which is mounted a tungsten filament cathode and a massive anode.

There are two voltages applied to X-ray tubes that are responsible for:

1. controlling the heating of the tungsten filament (cathode)

2. controlling energy of X-rays

Radioisotope Sources

Radioactive material shoots out X-rays.

X-ray Filters (discrete)

Metal foil, small thickness (0.01 cm).

X-ray Monochromators (continuous)

A single crystal mounted on a rotatable table that permits variation and precise determination of the angle between the crystal face and the collimated incident beam.

Three types of transducers used in X-ray instruments:

-gas-filled

-scintillation counters

-semiconductor

X-ray detectors are usually operated as _______.

photon counters

Photon Counting

Individual pulses of charge are produced as quanta of radiation, are absorbed by the transducer, and are counted; the power of the beam is then recorded digitally as the number of counts per unit of time.

Gas-filled Tranducers

When X-radiation passes through an inert gas, interactions occur that produce a large number of positive gaseous ions and electrons for each X-ray quantum.

Voltage regions of gas-filled transducers include:

1. V1-V2 (ionization chamber region)

2. V3-V4 (proportional counter region)

3. V5-V6 (Gieger region)

At voltages less than V1,

accelerating force is low and recombination occurs

V1-V2 (ionization chamber region)

# e- constant = # e- formed by X-ray photon (not useful, lack sensitivity).

V3-V4 (proportional counter region)

Gas amplification due to secondary ionization by accelerated e- (widely used in X-ray spectroscopy, dead time = 1 μs).

V5-V6 (Gieger region)

amplification is limited by the positive space charge

created as the faster-moving electrons migrate away from the slower positive ions.

Scintillation Counters

# of photons produced in scintillator is proportional to the energy of X-rays (shorter dead time = 0.25 μs).

Semiconductor Transducers

Absorption of a photon results in formation of a highly energetic photoelectron; intrinsic layer of a silicon detector functions like argon gas. Must be cooled down (N2)

Which transducer has the best sensitivity?

Semiconductor Transducers

X-ray Instruments include:

-Wavelength-Dispersive X-ray Fluorescence (WDXRF)

-Energy-Dispersive X-ray Fluorescence (EDXRF)

Two types of Wavelength-Dispersive X-ray Fluorescence are:

-signal channel, sequential (rotate crystal one wavelength after another)

-multichannel, simultaneous (no rotation, several detectors)

Energy-Dispersive X-ray Fluorescence includes:

-no monochromator

-low power X-ray source if needed (portable, can be moved toward the sample)

-detector kept in liquid N2 to cool

Compared to EDXRF, WDXRF has:

-high signal-to-noise

-lower detection limits (reduced throughput)

Compared to WDXRF, EDXRF has:

-simplicity, no moving parts

-absence of collimators and a crystal diffractor

-closeness of the detector

-cost about one fourth to one fifth the price of WDXRF

Which of the following components make use of a thin metal foil to isolate a nearly monoenergetic excitation beam?

Filters

3 multiple choice options

Energy dispersive system uses which of the following detectors

Semiconductor detector

3 multiple choice options

In energy dispersive system, the energy level of pulses is related to which of the following?

Element involved

3 multiple choice options

The analysis of X-ray beam by diffraction is similar to spectrum analysis carried out with a diffraction grating. TRUE or FALSE

True

The crystal used as X-ray grating has ________ dimensional lattice arrays?

three

2 multiple choice options

Which of the following can be done to avoid loss of intensities of X-rays due to absorption of long wavelength X-rays?

Air in the chamber must be replaced by helium

2 multiple choice options

Which of the following is the disadvantage of silicon semiconductor detector?

Can be operated only at low temperatures

2 multiple choice options