11 Energy Dispersive X-Ray Spectroscopy

Dr. Ajit K. Mahapatro

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Learning Objectives

From this module students may get to know about the following

i. X-Ray Generation

ii. Moseley’s Law

iii. Basic components of EDX

 

1. Introduction

Energy-dispersive X-ray spectroscopy (EDX) is an analytical technique used for elemental analysis or chemical characterization of materials. Each element in the periodic table has unique atomic structures and allow set of characteristic signals in the electromagnetic emission spectrum. Capturing the signals with respect to the variation in energy is the concept and fundamental principle of spectroscopy. In EDX, the interaction of X-ray excitation source with the sample is monitored by measuring the emission characteristics of X-rays ejected from a specimen after stimulating with a focused X-ray beam. At rest, an atom within the sample material contains ground state (or unexcited) electrons in discrete energy levels or electron shells bound to the nucleus. The incident beam excites an electron of the inner shell, ejecting it from the shell while creating a hole on the same place. An electron from the outer (higher-energy) shell fills the hole by releasing energy in the form of an X-ray radiation, of amount equivalent to the energy difference between the higher and lower shells. The counts of the X-rays emitted from the specimen in the energy spectrum is scanned and measured using EDX. As the energies of the emitted X-rays contain information of the difference in energy between two shells and atomic structure of the emitting element, the EDX provides estimation for the elemental composition in the specimen.

 

2. X-Ray Generation

Inelastic interaction of the electron beam within the specimen atoms produces two types of X-rays in SEM and peaks in the energy spectrum appears. These are:

  • Characteristic X-rays: Produced when electrons from the incident beam eject inner shell electrons from the specimen. Characteristic X-rays are identified as peaks on a background of continuum X- rays).
  • Continuum (Bremsstrahlung) X-rays: Produced when the electrons of the incident beam interact with nucleus of the specimen atoms.

Figure 1 indicates the EDX spectra for the characteristic and continuum (Bremsstrahlung) x-rays.

 


Figure 1. EDX spectra depicting characteristic and continuum (Bremsstrahlung) x-rays.
 

2.1. Production of Characteristic X –rays

The electrons of the incident high energy beam (E0) create a hole in the inner shell of the specimen atom, loses the energy (E) by transferring it to the ejected electron. An electron from the outer shell subsequently fills this hole in the inner shell. The energy difference between the inner shell electron and the outer shell electron is emitted as a characteristic X-ray quantum, as shown in Figure 2.


Figure 2. Origin of characteristic X-ray

 

2.1.1. Characteristic X-ray nomenclature

X-ray lines are named using a capital Roman letter indicating the shell containing the inner vacancy (K, L, M or N) with a Greek letter to specify the group corresponding to the line (i.e. α, β, etc.), and a number to denote the intensity of the line within the group (i.e., 1, 2, etc.). The characteristic X-ray lines are nomenclatured in accordance with the shell where the vacancy is created by the ejected electrons and the shell from which an electron migrates to fill the vacancy created in the inner shell. The schematics of the characteristics X-ray lines are represented as arrows in Figure 3. For example, if the initial vacancy occurs in the K shell and the vacancy of the filling electron migrates from the adjacent shell L, the X-ray is said to be emission of Kα line. If the electron migrates from the M shell (two shells away), the emitted X-ray is a Kβ X-ray.

Figure 3. Nomenclature for characteristic X-ray

 

Similarly, if an L-shell electron is ejected and an electron from the M-shell fills the vacancy, Lα radiation will be emitted. Within a given shell, there are electrons in orbitals differing in energy due to bonding effects and gives variable intensities. The Kα peak comprises of the Kα1 and Kα2 X-rays that appears very closely and unresolved in the EDX system, and termed as Kα1,2 doublet. It appears as the Kα peak at an energy between the two individual components and an intensity corresponding to the weighted average of both the Kα1 and Kα2. The most probable transition occurs between the adjacent energy shells e.g. when a K-shell vacancy is created the transition from L to K is the most probable one. Hence, Kα radiation possess greater intensity than Kβ radiation. As the energy difference between the M and K shells (Kβ radiation) is greater than the energy difference between the L and K shells (Kα radiation), Kβ radiation appears at higher energy compared to the Kα radiation, while progressing outward from the nucleus, the energy difference between adjacent electron shells decreases and hence, the energy released upon electron transitions between adjacent outer shells is less than that released for adjacent inner shell transitions. Hence, for a given atom, the energy trend for the radiations is as follows,

Mα radiation < Lα radiation < Kα radiation.

 

 

2.2. Continuum X-rays production

Continuum X-rays represent the background on which the characteristic X-ray peaks appear. A high value of peak-to background ratio is essential for proper identification of elements using the observed characteristic X-ray peaks. Interaction of the incident electrons with the Coulomb (electrical) field of the atomic nuclei leads to loss of energy from electrons in the beam that produce continuum X-rays. The distribution of this energy loss is continuous and not a characteristic feature of the atomic number of elements in the specimen. The closer the beam electron comes towards the nucleus of the specimen atom, the stronger is the interaction with the coulomb field of the atomic nucleus, and more energy is lost by the electrons of the beam and correspondingly, more energetic X-ray photons are emitted. The difference between the actual and observed intensities of X-ray emission spectra may occur due to absorption by the window of the detector that allows the emitted X-rays to pass through for detection. The intensity of the continuum background increases with probe current, atomic number, and accelerating voltage.

 

3. Moseley’s Law

The energies of the characteristic radiation for a given series of lines vary monotonically with the atomic number (Z) and is given by Moseley’s law,

E = C1 (Z- C2)2

where, E is the energy of the emission line for a given X-ray series (e.g. Kα), Z is the atomic number of the emitter, and C1 and C2 are constants. Moseley’s Law is the basis for elemental analysis. With the energy of a given K, L, or M line, the atomic number of the element producing that line can be determined. The X-rays of K, L and M series are having increased energy values for higher atomic number elements (as shown in Figure 4).

Figure 4. Curve depicting Moseley’s law.

Within the normal accelerating voltage range (15-20 keV) used for EDX analysis,

  • Light weight elements emit X-rays of the K series only.
  • Intermediate weighing elements emits X-rays of the L series or K and L serieses.
  • Heavy elements emit X-rays of the M series or L and M serieses.

 

4. Spatial Resolution

As we studied in lecture 1 of SEM, the inelastic interactions produce diverse effects including:

  • Secondary electrons
  • Phonon excitation (heating)
  • Cathodoluminescence (visible light fluorescence)
  • Continuum radiation (bremsstrahlung radiation)
  • Characteristic X-ray radiation
  • Plasmon production (secondary electrons)
  • Auger electron production (ejection of outer shell electrons)

X-rays are produced from deeper side of the interaction volume formed in the sample as compared to the secondary and backscatter electrons. Backscattered electrons originate from interaction volume more closely approximating that of X-rays, and therefore are a useful imaging signal to correlate with the X-ray analysis. In fact, the BSE signal is the signal of choice for correlation with X-ray maps. X-ray spatial resolution depends on the specimen’s density and overvoltage. The following equation shows a good estimate of X-ray interaction with the samples:

R = 0.064 (E 1.68 – Ec 1.68 ) / ρ

where, R is the spatial resolution in µm, E0 is the accelerating voltage in keV, EC is the critical excitation energy in keV, and ρ is the mean specimen density in g/cc.

 

5. Basic Components of EDX System

The EDX system detects the X-rays ejected out from the sample specimen. The schematic of the EDX system is shown in Figure 5. The absorption of an individual X-ray photon by the detector leads to the production of a photoelectron that utilizes its energy to form electron-hole pairs in a single crystal and subsequent formation of charge pulse with exposure to external electric field. The charge pulse is converted to a voltage pulse by passing through a preamplifier. The generated voltage pulse is proportional to the energy of the incoming X-ray photon. The signal is further amplified and shaped by a linear amplifier (pulse processor) and passed on to a multi-channel analyzer where the data is displayed as a histogram of intensity verses voltage. The main components of EDX system includes,

  • An X-ray detector that detects and converts X-ray into electronic signals.
  • A pulse processor, which measures the electronic signals to determine the energy of each X-ray detected and,
  • A Multi-Channel Analyzer (MCA), which displays and interprets the X-ray data.


Figure 5. Basic Components of EDX System

 

5.1. X-ray Detector

The EDX detector is a self contained vacuum system (called a cryostat) with cryogenic pumping created by liquid nitrogen cooling or alternative methods such as Peltier cooling. The basic components of X-ray detector are schematized in Figure 6 and includes collimator assembly, electron trap, cryostat, and electronic attachments.


Figure 6. Components of the X-ray detector

 

  • Collimator assembly: The collimator provides a limiting aperture through which X-rays pass through to reach the detector ensuring that only the X-rays from the limited area being excited by the electron beam are detected.
  • Electron trap: Electrons that enter the  detector  cause  background  artefacts.  The  electron trap is a pair of permanent magnets that strongly deflect any passing electrons that could cause background artefacts.
  • Window: The window helps maintaining vacuum by isolating the detector crystal from the chamber of the microscope while being as transparent as possible to low energy X-rays. Earlier, windows were composed of Be which did not allow low-energy X-rays (<0.9 keV) to pass through it, however modern windows are composed of polymers and allows low-energy X-rays (down to ~0.1 keV) to pass.
  • Crystal: The main material used for crystal is silicon (Si), and lithium (Li) is added to compensate for small levels of impurity. When an incident X-ray strikes the detector crystal, its energy is absorbed by a series of ionizations within the semiconductor and a number of electron-hole pairs are generated. The energy of the incoming X-ray is dissipated by the creation of a series of electron-hole pairs in the semiconductor crystal.


Figure 7. Mechanism in semiconducting crystal of X-ray detector

The electrons excited to the conduction band of the semiconductor are free to move within the crystal lattice. The excited electron leaves behind a hole, which behaves like a free positive charge within the crystal lattice. Under an applied bias between electrical contacts on the front and back of the crystal, the electrons and holes sweeps to the opposite electrodes producing a charge signal. The size of this signal (voltage step) is directly proportional to the energy of the incident X-ray.

  • Field Effect Transistor: The field effect transistor (FET) is the primary step of the amplification process that measures the charge produced in the crystal by an incident X-ray and converts it to a voltage output. During operation, charge is built up on the feedback capacitor.


Figure 8. Voltage steps induced in FET by X-ray
 

The sharp steps on the volt buildup (as shown in the Figure 8) are due to the charge created by each X-ray event. The voltage step size is proportional to the incident X-ray energy. This accumulating charge should be periodically restored to prevent saturation of the preamplifier.

  • Cryostat: The charge signals generated by the detector are small and can only be separated from the electronic noise of the detector if the noise is minimized by cooling the crystal and FET. The noise determines the resolution of a detector particularly at low energies. Generally, the full width half maxima (FWHM) of an X-ray peak is of the order of 2-10 eV. The actual FWHM is at least an order of magnitude greater than this due to noise in the system. The EDX detector is cooled using a continuous filling of liquid nitrogen held in a dewar. The vacuum is maintained at a level low enough to prevent the condensation of molecules on the crystal.

5.2. Pulse Processor

The signal (voltage step) from the preamplifier is transformed into a voltage pulse that is suitable for the multi-channel analyzer. Shaping and noise reduction of the signal are achieved by digital computation.


Figure 9. Working principle of pulse processor

The noise on the voltage ramp from the detector is effectively filtered out by averaging the signal. The time over which the waveform is averaged is called the process time (Tp) and can be controlled by the operator. The longer the Tp, the lower is the noise on the voltage ramp. If noise is minimized, the resolution of the peak in the spectrum improves, and can be easily separated or resolved, from another peak that is close in energy.

However, there is a trade-off between the process time and the speed of measuring the data. The longer the process time, the more time is taken to measuring each X-ray, and fewer are the events that can be measured. Productivity in the detector depends on the rate of counts measured, called the output count or the acquisition rate, rather than the input count rate. However, at very low count rates, the output and input count rates are proportional. As the input rate increases, the acquisition rate also increases, but large number of events are rejected because they arrive in a shorter time period than the processing time. This phenomenon is termed as pulse pileup. If input rates increase sufficiently, the proportion rejected will exceed the increase in measured events and the acquisition rate will start to decrease with further increasing in input rate. This situation is most dramatic for the longer process times.

 

5.3. Multi-Channel Analyzer (MCA)

The MCA takes the data from the pulse processor and displays it as a histogram of intensity (number of counts) verses voltage. The voltage range displayed on the x-axis is divided into a number (1024, 2048, etc.) of channels each corresponding to a given energy range (for example, 5,280 – 5,300 eV). The MCA takes the peak height of each voltage pulse, converts it into a digital value, and puts it into the appropriate channel. Thus, a count is registered at that energy level.

 

Summary

  • An EDX spectrum is essentially a histogram of the number of X-rays measured at each energy.
  • In the EDX system, the energy of the incoming X-ray is dissipated by the creation of a series of electron-hole pairs in the semiconductor crystal.
  • The electron-hole pairs are swept away by an applied bias to form a charge pulse which is further converted to a voltage pulse by a preamplifier.
  • The signal is further amplified and shaped by a linear amplifier (pulse processor) and passed on to a multi-channel analyzer where the data is displayed as a histogram of intensity vs voltage.
you can view video on Energy Dispersive X-Ray Spectroscopy

 

References

1. Corbari, L; et al.(2008) “Iron oxide deposits associated with the ectosymbiotic bacteria in the hydrothermal ventshrimp Rimicaris exoculata”

2. Joseph   Goldstein   (2003) “Scanning   Electron   Microscopy   and   X-Ray Microanalysis”,Springer. ISBN 978-0-306-47292-3.

3. John J. Friel. “X-ray and Image Analysis in Electron Microscopy”, Princeton Gamma-Tech.

4. Russ,  J.  C.  (1984) “Fundamentals  of  Energy  Dispersive  X-ray  Analysis”,  Butterworths.London.

5. Goldstein,  J.  I., et  al.  (2003) “Scanning  Electron  Microscopy  and  X-ray  Micronalysis”,  3rd ed.,Plenum Press, New York.