X-ray Detection and Analysis

Principles

When an electron from a K-shell is replaced by one from the second closest shell (M), it is designated as a Kβ event

Ka Kb

Certain events such as Mα, Lβ, and Kγ are only possible in atoms of sufficient atomic weight

The Complete Range of Possible Electron Transitions that Give Rise to K, L and M Characteristic X-Rays

Some transitions are prohibited!

An X-ray spectrum for a sample is composed of all the possible signals for that given set of elements.

These will differ in terms of energies (KeV) and probabilities (likelihood) scored as number of such signals collected over a given period of time.

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X-ray Energy in KeV

Each element has a family of characteristic X-rays associated with it Absorption edge energy: when a photon impinging on an atom carries an energy just above the binding energy of an electron in a specific shell of the atom. The photon can in this case interact with this electron shell and ionize the atom

In EDS: we use a much higher energy to excite the electrons to have higher excitation efficiency

There are energy peak overlaps among different elements, particularly those corresponding to x-rays generated by emission from different energy-level shells (K, L and M) in different elements. For example, there are close overlaps of Mn-Kα and Cr-Kβ, or Ti-Kα and various L lines in Ba. Particularly at higher energies, individual peaks may correspond to several different elements; in this case, the user can apply deconvolution methods to try peak separation, or look at a series of minor peaks if they are present, or simply consider which elements make "most sense" given the known context of the sample.

In microanalysis we only use the most intense lines, usually the α lines (or, if we can’t resolve them, we use the α and β line).

"Bremsstrahlung" means "braking radiation" and comes from the original German to describe the radiation which is emitted when electrons are decelerated or "braked" when they interact with the specimen.

Although they contribute to the total X-ray signal they contain no useful information because their energies are nonspecific and therefore are considered as part of the background .

Bremsstrahlung X-rays are the major part of the continuum X-ray signal that can escape from the deepest portion of the interaction region.

Clay sample and X-ray analysis

Bullet fragments (blue) can be identified on cloth fibers and distinguished from other metal pieces by their elemental composition

Forensic Applications

Gunshot Residue (GSR) Analysis

The proportion of elements present in GSR differ slightly and databases of GSR from different manufacturers can be used to identify what ammunition was used in a crime.GSR is often found on criminals and also on victims if shot at close range.

Gun-shot residual (GSR) analysis for forensics

X-ray mapping

Sandstone: K-feldspar (silicon, aluminium, potassium), albite (silicon, aluminium, sodium) and dolomite (calcium, magnesium). The bright blue particles in the silicon map highlight quartz grains.

Pulse ProcessorMeasures the electronic signals to determine the energy of each X-ray detected

X-ray DetectorDetects and convertsX-rays into electronic signals

AnalyzerDisplays and interpretsthe X-ray data

Cut-away diagram showingthe construction of a typical EDS detector.

FET

Crystal

Window

Collimator

Electron trap

Collimator to limit BSE and stray X-rays

Window usually made of beryllium (limited to sodium, atomic number 11) or thin plastic to detect down to boron (Atomic number 5) protects cooled crystal from air.

FET

Crystal

Window

Collimator

Detector : crystal silicon wafer with lithium added in. For each 3.8 eV from an X-ray, produce an electron and hole. This produces a pulse of current, the voltage of which is proportional to the X-ray energy. Must keep the crystal at LN temperature to keep noise to a minimum.

FET : The field effect transistor is positioned just behind the detecting crystal. It is the first stage of theamplification process that measures the charge liberated in the crystal by an incident X-ray and converts it to a voltage output.

FET

Crystal

Window

Collimator

Generation and measurement of electron-hole pairs in the crystal

Circuit diagram of the EDS detector (v=Q/C) Typical output voltage “ramp”

X-ray without and with a collimator

The collimator is made of parallel lead blades

The collimating method is unlike the optical lens collimator

Lithium doped Silicon (SiLi) crystal detector acts as a semiconductor that carries current in a rate proportional to the number of ionization events and acts as an indirect measurement of the energy contained in the X-ray.

The detector (and FET) must be cooled to minimize the current produced by the bias!

Li doping to neutralize residual charges produced by impurity in Si

Each ionized atom of silicon absorbs 3.8 eV of energy, so an X-ray of 3.8 KeV (e.g. Ca Ka 3.7 KeV) will ionize approximately 1000 silicon atoms.

The output voltage is proportional to the X-ray energy. This voltage must be reset (e.g. shininglight on the FET) to zero periodically since it can only be increased to a few volts.

One reason that the final lens of an SEM is conical in shape is so that the EDS detector can be positioned at a high take-off angle and inserted close to the specimen for a high solid angle.

Factors affecting signal collection

Distance between detector and X-ray sourceAngle at which detector is struckVolume of signal collected.

For a given angle of electron incidence, the length of the absorption path is directly proportional to the cosecant (1/sin φ ) of the take-off angle, φ

Take-off Angle

Solid Angle

The solid angle Ω of a detector is defined as angle of the cone of signal entering the detector. The greater the size of the detector surface area the greater will be the solid angle.

Larger SiLi crystals will be able to sample a larger volume of signal (better Ω) but because of imperfections in the crystal they have slightly greater noise and thus slightly lower resolution.

Fluorescence Yield(Ratio of x-ray emission to inner-shell ionization)

WDS

For a given wavelength λ there is a specific angle θ (Bragg’s angle) at which diffraction will occur. Bragg’s angle is determined by the d-spacing (interplanar spacing) of the crystal and the order of diffraction (n = 1, 2, 3….).

The Bragg angle q and the detector position can be changed by re-positioning the crystal and the detector along a circle called the Rowland circle

A comparison of two spectra collected with EDS and WDS shows how peak overlap and energy spread can serve to obscure the information in an EDS spectrum

New SDD detector approaches the performance of WDS