X-ray Diffraction- Interaction with Matter

Question 1

What contributes to absorption of x-rays?

Answer 1

Electronic transitions within atoms of the material.

X-ray scattering by atoms of the material so there is a loss of intensity in transmitted beam.

Question 2

What factors determine absorption?

Answer 2

Thickness of material. Density. Chemical composition. Wavelength of radiation.

Question 3

Formula for transmitted intensity

Answer 3

It=I0exp(-μx)
I0 is incident beam intensity 
x is sample thickness
μ is linear absorption coefficient 
It=I0exp(-μmρx)

Question 4

What does linear absorption coefficient depend on?

Answer 4

The substance, its density (its state) and the surface x-ray wavelength

Question 5

Formula for transmittance

Answer 5

T=It/I0=exp(-μx)

Question 6

Formula for absorbance

Answer 6

A=log10(I0/It)=2.303μx

Question 7

Mass absorption coefficient

Answer 7

Independent of density and physical state so is constant of a material. Units cm^2g^-1. Tabulated values for elements at various wavelengths.
μm=μ/ρ
μm=Σxi μmi
xi is mass fraction of each element

Question 8

Radiography

Answer 8

Used in medicine and non-destructive testing of metals. Need high transmission so use high energy white radiation from tungsten tube at 70kV or higher. Makes use of different mass absorption coefficients and densities to obtain contrast in images.

Question 9

Variation of mass absorption coefficient with wavelength equation

Answer 9

μm=kλ^3Z^3
k is constant
Z is atomic number?

Question 10

Graph of μm vs wavelength

Answer 10

Curve up from origin to peak. Then sharp discontinuity almost straight down to nearly 0 (absorption edge). Then shallower curve up.

Question 11

Above absorption edge

Answer 11

Absorption decreases with decreasing wavelength. Increasing photon energy reduces scattering. Hence μm decreases with λ

Question 12

At absorption edge

Answer 12

X-ray photons have sufficient energy to eject core shell electrons from atoms. This is true absorption. Hence μm increases rapidly at the absorption edge

Question 13

Below absorption edge

Answer 13

X-ray photons still have sufficient energy to eject core shell electrons. But shorter wavelengths are also more penetrating meaning reduced scattering. Hence μm decreases with λ.

Question 14

How does fluorescent radiation get emitted?

Answer 14

Electrons transition from higher shells to fill a core hole with the emission of x-ray photons at characteristic wavelengths

Question 15

How to get monochromatic radiation

Answer 15

Use characteristic radiation. For Cu, Kα is most intense so suppress Kβ by filtering it out. Use filter made of element with an absorption edge lying to the short wavelength side (β side) of the Kα radiation

Question 16

What does an x-day filter do?

Answer 16

Suppresses (doesn’t remove) unwanted characteristic radiation and continuous (lower background) radiation. Also reduces transmitted beam intensity

Question 17

What does conventional x-ray imaging make use of?

Answer 17

Density and scattering contrast. As opposed to x-ray absorption imaging making use of lead absorption edge

Question 18

How does x-ray absorption imaging work for detecting lead?

Answer 18

Compare two images. Wavelength just above absorption edge: little contrast because μ for lead is small. Wavelength just below absorption edge: high contrast because μ for lead is large.