Chemistry Reference
In-Depth Information
The latest type of microfocus sealed tubes, also called microsources, have been
introduced to the XRD field, thanks to several technical innovations [3-5]. The
microsource tube has a very small focal spot size in the range of 10-50 mm. The tube is
typically air cooled because the X-ray generator power is less than 50W. The
brilliance of a microsource can be as much as one to two orders of magnitude higher
than a conventional fine focus sealed tube. The X-ray optics for a microsource, either
multilayer or polycapillary, is typically mounted very close to the focal spot so as to
maximize the gain on the capture angle.
Correct and careful operation of an X-ray generator is critical for satisfactory
performance of a diffractometer. All X-ray tubes have a maximum power rating,
which defines the highest power input to the tube. A cathode current versus anode
voltage chart (or table) is normally supplied for a sealed tube or RAG. The tube
filament current is also given by the tube vendors. The following practices should be
adapted to increase the lift time of a generator. The cooling system should be checked
for the specified temperature, pressure, and flow rate before powering up the
generator. When increasing the generator power manually, always increase the
voltage first and then increase the current. When reducing the generator power,
always reduce the current first and then the voltage. To increase the lifetime of X-ray
tubes, run the generator at full power, only if it is necessary, and set the generator in
standby mode if the generator is not in use for extended time.
3.1.4 Absorption and Fluorescence
When X-rays pass through matter of any form, whether gas, liquid, or solid, only part
of the X-rays are transmitted; the other part of the X-rays are absorbed. The intensity
of the transmitted X-rays from a homogenous substance is given as
I ¼ I
0
e
mt
ð3
:
2Þ
where I
0
is the intensity of the incident X-ray beam, I is the intensity of the transmitted
beam, m is the linear absorption coefficient, and t is the thickness of the substance. The
value of the linear absorption coefficient depends also on the density of the substance.
A more consistent parameter for a specific element is called mass absorption
coefficient, which is equivalent to the linear absorption coefficient normalized by
the density, and is expressed as (m/r). Here r is the density of the absorption substance.
Eq. (3.2) can then be written as
I ¼ I
0
e
ðm=rÞrt
ð3:3Þ
Since the intensity of the transmitted X-ray beam is always less than that of the
incident X-ray beam, the X-ray is attenuated by the material. Therefore, the linear
and mass absorption coefficients are also called the linear attenuation coefficient and
the mass attenuation coefficient, respectively. The mass absorption coefficient of
an element is dependent on the wavelength of the X-rays. The mass absorption
coefficients (m/r) of most elements for the four most commonly used X-rays (Mo-K
a
,
