Chemistry Reference
In-Depth Information
Source, gate, and drain are developed as concentric rings inside the small anode
ring while the anode and gate of the JFET are connected by a micrometer-short
metal strip. By this means, parasitic electric capacitances of conventional bonds
are avoided. The minimized capacitance between anode and JFET allows
shorter shaping times and higher count rates and even a better spectral
resolution.
The sophisticated fabrication of the sensor chips leads to a very low leakage
current [56,57], which is proportional to the small detector volume so that
traditional cooling by liquid nitrogen quite normal for Si(Li)s is not necessary.
Consequently, the vibrational noise generated by the boiling bubbles of the
liquid nitrogen is completely avoided. A simple Peltier cooler ensures a
moderate thermo-electrical cooling down to
40
°
C. Consequently,
SDDs are maintenance-free and guarantee an absolutely vibration-free oper-
ation. The moderate cooling is sufficient for good spectral resolution (
<
130 eV
for Mn-K
α
) and a high count rate efficiency (up to 1
×
10
6
cps).
The sensor chip is usually mounted on a socket with a ring-shaped collima-
tor, electron trap, cooling studs, and electrical pins and bonds. This module is
enclosed in a cylindrical case with a thin window in order to protect the sensor
against visible light, air, dust, and moisture. A 7.5
μ
m thin beryllium window is
normally used. However, for the detection of light elements with 5
20
°
Cor
15 or
X-ray photons with low energies between 0.18 and 2 keV, respectively, an
ultrathin window of a 0.5
μ
m polymer foil is used, which hardly absorbs these
X-rays (polyimide or Kapton
of Du Pont). It may be supported by a fine Si
grid so that it becomes stable, leakage-free, and pressure resistant. Further-
more, a medium vacuum with about 100 Pa can be applied. For a typical air-
path of 0.5 cm the absorption is reduced below 3% for photons with energies
Z
0.1 keV (K
α
peak of beryllium).
The head of both detectors is shown in Figure 3.17a and b, demonstrating the
technical development within the last 45 years. Figure 3.18a shows a spectrum
of a type letter recorded by a Si(Li) detector. Figure 3.18b represents a
spectrum of a pigment mixture taken by an SDD. Elements like C, O, Cu,
As, S, and Ca can be detected by the SDD at photon energies between 0.2 and
4 keV. The peak width is in the range of only 40-110 eV so that the K
α
peaks of
these elements are clearly separated. Even boron at 0.184 keV has been
identified, namely, as boride inclusions of stainless steel [58].
3.6.3PositionSensitiveDetectors
A direct imaging of the information provided by X-rays is of general interest
and has been developed in the last decades. The most prominent 2D (two-
dimensional) X-ray detectors are applied for imaging in medicine and for
monitoring of X-ray diffraction patterns [59]. The simplest way to obtain
information about spatially resolved X-ray data is to use a Si photodiode
and to scan the area of interest. More comfortable arrangements are CCDs
(charge-coupled devices) normally used as image sensors for visible light.
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