superconductor. Since individual STJs are small and limited in count rate, there has
been an evolution in arrays of detectors, starting at 9 [197, 198], through 36 [199],
then >100 [200, 201], and there are hopes for arrays with >1000 elements
[201]. Resolutions on the order of 10 eV at 500 eV and even at 6 keV have been
achieved.
Table 5.4 Properties of different superconductors for STJ detectors [195]
Material
Energy gap (meV)
T c (K)
ΔE (@ 1 keV) (eV)
a
ΔE (@ 6 keV) (eV)
a
Niobium
1.55
9.3
4.2
10.2
Vanadium
0.82
5.4
3.0
7.5
Tantalum
0.70
4.5
2.8
7
Aluminum
0.18
1.2
1.4
3.5
Molybdenum
0.14
0.915
1.25
3.1
Hafnium
0.019
0.128
0.47
1.15
a Theoretical best resolution (FWHM)
Fig. 5.12 Top left: STJ detector operating principle. Top right: a dual superconductor STJ—most
of the X-rays are absorbed in the Nb layer. Middle left: a 9-element STJ array; pixels are 200 μ
x 200 μ [197]. Lower left: a 112-element STJ array. Individual pixels are 208 μ Â 200 μ
[200]. Lower right: superposition of signals from 36-element array [199]
5.13 Superconducting Tunnel Junction (STJ) Detectors
123