are the proper choice for nuclear scattering experiments,
since they fullfill the following severe demands:
- a good time resolution (100ps to about 1 ns),
- a good quantum efficiency (50% at 14.4 keV),
- low noise (0.01 photons/sec),
- high dynamic range and linearity (over nine decades)
- short recovery from large pulses.
The APDs used for these purposes are silicon based. The attenuation length of x-rays penetrating into the silicon is around 370 mm for a photon energy Eg of 14.4 keV and 35 mm for Eg = 6.4keV. Many of the absorption events result in the production of a fast electron having nearly the same energy as the incident photon. This fast electron is slowing down while generating other electron-hole pairs. The theoretical amount of one electron-hole pair created for every 3.6 eV is much lower in reality. A 10 keV electron travels around 1.4 mm in silicon. For active thicknesses of APDs of 100 mm the absorption event may therefore be regarded as a point like deposition of (less than Eg / 3.6 eV) electron-hole pairs. The electrons drift to the avalanche region with a velocity of 5 ps/mm. In the avalanche region the gain of the electron-hole pairs is between 10 and 1000.
The APDs normally used are large devices with 10x10 mm2 area and 100mm active thickness. The efficiency for such APDs for 14.4 keV is about 12% (EG&G). For NFS experiments this can be improved by inclining the APD. For an effective detector acceptance of 2.3 mm x 10 mm (grazing angle of 13 degrees) the efficiency was determined to be 57±3% and for 1.1mm x 10mm (grazing angle of 6 degrees) to be 74±4%. For NIS experiments the efficiency is about 94 %. This value is higher, because of the smaller attenuation length of 6.4 keV photons in silicon. The time resolution is better than 1 ns and up to 107 photons per second the detector behaves linearly in efficiency. The background is mainly originating from cosmic rays and is in the order of 0.01 counts/s.
Recently transparent avalanche photodiodes were used in NFS experiments. The latest 5x5 mm2 diodes (manufactured by EG&G Optoelectronics) are supported in ceramic frames and have only a thin layer of Au on the back. They have fast signal rise and fall times (less than 2ns), which significantly improves their recovery after large prompt pulses. A pair of such diodes in series gives 40% efficiency at 14.4 keV and a time resolution better than 2ns. In such a configuration, they allow good detector performance at prompt count rates about a factor of five larger than previously possible. Additional advantages include the possibility to position other detectors behind the transparent diodes, for example, optimized for detection at early times after the prompt pulse. More diodes can also be stacked in series, allowing for high efficiencies at energies up to 30 keV. A single detector shows saturation effects at count rates of more than 5x 106 counts/s (10% loss rate) in 1/3 fill, and thus, with a noise rate of about 0.01 counts/s, these diodes are single photon detectors that should have a dynamic range of 109 in the, now-standard, 2/3 fill pattern at the ESRF.