International Radiation Detectors, Inc.

Absolute X-ray Detectors

AXUV standard photodiodes have an effective silicon thickness of 30 to 105 microns. Thus, a fraction of photons with energies above 4000 eV will transmit through the active silicon reducing their quantum efficiency from the designed 100% value. As this reduction is solely caused by the limited silicon thickness, the AXUV diodes can also be used as absolute x-ray devices if silicon thickness is known. AXUV100 photodiodes (10mm x 10mm active area) with measured silicon thickness up to 55 microns with a measurement uncertainty of ±1.5 microns are available as standard products. The AXUV-20HE1 photodiodes have a silicon thickness of 425 microns and will have 100 % collection of photons up to 10 keV. However, because of high noise in these devices, use of the AXUV100 devices with known silicon thickness is recommended. Photodiodes with custom thicknesses can also be manufactured.

Theoretical responsivity as a function of x-ray energy may be obtained once the absorption A(eph) of the silicon layer is known. The absorption may be obtained from public sources such as LBL for photon energies up to 30 keV [1] and NIST for photon energies above 30 keV [2]. Once the absorption is known, the following formula may be used to calculate the responsivity

The value of 3.65 is an average value for electron-hole pair creation energy (eV) in silicon. The factor .98 accounts for 2% x-ray fluorescence yield in silicon for photons with energy larger than 1838 eV and includes estimates for reabsorption of a fraction of the fluorescent light. Silicon fluorescence yield has been experimentally measured for photons with energy up to 9 keV [3].

Figure 1 shows calculated responsivity for 45, 100 and 425 µm thick silicon. Figure 2 shows a comparison of responsivity calculated using NIST and LBL data for 53 µm silicon thickness.

Figure 3 shows recently obtained data for a AXUV100GX device with 104 µm thick silicon. The device was calibrated at PTB and compared against responsivity calculated using the above formula, with ideal results. Calculations included 2% reabsorbance of fluorescent photons.

1] Center for X-ray Optics, Lawrence Berkeley Laboratory

http://www-cxro.lbl.gov/optical_constants/ 

2] Physical Laboratory Physical Reference Data, National Institute of Science and Technology

http://physics.nist.gov/PhysRefData/XrayMassCoef/cover.html

3] J.L. Campbell et. al. "Experimental K-shell fluorescence yield of silicon"

J. Phys. B: At. Mol. Opt Phys., Vol. 31, 4765-4779 (1998).