Implementation of Noise Spectroscopy Using Biased Large-Area Photodiodes

The bremsstrahlung X-ray spectrum produced by X-ray sources used in cargo inspection systems is attenuated and modified by materials in the cargo in a Z-dependent way and then detected in a radiographic detector array. We have previously shown that it is possible to obtain spectral information indirectly by analyzing statistical fluctuations (the “noise”) in radiographic data, in a technique we call Noise Spectroscopy (NS) or Z-SCAN (Z-determination by Statistical Count-rate ANalysis). The technique is especially effective when the detector array consists of fast scintillation detectors and waveform digitization electronics, measuring both the waveform mean and variance during each X-ray pulse. This previous work, however, used photo-multiplier tubes, which are not especially suitable for a practical implementation. Here we describe the results of R&D performed to produce an effective version of the technique that lends itself to implementation in a single detector array that can be used both for radiography and for NS, with the same imaging spatial resolution as in conventional systems. We characterize the performance of biased photodiodes in combination with trans-impedance preamplifiers, read out with commercially available waveform digitizers. We report on experiments performed with a 16-channel detector array in a test beam, with test samples of different atomic number Z. We also report the implementation of an NS algorithm in an FPGA. In combination, we show that a practical implementation of Noise Spectroscopy in cargo inspection systems is feasible.