Recent developments in high-resolution-gamma-ray cargo-inspection technology

Under US Department of Homeland Security sponsorship, Spectral Labs Incorporated has developed a prototype high-resolution retrofit for an existing mobile VACIS, named the High-Resolution Imaging System (HiRIS). The legacy 256 NaI detectors in the VACIS detector column were replaced with 576 CsI detectors, more than doubling the pixel count. Using SiPMs to replace conventional PMTs allowed the packing of more detectors in the same VACIS detector enclosure. Legacy analog signal-processing electronics were replaced with advanced digital signal-processing electronics. Replacing gross counting in the legacy system with multichannel analysis of the counts from each detector will allow better control of detector crosstalk. The HiRIS detector modules were installed on a VACIS truck refurbished to as-new condition. Initial testing of the HiRIS prototype demonstrates enhanced spatial resolution by a factor of two as compared to the legacy system, without any degradation in throughput capability (20 containers per hour).

Thermal noise limit for ultra-high vacuum noncontact atomic force microscopy

The noise of the frequency-shift signal Δf in noncontact atomic force microscopy (NC-AFM) consists of cantilever thermal noise, tip–surface-interaction noise and instrumental noise from the detection and signal processing systems. We investigate how the displacement-noise spectral density d z at the input of the frequency demodulator propagates to the frequency-shift-noise spectral density d Δ f at the demodulator output in dependence of cantilever properties and settings of the signal processing electronics in the limit of a negligible tip–surface interaction and a measurement under ultrahigh-vacuum conditions. For a quantification of the noise figures, we calibrate the cantilever displacement signal and determine the transfer function of the signal-processing electronics. From the transfer function and the measured d z , we predict d Δ f for specific filter settings, a given level of detection-system noise spectral density d z ds and the cantilever-thermal-noise spectral density d z th. We find an excellent agreement between the calculated and measured values for d Δ f . Furthermore, we demonstrate that thermal noise in d Δ f , defining the ultimate limit in NC-AFM signal detection, can be kept low by a proper choice of the cantilever whereby its Q-factor should be given most attention. A system with a low-noise signal detection and a suitable cantilever, operated with appropriate filter and feedback-loop settings allows room temperature NC-AFM measurements at a low thermal-noise limit with a significant bandwidth.

Evaluating the Performance of a Commercial Silicon Drift Detector for X-ray Microanalysis

The development of large-area silicon drift detectors (SDDs) provides a significant improvement in X-ray micro-analysis, especially in scanning electron microscopes (SEMs) and electron microprobes, where high incident probe currents are possible. The resultant improved detection limits and/or speed of elemental mapping and analysis make the SDD the detector of choice for microanalysis. With the larger geometric collection efficiency and faster response times of these detectors, the higher input count rates can place significant demands on the performance and speed of the signal processing electronics.