Detector and efficiency

Two examples illustrate this with practical source strengths and detectors. Consider an electronic neutron source such as a DT generator with 10s neutrons/s, a detector of area 45 cm2 (typical 3-inch diameter detector) and efficiency 1. For distances of 100 cm and a mass of 1kg, we get 1.25 counts/s. Another view might be a truck scanner with 100 kg of material (factor of 102 increase in count rate) and distances of 200 cm (a factor of 16 decrease) which would be a factor of about 6 overall, 8cps. If we reduce the distance to 50 and 20 cm, we would get a factor of more than 100 or about 123 cps/kg, respectively. Just by way of reference, PELAN operating in those conditions for land mine detection gets about lOcps for a 100 g mass, so we are about right in this calculation. 

Although the molar absorptivities of these bands tend to be rather small, the instrumental state of the art, combined with superior data deconvolution routines, has progressed to the point where their measurement is relatively straightforward.78 The most important experimental requirements are the use of high-intensity stable light sources, low-noise sensitive detectors, and efficient methods for collection of the diffuse reflectance of the measurement. 

Since the Raman effect is intrinsically quite weak, relatively high-power lasers and sophisticated optical and electronic equipment are required to detect the Raman scattered photons. This accounts for the lengthy time that elapsed in the development of Raman spectroscopy. It experienced a rebirth in the 1960s with the invention of laser and its use as a light source. However, from the 1980s the advances in optoelectronics, particularly the development of compact lasers, detectors and efficient optical filters allowed lower cost, integrated instmments to be produced commercially. Consequently, Raman spectroscopy has been adopted as a routine method in many fields because it has been shown to be simpler and faster than alternative techniques. 

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