Self-scanning photodiode array detectors

The performance characteristics of four optoelectronic image detectors (OIDs) are discussed. The detectors discussed are the silicon intensified target vidicon (SIT), the intensified SIT, the intensified silicon photodiode array detector (ISPD), and the self-scanned photodiode array detector. The main objective of the paper is to provide research workers interested in applying OIDs to a particular application with comparative performance information so that the best detector for a particular application may be selected. 

Limited comparisons are also drawn for silicon intensified target vidicons (SIT) and intensified self-scanning photodiode array detectors (ISPD). 

As shown in the discussion above, there are a multiplicity of desirable and undesirable features of OID s that impact their general application as detectors in analytical atomic emission spectrometry. It is therefore appropriate to compare, in a critical and objective sense, the experimental figures of merit of these devices vis-a-vis the classical polychromator photomultiplier approach. These comparisons should be performed virtually on a continuing basis because of advances in performances, not only of the array detectors themselves but also in the associated spectroscopic excitation sources. An evaluation of the overall performance figures of merit of OID s when they are employed in conjunction with induction-coupled plasma excitation is of particular current interest because of the popularity that this source is attaining for the simultaneous determination of the elements at all concentration levels. In this paper we present such an evaluation for self-scanned, photodiode array detectors... 

Although most experts predict that third generation self-scanned solid state imagers will eventually become the OIDs of choice, at the present time, their compromised performance, low manufacture yield and therefore, limited commercial availability, greatly limit their use as spectrometric detectors. An exception to that are self-scanned photodiode arrays. 

Array detectors provide an attractive alternative to the polychrometers commonly used with ICP-AES, because they provide the capability of simultaneous detection of spectral lines and spectral background dispersed on hundreds of virtually independent optical channels. In this paper a critical evaluation of self-scanned photodiode arrays as radiation detectors for ICP-AES is presented. 

Several visible detectors have been used in CTR research as the encoding device behind an MCP for EUV spectroscopy. These have included a gated Vidicon (10), a CID camera (11), and most commonly, a self scanning photodiode array (PDA) (12, 13). The self scanning PDA has achieved widespread use and has shown itself to be a very useful and flexible detector. It is discussed in some detail in the next section. 

An intensified photodiode array (IPDA) detector for onedimensional spatial imaging in the EUV is shown in Figure 2. This type of detector was originally described by Riegler and Moore (14) It consists of a microchannel plate whose output is optically coupled to a self scanning photodiode array. An incident photon produces a photoelectron which is subsequently amplified by the MCP. The exiting electrons are proximity... 

An example of a solid state detector is the Reticon self-scanning photodiode array, which was specifically designed for spectroscopic applications [104]. These diode arrays contain 512 or 1024 silicon diode sensor elements on 25 jum centers corresponding to a density of 40 diodes mm" . Each diode is 2.5 mm high giving each element a slit-like geometry with a 100 1 aspect ratio. Beam registration problems do not apply to the diode array since the channel dimensions are defined by a photomask and hence the detector element size and position are completely reproducible. Solid state detectors do not suffer from lag [105] and althou they will bloom, the effect is much less severe than in a vidicon. 

Complete MCP s can be stacked to provide even higher gains. For response in the vacuum ultra-violet spectral region (50-200 nm) a SSANACON, self-scanned anode array with microchannel plate electron multiplier, has been used (36). This involves photoelectron multiplication through two MOP S, collection of the electrons directly on aluminum anodes and readout with standard diode array circuitry. In cases where analyte concentrations are well above conventional detection limits, multi-element analysis with multi-channel detectors by atomic emission has been demonstrated to be quite feasible (37). Spectral source profiling has also been done with photodiode arrays (27.29.31). In molecular spectrometry, imaging type detectors have been used in spectrophotometry, spectrofluometry and chemiluminescence (23.24.26.33). These detectors are often employed to monitor the output from an HPLC or GC (13.38.39.40). 

---