CEMS detectors

Experimental data show a sharp peak in the number of electrons (related to Mossbauer events) at energies below 20 eV. These electrons supply information on a surface layer to a depth of --5 nm. The detection of very low energy electrons offers the advantage of short data acquisition times (-77% of the electrons emitted from the Fe atom are low-energy Auger and shake-off electrons), and increases surface sensitivity compared to established procedures relying on the collection of electrons near 7.3 keV. CEMS detectors and techniques are summarized in Table 2. 

Discrimination of slower ions as compared to faster ions is observed with SEMs, [243] CEMs, and MCPs as well. [239,244,245] This means a reduction in sensitivity upon reduction of the acceleration voltage of a mass spectrometer, and of course, with increasing ion mass (Eq. 4.3). In order to reduce such effects and especially to improve sensitivity for high-mass ions, post-acceleration detectors have been developed. [236,245] In post-acceleration detectors the ions are accelerated immediatly in front of the detector by a voltage of 10-30 kV before they hit the first dynode or the first MCP. 

The names of both detectors reflect that these devices are channels which act as continuous dynode electron multipliers. If there is one channel, it is called a channeltron (channeltron electron multiplier, CEM), if many microchannels are used to form a plate it is called a microchannel electron multiplier plate (in short a microchannelplate, MCP, or channelplate), see Fig. 4.17. A comprehensive description of these devices is given in [Wiz79]. 

MCPs operate on the same principle as CDEMs. A microchannel plate is essentially a monolithic array of miniaturized CDEMs fabricated in a single wafer or disk of glass. The disk contains pores extending from the upper surface to the lower surface. These pores are known as channels and perform the same function as the interior of the tube in a CEM, but in contrast to a typical CEM, the entrance ends of the channels are not flared. The channels are typically 3 to 30 micrometers in diameter depending on the design. The length of the channels is set by the thickness of the disk, typically 200 to 1000 micrometers. MicroChannel plates can be fabricated in areas measured in cm, and because the disk-shaped profile forms a nearly flat stopping surface for ions, they are ideal detectors for TOE mass spectrometers. Burle and Hamamatsu are the major suppliers of these devices. 

A micro-evaporator can also be attached to modem microwave digesters and are designed to reduce the acid solutions to a low or even a dry state. The principle is that samples are exposed to microwave energy under vacuum to accelerate volume reduction. An auto-detect feature of the CEM MAR 5 system is that a detector is installed to inform the operator when the evaporation is complete and automatically stops operation. Acid fumes are neutralised by an integrated vapour scmbber system. 

The CEMS measurements were carried out in a specially designed reactor. In this reactor the samples of Fe foil were studied and treated with H /H S to obtain the iron sulfides. The detector employed for tne detection of the electrons was a He/10% CH flow counter connected in line with the reactor. The reactions of pure iron and iron sulfide with naphthoquinone were studied with this cell. The situ Mbssbauer measurements were performed using the system described in References 6 and 14. The coals used were a North Dakota lignite and an Australian Victorian Morwell Brown Coal... 

Fig. 2 Portion of the collinear laser-ion beam apparatus. QDl, QD2, electrostatic quadrupole deflectors CEM, channel electron multiplier DP, deflection plates PD, positive ion detector FC, Faraday cup ND, neutral particle detector CG, conducting glass plate AP, aperture MP, metal plate A, any element. The distance between QDl and QD2 is approximately 0.5 m.

Figure 1. TPEsCO apparatus using two-channel electron multiplier (CEM) electron detectors. Potentials on the several lens elements and deflectors are tuned manually for optimum electron pair detection and resolution.

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