Mossbauer instrumentation

Fig. 3.14 Left. NASA Mars-Exploration-Rover (artist view courtesy NASA, JPL, Cornell). On the front side of the Rover the robotic arm carrying the Mossbauer spectrometer and other instruments can be seen in stowed position. Right, robotic arm before placement on soil target at Victoria crater rim, Meridian Planum, Mars. The Mossbauer instrument MIMOS II with its circular contact plate can be seen, pointing towards the rover camera. See also Sect. 8.3...

Because of the complexity of sample preparation, backscatter measurement geometry (see Fig. 3.19) is the choice for an in situ planetary Mossbauer instrument [36, 47 9]. No sample preparation is required, because the instmment is simply presented to the sample for analysis. On MER, the MIMOS II SH is mounted on a robotic arm that places it in physical contact with the analysis target (e.g., rock or soil) [36, 37]. 

The miniaturized Mossbauer instruments have proven as part of the NASA Mars Exploration Rover 2003 mission that Mossbauer spectroscopy is a powerful tool for planetary exploration, including our planet Earth. For the advanced model of MIMOS II, the new detector technologies and electronic components increase sensitivity and performance significantly. In combination with the high-energy resolution of the SDD, it will be possible to perform XRF analysis in parallel to Mossbauer spectroscopy. In addition to the Fe-mineralogy, information on the sample s elemental composition will be obtained. 

Morris, R.V. et al. 2008. Iron mineralogy and aqueous alteration from Husband Hill through Home Plate at Gusev Crater, Mars Results from the Mossbauer instrument on the Spirit Mars Exploration Rover. Journal of Geophysical Research, 113, E12S42,... 

There have been a number of regional and international conferences devoted to industrial applications of Mossbauer spectroscopy or specific problems of materials science. The scope of problems addressed is extremely wide and ranges over all the above-mentioned problems. One can expect the contributions of Mossbauer spectroscopy in industry to divide into three areas (1) as a research tool, (2) in quality control, and (3) for in-service evaluation. Unfortunately, there are still very few Mossbauer instruments in use for quality control, or for in-service evaluation of materials or surface. More than 99% of Mossbauer publications cover use of the technique as a research tool. 

In this chapter, we present the principles of conventional Mossbauer spectrometers with radioactive isotopes as the light source Mossbauer experiments with synchrotron radiation are discussed in Chap. 9 including technical principles. Since complete spectrometers, suitable for virtually all the common isotopes, have been commercially available for many years, we refrain from presenting technical details like electronic circuits. We are concerned here with the functional components of a spectrometer, their interaction and synchronization, the different operation modes and proper tuning of the instrument. We discuss the properties of radioactive y-sources to understand the requirements of an efficient y-counting system, and finally we deal with sample preparation and the optimization of Mossbauer absorbers. For further reading on spectrometers and their technical details, we refer to the review articles [1-3]. 

Solid-state detectors based on silicon- or germanium-diodes possess better resolution than gas counters, particularly when cooled with liquid nitrogen, but they allow only very low count rates. PIN diodes have also recently become available and have been developed for the instruments used in the examination of Martian soils (Sects. 3.3 and 8.3). A very recent development is the so-called silicon-drift detector (SDD), which has very high energy resolution (up to ca. 130 eV) and large sensitive detection area (up to ca. 1 cm ). The SNR is improved by an order of magnitude compared to Si-PIN detectors. Silicon drift detectors may also be used in X-ray florescence spectroscopy, even in direct combination with Mossbauer spectroscopy (see Sects. 3.3 and 8.3). 

Physically, the MIMOS II Mossbauer spectrometer has two components that are joined by an interconnect cable the sensor head (SH) and electronics printed-circuit board (PCB). On MER, the SH is located at the end of the Instrument Deployment Device (IDD) and the electronics board is located in an electronics box inside the rover body. On Mars-Express Beagle-2, a European Space Agency (ESA) mission in 2003, the SH was mounted also on a robotic arm integrated to the Position... 

Fig. 3.15 Left External view of the MIMOS II sensor head (SH) with pyramid structure and contact ring assembly In front of the Instrument detector system. The diameter of the one Euro coin is 23 mm the outer diameter of the contact-ring is 30 mm, the inner diameter is 16 mm defining the field of view of the Instrument. Right. Mimos II SH (without contact plate assembly) with dust cover taken off to show the SH Interior. At the front, the end of the cylindrical collimator (with 4.5 mm diameter bore hole) Is surrounded by the four SI-PIN detectors that detect the radiation re-emltted by the sample. The metal case of the upper detector is opened to show its associated electronics. The electronics for all four detectors Is the same. The Mossbauer drive is inside (in the center) of this arrangement (see also Fig. 3.16), and the reference channel is located on the back side In the metal box shown In the photograph...

Adjustable Workbench (PAW) instrument assembly. The SH shown in Figs. 3.15 and 3.16 contains the electromechanical transducer (mounted in the center), the main and reference Co/Rh sources, multilayered radiation shields, detectors and their preamplifiers and main (linear) amplifiers, and a contact plate and sensor. The contact plate and contact sensor are used in conjunction with the IDD to apply a small preload when it places the SH holding it firmly against the target. The electronics board contains power supplies/conditioners, the dedicated CPU, different kinds of memory, firmware, and associated circuitry for instrument control and data processing. The SH of the miniaturized Mossbauer spectrometer MIMOS II has the dimensions (5 x 5.5 x 9.5) cm and weighs only ca. 400 g. Both 14.4 keV y-rays and 6.4 keV Fe X-rays are detected simultaneously by four Si-PIN diodes. The mass of the electronics board is about 90 g [36],... 

Because instrument volume and experiment time must both be minimized for a planetary Mossbauer spectrometer, it is desirable in backscatter geometry to illuminate as much of the sample as possible with source radiation. However, this... 

Fig. 8.27 NASA Mars-Exploration-Rover artist view (courtesy NASA, JPL, Cornell). On the front side of the Rover, the robotic arm (IDD) carrying the Mossbauer spectrometer and other instruments can be seen...

The instrument MIMOS 11 is extremely miniaturized compared to standard laboratory Mossbauer spectrometers and is optimized for low power consumption and high detection efficiency (see Sect. 3.3) and [326, 327, 336-339]. All components were selected to withstand high acceleration forces and shocks, temperature variations over the Martian diurnal cycle, and cosmic ray irradiation. Mossbauer measurements can be done during day and night covering the whole diurnal temperature... 

Fig. 8.29 The flight unit of the MEMOS II Mossbauer spectrometer sensor head (for the rover Opportunity), with the circular contact plate assembly (front side). The circular opening in the contact plate has a diameter of 15 mm, defining the field of view of the instrument...

Cosine smearing. Because instrument volume and experiment time must both be minimized for a planetary Mossbauer spectrometer, it is desirable in backscatter geometry to illuminate as much of the sample as possible with source radiation. However, this requirement at some point compromises the quality of the Mossbauer spectrum because of an effect known as cosine smearing [327, 348, 349] (see also Sects. 3.1.8 and 3.3). The effect on the Mossbauer spectrum is to increase the linewidth of Mossbauer peaks (which lowers the resolution) and shift their centers outward (affects the values of Mossbauer parameters). Therefore, the diameter of the source y-ray beam incident on the sample, which is determined by a... 

Thermal Emission Spectrometer) instrument indicated the metallic nature of the rock [340]. Observations made with the panoramic camera and the microscopic image revealed that the surface of the rock is covered with pits interpreted as regmaglypts and indicate the presence of a coating on the surface. The a-Particle-X-ray spectrometer (APXS) and the Mossbauer spectrometer were used to investigate the undisturbed and the brushed surface of the rock. Based on the Ni and Ge... 

Fig. 8.41 Left. Comparison of SNR of 14.4 keV Mossbauer spectra, taken with a Si-PIN detector system (MER instrument four diodes) and with a SDD detector system (advanced MIMOS instrument only one diode chip) Right. X-ray spectrum of a basalt (Ortenberg basalt see [366, 371], taken with a high resolution Si-drift detector system at ambient pressure (1 atm), demon-...

Introduce instrumental techniques used in analysis of the bioinorganic systems I will lecture on (Chapter 3 Instrumental and Computer-Based Methods). Typically, these would be electron paramagnetic resonance (EPR) and Mossbauer spectroscopies not often covered in undergraduate instrumental analysis courses plus X-ray diffraction and NMR techniques used for structural analyses of metalloproteins and their small molecule model compounds. 

The Mossbauer spectroscopy cell used in this investigation has been described in detail elsewhere (9). The Mossbauer spectroscopy instruments and the fitting routine used are also described elsewhere (9). All isomer shifts are reported relative to metallic iron at room temperature. 

The Mars Pathfinder rover carried an Alpha Proton X-ray Spectrometer (APXS), and the two Mars Exploration Rovers (MER - Spirit and Opportunity) carried Alpha Particle X-ray Spectrometers (also called APXS, but in this case more precise versions of the Pathfinder instrument, though without the ability to monitor protons for light element analyses). These instruments contained radioactive curium sources (Fig. 13.16) whose decay produced a-particles, which irradiated target rocks and soils. The resulting characteristic X-rays provided measurements of major and minor element abundances. The MER rovers also carried Mossbauer spectrometers, which yielded information on iron oxidation state. 

A typical Mossbauer effect spectrometer consists of a transducer which imparts controlled motion of a radioactive source, the sample itself, a suitable radiation detector, and certain necessary electronic instrumentation. Such a system is diagrammed in Figure 2A. 

The electronic equipment used is now commercially available from three or four sources. Ours consisted of a Nuclear Science Instruments Mossbauer spectrometer and a Northern Scientific pulse height analyzer. The spectra we obtained were all fitted (by least squares) by an IBM 360 computer (7) used in a time-shared system. Plots were also computer-produced. All of our spectra, whether obtained by transmission or by scattering, are shown with the ordinate as fractional absorption relative to the off-peak background. 

In terms of what is measured or observed, there are (1) portions of the electromagnetic spectrum gamma-ray, cosmic ray, x-ray, ultraviolet, infrared, far-infrared, microwave, and radiowave instruments (2) regions pertaining to the energies of particles beta ray (electrons), protons, neutrons, and mass associated instruments and (3) instruments dealing with other spectra such as radioactive decay and Mossbauer effects. 

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