Spectrometer Mossbauer

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]. 

Mossbauer spectra are usually recorded in transmission geometry, whereby the sample, representing the absorber, contains the stable Mossbauer isotope, i.e., it is not radioactive. A scheme of a typical spectrometer setup is depicted in Fig. 3.1. The radioactive Mossbauer source is attached to the electro-mechanical velocity transducer, or Mossbauer drive, which is moved in a controlled manner for the modulation of the emitted y-radiation by the Doppler effect. The Mossbauer drive is powered by the electronic drive control unit according to a reference voltage (Fr), provided by the digital function generator. Most Mossbauer spectrometers are operated in constant-acceleration mode, in which the drive velocity is linearly swept up and down, either in a saw-tooth or in a triangular mode. In either case. 

Most Mossbauer spectrometers use triangular velocity profiles. Saw-tooth motion induces excessive ringing of the drive, caused by extreme acceleration during fast fly-back of the drive rod. Sinusoidal operation at the eigen frequency of the vibrating system is also found occasionally and... 

The y-detector of a Mossbauer spectrometer converts the incident y-photons into electric output pulses of defined charge (see Sect. 3.1.6). The detector signals are electronically amplified and shaped by an amplifier network to obtain strong needle pulses with well-defined rise time, so that the pulse height is proportional to the energy of the incident photon. The amplifiers are usually adjusted to obtain... 

In summary, pulse-height analysis (PHA) prior to a Mossbauer measurement is an essential step in tuning a Mossbauer spectrometer. PHA allows the adjustment of the y-detection system to the Mossbauer photons and the reduction of noise by rejecting nonresonant background radiation. 

Fig. 3.10 Variation of the spectrometer aperture as a function of the source motion for Mossbauer spectrometers operated in constant acceleration mode with triangular velocity profile, and the resulting nonlinear baseline distortion of the unfolded raw spectra. For simplicity a point-source is adopted, in contrast to most real cases (Rib mm active spot for Co in Rh)...

The Mossbauer spectrometer will typically divide the velocity scale into 256 channels. For a 0.93 GBq source (25 mCi), the total count rate of photons arriving at the detector and having the proper pulse-height is usually about C = 20,000 counts s Only about 85% of these will be 14.4 keV radiation the others are... 

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...

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... 

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],... 

Fig. 3.16 Schematic drawing of the MIMOS II Mossbauer spectrometer. The position of the loudspeaker type velocity transducer to which both the reference and main Co/Rh sources are attached is shown. The room temperature transmission spectrum for a prototype internal reference standard shows the peaks corresponding to hematite (a-Fe203), a-Fe, and magnetite (Fe304). The internal reference standards for MIMOS II flight units are hematite, magnetite, and metallic iron. The backscatter spectrum for magnetite (from the external CCT (Compositional Calibration Target) on the rover) is also shown...

Fig. 3.19 Schematic illustration of the measurement geometry for Mossbauer spectrometers. In transmission geometry, the absorber (sample) is between the nuclear source of 14.4 keV y-rays (normally Co/Rh) and the detector. The peaks are negative features and the absorber should be thin with respect to absorption of the y-rays to minimize nonlinear effects. In emission (backscatter) Mossbauer spectroscopy, the radiation source and detector are on the same side of the sample. The peaks are positive features, corresponding to recoilless emission of 14.4 keV y-rays and conversion X-rays and electrons. For both measurement geometries Mossbauer spectra are counts per channel as a function of the Doppler velocity (normally in units of mm s relative to the mid-point of the spectrum of a-Fe in the case of Fe Mossbauer spectroscopy). MIMOS II operates in backscattering geometry circle), but the internal reference channel works in transmission mode...

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... 

The miniaturized Mossbauer spectrometer MIMOS II has been used already in several terrestrial applications which would not have been possible before. A number of other possible terrestrial applications, for example, in the field, in industry, and fundamental research, are under consideration. With the new generation of the Mossbauer spectrometer MIMOS 11, the method itself can be applied to numerous new fields in research, environmental science, planetary science, and many other fields. Because of this reason, Mossbauer spectroscopy may become a more widely used method than it is today. 

Rather sophisticated applications of Mossbauer spectroscopy have been developed for measurements of lifetimes. Adler et al. [37] determined the relaxation times for LS -HS fluctuation in a SCO compound by analysing the line shape of the Mossbauer spectra using a relaxation theory proposed by Blume [38]. A delayed coincidence technique was used to construct a special Mossbauer spectrometer for time-differential measurements as discussed in Chap. 19. 

Klingelhofer, G. et al. 2003. Athena MIMOS II Mossbauer spectrometer investigation. Journal of Geophysical Research, 108(E12), 8067, doi 10.1029/2003JE002138. 

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