Velocity calibration

Cahbration spectra must be measured at defined temperamres (ambient temperature for a-iron) because of the influence of second-order Doppler shift (see Sect. 4.2.1) for the standard absorber. After folding, the experimental spectrum should be simulated with Lorentzian lines to obtain the exact line positions in units of channel numbers which for calibration can be related to the hteramre values of the hyperfine splitting. As shown in Fig. 3.4, the velocity increment per channel, Ostep, is then obtained from the equation Ustep = D,(mm s )/D,(channel numbers). Different 

The calibration procedure is sufficiently accurate for Doppler velocities in the regime 0 to 10 mm s beyond this range, laser calibration is more suitable. Calibration with ot-iron, as described, can also be used for Mossbauer measurements with other isotopes, e.g., Ni, Au, and Ir, for which suitable standard absorbers are not available (provided that the Doppler velocity range of interest is not significantly greater than 10 mm s ). This, of course, requires that the spectrometer is temporarily equipped with a Co source and an a-iron absorber. 

Interestingly, the correct polarity of the Mossbauer drive can be checked by using the isomer shift of oc-iron with respect to the materials in Table 3.1. After folding of the raw data, the center of the calibration spectrum without further correction must be at —0.12 mm s relative to the Co/Rh source material. 

Mossbauer spectra of Dy, Sb, and other isotopes may show resonances at 

The number of beatings in the output signal U t) can be recorded as a function of the channel number simultaneously to the Mossbauer spectrum (usually in a second part of the MCA memory). Accordingly, the number of beats stored in each channel per velocity sweep divided by the dwell time of the MCA channels yields the specific average velocity for each detection channel. 

The window material used in vacuum cryostats is usually beryllium, aluminium, or aluminised mylar. It should be noted, however, that commercial beryllium and aluminium often contain sufficient iron to give a detectable Fe (14 keV) resonance, and this has been known to cause problems when working with this isotope. 

Temperatures above 300 K are sometimes desirable for Fe work, and a controlled-temperature furnace is then required [68, 69]. The sample is frequently sandwiched between thin discs of beryllium, graphite, or aluminium (m.p. 660°C) attached to an electric heating coil in a vacuum. Alternatively [70], the sample can be placed in a furnace with thin entrance and exit windows and containing an atmosphere of hydrogen to prevent oxidation. A vacuum furnace capable of providing temperatures up to 1000°C has been described [71] and temperatures of 1700°C have been reached with a helium-filled oven with beryllium windows [72]. 

High-pressure work has been published by relatively few laboratories because pressure cells require special experience and engineering [73-75]. 

If a large external magnetic field is to be applied to an absorber, this is usually done with a superconducting magnet installation which also requires a liquid helium cryostat for its operation [76]. Again, several commercial instruments are available. 

One of the more difficult experimental aspects of Mossbauer spectroscopy is the accurate determination of the absolute velocity of the drive. The calibration is comparatively easy for constant-velocity instruments, but most spectrometers now use constant-acceleration drives. The least expensive method, and therefore that commonly used, is to utilise the spectrum of a compound which has been calibrated as a reference. Unfortunately, suitable international standards and criteria for calibration have yet to be decided. As a result, major discrepancies sometimes appear in the results from different laboratories. The problem is accentuated by having figures quoted with respect to several different standards, necessitating conversion of data before comparison can be made. However, calibration of data from an arbitrary standard spectrum will at least give self-consistency within each laboratory. 

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A calibration facility must produce the desired velocity range for the meter to be calibrated. The air temperature should be kept constant over the test to ensure constant density. For thermal anemometers, velocity calibration only is not sufficient. They should also be checked for temperature compensation. In the case of omnidirectional probes, sensitivity to flow direction should be tested. In the case of low-speed (thermal) anemometers, their self-convection error should be measured, and, for instruments measuring flow fluctuation (turbulence), dynamic characteristics testing should be carried out as well. ... 

During the mission, the magnetite CCT was measured in several runs to verify the functionality of MIMOS II. The well-known Mossbauer parameters of magnetite were used for velocity calibration, as shown in Fig. 3.22 for different temperatures. This kind of measurement was done in the laboratory with the flight units as a function of temperature to be used as a reference for the measurements on Mars. Figure 3.22 shows the Mossbauer spectra of the CCT at different mean temperatures. 

Dr. Spijkerman I still feel we should have a doublet because it is so time consuming to set up a spectrometer for iron, determine the velocity calibration, and then have to take oflF the source, change the detector and everything around to do a tin measurement. If we can get a standard for tin I think we should have a doublet that makes a velocity calibration possible. I would like to see a doublet that has approximately 3 or 4 mm./sec. (quadrupole) splittings—artificial if need be. 

Dr. Thomas No, in general. I was thinking of it as a velocity calibration, not as a zero point calibration. What is the situation on obtaining standards which give a velocity calibration over a wider range so that very accurate estimates of this splitting are not necessary ... 

Further identification of the particles is made with 57Fe Mossbauer spectroscopy. Mossbauer spectra were recorded with a conventional constant acceleration spectrometer with 57Co in Rh matrix as a y-ray source. Velocity calibration was made using a 5-pm a-Fe foil at 293 K. Figure 1.6.10 shows the Mossbauer spectra of the sample recorded at 293 K and 4.2 K. Spectra were fitted with theoretical... 

There are a number of cases, however, for which the above indirect velocity calibration is not well suited. The relationship between velocity and position... 

A similar absolute calibration method uses a diffraction grating on the end of the drive shaft opposite to the y-ray source (30). In either case, however, these methods of absolute velocity calibration provide a determination of the drive shaft motion during collection of all Mdssbauer spectra, and as such offer an important advantage over indirect methods of velocity calibration. 

FIGURE 2.1 Energy levels, isomer shift, and quadrupole splitting for 57Fe. The Mossbauer spectrum shown was recorded at T— 4.2 K with a sodium nitroprusside absorber, a diamagnetic (.S 0) compound often used for velocity calibrations. The isomer shift, indicated by the vertical dashed line, has the value 5 — —0.18 mm/s relative to Fe metal. 1 mm/s Doppler shift... 

The Doppler shift, even with a near co-linear geometry, is not a serious limitation to the precision, at least at the level of 0.002 cm-1 for the 2Si/2 2P3/2 interval. We are pursuing a set-up using a 5° intersection angle, with two 13C16C>2 lasers. The beam velocity calibration will make use of previously accurately measured transitions in heliumlike ions. 

Figure 1 Mossbauer spectrometer with velocity calibrator Mossbauer drive and detector. The sample is placed between drive and detector. For measurements at cryogenic temperatures, the sample is moimted in a cryostat equipped with Mylar or beryllium windows. (Reproduced with permission Wissel Gmbh)...

FIGURE 1 Velocity calibration of a M ssbauer spectrometer. The spectrum shown (a) is of metallic iron at room temperature. Line positions are given in channels and line widths in mm/s. The velocity calibration constant (b) is derived from the known energy differences between various components of the magnetic hyper fine spectrum. In the present data a differential nonlinearity of about 1 percent is observed. Such spectrometer nonlinearity may become a source for significant systematic errors in high-resohition experiments. 

Several systems for a more direct absolute-velocity calibration have been developed. In one such method [31], the output of the monitoring pick-up... 

Flow Through Safety Rods. The shim-safety rods afso require a cooling velocity of 30 ft/sec through the fuel section. Measurements in a previous experiment indicated a pressure drop of 40 psi for this velocity. Calibration of the Mock-Up rods agreed with this figure. The curve of velocity versus pressure drop is presented in Fig. A4.L. 

The Mossbauer measurement requires the generation of a precise, controllable relative motion between the source and the absorber. A large variety of drive systems has been developed. The majority of drives work on electromechanical, mechanical, hydrauHc, and piezoelectric principle. The spectrometers can be classified into constant-velodty spectrometers and velocity-sweep spectrometers. The mechanical drives, hke a lead screw or a cam, move with constant velocity. They have advantages for the thermal scan method and because their absolute velocity calibration is straightforward. The velocity-sweep spectrometers are usually of electromechanical nature (like loudspeaker-type transducers) and normally used in conjunction with a multichannel analyzer. The most commonly used M(t) functions are rectangular (constant velocity), triangular (constant acceleration), trapezoidal, and sinusoidal. A typical Mossbauer spectrometer is shown schematically in O Fig. 25.24. 

The block scheme of a Mossbauer spectrometer. A, absorber CR, cryostat with temperature controller TC (optional, for low-temperature measurements) S, source moved by velocity transducer VT of driving unit DR FG, function generator VC, velocity calibrator (optional) LI, laser interferometer (optional) DET, detector HV, high-voltage power supply PA, preamplifier AM, amplifier SCA, single channel analyzer MCA, multichannel analyzer and PC, computer, OP, output... 

The M5ssbauer spectrum is usually obtained as the transmitted or scattered intensity (in counts) vs. the Doppler velocity (in mm/s). However, the data provided by the analyzer are normally given in the form of counts vs. channel number. The correspondence between channel numbers and velocity is established by velocity calibration. 

The velocity calibration of the Mossbauer spectrometer is performed either by measuring the Mossbauer spectrum of standard materials or by a calibrator instrument measuring the absolute velocity of the source relative to the absorber. The latter can be achieved by counting the fringes from a Michelson interferometer with a laser source connected to the multichannel analyzer, and thus the velocity for each channel can be obtained. 

The essential components of a modem Mdss-bauer spectrometer as illustrated in the block-diagram of Figure 5 are the velocity transducer, the wave form generator and synchronizer, the multichannel analyzer, y-ray detection system, a cryostat or oven for low and temperature dependent measurements, a velocity calibration device, the source and the absorber, and a read-out unit. 

D.G. Agresti, M.D. Dyar, M.W. Schaefer, Velocity calibration for in situ Mdssbauer data from Mars. Hyp. Interact. 167, 845-850 (2006)... 

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