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2 Preparation of Mössbauer Sources and Absorbers

2 Preparation of Mössbauer Sources and Absorbers

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46



3 Experimental



If radioactive sources are to be prepared for specific M€ossbauer isotopes, which

are commercially not available, there are a number of criteria to be considered. First

of all, if there are several different nuclear transitions leading to the excited nuclear

level of interest, one should preferentially choose the one that leads to the highest

intensity of M€

ossbauer quanta and has the longest half-life of the precursor nucleus.

The chemical composition of the source material should be such as to obtain narrow

and intense emission lines with low background from Compton scattering and XRF.

Any electric quadrupole or magnetic hyperfine perturbation would split or at least

broaden the emission lines, which in turn reduces the spectral resolution and

renders the evaluation cumbersome. The Debye–Waller factor should be as large

as possible (corresponding to a transition energy as small as possible) to obtain the

highest possible resonance absorption. The source material should be chemically

inert during the lifetime of the source and resistant against autoradiolysis. Various

methods of source preparation are outlined in Chap. 7.



3.2.1



Sample Preparation



The sample for a M€

ossbauer absorption measurement can be a plate of solid

material, compacted powder or a frozen solution, if it contains the M€ossbauer

isotope in sufficient concentrations and if the g-radiation can penetrate the material.

The first condition is almost trivial, low concentrations of the resonance nuclei

cause low signals and exceedingly long acquisition times, but the second condition

may be more severe than suspected, because the M€ossbauer g-rays are soft and get

strongly absorbed in many materials by nonresonant mass absorption, particularly

when heavy atoms (like chlorine or higher) are present in appreciable amounts.

The electronic absorption can be neglected only for samples with a high content of

the M€

ossbauer isotope and sufficiently light other atoms such as C, N, O, and H.

(Then the optimum absorber thickness is limited only by the onset of spectral

distortions when samples are too thick). High mass absorption of the 14.4 keV

radiation for 57Fe spectroscopy is already an issue when samples are made of glass

or enclosed in glass because of the absorption coefficient of silicon (N ¼ 14,

me ¼ 14 cm2 gÀ1). The choice of a good absorber thickness in such cases is a

compromise between affording a strong signal and avoiding low count rate due to

nonresonant g-attenuation. Optimization of this ratio is the topic of this section.

3.2.1.1



Basic Considerations



The strength of a M€

ossbauer signal is determined by the effective thickness of the

absorber, t ¼ fANMs0 (2.27). This dimensionless factor includes the number NM of

M€

ossbauer nuclei per square centimetre, the Debye–Waller factor fA of the absorber

material, and the resonance cross-section s0 of the M€ossbauer isotope. For a multiline

spectrum, the result must be split into separate values for each line, which are

obtained by weighting t with the relative transition probability of each line.



3.2 Preparation of M€

ossbauer Sources and Absorbers



47



The relative absorption depth of the M€

ossbauer line is determined by the product

ossbauer source and the fractional absorption

of the recoil-free fraction fS of the M€

e(t) of the sample, absr ¼ fSÁe(t), where e(t) is a zeroth-order Bessel function ((2.32)

and Fig. 2.8). Since e(t) increases linearly for small values of t, the “thin absorber

approximation,” e(t) % t/2, holds up to t % 1. On the other hand, values as small as

t ¼ 0.2 may cause already appreciable thickness broadening of the M€ossbauer lines,

according to (2.31), Gexp % 2Gnat(l ỵ 0.135t). In practice, therefore the sample

thickness may be limited to values of t % 0.2–0.5 to keep the resulting distortions

of line areas, heights, widths, and shape at a tolerable level.8

Using the value t ¼ 0.2 for the effective thickness, the amount of resonance

nuclei (57Fe) for a good “thin” absorber can be easily estimated according to the

relation NM ¼ t/(fAÁs0). For a quadrupole doublet with two equal absorption

peaks of natural width and a recoil-free fraction of the sample fA ¼ 0.7 one obtains

for the concentration of a good thin absorber: NMthin ¼ 2.23 Á 1017 atoms per cm2

or 21.1 mg cmÀ2 (the resonance cross-section for 57Fe is s0 ¼ 2.56 Á 10–18 cm2).

One milligram of natural iron, having 2.18% of 57Fe, corresponds to 2.35 Á 1017

resonance nuclei, or 0.386 mmol 57Fe.

Thus, one may state as a rule of thumb that an absorber should have about

21 mg cmÀ2 of 57Fe (0.37 mmol cmÀ2) or ca. 1 mg cmÀ2 of natural iron (17.9 mmol

cmÀ2) for a symmetric quadrupole doublet with natural line width.

The corresponding absorption depth for t ¼ 0.2 is 0.074, or 7.4% of the baseline

counts (absr ¼ 0.7 Á t/2). In practice, this value can be hardly accomplished

because of inhomogeneous line broadening arising from various sources and

because of increased background due to the contributions from nonresonant radiation reaching the detector. The experimental absorption depth for a quadrupole

doublet recorded with an absorber having 1 mg Fe per cm2 will not be more than %

5% if the intrinsic line width is 0.26 – 0.28 mm sÀ1 as is often found for inorganic

compounds, whereas 2Gnat ¼ 0.194 mm sÀ1.

One can also infer in turn from these arguments that the relative absorption depth

of a M€

ossbauer line should not exceed 10–15%, because of the increasing thickness

broadening and the related line distortions.



3.2.1.2



Counting Statistics and Acquisition Time



The M€

ossbauer 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 ¼ 20,000

counts sÀ1. Only about 85% of these will be 14.4 keV radiation; the others are

8

It is difficult to give an exact limit because the impact of thickness broadening depends on the

intrinsic width of experimental lines [31], which often exceeds the natural width 2Gnat by

0.05À0.1 mm sÀ1 for 57Fe as studied in inorganic chemistry. This inhomogeneous broadening,

which is due to heterogeneity and strain in the sample, causes a reduction of the effective

thickness. Rancourt et al. have treated this feature in detail for iron minerals [32].



48



3 Experimental



Fig. 3.12 Contributions to a M€

ossbauer spectrum given in counts per channel. Nb is the nonresonant background from scattered g-radiation and X-ray fluorescence in source and absorber



nonresonant background from Compton scattering, etc. If the spectrometer efficiency is 66%, the counting rate of 14.4-keV M€

ossbauer pulses per channel will be

typically C0 % 40 counts per second per channel (¼ C Á 0.85 Á 0.66/256). Thus,

when the mean numbers of counts collected in and off resonance after a certain

acquisition time Dt are denoted N0 and N1, respectively (see Fig. 3.12), the

absorption depth of the spectrum will be Ns ¼ (N1 À N0), or Ns % absrÁC0 ÁDt.

Since g-emission is a stochastic process, the collected data will scatter around

their mean values. The statistics of this result are described by the Poisson distribution

pffiffiffiffi

which for large numbers N approaches a Gaussian of half-width s ¼ N  DN.

Whenever the baseline at N1 is exactly known (from a line fit, etc.), the statistical

error or “noise” of the M€

ossbauer signal is essentially given by the distribution

of counts

at

resonance.

Since

for sufficiently

small values of absr the distribution

pffiffiffiffi

pffiffiffiffiffiffiffi

width N 0 is nearly the same as N1 , we can adopt the approximation DNS %



pffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffiffiffiffi

N1 ¼ C0 Á Dt. A common and convenient expression for the quality of the data

is the SNR, which we can define here by the simple approximation SNR

¼ NSffi/DNS.

pffiffiffiffiffiffiffiffiffiffiffiffi

Substitution of the relations for NS and DNS yields SNR ¼ absr Á C0 Á Dt, and

resolving for the Dt finally gives us the acquisition time.

À

Á

Dt ¼ SNR2 = abs2r Á C0 :



(3.1)



Hence, if we require a SNR of 40 for the spectrum, the acquisition time Dt will

be 16,000 s or about 4 Á 3=4 h for the aforementioned conditions (absr ¼ 0.05,

C0 ¼ 40 counts sÀ1 per channel).



3.2.1.3



Minimal Thickness of a M€

ossbauer Sample



The fact that according to (3.1) the run time for a spectrum is inversely proportional

to the square of the peak absorption absr and that the “noise” improves only with the



3.2 Preparation of M€

ossbauer Sources and Absorbers



49



square root of time implies also that there is a lower limit for the effective thickness

of a M€

ossbauer absorber. If, for instance, the iron content of a sample is ten times

less than that of a “good” sample as discussed earlier (t ¼ 0.2), the acquisition time

will be 100 times longer, which means Dt % 445 h. Since this may be at the limit of

acceptable acquisition time, a sample for 57Fe spectroscopy should have an effective thickness of at least tmin % 0.02, corresponding to 0.1 mg cmÀ2 of natural iron

or 0.039 mmol of 57Fe cmÀ2.



3.2.2



Absorber Optimization: Mass Absorption and Thickness



The thickness of a M€

ossbauer sample affects not only the strength of the M€ossbauer

signal but also the intensity of the radiation arriving at the detector because the

g-rays are inherently attenuated by the sample because of nonresonant mass

absorption caused by the photo effect and Compton scattering as mentioned earlier.

The counting rate C in the detector decreases exponentially with the density of the

absorber,

C ¼ C0 expðÀt0 Á me Þ;



(3.2)



where me is the total mass absorption coefficient of the sample given in cm2 gÀ1,

and t0 is the thickness of the sample, or the area density given in gÀ1 cm2. The

attenuation limits the statistical uncertainty of the number of counts N collected

in

pffiffiffiffi

each channel after a certain time Dt because of the relation DN ¼ N (seen

earlier). Since the M€

ossbauer signal increases almost linearly with the thickness

of the sample, whereas the count rate decays exponentially, there must exist an ideal

absorber thickness for which the measuring time is minimal and the SNR for a

certain acquisition time is maximal.

Particularly for “thin” M€

ossbauer absorbers with a low concentration of the

resonance nuclide and high mass absorption, it may be problematic to apply the

recommendation for sample preparation (t % 0.2), because the resulting electronic

absorption may be prohibitively high. In such a case, it may pay well to optimize the

absorber thickness, i.e., the area density t0 . To this end, following the approach of

Long et al. [33], we adopt the general expression:

.pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

SNR ẳ NS

DN1 2 ỵ DN0 2 ;

(3.3)

where NS ẳ N1 À N0 represents the signal amplitude in counts and N0 and N1 are

the counts per channel in and off resonance (Fig. 3.12). A relative simple solution to

the problem can be obtained for absorbers with low signal (t 1) when the

nonresonant background Nb can be neglected. Long et al. have shown [33] that

the variations of SNR then obey the plain exponential.

0



SNRðt0 Þ / t0 eÀt me =2 :



(3.4)



50



3 Experimental



Fig. 3.13 Signal-to-noise

ratio of M€ossbauer spectra as

a function of the area density

t0 of the sample for thin

absorbers (t 1) and

negligible nonresonant

background, Nb ( N1



Interestingly, the curve shape of SNR(t0 ) does not depend explicitly on the

concentration of M€

ossbauer nuclei in the sample. The function has a maximum at

t0 me ¼ 2 (see Fig. 3.13), as one can easily verify from the root of the first derivative.

The ideal thickness of a M€

ossbauer absorber is therefore given by [33]:

t0opt ¼ 2=me ðfor low nonresonant background; Nb ( N1 Þ



(3.5)



When Nb cannot be neglected, the analysis yields [33]

t0opt ¼ 1=me ðfor high nonresonant background ðNb > N1 =2Þ:



(3.6)



The derivation of the expressions (3.3)–(3.6) is found in Appendix A (cf. CDROM). Since for most 57Fe-spectra the level of nonresonant background counts, Nb,

may be in the range of 10–30% of the total counts, the absorber thickness is usually

best adjusted to a value between the limits given above. The maximum of SNR(t0 ) is

naturally rather broad, such that deviations from t0 opt of even Ỉ50% are fairly

immaterial.

Conclusions. A M€

ossbauer sample with a low content of the resonance nuclide has

ideal thickness when it attenuates the incident radiation by ca. 63–85%

(me Át0 ¼ 1–2, C0 /C0 % eÀ1 – eÀ2). However, the optimization should be subordinated to the requirement of a “thin absorber” having an effective thickness t < 1 to

avoid excessive line broadening.

3.2.2.1



Mass Absorption Coefficients



The mass absorption coefficient me used in (3.5)–(3.6) is obtained from tabulated

mass absorption coefficients (given in cm2 gÀ1) and the mass fractions ci for the

elements in the sample:

me ¼



X



ci mi :



(3.7)



Mass fractions for molecules are easily obtained from the relation ci ¼ Mi/M,

where Mi is the product of the atomic mass of the ith element and its abundance in



3.2 Preparation of M€

ossbauer Sources and Absorbers



51



Table 3.3 Mass fractions and mass absorption coefficients for C1610H3624N110FeP101F606

Mia

ci

me,i (cm2 gÀ1)

cime,i (cm2 gÀ1)

C

12 Á 1,610

0.493

0.87

0.429

H

1 Á 3,624

0.093

0.387

0.036

N

14 Á 110

0.039

1.4

0.055

Fe

55.85 Á 1

0.001

64

0.064

P

31 Á 101

0.079

14.2

1.122

F

19 Á 606

0.294

2.7

0.794

∑ ¼ 2.499 cm2 gÀ1

a

Atomic mass times abundance per formula unit; the total molar mass is M ¼ 39,228 g molÀ1



the formula unit, and M is the total molar mass of the compound. Atomic mass

absorption coefficients for the 14.4 keV radiation of 57Fe are summarized in

Appendix B (cf. CD-ROM) and can also be found in [33, 34].

An Example

For illustration, we shall demonstrate the thickness optimization of the iron(III)

bis-azide complex, [(cyclam)FeIII(N3)2]PF6, mixed with a 100-fold excess of

(TBA)PF6 (tetrabutylammonium–hexafluorophosphate), which is often used as a

conducting electrolyte in electrochemistry. The iron compound has the chemical

composition C10H24N10FePF6 and a molar mass of M ¼ 485.2 g molÀ1. (TBA)PF6

has the composition C16H36NPF6 and a mass of M ¼ 387.428 g molÀ1. We define

an effective composition of the mixture by summing up the composition

of the compound and one hundred times that of (TBA)PF6, yielding

C1610H3624N110FeP101F606 with an effective mass of M ¼ 39,228 g molÀ1. The

data for the participating atoms are compiled in Table 3.3. The total mass absorption coefficient is found to be me ¼ 2.499 g cmÀ2, which according to (3.5)

yields the ideal absorber thickness t0 opt ¼ 2/2.499 cm2 gÀ1 ¼ 0.89 g cmÀ2,

corresponding to about 1.3 mg of natural iron per cm2!

3.2.2.2



Solvents, Solutions, and Powders



Mass absorption increases strongly with the atomic number Z. For the 14.4 keV

radiation of 57Fe, the coefficient follows approximately the relation me % 0.003 Á Z3

from oxygen to krypton. Therefore, organic solvents containing sulfur or chlorine

are virtually opaque to the M€

ossbauer radiation. The sulfur component of a 2 mm

layer of dimethylsulfoxide (DMSO) absorbs %70% of the M€ossbauer radiation

(t0 ¼ 1.1 g cmÀ2) [35]. Even worse is dichloromethane (CH2Cl2), having an

absorption coefficient of 16.83 cm2 gÀ1. A layer of 0.1 g cmÀ2, which is only

0.75 mm thick (r ¼ 1.33 g cmÀ3), absorbs about 82% of 14.4 keV radiation.9 For

the same reason, chlorinated polymers (PVC) or glass should not be used for

9



The mass absorption of a solution of an iron complex in CH2Cl2 is usually entirely dominated by

the solvent. With me ¼ 16.83 cm2 gÀ1, the optimized sample thickness t0 opt is between 0.059 and

0.119 g cmÀ2, or 45–89 ml cmÀ2, which corresponds to a layer thickness of 0.45–0.89 mm.



52



3 Experimental



making sample holders. Water has an absorption coefficient of me ¼ 1.7 cm2 gÀ1,

whereas a typical protein has me ¼ 1.3 cm2 gÀ1 [35]. The ideal thickness of protein

samples with water as the solvent is about 0.8 g cmÀ2, which corresponds to a layer

thickness of about 8 mm.

Very thick absorbers may be required for applied-field measurements to achieve

reasonable absorption depths and measuring times because the M€ossbauer spectra

are usually split into several hyperfine components. Here the iron content may be as

large as %100 mg 57Fe per cm2 (1.75 mmol 57Fe per cm2), which would correspond

to t % 1 for a two-line spectrum. For studies of frozen solutions, 57Fe concentrations of 1 mM are desirable for each nonequivalent iron site [35].

M€

ossbauer absorbers should be reasonably homogeneous. Holes must be

avoided, particularly when particles or frozen liquids are mounted. A mixture of

grains having large thickness with spots of vanishing iron content may yield

thickness broadening and extremely small absorption. Powder material must be

fixed to avoid sliding. Moreover, minor “roughness” of the absorber layer hardly

affects the performance of the spectrometer because the g-beam does not pass

through any optical component.

3.2.2.3



Isotope Enrichment



Compounds with natural iron can be measured in nonchlorinated organic solvents

or water when the concentration is at least 35–50 mM (sample thickness 6–8 mm).

Since the natural abundance of 57Fe is only 2.18%, the M€ossbauer isotope can be

enriched up to 40 times to lower the sample concentration if necessary. The stable

isotope is available as metal sheets, metal powder, or oxide with about 95% of the

isotope 57Fe. Proteins are usually prepared by in vivo enrichment up to 90% of 57Fe;

in vitro enrichment is also possible if the iron site of the natural protein can be

metal-depleted and reconstituted with the M€

ossbauer isotope ions in the original

oxidation state. An interesting aspect of both methods is the possibility of selective

enrichment of certain iron sites if multicomponent enzymes are to be studied. For

the enrichment of synthetic compounds, it might be favorable not to use the highest

isotope concentration, but rather to synthesize a larger amount of the substance with

only 30–50% enrichment. Handling is facilitated, and good absorbers can be easily

prepared, if solubility of the compound is sufficiently high.



3.2.3



Absorber Temperature



The recoil-free fraction fA of transition metal complexes or proteins in frozen

solution can be as small as 0.1–0.3, when measured just below the melting point,

but the f-factor increases strongly when the temperature is lowered to liquid

nitrogen temperatures (77 K), and at liquid helium temperatures (4.2 K) it may

reach values of 0.7–0.9 [35]. This makes a substantial difference to the acquisition

time of the spectra because of the square dependency on the signal (3.1).



3.3 The Miniaturized Spectrometer MIMOS II



53



Paramagnetic compounds are measured at liquid helium temperatures to slowdown spin relaxation when magnetic hyperfine parameters are to be evaluated, and

to increase the difference in Boltzmann population of the magnetic sublevels. For

half-integer spin systems, the application of fields of only a few milli-Tesla may

induce sizable magnetic hyperfine splitting in the static limit of slow relaxation

without line broadening (see Sect. 4.6 and Chap. 6).

M€

ossbauer spectra can yield valuable information about the abundance of different M€

ossbauer sites in a sample from the relative intensities of the corresponding

subspectra if the f-factors are known. These, however, depend critically on temperature. Differences for the individual sites must be expected at ambient temperatures; however, they all vanish at 4.2 K because f(T) approaches one for T ! 0

(see (2.15)). Often measurements at 80 K are sufficient for reliable estimates of the

true intensity ratio of M€

ossbauer subspectra.



3.3



The Miniaturized Spectrometer MIMOS II



G€

ostar Klingelh€

ofer10



3.3.1



Introduction



Iron-57 M€

ossbauer spectroscopy has been used in earth-based laboratories to study

the mineralogy of iron-bearing phases in a variety of planetary samples, including

lunar samples returned to earth by American Apollo astronauts and Soviet (Russian)

robotic missions, and meteorites that have asteroidal and Martian origin (see

[36, 40] and references therein). In the early 1990s, the development of a

miniaturized M€

ossbauer spectrometer was initiated by E. Kankeleit at the

Technical University of Darmstadt, originally for the Russian Mars mission

Mars-92/94. This development was led by Klingelh€ofer, who brought the project

to completion for the NASA Mars Exploration Rover 2003 mission and the ESA

Mars-Express Beagle 2 mission at the Institute of Inorganic Chemistry, University of Mainz. The Beagle 2 mission failed, but the Mars Exploration Rovers

(MER) (Fig. 3.14; see also Sect. 8.3) Spirit and Opportunity landed successfully

on the Red Planet in January 2004. The goal of the mission was to search for

signatures of water possibly from the past. Both Rovers, Spirit and Opportunity

[37–39, 53–55, 60, 62], carry the miniaturized M€ossbauer spectrometer MIMOS II

[36] as part of their scientific payload. For more details, see Sect. 8.3. In the

following section we describe the technical details and specialities of the miniaturized M€

ossbauer spectrometer MIMOS II.

10



Institut f€ur Anorganische Chemie und Analytische Chemie, Johannes Gutenberg-Universit€at

Mainz, Staudingerweg 9, 55099 Mainz, Germany; e-mail: klingel@uni-mainz.de



54



3 Experimental



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 M€

ossbauer spectrometer and other

instruments can be seen in stowed position. Right: robotic arm before placement on soil target at

Victoria crater rim, Meridiani Planum, Mars. The M€

ossbauer instrument MIMOS II with its

circular contact plate can be seen, pointing towards the rover camera. See also Sect. 8.3



3.3.2



Design Overview



The MIMOS instrument II is extremely miniaturized compared to standard laboratory M€

ossbauer spectrometers and is optimized for low power consumption and

high detection efficiency [36, 40, 41]. All components were selected to withstand

high acceleration forces and shocks, temperature variations over the Martian

diurnal cycle, and cosmic ray irradiation. Because of restrictions in data transfer

rates, most instrument functions and data processing capabilities, including acquisition and separate storage of spectra as a function of temperature, are performed by

an internal dedicated microprocessor (CPU) and memory. The dedicated CPU is

also required because many M€

ossbauer measurements are done at night when the

rover CPU is turned off to conserve power. High detection efficiency is extremely

important to minimize experiment time. Experiment time can also be minimized by

using a strong 57Co/Rh source. Instrument internal calibration is accomplished by a

second, less intense radioactive source mounted on the end of the velocity transducer opposite to the main source and in transmission measurement geometry with

a reference sample (see also Sects. 3.1.3 and 3.3.6). The spectrometer can also be

calibrated using an external target (e.g., Fe-metal or magnetite) on the rover

spacecraft.

Physically, the MIMOS II M€

ossbauer 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



3.3 The Miniaturized Spectrometer MIMOS II



55



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 reemitted 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 M€

ossbauer 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 57Co/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 M€

ossbauer spectrometer MIMOS II

has the dimensions $ (5 Â 5.5 Â 9.5) cm3 and weighs only ca. 400 g. Both 14.4 keV

g-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].



3.3.2.1



M€

ossbauer Sources, Shielding, and Collimator



To minimize experiment time, the highest possible source activity is desirable, with

the constraint that the source line width should not increase significantly (maximum

by a factor of 2–3) over the $9–12 months’ duration of the mission. Calculations

and tests indicate an optimum specific activity for 57Co at 1 Ci per cm2 [42, 43].

Sources of $350 mCi 57Co/Rh with a specific activity close to this value and

extremely narrow source line width (<0.13 mm sÀ1 at room temperature), given



56



3 Experimental



Velocity [mm s–1]



Velocity [mm s–1]



Fig. 3.16 Schematic drawing of the MIMOS II M€

ossbauer spectrometer. The position of the

loudspeaker type velocity transducer to which both the reference and main 57Co/Rh sources are

attached is shown. The room temperature transmission spectrum for a prototype internal reference

standard shows the peaks corresponding to hematite (a-Fe2O3), a-Fe, and magnetite (Fe3O4). 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



the high activity, were produced by Cyclotron Co. Ltd. (Russia) in custom-made

space-qualified Ti-holders, tested successfully, and mounted in flight instruments

approximately 90 days prior to launch. The rhodium matrix precludes additional

line broadening at lower temperatures on Mars.

The effective shielding of the detector system from direct and cascade radiation

from the 57Co/Rh source is also very important. A graded shield consisting of

concentric tubes of brass, tantalum, and lead was selected. The thickness and the

shape of different parts of the shielding were optimized so that nearly zero direct

122 and 136 keV radiation (emitted by the 57Co source) was in a direct line with the

detectors (see Fig. 3.16).

The shielding also acts as the collimator which fixes the diameter of the target

that is illuminated by g-rays. As discussed previously, this diameter is as large as

possible to minimize experiment time within the constraint of acceptable cosine

smearing [44, 50, 51]. The measure used for acceptable cosine smearing was the

ability to reliably resolve the strongly overlapping spectra of hematite and maghemite in a 1:1 mixture of those oxides. A series of experiments with this mixture and

the pure oxides were conducted at constant source intensity and variable collimator

diameters between 4.5 and 7.1 mm, which correspond to illumination diameters of



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