METHODOLOGY OF "LESS USED" MÖSSBAUER ISOTOPES

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METHODOLOGY OF ”LESS USED” MÖSSBAUER

ISOTOPES

G. Kalvius, F. Wagner, W. Potzel

To cite this version:

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MOSSBA

UER I S 0 TOPES AND TECHNIQUES.

METHODOLOGY OF

a

LESS USED

u

MOSSBAUER

ISOTOPES

(*)

G. M. KALVIUS, F. E. WAGNER and W. POTZEL

Physik Department, Technische Universitat Miinchen, D-8046 Garching, James Franckstr., Germany

Rhumk. - On donne un aperqu des parametres importants pour une vingtaine de resonances qui ne sont pas encore utilisks de fa~on courante en spectroscopie Mossbauer, quoiqu'elles pr6sentent genkralement des possibilites potentielles intkressantes.

On indique en particulier la technique de preparation des sources et la sensitivitd des resonances pour les differentes interactions hyperfines. On discute brihvement quelques problemes expkrimen- taux lies a ces rksonances.

Abstract.

-

A survey is presented of useful parameters of some twenty resonances which so far have found only limited use in general applications of Mossbauer spectroscopy although they possess a good potential for such measurements. Described are in particular source preparation techniques and the sensitivity of the resonances for the different hyperfine interactions. A short discussion of main experimental problems connected with each of these resonances is also included.

1. Introduction.

-

This survey is written with the aim to encourage wider use of Mossbauer resonances other than 57Fe, '19Sn and '"Eu. Amongst the isotopes and resonances available, we have selected those which have already found a certain spectrum of applications. Exotic resonances which are at present not sufficiently well-established for general use (e. g. 63Ge or 67Zn), resonances in radioactive nuclei (with the exception of the quasistable isotopes lZ9I and 237Np), resonances with inherently too poor a resolution to be useful for the determination of hyperfine parameters (e. g. 1 5 9 ~ b ) , and resonances which require special techniques for their observation (e. g. 139La) have been omitted. This does not necessa- rily mean that such resonances cannot find wider use in the near future, as the experimental techniques develop.

2. General remarks. - The fairly wide use of the resonances 57Fe, l19Sn and l5'Eu is in part due to the fact that their source activities are long-lived and easily available. Furthermore, experiments can be carried out at room temperature. For these isotopes one usually purchases ready made Mossbauer sources. If spectra have to be taken at low temperatures (e. g. for the investigation of magnetic materials with low transition temperatures). it is common practice to keep the source at room temperature and to cool the absorber only in a horizontal-beam transmission cryostat [85], [86], [87]. This configuration allows a simple spectrometer design if the Doppler motion is fed to the source.

(*) Supported by the Bundesministerum fur Forschung und

Technologic Federal Republic of Germany.

For most Mossbauer isotopes discussed here, commercial sources are either impractical because of the short half-life of the activity or simply unecono- mical. In some cases it may be reasonable to purchasse the carrierfree activity but then to prepare the Moss- bauer source oneself. This aspect is discussed in section 4 for the various isotopes. The majority of Mossbauer transitions described in the following require also that both the source and the absorber be cooled. Then a vertical beam geometry, where the source (or the absorber) is driven by a transducer mounted on top of the cryostat (see Fig. 1) is the commonly used spectrometer. The long drive rod necessary for transmitting the Doppler motion requires special attention in mechanical design. Otherwise it may be the origin of sizeable deviations in the velocity wave form particularly when the drive system is operated in the constant acceleration or (even more serious) in the flyback mode. These problems are eased considerably if sinusoidal motion is used. Then, if a multiscaler device operated in time mode is employed for data storage, a non-linear velocity scale will result [88]. The least squares fitting routine must include a procedure for the linearization and folding of spectra.

Whenever possible, the source compound should be metallic. This avoids problems with the so-called after effects of nuclear decay (e. g. multiple emission lines) [89]. It also makes the handling of radioactivity much easier, provided that one can use either small ingots or roll the material into sufficiently thin foils. The former is usually possible only for gamma transitions exceeding 60 keV since electronic absorption is then a minor problem. Figure 2 shows the design of a standard source holder as used in our

A 2

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C6-658 G. M. KALVIUS, F. E. WAGNER A N D W. POTZEL

GAMMA RAY WINDOW CALIBRATION SOURCE

M ~ S S B A U E R DRIVE ELECTRICAL DRIVE HOUSING FEED THRU \

EXCHANGE

GAS L I N E GAS GAUGE GE

He PUMP L I N E He F I L L 1

RIVE GUIDANCE SPRINGS BSORBER MOUNT DURCE HEATER HERMAL SHIELD BSORBERHEATER HERMOMETER --STYROFOAM COVER THERMAL CONTACTSPRINGS L N CONDUCTION SHIELD SAFETY VALVE BAFFLES L I O NITROGEN L l O HELIUM EXCHANGE GAS DRIVE TUBE CENTER T U B E SUPPORT TUBE

CENTER TUBE INDIUM SEAL

GAMMA RAY COLLIMATOR COOLED GE (LI) 1 ' Y

DETECTOR

FIG. 1.

-

Liquid Helium cryostat with vertical beam axis for Mossbauer experiments. Both source and absorber are located in the exchange gas column. The doppler motion is fed to

the absorber

laboratory. It is coupled to the drive rod via a screw connection. The inner plastic container is sealed and fastened to the outer aluminium casing by applying vacuum grease.

Fully sealed containers should be used for powder sources. Faults in design of such source holders may lead to radioactive accidents : In the majority of low temperature spectrometers the source is immersed in the liquid coolant. If tiny leaks exist the liquid seeps into the container which then explodes upon warm-up. Such leaks may develop in time, either by stress experienced in thermal cycling or by radiation damage caused by the source activity. The latter is particularly a problem with activities which emit high energy P-rays (e. g. 170Tm) or with containers sealed with epoxy. If sealed off quartz containers are used, a double enclosure is recommended. The outer ampoule should routinely be inspected for strong coloration. Aluminum containers are a better choice. To seal them perma- nently helium arc or better electron beam welding equipment is needed. In our laboratory we have used a container sealed with an indium O-ring. Its design is basically similar to the absorber holder shown in

Aluminium

P l e ~ ~ g h s s or Nylon

FIG. 2.

-

Simple source holder : a) top of outer container with connection to drive rod ; b) spacer ; c) top of inner container ; d ) bottom of inner container ; e) bottom of outer

container.

figure 3 which is used for measurements on radioactive

samples (e. g. 237Np). The active compound is filled into an inner plastic container inside a glove box or ,a plastic glove bag or a radioactive hood. This contai-

Indium O- seal 2 Rubber 0 - Rings Absorber Substance

a

Nylon

a

Aluminium

RG. 3.

-

Sealed absorber holder for radioactive powders (e. g. 237Np). The total tickness of A1 in the y-ray beam is

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ner is loosely sealed with vacuum grease. The outer walls of the plastic container will usually be slightly contaminated with the sample activity. In the lock of the glove box or in the hood the plastic container is therefore put into a thin-walled aluminum container which is again sealed by vacuum grease. The sample can now safely be handled in the open, where it is finally sealed in the aluminum container equipped with the indium O-ring. Some of the danger connected with the helium seepage described above can be avoided if source and absorber are located inside an exchange gas column in the cryostat and not in the liquid itself. The exchange gas pressure is kept at several Torr.

Another problem is sometimes encountered with powder sources : If loosely packed inside a sealed evacuated container, the heat contact to the coolant may be extremely poor and the cool down is very sluggish. Furthermore, when used in spectrometers where the Doppler motion is applied to the source the loose packing of the powder may lead to grain motion and thus cause distortions in line shape. Due to variations in thermal contraction of different materials one can not always be sure that a powder is tightly pressed at low temperatures.

Neutron activation of samples (powder and foils) is usually done in sealed quartz ampoules. For irradiations of a few hours polyethylene ampoules can be used. Both types of ampoules need not be re-packed after irradiation in most cases. They should, however,

be used inside a sealed outer container as described above. Cyclotron irradiations of powders are difficult because of the strong local heating by the beam. It is best to press the material into a small bore in a Cu block and to cover it with a very thin A1 or Ni foil. Such, or similar, irradiation facilities are usually available at any cyclotron. Even if foils are irradiated, the maximum tolerable power dissipated in the foil limits the beam current. Good heat contact to the watercooled target mount is important, and defocuss- ing of the beam spot may be helpful.

3. Tables.

-

The transitions considered are listed in table I, together with a compilation of relevant nuclear parameters. They are : the resonant energy

(E,), the spin and parity of the excited state (I:), the

spin and parity of the groundstate (I:), the multipola-

rity of the y-ray (Multipol)., the half-life of the excited state (TI,,), the magnetic dipole moment of the excited state (p,) the magnetic dipole moment of the ground-state (pg), the quadrupole moment of the excited state (Qe), the quadrupole moment of the groundstate (Q,), the isomer shift calibration constant ( E ) , and the minimum observable linewidth ( Wo). The isomer shift calibration constant a is defined by

S = a.Ap(0) where S is the isomer shift and Ap(0) is the electronic charge density at the nucleus. The minimum observable linewidth is given by

- Ele- Z ment A - - Multipo. (@) TI;,

1

pi Y.

I

Qe

Q

.

1

wo

11s n. m. n. m. barns barns a: mmls mm/s

(a) Higher admixtures are given only if larger than 0.1 %.

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Resonant energy Isotope keV

-

Source temp. K Source Production

activity Tllz reaction Source matrix

-

61Co 99m 64Ni(p, a) NiV (14 %) ("1

9 9Rh 13d 99Ru(d, 2n) Ru metal

Remarks

-

Irradiate alloy made of enriched 64Ni ; use without further processing.

Natural Ru target ; no chemical separation necessary ; use without further processing.

lzlmSn 50a "WSn(n, y) BaSnO 3 High flux, long irradiation necessary ; better purchase activity ; make compound after irradiation.

Activity is diffused into Cu after irradiation ; enriched targets ; activity can be purchased.

127mTe 109d 126Te(n, y) ZnTe ("1

Irradiate enriched targets ; Compound prepared after irradiation.

12gmTe 34d 128Te(n, y) ZnTe Irradiate enriched targets ; small activation crossection ; compound prepared after irradiation ; 1291 is slightly radio-

active.

Irradiate enriched targets ; no processing after irradiation.

Natural Sm203 can be irradiated ; no processing after irradiation.

Irradiate enriched targets ;

--

200d cooling ; irradiate compound ; activity can also be purchased and diffused (100 ppm) into Pd.

ls5Eu 4.86a see l55Gd (87 keV)

Irradiate compound of enriched Gd ; emission line is 10 x broadened ; no processing after irradiation.

161Tb 6.9d see 161Dy (26 keV)

Prepare compound before irradiation and use without further processing (165H0 is 100 % isotope).

Prepare alloy of 10 wt % Er in A1 from enriched 168Er metal before irradiation ; use without further processing. Prepare : 10 wt % of Tm-in A1 before irradiation (169Tm is 100 % isotope) ; use without further processing.

Prepare alloy of about 10 wt % 170Er in Al. Irradiate in a reactor, then use without further processing.

178Ta (22d) IslTa(d, 5n) + Ta metal 178W@, 22d) + 178Ta

Irradiate natural Ta foil (100 % 18lTa) with 45 MeV deuterons source half-life is given by 178W activity ; 178W emits no gamma rays ; use without further processing.

181W 140d ls0w(n, Y) W metal

("1 Irradiate enriched target in high flux reactor ; diffuse into W single crystal ; line about 10 x natural width.

1 szw 100

(a) Other sources

l8zTa 115d 181Ta(n, y) Ta metal Irradiate natural Ta metal ; use without further processing.

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C6-662 G. M. KALVIUS, F. E. WAGNER AND W. POTZEL TABLE I11 (Continued)

Resonant

energy Source Production

Isotope keV activity Tilz reaction Source matrix

-

1890s 69 189Ir 13.3d 1890s(d, 2n) Ir metal Source temp. K

-

Remarks

-

Irradiate enriched target ; chemical separation and conversion into Ir metal.

1890s 36 l89Ir 13.3d see 1890s (69 keV)

193Ir 73 1930s 31h 1920s(n, y) 0 s metal ( 9

Irradiate enriched targets of 1920s metal. No further processing required. Small line broadening.

Irradiate natural Pt target ; chemical separation of 195Au (or use cornmer- cially bought activity) ; diffuse into Pt foil.

195Pt 99 195Au 183d 195Pt(d, 2n) Pt metal (")

Irradiate Pt foil (natural or enriched in 196Pt) ; use without further processing.

197Au 77 197Pt 18h 196Pt(n, y) Pt metal

("1

2 3 7Np 60 241Am 433a Neutron capture Am in Th

( 9 in reactor fuel (a)

element (a) Other sources are available ; see text.

Source has to be purchased ; line 20 times natural width ; 237Np is a-active. Resonant isotope

-

61Ni "Ru Compound

-

Ni0.86v0.14 Ru-metal or K4Ru(CN), . 3 H,O Remarks

-

Same alloy is used as source matrix.

Ru-metal shows a slight broadening due to unresolved quadrupole interactions. The cyanide compound gives

a

narrower line but a smaller resonance effect.

InSb ZnTe NaI CsI see lZ7I SmF, S m 2 0 3

CsI is easier to handle since it is not hygroscopic, but has a larger electronic absorption.

SmF, gives a narrower line but is difficult to make and not stable over long times. Sm,O, is easily available and will suffice for most practical applications, but gives slightly broadened line. The alloys give good single lines at 4.2 K but are not easily

available. For most practical purposes Eu203 which gives a line of roughly twice natural width will be a reasonable choice. Gives a good single line but has a rather low Debye Temperature. Can be used down to 4.2 K. At room temperature DyH, is appli-

cable.

Gives single line at 4.2 K. Gives single line down to 1 K.

The alloy has the higher Debye Temperature. ErH, TmA1, Yb metal, YbAI, see 170Yb HfC, HfZn, Ta metal W metal K20sC16 Ir metal Pt metal Au metal

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METHODOLOGY OF << LESS 1

The various compilations of those parameters given in the literature all differ somewhat from each other. The values given here are mostly, but not always, the same as in MEDI 1974 [I]. The values for a have been calculated from the table given by Kalvius and She- noy [2]. These authors solely rely on Dirac-Fock- Slater calculations for free atoms to obtain their electron density calibrations. The reader should therefore use those numbers with care.

Table I1 gives numbers useful for planning an experiment with the resonances listed in table I. This includes the resonant energy (E,), the natural abundance of the Mossbauer isotope in

%

(IA), the resonant absorption cross section (o,, see definition below), the recoilfree fractions (f) for Debye temperatures of the sample of 8, = 150 K and 8, = 250 K and for measuring temperatures of T = 300 K, 77 K, and 4.2 K, as well as the standard absorber thickness (d, see definition below) for the listed values of the recoilfree fraction. The resonant absorption cross section is given by :

where b is the branching ratio (= 1 for the first excited state) and a, is the total conversion coefficient. The standard absorber thickness d is the thickness of an absorber having t , = n. o,. f = 2 (i. e. giving a resonant absorption strength of l/e) [3]. It is given in mg/cm2 of the natural isotopic mixture of the Mossbauer element and for natural linewidth in both the source and the absorber. The values of f were calculated using the Debye model 131.

In table I11 the source isotopes and other relevant information are listed. The half-lives of the source activities are given in minutes (m), days (d) or years (a). Also given are the commonly used production reaction, matrices suitable for obtaining a single emission line (Source Matrix), the temperature at which these sources are mostly used (Source Temp.), and finally some pertinent remarks about the preparation of the sources. More details can be found in the text of section 4. In table IV we list compounds which may serve as single line absorbers in cases where the shape of the emission spectrum of a source material is under investigation.

4. Short survey of isotopes. - In the following we present a brief discussion of the most important experimental aspects together with a list of pertinent references for each of the resonances under consideration.

61Ni (67.41 IceV). - The parent activities feeding the 67 keV state are either 61Co(99m) or 61Cu(3.2h). Although the latter has the longer half-life and can be produced in a Cu matrix (single line source !) it has hardly been used because of its complex decay scheme. "Co is produced either by irradiation of 64Ni with

N 20 MeV protons in a cyclotron (64Ni(p, LY)~'CO) or

by a photoreaction on 62Ni (i. e. 62Ni(y, P)~'CO) with Bremsstrahlung from an electron accelerator (electron energy N 20 MeV) [4]. Isotopically enriched targets

have to be used in both cases. To avoid line broadening due to large magnetic hyperfine fields at low temperatures, alloys of NiV or NiCr (about 15

%

V or Cr) are commonly used as sources. They give practically natural linewidth at 4.2 K with an f-factor around 0.15. N N is the better choice. Alloy targets can be prepared before irradiation and can be re-used. They should, however, be reannealed periodically to avoid the accumulation of radiation damage. Annealing directly after the irradiation does not seem to be necessary.

Gammaray detection is simple if 61Co is used. A scintillation detector is sufficient and emphasis should be given to high counting speeds rather than to high resolution (see also [5]). Sources can be made very strong. The background (and thus the observable resonance effect) changes somewhat with time and proper care has to be taken in case of f-factor measurements. It is useful to employ an automatic system which reduces the source-detector distance with time in order to keep the countrate at its permissible maximum. Measurements can be performed to about 200 K if the source is kept at 4.2 K and absorbers enriched in

6 1 ~ i are used. Enriched absorbers are generally required for precision measurements. The resonance has moderately good resolution for hyperfine splittings. However, full resolution is practically never achieved and precision data analysis calls for the use of fitting procedures employing the full transmission integral [13]. In this case the effective absorber thickness has to be known and the remarks concerning $measurements apply. If investigations of isomer shifts are performed the large second order Doppler effect [6] has to be taken into account.

The 6 1 C ~ activity may be produced via the reactions 60Ni(d, n)%u or 6 3 C ~ ( y , 2 n ) 6 1 ~ u . The latter requires electron energies around 45 MeV but would produce directly a single line source. Enriched metal targets have to be used.

Literature : [4, 6 , 71.

99Ru (90 keV). - The source activity is 99~h(13d). It is produced by cyclotron irradiation via

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C6-664 G . M. KALVIUS, I?. E. W 'AGNER AND W. POTZEL

be achieved if chemical separation of the activity is performed after irradiation. Then one may use Rh metal as a matrix, which is cubic. Its drawback is a smaller recoilfree fraction. Gammaray detection is simple, a scintillation or a solid state detector (Ge-Li) may be employed. Useful temperature range for chemical compounds is

<

100 K, metallic compounds often have high Debye temperatures (8, E 350 K) which

extends the temperature range upwards. Compounds with natural isotopic abundance of Ru can be used as absorbers. The resonance has good sensitivity for isomer shifts. In most cases quadrupole spectra can be fitted as simple two line patterns since the splitting of the groundstate is small compared to that of the I =

4

excited state. Resolution for magnetic interactions is only moderate since the high nuclear spins involved give rise to a complex hyperfine pattern. Internal fields must exceed 200 kOe in order to produce reasonably charac- teristic structures. The transition is of mixed M1

+

E2 character which has to be taken into account in data analysis. Sources can be obtained commercially but are expensive (short TI,, !).

Literature : [8, 9, 10, 111.

121sb (37 keV). -This resonance has already found fairly widespread use. The source activity is 121mSn(50a). It is produced by the lZoSn(n, y)121mSn reaction. To achieve a reasonable source strength long irradiations in a high flux reactor are required, but sources can be obtained commercially and the long halflife makes up for the high price. The most common source compound is BaSnO,. It gives nearly natural linewidth but has the slight drawback of a large isomer shift. Gammaray spectroscopy is a bit tricky because of the vicinity of strong K-X-rays. Best is in principle a Si(Li) or, a thin Ge(Li) detector, which easily resolves the 37 keV line and has good detection efficiency. A large diameter Xe-filled proportional

counter may be used as well. In this case it is best to set the single channel analyser at the 8.5 keV escape peak (or to use two SCA, one set at the 37 keV full energy peak and the other at the escape peak). The same may be done using a thin NaI (Tl) scintillator with some reduction in signallbackground ratio but higher counting efficiency.

The source is best kept at 4.2 K or at 77 K. Absorber temperatures extend to 200 K for chemical compounds. Alloys usually have a low Debye temperature and thus are restricted to the liquid nitrogen range. The poor specific activity of lZ1Sb results in large area (- 2 cm2) sources and correspondingly large absorber areas. The cosine velocity smear may be substantial under these circumstances, particular for lines with large shifts. The sensitivity of the resonance is very good for isomer shifts and fairly good for quadrupole interactions and magnetic splittings. The analysis of only partially resolved spectra must be carried out using a transmission integral fitting routine [13]. Recently, it has also been shown that 121Te with a half-life of 17d may be

used as a source [16]. The activity is prepared by the reaction lZ1Sb(p, n)"l~e. Natural antimony is irradiated by 20 MeV protons. Chemical separation is performed after irradiation and the source compound ZnTe is prepared. This source gives only a weak resonance because of poor signal to noise ratio arising from the complex decay scheme. It should only be used in special circumstances.

Literature : [12, 13, 14, 15, 161.

1 2 5 ~ e (35.46 keV).

-

Possible source activities are lZ5I(60d), lZ5"Te(58d) and lZ5Sb(2.7a). The lZ5Sb is preferable since it has the longest half-life and can easily be incorporated in materials having a good recoilfree fraction. Its drawback is a slightly more complex gammaray spectrum. The production method is lZ4Sn(n, y)125mSn(/?-, 9.5m) .+ 12%b. (Some '25Sn(~,,z = 9d) is also produced. The source should therefore be allowed to cool for about two weeks.) Enriched target material is necessary. The lZ5Sb activity can also be obtained commercially (Sb in HCl solution). Ready-made Mossbauer sources are available, too. Common source material is lZ5Sb in Cu which gives practically natural linewidth. The annealing procedure is described in [17]. Source temperatures should not exceed 77 K but 4.2 K is preferable. Gammaray counting is best performed with a Si(Li) or Ge(Li) detector. If a Xe-filled proportional counter is used, the channel is best set to the 5.8 keV escape peak which is only partially resolved from the X-rays of the Cu source matrix. Scintillation counters may be used but they give a marked reduction in resonance effect due to poorer signal-to-background ratio (see also remarks on lZ1Sb). Since Te compounds generally have low Debye-temperatures, the useful temperature range is restricted to 77 K or below. Natural Te suffices generally as absorber material. Only in special applications enriched material is required. The resonance has only moderate resolution for all hyperfine interactions. The simple resonance patterns, however, allows one to extract reliable information even from only partly resolved spectra provided thickness effects etc. are taken into consideration.

The other sources that have found some application are lZ5I in Cu and Zn l Z 5 " ~ e . Both, however, give poorer linewidth than lZ5Sb in Cu. The lZ5I activity is produced via lZ3Sb(a, 2n)lZ51 and then chemically separated. It may be purchased commercially. The lZ5"Te activity is produced by neutron capture from enriched lZ4Te. The capture cross section is very small, high fluxes and long irradiations are necessary. The ZnTe is prepared after irradiation.

Literature : [17, 18, 19, 201.

1 2 9 ~ (27.8 keV).

-

Parent activities are either

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reactor is close, however, working with 12'Te is simple is moderate, and the resonance patterns are complex and efficient since very strong sources can be prepared. due to the high nuclear spin states involved. Maximum In this case, one irradiates 6 6 ~ n ' 2 9 ~ e prepared from isomer shifts are about 0.5 W,. Only very limited enriched isotopes in order to avoid neutron activation data exist to date with this resonance.

of other Zn isotopes. I t gives practically no other Literature [26, 27, 281. activity and can -be re-irradiated promptly. The

single emission line has a FWHM x 1.5 W,. 12'"'~e is produced by neutron activation of enriched '28Te, and the ZnTe is prepared after irradiation. Gammaray detection is done either with a Si(Li) or a NaI(T1) detector. The latter will not discriminate against the K-X-ray background, but the resonance effect is not too seriously affected by this. A critical absorber of 0.1 mm In may be used to improve the signal to background ratio. It should be placed directly behind the absorber facing the NaI-detector. 1291 is weakly 8-active

( T I , , = 1.7 x 107a). This does not affect the Moss- bauer experiment, but lZ9I is a health hazard and care has to be taken in working with it. It can be obtained commercially at moderate cost in sufficient quantity since it is a common fission product. Amounts

1 5 3 E ~ (103 key).

-

Most investigations of Eu and its compounds use the 21 keV resonance of l 5 'Eu. For

some experiments the high transition energy may be of advantage (e. g. in high pressure measurements). The sensitivity for isomer shifts is comparable t o the 151Eu resonance. Magnetic splittings are somewhat less resolved. For the determination of electric field gradients both resonances are not very well suited. With 1 5 3 ~ ~ , temperatures are limited to the liquid

He range.

Source activities are 153Sm and 153Gd. The former is produced by neutron activation of natural Sm. Its disadvantage is the short half life (47h). The longer lived source (242d) of '53Gd may be produced by one of the following reactions

around 10 mg are required to make an absorber. The 152Gd(n, 153E~(d, 2n)153Gd

useful temperature range is limited to 200 K since Debye temperatures are usually small. The source must be kept at least at 77 K, but 4.2 K is preferable. The resonance in 12'1 has excellent resolution for all hyperfine interactions, but the high cost of 12'1 and the difficulties in handling it may suggest the use of the 57.6 keV resonance in "71, the only stable isotope of iodine.

Literature : [20, 21, 22, 231.

1271 (57.6 keV).

-

The main drawback of this resonance when comvared to the 28 keV transition in ','I is the roughly ten times poorer resolution. Further- more, the useful temperature range is restricted virtually to liquid helium temperature. The source activity is 127mTe(109d). Sources are prepared by neutron activation of 126Te which should be enriched because of the small cross section and to avoid the production of 125mTe. The ZnTe is prepared after irradiation. Gammaray counting is without problems, a NaI(T1) detector will do.

Literature : [24, 25, 22, 201.

149Srn (22.5 keV).

-

The source activity is 1 4 9 ~ ~ ( 1 0 6 d ) which can be produced by cyclotron irradiation 'via the 1 5 0 ~ m ( p , ieaition. A target of enriched 1 5 0 ~ m , 0 3 can be irradiated in a cyclotron and used without further treatment. At room temperature this source gives a FWHM x 1.6 W,. At low temperatures the emission line broadens markedly. A narrower line might perhaps be obtained with other source matrices. For instance, the techniques developed for other rare earth resonances could be used. Gammaray detection is done either with a Si(Li) detector or a Xe-proportional counter. The useful temperature range for absorbers is up to 1 000 K, and Sm of natural isotopic abundance may be used. The resolution of this resonance for hyperfine splittings

or 153E~(p, n)153Gd. The neutron capture process is hampered by the low natural abundance of 15,Gd. The required enriched targets are therefore expensive. With the cyclotron targets chemical separation should be performed after irradiation because of resonant self- absorption. The 153Gd activity can, however, be obtained commercially. With 153Sm as parent isotope one uses simply Sm203 as the source compound which results in a tolerable line broadening (-- 2 W,). SmRh2 gives a better linewidth. i t can be prepared before irradiation. Gammaray detection by either a NaI(T1) or a Ge(Li) detector is easy in this case. Using the 153Gd source again an oxide matrix is most easily prepared and will do in most cases. Gammaray detection is more tricky here because the 97 keV gamma ray is strongly emitted. These gammarays are also absorbed resonantly. Only a Ge(Li) detector with good resolution (at high countrates) will provide full separation. Otherwise two superimposed resonance patterns will be recorded. Since the 97 keV resonance is about 20 times wider than the 103 keV resonance a separation by an appropriate fitting routine is, however, not too difficult.

Literature : [29, 301.

155Gd (87 keV). -The source activity is f 5 5 ~ ~ with T I ! , x 5a [31]. It is produced by neutron activation via the reaction 154Sm(n, p) 155Sm(/3, 22m) 1 5 5 ~ ~ .

Highly enriched (> 95 "/,) '54Sm has to be used and irradiation times of the order of a month in a good reactor are necessary. A problem is the high

neutron capture cross-section of 1 5 5 E ~ which easily leads to a burnup of this isotope in high neutron fluxes. Despite the use of highly enriched materials the source has to cool for about 6 months to reduce the amount of the other radioactive isotopes formed. A

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C6-666 G . M. KALVIUS, F. E. WAGNER AND W. POTZEL

produced before irradiation. An alternative way to obtain a source is to purchase the carrierfree 1 5 5 E ~ activity in solution. For a source one diffuses about 100 ppm of 1 5 5into a Pd matrix [33]. SmPd, and Eu ~ ~

in Pd both give nearly the natural linewidth at 4.2 K (FWHM w 1.5 Wo). Gammaray spectroscopy requires a Ge(Lij detector. Natural Gd compounds can be used as absorbers. The temperature range can be extended to 77 K if the source is kept at 4.2 K and the absorber material has a reasonable Debye Temperatire. This resonance has only recently seen a wider application and should be useful in the study of rare earth materials. Its resolution for all types of hyperfine interactions is good to moderate. Magnetic hyperfine fields are usually small in Gd since there is no orbital contribution. For ion implantation measurements in particular, it is more useful to employ the short lived parent activity 155Tb(5d). It can be produced from a natural Tb target via the 159Tb(p, 5n) 155Dy(p, 10h) l S 5 ~ b reaction using 50 MeV protons from a cyclotron. After irradiation the Dy activity is chemically separated from the target and used as the starting material for implantation, It could also be diffused into a Pd matrix [91].

Literature : [32, 33, 34, 351.

155Gd (105 keV).

-

155Gd possesses a second useful Mossbauer transition at 105 keV. Normally one would use the 87 keV resonance for all applications. For special applications, however, the higher energy may be useful. For example, in case of large asymme- tries in quadrupole spectra measurements with both resonances allow a distinction between texture effects and the Goldanskii-Karyagin effect. The same source can be used for both resonances.

Literature : [32, 32~1.

161Dy (26 keV). - This resonance has already found rather widespread application. The source activity is 16'~b(6.9d) p o d i c e d via the reaction 160Gd(n, y) 16'Gd(p, 3.7m) 61Tb. Enriched 16dGd is not required for source production but gives better results and is generally worth its price. The source compound is GdF, which is prepared by heating 160Gd,03 repeatedly with (NH,) F.HF in an argon stream 1361 before irradiation. It gives a FWHM w 10 Wo. Because of the excellent resolution of the I6'Dy resonance this line broadening can easily be tolerated, but the resulting reduction in depth of the resonance lines has to be taken into account.

A source of Dyo.,GdO.,F3 has been suggested by [37] but gives little improvement over a well-prepared (dry process) GdF, source. The 16'Tb activity can also be obtained carrier free by ion exchange after irradiation. Sources of 16'Tb in CeOz have thus been prepared which give a single line of FWHM z 3 Wo (against a magnetically split Dy-metal absorber) plus some broad nearly structureless background which can be neglected in most cases. The single line contains about 40

%

of the recoilfree gammarays [38]. The best linewidth

(about 2 Wo against Dy metal) has been reported [39] for a source of 160~d,Ti0,. The source temperature must be kept at 150 K. At this temperature the source emits the single line in addition to a broad background which contains most of the gammaray activity. The compound made from enriched 16'Gd was neutron activated. Thus this source has a low efficiency. Absorbers can be prepared from natural Dy, the useful temperature range extends to 1 000 K. The resolution is excellent for all hyperfine interactions. The 26 keV transition has El multipolarity and a non-negligible dispersion term in the absorption cross section leads to asymmetric lineshapes. This has to be considered in precision data analysis [40].

Literature : [36, 38, 39, 411.

16'Dy (75 keV). - For some applications the 75 keV transition of 161Dy may be used instead of the 26 keV resonance. The resolution is nearly ten times worse, which makes the source problem less stringent. Tempe- ratures are limited to the cryogenic range. The source activity is again 161Tb (see I6'Dy (26 keV)) and the recommended source compound is GdF, which gives FWHM z 4 Wo.

1 6 6 ~ r (81 keV). - The source activity, 166Ho(27h) is easily produced if a reactor is nearby. High neutron fluxes are not required since 1 6 5 ~ o is the only isotope in natural Ho and since the capture cross section is large. Sources have been prepared by arc melting of 10 wt.

%

Ho with aluminum, which results in a mixture of HoA1, and Al. The alloy can be irradiated. Such sources have to be kept at 25-30 K in order to prevent magnetic ordering. Probably due to parama- gnetic relaxation, the FWHM is still around 3.5 Wo. This, however, gives sufficient resolution for most hyperfine studies. More practical is a source of ( H O ~ . , Y ~ . ~ ) H , which can be used at 4.2 K, giving also a single line of FWHM

w

4 Wo [44]. This compound is also prepared before irradiation. Absorbers can be made of Er of natural isotopic abundance. The temperature range is up to 100 K if the source is kept at cryogenic temperatures. Resolution for magnetic and electric quadrupole interactions is good. Isomer shifts are too small to be measurable. A small dispersion term [45] is present. If this is neglected in the data analysis, spurious isomer shifts may be returned by the least squares fit. The gammarays are best detected with a Ge(Li) detector. A scintillation detector may also be used with a slight loss in signal to noise ratio. In this case it is of advantage to reduce the X-ray background with a 2 g/crn2 Cu filter placed behind the resonant absorber.

Literature : [46, 47, 48, 441.

169Tm (8.4 key).-The source activity is

(12)

rolled to foils of

-

50 pm thickness. It can be prepared before irradiation. Such sources are usually kept at room temperature but may be cooled down to

-

30 K without serious line broadening. It is advisable, but not mandatory to use isotopically enriched 16'Er. The major difficulty then is to reduce the commercially available 1 6 8 ~ r 2 0 3 to the metallic form. A rather straightforward procedure is the reduction of ErC1, by Al powder [49]. Recently enriched rare earth metals have also become available commercially upon request from the suppliers of enriched isotopes. Sources prepared with natural Er metal have to cool after irradiation for about a week. Even then a loss in resonance absorption by a factor of two or more has to be tolerated due to increased X-ray background. Er203 (or ErF,) may be used as a source if heated to

-

550 K. At this temperature a minimum in the quadrupole interaction occurs [50]. Due to the low y-ray energy all windows should be made of Be (or mylar if only low temperatures are employed). All sources described give PWHM z 2 Wo. Gammaray detection is usually done with an Argon-filled proportional counter (optimal thickness is about 10 cm dia). The 8.4 keV gammaray is surrounded by Tm and Er L-X-rays which arise from the fast electrons of the P-decay of 16'~r. Separation is not possible even with high resolution detectors. Therefore even a thin Si(Li) detector brings very little improvement. Usually a broad, nearly structurless peak around 8 keV is seen in the y-ray spectra. The single channel should be set to the high energy part of this peak. 1 6 ' ~ m is the only naturally occurring isotope. It is often difficult to make the resonant absorbers thin enough (typically 5- 10 mglcm2). Only very fine-grain powders should be used. The useful temperature range extends to about 2 000 K. The sensitivity for quadrupole and magnetic splittings is very good, isomer shifts, however, are too small to be normally observed. A typical magnetic hyperfine pattern extends to about

+

700 mm/s which makes special demands on the velocity drive system [86].

It might appear that 16'Yb(32d) which can easily be prepared by neutron activation of 16'Yb is the better choice as parent activity than '"Er. The 8.4 keV state is indeed strongly populated in this decay although several other gamma rays appear as well. Unfortuna- tely, the L-X-ray background is about a thousand times stronger than with 16'Er. This makes the signal t o background ratio so poor that the resonant effect cannot be observed.

Literature : [50, 46, 51, 52, 531.

170Yb (84 keV). - T h e source activity is 170~m(130d) produced by neutron activation of natural Tm since 1 6 ' ~ r n is the only stable isotope of this element. The usual source compound,

-

10 wt.

%

of Tm in A1 actually is TmAI, in an A1 matrix. The alloy is easily prepared by arc melting before irradiation and can be rolled to foils. Strong sources (several

Curies) can be obtained, since the neutron capture cross section is large. Even at 4.2 K this source material gives a linewidth of about 1.2 W,. Gammaray detection requires only a scintillation detector. A 2 g/cm2 Cu filter is advisable to reduce the X-ray background. 170Yb has only 2

%

natural abundance. At 4.2 K and with strong sources absorbers prepared from natural

Yb can be used. Metallic compounds of Yb often have

rather low Debye temperatures. The use of enriched materials eases the data collection significantly and is generally recommended. The useful temperature range can then be extended to 100 K (source at 4.2 K). Sensitivity to magnetic and quadrupolar splittings is good, isomer shifts are rather small. When evaluating isomer shift data a small dispersion term [45] has to be included in the lineshape.

Literature : [46, 54, 55, 561.

l7IYb (67 keV).

-

The higher isotopic abundance (- 14

%)

of 171Yb makes it easier to use this resonance than 170Yb. Absorbers prepared from natural Yb compounds can be used. For hyperfine splittings as well as isomer shifts the resolution is somewhat, but not significantly poorer than with 170Yb7 but the hyperfine patterns are more complex. The dispersion term in the absorption cross section [45] is smaller. The source activity is 17'Tm with a convenient half- life of about 2a. Sources are easily prepared by neutron irradiation of an Er-A1 alloy, preferentially one made from enriched 170Er (see also the remarks on the preparation of sources for measurements with 16'Tm). The l7'Er produced by neutron capture from 170Er decays with a half-life of about 7h to the 17'Tm source activity. The sources should be allowed to cool for about a month after irradiation. For the detection of the gammarays a Ge(Li) detector should be used, because only then the 65 keV gammarays can be resolved from the YbK, X-rays. When no Ge(Li) device is available, a scintillation detector together with a filter of 1.5 g/cm2 Cu can be used but will bring some deterioration of the signal-to-background ratio. The accessible temperature range extends to liquid nitrogen.

Literature : [46, 57, 55, 58, 591.

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C6-668 G. M. KALVIUS, F. E. W 'AGNER AND W. POTZEL

absorbers. The resonance is limited to temperatures near the liquid He range.

Literature : [60].

lS1Ta (6.3 keV).

-

This was considered an <( exo-

tic )) resonance only a few years ago but has been

widely applied since then. Its dominant feature is its extremely high resolution for all types of hyperfine interactions and especially for isomer shifts. Hyperfine patterns are complex due to the high nuclear spins. The multipolarity is El. Together with a large conversion coefficient this leads to a strong dispersion term in the absorption cross section and consequently to rather asymmetric line shapes. These must be taken into account by the data analysis [61]. The velocity drive is not the most difficult problem in lS1Ta work. A standard electromechanical system will suffice, although decoupling from environmental noise may be necessary. The main problems, usually arise with the preparation of sources and absorbers ; in addition, because of the extremely large isomer shifts, it may often be difficult to find the resonance. With metallic systems, however, these difficulties can be overcomz, since an isomer shift scale has been established for alloys. Measurements on chemical compounds sofar have seldom been successful.

As source activity one uses 181W(130d), since lS1Hf has a rather complex decay scheme. The easiest way to produce the lS1W activity is by neutron capture on enriched lS0W.

Because of electronic absorption the useful source thickness is only a few mg/cm2. Therefore a high specific activity is required. This can be obtained by prolonged irradiation of highly enriched lS0W (2 1 month) in a high flux reactor

(2

5 x 1014n/cm2 s). Another possibility is to produce 18'W by cyclotron irradiation via the lSITa(d, 2n)181W or lS1Ta(p, n)lS1W reactions. The lS1W can then be separated chemically from the target material. A good matrix is W metal particularly if the activity is diffused into a single crystal. Annealing at

-

2 500 K in a

vacuum of about l o F 9 torr is required to clean the metal from absorber gasses. Sources are usually used at room temperature and give

FWHM

R. 10 W,. This represents no serious loss in resolution. l S 1 ~ a is virtually 100

%

abundant in the natural element. The difficulty in preparing absorbers is to make them sufficiently thin and still uniform. For Ta metal a typical thickness is 12 pm. Mechanical stiffness is then also a problem. The absorber has to be outgassed in ultrahigh vacuum at 2 500 K. The gammarays are detected with an argon-filled proportional counter. The gammaray is only partially resolved from the L-X-rays. A thin Si(Li) detector could also be used. Windows for the gammarays should be made of thin Be. Useful temperature range exceeds 2 000 K.

Literature : [61, 62, 631.

lS2W (100 keV). - This is an easy resonance of good resolution for hyperfine splittings (especially

quadrupole interactions). Isomer shifts, however, are extremely small and very difficult to measure.

A

small dispersion term must be included in the analysis of precision data [45]. The source activity is l S 2 ~ a ( 1 15d), which is prepared by neutron activation of natural Ta. A convenient source matrix is Ta metal. If this material is free of absorber gasses one obtains virtually the natural linewidth. Gammaray detection is done either with a Ge(Li) or a scintillation detector. Natural W compounds may be used as absorbers. With strong sources at 4.2 K the useful temperature range may be extented to N 100 K.

lS90s (69 keV). - Source activity is lS9Ir(13d) which is prepared by cyclotron irradiation, e. g. using the lS90s(d, 2n)lS9Ir reaction and enriched lS90s targets. The lS9Ir activity should be chemically separated and converted to the metal with the addition of some natural Ir [66]. Natural 0 s compounds may serve as absorbers, the useful temperature range extends to

--

100 K. Gammaray detection requires a Ge(Li) diode. Even then only with instruments having very good resolution it is possible to separate the 69 keV gammarays from the KP X-rays. It normally

suffices to set the single channel simply at the correct energy even if no structure in the pulse height spectrum appears. The resonance is sensitive to hyperfine splittings but not for isomer shifts. The M1 radiation has a substantial E2 admixture which must be taken into account in the data evaluation.

lS90s (36.2 keV). -This resonance has a fairly good sensitivity for isomer shifts but poor resolution for hyperfine splittings. Thus experiments with 0 s usually call for the combined use of the 36.2 keV and 69 keV resonances. Sources and absorbers are basically the same in both cases. Gammaray detection is best done with a Si(Li) diode. The 36 keV radiation is pure MI.

Literature : [68].

1931r (73 keV).

-

Source activity is lg30s(31h) which is obtained by neutron activation of enriched lg20s metal. This source matrix gives a slightly broadened line with

FWHM

x 1.5

W,,

because of a small quadrupole splitting in the hcp osmium metal. Nor- mally this linewidth is satisfactory for all investigations. A sharper emission line can be obtained with an alloy of 0.5

%

0 s in V or Nb. In both cases the alloy can be prepared before irradiation. It should be kept in mind that V and Nb are excellent getter materials. Heavy gas loading of the source matrix may easily broaden the emission line. Natural Ir compounds are used as absorbers, the useful temperature range extends to 100 K. Gammaray detection is best done with a Ge(Li) detector. Separation from the Kp X-ray

peak is impossible. The window of the single channel analyzer is simply set on the Kg peak. The 73 keV

(14)

METHODOLOGY OF ct LESS USED n MOSSBAUER ISOTOPES C6-669

types of investigations. The M1

+

E2 mixing ratio must be considered in the data analysis.

195Pt (99 keV).

-

Source activities are either 195A~(183d) or 195mPt(4.1d). Cyclotron irradiation (195~t(d, 2n)195Au or 195Pt(p, n)195A~) of natural Pt targets with following chemical separation is used to obtain 1 9 5 A ~ . This activity is also commercially available. It is best diffused into a foil of Pt metal. 1 9 5 m ~ t can be produced by neutron capture on enriched lg4pt. If a reactor is at hand, this source is easier obtained and cheaper, because the lg4Pt foils can be re-irradiated. Gammaray detection is performed either with a Ge(Li) or a scintillation detector. Natural Pt compounds serve as absorbers. The temperature range is limited to the liquid He region. No quadrupole splittings have been observed to date. Magnetic splittings are only partially resolved, the internal field should exceed N 300 kOe for reliable analysis. Isomer

shifts are small but can be measured reasonably well. Literature : [71, 721.

1 9 7 A ~ (77 keV). - This again is nearly a standard

resonance with good resolution for all types of investigations. The source activity mostly used is 197Pt which only suffers from its fairly short halflife of TI,, = 18h. The other parent isotope, 197Hg, has only slightly longer halflife (TI,, = 65h). Both source activities are prepared by neutron activation. Enriched 196Pt is of advantage but not essential. Foils of lg6Pt metal may be re-irradiated. Natural Hg can be used to produce the 197Hg activity. Best is to prepare an alloy of about 20

%

Hg in Pt before irradiation.

The Pt metal matrix gives a source with a single emission line of practically natural width. lg7Au is the only stable gold isotope. The useful temperature range extends to about 200 K with strong sources kept at

4.2 K. A Ge(Li) diode is the recommended detector,

but separation from Kg X-rays is impossible. The

single channel window should be set on the Kg peak. A

scintillation detector may also be used. Then the y- line is hidden in the high energy part of the K X-rays and the single channel window should be set to the upper half of the peak. Data analysis must include the 10

%

E2 admixture to the M1 radiation.

Literature : [73, 74, 75, 76, 77, 781.

237Np (60 keV). - The price and availability of 237Np is comparable to those of the enriched isotopes needed for work with some of the other Mossbauer isotopes. Its radioactivity is moderate and chemical compounds can be prepared with appropriate precau- tions. Intermetallic compounds require special techniques and efforts. Their production will remain limited to suitably equipped laboratories. For a typical absorber, about 100 mg of 237Np is needed, but in special geometries this amount may be reduced by a factor of 10 by reducing the diameter of the absorber and using a rather point-like source. The resonance has excellent sensitivity for all types of investigations. The useful

temperature range with strong sources may extend to 300 K 1791. Because of the health hazard connected

with the a-activity, Np absorbers should be tightly encapsulated. A suitable container is shown in figure 3. It has been described earlier in section 3.

The most commonly used source activity, 241Am, has a halflife of 433a. Such a source must be purchased. The specific activity of 241Am is

-

3.4 mC/mg. Sources containing between 10 to 50 mg of Am are recommended. The maximum allowable thickness of an Am metal foil is around 50 mg/cm2. The sources should be kept in vacuum sealed containers. This is best achieved by placing the source foil inside a thin- walled stainless steel container which is subsequently sealed by electron beam welding. Another technique is to use a copper container and to seal it by a soft solder joint. Caution should be exercised in using plastic materials because of the heavy radiation damage caused by the a-rays. Another possible parent is 237U which is obtained by neutron activation of 236U. Very pure 236U has to be used, ortherwise the hard task of chemical separation of fission products arises. Higher enriched 2 3 6 ~ is expensive and difficult to obtain. The halflife of 6.75d makes this source inconvenient, too. It has seen little use recently [81]. Sources consist of 241Am, either as Am metal or as an alloy of N 5

%

Am in Th. At 4.2 K the alloy source

gives a single line of

--

15 W, ; the metal source emits roughly twice that width because of unresolved quadrupole interactions. Even with those broad lines the resolution is excellent. Sources have to be checked carefully for multiple emission lines. Sometimes the second line is rather weak but has to be considered in high precision experiments. Experience also shows that sources often deteriorate after several years and emit strong additional lines for unknown reasons. The electronic absorption coefficient for the 60 keV gammaray differs little between Th and Am. Thus the maximum possible source activity is weaker by an order of magnitude for the Th alloy. At present, the best choice is a combination of an Am metal and an AmTh source. The former is used for general applica- tionzthe latter for the few cases where higher resolution is essential. Long data collection times can then be tolerated.

The gammaray spectrum is very clean and the activity of the absorber causes little additional background. A simple scintillation spectrometer suffices and effort should rather be spent to obtain high counting speeds [5]. Since the 60 keV gammaray is of pure E l multipolarity a noticeable dispersion term must exist [40], which has only been determined recently [go]. It has not been taken into account in the spectra published to date.

Literature : [80, 81, 82, 83, 841.

Acknowledgment.

-

The authors would like to thank Dr. L. Asch for her help in compiling the data

(15)

C6-670 G. M. KALVIUS. F. E. WAGNER AND W. POTZEL

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