MÖSSBAUER EXPERIMENTS WITH HIGH ENERGY GAMMA RAYS : THE 158 keV TRANSITION IN 199Hg

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MÖSSBAUER EXPERIMENTS WITH HIGH

ENERGY GAMMA RAYS : THE 158 keV

TRANSITION IN 199Hg

W. Koch, F. Wagner, D. Flach, G. Kalvius

To cite this version:

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JOURNAL DE PHYSIQUE Colloque C6, suppliment au no 12, Tome 37, De'cembre 1976, page C6-693

MOSSBAUER EXPERIMENTS WITH

HIGH

ENERGY

GAMMA

RAYS

:

THE 158

keV

TRANSITION IN lg9Hg

(*)

W. KOCH, F. E. WAGNER, D. FLACH and G. M. KALVIUS Physik Department, Technische Universitat Miinchen, D-8046 Garching, Germany

Rbum6.

-

On a 6tudi4 la resonance Mossbauer de 158 keV de 199Hg dans une gkomCtrie de transmission en mesurant le courant integral du detecteur, aussi qu'en experiences de retrodiffusion. A partir des deplacements isomeriques observCs entre HgFz et les sources de 199Au dans les matrices de Pt, V, Nb et Rh, on estime une valeur de A

<

r2

>

=

+

1 x 10-3 fm2 pour le changement du rayon quadratique moyen nuclkaire.

Abstract.

-

The 158 keV Mossbauer resonance in 199Hg has been studied both in a transmission geometry by measuring the integral detector current and in backscattering experiments. From the isomer shifts observed between HgFz and sources of 199Au in Pt, V, Nb and Rh matrices one estimates A

<

r2

>

x

+

1 x 10-3 fmz for the change mean square nuclear charge radius.

1, Introduction.

-

Mossbauer experiments with gammarays having energies above approximately 100 keV become increasingly difficult because of the small recoilfree fractions then encountered even in heavy elements. The 158 keV transition in lg9Hg presents particular difficulties, mainly due to the inherently low effective Debye temperatues of mercury in most of its compounds and alloys.

To obtain Mossbauer spectra of the 158 keV gammarays within measuring times of the order of a few days, two approaches may be taken : One is to perform a standard transmission experiment with a very high gammaray intensity. In principle, in this way sufficient statistical accuracy can be obtained within a reasonable length of time, but even with fast counting electronics, one can hardly increase the countrate above several times lo6 s-I [I]. Higher rates of events in a detector like a scintillator or a Ge(Li) crystal can, however, be handled if the integral detector current is measured rather than individual pulses [2, 31.

Alternatively, one can use a backscattering geometry instead of the transmission arrangement. Again with sources of typically a few Curies, one then gets countrates that can easily be handled by a Ge(Li) detector, but the observed resonance effect can be 10'-lo3 times larger than in a transmission experiment (see, e. g. 141). Both the current integration and the backscattering technique are restricted to Mossbauer transitions that are fed by simple nuclear decay schemes, the former because of the inherent complete loss of energy resolution and the latter (*) Research performed under the auspices of the Bundes- ministerium fiir Forschung und Technologie and with financial support from the Gesellschaft fiir Kernforschung mbH, Karl- sruhe.

because the strong Compton background produced by gammarays having higher energies than the Mossbauer line can easily swamp the elastic scattering peak.

The decay of lg9Au which feeds the 158 keV level of lg9Hg fulfills this requirement. We have therefore performed both types of experiment in an effort to study this Mossbauer resonance.

2. Experiments and results.

-

All experiments were carried out at liquid He temperature. The backscattering setup has been described elsewhere 141. The experiments using the integral current technique were performed in a transmission arrangement with a moving absorber to reduce the geometry effect in the gammaray intensity impinging on the detector. The latter was a NaI(T1) scintillator mounted on a EM19656 photomultiplier, whose anode output signal was amplified and then fed into a Hewlett- Packard model 2212A voltage-to-frequency converter. The output pulses of the converter were stored into a PDP8 computer operated in the usual time-mode regime [I].

The absorber or scatterer material was HgF, in all cases. This compound has the cubic fluorite structure, and preliminary experiments with a variety of mercury compounds as sources for lg7Au Mossbauer experiments indicate that it has an out- standingly high effective Debye temperature among the mercury compounds.

The Ig9Au source activity (T,;, = 3.13 d) was produced by neutron capture in lg8Pt enriched to about 95

%.

For both the scattering and the transmission experiments, between 100 mg and 200 mg of 19'Pt were irradiated for about one week in a neutron flux of loL4 S - I cm-'. Besides sources of

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C6-694 W. KOCH, F. E, WAGNER, D.'

FLACH

AND G.

hi.

KALVIUS

Pt metal, alloys of the composition Pto.05Rho.95, Pto.02Vo.os and Pto.95Nbo.9, were studied. Since these matrix elements produce little or no harmful gammarays, the alloys could be prepared before the reactor irradiation:

Figure 1 shows a backscattering Mossbauer spec- trum obtained with a ' " ~ t metal source. A trans- mission spectrum taken also with a Pt metal source is shown in figure 2. Although the resonance effect is about 30 times larger in the scattering arrangement, it turned out that the time needed to measure spectra of comparable statistical accuracy was somewhat shorter in the transmission experiments. This geometry was therefore chosen for the experiments with the alloy sources (Fig. 2).

. ..

I

- 2 - 1 0 1 2

V E L O C I T Y ( r n r n l s )

FIG. 1.

-

Mossbauer backscattering spectrum obtained with a source of 199Au in Pt metal and a scatterer of 1 g/cm2 of

HgF2.

FIG. 2.

-

Mossbauer transmission spectra obtained with sources of 199Au in Pt metal and a Pto.osRh0.9s alloy and an

absorber of 730 rng/cmz of HgF2.

The observed linewidths are at most 1.5 times the natural width of Wo = 2 Ac/z Eo = 0.70(2) mm/s resulting from the lifetime [5] of the 158 keV level. In all experiments negative isomer shifts ranging between about

-

1.0 mm/s and - 1.5 mm/s were observed. These shifts are summarized in figure 3.

197

AU I S O M E R SHIFTS ( m m l s )

FIG. 3. - Isomer shifts observed for 199Hg in Pt, Rh, Nb and V matrices plotted versus the corresponding shifts for the 77 keV Massbauer resonance in 197Au [6]. The 197Au shifts are with respect to an absorber of metallic Au. The 199Hg shifts are given relative to HgF2. Their sign has been changed with respect to the experimental results (see Fig. 1 and 2) in order to be consistent with the absorber convention

used for the 197Au shifts.

3. Discussion.

-

The substantial isomer shifts indicate that the 158 keV transition goes along with a fairly large change A

<

r 2

>

of the mean square nuclear charge radius. The observation of isomer shifts in several transition metal hosts provides a possibility to obtain the sign and even an estimate of the magnitude of A

<

r 2

>,

since the systematics of the behaviour of the electron densities at transition metal nuclei in metallic 3d, 4d and 5d hosts are fairly well established [6-81. Thus, from Mossbauer data for 57Fe, 9 9 ~ u , lS90s, 1931r, lg5Pt and lg7Au, it has become evident that there exist approximately linear correlations between the isomer shifts of the different Miissbauer resonances in such alloy sys- tems [8]. It seems reasonable to assume that these systematics can be extended to include the 199Hg isomer shifts. If this is the case, one expects a linear correlation between the isomer shifts of lg7Au and lg9Hg in transition metal matrices. This expectation is supported by the plot of the shifts observed for 19'Hg versus those for lg7Au in the same host matrices. The positive slope of the correlation revealed by figure 3 shows that the sign of A

<

r2

>

is the same for Ig9Hg as for Ig7Au, for which a positive value of A

<

r2

>

is well established [9].

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M~~SSBAUER EXPERIMENTS WITH HIGH ENERGY GAMMA RAYS : THE 158 keV TRANSITION C6-695 alloys, one has to make a plausible assumption on the

ratio of the electron density differences at Au and Hg nuclei in transition metal hosts. Although the mecha- nism governing these isomer shifts is not very well understood, the isomer shift results for lg31[r, 19'Pt and 19'Au suggest that the electron density differences increase by roughly a factor of 1.5 between neigh- bouring elements. Assuming the same increase to take place between Hg and Au, and taking

from figure 3, we obtain

With A

<

r 2

>,,

= 8.6

x

l o v 3 fm2 for the 77 keV transition in lg7Au [9], this corresponds to

Since

A

<

r2 >Hg is positive, the shift of

-

0.96(2) mm/s observed between a Pt metal source and a HgF, absorber or scatterer means that the electron density at the Hg nuclei is smaller in the fluoride than in the Pt metal host. This appears reasonable when one considers that the Au(1) halides, which have a formal 5di0 electron configuration like the Hg(I1) compounds, also exhibit lower electron densities than Au in all its known alloys [6- 8, 10, 111. However, compared with the range of

isomer shifts spanned by the studied Hg alloys, and again taking the 1 9 7 A ~ isomer shifts as a reference standard, the shift between Hg in Pt and in HgF, seems rather large. This may either reflect an intrinsic difference in the behaviour of 1 9 7 A ~ and I9'Hg alloy isomer shifts, or the HgF2 lattice may give rise to particularly low electron densities. The latter idea is indeed supported by our results of 1 9 7 A ~ Mossbauer measurements with sources of neutron activated HgF,, in which Au species having a consider-. ably lower electron density than the Au(1) halides have been observed [12].

The A

<

r2

>

value for the 158 keV transition derived above can be compared with muonic isomer shift data [13, 141, which yield positive values of

A

<

r2

>

for both the 158 keV and the 208 keV transition in i99Hg, both of the order of a few times fm2. In view of the crudeness of the electron density calibration used to interpret our Mossbauer data, and of the inherent problems encountered in the interpretation of muonic isomer shift data in terms of a change of the mean square nuclear charge radius, the agreement can be considered as rather good, but the fact that the value derived from the muon data is higher than our value seems to indicate that our estimate of A

<

r 2

>

is somewhat too low. Acknowledgement.

-

We wish to thank Dr. JS. Sep- pelt for preparing the mercury difluoride and N. Hal- der for advice on problems of electronics.

References

[I] FORSTER, A., HALDER, N., KALVIUS, G. M., POTZEL, W. and ASCH, L., J. Physique Colloq. 37 (1976) C 6. [2] KANKELEIT, E., << Some Technical Developments in Moss-

bauer Spectroscopy

>>.

In : Proceedings of the Inter- national Conference on Mossbauer Spectroscopy, Cracow, Poland. Eds. A. 2. Hrynkiewicz and J. A. Sawicki, 1975, Vol. 2, p. 43.

[3] VIEGERS, M. P. A. and TROOSTER, J. M., Nucl. Instrum. Methods 118 (1974) 257.

[4] WAGNER, F. E., J. Physique, Colloq. 37 (1976) C6-673. [5] EDELSTEIN, W. A. and POUND, R. V., Phys. Rev. B 11

(1975) 985.

161 WAGNER, I?. E., WORTMANN, G. and KALVIUS, G. M.,

Phys. Letters 42A (1973) 483.

[7] KAINDL, G., SALOMON, D. and WORTMANN, G., Phys. Rev. B 8 (1973) 1912.

[8] WAGNER, F. E. and WAGNER, U., In : M6ssbauer Isomer Shifts. Eds. G . K . Shenoy and F. E. Wagner (North Holland, Amsterdam) 1977, Chap. 8a.

[9] KALVIUS, G. M. and SHENOY, G. K., Atomic Data and Nuclear Data Tables 14 (1974) 639.

[lo]

BARTUNIK, H. D., POTZEL, W., MOSSBAUER, R. L. and KAINDL, G., Z. Phys. 240 (1970) 1 .

[ l l ] FALTENS, M. 0. and SHIRLEY, D. A., J. Chem. Phys. 53 (1970) 4249.

[12] FLACH, D., Thesis, Technical University of Munich 1975. [13] WALTER, H. K., NUCI. Phys. A 234 (1974) 504.