HAL Id: jpa-00215764
https://hal.archives-ouvertes.fr/jpa-00215764
Submitted on 1 Jan 1974
HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.
L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.
TIME DISTRIBUTION OF MÖSSBAUER SCATTERED RADIATION
H. Drost, K. Palow, G. Weyer
To cite this version:
H. Drost, K. Palow, G. Weyer. TIME DISTRIBUTION OF MÖSSBAUER SCATTERED RADIA- TION. Journal de Physique Colloques, 1974, 35 (C6), pp.C6-679-C6-681. �10.1051/jphyscol:19746149�.
�jpa-00215764�
METHODOLOGY.
TIME DISTRIBUTION OF MOSSBAUER SCATTERED RADIATION
H. DROST, K. PALOW and G. WEYER Institut fur Atom- und Festkorperphysik
Freie Universitat Berlin, Berlin (West), D-1000 Berlin 33, Boltzmannstr. 20, Germany
Resume. — Nous avons 6tudie par la methode des coincidences retardees la distribution en temps du rayonnement reemis par un absorbant Mossbauer. Des effets d'interference dus a l'exci- tation coherente resonnante peuvent etre observes sur la distribution en energie ou en temps du rayonnement transmis ou r6emis. D'apres la th6orie, des oscillations caracteristiques doivent appa- raitre en fonction du temps avec une frequence associee a la difference d'energie entre le rayonne- ment incident et la reponse de 1'absorbant. Les mesures ont et6 realisees avec le rayonnement Mossbauer a 14.4 keV du 57Co par observation des electrons de conversion r6emis par un absorbant Mossbauer (diffuseur). Les ecarts observes entre la theorie et Pexperience peuvent etre interprets par l'introduction d'un effet quadrupolaire dans le materiau absorbant et par un elargissement inhomogene de la raie de la source et de 1'absorbant.
Abstract. — The time distribution of radiation reemitted from a Mossbauer absorber has been investigated in delayed coincidence experiments. Interference effects due to coherent resonance excitation are observable in the energy or time distribution of the transmitted or reemitted (scatter- ed) radiation. From theory, characteristic oscillations in the time dependence are expected with minima at times related to the energy difference between incident radiation and absorber response.
Measurements were carried out with the 14.4 keV Mossbauer radiation of 57Co by observation of the conversion electrons reemitted from a Mossbauer absorber (scatterer). Deviations from theory obtained in the experimental results are discussed with regard to a possible quadrupole splitting of the absorber material and inhomogeneous line broadening of source and absorber.
The process of Mossbauer resonance absorption and its influence on the energy or time distribution of the transmitted or scattered radiation has been discussed in detail in the literature [1-6]. The intensity of the re- emitted radiation from a Mossbauer absorber has been calculated using both classical and quantum mecha- nical dispersion theory, assuming Lorentzian shapes for the incident radiation and the absorber response, i. e.
the shape of the intermediate Mossbauer state. It turns out that the process can be described in a simple classical model treating the Mossbauer absorber as a medium of damped harmonic oscillators. The ampli- tude and phase of each Fourier component of the incident radiation are changed due to a complex index of refraction. The intensity of the scattered y-radiation, or reemitted conversion electrons in the case of internal conversion after resonance excitation can then be calculated by coherent summation of the amplitudes or intensities, respectively, over all scattering nuclei- The time distribution obtained for the intensity depends on the frequency difference Aco and on the linewidths of the incident radiation and the absorber response. An absorber thickness /?, describing the linewidths, enters into the calculation for the case of a very thin source and a homogeneous broadening of the absorber.
In the case of full resonance overlap a time distribu- tion of the form r2. e_'/ T is calculated for source and scatterer having natural linewidth (j3 = 0) and of the form t.e~'ix in the limiting case of infinite broadened scatterer response (e. g. for very thick scatterers).
Here x is the lifetime of the Mossbauer state. For Am # 0 additional oscillations in the time distribution are obtained with minima at times related to the fre- quency shift.
Sensitive experimental studies of this process are possible using the 14.4 keV Mossbauer transition of
57Fe by observing the reemitted conversion electrons.
With a conversion coefficient of a ^ 9 most of the resonantly absorbed radiation will be followed by internal conversion. The lifetime of- this state (T = 141 ns) is sufficiently long to allow time differen- tial measurements in coincidence experiments. A Mossbauer resonance counter was used in the experi- mental arrangement. The system consisted of a gas- filled parallel plate avalanche counter [7] for the detec- tion of conversion electrons, with a thin layer of K4Fe(CN)6-3 HzO as absorber inside the sensitive volume of the counter. The effective thickness of the layer is limited to about 100 ug/cm2 of 57Fe by the range of the conversion electrons, the total thickness
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19746149
C6-680 H. DROST, K. PAL( >W AND G. WEYER being 250 pg/cm2. Sources of 57Co in Pd and 5 7 ~ o in
Pt were fixed to the entrance window of the resonance counter. A constant frequency shift is then given by the isomer shift between source and absorber. The 122 keV y-radiation preceeding the Mossbauer transition in the source was detected in a NaI(T1)-detector and used as time-zero signal. Supplementing Mossbauer spectra were taken with this system to evaluate the actual absorber thickness from the experimental linewidth.
The measured time spectra contained background contributions due to the detection of random coinci- dences, of non-coincident radiation, and a pure expo- nential contribution from photoelectrons generated by incident Mossbauer radiation in the absorber material.
The latter contribution is rather substantial especially in cases with small resonance overlap. For the 57Co in Pt source this correction was nearly two times larger than the true resonance effect. To determine these background contributions, additional measurements have been carried out in which the resonance was destroyed by vibrating the source at high velocity. The figures always show results corrected for background
contributions.
The frequency shifts applied were A w = 2.4 T o and A o = 4.1 r0 for the 57Co in Pd and in Pt sources, respectively. The depths of the minima in the time dis- tribution depend sensitively on the linewidth of the source and the absorber thickness. For 57Co in Pd, upper limits of the thickness parameter j? are j? = 0.1 for the absorber alone, andP = 0.35 including inhomo- geneous line broadening of the source, as evaluated from the Mossbauer spectra obtained with the iden- tical system.
The measured time spectra show considerable devia- tions from theoretical curves calculated with these parameters. Relatively good agreement is obtained for the positions of the minima, although a shift to later times is observable. But no agreement was found for the shapes of the curves, especially the depths of the
FIG. I. - Time distribution of conversion electrons reemitted from a Mossbauer absorber. The frequency shift between source and absorber was 2.4 To. The dashed curves give theoretically expected distributions with the assigned parameters. The solid
curve is an approximate fit to the data.
first minima show drastic deviations from theory. In figure 1 theoretical curves are shown for 5 7 ~ o in Pd which should be compared to the measured data. One curve is calculated with P = 0.7, which should be twice the experimental upper limit. The other curve is cal- culated with P = 0.5, assuming a quadrupole splitting of the absorber of 0.5 ro. Both curves show only quali- tative agreement with the measured time distribution.
In the case of 5 7 ~ o in Pt, the observed deviations from theory are of the same form.
FIG. 2. - Time distribution of conversion electrons reemitted from a Mossbauer absorber. The frequency shift between source
and absorber was 4.1 To.
Similar measurements on the resonance absorption of the 14.4 keV y-radiation of 57Fe have been performed earlier [5, 6, 8-11]. Neuwirth [5] detected the 6.4 keV K-X-rays following internal conversion ; however, he only considered the case A o = 0. His results were in agreement with theory within a 5 % experimental error. The measured time distribution approached the form t.e-'Ir as it is expected for the relatively thick absorber used. Thieberger et al. [6] measured directly the resonantly scattered y-rays. For very thick scatte- rers the measured spectra were of the form r. e-'Ir, also in agreement with theory. However, for a thin scatterer and frequency differences A w # 0 the spectra obtained showed discrepancies. The distinct minima expected from theory were not observed. Several possible expla- nations for this were discussed by the authors, espe- cially the problem of a possible line broadening. It appears that these can not account for the observed deviations.
In a previous measurement from this laboratory [12]
the oscillations in the time distribution of the scattered radiation for cases with A o # 0 were found for the first time. However, deviations appeared, qualitatively similar to those found by Thieberger and remained unexplained. In the present experiments, the resonance counter was improved with regard to the signal to noise ratio, further increasing the experimental accu- racy. Nonetheless, systematic deviations are still present and the question of their significance remains. Although
homogeneous and inhomogeneous line broadenings are quadru~ole interaction ("1. It can be seen from figure 1 involved, they alone cannot account for the observed that an additional quadru~ole broadening will reduce deviations, if one assumes Lorentzian shapes, both for the deviations between theory and experiment consi- the incident radiation and absorber response. On the derably. In particular, the shift of the minima shows the other hand, it has been proposed that the line broaden- same direction as experimentally observed. This seems ing of the K,Fe(CN),.3 H,O absorber is due to to be a significant result, because the position of the minima is hardly affected by incorrect background
(*) NEUWIRTH, W., private communication. corrections in the data treatment.
References
[I] HAMERMESH, M., Argonne Nat. Lab., Rep. ANL-6111 [9] LYNCH, F. J., HOLLAND, R. E. and HAMERMESH, M.,
(1961). Phys. Rev. 120 (1 960) 51 3.
[2] HARRIS, S. M., Phys. Rev. 124 (1961) 1178.
[3] WONG, M. K. F., PYOC. Phys. Soc. 85 (1965) 723. [lo] Wu, C. S., LEE, Y. K., BENZCER-KOLLER, N. and SIMMS, P., [4] KERLER, W. and PETZOLD, J., 2. Phys. 186 (1965) 168. Pfzys. Rev. Lett. 5 (1960) 423.
[5] NEUWIRTH, W., Z. Phys. 197 (1966) 473. [ l l ] HOGASEN, H., BERGHEIM, K. and SKARSVAG, K., Physica [6] THIEBERGER, P., MORAGUES, J. A. and SUNYAR, A. W., Norvegica 1 (1963) 153.
Phys. Rev. 171 (1968) 425.
[7] CHRISTIANSEN, J., 2. Angew. Phys. 4 (1952) 327. [12] DROST, H., V. LOJBWSKI, H., PALOW, K., WALLENSTEIN, R.
[8] HOLLAND, R. E., LYNCH, F. J., PERLOW, G. J. and HANNA, and WEYER, G., 5th Int. Conf. on Mossbauer Spectro- S. S., Phys. Rev. Lett. 4 (1960) 181. metry, Bratislava (1973).
