Fe57 POLARIMETRY BASED ON QUADRUPOLE INTERACTION

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Fe57 POLARIMETRY BASED ON QUADRUPOLE

INTERACTION

U. Gonser, H. Sakai, W. Keune

To cite this version:

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JOURNAL DE PHYSIQUE Colloque C6, suppliment au no 12, Tome 37, DPcembre 1976, page C6-709

P0LAIUMJCL"I'Y

BASED ON QUADRUPBLE INTERACTION

U. GONSER, H. SAKAI (*) and W. KEUNE (**)

Fachbereich Angewandte Physik, UniversitM des Saarlandes, 6 600 Saarbriicken, Germany

R&nm6.

-

Un polarim5tre construit B partir de monocristaux de LiNb63 (C057 comme source (polariseur) et de FeC03 (siderite) comme absorbeur) est dkcrit. Les interactions quadrupolaires des deux matkriaux sont presque t5gales en grandeur mais contraires en signe et de plus le param5tre d'asymktrie q est presque nul.

Abstract.

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A quadrupole Fe57 polarimeter consisting of single crystals of LiNbO3 : CoJ7 as source (polarizer) and of Fee03 (siderite) as absorber (analyzer) is described. The quadrupole interactions of the two materials are nearly equal in magnitude but opposite in sign and in addition the asymmetry parameter q ss 0.

The polarization dependence in Mossbauer spectroscopy has been worked out in a number of theore- tical treatise [I-101 and various techniques have been used t o carry out polarimetry experiments, mostly based on transversely magnetized Co57 a-Fe sources [lo-271 and/or by selective excitation of nuclear sublevels [28] or filter techniques [13, 29-32]. These methods have disadvantages and difficulties are encountered in the analysis of the spectra. In the former case by the multiplicity resulting from the six linearly polarized source lines and in the latter cases by the inefficiency of the selective excitation or of the filters in terms of intensity, degree of polarization and non-Lorentzian lines. Sources exhibiting quadrupole hyperfine interaction will circumvent some of these problems. The usefulness of a quadrupole split Co57 source for a polarimeter depends on the fulfillment of the following conditions :

1. The material can be grown as single crystal and cutting of thin plates in various directions is possible.

2. Co5' can be incorporated with a unique charge state and a crystallographically unique resonance site. 3. The lattice site should have axial symmetry

(q x 0) with unique orientation.

4. The quadrupole splitting should be large or at least well resolved.

polarized, thus, at least one line is suitable in a Moss- bauer polarimeter as a source andfor as an absorber of polarized recoil free y-rays.

Note that lattice vibrational isotropy is not required. For an appropriate orientation in a single crystal

arrangement one Debye Waller factor is involved and will determine the intensities of both quadrupole split lines. To the contrary in polycrystalline materials lattice vibrational anisotropy will cause unusual relative line intensities (Goldanskii-Karyagin effect). Iron doped LiNbO, has become of technological importance mainly as a useful high-resolution optical information storage material. It was of interest to us to obtain information regarding the charge and defect state of the iron ions [33, 341. As a byproduct we found that single crystals of L ~ N ~ O , ( C O ~ ~ , Fe57) as source as well as absorber satisfy the above men- tioned conditions, while the common Fe compounds seem to defy these conditions. LiNbO, crystals were doped with Co57 or Fe57 and an annealing treatment at 800 OC for 24 h in pure Ar atmosphere reduces the cobalt or the iron entirely, at least, no trace of the three-valent state could be detected by Mossbauer spectroscopy. A conventional Mossbauer spectrum of LiNbO, : C O ~ ~ , Fe57 in conjunction with a single line absorber or source, respectively, exhibits a quadrupole splitting

y-radiation corresponding t o the transitions at room temperature and a negative quadrupole f

3

&

rt_

observed perpendicular t o the principal hyperfine interaction. With a single crystal oriented

axis of the electric field gradient (q = 0) is linearly with the c-axis perpendicular to the y-ray propagation polarized while the y

-

radiation corresponding to direction one obtaines relative line intensities close the transitions f

4 +

f + i s only partially linearly to 0.6(3 :5) indicating axial symmetry at the Fez+ ion sites along the c-axis. If both, source and absorber,

(*) Kyoto University, Osaka, Japan. exhibit quadrupole split spectra one can expect 4 lines,

(**) Gesamthochschule Duisburg, 4100 Duisburg, Germany. and under the condition that the (quadrupole splitting

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C6-710 U. GONSER, H. SAKAI AND W. KEUNE ( s o u r c e ) ( o b s o r b e r )

3

'

2112 2 3 1 2 t 312 ? I 1 2

*

112 L i ~b

4:

Fe C 0, ( s o u r c e ) (obsor ber

Fig. 1 and Fig. 2,

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Mossbauer spectra of single crystals oriented with their c-axes perpendicular to the y-ray propagation direction. (a) c-axes fo crystals were parallel and (b) c-axes were perpendicular to each other.

is the same the number of lines reduces to three ;

i. e. degeneracy of the center line at zero velocity occurs. Spectra of LiNbO, : Co57 VS. LiNbO, : FeS7 with c-axes perpendicular to the y-ray propagation direction and with the axes parallel (a) and perpendicular (b) to each other are shown in figure 1. At the bottom the nuclear level scheme is indicated. For a quadrupole ~e~~ polarimeter the mineral siderite (FeC03) is of particular interest as an absorber (analyzer) in conjunction with LiNbO, : CoS7 as source (polarizer). The J?e2+ in FeCO, (rhombohedra1 structure)' has axial symmetry and natural single crystals of this mineral are available. Furthermore, FeCO, exhibits a quadrupole splitting

(dEo

= 1.798

+

0.004 mm/s at room temperature) which is similar' to the value of LiNbO, : Fe5', however, the quadrapole coupling constant is positive. Spektra of single 'crystals of LiNbO, : Cos7 and F e C Q ate 'shown in figure 2, again with c-axes

oriented perpendicular to the y-ray propagation direction. At the bottom the nuclear level scheme is indicated. In (a) the two axes were parallel and in (b) they were perpendicular to each other. Of particular interest B the line on the side of positive velocity

( m

+

1.8 mm/s) because it results from the +_

4 e

f. $

transitions in source and absorber corresponding to linearly polarized y-rays. In the perpendicular arrangement (b) the FeCO, absorber is transparent for polarized y-rays, only a small residual absorption contribution can be detected which is most likely due to imperfections in the mineral FeCO, single crystal and also'due to misalignment of the polarimeter.

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Fe57 POLARIMETRY BASED ON QUADRUPOLE INTERACTION C6-7 1 1

the analyzer depends on the angle o between the automatic Mossbauer polarimeter [23] with magne- two c-axes of polarizer and analyzer and has sinusoidal tized a-Fe is simplified by the use of LiNbO, : Co5' form, corresponding to a Malus curve in optics : and FeCO, arrangement because no permanent

magnets are needed.

R(o) =

Ro

-

Ri

cos 2 o

Acknowledgement.

-

This work was supported

Ro

contains the non-resonant background and Ri by the Deutsche Forschungsgemeinschaft under is the amplitude of the Malus curve. The described SFB 130.

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