372596239

48

RIKEN Accel. Próg. Rep. 24 (1990)

111-2-15. High Resolution L X-Ray Angular Distribution Measurement

on Terbium

T. Papp, Y. Awaya, A. Hitachi, T. Kambara, Y. Kanai, and T. Mizogawa

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It was found, in the L₃-subshell angular correlation⁰ and angular distribution^2,3* studies, that the angular distribution of X rays reflects the effect of magnetic quadrupole (M2) mixing into the electric dipole (El) transition. The M2/ El mixing ratios experimentally obtained devi-ate significantly from theory^4> around 60. In these experiments semiconductor detectors were used to detect X rays. This type of detectors well resolve the Li and L_a peak; however the taił of the intense L* peak lies under the Li peak, caus-ing a systematic error in the evaluation of the mixing ratio. The large discrepancy between theory and experiments would be verified from an angular distribution measurement by a higher-resolution detection techniąue.

Thus we carried out such a measurement, detecting X rays with a crystal spectrometer in which a position-sensitive proportional counter was incorporated.⁵⁾ N²⁺ ions of 0.553 MeV/amu obtained from the RILAC were allowed to impinge on a Tb target of 4 /^m in thickness, tilted by 45° to an incoming beam, and the angular distribution was measured at eight different angles in the rangę of 95°-155°.

Usually, the L*i,₃-X ray emission is regarded as isotropic and is used for a normalization standard in angular distribution measurements. How-ever, in heavy ion-atom collisions with a simulta-neous multiple outer-shell ionization, the isotropic angular distribution of the L_0U3-X ray has to be verified experimentally. Therefore the angular distribution measurement was perfor-med in two steps: First the ratio of to L_a transitions was studied using a LiF (220) crystal. Since the Bragg angle for the L_a transition is 44° and the polarization sensitivity is very close to unity, the L_a intensity measured has an isotropic angular distribution, and can be used for normalization. From the measurement, the ratio of L^_lt3 to L_a transitions and thus the L^_lł3 transition were found to have an isotropic angular distribution. In the second step the angular distribution of the L-X rays was studied with a LiF (200) analyzer crystal. The intensity of the L₃-X rays

were normalized to the L_PXt3 transitions.

Figurę 1 shows an example of the L X-ray spectra. The spectra were evaluated by assuming a Voigt peak shape.

■	La
1-31,3	A , \
\	. 1
f 1 ^LP2,1S	1 %
A J \ - /\ r \	i \ L,
i \ / N-Y*'*'-** J ^v
■ 1_	^1 ■

200    300    400    500    600

Wavelength(Channel)

Fig. 1. L X-ray spectrum of Tb, ionized by 0.553 MeV N-ion impact, and measured with a crystal spectrometer using a LiF(200) crystal.

A large anisotropy was observed for the L₃ X-ray lines. By assuming that the crystal was perfect, a value of —0.20±0.04 was obtained for the anisotropy parameter for the Li transition normalized to the L*i,₃. The anisotropy was found two times larger for the L^.is transition than for the L_a. The ratio of the anisotropy parameters of the normalized Li and L_a transitions was evaluated to be 2.76±1.5 and 3.25± 1.9, assuming perfect and mosaic crystal struc-ture respectively. These results verify the dis-agreement with the theory observed in the earlier measurement carried out with semiconductor detectors.

References

1)    A.L. Catz: Phys. Rev., A2, 634 (1970); E.S. Macias and M.R. Zalutsky: ibid., A9, 2356 (1974).

2)    T. Papp: High Energy Ion-Atom Collisions (eds. D. Berenyi and G. Hock), Lecture Notes in Physics, 294, Springer-Verlag, Berlin Heidelberg, p. 204 (1988).

3)    T. Papp and J. PSlinkćis: Phys. Rev., A38, 2686 (1988).

4)    J.H. Scofield: ibid., 179, 9 (1969); J.H. Scofield: ibid., A10, 1507 (1974).

5)    A. Hitachi, H. Kumagai, and Y. Awaya: Nuci. In-strum. Methods, 195, 631 (1982).