TWO ELECTRON, ONE PHOTON TRANSITION PROBABILITY IN Al FOLLOWLNG DOUBLE K-SHELL IONIZATION BY ELECTRON IMPACT

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TWO ELECTRON, ONE PHOTON TRANSITION PROBABILITY IN Al FOLLOWLNG DOUBLE K-SHELL IONIZATION BY ELECTRON IMPACT

A. Kumar, J. Auerhammer, H. Genz, A. Richter

To cite this version:

A. Kumar, J. Auerhammer, H. Genz, A. Richter. TWO ELECTRON, ONE PHOTON TRANSITION PROBABILITY IN Al FOLLOWLNG DOUBLE K-SHELL IONIZATION BY ELECTRON IMPACT.

Journal de Physique Colloques, 1987, 48 (C9), pp.C8-665-C8-668. �10.1051/jphyscol:19879115�. �jpa-

00227223�

JOURNAL DE PHYSIQUE

Colloque C9, suppl6ment au n012, Tome 48, dbcembre 1987

TWO ELECTRON, ONE PHOTON TRANSITION PROBABILITY IN A1 FOLLOWING DOUBLE K-SHELL IONIZATION BY ELECTRON IMPACT(')

A. K U M A R ( ~ ) , J. AUERHAMMER, H. GENZ and A. RICHTER

Institut fiir Kerphysik, Technische Hochschule Dannstadt, 0-6100 D a m s t a d t , F.R.G.

Abstract:

-

The decay possibilities of atoms with two K-shell vacancies have been studied by bombarding a thin (465 pg/cm2) A1 target with elec- trons of Eo= 2 0 keV. Besides the more common rob ability involving a sequential decay, that leads to a K - ~ + K-~L-' transition with the emission of a Ka hypersatellite X ray

-

called one-electronlone-photon transition (OEOP) -, we have also investigated the weak correlated two- electron,one-photon transition (TEOP) by spectroscopy of the emitted X rays using a high resolution crystal spectrometer. The two transitions were recorded with different crystals always relative to the intensity

to the Ka line which originates from a single K-shell vacancy. The branching-ratio BR = TEOP/OEOP was found to be BR = (1.2+0.4)10-3, which

is in qualitative agreement with theoretical predictions and close to previous heavy-ion induced experiments.

1

.

Introduction

Although multielectron transitions were predicted to occur since a long time, it took fifty years

1 1 1

until their experimental discovery in heavy ion collisions. Since then two-electronlone-photon transitions have been detected in a variety of experiments, which generated a great deal of interest in the decay scheme of multi-ionized atoms. Several models, each of them were based on the configuration mixing of the elec-

tronic states of electrons and the change of the average potential of electrons in the independent particle model, were proposed to explain

("~u~ported by the Deutsch Forschungsgemeinschaft

(2)~resent address : California State University, Long Beach. CA 90840. U.S.A.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19879115

C9-666 JOURNAL DE PHYSIQUE

the decay mechanisms. In the "shakedown" model by %erg et al. [ 2 ] the branching ratio BR is predicted by the relation

h h

where I(Kaa) and I(Ka ) are the observed intensities of the TEOP and OEOP transitions, E l is the average hypersatellite (Ka h ) energy, E2 is the average two-electron (Kaa) transition energy and Do h ( I s2s) is the ls2s radial overlap integral, which is predicted to be approximately equal to 0.187/2. The comparison of the various theoretical predictions with the experimental findings of these very weak transitions (1(Kaa)j h I(Ka) = lo-') exhibits agreement within an order of magnitude.

It was a recent experiment

131,

in fact the only one in which the double vacancies were produced by electron bombardment, that found for several first transition series elements branching ratios which were some three orders of magnitude higher than almost all the previous ex- periments and theoretical predictions. In view of this variance the present experiment was performed to clarify the situation. Since the branching ratio decreases with the increase of the atomic number (see

Eq. l ) aluminum was chosen as a target. According to Eq.1 we find that

the branching ratio of A1 (Z=13) is BR=1.4.10-~.

2. Experiment

The experiment was performed at our superconducting pilot accelera- tor facility. The details of the set up are described elsewhere [ 4 1 . Essentially, an electron gun was used as a source of the electron beam with an energy of 20 keV

,

which was focussed on a thin A1 target placed 2 m from the electron gun. After traversing through the thin target the electrons were collected in a Faraday cup. The emitted characteristic radiation was studied perpendicular to the beam axis with a high resolution flat Bragg spectrometer with either a TRAP or a PET crystal. The PET crystal was used in the study of the two-electron, one-photon transitions and the TRAP crystal for the ~a~ intensity, while the Ka transitions were investigated with both crystals.

The experiment was performed in two steps. First we determined the OEOP transition probability for double K-shell vacancies relative to a single K-shell vacancy by detecting the intensity of the hypersatellite relative to the Ka line following the procedure outlined before C51.

Accordingly, the intensity of the TEOP transition was detected relative to the Ka radiation. The total spectrum obtained after about four weeks of continous beam time is presented in Fig.1. Based on an absolute energy calibration the transitions indicated in the figure could be

I

Two electron, one-photon transitior

I

^l3

^{At K,, " Spectrum}

"1

E,=20 k e V

h PET crystal

I

Bragg Angle

Fig.1

-

Aluminum two-electronlone-photon spectrum obtained by bombarding a thin (465 yg/cm2) AL target with electrons of 20 kev energy using a flat TRAP crystal. The TEOP transition under in- vestigation is indicated by IZaah.

assigned unambiguously following the designation by Tanis et al. [61.

One clearly observes besides the Kaa transition searched for many sa- h tellites originating from atomic states with additional L vacancies.

The reported intensities were corrected for absorption in the target, the crystal reflectivity and the detector efficiency. Figure 1 exhi- bits clearly that a high resolution crystal spectrometer is needed to resolve the satellites in order to compare only the Kaa transition pro- h bability with theory.

3. Results and Discussion

h h

From the independent measurements of I (Ka ) /I (Ka) and I (Kaa) /I (Ka) we deduced a branching ratio of BR = (1 .2

+

0.4)

lo-"

which is in accordance with the findings of Stoller et al. [7] and of the same or- der of magnitude as the results by Schuch et al. [8]. The present re- sult is also quite close to several theoretical predictions [2,9,101.

The comparison of the branching ratio for different modes of excitation is possible since the hypersatellite and the TEOP transitions have the

C9-668 JOURNAL DE PHYSIQUE

same i n i t i a l s t a t e c o n f i g u r a t i o n p r o v i d e d no f u r t h e r m u l t i p l e i o n i z a - t i o n s h a v e o c c u r e d . The b r a n c h i n g r a t i o t h e n r e f l e c t s t h e c o r r e l a t i o n s t r e n g t h between t h e i n n e r e l e c t r o n s .

F i n a l l y , it c a n b e s t a t e d t h a t due t o t h e improved t e c h n i q u e u s e d i n t h e p r e s e n t e x p e r i m e n t a new p r e c i s e v a l u e f o r t h e b r a n c h i n g r a t i o i s o b t a i n e d . The p r e s e n t e x p e r i m e n t c o n t r a d i c t s t h e r e s u l t o f t h e work by Salem e t a l . [ 3 1 b u t s u p p o r t s t h e o t h e r e x p e r i m e n t a l and t h e o r e t i c a l p r e d i c t i o n s .

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