K- TO K- SHELL CHARGE TRANSFER IN CLOSE COLLISIONS OF HYDROGEN-LIKE Kr IONS ON Mo

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K- TO K- SHELL CHARGE TRANSFER IN CLOSE

COLLISIONS OF HYDROGEN-LIKE Kr IONS ON Mo

G. Wintermeyer, V. Dangendorf, M. Gabr, T. Kambara, P. Mokler, S. Reusch,

H. Schmidt-Böcking, R. Schuch, I. Tserruya

To cite this version:

JOURNAL DE PHYSIQUE

Colloque Cl, supplbment au n O l , Tome 50, j a n v i e r 1989

K- TO K- SHELL CHARGE TRANSFER IN CLOSE COLLISIONS OF HYDROGEN-LIKE Kr IONS ON MO

G. W I N T E R M E Y E R , V. D A N G E N D O R F * , M. GABR, T. KAMBARA", P. MOKLER*.',

S. REUSCH*.', H. S C H M I D T - B O C K I N G * , R. SCHUCH"" and I. TSERRUYA"'^^ Physikalisches Znstitut der Universitat Heidelberg, D-6900

Heidelberg, F.R.G.

' ~ n s t i t u t fiir Kernphysik der Universitat Frankfurt, D-6000 Frankfurt, F.R.G.

Atomic Processes Laboratory, RIKEN, Wako Saitama 351 01, Japan *'*~esellschaft fiir Schwerionenforschung, 0-6100 Darmstadt, F.R.G.

* * * * _{Wanne Siegbahn Institute of Physics, 5-10405 Stockholm, Sweden}

.*.*.

_{Nuclear Physics Department, Weizmann Institut of Science, Israel}

RQsum6 Nous avons ktudid la production et le partage destrous en K - couche avec les collisions entre des ion de Kr ressemblant B H et Li ayant 5.5 MeV/nucl. et une cible de MO Ctat solide. Nous avons mesurC la dCpendance de la proba- bilitk des trous en K - couche de collisions de 700 fm jusqu'a la region des colli- sions nucldaire du paramktre de choc du projectile et de la cible. Les rksultats sont interpretds en applicant le modkle des orbitals molkculaires.

Abstract The production and sharing of K - shell vacancies has been investi- gated with collisions of H

- like and Li

- like 5.5 MeV/nucl. Kr ions on a MO solid target. We measured the impact parameter dependence of the projectile and target K - vacancy probability for collisions ranging from 700 fm down to the region of nuclear collisions. The results are interpreted in terms of the molecular orbital model.

I. Introduction

The molecular orbital theory proved to be a useful1 tool in explaini~lg some features of ion - atom collisions. Charge transfer and excitation processes are described by couplings between quasimolecular states in a two center potential. These states can only be formed if the electrons under consideration have enough time to adjust to the transient binuclear potential. The collision therefore must be slow enough. This 'slowness' generally is defined by the condition v1

> v,-

.

v1 is the impact velocity while v,- is the Bohr velocity of the orbital being invested. In near symmetric collisions

( Z P / Z ~

% 1 ) changes in the K

- shell charge state are dominantly induced by couplings between the Ism, 2pa and 2pn

molecular orbital.

Creation of K - vacancies in target and projectile has been observed for a varity of close

collision systems (see e.g. 1). The experimental results for gastargets could be extensively explained due t o 2pn - 2pa rotational coupling transfering L - vacancies into the K shell of the lighter collision partner. Similar measurements using solid state targets show dis- crepancies with theory which do not occur when using gastargets and which are still not explained. l s o - 2pa radial coupling shares the vacancies between target and projectile K shell in the outgoing part of the collision

.

There is a large spatial separation between the regions where the two couplings already mentioned become eEectiv. The rotational coupling between 2pn

and

2pa gets largest in the range of closest approach where the rotation of the internuclear axis has its maximum. The lsa - 2pa radial coupling occurs further out in the region where the electronic wavefunction of the'innermost shell changes from atomic towards molecular character.

JOURNAL DE PHYSIQUE

Using hydrogenic projectiles some of the K

-

vacancies brought into the collision will be transfered to the target K shell via l s a - 2pa radial coupling. The transitions can occur during the incoming and the outgoing part of the trajectory. Therefore the amplitudes for vacancy transfer have to be added coherently causing a n oscillating behavior in the impact parameter or collision energy dependence of the vacancy transfer probability. Such a structure first was observed for light systems (2), later for moderately heavy systems using gaseous targets (3). The principal behavior of such systems can be explained with a model just dealing with the l s a and the 2pu level, an analytical formula for this given in (4). Better agreement with experimental d a t a is achieved with a 3 - state calculation (5) and a multistate atomic expansion (6).

Measurements of resonant K to K charge transfer with hydrogenic projectiles using solid targets are only rarely reported till now. Motivation for using solid targets arises from the idea to measure I< to I< charge transfer a t very small impact parameters down to nuclear grazing, where low event rates resulting from small scattering cross sections partly could b e compensated by using thicker, i.e. solid targets.

A presently already established technique to produce highly charged slow ions is the accel- eration - deceleration

-

stripping method (4,7), which allows to produce sufficiently slow hydrogenic ions u p to Z x 40 at existing accelerator facilities. We decided to go close to the edge of the presently attainable region and measured I<

- vacancy probabilities for

Kr33+, MO a t small impact parameters ranging down to nuclear grazing. The maximum possible deceleration would have been down to 4 MeV/nucl., for staying above the Coulomb barrier and for getting a higher beam intensity we decelerated only to 5.5 MeV/nucl.

11.

Experiment

The Krypton beam was produced a t the UNILAC of the Gesellsehaft fiir Schwerionen- forschung, Darmstadt, FRG. Kr ions with charge state q = 1'2f were accelerated in the Wideroe and Alvarez part of UNILAC up to 11.4 MeV/nucl.. At this energy the Kr ions were stripped by a 100 pg cm2 C foil, reaching charge states up to ~ r ~ ~ + . Deceleration of the now highly charged beam was performed with 14 single gap cavities, giving us the desired 5.5 Mev/nucl. Kr3'+, resp. Kr33+ beam.

Collimators

T

Ge(1) ( 2 ~ ) MWPC PPAD FC

I

Fig.1 The Experimental Set - Up

A

multiwire proportional counter MWPC was used for detecting particles between 10" and 60" with an angular resolutionof l". The X

-

ray detectors were set in coincidence with the PPAD and the MWPC. For determining absolute probabilities we measured the single scattering angle spectrum of

articles

being detected by the MWPC. Each coincident event contained information about the X

- ray energy, the scattering angle and the coincidence

time between one of the X - ray detectors and one of the particle detectors.

The target thickness dependence of the MO K - X ray yield was measured using targets of 0.8 Pg/cm2, 5.94 ,ug/cm2, 7.8 Pg/cm2, 12.8 Pg/cm2 and 200 Pg/cm2 evaporated on 10 pg/cm2 C backing. For the I< - t o I< transfer measurement we used the 5.94 pg cm2 MO target. The thicknesses of the different targets was measured a t the 2 MV tandem accelerator of the Institut fiir Kernphysik, Universitat Frankfurt, FRG where we measured the yield of backscattered particles while bombarding the taget with 0.5 Mev protons. The accuracy in determining the target thickness was better then 1 %. The homogenity of the target was controlled by an electron microscope survey which showed us a smooth surface structure with a resolution of 50 nm.

111. D a t a Analysis

For every scattering angIe region we got a spectrum of X - ray energies in coincidence with particles scattered in the respective Ella6 - range. These spectra containing the total coincidence yield had to be corrected for random X

- ray

- particle coincidences.

10

20

30

X

-

ray

energy

[

keV1

Fig.2 X

-

ray energy spectrum of true coincidences with particles scattered between 0.3' and 0.4'

The true to random ratio varied from 10 : 1 the Kr and MO lines a t I<r33+ + MO and the MO lines a t Kr35+ + MO up to 3 : 1 for the Kr lines at Kr3'+ + MO. The latter value results from the large total yield of ICr X rays when using projectiles with a K vacancy. Fig. 2 shows an example for an X - ray energy spectrum after random substraction. A clear separation between the Kr K, and Kg and the M O K, and Kg lines can be seen. The contents of these 4 K

- lines was determined by fitting them separately with Gauss -

shaped curves. For obtaining K

- vacancy probabilities the number of true coincident Kr

and MO events were divided by the total number of scattered particles, these values being corrected for X - ray detector efficiency, solid angle and fluorescence yield.

Cl-178 JOURNAL DE PHYSIQUE

statistical errors of the measured probabilities. Furthermore we have a systematical error resulting mainly from uncertainities in determining the correct solid angle and fluorescence yield. For fluorescence yield we took the neutral atom values W K , = 0.646 and w ~= ,

0.748 (8). We estimate the systematical error from all these corrections in the order of f

30 %.

IV. Results a n d Discussion

a) T a r g e t Thickness Effects

The

charge state distribution of an ion beam traversing a target with finite thickness can be changed. In an hydrogenic beam some of the K

- holes will be lost while others will be

newly created. The fraction of ions

Y(X)

having a

K

-

vacancy after traversing a target thickness

X

will change according to the following equation describing destruction and newproduction of projectile K

- vacancies

(10):

U" is the cross section for producing projectile K - vacancies in collisions with target ions and

u = Q v + ~ c + ~ ~

uc +a, is the sum of the cross sections for K

- vacancy loss due to electron capture

in the projectile K

-

shell ac and for direct decay of the K

- vacancy

a,. Assuming that target I( - vacancies can only be produced in collisions with projectiles bearing a K - hole one has to integrate eq. (1) to get an expression for the total yield of target X - rays in a target with thickness T :

O K is the cross section for production of target K - vacancies in collisions with projectiles bearing a I<

- vacancy,

W the target fluorescence yield.

torget thickness [vglcrn2]

Fig.3 Normalised yield of target K - X - rays measured for 5 different target thicknesses. For the solid line see text.

We have been measuring the MO K X - ray yield for 5 different target thicknesses between 0.8 pg cm2 and 200 pg cm2 (Fig 3). These values were fitted with a curve using eq. 2, thus giving us av = 2.65

Mb

f 15 % and a = 9.78 Mb f 12 % as fitparameters. Using the relation

a, = ( n w ) - l

of our experiment the fraction of K

-

holes surviving the traversion of a 5.94 pg cm2 MO target is meaningfull. Setting the received values for a and a v into eq. 1 tells us that 75.9

% of the K

-

holes are not destroyed. Including the vacancies which are newly produced 77.1 % of the projectiles have a K - vacancy after passing the target.

b)

K

-

K C h a r g e T r a n s f e r

For collisions of hydrogenic Kr on MO tab.1 shows the K - vacancy probability in the pro- jectile ion or the target ion after the collision. If there would be neither new production nor destruction of K

- holes, which happens if we only allow l s a - 2pa coupling to occure,

we would expect the P values in tab.1 to be unity for all impact parameters. For impact parameters larger than 200 fm the observed values are in good agreement with this as- sumption. For smaller impact parameters the total probabiliy eventually gets larger, which could be explained by 2 p r - 2pa rotational coupf ng having its cinematic maximum in this region. The small count rate for these impact parameters resulting in large statistical errors makes a final conclusion difficult.

The impact parameter dependence of

I<

- hole production in projectile and target for collisions with Li - like Kr ions show the same behavior reported before for lighter systems (see e.g. 1) employing solid state targets, i.e. a shift of the adiabatic peak towards smaller impact parameters and filling of the minimum between adiabatic and kinematic peak in comparision with the rotational coupling model. The data can be adjusted in height with theoretical values calculated for an empty 2 p r level (11) by scaling the theoretical values

Tab.1 Total K - vacancy probability after the collision of Kr3'+ + MO. AP is the statistical error.

with the factor F = 0.34

.

F can be interpreted as the fraction of vacancies in the 2 p ~ level during the collision.

Fig.4a shows the ratio R of the measured K - vacancy probabilities in dependence of the impact parameter for Kr3'+ _t MO.

Cl-180 JOURNAL

DE

PHYSIQUE

account, adding coherently the transition amplitudes for both passages of the interaction region. This calculation is in bad agreement with the experimental data. Agreement gets better for curve b - d in Fig.4a. These 3 calculations were done following the method of Piacentini and Salin (5), taking into account l s a - 2 p r and 2pa - 2p7r rotational coupling and l s u - 2pa radial coupling. We calculated curve b, c and d assuming different potentials for target and projectile during the collision. The initial relations between the energy levels of the hydrogenic projectile and the neutral target will be changed during the collision due to changes in the stage of ionisation. By selecting different potentils we have been able to bring experiment and theorie closer, nevertheless there are still significant differences between both.

10 100 LOO 800

b [ f m l

Fig.4a) Measured values for the ratio of vacancy probabilities PMo(b)/PK,(b) with hy- drogenic Kr on MO. Curve a, solid line: 2 state calculation. Curve b, dotted line, curve c, dashed line and curve d, dash - dotted line are 3 state calculations.

l ' I l

10 100 LOO 800

Fig.4b Same experimental values as Fig.4a for collisions of Li

- like Kr ions on MO. The

solid line shows the K - K transfer probability for single passage following (12)

V. Conclusion

Acknowledgement

This work was supported by Bundeminister fiir Forschung und Technologie. References

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