Four-nucleon transfer reaction induced by 10B on 12C

Nuclear Physics A157 (1970) 297-304;

@

North-Holland

Publishing

Co., Amsterdam

Not to be reproduced by photoprint or microfilm without written permission from Ihe publisher

FOUR-NUCLEON INDUCED K. D. HILDENBRAND,

H. H. GUTBROD,

Max-Planck-Institut Physikalisches

TRANSFER REACTION BY “B ON “C

Institut

W. VON OERTZEN

and R. BOCK

fiir Kerphysik, Heidelberg and der Universitiit Marburg, Germany

Received

26 June

1970

Abstract: The four-nucleon transfer reaction induced by ‘OB on 12C has been studied above the Coulomb barrier. The population of final states of I60 is less selective than in other fournucleon transfer reactions induced by bLi, 7Li, or laO. This fact as well as the structureless angular distributions can be explained by the complexity of the structure of the ground state of l”B, which has large components with a lower degree of symmetry than the a-particle.

E

NUCLEAR REACTIONS 12C(10B, 6Li), E = 18 and 28 MeV; measured a(8). 1°B ground state deduced symmetry component. ‘“0 deduced selective population of states. Natural targets.

1. Introduction

The study of four-particle excitations in nuclei by four-nucleon transfer reactions induced by heavy ions can be achieved by many different projectiles which are accessible for acceleration. However, the features of the transfer reaction and the information obtainable from the experimental data is expected to depend strongly on the structure of the projectile. The use of 6Li and 7Li nuclei as projectiles for the study of states with a large a-particle width has proved to be a very powerful tool in nuclear spectroscopy ‘). This is mainly due to the fact, that both nuclei have, as their strongest configuration for the decomposition into four particles plus deuteron or triton, respectively, the four nucleons in their lowest energy and symmetry state (alpha particle). These two nuclei show high degrees of cluster isolation and the low binding energy of the a-par title in these nuclei favors their description in terms of alpha-plusdeuteron or alpha-plus-triton cluster, respectively. The l”B nucleus which also has a rather small binding energy of 4.46 MeV for the decomposition into (a+ 6Li), nevertheless was predicted to have a rather small aparticle reduced width. The l”B induced four-nucleon transfer reaction should therefore exhibit rather different properties as compared to the Li induced reactions. States which consist of multi-particle excitations without the strong correlation of the aparticle type should be excited as well. A study of the reactions induced by l”B on 12C was therefore performed with the aim of observing states in 160 via the four-nucleon transfer. The structure of the 297

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states in I60 is rather well known both from single-particle transfer reactions “) as well as from multi-nucleon transfers 4), and the experimental results are in good agreement with theoretical expectations 5p“). The population of final states in I60 with known structure can therefore be used to discuss the reaction mechanism involved and the structure of the l”B nucleus. 2. Experimental technique The experiments were performed with the EN and MP tandem accelerators at two energies, 18 and 28 MeV. Some spectra were taken at 45 MeV. The negative l”B ions GAS-ENTRY

\GUARD-RING COUNTER-WIRE

@l TEFLON

m BRASS

Fig. 1. Scheme of the proportional

SI -DETECTOR

counter telescope.

Fig. 2. Two-dimensional display of the coincident events in the dE (horizontal) and E (vertical) plane. From right to left the intensity lines correspond to 14N, 12C, l“B, 6Li and m-particles.

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REACTION

for acceleration in the tandem were obtained either by the method of Hortig and Schubert ‘) or from the penning source installed at the EN and MP tandem accelerators by Heinicke and Baumann “). The targets consisted of self-supporting carbon foils with a thickness between 50 and 100 @gjcm2. The particle identification was achieved by AE-E telescopes. The AE counter was a proportional counter of 5 cm effective length with a diameter of 28 mm. A scheme of the proportional counter

f 0

0

20

20

40

40

80

60

100 CHANNEL-NUMBER

60

80 100 CHANNEL-NUMBER

Fig. 3. Energy spectra of Li nuclei from four-nucleon transfer reactions induced by ICI3 of 28 MeV and 45 MeV.

telescope is shown in fig. 1. The 30 pm tantalum wire is surrounded by guard rings in order to restrict the charge collection to regions of homogeneous field. A surface barrier counter is mounted in the gas atmosphere and measures the energy of the nuclei which remains after passage through the gas. The counting gas is a mixture of

300

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90 % AC and 10 % CH,, with pressures between 50 and 100 Torr adjustable and controlled by a Cartesian manostat. The entrance window consists of 50 pg/cm2 movital foils. They are obtained by letting a droplet of the liquid solution of movital expand on water. With this identification system a resolution in energy loss AE x MZZIE of 4 % was obtained. This allowed complete separation of elements and of the isotopes of Li and Be. 3. Experimental results The energy spectra of the Li nuclei correspon~ng to the transfer of four nucleons at 28 MeV and 45 MeV are shown in the fig. 3. The energy resolution of 120 keV was sufficient to separate most of the peaks in 160 except for the doublet at 6 MeV. In some spectra the 7 MeV peak can be identified as being mainly the 2+ state at 6.92

‘2Cfm~.%)‘% * E LAB = 28 MeV Exe

k’ ”

1’ i

rt”

Energy

in MeV

“4

‘2C(‘oB,6Li?60 E ~B=18MeV

(G,6.92+712)

(G.6.06+6.13)

00 “ic:

0.6 : 0.4 0.2 0.1 II

(G.G)

0.06 0.04 0.02 -

Fig. 4. Angular distributions of the reaction 12C(10B, 6Li)‘60 at 18 MeV. The curves are drawn to guide the eye.

Fig. 5. Angular distributions of the reaction 12C(‘oB, 6Li)160 at 28 MeV. The curves are drawn to guide the eye.

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MeV, a lower intensity for the 7.12 MeV state shows up as a shoulder on the highenergy side. The spectrum at 45 MeV (fig. 3b) shows a large rise of intensity at excitation energies of the final nucleus above 12 MeV. This intensity is mainly due to the break-up of l”B into (jLi and c(. The break-up of “B was also the main reaction observed in experiments of 45 MeV “B on targets of SiOz , 24Mg 40Ca and 58Ni. For the targets heavier than 24Mg no transitions have been observdd with cross sections larger 50 pb. Fig. 3b also shows a spectrum obtained with the SiO, target with the same experimental conditions and clearly illustrates the position in energy of the 6Li nuclei due to the break-up of the projectile. Other strong reactions observed are the proton transfer “C( “B, 9Be)13N and the deuteron exchange 12C(10B, “C)“B. The angular distributions of the four-nucleon transfer reactions leading to final states in I6 0 are shown in figs. 4 and 5. Angular distributions of the elastic scattering of l”B on “C have been shown in refs. 9, 4b). 4. Discussion

of the results

The results of these measurements will be discussed only qualitatively. In view of the complexity of the reaction a quantitative analysis of the (log, 6Li) reaction is rather difficult even if finite range DWBA calculations can be performed. This statement is indeed the result of the following discussion. In the reaction 12C(loB, 6Li)12C all states except the ground state of 160 are excited with comparable intensity. The selectivity in the population of 160 states observed in other four-nucleon transfer reactions “) does not exist in (lOB, 6Li) reaction. The ground state of 160 is known to have a closed p-shell configuration with very small admixtures of particle-hole excitations “). As in all multi-particle transfer reactions “) this state is very weakly populated because the transfer of several particles goes preferentially to states with collective properties. The rotational band starting at 6.06 MeV is strongly excited. Of special interest is the population of the 2- state at 8.88 MeV. This state has unnatural parity and cannot be excited by aparticle transfer on 12C [this state is only weakly excited in other four-nucleon transfer reactions, refs. ‘74)]. The intensity of the transition and the form of the angular distribution which is nearly identical with that of transitions to other excited states, excludes the possibility that the state is populated strongly by a compound nuclear reaction, and also excludes a two-step process via the 2’ state in “C at 4.44 MeV. The maxima in the angular distributions at 28 MeV at approximately 25” c.m. correspond to the angle of grazing collision 8, calculated by the classical relationship R min The parameter

=

Rint = ro(Af+A$)

for the interaction

= (q/k)(l +cosec +Oo).

radius r. which corresponds

to the minimum

dis-

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tance for a particle in Rutherford orbits is 1.55 fm and is in agreement with determinations of this phenomenological parameter from the analysis of elastic scattering data ‘). Thus the form of the angular distribution corresponds to the semi-classical picture developed by McIntyre et al. lo ) for the transfer of nucleons between complex nuclei. As the impact parameter decreases (and the scattering angle increases) the increasing overlap of the nuclear wave functions leads to a rise of the differential cross section with scattering angle. At a certain impact parameter corresponding to a minimum distance Rmin and a scattering angle do, the absorption starts to reduce strongly the transfer cross section. However, the low value of the Coulomb parameter q = 2.8 should allow diffraction structures at 28 MeV. The lack of structure in the angular distributions [in contrast to other four-nucleon transfer reactions “)I is expected to be related to the specific structure of the l”B nucleus and to the reaction mechanism, which is determined by these specific properties. From experimental investigations of four-nucleon pickup reactions on 1°B, like (d, 6Li) [ref. 11)] and (3He, 7Be) [ref. ‘“)I, wh’ic h are known to correspond to uparticle pickup reactions to a high degree, it can be concluded that 1°B has a small width for the decomposition into (6Li+ti). The main strength of these reactions is observed to lead to the 6Li(3+) state at 2.18 MeV, which is not observable in this reaction because this state is unstable with respect to particle emission. Those measurements are in qualitative agreement with calculations of Rotter “) based on the intermediate coupling shell model. However, the a-pickup reactions as well as the strong intensity of the break-up of “B into 6Li and an a-particle observed in the present work (it was observed also by Chasman et al. ‘“)) clearly show that the cluster isolation in l”B is stronger than predicted by the shell model ‘). The “B ground state can be written in the intermediate coupling shell model as ’ “) 1°B 3+ = c11(6Li+ [4]11D)+d2(6Li+ [4]‘lG) +&(6Li+ [31]13D3)+B2(6Li+ [31]13D2) +/?3(6Li+ [31]‘3F4)+y(6Li+ [22]‘lD,)+. . . +B4(6Li+ [31]13F3) +B#Li+ [31]13F2). The coefficients have the values d, = 0.068, f.?3= -0.25,

ti2 = -0.116, y = 0.25,

& = -0.34, b4 = -0.08,

p2 = 0.24, /& = 0.27.

This large variety of symmetry components with comparable strength for the four nucleons in the p-shell allows the population of states states with different structures. The positive parity states in ’ 60, which form rotational bands, are mainly described by a 12C core plus four nucleons in the [4] symmetry with small components with [31] symmetry. Thus, the transitions to the states in 160 which form the rotational band starting at 6.06 MeV will consists of several amplitudes. For the transfer of the

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four nucleons in the [4] symmetry already two amplitudes will contribute with angular momentum transfer I = 2 and 4 due to the [4] D and [4] G components in l”B (they are again due to the high spin of rOB_ of 3). The high value of the angular momentum transferred already would be sufficient to explain the structureless angular distributions. The small components of the [31] symmetry in the wave functions of the final state will lead to appreciable additional amplitudes, due to the large components of this symmetry in r’B_. The total sum of all amplitudes due to all possible symmetry groups is not expected to lead to localisation in angular momentum space and will consequently lead to structureless angular distributions. The 2- state at 8.88 MeV has a strong component with [31] symmetry. The population of this state is easily explained by the structure of the l”B projectile; the population of this state indeed proves the existence of this symmetry component in r”B. Again several amplitudes with high angular momentum transfer will contribute and no structure is observed in the angular distribution. 5. Summary and conclusions The four-nucleon transfer reaction induced by r”B on “C shows marked differences from other four-nucleon transfer reactions induced by Li, 160, 19F and 20Ne [see ref. “)I. The final states of 160 are less selectively populated and the angular distributions do not show the strong diffraction structures observed in the other fournucleon transfer reactions. These differences are due to the specific structure of the “B nucleus. The cross section of a four-nucleon transfer reaction consists of a coherent sum over all structural components which occur simultaneously in the initial and final state “). For “B several structural components contribute to the transitions and the spectrum of excitation of final states in I60 is not expected to correspond to the spectrum of the a-widths in I60 [ref. “)I. The component with symmetry [4] which corresponds to the z-particle is smaller or comparable with other components with symmetries [31] or [22]. These components have large intrinsic relative angular momenta due to the high spin of the “B nucleus. Thus the transition amplitude for a certain final state consists of a sum of several amplitudes with transferred angular momenta of 2 and 4, at least. Four-nucleon transfer reactions observed with other projectiles like ‘Li or 160 where the cr-particle can have a relative angular momentum of 1 or zero, have clearly shown that diffraction structures are not observed in the angular distributions if the transferred angular momentum is large or if several amplitudes contribute to the reaction. The population of the final states as well as the structureless angular distributions (in spite of small q) are consistently described by the complex structure of the l”B nucleus. The population of states of unnatural parity can easily be explained by the corresponding transfer of a nucleon group in lower symmetries and the cross section of these four-nucleon transfers is comparable with those of the highest possible degree of symmetry.

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The authors are indebted to Prof. Gentner and Prof. Schmidt-Rohr for their interest and support. Special thanks are due to Dr. Hortig and Dr. M. Mtiller, as well as to Dr. Baumann and Dr. Heinicke for their help in preparing the boron beam. In this context the efforts made by Mr. Schmidt and his colleagues are gratefully acknowledged. Thanks are due to Mrs. Schlotthauer-Voos and Mr. Bohlen for their help during the experiments. We thank Dr. Yoshida for many clarifying discussions. References 1) R. Middleton, Proc. of Heidelberg Conf. on nuclear reactions induced by heavy ions, eds.

2) 3) 4.a)

b) 5) 6) 7) 8) 9) 10) 11)

12) 13) 14) 15)

R. Bock and W. R. Hering (North-Holland, Amsterdam, 1970) 263; K. Bethge, lot. cit.. p. 277; A. A. Ogloblin, lot. cit., p. 231; I. Rotter, Fortschr. Phys. 16 (1968) 195; Nucl. Phys. A135 (1969) 378 W. Bohne, H. Homeyer, H. Lettau, H. Morgenstern, J. Scheer and F. Sichelschmidt, Nucl. Phys. A128 (1969) 537 and refs. therein J. C. Jacmart, M. Liu, J. C. Roynette, C. Caverxasio, F. Pougheon, C. Stephan, M. Riou and W. v. Oertzen, Proc. of Heidelberg Conf. on nuclear reactions induced by heavy ions, eds. R. Bock and W. R. Hering (North-Holland, Amsterdam, 1970) p. 128; W. von Oertzen, H. G. Bohlen, H. H. Gutbrod, K. D. Hildenbrand, U. C. Voos and R. Bock, lot. cir., p. 156 A. P. Zuker, B. Buck, J. B. McGrory, Phys. Rev. Lett. 21 (1968) 39; G. E. Brown and A. M. Green, Nucl. Phys. 75 (1966) 401 W. H. Bassichis and G. Ripka, Phys. Lett. 15 (1965) 320 W. Gentner and G. Hortig, Z. Phys. 172 (1963) 353; K. Schubert, thesis, Heidelberg (1966) E. Heinicke and H. Baumann, Nucl. Instr. 74 (1969) 229 U. C. Voos, W. von Oertxen and R. Bock, Nucl. Phys. Al35 (1969) 207 J. A. McIntyre, T. L. Watts and F. C. Jobes, Phys. Rev. 119 (1960) 1331 H. H. Gutbrod, H. Yoshida and R. Bock, Proc. of Heidelberg Conf. on nuclear reactions induced by heavy ions, eds. R. Bock and W. R. Hering (North-Holland, Amsterdam, 1970) p. 311; to be published C. D&ax, H. H. Duhm and H. Hafner, Nucl. Phys. Al47 (1970) 488 R. Ollerhead, C. Chasman and D. A. Bromley, Proc. Third Conf. on reactions between complex nuclei (1963) eds. A. Ghiorso, R. Diamond, H. Conzett, p. 191 H. Yoshida, private communication I. Rotter, Nucl. Phys. A135 (1969) 378