REACTIONS 14N(6Li, 7Li)13N AND 14N(6Li, 7Be)13C AT 32 MeV

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Submitted on 1 Jan 1971

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REACTIONS 14N(6Li, 7Li)13N AND 14N(6Li, 7Be)13C AT 32 MeV

B. Zeidman, U. Strohbusch, A. Richter, K. Groeneveld

To cite this version:

B. Zeidman, U. Strohbusch, A. Richter, K. Groeneveld. REACTIONS 14N(6Li, 7Li)13N AND 14N(6Li, 7Be)13C AT 32 MeV. Journal de Physique Colloques, 1971, 32 (C6), pp.C6-295-C6-297.

�10.1051/jphyscol:1971670�. �jpa-00214888�

JOURNAL DE PHYSIQUE Colloque C6, supplkment au no

11-12,

Tome 32, Novembre-Dkcembre 1971, page C6-295

REACTIONS 14N('Li, 7Li)13N AND 14N('Li, 7Be)13C AT 32 MeV (*)

B. ZEIDMAN, U. STROHBUSCH

(**), A.

RICHTER

(***)

and K. 0. GROENEVELD

(****)

Argonne National Laboratory, Argonne, USA

Rhumb. - Les distributions angulaires des reactions de transfert d'un nuclhon 14N(6Li, 7Li)13N et 14N(6Li, 7Be)13C ont

kt6

6tudiees

a

E(6Li)

= 32

MeV et sont comparhes

a

des calculs en ondes dkformies avec portee finie et des facteurs spectroscopiques thkoriques.

Abstract.

- Angular distributions for the one-nucleon transfer reactions

1

4N(6Li, 7Li)

1 3N

and 14N(6Li7 7Be)l3C were investigated at E(6Li)

= 32

MeV and were compared with linite-range distorted-wave calculations and theoretical spectroscopic factors.

One of the most fundamental interactions between complex ions is single-nucleon transfer which, when studied with light projectiles, has yielded much of the nuclear-structure information currently available. We have investigated 'Li-induced neutron and proton .,

pickup from 14N via the 14N(6Li, 7~ig.s.)i3~g.s.,

1 4( ⁶

L ,

~

i 7Lil,t)13Ng.s., 1 4 N ( 6 ~ i , 7Beg.s.)13Cg.s., and

i 4( ⁶

L i

~

, 7Bel,J13Cg.,. reactions, which are particu- larly interesting because their reaction products form the 7Li-7Be and 3C-1

^3~

isobaric-analogstate doublets. Therefore the spectroscopic factors for neutron and proton transfer should be equal if the mass-7 and mass-13 nuclei involved are true isobaric doublets and if the reaction mechanism at a n energy far above the Coulomb barrier conserves isospin.

We investigated the effects of isospin impurity by comparing the relative cross sections for reactions to analog final states, and in addition we demonstrated the ability of DW calculations to predict the magnitude of absolute cross sections for neutron and proton transfer.

Our experiment was performed in an 18-in scattering chamber where a melamine (CH,N2), target was bombarded with 32-MeV 6 ~ i ions from the Argonne EN tandem Van de Graaff. Spectra of 'Li, 7Li and 7Be emerging from the target were recorded simultaneously in separate quadrants of a 4096- channel analyzer

;

typical spectra are displayed in figure 1. The outgoing particles were detected and identified electronically by use of a counter telescope

;

(*)

Work performed under the auspices of the U.

S.

Atomic Energy Commission.

(**)

Permanegt address

:

Physikalisches Institut

der Univer-

sitat Freiburg, Germany.

(***)

Present address

:

Institut

fur

Experimentalphysik

der

Ruhr-Universitat

Bochum,

and Max-Planck-Institut

fur

Kern- physik, Heidelberg, Germany.

P (****)

Present address

:

Institut

fur Kernphysik der

Univer-

sitat

Frankfurt, Germany.

-

0 t 7

L i S P E C T R U M F R O M

g 3201

1 4 ~ ( 6 ~ ~ , 7 ~ i ) 1 3 ~

B e SPECTRUM FROM

C H A N N E L S

FIG. 1.

- Spectra from reactions

induced

by

32-MeV

6Li ions incident

on

14N.

The

6Li, 7Li,

and

7Be spectra were obtained

simultaneousIy

with a counter telescope at

@lab = 140. In the

6Li

spectrum,

the numbers are the excitation

energies (MeV) of the

states. The notation [i,

j]

indicates the ith and

jth

excited

states of C

and

D,

respectively, in the reaction 14N(6Li,

C)D.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1971670

C6-296 B. ZEIDMAN, U. STROHBUSCH, A. RICHTER AND K. 0. GROENEVELD

the resolution was of the order of 100 keV (FWHM).

The data were normalized both by current integration and by a monitor counter which also measured the deterioration of the target during the course of the bombardment.

The angular distributions over the range

O,,, ⁼

7-440 (0 ,.,.

⁼

10-65O) are shown in figure 2.

FIG. 2.

-

Angular distributions for 14N(6Li, 6Li)14N, 14N(6Li, 7Li)13N, and 14N(6Li, 7Be)l3C. The curve through the elastic-scattering data points represents an optical-model fit. The curves through the reaction data are D W results as discussed

in the main text.

The errors in the relative cross sections for 6Li scatte- ring and the production of 7Li and 7Be are determined from statistical errors only. The errors in the absolute cross sections are estimated to be approximately

15 %.

As is seen in figure 2, the angular distributions for all four transfer reactions are similar in shape but the experimental cross sections differ in magnitude, the ratios being

where the nuclide indicated in a square bracket is the particle detected. The cross-section ratios R, would be expected to differ from 1 because the ground state and excited state of a mass-7 nucleus have different structures. After removing a purely kinematical factor by multiplying the ratio R , by the ratio of

the relative wave numbers of the mass-7 nuclei in the exit channels, about a 15 % difference between the corresponding Be and Li cross sections remains.

If there were pure isospin \symmetry, this ratio should be unity.

In order to understand both o m angular distri- butions and our results for R, and R,,

we

have used a finite-range D W program [I]. The optical-model parameters employed (Table I) were obtained from fitting the 6 ~ i elastic-scattering data obtained in the present work. For the outgoing channel, the only change in parameters was t o make the appropriate change in charges and masses. The values of the orbital angular-momentum transfer

L are severely restricted

by triangle selection rules and by parity. In the case of p-shell nuclei, only L

=

0 and L

=

2 are treated by the program.

Optical-model parameters (well depths in MeV and geometric parameters in

F) used in

a potential of the form V(r)

=

- V0(1 + ^ex)-'

-t-

4 iWD(d/dxl) (1 + ^ex')-' + V,, where x

=

(r - R)/a, x'

= ( r

- R')/ar, and V,

is the Coulomb potential

Assuming the same spectroscopic factor for neutron and proton transfer, we find that a t the energy E ( ~ L ~ )

=

32 MeV distortion effects are minor and hence not the reason for R,

#

1. If the calculation is carried through with identical form factors for the bound-state wave functions, distortions have no sizable effect on the cross section. The calculations reproduce the experimental ratio R,

=

1.20 only if the proper neutron and proton form factors are used.

Thus the difference between the cross sections must originate in the difference between the bound-state wave functions. Calculations show that in both the initial and final states the proton wave function is slightly larger than the neutron wave function in the exterior region where the transfer reaction occurs.

The reason for the large difference between the cross sections is that the proton wave function is larger in both the initial and the final nuclei, and the resultant amplification of small differences finally leads to the experimentally observed ratio R,.

If the mechanism involved in (6Li, 7Li) and (6Li, 7Be) is simply the pickup of a nucleon, then the cross section is proportional [I] to C S1 S, oFW(0), where

L

oFW(8) is the finite-range DW cross section for the

nucleon transfer. The spectroscopic factors

S1

and

S,,

which measure the fractional parentage of the

13C

+ p (or 13N + n) and 6Li + p (or 6Li

i-

n)

systems, are taken from the calculation of Cohen and

Kurath [2]. No arbitrary free parameters are available

to normalize the results (Fig. 2) of the finite-range

REACTIONS 14N(6Li9 7Li)13N AND 14N(6Li, 7Be)l3C AT 32 MeV C6-297

calculations, and no cut off was used. All curves fit both the shapes and absolute magnitudes of all four angular distributions reasonably well, and consequently the ratio R, is reproduced within its experimental uncertainty.

In summary, the single-nucleon transfer reactions (6Li, 7Li) and (6Li, 7Be) on 14N have been measured with good resolution at an energy well above the Coulomb barrier. With the use of finite-range DW

calculations, the differences between the cross sections for these mirror reactions are understood in terms of the differences between the neutron and proton form factors at large distances. Thus no sizable isospin impurities have been detected in the mechanism of the heavy-ion transfer reactions investigated here.

Acknowledgement.

- We wish to thank W. Toboc- man for kindly providing his code.

References

[I]

SCHMITTROTH

(F.),

TOBOCMAN

(W.)

and GOLESTANEH

( A . A.), Phys. Rev., 1970, C 1, 377.

[2]

COHEN

(S.)

and KURATH

(D.), Nucl. Phys., 1967, A 101, 1.