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HAL Id: jpa-00208312

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Submitted on 1 Jan 1975

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Study of the first excited state in 5Li

R.M. Gagne, C.M. Fou, S. Ward

To cite this version:

R.M. Gagne, C.M. Fou, S. Ward. Study of the first excited state in 5Li. Journal de Physique, 1975,

36 (9), pp.759-763. �10.1051/jphys:01975003609075900�. �jpa-00208312�

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STUDY OF THE FIRST EXCITED STATE IN 5Li

R. M. GAGNE, C. M. FOU and S. WARD (*) Department of Physics, University of Delaware

Newark, Delaware 19711, U.S.A.

(Reçu le 24 janvier 1975, accepté le 23 avril 1975)

Résumé.

2014

La réaction 6Li(3He, 03B1)5Li(03B1)p a été étudiée à 1,8 MeV incident par la méthode des coincidences (03B1 - 03B1), pour les positions angulaires 03B81

=

25°, 35°, 40° et 03B82

= -

150°, dans le but de déduire les caractéristiques du 1er état excité de 5Li. Tous les spectres expérimentaux sont

bien expliqués si l’on prend pour ce niveau Ex

=

3,2 ± 0,2 MeV et 0393

=

1,5 ± 0,5 MeV.

Abstract.

2014

The reaction 6Li(3He, 03B1)5Li(03B1)p has been studied with a 1.8 MeV incident 3He beam.

Coincidence spectra (03B1 - 03B1) were measured at 03B81

=

25°, 35°, 40° and 03B82

= -

150°. The purpose

was to locate the first excited state of 5Li. The analysis yields

Ex

=

3.2 ± 0.2 MeV and 0393

=

1.5 ± 0.5 MeV . Classification

Physics Abstracts

4.370 - 4.410

1. Introduction.

-

The latest compilation of spec-

troscopic information [1] about the first excited state of ’Li still shows an excitation energy of 5-10 MeV and a width of 5 ± 2 MeV. Recently, Vignon et al [2]

have attempted a two-detector study of the ’Li + 3He

reaction and given for the location and width of this state in 5Li, Ex ~ 7.5 MeV, r = 5-10 MeV. However Kraus and Linck [3] based on a new analysis of the

p + 4He and n + 4He scattering data showed that the first excited state of 5Li should be at 2.3 + 0.5 MeV with a width of 9.0 ± 1.5 MeV. This low excitation energy is in better agreement with the excitation energy of the first excited state of 5He which should be 2.0 ± 0.5 MeV according to their analysis and

4 ± 1.5 MeV according to the recent compilation of experimental data [1]. In view of this, it was felt that

a new attempt similar to that of Vignon et al should

be made to see whether better measurements can be obtained with different sets of detector arrangements.

2. Expérimental apparatus and procédure.

-

The

reaction was induced by a 1.8 MeV 3He beam from

the 2.5 MV van de Graaff accelerator of University of

Delaware. The choice of the low incident energy was

made to ensure a most stable operation of the acce-

lerator for long coincidence-measurement time, to

reduce the number of intermediate states. The target

was made by vacuum evaporation of isotopically

enriched 6Li on a 35 pg/cm2 aluminium backing.

Two 100 pm detectors were used to measure the a-a

(*) N.S.F. Undergraduate Summer Research Participant from

Bucknell University, Lewisburg, Pennsylvania 17837.

coincidences in a 45 cm scattering chamber. The

angular resolution of the detectors was 1.1 °. One detector was fixed at -150° relative to the beam direc- tion where the count-rate of elastically scattered 3He

is low and the kinematic broadening is small. The other detector was protected by a thin mylar foil of appropiate thickness, which absorbs elastically scatte-

red 3He particles, and was placed at forward angles.

Standard coincidence set-up with zero-crossover

timing and 100 ns resolving time was used. The

thinness of the detector and the coincidence requi-

rement made the particle identification unnecessary.

Kinematic calculations shown in figure 1, shows that, with the movable detector set at 35° relative to the beam direction, the kinematics locus is situated away from all known competing intermediate states, namely

’Li ground state, 8Be 16.6, 16.9 and 11.4 MeV states.

This arrangement should yield the best possible

measurement for the first excited state of 5 Li. For

comparison and calibration purposes coincidence measurements were also made at 25° and 40°. Charge

collection for each coincidence spectrum was 40,000 c

which corresponds to more than 60 hours of data accumulation time. Energy calibration of the total system was obtained using the 6Li(p, a) 3He, 6Li(p, p) 6Li, 160(p, p) 160 reactions and an 241Am a-particle source. Energy loss in the mylar foil were properly taken into account by using the energy loss formulae [4].

3. Expérimental result.

-

The contour plot of all

three coincidence spectra are shown in figure 2. Direct comparisons with the kinematic calculations in figure 1

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:01975003609075900

(3)

760

FIG. 1.

-

Kinematic calculations and energy calibrations for the

6Li(3He, a)$Li(a)p reaction in a 64 x 64 data storage with 01 =25°, 35° and 40° and BZ

= -

150°. The loci are corrected for 2-particle

energy losses in the mylar foil in front of the a-detector at 91. The

dashed lines indicate the position of the first excited state assumming Ex

=

3.0 MeV. The intermediate ’Be states at 11.4, 16.6 and 16.9 MeV from the competing 6Li(3He, p)8Be(a)a reaction are

also indicated.

- -

FIG. 2.

-

Two dimensional contour plot of the experimental

data (25°, - 150°), (35°, - 150°) and (40°, - 150°). Positions

marked by letters A, B, C, D are the groups corresponding to the

first excited state of SLi. Position marked by E is due to the mixing-in

of the ground state of ’Li. Cor visual purpose, only a few sizes of dot

are used to indicate the distribution of events. For the 35° and 40°

loci : (e) 1-5 ( x ) > 5. For the 25° locus : (e) 1-30, ( x ) 31-100, (e) 1 O 1-200, ( a) > 200.

are possible because the theoretical curves were

corrected for energy losses in the mylar foil. Good agreements were observed for the location of the 16.6 and 16.9 MeV states of 8Be and the ground state of 5 Li.

There is very little contribution from the 11.4 MeV state of 8Be. This is understandable because of the low incident beam energy and the large 4+ spin of this

state which make its excitation unlikely. This argument and observation has also been made by others at low

incident energies [5, 6]. In fact the best measurement and strong evidence for the 11.4 MeV state were

recently obtained at higher incident energies [7, 8]

in the ’Li(a, t) reaction with Ea

=

50 MeV and

9Be(d, t) with Ed

=

26.4 MeV supporting the above argument. There are clear indication of the first excited state of ’Li on the 35° and 400 loci between the points AB and CD shown in figure 2. Projections of

these two sections onto an axis diagonal to the 64 x 64

block in a way described in a previous publication [9]

are shown in figure 3. These projections both show

a double-hump structure. At the point E in figure 2

in the upper part of the (35°, - 150°) locus indication of the mixing in of the ground state of 5 Li, because of

its width and the width of the detector angle, is obser-

ved.

FiG. 3.

-

Projection of the (35°, - 1500), (40°, - 1500) experi-

mental data onto a diagonal of the 64 x 64 display. Every data storage site is split into two halves, and all the halves along the same

direction perpendicular to the diagonal are summed. Both projected spectra show a minimum between the observed groups correspond

to -5Li first excited state marked by A, B, C, D. The projected phase

space factors described in the text are also shown for comparison.

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FIG. 4.

-

The (25°, - 150°) locus are projected onto the two energy-

axes. Shown in the figure are the positions of the groups correspond- ing to the known states in 8Be and 5 Li. Complete agreement was obtained between the data and the theoretical prediction based

on energy calibration, mylar foil correction and the known exci- tation energies and widths. The broadening of the 16.6 and 16.9 MeV states of ’Be are due to the coincidence with the first excited state of 5 Li. The structures around the ground state of 5 Li are due to the

4. Analysis.

-

Analysis of the data can be done best along the kinematic loci. However, because of the small number of channels along each axis for data storage (64 x 64), it is impossible to integrate the

events in an unambiguous way along the curve.

Therefore events are summed and projected on to

the two axes for analysis. They are shown in figure 4, 5, 6. The projections of the (25°, - 150°) data show the

comparison of the positions of the expected ’Be

and 5Li intermediate states mainly for energy cali- bration purpose. For the (35°, - 150°), (40°, - 150°) data, the projected spectra are first divided by the projected phase space factor. These factors are

calculated by multiplying the phase space factor along

the kinematic locus as given by the formula of Kim and Kane [10] and the slope of the section of the locus lying within the boundary of that particular pulseheight analyser channel. Thus the channel reso-

overlapping of the two groups of events both correspond to the ground state of 5 Li on that locus. Slight relative shift deviating from

the kinematically predicted perfect coincidence might have occurred due to finite angular width and angular uncertainty (0.50) of one

or both of the two detectors. The solid lines are calculated using

the Breit-Wigner shape using published widths. The broken line indicates the calculated projected phase-space factor. For the

ground state of 5 Li, overlapping interference was considered.

lution is properly taken into consideration. Since the cross-section along the projection is given by

where PPSF is the projected phase-space factor and 1 M 12 is the transition matrix squared, the experi-

mental data so modified can be compared directly to 1 M 12. We have chosen here for all intermediate states a Breit-Wigner shape i.e.,

For the (25°, -150°) projections, we used the published

values for excitation energies and widths for the

(5)

762

FIG. 5.

-

Projected spectra of the (35B - 150°) data in semi-log plot. Also shown are the calculated projected phase space factor in solid curves. The difference between this curve and the data

points represents the transition matrix squared. Using Breit Wigner amplitude with Ex

=

3.2 MeV and r

=

1.5 MeV, the differential cross-section obtained is in good agreement with the data. No

a)

m

FIG. 6.

-

Projected spectra of the (40°, - 150°) data in semi-log plot. Solid curves are the projected phase space factors and the theoretical fits assuming Breit-Wigner amplitude using the same

comparison is attempted for the high energy end of the projection

because of the vertical slope of the locus at that end which made the

projection somewhat ambiguous. There is considerably more mixing-in of the events corresponding to the ground state of ’Li (F

=

0.5 MeV) than in the - 1500 axis projection. Not much

contribution from the 11.4 MeV state of 8Be is observed.

parameters as in figure 5. Also indicated are the fits to the 11.4 MeV

state of 8Be in broken lines, interférence terms are not included.

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ground state of 5Li and the 16.6, 16.9 MeV states of 8Be [1]. Because of the overlap with the first excited state of 5 Li no détail fit taking interference into consideration was obtained for the 16.6 and 16.9 states.

However, good agreement was obtained for the

positions indicating reliable energy calibration. For the ground state of 5Li because the two peaks, due to exchange symmetry of the two a-particles, nearly overlapped the amplitudes are added and then squared

to account for the so called order of emission inter- ference. The agreement obtained is quite good.

Therefore we use the same method to extract infor- mation about the excitation energy and width for the first excited state of ’Li in the (35°, - 150°) and (40°,

-

150°) data. Because of the observed minimum between the two groups (Fig. 3) in both loci, a narrow

width with little overlap seems to be the best choice for fitting the data since a constructive interference is expected. We obtained

which fit all four projections. Uncertainties of these two values were determined from the onset of noti- ceable worsening of agreements with experimental

data by changing these two values independently.

Possibility of these structures being due to proximity

rescattering can be ruled out on kinematics ground.

And, accepting these excitation energy and width, the relative velocity between the 5 Li* and the a particle is roughly 0.12 C. The separation between 5Li*

and a after the elapse of the life time of this first excited state is approximately 8 fm. Therefore the possibility

of this structure being affected by the third particle

is also very unlikely.

5. Summary and discussion.

-

The present work showed that by chosing the special arrangement (35°, - 150°) of the two a-particle detectors in a kine-

matically complete experiment 6Li(3He, aa) p it was

possible to’ isolate the first excited state of 5 Li on the kinematic locus. Analysis of the data yield excitation

energy and width of the state ; Ex

=

3.2 ± 0.2 MeV

and r

=

1.5 + 0.5 MeV. These values are in better agreement with the recent compilation of experimental

data [1] for the first excited state of ’He which are

Ex

=

4.0 + 1.5 MeV, r

=

4.0 ± 1.5 MeV respecti- vely. They also render support to the recent analysis

of Kraus and Linck [3]. Furthermore if one assumes

that the first excited state and the ground state of 5 Li correspond to the 1 P1/2 and 1 P3/2 shell model states, then a spin-orbit splitting of 3 MeV is reasonable.

Because the spin-orbit splitting is supposedly propor- tional to 1 + 1/2, taking the 1 d5/2’ 1 d3/2 splitting

in 170 to be 5 MeV, the 1 p splitting tums out to be 3 MeV.

References

[1] AJZENBERG-SELOVE, F. and LAURITSEN, T., Nucl. Phys. A 227 (1974) 1.

[2] VIGNON, B., CAVAIGNAC, J. F. and LONGEQUEUE, J. P., J.

Physique 30 (1969) 913.

[3] KRAUS, L. and LINCK, I., Nucl. Phys. A 224 (1974) 45.

[4] WHALING, W., Handb. Phys. 34 (1959) 213.

[5] ASSIMAKOPOULOS, P. A., GANGAS, N. H. and KOSSONIDES, S., Nucl. Phys. 81 (1966) 305.

[6] VALKOVIC, V., JACKSON, W. R., CHEN, Y. S., EMERSON, S. T., PHILLIPS, G. C., Nucl. Phys. A 96 (1967) 241.

[7] LAMBERT, J. M., TREADO, P. A., BEACH, L. A., THEUS, R. B.

and PETERSEN, E. L., Nucl. Phys. A 152 (1970) 516.

[8] SONNEMANS, M. A. A., WAAL, J. C. and VAN DANTZIG, R., Phys. Rev. Lett. 31 (1973) 1359.

[9] FOU, C. M. and GAGNE, R. M., Z. Phys. 267 (1974) 331.

[10] KIM, Y. E. and KANE, J. V., Rev. Mod. Phys. 37 (1965) 519.

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