Abstract
The spectroscopic study of unbound states in the 6Li nucleus was performed by measuring the energy spectrum of the alpha-particles emitted from the transfer reaction ³He(7Li, alpha)6Li. The 7Li beam at E Lab = 31.2MeV was produced at the São Paulo Pelletron accelerator. A ³He gas cell was used as target. alpha-Particle spectra were measured from thetaLab = 8º up to thetaLab = 20º with steps of 1º. Due to its positive Q = +13.3MeV, the ³He(7Li, alpha)6Li reaction favours the population of states in 6Li at high excitations energies up to about 20MeV. We observed resonances at 17.29MeV, 15.31MeV, and a new state at 12.45MeV, in addition to all previously known states of 6Li. An <FONT FACE=Symbol>Â</FONT>-Matrix analysis was performed and the positions and widths of these states were extracted.
Spectroscopy of 6Li using the 3He(7Li, a)6Li reaction
R. Kuramoto; R. Lichtenthäler; A. Lépine-Szily; V. Guimarães; G. F. Lima; E. Benjamim; P. N. de Faria
Departamento de Física Nuclear, Instituto de Física da Universidade de São Paulo, 05508-090, São Paulo, SP, Brazil
ABSTRACT
The spectroscopic study of unbound states in the 6Li nucleus was performed by measuring the energy spectrum of the a-particles emitted from the transfer reaction 3He(7Li, a)6Li. The 7Li beam at ELab = 31.2MeV was produced at the São Paulo Pelletron accelerator. A 3He gas cell was used as target. a-Particle spectra were measured from qLab = 8º up to qLab = 20º with steps of 1º. Due to its positive Q = +13.3MeV, the 3He(7Li, a)6Li reaction favours the population of states in 6Li at high excitations energies up to about 20MeV. We observed resonances at 17.29MeV, 15.31MeV, and a new state at 12.45MeV, in addition to all previously known states of 6Li. An Â-Matrix analysis was performed and the positions and widths of these states were extracted.
1 Introduction
The 6Li nucleus is one of lightest nuclei with a known sequence of excited levels. In general the theoretical predictions are in fair agreement with the experimental results only for the first six levels up to ~6MeV excitation[1, 2]. The spectroscopy of 6Li nucleus has been performed through the 3He(7Li, a)6Li reaction which has a binary exit channel. The mechanism of this reaction provide a transfer of a triton cluster to the target nuclei 3He to lead to states in the 6Li nucleus. Due to the binary character of the reaction mechanism, the ground-state and resonant states of 6Li are observed in the a-particle spectrum. Moreover, the high positive Q = +13.3MeV of the 3He(7Li, a)6Li reaction, favours the population of resonant states up to high excitation energies in 6Li nucleus (20MeV). This reaction was performed in three previous works [3, 4, 5]. However, due to the poor statistics and insufficient experimental resolution, none resonances above 5Li + n threshold (5.4MeV) were observed. Only structures in the a-particles spectra near the 3He+t threshold (15.8MeV) were reported. In this work, we intend to populate resonant states in the 6Li nucleus near the 3He + t threshold, using the 3He(7Li, a)6Li reaction. The Â-matrix formulas[6, 7] were used to fit the a-particle experimental spectra in order to extract the positions and reduced widths of the 6Li resonances.
2 Experimental Procedure
A 31.2MeV beam of 7Li was provided by the São Paulo Pelletron accelerator with a average current about 70nA. We used a gas target cell with kapton windows of 3.5mg/cm2 thickness. The cell was filled with 99.95% isotopically enriched 3He up to the pressure 255mbar. The a-particles products of 3He(7Li, a)6Li reaction were detected by a E DE telescope which consisted of two Si surface barrier detectors, conected to a system of multiparametrical analysis. The DE segment of the telescope had a thickness of 150mm, while the E segment had a thickness of 700mm. A rectangular double-slit system was used to prevent particles scattered in the windows of the gas cell from entering the detectors. This double-slit system defined a solid angle of the order of 0.5msr, depending on the detection angle. The dimensions and the efficiency of the experimental setup (double-slit system and gas target cell) were calculated by a Monte Carlo code[8].
Energy spectra of a-particles have been obtained in the angle range 8° 24° in the laboratory system. The typical energy spectra of a-particle from the 3He(7Li, a)6Li reaction is shown in Fig. 1. The a-particles are easily identified in the biparametrical spectrum and projected onto the total energy axis (E + DE). As indicated in the a-particle spectrum, we observe all the low-lying states in 6Li up to 6MeV[2].
3 Analysis
As a first step in the analysis, the energy axis of the alpha particles spectra was converted into excitation energy of the recoil nucleus 6Li, supposing a binary process. In order to improve the statistics of the 6Li excitation energy spectrum, the spectra acquired in the angles of 12° and 14° in the laboratory system were summed. The result of this process is shown in Fig. 2.
Using the one-channel, one-level Â-matrix[6, 7] , we attempt to fit the 6Li excitation energy spectrum with a function of the form:
The [g(E t)*b(E)]i term is the convolution of a Gaussian and a Breit-Wigner functions:
where, i indicate the levels of the 6Li nucleus, 
is the gaussian FWHM, which corresponds to the experimental resolution, Pi and Si are the penetration factor and shift factor, which are functions of E, Bi is the constant boundary condition parameter, and 
is the reduced width. The second term in Eq. (1) is the 6Li ® a + d penetrability background[9, 10]. The third term is the phase-space background[11, 12], where j indicates the 3-body (6Li ® a + d, 6Li ® 5Li + n and 6Li ® 3He + t) or 4-body breakup (6Li ® a + n + p), which depends on the threshold energy of 6Li nucleus. 
i, 
and 
j are normalization constants.
The fit of the 6Li excitation energy spectrum was performed by the c2 minimization, which was carried out by Simulated Annealing[13] combined with the Downhill Simplex Method[4] using the computer code AMEBA[8]. In this way, we included contributions from the ground-state and the known first five resonances in 6Li. Above 6MeV three more resonant states were considered near the 3He + t threshold (15.8MeV). The overall energy resolution of about 445keV was obtained via the ground-state fit. The background was described through the 3-body (6Li ® a + d and 6Li ® 3He + t) and 4-body breakup (6Li ® a + n + p), which were calculated using the phase-space model.
The best fit results are given in Tab. I. Our quality of fit is given by 
= 1.57. The values of Ei and 
= 2
agree reasonably with the energies and widths given in Ref. [2]. The resonance at 17.29MeV was included recently in the 6Li energy levels diagram[2]. The resonance at 15.31MeV have been also observed before[1], but it was not considered in the last compilation[2]. Thus, in this spectrum there is a confirmation of the resonances at 15.31MeV and 17.29MeV. Furthermore, we identify a new resonant state in 6Li at 12.45MeV not observed yet.
4 Conclusion
In summary, we performed measurements of the alpha particle spectra emmitted from the 3He(7Li, a)6Li reaction at ELab = 31.2MeV. The first six known states of 6Li have been observed. Above Ex = 6MeV three resonances have been observed at 12.45MeV, 15.31MeV and 17.29MeV. The 3He + t threshold is located at Ex = 15.79MeV. An Â-matrix analysis of the a-particles spectra plus background was performed and the position and widths of the resonances have been determined. The best fit results are given in Tab. 3. The main results of the present study are the confirmation of resonances at 15.31MeV and 17.29MeV and the identification of a new resonant state at 12.45MeV in 6Li.
Acknowledgments
The authors would like to acknowledge the finacial support by FAPESP.
Received on 13 October, 2003
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Publication Dates
-
Publication in this collection
26 Oct 2004 -
Date of issue
Sept 2004
History
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Accepted
13 Oct 2003 -
Received
13 Oct 2003
