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4Li (1992TI02)

(See Energy Level Diagrams for 4Li)


The stability of 8B against particle decay (1988AJ01), in particular against decay into 4He + 4Li, sets an upper limit of 1.7 MeV on the separation energy of 4Li into p + 3He (1952SH44). The instability of 4H against particle decay (see 4H, GENERAL section) makes the particle stability of 4Li very unlikely, since the Coulomb energy of 4Li is approximately 1.7 MeV larger than that of 4H (1963WE10), and the nuclear energies should be identical because of charge symmetry. Indeed all decisive tests of the stability of 4Li have failed. Searches for its beta decay have given negative results (see reaction 1). Indirect proof of the non-existence of 4Li can be provided by a measurement of the solar neutrino flux which would be strongly influenced by the existence of 4Li. See (1968ME03). For other theoretical work on 4Li, see (1974ST14, 1979HU02, 1981KA39, 1988CO15).

The level structure of 4Li presented here is based on an R-matrix analysis (1983HA1N) that gives a good representation of all the p + 3He scattering data at proton energies below 20 MeV. BW resonance parameters from that analysis are given in Table 4.24 preview 4.24 (PDF or PS) and shown in Fig. 3. The spin-correlation and 3He analyzing-power data included in the p-3He analysis determined that the lower 1- level is primarily in the 3P state, while the upper 1- is primarily in the 1P state, removing the ambiguities in the earlier phase-shift solutions, as was discussed in the previous compilation (1973FI04).

As in the case of the 4H levels (see 4H, GENERAL section), which were based on the 4Li parameters, all the levels are at least 1 MeV lower than they were in (1973FI04). The only significant difference between the 4H and 4Li levels is in the position of the ground state above the nucleon-trinucleon threshold, as would be expected from the simple model used to obtain the 4H parameters. Again, the parameters of the analysis predict very broad, positive-parity, T = 1 states in the Ex = 15 - 20 MeV range and antibound P-wave states that cannot yet be identified in the data. The known T = 1 levels in the A = 4 nuclei are summarized in the isobar diagram of Fig. 4.

The S-matrix poles resulting from the analysis are all far from the real axis with large decay widths Γ, while their residues are relatively small, leading to small values of the strengths. Although the connection is not clear at this point, the small residues for these poles may be connected with the anomalously small widths that have been observed in recent experiments (1990BR14, 1990BR17) that detect 4Li states in the particle spectra of breakup reactions. It may even be possible that these experiments are not detecting the 2-and 1-states as they assume, but positive-parity states (0+ and 1+) whose S-matrix poles are much lower in energy than are the KR-matrix poles.

1. 4Li(β+)4He Qm ~ 21.7 not observed

As noted in the previous compilation (1973FI04), the positron decay of 4Li, if it were particle stable, must lie close to 20 MeV, and all searches for 4Li beta decay with the expected energy have yielded negative results. Since the previous compilation, no evidence for this decay has been reported.

2. 3He(p, γ)4Li Qm ~ -2.9 not observed

This reaction has not been observed. Upper limits have been set based on the absence of observable beta decay (1973FI04).

3. 3He(p, p)3He

Measurements of cross sections, polarizations and analyzing powers reported since the previous compilation (1973FI04) are summarized in Table 4.25 preview 4.25 (PDF or PS), and methods of analysis are indicated. No evidence for narrow levels is reported. A discussion of the general features of the earlier cross section data as it concerns possible states in 4Li is given in (1973FI04).

Recent polarization measurements are reviewed, and analyses and comparisons with model calculations are presented in (1987GR08). Phase-shift analyses are reported in (1972BO15, 1978SZ06, 1985BE39, 1985SA22), in addition to those carried out in connection with experiments summarized in Table 4.25 preview 4.25 (PDF or PS). A considerable amount of theoretical work bearing on 3He(p, p)3He has been carried out. Microscopic calculations for the 4Li continuum presented in (1977BE40) include 3He(p, p) differential cross sections as well as 4Li and 4H level positions and widths. A cluster model study by (1979FU05) predicts the ordering of T = 1 negative-parity states in A = 4 to be 2-, 1- (triplet-main) 0-, 1- (singlet-main). In the microscopic multichannel resonating group calculations of (1981HO04) the T = 1 level order is predicted to be 2-, 1- (triplet), 1- (singlet), 0-.

In (1987FI03) a review of recent progress in four-body scattering and breakup reactions in the integral equation approach (IEA) is presented. Theoretical studies carried out with various formulations of (IEA) are (1976SA02, 1977BA46, 1985SO07, 1986FO07, 1987FI03). See also (1980BA55, 1982BL15, 1983BL15). Extensive use of the Glauber multiple scattering theory has been made to describe the scattering cross section in the intermediate energy region, for example (1973NA06, 1976FR12, 1979ME08, 1981BI08). See also (1976LE32, 1978PE20). Optical model calculations of 3He(p, p) have been reported in (1979SH06, 1984LA20, 1984PA09, 1985SA22, 1986LA02, 1990LA01). The mechanism of two-nucleon exchange in backward scattering is explored in (1989LA26). The specific distortion effect of the three-nucleon cluster in the p + 3He system was studied in a resonating-group formulation by (1986SH12). A non-relativistic field theoretic formalism was used (1979FO08, 1984FO08) to develop a soluble four-body model and calculate scattering cross sections. Time-reversal violating effects in low-energy 3He(p, p) scattering were calculated and found to be very small (1977SI11). For other theoretical work related to 3He(p, p)3He see (1972KI13, 1973KI07, 1974LY02, 1975BA05, 1975KI11, 1975RU01, 1979KA19, 1983LY07, 1984LA25, 1987AB13).

4. 3He(p, d)1H1H Qm = -5.494

Early measurements of particle spectra from the 3He(p, d)1H1H reaction are tabulated in the previous compilation (1973FI04), and a discussion of information obtained on pp final-state interactions (FSI) and pp and pd quasi-free scattering (QFS) processes is presented. More recent experiments on 3He(p, d)1H1H are summarized in Table 4.26 preview 4.26 (PDF or PS).

Measurements of continuum cross sections and analyzing powers reported in (1985WE12) were compared with distorted-wave impulse approximation calculations based on quasi-free nucleon and deuteron knockout. Results of this comparison clearly indicated the need for including deuteron knockout. Several experiments on 3He(p, d) have been carried out to investigate the question of dibaryonic resonances. A review of experiments including 3He(p, d) and other reactions was presented in (1986GA15). Missing mass spectra reported in (1987TA17, 1987TA20) show narrow structures possibly associated with B = 2, T = 1 quantum numbers. The structures observed have masses and widths (Mx = 2.240 ± 0.005, Γ1/2 ~ 0.016 ± 0.003 GeV; Mx = 2.192 ± 0.003, Γ1/2 ~ 0.025 ± 0.006 GeV; Mx = 2.121 ± 0.003, Γ1/2 ~ 0.025 ± 0.002 GeV). An independent investigation reported in (1988SA33) found narrow structure in the missing-mass dependence of analyzing power that showed significant correspondence with previous reports and predictions of theory.

5. 3He(p, n)1H1H1H Qm = -7.718 not observed

The previous compilation (1973FI04) lists a limited number of measurements of neutron spectra from 3He(p, n)1H1H1H and notes there is no evidence for a neutron group below the four-body threshold. No recent work has been reported.

6. 3He(d, n)4Li Qm ~ -2.225

As noted in the previous compilation (1973FI04) this reaction was reviewed in (1974AJ01), and early work setting an upper limit to the cross section was cited. No new measurements have been reported. See, however, the work on primordial nucleosynthesis discussed in (1991RI03).

7. 3He(d, n)3He + 1H Qm = -2.225 Eb = 16.387

This reaction was reviewed in the previous compilation (1973FI04) and by (1974AJ01). No evidence was cited for states of 4Li. No new work has been reported.

8. 3He(3He, d)3He + 1H Qm = -5.494 Eb = 11.489

This reaction was reviewed in the previous compilation (1973FI04) and by (1974AJ01). Upper limits are quoted for the cross section. Two experiments in which the products of 3He(3He, d) reactions were studied have been reported since (1973FI04). The first of these (1977DA11) searched for high energy deuterons from 3He(3He, d)4He + e+ + ν at E(3He) = 15 and 20 MeV in connection with the solar neutrino problem. The other experiment examined quasi-free processes in which at least two of the final-state particles were charged. Beam energies were 50, 65, and 78 MeV. The kinematic conditions that were chosen favored the dominance of d-3He quasi-free processes over sequential decay modes through 4Li or 5Li. No evidence for 4Li was observed in either of these experiments.

9. Λ4He(π-)1H + 3He Qm = 35.394

Early work on this reaction was summarized in the previous compilation (1973FI04). More recently, extensive reviews of experimental and theoretical work on hypernuclei were presented in (1975GA1A, 1978PO1A, 1990CO1D, 1990OS1A). A theoretical study (1985LY1A) found that the polarization of the protons and tritons in the reaction is largely determined by the strong interaction in the p-3He system. Theoretical analyses of the binding energies of the ground and excited states of Λ4He and Λ4H are presented in (1982KO13, 1987YA1M). See also (1988MA09). Coulomb effects and charge symmetry breaking are discussed in (1985BO17). A four-body calculation of the 0+ - 1+ binding energy difference is reported in (1988GI1F). Non-mesonic decays are discussed in (1985TA1E, 1986SZ1A, 1990CO1D, 1990OS1A). Evidence for the existence of a Σ-nucleus bound state formed in a (K-, π-) reaction on 4He was reported in (1989HA39). See also (1989HA30, 1989HA39, 1990HA08, 1990HA11). The possibility of forming doubly-strange Ξ-hypernuclei is considered in (1983DO1B). Theoretical discussions of Σ-hypernuclei are given in (1990HA1B, 1990HA08, 1990HA11, 1990OK03).

10. (a) 4He(π+, p)3He Qm = 120.28
(b) 4He(π+, 2p)2H Qm = 114.79
(c) 4He(π+, 2p)N + 1H Qm = 112.56
(d) 4He(π+, π0)4Li* Qm = -14.69

Measurements of cross sections and proton spectra from reaction (a) reported since the previous compilation (1973FI04) are summarized in Table 4.27 preview 4.27 (PDF or PS). The emphasis in these measurements is on the study of pion scattering and absorption contributions. A review of the experimental and theoretical situation with respect to pion absorption in nuclei is presented in (1985OH09), and an isobar-hole model calculation is described for 4He which takes into account large pion distortion effects and predicts the main features of the cross section correctly. One experimental study of reaction (b) was reported (1981AS10). The relative absorption ratio of a pion by T = 0 and T = 1 pairs was determined. Calculations of this ratio have been carried out utilizing a standard theory of Δ-isobar excitations (1982TO18) and by a unitary isobar model (1984SI03). Earlier work reported in (1974WI19) obtained amplitudes and cross sections for the (π+, 2p) reactions in a treatment based on field theory. No new data on reaction (d) have been reported. A calculation of inelastic pion-nucleus collisions and pion absorption from the Boltzmann equation which included the (π+, π0) reaction is described in (1979HU02).