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4H (1992TI02)

(See Energy Level Diagrams for 4H)


The stability of the first excited state of 8Li against decay into 4He + 4H (1988AJ01) sets an upper limit for B(4H) ≤ 3.53 MeV (1965BA1A). This also sets a lower limit to the β- decay energy 4H → 4He of 17.06 MeV. The upper limit of the β- decay energy would be 20.60 MeV, if 4H is stable against decay into 3H + n. (1965BA1A) give estimates for the expected half-life of the beta decay: if Jπ(4H) = 0-, 1-, 2-, τ1/2 ≥ 10 min; if Jπ(4H) = 0+, 1+, τ1/2 ≥ 0.03 sec. Experimentally there is no evidence for any β- decay of 4H (see reaction 1), nor has particle-stable 4H been observed (see reactions 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19). Evidence for a particle-unstable state of 4H has been obtained in 7Li(π-, t)3H + n (see reaction 16) at 8 ± 3 MeV above the unbound 3H + n mass with a width Γ < 4 MeV. See also reaction 17. For other theoretical work see (1976JA24, 1983VA31, 1985BA39, 1989GO24).

The level structure of 4H presented here is obtained from a charge-symmetric reflection of the R-matrix parameters for 4Li (see 4Li, GENERAL section) after shifting all the p-3He Eλ's by the internal Coulomb energy difference ΔEC = - 0.86 MeV. The parameters then account well for measurements of the n-3H total cross section (1980PH01) and coherent scattering length (1985RA32), as is reported in (1990HA23). The BW resonance parameters from that analysis for channel radius ant = 4.9 fm are given in Table 4.1 (PDF or PS) and are shown in Fig. 1. The levels are located substantially lower in energy than they were in the previous compilation (1973FI04), as will be true for all the T = 1 levels of the A = 4 systems. The 4Li analysis unambiguously determined the lower 1- level to be predominantly 3P1 and the upper one to be mainly 1P1; that order is preserved, of course, in the 4H levels. In addition to the levels given in Table 4.1 (PDF or PS), the analysis predicts very broad positive-parity states at excitation energies in the range 14 - 22 MeV, having Γ ≫ Ex, as well as antibound P-wave states approximately 13 MeV below the 2- ground state. Parameters were not given for these states in the table because there is no clear evidence for them in the data.

The structure given by the S-matrix poles is quite different, however. The P-wave resonances occur in a different order, and the positive-parity levels (especially for 0+ and 1+) are much narrower and lower in energy. It is possible that these differences in the S-matrix and KR-matrix pole structures, which are not yet fully understood, could explain the puzzling differences that occur when these resonances are observed in the spectra of multi-body final states (see 4Li, GENERAL section).

1. 4H(β-)4He Qm ≈ 20.6 not observed

As noted in the previous compilation (1973FI04), all searches for the beta decay of 4H have yielded negative results. No new work has been reported.

2. 3H(n, γ)4H Qm ≈ 0 not observed

The previous compilation (1973FI04) notes that this reaction has not been observed, but cites some work yielding upper limits for the cross section. No new work has been reported.

3. 3H(n, n)3H

Measurements of cross sections, polarization, and analyzing power for 3H(n, n)3H are summarized in Table 4.2 (PDF or PS). Earlier work is reviewed in the previous compilation (1973FI04). See also (1972SE23) for a summary of early data on this reaction.

A review of progress in four-body scattering and breakup reaction calculations in the integral equation approach is presented in (1987FI03). Calculations of 3H(n, n) scattering carried out in this approach are described in (1976FO13, 1976KH01, 1976TJ01, 1978KR01, 1983LE22, 1986FO07). An R-matrix prediction of n-3H cross sections and scattering lengths from p-3He scattering data is described in (1990HA23). A microscopic calculation for the 4H and 4Li continuum in which structure and reactions were treated on an equal footing was done by (1977BE40). 4H and 4Li level positions and widths were calculated. In (1979FO08) a soluble model of the four-nucleon system was developed using a nonrelativistic field theoretic formalism. A four-body calculation of the 4N system was carried out (1984FO08) to describe low energy phase shifts and cross sections. The pseudostate method in the resonating group formulation was used (1986SH12) to study distortion effects of the three-nucleon cluster in n + t and p + 3He. See also (1989PO23).

4. 3H(n, d)2n Qm = -6.257

The previous compilation (1973FI04) lists several measurements of the 3H(n, d)2n reaction and notes that forward-angle deuteron spectra are sharply peaked at high energy indicating a strong final-state interaction between two neutrons.

More recently only one additional measurement has been reported. Differential cross section measurements at En = 14.1 MeV were carried out (1976SH20) for deuteron angles 4°, 10°, 20°, 30°, and 40°. The measured energy spectra were represented satisfactorily by a Faddeev calculation which took into account the nn final-state interaction.

A soluble model involving four interacting particles and utilizing a field theoretic formalism was discussed in (1976FO13). Total cross sections for 3H(n, d)2n were calculated. A calculation described in (1982WU03) considered quasi-free scattering and d + n cluster structure, and deduced reaction kinematics and cutoff radius effects.

5. 3H(n, p)3n Qm = -8.482

Measurements of proton energy spectra are summarized in the previous compilation (1973FI04). In the only measurement reported since, polarized thermal neutrons were used (1980BOZH), and the measured asymmetry was used to determine the upper limit of the P-odd asymmetry coefficient. See also the review of resonances in three-particle nuclei (1989MO24).

6. 3H(d, p)3H+n Qm = -2.225

A summary of early measurements of the 3H(d, p) reaction is given in the previous compilation (1973FI04). Proton spectra show no evidence of formation of 4H. Observed structure in the proton spectra is attributed to final-state interactions in the 3H + n system. Upper limits for a bound state are discussed. However the recent work of (1990BL14) on a kinematically-complete measurement of the equivalent 2H(t, tp)n reaction for Et = 35 MeV finds evidence for a 3H ground state with Eres = 3.1 MeV, γ2 = 2.3 MeV. This work includes a review of 4He ground-state parameters from recent experiments. See also (1973SL03, 1977WE03, 1981SE11, 1982SE08, 1985FR01, 1986BE35, 1987GO25, 1989GR22, 1990AM04, 1990BL14, 1990BR14, 1990BR17). Since the previous compilation (1973FI04), measurements of vector and tensor analyzing power at Ed = 3 - 6 MeV and an R-matrix analysis were reported (1980CL1A, 1980DE1A). At T ≈ 300 K, thermonuclear reaction rates for 3H(d, p) and several other reactions of interest to astrophysics were calculated (1989SC25). The distribution of relative velocities between particles was described by the Maxwell-Boltzmann distribution. Results were compared with published experimental results.

7. 3H(t, d)3H + n Qm = -6.257

The previous compilation (1973FI04) cites measurements in which structure was observed in the deuteron spectra from the 3H(t, d) reaction, but the structure was attributed to final-state interactions in the 3He + n system. Other experiments noted in (1973FI04) have determined upper limits for the cross section. No new measurements on this reaction have been reported.

8. (a) 4He(γ, π+)3H + n Qm = -160.7
(b) 4He(γ, π0)3H + p Qm = -154.8
(c) 4He(γ, p)3H Qm = -19.81

Measurements of reaction (a) cited in the previous compilation (1973FI04) show some evidence for a 3H + n final-state interaction, but other explanations are possible. No new work has been reported. Angular distribution measurements (1989GL06) of reaction (b) were analyzed in terms of quasi-free, exchange, and quasi-elastic reaction mechanisms. Measurements of the asymmetry in angular distributions for reaction (c) induced by polarized photons of energies 40 MeV and 120 - 125 MeV were reported in (1988GA29, 1989VI05). A theoretical investigation of structure effects in the E3 cross section for reaction (c) is described in (1989BE07). See also the analysis of (1988TE04, 1989VO01).

9. 4He(e-, ν)3H + n Qm = -20.596

The previous compilation (1973FI04) cites one calculation, but no measurements on this reaction. No new work has been reported.

10. 4He(π+)3H + n Qm = 118.460

The previous compilation (1973FI04) cites one measurement, but no evidence for a 3H + n final-state interaction. No new work has been reported.

11. (a) 4He(π-, γ)4H* Qm = 115.6
(b) 4He(π-, π-π+)4H* Qm = -163.5
(c) 4He(π-, n)3H Qm = 118.5
(d) 4He(π-, 2n)2H Qm = 112.2
(e) 4He(π-, 3n)1H Qm = 110.0

The previous compilation (1973FI04) discusses measurements (1972BI09) of the energy spectrum of γ-rays from π- capture which show structure attributed to capture to a Jπ = 2-, Ex = 3.4 MeV state (apparent g.s. of 4H) and two strongly mixed 1- levels at Ex = 5.1 and 7.5 MeV. No new 4He(π-, γ)4H* measurements have been reported. A review of recent data significant in determining the characteristics of the (π-, γ) reaction on light nuclei was presented in (1982GM02). A calculation of the total radiative capture rate from 4He pionic atoms is described in (1988WE01).

Early experimental and theoretical work on reaction (c) is summarized in (1973FI04). More recently, measurements of differential cross sections were reported at Eπ = 100, 200, 290 MeV (1978KA01), and at Eπ = 285, 428, 525, 575 MeV (1981OR01, 1982OR06). The π- absorption was found to proceed by a two-nucleon mechanism with an energy dependence consistent with formation of a J = 1/2 resonance in the intermediate state. Measurements of the energy spectra of single neutrons, protons, and deuterons following absorption of stopped π- in 4He were reported in (1981CE01). An analysis of the 4He(π-, γ)4H reaction at 290 MeV utilizing the effective channel approach was presented in (1981RE01). The reaction was studied in the resonance region within the framework of a Δ-hole model (1983HI11).

No new measurements of reactions (d) or (e) have been reported. However, a cluster-model study of 4He(π-, 2n)2H was reported in (1986GE08), and in (1987GE06) it was suggested that measurements carried out for pionic capture at rest could yield information on the D-state of 4He.

12. 4He(n, p)3H + n Qm = -19.814

Experimental work cited in the previous compilation (1973FI04) showed structure in the proton spectra from the 4He(n, p)3H + n reaction that was interpreted as indicating an excitation of the giant resonance 1- states in 4H. It was noted that the 2- ground state of 4H does not appear to be populated appreciably. No new work on this reaction has been reported.

13. 6Li(γ, 2p)3H + n Qm = -23.514

The previous compilation (1973FI04) cites no reported measurements, but notes upper limits for the production cross section of 4H in earlier reviews. No new work has been reported.

14. (a) 6Li(π-, d)3H + n Qm = 117.0
(b) 6Li(π-, np)3H + n Qm = 114.8

Measurements of coincident charged-particle spectra discussed in the previous compilation (1973FI04) show structure which has been interpreted either in terms of a final-state interaction in 3H + n or in terms of levels in 4H. The yield for reaction (a) is stated to be (1.0 ± 0.5) × 10-4 of all possible π- + 6Li reactions. Early measurements of neutron-proton energy-summed coincident spectra and n-p angular distributions from reaction (b) are also cited in (1973FI04). More recently, measurements of energy spectra versus angle for reactions (a) and (b) were carried out (1983HE17) for stopped pions. The quasi-deuteron mechanism was found to be the dominant process in the reaction. Other measurements for reactions (a) and (b) with stopped pions were reported in (1985DO19) and the results discussed in terms of sequential reaction mechanisms as well as cluster absorption. Angular correlations of the cross section for reaction (b) were measured (1986YO06) at Eπ = 70 MeV, and the components of the two-nucleon absorption cross section were extracted. See also (1987KO47).

A calculation of the energy spectra and angular correlations of single and correlated spectra for absorption of stopped pions was described in (1981CH03).

15. 7Li(γ, 2p)4H + n Qm ≈ -33.582 not observed

The previous compilation (1973FI04) lists no observations of the 7Li(γ, 2p)4H + n reaction, but cites experimental work in which upper limits for the production cross section of 4He were set as revealed by its beta decay. No new measurements have been reported.

16. 7Li(π-, t)3H + n Qm = 116.0

The previous compilation (1973FI04) states that no evidence for a particle-stable state of 4H has been found, but notes that charged-particle spectra for π- capture at rest indicate structure which is interpreted as a 3H + n final-state interaction. Other measurements are cited, and proposed states in the unbound 3H + n system are discussed. More recently the charged particles from the breakup of 7Li following π- capture have been identified in coincidence (1979ME13), and strong evidence for a particle-unstable 4H state is obtained. The energy of the resonance was found to be 8 ± 3 MeV above the unbound 3H + n mass with a width Γ < 4 MeV. A review of previous experimental searches for particle unstable 4H is included.

17. 7Li(n, α)3H + n Qm = -2.468

No evidence for a particle-stable state had been obtained at the time of the previous compilation (1973FI04). Upper limits (σ ≤ 2.2 mb) were quoted. A recent measurement (1986KN06) of the 4He production cross section for an average neutron energy of 14.95 MeV gave σ = (0.336 ± 0.016) b. Measurements of coincident energy spectra of alpha particles were reported in (1986MI11, 1986MI14) for En = 14.6 MeV, and it was suggested that the observed structure might be attributed to states of 4H. See also the measurements of (1986SH33).

18. 9Be(7Li, 4H)12C Qm ≈ 3.233 not observed

The previous compilation (1973FI04) notes one particle-identification measurement on this reaction with no 4H particle groups detected. No new work has been reported.

19. 10B(7Li, 13N)4H Qm ≈ -1.409 not observed

The previous compilation (1973FI04) cites one measurement which determines an upper limit on the cross section and concludes that B(4H) ≤ 0.16 MeV. No new work has been reported.