(See Energy Level Diagrams for 8B)
GENERAL: References to articles on general properties of 8B published since the previous review (1988AJ01) are grouped into categories and listed, along with brief descriptions of each item, in the General Tables for 8B located on our website at (nucldata.tunl.duke.edu/nucldata/General_Tables/8b.shtml).
The β+ decay leads mainly to 8Be*(3.0). The half-life is 770 ± 3 msec; log ft = 5.6 (1974AJ01). There is also a branch to 8Be*(16.63), and evidence for population of an 8Be intruder state at Ex ≈ 9 MeV. See reactions 24 and 27 in 8Be. See also references cited in (1988AJ01).
A new β-NMR technique (NNQR) was used to measure the quadrupole moment of 8B, |Q(8B, 2+)| = 68.3 ± 2.1 mb (1992MI18, 1993MI35). The large quadrupole moment was reported as the first evidence of a proton halo in 8B.
The tilted foil technique was used to polarize atomic 8B nuclei. The polarization was transferred to the nucleus via the hyperfine interaction and the resulting β-decay asymmetry indicated that the polarization was saturated at 3.71 ± 0.28% (1993MO34).
The β-decay of 8B provides the high-energy neutrinos that are measured by large volume neutrino detectors that are attempting to resolve the "solar neutrino problem". The neutrino energy spectrum from 8B β-decay, which is essential to interpret the data from these detectors, has been measured and evaluated in (1987NA08, 1996BA28, 1999DE33, 2000OR04, 2003RE26, 2003WI16). The 8B neutrino absorption cross sections (± 3σ) for Cl and Ga are σCl = 1.14 ± 0.11 × 10-42 cm2 and σGa = 2.46+2.1-1.1 × 10-42 cm2 (1996BA28). However, the results of (2000OR04) suggest a harder neutrino spectrum than that used by (1996BA28).
For comments about the weak neutral current interaction in 8B β-decay see (1989TE04, 1992DE07, 2003SM02). For theoretical discussion of 8Be levels that are involved in the decay see (1989BA31, 1993CH06, 2000GR07, 2002BH03) and reaction 27 in 8Be.
At Ed = 300 and 600 MeV 8B*(0, 0.77, 2.32) are populated: see (1984AJ01).
Angular distributions for the n0 group have been reported at E(3He) = 4.8 to 5.7 MeV: L = 0. Two measurements for the Ex of 8B*(0.77) are 767 ± 12 and 783 ± 10 keV [Γ = 40 ± 10 keV]: see (1974AJ01) and 9B.
Angular distributions and analyzing powers have been measured for the transitions to 8B*(0, 0.77, 2.32) at Ep = 199.2 MeV (1987CA06) and at 280, 345 and 489 MeV (1988HU11): the Ay to 8B*(2.32) is characteristic of that to a stretched high-spin, two-particle one-hole final state [Jπ of 8B*(2.32) is 3+] (1987CA06).
Absolute cross sections have been measured for Ep = 112 keV to 10.0 MeV. See also (1984AJ01) and references cited in (1988AJ01). Resonances are observed at Ep = 720 and 2497 keV: see 8.17 (in PDF or PS). An R-matrix evaluation of (p, γ) and (p, p') [reaction 7] data supports the existence of a 2- level at Ex = 3 - 4 MeV (2000BA46), and a 1+ resonance is predicted at Ex ≈ 1.4 MeV (2000CS01). See however (2001RO32) and reaction 9.
Direct measurements of 7Be(p, γ) at low energies are typically carried out by measuring β-delayed alpha particles from decay of the residual 8B nucleus. However, systematic errors associated with 8B backscattering losses from the target prior to counting have become a concern, based on new measurements and Monte Carlo calculations (see (1998ST20) and reaction 9 in 8Li).
A review of astrophysical reaction rates (1998AD12) favored the measurements of (1982FI03) and deduced a value of S(0) = 19+4-2 eV · b, however, several measurements [see 8.18 (in PDF or PS)] have been reported since this review. See other overviews of direct and indirect measurements in (2001MO32, 2001MU20, 2002MO11, 2003DA30, 2003MO23, 2003MO28): for cluster model calculations see (1988DE38, 1988KO29, 1993DE30, 1993RO04, 1994DE03, 1995CS01, 1997CS07, 1998CS03, 1998MO13, 2000CS03); for direct-plus-resonances and R-matrix calculations see (1987KI01, 1988BA29, 1993KR18, 1995BA36); and for shell model calculations see (1996BR04, 1998BE44). See also (1992SC22, 1993TR06, 1994SC14, 2003CH79, 2003PA33).
The role of electron screening and other effects, for example, 7Be deformation, are discussed in (1994KA02, 1997CS07, 1997NU01, 1998BE1Q, 2000LI13). The correlation of the capture rate with properties such as the 8B quadrupole-moment and the 8B valence proton spatial distribution is discussed in (1993RI04, 1996BR04, 1998CS03, 2000CS03, 2000JE10, 2001CS03).
The nature of the shape of the S-factor as the proton capture energy approaches zero is discussed in (1998JE04, 1998JE10, 1998JE11, 2000BA09, 2000BB09, 2000JE10, 2002MU16). The authors of (1998JE10, 2000VE01) suggest that S(20 keV) is more relevant than S(0) since the Gamow energy is ≈ 20 keV, and they suggest that the extrapolation of the reaction rate to 20 keV has less uncertainty than the extrapolation to zero energy proton capture.
The time reversed reaction 8B + γ → 7Be + p has been measured by exciting 8B nuclei in the Coulomb field of high-Z target nuclei and detecting the 7Be and proton products (1994MO33, 1998KI19, 1999IW03, 2001DA03, 2001DA11, 2002DA15, 2002DA26, 2003HA30, 2003SC14). The 7Be(p, γ)8B cross sections are related to the photodisintegration cross sections by the Detailed Balance Theorem. Resulting values of S(0) are 18.9 ± 1.8 eV · b (1998KI19; RIKEN), 18.6 ± 1.2(expt..) ± 1.0(theor.) eV · b (1999IW03, 2003HA30, 2003SC14; GSI), and 17.8+1.4-1.2 eV · b (2001DA03, 2001DA11, 2002DA15, 2002DA26; MSU). The field of virtual photons that induce breakup can excite the 8B mainly via E1 and E2 multipolarities; however, the proton capture reaction is dominated by E1 strength. Since the numbers of E1 and E2 virtual photons created in the Coulomb field of the target are calculable, depending on projectile energy and impact parameter, the ratio of σ(E2)/σ(E1) in the Coulomb dissociation experiments was deduced from asymmetries in, for example, the measured angular distributions. Values for the ratio, which depends on the relative p + 7Be energy and theory that is used to determine the E2 strength, range from (0.5 to 5) × 10-4 at Ecm = 0.6 MeV (1997KI01, 1999IW03, 2001DA03, 2001DA11, 2002DA15, 2003SC14). See also (1996KE16, 1996VO09). Calculated estimates of the σ(E2)/σ(E1) ratio in Coulomb dissociation are given in (1994LA08, 1995GA25, 1995LA17, 1996BE83, 1996ES02, 1996SH08, 1997TY01, 1999BB07, 1999DE23, 2002BE76, 2003FO07). Interference between nuclear and Coulomb mechanisms is discussed in (1997TY01, 1998DA15, 1998NU01, 2003MA88). See also (1993TI01, 1994TY03, 1996RE16, 1997CS02).
Calculations showing the relationship between the low-energy astrophysical S-factor for 7Be(p, γ) and the asymtotic normalization coefficient (ANC) for (7Be, 8B) reactions are presented in (1990MU13, 1994XU08, 1995MU10, 1997TI03, 1998GR07, 2000JE10, 2003TI13). See also reactions 8 and 11.
The 7Be(p, p) scattering was measured at Ecm = 0.3 - 0.75 MeV using a 7Be beam (2003AN29). The data were analyzed in an R-matrix analysis and indicate Eres = 634 ± 5 keV and Γres = 31 ± 4 keV for the 1+ first excited state. Scattering length of a01 = 25 ± 9 fm (channel spin I = 1) and a02 = -7 ± 3 fm (channel spin I = 2) were also deduced from the data.
At E(7Be) = 32 MeV (1998GO16), two resonances were prominent in the inverse kinematics scattering excitation function, Ex = 2.32 ± 0.02 MeV, Γ = 350 ± 30 keV, Jπ = 3+ and Ex = 2.83 ± 0.15 MeV, Γ = 780 ± 200 keV, Jπ = 1+, though poor statistics in the measurement prevent a firm acceptance of the 2.83 MeV level. In addition there was evidence for a broad 2- or 1- level at ≈ 3 MeV. At E(7Be) = 25.5 MeV the Ex = 2.32 MeV Jπ = 3+ level was observed with an additional level at Ex = 3.5 ± 0.5 MeV, Γ = 8 ± 4 MeV (2001RO32). An R-matrix analysis of the interference between the 2.32 and 3.5 MeV levels indicates Jπ = 2- for the higher state. In the later work, the 1+ state at Ex = 2.8 MeV, suggested by (1998GO16), was not necessary to obtain a good fit to the data. In addition there was no evidence for a level at Ex = 1.4 MeV that had been suggested by (2000CS01): see reaction 6.
The total 2H(7Be, n) cross section was measured at E(7Be) = 26 MeV (σtot = 58 ± 8 mb) and was evaluated to determine the 8B → 7Be + p asymptotic normalization coefficient (ANC) C2p3/2 = 0.711 ± 0.092 fm-1. This can be related to the 7Be(p, γ) astrophysical capture rate and indicates S17(0) = 27.4 ± 4.4 eV · b (1996LI12, 1997LI05). Re-analysis of the data using better optical model parameters indicates a smaller ANC and a reduced value of S17(0) = 23.5 ± 3.7 eV · b (1998GA02, 1999FE04). To remove the dependence on the optical model parameters, (2003OG02) performed a Continuum-Discretized coupled channels calculation using the spectroscopic factors S = 0.849 (1987KI01), from this they deduce S17(0) = 20.96 eV · b.
Angular dependent differential cross sections were measured for 9Be(7Li, 8He)8B from 0° to ≈ 12° at E(7Li) = 350 MeV. States in 8B were observed at 0, 0.770 and 2.32 MeV (2001CA37).
At Ep = 49.5 MeV [see (1974AJ01)] and 51.9 MeV (1983YA05) angular distributions have been measured for the tritons to 8B*(0, 2.32): L = 2 and L = 0 + 2 leading to Jπ = 2+ and 3+, respectively. Measurements of Ex for 8B*(2.32) yield 2.29 ± 0.05 MeV and 2.34 ± 0.04 MeV [Γlab = 0.39 ± 0.04 MeV]. 8B*(0.77) is also observed: see (1974AJ01).
In reaction (a) the asymptotic normalization coefficient (ANC), C23/2, for 8B → 7Be + p was determined by measuring differential cross sections for 10B(7Be, 8B) from 0° to ≈ 35° at E(7Be) = 84 MeV. The value of C2p3/2 = 0.398 ± 0.062 fm-1 was deduced which, together with C21/2/C23/2 = 0.157, corresponds to S17(0) = 17.8 ± 2.8 eV · b (1999AZ02). For reaction (b) C2p3/2 = 0.371 ± 0.043 fm-1 was measured in 14N(7Be, 8B) at E(7Be) = 85 MeV, and S17(0) = 16.6 ± 1.9 eV · b was deduced (1999AZ04).
A re-evaluation of the data from (a) and (b) using improved model parameters leads to revised values and a weighted average of C2p3/2 = 0.388 ± 0.039 fm-1 which corresponds to S(0) = 17.3 ± 1.8 eV · b (2001AZ01, 2001GA19, 2002GA11). In addition, the C2p3/2 gives Rr.m.s. = 4.20 ± 0.22 fm for the valence proton (2001CA21). See also 13C(7Li, 8Li)12C [reaction 27 in 8Li] for a determination of the ANC from charge symmetry.
At E(3He) = 72 MeV the first T = 2 state is observed at Ex = 10.619 ± 0.009 MeV, Γ < 60 keV: dσ/dΩ (lab) = 190 nb/sr at θlab = 9°. No other states are observed within 2.4 MeV of this state. 8B*(0, 0.77, 2.32) have also been populated: see (1979AJ01).
The pion absorption mechanism, which has a characteristic of high energy transfer and small momentum transfer, was studied at E(π+) = 100 and 165 MeV (2002HU06). The role of 2-step processes, such as pion scattering prior to absorption and nucleon pickup after absorption, is discussed, and simple models for neutron-pickup final state interactions are presented and shown to reasonably represent the data.
Elastic scattering of 8B on 14C was calculated in a folding potential model. Results suggest that scattering of exotic nuclei from non-(N = Z) nuclei could reveal new information about the nuclear potentials, particularly in cases where rainbow effects are observed (1998KN02).
A measurement to determine muon induced background rates in large-volume scintillation solar neutrino detectors found σ = 4.16 ± 0.81 μb and 7.13 ± 1.46 μb for natC(μ, 8B) at Eμ = 100 and 190 GeV, respectively (2000HA33).
Angular distributions of 7Be following the breakup of 8B on a 58Ni target were measured at E(8B) = 25 - 75 MeV to evaluate the importance of Coulomb-nuclear interference effects (2000GU05).
The measured total reaction cross sections for nuclear processes are related to the 8B r.m.s. radius and valence proton r.m.s. radius in simple Glauber-type models. The cross sections range from σtot ≈ 800 mb and σ(proton removal) ≈ 95 mb at E(8B) = 1471 MeV/A on a 12C target to σtot ≈ 1.95 b at E(8B) ≈ 15 MeV/A on Si (1995WA19, 1996NE06). These cross sections correspond to 8B r.m.s. radii around 2.43 ± 0.01 fm (1996OB01); the valence proton r.m.s. radius deduced from the proton removal cross-section measurements is model dependent and values in the range of 3.97 ± 0.12 fm (1996NE06) to 6.83 fm (1995SC10) are deduced. See also (1997KN07, 1998SH09, 1999KN04). A review of nuclear sizes deduced from interaction cross sections is in (2001OZ04).
Measurements of the parallel momentum distribution of 7Be fragments following the breakup of 8B projectiles are reported in (1995SC10, 1996KE16, 1996NE06, 1997SC03, 1998DA14, 1999SM04, 2000CO31) and are interpreted in Serber-type models as reflecting detailed information about the 8B valence proton wave function. At E(8B) = 1.47 GeV/A the momentum distribution widths from breakup on C, Al and Pb are ΓFWHM ≈ 81 ± 6 MeV/c (1995SC10). This width is much narrower than that expected from the breakup of nuclei with "normal" densities and was interpreted as an indication of a proton halo in 8B. However, at energies near 40 MeV/A the momentum distribution of 7Be fragments from 8B breakup range from Γ = 62 ± 3 MeV/c on an Au target (mainly Coulomb breakup processes) (1996KE16) to Γ = 95 ± 7 MeV/c on a Si target (mainly nuclear breakup processes) (1996NE06); this is an indication that at this energy, simple Serber-type models are not adequate to explain the observed momentum distributions since the breakup mechanisms play a role in determining the observed distributions.
By evaluating fragment momentum distributions in more complex models, it was suggested that the asymmetric 7Be fragment momentum distribution from 8B breakup on Au at 41 MeV/A reflects the interference of E1 and E2 contributions in Coulomb Dissociation and gives information about the relative E2/E1 strength (1996ES02, 1996KE16). A high-resolution measurement of the asymmetric distribution from breakup on Pb at E(8B) = 44 and 81 MeV/A deduced that σ(E2)/σ(E1) ≈ 6.7 × 10-4(+2.8-1.9) at Erel.(p + 7Be) = 0.6 MeV (1998DA14). A more precise value of σ(E2)/σ(E1) ≈ 4.9 × 10-4 +1.5-1.3 at Erel. = 0.6 MeV was deduced by including measurements at E(8B) = 83 MeV/A (2002DA15).
Breakup cross sections and 7Be core-like fragment momentum distributions are analyzed in a modified Glauber model to obtain asymtotic normalization coefficients (ANC) for the 8B → 7Be + p reaction (2004TR06). In this analysis of breakup data, the value S17(0) = 18.7 ± 1.9 eV · b is deduced.
At E(8B) = 936 MeV/A, the ratio of (7Be*(0.429) + γ)/7Be production was measured on C and Pb targets (2002CO04, 2003CO06, 2003ME16). The measurements indicate a 13.3 ± 2.2% component of 7Be*(0.429) in the ground state of 8B (2003ME16). Spectroscopic factors for 7Be*(0, 0.43) were deduced from measurements of 12C(8B, 7Be) at E(8B) = 76 MeV/A; C2S = 1.036 and 0.220, respectively (2003EN05).