
^{8}B (2004TI06)(See Energy Level Diagrams for ^{8}B) GENERAL: References to articles on general properties of ^{8}B published since the previous review (1988AJ01) are grouped into categories and listed, along with brief descriptions of each item, in the General Tables for ^{8}B located on our website at (nucldata.tunl.duke.edu/nucldata/General_Tables/8b.shtml). See also 2 in (1988AJ01) [Electromagnetic Transitions in A = 510] (in PDF or PS), 8.15 [Table of Energy Levels] (in PDF or PS) and 8.16 [Electromagnetic transitions in ^{8}B] (in PDF or PS).
Q = 68.3 ± 2.1 mb (1992MI18, 1993MI35).
The β^{+} decay leads mainly to ^{8}Be*(3.0). The halflife is 770 ± 3 msec; log ft = 5.6 (1974AJ01). There is also a branch to ^{8}Be*(16.63), and evidence for population of an ^{8}Be intruder state at E_{x} ≈ 9 MeV. See reactions 24 and 27 in ^{8}Be. See also references cited in (1988AJ01). A new βNMR technique (NNQR) was used to measure the quadrupole moment of ^{8}B, Q(^{8}B, 2^{+}) = 68.3 ± 2.1 mb (1992MI18, 1993MI35). The large quadrupole moment was reported as the first evidence of a proton halo in ^{8}B. The tilted foil technique was used to polarize atomic ^{8}B nuclei. The polarization was transferred to the nucleus via the hyperfine interaction and the resulting βdecay asymmetry indicated that the polarization was saturated at 3.71 ± 0.28% (1993MO34). The βdecay of ^{8}B provides the highenergy neutrinos that are measured by large volume neutrino detectors that are attempting to resolve the "solar neutrino problem". The neutrino energy spectrum from ^{8}B βdecay, which is essential to interpret the data from these detectors, has been measured and evaluated in (1987NA08, 1996BA28, 1999DE33, 2000OR04, 2003RE26, 2003WI16). The ^{8}B neutrino absorption cross sections (± 3σ) for Cl and Ga are σ_{Cl} = 1.14 ± 0.11 × 10^{42} cm^{2} and σ_{Ga} = 2.46^{+2.1}_{1.1} × 10^{42} cm^{2} (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 ^{8}B βdecay see (1989TE04, 1992DE07, 2003SM02). For theoretical discussion of ^{8}Be levels that are involved in the decay see (1989BA31, 1993CH06, 2000GR07, 2002BH03) and reaction 27 in ^{8}Be.
At E_{d} = 300 and 600 MeV ^{8}B*(0, 0.77, 2.32) are populated: see (1984AJ01).
Angular distributions for the n_{0} group have been reported at E(^{3}He) = 4.8 to 5.7 MeV: L = 0. Two measurements for the E_{x} of ^{8}B*(0.77) are 767 ± 12 and 783 ± 10 keV [Γ = 40 ± 10 keV]: see (1974AJ01) and ^{9}B.
Angular distributions and analyzing powers have been measured for the transitions to ^{8}B*(0, 0.77, 2.32) at E_{p} = 199.2 MeV (1987CA06) and at 280, 345 and 489 MeV (1988HU11): the A_{y} to ^{8}B*(2.32) is characteristic of that to a stretched highspin, twoparticle onehole final state [J^{π} of ^{8}B*(2.32) is 3^{+}] (1987CA06).
See ^{6}H.
Absolute cross sections have been measured for E_{p} = 112 keV to 10.0 MeV. See also (1984AJ01) and references cited in (1988AJ01). Resonances are observed at E_{p} = 720 and 2497 keV: see 8.17 (in PDF or PS). An Rmatrix evaluation of (p, γ) and (p, p') [reaction 7] data supports the existence of a 2^{} level at E_{x} = 3  4 MeV (2000BA46), and a 1^{+} resonance is predicted at E_{x} ≈ 1.4 MeV (2000CS01). See however (2001RO32) and reaction 9. Direct measurements of ^{7}Be(p, γ) at low energies are typically carried out by measuring βdelayed alpha particles from decay of the residual ^{8}B nucleus. However, systematic errors associated with ^{8}B 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 ^{8}Li). 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 directplusresonances and Rmatrix 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, ^{7}Be deformation, are discussed in (1994KA02, 1997CS07, 1997NU01, 1998BE1Q, 2000LI13). The correlation of the capture rate with properties such as the ^{8}B quadrupolemoment and the ^{8}B valence proton spatial distribution is discussed in (1993RI04, 1996BR04, 1998CS03, 2000CS03, 2000JE10, 2001CS03). The nature of the shape of the Sfactor 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 ^{8}B + γ → ^{7}Be + p has been measured by exciting ^{8}B nuclei in the Coulomb field of highZ target nuclei and detecting the ^{7}Be and proton products (1994MO33, 1998KI19, 1999IW03, 2001DA03, 2001DA11, 2002DA15, 2002DA26, 2003HA30, 2003SC14). The ^{7}Be(p, γ)^{8}B 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 ^{8}B 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 + ^{7}Be energy and theory that is used to determine the E2 strength, range from (0.5 to 5) × 10^{4} at E_{cm} = 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 lowenergy astrophysical Sfactor for ^{7}Be(p, γ) and the asymtotic normalization coefficient (ANC) for (^{7}Be, ^{8}B) reactions are presented in (1990MU13, 1994XU08, 1995MU10, 1997TI03, 1998GR07, 2000JE10, 2003TI13). See also reactions 8 and 11.
The ^{7}Be(p, p) scattering was measured at E_{cm} = 0.3  0.75 MeV using a ^{7}Be beam (2003AN29). The data were analyzed in an Rmatrix analysis and indicate E_{res} = 634 ± 5 keV and Γ_{res} = 31 ± 4 keV for the 1^{+} first excited state. Scattering length of a_{01} = 25 ± 9 fm (channel spin I = 1) and a_{02} = 7 ± 3 fm (channel spin I = 2) were also deduced from the data. At E(^{7}Be) = 32 MeV (1998GO16), two resonances were prominent in the inverse kinematics scattering excitation function, E_{x} = 2.32 ± 0.02 MeV, Γ = 350 ± 30 keV, J^{π} = 3^{+} and E_{x} = 2.83 ± 0.15 MeV, Γ = 780 ± 200 keV, J^{π} = 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(^{7}Be) = 25.5 MeV the E_{x} = 2.32 MeV J^{π} = 3^{+} level was observed with an additional level at E_{x} = 3.5 ± 0.5 MeV, Γ = 8 ± 4 MeV (2001RO32). An Rmatrix analysis of the interference between the 2.32 and 3.5 MeV levels indicates J^{π} = 2^{} for the higher state. In the later work, the 1^{+} state at E_{x} = 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 E_{x} = 1.4 MeV that had been suggested by (2000CS01): see reaction 6.
The total ^{2}H(^{7}Be, n) cross section was measured at E(^{7}Be) = 26 MeV (σ_{tot} = 58 ± 8 mb) and was evaluated to determine the ^{8}B → ^{7}Be + p asymptotic normalization coefficient (ANC) C^{2}_{p3/2} = 0.711 ± 0.092 fm^{1}. This can be related to the ^{7}Be(p, γ) astrophysical capture rate and indicates S_{17}(0) = 27.4 ± 4.4 eV · b (1996LI12, 1997LI05). Reanalysis of the data using better optical model parameters indicates a smaller ANC and a reduced value of S_{17}(0) = 23.5 ± 3.7 eV · b (1998GA02, 1999FE04). To remove the dependence on the optical model parameters, (2003OG02) performed a ContinuumDiscretized coupled channels calculation using the spectroscopic factors S = 0.849 (1987KI01), from this they deduce S_{17}(0) = 20.96 eV · b.
Angular dependent differential cross sections were measured for ^{9}Be(^{7}Li, ^{8}He)^{8}B from 0° to ≈ 12° at E(^{7}Li) = 350 MeV. States in ^{8}B were observed at 0, 0.770 and 2.32 MeV (2001CA37).
At E_{p} = 49.5 MeV [see (1974AJ01)] and 51.9 MeV (1983YA05) angular distributions have been measured for the tritons to ^{8}B*(0, 2.32): L = 2 and L = 0 + 2 leading to J^{π} = 2^{+} and 3^{+}, respectively. Measurements of E_{x} for ^{8}B*(2.32) yield 2.29 ± 0.05 MeV and 2.34 ± 0.04 MeV [Γ_{lab} = 0.39 ± 0.04 MeV]. ^{8}B*(0.77) is also observed: see (1974AJ01).
In reaction (a) the asymptotic normalization coefficient (ANC), C^{2}_{3/2}, for ^{8}B → ^{7}Be + p was determined by measuring differential cross sections for ^{10}B(^{7}Be, ^{8}B) from 0° to ≈ 35° at E(^{7}Be) = 84 MeV. The value of C^{2}_{p3/2} = 0.398 ± 0.062 fm^{1} was deduced which, together with C^{2}_{1/2}/C^{2}_{3/2} = 0.157, corresponds to S_{17}(0) = 17.8 ± 2.8 eV · b (1999AZ02). For reaction (b) C^{2}_{p3/2} = 0.371 ± 0.043 fm^{1} was measured in ^{14}N(^{7}Be, ^{8}B) at E(^{7}Be) = 85 MeV, and S_{17}(0) = 16.6 ± 1.9 eV · b was deduced (1999AZ04). A reevaluation of the data from (a) and (b) using improved model parameters leads to revised values and a weighted average of C^{2}_{p3/2} = 0.388 ± 0.039 fm^{1} which corresponds to S(0) = 17.3 ± 1.8 eV · b (2001AZ01, 2001GA19, 2002GA11). In addition, the C^{2}_{p3/2} gives R_{r.m.s.} = 4.20 ± 0.22 fm for the valence proton (2001CA21). See also ^{13}C(^{7}Li, ^{8}Li)^{12}C [reaction 27 in ^{8}Li] for a determination of the ANC from charge symmetry.
At E(^{3}He) = 72 MeV the first T = 2 state is observed at E_{x} = 10.619 ± 0.009 MeV, Γ < 60 keV: dσ/dΩ (lab) = 190 nb/sr at θ_{lab} = 9°. No other states are observed within 2.4 MeV of this state. ^{8}B*(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 2step processes, such as pion scattering prior to absorption and nucleon pickup after absorption, is discussed, and simple models for neutronpickup final state interactions are presented and shown to reasonably represent the data.
Angular distributions from quasielastic scattering of ^{8}B on ^{12}C were measured at 40 MeV/A (1995PE09). Analysis of the data appears consistent with a proton halo (1995FA17, 1996KN05, 1997PE03).
Elastic scattering of ^{8}B on ^{14}C 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 largevolume scintillation solar neutrino detectors found σ = 4.16 ± 0.81 μb and 7.13 ± 1.46 μb for ^{nat}C(μ, ^{8}B) at E_{μ} = 100 and 190 GeV, respectively (2000HA33).
Angular distributions of ^{7}Be following the breakup of ^{8}B on a ^{58}Ni target were measured at E(^{8}B) = 25  75 MeV to evaluate the importance of Coulombnuclear interference effects (2000GU05).
Inclusive measurements of ^{8}B breakup have been reported: see 8.19 (in PDF or PS). The measured total reaction cross sections for nuclear processes are related to the ^{8}B r.m.s. radius and valence proton r.m.s. radius in simple Glaubertype models. The cross sections range from σ_{tot} ≈ 800 mb and σ(proton removal) ≈ 95 mb at E(^{8}B) = 1471 MeV/A on a ^{12}C target to σ_{tot} ≈ 1.95 b at E(^{8}B) ≈ 15 MeV/A on Si (1995WA19, 1996NE06). These cross sections correspond to ^{8}B r.m.s. radii around 2.43 ± 0.01 fm (1996OB01); the valence proton r.m.s. radius deduced from the proton removal crosssection 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 ^{7}Be fragments following the breakup of ^{8}B projectiles are reported in (1995SC10, 1996KE16, 1996NE06, 1997SC03, 1998DA14, 1999SM04, 2000CO31) and are interpreted in Serbertype models as reflecting detailed information about the ^{8}B valence proton wave function. At E(^{8}B) = 1.47 GeV/A the momentum distribution widths from breakup on C, Al and Pb are Γ_{FWHM} ≈ 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 ^{8}B. However, at energies near 40 MeV/A the momentum distribution of ^{7}Be fragments from ^{8}B 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 Serbertype 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 ^{7}Be fragment momentum distribution from ^{8}B 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 highresolution measurement of the asymmetric distribution from breakup on Pb at E(^{8}B) = 44 and 81 MeV/A deduced that σ(E2)/σ(E1) ≈ 6.7 × 10^{4}(^{+2.8}_{1.9}) at E_{rel.}(p + ^{7}Be) = 0.6 MeV (1998DA14). A more precise value of σ(E2)/σ(E1) ≈ 4.9 × 10^{4} ^{+1.5}_{1.3} at E_{rel.} = 0.6 MeV was deduced by including measurements at E(^{8}B) = 83 MeV/A (2002DA15). Breakup cross sections and ^{7}Be corelike fragment momentum distributions are analyzed in a modified Glauber model to obtain asymtotic normalization coefficients (ANC) for the ^{8}B → ^{7}Be + p reaction (2004TR06). In this analysis of breakup data, the value S_{17}(0) = 18.7 ± 1.9 eV · b is deduced. At E(^{8}B) = 936 MeV/A, the ratio of (^{7}Be*(0.429) + γ)/^{7}Be production was measured on C and Pb targets (2002CO04, 2003CO06, 2003ME16). The measurements indicate a 13.3 ± 2.2% component of ^{7}Be*(0.429) in the ground state of ^{8}B (2003ME16). Spectroscopic factors for ^{7}Be*(0, 0.43) were deduced from measurements of ^{12}C(^{8}B, ^{7}Be) at E(^{8}B) = 76 MeV/A; C^{2}S = 1.036 and 0.220, respectively (2003EN05).
