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## 12Be (2017KE05)

(See 12.2 (in PDF or PS) and Energy Level Diagrams for 12Be and Isobar Diagram)

<(rmatterrms)2>1/2 ≈ 2.71 ± 0.06 fm (2012IL01).

We accept ΔM = 25077.8 ± 1.9 keV (2012WA38). See precise 12Be mass measurements: ΔM = 25078.0 ± 2.1 keV (2010ET01: TITAN), ΔM = 25068 ± 13 keV (2009GA24: MISTRAL), ΔM = 25075.7 ± 4.2 keV (1994FO08: 10Be(t, p)) and ΔM = 25078 ± 15 keV (1978AL10: 10Be(t, p)). Earlier measurements indicated ΔM = 25050 ± 50 keV (1974BA15: 14C(18O, 20Ne)) and ΔM = 24950 ± 100 keV (1971HO26: 7Li(7Li, 2p)).

An analysis of T = 2 isobaric analogue states for A = 12 nuclei is reported in (2012JA11) including quadratic, cubic and quartic forms of IMME. See also (2006KO02, 2009BA41, 2010ET01, 2014MA56).

The 2s1/2 → 2p(1/2, 3/2) transition in 12Be was measured and compared with that of 9Be; based on the isotope shift data and Rc(9Be) = 2.519 ± 0.012 fm (1972JA10), Rc(12Be) = 2.503 ± 0.015 fm is deduced (2012KR04). See also (2015KA02).

 1. 12Be(β-)12B Qm = 11.7084

Measured values of the 12Be half-life are T1/2 = 21.49 ± 0.03 ms (2001BE53, 2002BE53), 26.1 ± 2.4 ms (1994KE06), 21.32 ± 0.06 ms (1994RE1R: see also (1991RE02)), 21.3 ± 2.2 ms (1986CU01) and 24.0 ± 1.0 ms (1984DU15), 24.4 ± 3.0 ms (1978AL10): the weighted average of these values is 21.46 ± 0.05 ms. The β-delayed neutron probability is (0.50 ± 0.03)% (1999BE53). See also Pn = (0.9 ± 0.4)% (1991RE02); and earlier measurements which placed an upper limit of Pn ≤ 1% (1978AL10, 1984DU15). Observation of β-delayed neutrons provide evidence that at least two 12B states are fed in the decay, though no detailed decay scheme has been experimentally confirmed (1994KE06). Assuming a (99.50 ± 0.03)% branching ratio to the ground state gives log ft = 3.795 ± 0.002 for decay to 12Bg.s.. (1997SU12) suggest the large log ft indicates "tremendous breaking of the neutron closed shell core;" a 65% breaking of the closed p-shell is estimated in (1998SU17). See also (1993CH06).

 2. (a) 1H(12Be, 12Be) Eb = 15.8047 (b) 1H(12Be, 12Be')

At E(12Be) = 53.8 MeV the angular distribution for scattering to 12Be*(2.1[Jπ = 2+]) was measured and analyzed in a coupled channels analysis (2000IW02). Scattered 12Be ions and γ-rays were measured. Analysis indicates a deformation length, δ = 2.00 ± 0.23, and supports evidence that N = 8 shell closure is not present in 12Be. See also (2008TA02). At E(12Be) = 704 MeV/A the differential cross sections for small-angle elastic scattering were measured (2012IL01); Rmatterrms ≈ 2.71 ± 0.06 fm is deduced. For calculations on p-Be elastic scattering see (2012FA14, 2012FO08, 2013CA04, 2014FA15).

Protons corresponding to 12Be*(0, 2.1, 2.7, 4.56, 5.7, 8.60 ± 0.15, 10.00 ± 0.15, ≈ 14 MeV) were observed in elastic and inelastic scattering of 55 MeV/A 12Be on 1H (1995KO10, 1995KO27); states at 8.6 and 10.0 MeV have Γ ≤ 0.5 MeV. He+He cluster structures are suggested for 12Be*(10.0, ≈ 14) based on p + Be and p + He correlations observed in the particle-coincidence data. The kinematic reconstruction of 8He + α and 6He + 6He products, for example, from 12Be breakup reactions on 1H and 12C targets yielded evidence for several states in the 10 MeV ≤ Ex ≤ 25 MeV region (1999FR04, 2001FR02, 2007CH81); see also (2005GA31, 2016KO22), reaction 18 and 12.4 (in PDF or PS).

 3. 1H(14Be, 13Be → 12Be + n) Qm = 0.9541

States at 12Be*(0, 2.1, 2.7) are involved in the decay of 13Be states populated in the 1H(14Be, 13Be) reaction at E(14Be) = 304 MeV/A (2013AK02).

 4. 2H(11Be, 12Be+p) Qm = 0.9461

In (2010KA03, 2010KA24), angular distributions of protons (130° ≤ θlab ≤ 160°) and 12Be (0.8° ≤ θlab ≤ 2.7°) ejectiles were measured for E(11Be) = 5 MeV/A (2010KA03, 2010KA24). A DWBA analysis finds s-wave spectroscopic factors of S = 0.28+0.03-0.07, 0.10+0.09-0.07, 0.73+27-40 and ≈ 0.35 for 12Be*(0[Jπ = 0+], 2.11[2+], 2.24[0+], 2.68[1-]). The low neutron binding energy and sizeable spectroscopic factors are viewed as evidence for a possible halo structure for the isomeric 12Be*(2.24) state. At E(11Be) = 2.8 MeV/A (2013JO06) the scattered protons (8° ≤ θlab ≤ 152°) and de-excitation γ-rays from 12Be*(2.1, 2.7) were detected; spectroscopic factors of S = 0.15+0.03-0.05, 0.075 ± 0.025, 0.40 ± 0.13 and 0.27 ± 0.15 were deduced for 12Be*(0, 2.11, 2.24, 2.68). In addition, analysis of the delayed radiations from 12Be*(2.24) indicated T1/2 = 247 ± 15 ns with (87.3 ± 3.5)% decay to 12Beg.s. via pair production and 12.7% via γ-decay to 12Be*(2.11). See also (2009JOZY). See comments on the spectroscopic factors in (2012FO11) and (2003ZE06).

 5. 4He(12Be, 12Be')

At E(12Be) = 60 MeV/A angular distributions for scattering to 12Be*(2.1, 2.7) were measured (2004SH24, 2004SH29). DWBA analysis is consistent with Jπ = 2+ and 1-, respectively. Angular distributions for the corresponding γ-rays are measured and analyzed in (2002MA79).

Invariant mass spectroscopy of the 6He-6He and α + 8He particle coincidences populated with E(12Be) = 60 MeV/A projectiles (2004SA32) resolved peaks at 12Be*(10.9, 11.3); DWBA analysis indicates Jπ = 0+ and 2+, respectively. Further evidence of a peak at Ex = 11.8 MeV is observed in the 6He-6He data, and evidence of a peak at Ex ≈ 10 MeV is found in the α+8He data. See also (2011OG09).

 6. 7Li(12B, 7Be)12Be Qm = -12.5703

Angular distributions for the 12B(7Li, 7Be) charge exchange reaction were measured in inverse kinematics at E(12B) = 80 MeV/A (2012ME05) as a means to determine B(GT) = 0.214 ± 0.051 for the transition between 12Be*(2.25) and 12Bg.s.. The value is deduced by normalization to the known B(GT) = 0.184 ± 0.008 for the 12Beg.s.12Bg.s. transition.

 7. 9Be(12C, 9C)12Be Qm = -42.6404

States at 12Be*(0[Jπ = 0+], 2.1[2+], 4.56[2+], 5.70[4+, 3-], 6.30 ± 0.05, 7.53, 8.23, 8.81, 10.18, 10.83, 11.44, 12.46, 14.7, 16.7) were populated in the 3-neutron stripping reaction at E(12C) = 231 MeV (2004BO23, 2008BO37). The 12Be*(7.53, 8.23, 8.81, 10.18, 12.46) states are suggested as a K = 1- rotational band. Level spacing suggests Jπ = 6+ for 12Be*(14.7) if it is a member of the ground state rotational band.

 8. 9Be(13B, 12Be)10B Qm = -9.2180

Single-proton removal reactions from 13B were utilized in a search for neutron unbound states in 12Be (2014SM03). A state at E(11Be + n) = 1243 ± 12 keV (Ex = 4412 ± 16 keV) was observed with L = 1 and Γ = 634 ± 60 keV; it is presumed that 11Beg.s. is populated implying J = (2-). No states are observed in the 10Be + 2n analysis. Theoretical opinions given in (2016FO02, 2016FO03, 2016FO26) suggest the state is either a Jπ = 0+ or Jπ = 2+ state that likely decays to 11Be*(0.32).

 9. 9Be(15N, 12N)12Be Qm = -30.9660

At E = 240 MeV states at 12Be*(5.7, (6.4[Jπ = 0+]), 7.40[2+], 9.3, 10.7[4+], 14.6[6+], 19.2, 21.7[8+]) are observed (2002BO16). The Jπ values are assumed based on systematics corresponding to a rotational band built on 12Be*(6.4). See also (2003BO24, 2003BO38, 2003BO50, 2008BO37).

 10. 9Be(18O, 12Be)15O Qm = -17.3678

In (2003SH06) a 12Be beam was produced by fragmenting 100 MeV/A 18O projectiles in a 9Be target. The first report of 12Be*(2.25) found evidence in the in-flight decay of the state via detection of the sequentially emitted γ-rays, 12mBe*(2.25) → 12Be*(2.107) → 12Beg.s.. The 12Be ions were detected in coincidence with the γ-rays. The angular correlations of the two γ-rays determine Jπ = 0+ for the state. Furthermore, 511 keV annihilation γ rays were also detected following e+ + e--decay, indicating a E0(0+ → 0+) decay branch to the ground state. The branching ratio for decay to either 12Be*(2.107)[2+] or 12Beg.s. was found as E2 = (17 ± 2)% and E0 = (82 ± 2)%, respectively.

A subsequent endeavor also produced a 12Be beam via fragmentation of a E(18O) = 100 MeV/A beam; the 12Be ions were implanted in a thick polyethylene block (2007SH34). Decay of the 12mBe*(2.251) state was observed via the γ-ray emission cascade through 12Be*(2.107), as well as observation of the 511 keV annihilation γ-rays following e++e--pair emission. The γ-ray energies of 144 and 2107 keV in the Jπ = 0+ → 2+ → 0+ decay cascade lead to Ex = 2251 ± 1 keV for 12mBe. The mean lifetime for the two decay branches is τm = 331 ± 12 ns. The branching ratio E2 = (18.3 ± 1.4)% and E0 = (81.7 ± 1.4)% is observed; when combined with (2003SH06) the average is E2 = (17.7 ± 1.5)% and E0 = (82.3 ± 1.5)%, which yields B(E0) = 0.87 ± 0.03 e ⋅ fm2 and B(E2) = 7.0 ± 0.6 e2 ⋅ fm4. See also (2003SH35, 2007HA50) and (2012FO08). See (2008RO03) for the prediction of a Jπ = 0- isomeric state near Ex = 2.5 MeV.

 11. 10Be(t, p)12Be Qm = -4.8095

Levels are reported in measurements at Et = 12 to 17 MeV; see 12.3 (in PDF or PS). At Et = 12 MeV 12Be*(2.10) is populated [Ex = 2110 ± 15 keV], and (p, γ) angular correlations lead to J = 2 (1978AL10). The Q-value 4808.3 ± 4.2 keV was deduced in (1994FO08), which compares with Q = 4809 ± 15 keV from (1978AL10). See also (2006FO11, 2011FO04, 2013FO30, 2013GA48, 2014FO04, 2014FO11, 2014PO08, 2015FO06) and (1980AJ01).

 12. 10Be(14C, 12Be)12C Qm = -9.4504

At E(14C) = 87.7 MeV analysis of the invariant mass spectra constructed from the 6He + 6He and 4He + 8He relative energies found no evidence for 12Be excited states (2006CU01).

 13. 10Be(14N, 12N)12Be Qm = -26.9450

States at 12Be*(0, 2.1, 4.56, 5.70, 6.25 ± 0.05, (11.2)) were populated in the 2-neutron stripping reaction at E(14N) = 216 MeV (2003BO24, 2008BO37).

 14. 12C(π-, π+)12Be Qm = -25.0778

At Eπ- = 164 MeV, 12Be*(0, 2.10 + (2.71)) are populated (1978SE07). See also (2007FO05) for Eπ- = 120, 180 and 240 MeV.

 15. 12C(12Be, 10Be)14C Qm = 9.4504

The width of the 10Be longitudinal momentum distribution following E(12Be) = 56.75 MeV/A breakup on a 12C target is FWHM = 194 ± 9 MeV/c (1993ZA04). See also (1990LI39, 1997OR03, 2000BH09, 2003TI10, 2004YA05, 2010BA03).

 16. natC(12Be, 11Be)13C Qm = 1.7756

The neutron removal cross sections was measured at E(12Be) = 39.3 MeV/A. States at 11Be*(0, 0.32, 1.78, 2.69, ≈ 4) were populated (2005PA68, 2006PA04). The spectroscopic factors S = 0.56 ± 0.18, 0.44 ± 0.08, 0.48 ± 0.06, 0.40 ± 0.07 were deduced, respectively. The significant feeding of 11Be*(1.78[Jπ = 5/2+]) implies a ν(0d5/2)2 component in the 12Beg.s.. See also (2004BR17, 2004GO23, 2004IT08).

 17. 12C(12Be, 12Be) Eb = 31.0294

The angular distribution for quasielastic scattering of 12Be on 12C was measured at E(12Be) = 56 MeV/A (1994ZA02), and optical model parameters were deduced from a coupled-channels analysis. See also (1996TH01) who analyzed the data of (1994ZA02) and deduced a dominant (p1/2)2 configuration in the 12Be ground state with a possible 25% (sd)2 admixture. See also (1994ME15, 1995BE26, 1995TH04, 1999KN04, 2000PA14, 2001OZ04).

 18. (a) 12C(12Be, 12Be') (b) 12C(12Be, Breakup)

Unbound 12Be states with Ex = 10 to 25 MeV were studied by measuring 6He + 6He and α+8He breakup coincidences and reconstructing the excitation energy from kinematics (1999FR04, 1999OR07, 2001FR02: E(12Be) = 378 MeV), (2007CH81: E(12Be) = 600 MeV) and (2014YA08, 2015YA05: E(12Be) = 348 MeV). 12C and (CH2)n targets were used. Observed states are shown in 12.4 (in PDF or PS).

In (2001FR02) the states are associated with cluster rotational states that could indicate a quasi-molecular α-4n-α structure in 12Be. However, the high statistics measurements of (2007CH81) failed to find evidence supporting many states observed by (2001FR02); specifically several key members of the suggested rotational band were absent from the spectra. In (2007CH81) the relative energies of p + 11Li and t + 9Li breakup particles were reconstructed; in the p + 11Li data, a state at Ex = 25 MeV with Γ ≈ 370 keV is suggested as the IAS of 12Lig.s.. The array configuration used in (2014YA08, 2015YA05) emphasized a high sensitivity for low relative energy breakup events. New states were identified and the angular distributions were analyzed via DWBA to obtain Jπ values. At Ex = 10.3, a Jπ = 0+ level is identified as the bandhead of a 4He + 8He molecular rotational band with a transition strength of 7.0 ± 1.0 fm2 for the isoscalar monopole transition.

The cross sections for 12Be breakup into 6He + 6He, α + 8He, α + 6He (+ 2n), 11Be (+ n), 10Be (+ 2n), 9Be (+ 3n), and 8Be (+ 4n) were measured at E(12Be) = 41.8 MeV/A (2004AS02). The cluster cross sections are dominated by breakup into 2α + neutrons, suggesting a α-4n-α structure. See also theoretical analysis of cluster states in (2000BB06, 2000IT07, 2001DE20, 2001IT05, 2002IT09, 2003FI15, 2004FI04, 2004FI11, 2004KA21, 2004KA34, 2005KA50, 2007FR22, 2007PE26, 2008IT05, 2008IT07, 2008RO12, 2010DU08, 2010WO02, 2011DU11, 2011IT02, 2012GA33, 2012IT01, 2012IT02, 2012IT04, 2013TA01, 2014IT02).

 19. 12C(14Be, 12Be')14C Qm = 11.8523

At E(14Be) = 75 MeV/A the 6He + 6He breakup events were detected (2002SA65). Peaks at the decay energies E(6He + 6He) ≈ 1.7, 2.9, 4.6 and 5.6 MeV were observed; these correspond to Ex ≈ 11.8, 13.0, 14.7 and 15.7 MeV. Analysis of the angular correlation data for 12Be*(11.8) suggests J = 0.

 20. (a) 12C(14C, 14O)12Be Qm = -30.06534 (b) 14C(12C, 14O)12Be Qm = -30.06534

At E(14C) = 335.9 MeV 12Be*(0, 2.1, 2.7, 4.56, 5.7, 7.2 ± 0.1, 9.3 [Γ = 2.0 ± 0.3 MeV]) are populated (1995VO05, 2002BO16). For reaction (b) at 231 MeV, 12Be*(0, 2.1, 4.56, 9.6, 18.95) are observed (2008BO37). Although the Ex = 2.24 MeV state is unresolved from the strongly populated Ex = 2.1 MeV state, the authors suggest a rotational band based on 12Be*(2.24 [0+2], 4.56 [2+2], 9.6, 18.95). See also (2015MA03, 2015CA08).

 21. 14C(14C, 16O)12Be Qm = -14.3010

At E(14C) = 335.9 MeV, 12Be*(0, 2.10, 4.56, ≈ 5.7) are populated (1995BO10).

 22. (a) 28Si(12Be, X) (b)208Pb(12Be, X)

At E(12Be) = 30 to 60 MeV/A the total reaction cross sections were measured on 28Si and 208Pb targets (2001WA40). Data are compared with a Glauber model. Neutron removal cross sections indicate σ2n > σ1n, owing to the ease of removing the second neutron.

See also measurements in (1988TA10) and (1990LI39, 1990LO10, 1993FE12, 1996AL24, 1999KN04, 2000BH09, 2000CA33, 2001OZ04, 2002BR01, 2003CA07, 2006BH01, 2006SH20).

 23. 197Au(12Be, 12Be'γ)

At E(12Be) = 42.9 MeV/A, the γ-rays associated with inelastic scattering to 12Be*(2107 ± 3, 2693 ± 5) on a 1 mm thick Au target were observed at θlab = 150° (2009IM01). Using τm = 1.9 ± 0.5 fs (2000IW03) for the lifetime of 12Be*(2693), the spectrum was analyzed using the Doppler shift attenuation method (or DSAM technique) to deduce the lifetime of the Ex = 2107 keV Jπ = 2+ state. The mean lifetime τm = 2.5 ± 0.7 (stat.) ± 0.3 (sys.) ps, which corresponds to B(E2) = 8.0 ± 2.2 (stat.) ± 0.8 (sys.) e2 ⋅ fm4, was deduced in an analysis that considered cascade feeding from the higher-lying Ex = 2251 keV Jπ = 0+ isomeric state. See also discussion in (2004TH07, 2005TH06, 2008UM02, 2012FO08, 2012LI32, 2012YU07, 2013MA53, 2013MA60, 2014LI39).

 24. 208Pb(12Be, 12Be'γ)

At E(12Be) = 53.3 MeV/A Coulomb excitation of the Ex = 2110 ± 20 and 2680 ± 30 keV states of 12Be was measured (2000IW03). The Ex = 2.68 MeV state was strongly populated, leading to a Jπ = 1- assignment and B(E1) = 0.051 ± 0.013 e2 ⋅ fm2. The low energy and significant strength of the 1- state is seen as further evidence of the ending of N = 8 "magicity" in 12Be. A further study in (2002IW01) deduced the quadrupole deformation parameter, δ = 2.00 ± 0.23 fm, for 12Be*(2.11) which corresponds to β2 ≈ 0.7. See also (1995AN20, 1997BO08, 2001SA06, 2001SU10, 2002SA12, 2002SH44, 2003SH26, 2003SH35, 2005NA06, 2005NA42, 2006GU07, 2010BL08).