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¹¹Li (2012KE01)

(See 11.1 (in PDF or PS) and Energy Level Diagram for ¹¹Li and Isobar Diagram )

We accept the most precise measurement of the ¹¹Li mass M = 11.04372361 ± 0.00000069 u which yields a mass excess of 40728.28 ± 0.64 keV (2008SM03: TITAN). This values differs from Audi et al. (2003AU03) by 70 keV and results in the value S_2n = 369.15 ± 0.65 keV. Other precise mass measurements have indicated ΔM = 40719 ± 5 keV (S_2n = 378 ± 5 keV) (2005BB01, 2008BA18, 2009GA24: MINSTRAL). Values derived from reaction Q-values, in terms of the 2-neutron separation energy, are S_2n = 363 ± 22 keV (2009RO04), 295 ± 35 keV (1993YO07), 340 ± 50 keV (1991KO1U), 320 ± 120 keV (1988WO09) and 170 ± 80 keV (1975TH08).

The nuclear charge radius derived using ΔM = 40728.28 ± 0.64 keV is reported as, for example, R_rms^charge = 2.427 ± 0.016 (measurement) ± 0.030 (reference) fm (2008SM03) and R_rms^charge = 2.48 ± 0.04 (2011NO11); also see (2006PU03, 2006SA52, 2010PU01).

1. 11Li(β-)11Be

Qm = 20.5513

Published values of the ¹¹Li half-life are 8.99 ± 0.10 ms (1997MO35), 8.2 ± 0.2 ms (1996MU19), 8.5 ± 0.2 ms (1974RO31) [8.8 ± 1.2 ms and 9.0 ± 0.8 ms are also independently measured in (1974RO31)], 8.83 ± 0.12 ms (1981BJ01), 7.7 ± 0.6 ms (1986CU01) and 8.5 ± 1.0 ms (1969KL08). An unpublished value of 8.4 ± 0.2 ms (1995RE1M) is referenced in (2003AU03). The weighted average of all measurements is 8.75 ± 0.07 ms; however, this value has poor overlap with measurements and indicates that uncertainties are significantly underestimated in some cases. We accept the (2003AU03) value, 8.75 ± 0.14 ms, which is obtained by enlarging the uncertainty of the weighted average.

The β-decay is complex and details reported for branching ratios are ambiguous. Most of the decay populates low-lying states in ¹¹Be (> 94%); all but ¹¹Be*(0, 0.32) are unstable with respect to neutron emission. At higher excitation energies the ¹¹Be states are also open to deuteron, triton, α and ⁶He emission. Measurements of γ-rays following ¹¹Li β-decay have provided new information on ¹⁰Be and ¹¹Be states; see ¹¹Be reaction 15 and (1996MU19, 1997AO01, 1997AO04, 1997BO01, 1997BO03, 1997MO35, 1997MU06, 2003FY01, 2004FY01, 2004HI24, 2004SA46, 2005HI03, 2008MA34, 2009MA54).

The β-delayed particle emission branching ratios appear to show discrepancies. Details are given in 11.14 (in PDF or PS). A summary suggests: β-1n = 86.3 ± 0.9 %, β-2n = 4.1 ± 0.4 %, β-3n = 1.9 ± 0.2 % and P_n = 100.3 ± 1.4 % using P_{¹¹Be*(0.32)} = 7.7 ± 0.8 % (2005HI03). A precise determination of the β-d branch for low-energy deuterons is complicated by contamination from β-delayed alpha particles though published values indicate β-d = (1.30 ± 0.13) × 10^-2 % (2008RA23); β-t = (0.93 ± 0.08) × 10^-2 % (2009MA72); β-α = 1.7 ± 0.3 %. A theoretical analysis suggests β-(p + n) ≈ 10^-10 which is roughly six orders of magnitude smaller than the β-d ratio (2010BA44).

Studies of β-decay to ¹¹Be states near E_x = 18.5 MeV (1996MU19, 1997BO03, 1997MU06, 2008MA34) have searched for β-delayed deuterons, which may indicate a deuteron-halo state in ¹¹Be that is analogous to the 2-neutron halo ground state of ¹¹Li; also see (1995OH02, 1995ZH31, 2004KU27, 2006BA73, 2011TU07). The β-decay shows retardation because of poor overlap of the initial and final wavefunctions (1995OT01). The s-wave and p-wave components in the 2-neutron valence wavefunctions are discussed in, for example, (1994SU12, 1996SU23, 1997BO01, 1997SU12, 2002SU16). See also (1997RI04, 2003SU04).

A β-NMR technique was used to measure the quadrupole moment of ¹¹Li (1992AR07). The measured quantity |Q(¹¹Li)/Q(⁹Li)| = 1.088 ± 0.015 implies Q(¹¹Li) = -33.3 ± 0.5 mb (2008NE11): negative sign assumed from theory. Also see Q(¹¹Li) = -31.2 ± 4.5 mb (1992AR07) and (1993NE08, 1994AR19).

2. 1H(11Li, 1H)

Scattering of ¹¹Li ions from ¹H has been measured at energies from 62 to 700 MeV/A (see 11.2 (in PDF or PS)). The elastic scattering distributions have been analyzed to evaluate possible signatures of the 2-neutron halo and to determine the nuclear size; R_rms^matter = 3.62 ± 0.19 fm (2002EG02) and 3.71 ± 0.20 fm (2006AL16, 2006DO02). At E(¹¹Li) = 75 MeV/A states are reported at 0, 1.25 ± 0.15, 3.0 ± 0.2, (4.90 ± 0.25), (6.40 ± 0.25), and 11.3 ± 0.35 MeV (1996KO02, 1997KO06). Evidence for an L = 1 state with E_x = 1.3 MeV (Γ = 0.75 ± 0.60 MeV) is analyzed in (1992FAZV, 1997KA19, 1997KA42, 1997KO11, 1998KA33, 1999KA68, 2001CR06, 2002CR06, 2004ER07); the state is attributed to either the soft-dipole resonance or 2-neutron removal threshold effects. See (1993HI04, 1994CH07, 1996CR06) for comments on the influence of the ⁹Li core. Also see (1991AL13, 1993AL06, 1993BE05, 1993KO44, 1993SU04, 1995SA33, 1996CR06, 1997CO11, 1997KN07, 1997KO12, 1998AN25, 1998DO16, 1999AL13, 1999CR02, 1999GR34, 2000GU19, 2000KA04, 2001CR02, 2001KI29, 2002CR02, 2002GU02, 2004AL09, 2004ER05, 2005KA21, 2005KI22, 2009HA04) and reaction ¹²C(¹¹Li, ¹¹Li).

3. 1H(11Li, 9Li)3H

Qm = 8.1127

Two techniques were used to determine the ¹H(¹¹Li, ⁹Li)³H reaction Q-value (2009RO04). First, a complete kinematic reconstruction of the ejectiles yielded Q = 8.123 ± 0.025 MeV. Second, two-body reaction recoil energy kinematics of ejectiles at 180° yielded Q = 8.106 ± 0.042 MeV. The weighted average is Q = 8.119 ± 0.022 MeV, which corresponds to S_2n = 363 ± 22 keV.

At E(¹¹Li) = 3 MeV/A the two halo neutron transfer reaction is studied by measuring the angular distribution of ⁹Li*(0, 2.69) (2008TA13); n-n correlations and the reaction mechanism are discussed. The population of ⁹Li*(2.69: J^π = 1/2^-) suggests a 1⁺ or 2⁺ configuration of the halo neutrons. Also see analysis of a proposed phonon mediated pairing interaction given in (2010PO08).

4. 10Be(14C, 13N)

Qm = -30.4465

At E(¹⁴C) = 334.4 MeV states corresponding to ¹¹Li*(0, 2.48 ± 0.07, 4.86 ± 0.07, 6.22 ± 0.08) were observed with Γ = 1.2 ± 0.2, < 0.1 and < 0.1 MeV for the excited states, respectively (1995BO15, 1995VO05).

5.	(a) 11B(π-, π+)		Qm = -32.0605
	(b) 11B(K-, K+)		Qm = -32.0605

The radius of ¹¹Li determined from pion double charge-exchange reactions on ¹¹B at T_π = 164 MeV is R_rms^neutrons = 3.28^+0.24_-0.33 fm, and the radius of the valence neutrons is R_rms^valence = 5.1^+0.6_-0.9 fm (1991GI06). See also (1992ES02, 1994LE06, 1995HA31). For reaction (b) see (1997YA11).

6.	(a) 12C(11Li, 11Li)
	(b) 28Si(11Li, 11Li)

At E(¹¹Li) = 246 MeV/A, analysis of a complete three-body kinematical measurement of ¹¹Li breakup on a ¹²C target indicates the reaction mechanism is ¹¹Li inelastic scattering to unbound states at E_x = 1.31 ± 0.05 and 2.52 ± 0.27 MeV with Γ = 0.26 ± 0.24 and 2.91 ± 0.72 MeV, respectively (2007SI24). Note: these excitation energies are adjusted upward by 70 keV to account for the change in the accepted ¹¹Li mass and 2n separation energy.

Elastic and quasi-elastic scattering of ¹¹Li ions on ¹²C and ²⁸Si have been reported at energies from 30 to 60 MeV. Optical model analysis of the angular distributions yield parameters that are significantly different from standard parameters (reaction (a): 1992KO14, 2003PE01) and (reaction (b): 1993LE14). The effects of refraction and absorption are analyzed in (1991SA02, 1992AL05, 1993ME02, 1995DA05, 1995EV03, 1996EV01, 1997MO42, 2000MO34).

See also (1991CA14, 1991TA21, 1992TA16, 1992VE03, 1993DA09, 1993SU04, 1993TH01, 1994AL02, 1994CA07, 1994HU04, 1994SA16, 1994SK04, 1995AL01, 1995AL02, 1995AN06, 1995CO01, 1995FA17, 1995FO08, 1995GA24, 1995HU08, 1995KH11, 1996CA01, 1996KN02, 1996RA18, 1996UE01, 1997CH32, 1997KN07, 1997MO24, 1998CH18, 1998MO27, 2000BO45, 2000PA14, 2006DL01, 2011IB02) and reaction 2.

7. 14C(π-, p + d)

Qm = 81.4370

The kinematic reconstruction of p + d pairs resulting from the capture of stopped π^- on ¹⁴C indicates population of ¹¹Li*(1.09 ± 0.07, 2.14 ± 0.12, 3.70 ± 0.13) (1998GO24) and ¹¹Li*(0.92 ± 0.15, 2.29 ± 0.25, 3.90 ± 0.25) with Γ ≈ 0.3, ≈ 0.7 and < 0.2 MeV, respectively (2010GU04). There was no evidence for breakup into ⁹Li_g.s.+ di-neutron; however in the high-excitation energy part of the spectrum the two neutron pairs corresponding to breakups via ⁹Li*(2.69, 4.31) were strongly correlated. Note: these excitation energies are adjusted upward by 70 keV to account for the change in the accepted ¹¹Li mass and 2n separation energy.

8. 14C(11B, 11Li)

Qm = -37.0481

At E(¹¹B) = 32 MeV the ground state of ¹¹Li was observed, and the Q-value for the reaction was measured as Q = -37.120 ± 0.035 MeV (1993YO07, 1995BE30). This corresponds to S_2n = 295 ± 35 keV. See also (1992PEZT, 1993BEZO).

9. 14C(14C, 17F)11Li

Qm = -36.6403

At E(¹⁴C) = 335.9 MeV states corresponding to ¹¹Li*(0, (1.2), 2.45 ± 0.10, 4.84 ± 0.10, 6.22 ± 0.10) were populated with Γ = 1.2 ± 0.3, < 0.1 and < 0.1 MeV for the states above 2 MeV, respectively (1995BO15, 1995VO05). These states were tentatively interpreted as excitations of the ⁹Li core (1995VO05).

10.	(a) 1H, 9Be, 12C, 27Al, Si, Pb(11Li, n)
	(b) 1H, 2H, 9Be, 12C, 27Al, Si, Ni, 93Nb, 181Ta, 197Au, Pb(11Li, 9Li)
	(c) 1H, 2H, 9Be, 12C, 27Al, Cu, Pb(11Li, 2n + 9Li)

Studies of ¹¹Li have been carried out via measurement of interaction cross sections and breakup reaction cross sections, see 11.3 (in PDF or PS). The anomalously large cross sections observed in the studies can be related to the extent of the valence neutrons by various reaction models (1990HA15, 1990LI39, 1991MU19, 1992TA15, 1994BEZX, 1996AL13, 1996WA27, 2000EV03, 2001EG02, 2001OZ04). Also see (1990BE29, 1992OG02, 1992SA11, 1992SO15, 1992YA02, 1993BA71, 1993MA25, 1995PE19, 1996AL24, 1996BU09, 1997ES07, 1997FO04, 1997TO04, 1997ZA08, 1998BE09, 1998KN03, 1999KA67, 1999KN04, 2000GA20, 2000GA31, 2001BH02, 2003KH10, 2004CA18, 2004LO12). Measurements of the parallel and transverse momentum distributions of outgoing fragments can be related to the extent of the neutron spatial distribution using the uncertainty principle, but details of the reaction mechanism and final state interactions influence the measurements (1990UT01, 1992TA15, 1992ZH05, 1993BE45, 1993KO11, 1993LEZR, 1994JO04, 1995HA17, 1995OR02, 1995ZH13, 1995ZI03, 1996GA09, 1997GA04, 1997GA10, 1997OR03, 1997ZI04, 2001AX01, 2007SI24). Also see (1991ZH11, 1992BE43, 1992SH09, 1993ES02, 1993RO16, 1995BA32, 1995ES01, 1995OG04, 1996BA68, 2000BA47, 2001GA09, 2004BE45, 2009SH25). The measurements appear consistent with an R_rms^matter size that is greater than 3 fm and a valence neutron "halo" that extends to R_rms^valence = 5 fm or more. Comments on the interference effects between nuclear and Coulomb breakup components are given in (1990BE29, 1993BA71, 1993EV02, 1993IV01, 1996DA03, 2000GA31).

The complete kinematical detection of ⁹Li + 2n following Coulomb breakup on high-Z targets permits a determination of the dipole strength distribution in the region just above the neutron binding threshold. At E(¹¹Li) = 280 MeV/A (1997ZI04) the excitation strength function was decomposed into two Gaussian components; analysis indicates peaks at E_x = 1.1 ± 0.1 and 2.5 ± 0.2 MeV with Γ = 0.7 ± 0.2 and 2.1 ± 0.6 MeV, respectively. Note: these excitation energies are adjusted upward by 70 keV to account for the change in the accepted ¹¹Li mass and 2n separation energy. The two peaks, which were assumed to have E1 character, carried 1.2 ± 0.3 % and 7 ± 2 % of the energy weighted sum rule. At E(¹¹Li) = 30 MeV/A (1996GA08, 1996IE01) report that the dipole strength function is consistent with a resonance at E_res = 0.7 MeV, Γ = 0.8 MeV, but considering Coulomb re-acceleration of the charged particles in the Z-field of the target indicates a negligible lifetime for any resonant state and presents a contradiction to any soft-dipole resonant state behavior (2001GA22). A measurement of the relative three-body breakup energy at E(¹¹Li) ≈ 70 MeV/A (2005NA40, 2006NA21, 2006NA39, 2007NA22) showed significant strength at low energies, peaking at E_rel ≈ 0.3 MeV. This corresponds to E_x ≈ 0.6 MeV with Γ ≈ 0.6 MeV; the corresponding strength for 0 ≤ E_x ≤ 3 MeV is B(E1) = 1.42 ± 0.18 e² ⋅ fm². Also see (1990BE04, 1990BE07, 1990HA33, 1990HO26, 1990SA41, 1990SU16, 1991FA09, 1991HO06, 1991HU03, 1991TE01, 1992BA63, 1992BE40, 1992CA20, 1992ES01, 1992SA10, 1992SU06, 1993BE05, 1993CA34, 1993GO17, 1993ZHZV, 1994TY02, 1995FU12, 1995RO13, 1996CA12, 1997BO08, 1998CO11, 1998CO22, 1999GA08, 1999KA68, 2000FO09, 2001FI14, 2001SA79, 2003MY03, 2003MY04, 2004ER05, 2004ER07, 2004MY01, 2006AO02, 2007BE58, 2007ES04, 2007MY04, 2007SA41, 2009HA30).

A simple interpretation of ¹¹Li describes the system as a ⁹Li core with the two valence neutrons bound in a simple potential (1987HA30). In a search for possible evidence of a di-neutron, the data have been evaluated with an emphasis on measurements of the correlation of the valence neutrons after breakup (1992BE40, 1995IS04, 1995SH14, 1996CH38, 1996GA08, 1996IE01, 1997KU07, 1997LU08, 1997ZI04, 1998GA37, 2000EV03, 2000EV04, 2000MA12, 2001DA17, 2004PE01, 2004PE08, 2004WE05, 2009HA37, 2010HA10). Further studies emphasize the determination of s-wave to p-wave contributions for the valence neutron wavefunctions and implications for ¹⁰Li (1992ZH05, 1995ZI03, 1996GA09, 1997GA04, 1999SI08, 2003MY03, 2004MY01). Overviews of the experimental work can be found in (1993KO11, 1994JO04, 1997OR03, 2001EG02, 2001OZ04, 2002BR01, 2002WA08). In all cases the emitted neutron pairs show no strong correlations, arguing against the existence of a di-neutron in ¹¹Li.