(See Energy Level Diagrams for 11Be)
Mass of 11Be: The mass excess of 11Be is given as 20.181 ± 0.015 MeV (1965MA54) [based on (1962PU01, 1965RY01)] and as 20.174 ± 0.007 MeV (1970GO11). (1971WA1E) adopt 20.177 ± 0.006 MeV, and so do we. See also (1974TH01).
The decay proceeds to 11B*(0, 2.12, 5.02, 6.79, 7.98, 9.87) with Jπ = 3/2-, 1/2-, 3/2-, 1/2+, 3/2+ and 3/2+, respectively: see Table 11.16 (in PDF or PS) (1971AL07). The half-life is 13.81 ± 0.08 sec (1970AL21). See also (1967FL16, 1968AJ02). The nature of the decay indicates J = 1/2 or 3/2, even parity for the ground state of 11Be: see (1971AL07).
Proton groups have been observed to many states of 11Be by (1962PU01: Et = 14 MeV) and (1972AJ01: Et = 20 MeV): see Table 11.2 (in PDF or PS). The first excited state has an excitation energy of 319.8 ± 0.2 keV and τm = 0.18 ± 0.06 psec, corresponding to a very strong E1 transition of 0.33 W.u. (1971HA25). The Jπ of 11Be*(0.32) is 1/2-, as determined by a study of the yield of 320 keV γ-rays as a function of time in μ- capture by 11B (1968DE20). The strength of the E1 transition fixes Jπ of 11Beg.s. to be 1/2+ or 3/2+, using the parity information obtained from the nature of the β- decay of the ground state [see reaction 1]. Jπ limits for 11Be*(1.79, 2.69, 3.41) are derived from angular distribution studies (1962PU01, 1964HI08). 11Be*(5.25, 6.72, 8.84) are strongly populated at Et = 20 MeV indicating that these states have a large overlap with 9Beg.s. + two neutrons (1972AJ01). See also (1968AJ02) and (1970AL21, 1971AL07).
See 15C in (1970AJ04).
The capture cross section is < 1 mb (E.T. Jurney, private communication).
Angular distributions of the p0 and p1 groups have been measured at Ed = 6 MeV (1970GO11: p1 only) and 12 MeV (1970AU02): ln = 0 [and therefore Jπ = 1/2+ for 11Be(0)] and 1, S = 0.73 ± 0.06 and 0.63 ± 0.15, respectively (1970AU02). Ex = 318 ± 7 keV for 11Be*(0.32) (1970GO11). See also (1973GO09) and 12B.