
^{11}Be (1975AJ02)(See Energy Level Diagrams for ^{11}Be) GENERAL: See also (1968AJ02) and Table 11.1 [Table of Energy Levels] (in PDF or PS). Model calculations: (1970TA1J, 1973SA30, 1973SP02, 1974IR04). Special reactions: (1969AR13, 1971AR02, 1972VO06, 1973BA81, 1973KO1D, 1973WI15). Muon capture (See also reaction 2.): (1967DE1E, 1968DE20, 1969BE41, 1970VA24, 1971BE57, 1973MU1B). Pion reactions: (1962DY1A, 1971BE85). Mass of ^{11}Be: The mass excess of ^{11}Be is given as 20.181 ± 0.015 MeV (1965MA54) [based on (1962PU01, 1965RY01)] and as 20.174 ± 0.007 MeV (1970GO11). (1971WA37) adopt 20.177 ± 0.006 MeV, and so do we. See also (1974TH01).
The decay proceeds to ^{11}B*(0, 2.12, 5.02, 6.79, 7.98, 9.87) with J^{π} = 3/2^{}, 1/2^{}, 3/2^{}, 1/2^{+}, 3/2^{+} and 3/2^{+}, respectively: see Table 11.16 (in PDF or PS) (1971AL07). The halflife 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 ^{11}Be: see (1971AL07).
Proton groups have been observed to many states of ^{11}Be by (1962PU01: E_{t} = 14 MeV) and (1972AJ01: E_{t} = 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 τ_{m} = 0.18 ± 0.06 psec, corresponding to a very strong E1 transition of 0.33 W.u. (1971HA25). The J^{π} of ^{11}Be*(0.32) is 1/2^{}, as determined by a study of the yield of 320 keV γrays as a function of time in μ^{} capture by ^{11}B (1968DE20). The strength of the E1 transition fixes J^{π} of ^{11}Be_{g.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]. J^{π} limits for ^{11}Be*(1.79, 2.69, 3.41) are derived from angular distribution studies (1962PU01, 1964HI08). ^{11}Be*(5.25, 6.72, 8.84) are strongly populated at E_{t} = 20 MeV indicating that these states have a large overlap with ^{9}Be_{g.s.} + two neutrons (1972AJ01). See also (1968AJ02) and (1970AL21, 1971AL07).
See ^{15}C in (1970AJ04).
The capture cross section is < 1 mb (E.T. Jurney, private communication).
Angular distributions of the p_{0} and p_{1} groups have been measured at E_{d} = 6 MeV (1970GO11: p_{1} only) and 12 MeV (1970AU02): l_{n} = 0 [and therefore J^{π} = 1/2^{+} for ^{11}Be(0)] and 1, S = 0.73 ± 0.06 and 0.63 ± 0.15, respectively (1970AU02). E_{x} = 318 ± 7 keV for ^{11}Be*(0.32) (1970GO11). See also (1973GO09) and ^{12}B.
See ^{11}Li.
See (1974BA15).
