<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si1.gif" overflow="scroll"><mml:mn>2</mml:mn><mml:mi>α</mml:mi><mml:mo>+</mml:mo><mml:mi>t</mml:mi></mml:math> cluster structure in 11B

The cluster structures of the excited states in 11B are studied by analyzing the isoscalar monopole and quadrupole strengths in the B11(d,d′) reaction at Ed=200MeV. The excitation strengths are compared with the predictions by the shell-model and antisymmetrized molecular-dynamics (AMD) calculations. The large monopole strength for the 3/23− state at Ex=8.56MeV is well described by the AMD calculation and is suggested to be an evidence for a well developed 2α+t cluster structure.