Evidence for two-particle core polarization in the isomeric 29/2+ and 21/2− states of the <img src="/fulltext-image.asp?format=htmlnonpaginated&src=W5116687R4N86263_html\10050_2005_Article_BF01410012_TeX2GIFIE1.gif" border="0" a

Usingα-particles in the energy range 42–51 MeV, the²⁰⁹Bi(α, 4n)²⁰⁹At reaction was studied with the intention of investigating the properties of excited high spin states in²⁰⁹At. In-beam prompt and delayed gamma-ray and conversion electron spectroscopy reveal two isomeric states at 2429 and 1428 keV having the half-lives of 875±100 ns and 25±1 ns, respectively. The spins and parities of these states are found to be 29/2⁺ and 21/2⁻, respectively. Theg-factors of the isomeric states are 1.06±0.02 and 0.88±0.06, which, within the experimental errors, are the same as theg-factors of the corresponding states in²¹¹At. This suggests that the isomeric states are due to the <img src="/fulltext-image.asp?format=htmlnonpaginated&src=W5116687R4N86263_html\10050_2005_Article_BF01410012_TeX2GIFIE3.gif" border="0" alt=" $$\pi \left( {h_{{9 \mathord{\left/ {\vphantom {9 2}} \right. \kern-\nulldelimiterspace} 2}}^2 } \right)_{8^ + } i_{1 {3 \mathord{\left/ {\vphantom {3 2}} \right. \kern-\nulldelimiterspace} 2}} v\left( {j^{ - 2} } \right)_{0^ + } $$ " /> and <img src="/fulltext-image.asp?format=htmlnonpaginated&src=W5116687R4N86263_html\10050_2005_Article_BF01410012_TeX2GIFIE5.gif" border="0" alt=" $$\pi \left( {h_{{9 \mathord{\left/ {\vphantom {9 2}} \right. \kern-\nulldelimiterspace} 2}}^3 } \right)v\left( {j^{ - 2} } \right)_{0^ + } $$ " /> configurations, respectively. The deviation between the calculated and experimentally found excitation energies of the isomeric levels are −146±26 and +72 ±26 keV. This deviation which in cases of a²⁰⁸Pb-core usually is smaller than 15 keV, is likely to be due to the neglect of the interaction between two particles via the exchange of virtual 2⁺ phonons in the²⁰⁶Pb core. This two-particle polarization interaction is found to be approximately described by aP₂-force.