Experimental evidence for low-lying quadrupole isovector excitation of $$^{208}$$Po

Abstract

We present the results from an experiment dedicated to measure the lifetime of the 2 + 2 state, candidate for the one-phonon mixed-symmetry state, of 208 Po. This nucleus was studied in the α-transfer reaction 204 Pb( 12 C, 8 Be) 208 Po and the lifetime of the 2 + 2 state was determined by utilizing the Doppler-shift attenuation method. The experimental data show that the 2 + 2 state decays with a sizable M 1 transition to the 2 + 1 state revealing its isovector nature. Keywords Lifetimes • Electromagnetic transitions • Transfer reactions • 190≤ A ≤219

Microscopic structure of the one-phonon 2+ states of Po208

“…Indeed, the data from Ref. [17] show that 38.0(3)% from the feeding of the 2 + 1 state is coming from the 4 + 1 → 2 + 1 transition which is in agreement with the data from the present experiment where this feeding is determined to be 40(4)%. Since the decay of the 4 + 1 state is the only well visible feeder to the 2 + 1 in the present data, the feeding states for the states of interests are taken from Ref.…”

“…The depicted levels are those that have been populated in our previous study, see Ref. [17]. In the present data, however, only the most intensive transitions can clearly be observed [cf.…”

“…In this respect, the relatively low B(M 1; 2 + 1,ms → 2 + 1 ), compared to the ones in the cases of single-isolated MSSs, observed in 212 Po [14] can be attributed to the low quadrupole collectivity in this nucleus [25]. In 208 Po which is the neutron particle-hole mirror of 212 Po, a similar low B(M 1) strength between the 2 + 2 and the 2 + 1 states has been observed [17]. On this basis the 2 + 2 state of 208 Po has been assumed to be a fragment of the one-phonon mixed-symmetry state.…”

“…1(b)]. In order to estimate the relative contributions of the feeding transitions to the intensities of the 686-keV and 660-keV lines we used the data from the previous measurement [17] which has utilized also the αtransfer reaction 204 Pb( 12 C, 8 Be) 208 Po at a beam energy of 62 MeV. As a result, we can expect similar relative population of the excited states of 208 Po.…”

“…Several MSSs are also present in the mass A ≈ 130 region [6,[8][9][10][11][12][13]. Recently, a few examples of MSSs have been found in weakly collective nuclei around the doubly-magic core 208 Pb [14][15][16][17].…”