We present an experimental investigation of thermal phase transition in atomic nuclei by measuring the γ rays from the decay of the giant dipole resonance in Tm169 populated by using the reaction He4+Ho165. The systematic measurement confirms the prolate shape, similar to the ground-state value, till temperature T=1.23 MeV. Moreover, the present data, together with the previous experimental studies, point towards the persistence of prolate shape with deformation similar to that of the ground state till T=1.5 MeV. In addition, the emergence and evolution of thermal fluctuations observed directly in the experiment suggest that the sharp phase transition from prolate to near spherical at T≈1.7 MeV will not be evident experimentally due to statistical fluctuations owing to the finite size of the nucleus.

• Received 6 November 2018
• Revised 9 January 2019

DOI:https://doi.org/10.1103/PhysRevC.99.024315