An earlier measurement on the 4+ to 2 + radiative transition in 8 Be provided the first electromagnetic signature of its dumbbell-like shape. However, the large uncertainty in the measured cross section does not allow a stringent test of nuclear structure models. The present paper reports a more elaborate and precise measurement for this transition, via the radiative capture in the 4 He+ 4 He reaction, improving the accuracy by about a factor of three. The ab initio calculations of the radiative transition strength with improved three-nucleon forces are also presented. The experimental results are compared with the predictions of the alpha cluster model and ab initio calculations.PACS numbers: 21.60. De, 23.20.Js, 24.30.Gd, 27.20.+n The nucleus 8 Be is a classic example of the occurrence of alpha clustering [1] in nuclei. Its formation from two alpha particles provides an intermediate step in the synthesis of 12 C [2] from the fusion of three alpha particles inside the stars. The nucleus is also the stepping stone to understand alpha-clustering in heavier self-conjugate 4n nuclei. The dumbbell-shaped nucleus exhibits rotational states manifested as resonances in the alpha-alpha scattering system. The electromagnetic transition between the excited resonant states in 8 Be, with spin-parities of 4 + and 2 + , was reported earlier [3] in order to provide a test for its alpha cluster structure. The measurements were made at two beam energies, on and off the 4 + resonance, by detecting the transition gamma rays in coincidence with the two alpha particles arising from the decay of the 2 + final state. However, the measured cross section (with an uncertainty of ∼33%) and the inferred reduced electromagnetic transition rate were not precise enough to provide a stringent test for various models like the cluster model [4] and ab initio quantum Monte Carlo model [5]. The uncertainty arose mainly due to the large background of 4.44 MeV gamma rays originating from the interaction of the incident beam with the window of the chamber holding the helium gas target. The present work, using essentially the same method, is aimed at a more accurate measurement and also at more beam energies straddling the 4 + resonance. The essential aspects in this improved measurement are a better pixelisation of the alpha particle detectors, a more efficient and segmented gamma ray detector and a better shielding of the gamma rays from the beam-window interaction mentioned above.The experiment was carried out using beams of 4 He from the BARC-TIFR Pelletron Linac Facility at TIFR, Mumbai at energies of 19−29 MeV. The beam current was about 1 pnA on the target. The schematic of the experimental setup is shown in Fig. 1. The γ-rays were detected in a BGO detector array with a photopeak efficiency of about 23% at E γ =8 MeV. The array consisted of 38 hexagonal cross section detectors, of length 76 mm and a face to face distance of 56 and 58 mm (in two groups), encased in thin aluminum housing. These were mounted in close packed groups of 19 each p...

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