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In 2016, the announcement of an anomaly in the Internal Pair Creation (IPC) in the isoscalar magnetic dipole transition in $^8$Be [1] triggered an effort worldwide to investigate this phenomenon. According to the model of Rose for the IPC process [2,3], the $e^+e^-$ angular correlation distribution drops quickly with the $e^+e^-$ relative angle. In contrast, a peak-like behavior was observed at around 140$^{\circ}$. This result was interpreted as the creation and subsequent decay of a previously unknown neutral boson, named X(17), with a mass of $m_o c^2 = 16.7 \pm 0.35(stat)$~MeV.
At the Laboratori Nazionale di Legnaro (LNL-INFN), a new $e^+e^-$ pair spectrometer was built and commissioned. The EJ2000 (polyvinyl toluene) scintillator material was used in its construction. The project aims to measure the angular correlation distribution of the $e^+e^-$ pairs from the IPC process in light nuclei. In 2023 and 2024, the first experimental campaign occurred at the AN2000 accelerator facility at LNL-INFN. LiF targets were irradiated to study the proton-induced nuclear reactions $^7$Li($p$,$e^+e^-$)$^8$Be and $^{19}$F($p$,$\alpha e^+e^-$)$^{16}$O. The $1^+_1 \rightarrow 0^+_1$ and $1^+_2 \rightarrow 0^+_1$ electromagnetic transitions in $^8$Be were been studied. The interest in those transitions lies in that anomalies have been reported [1,4]. In addition, the $0_2^+ \rightarrow 0^+_1$ electromagnetic transition has been taken as a reference due to the high $e^+e^-$ emission. This work reports the first results of this experimental campaign, providing a panorama of the new opportunities for studying the IPC process using this new $e^+e^-$ pair spectrometer.
[1] F.W.N. de Boer et al, Phys. Let. B
[2] E. Rose, Phys. Rev. 76 (1949) 678.
[3] E. Rose, Phys. Rev. 78 (1950) 184.
[4] J. Krasznahorkay et al., Phys. Rev. Let. 116 (2016) 7.
