Electron-positron angular correlations were measured for the isovector magnetic dipole 17.6 MeV ($J^{\pi }=1^{+}$, $T=1$) $\text{state}\to \text{ground}$ state ($J^{\pi }=0^{+}$, $T=0$) and the isoscalar magnetic dipole 18.15 MeV ($J^{\pi }=1^{+}$, $T=0$) $\text{state}\to \text{ground}$ state transitions in ${}^8\mathrm{Be}$. Significant enhancement relative to the internal pair creation was observed at large angles in the angular correlation for the isoscalar transition with a confidence level of $>5\sigma$. This observation could possibly be due to nuclear reaction interference effects or might indicate that, in an intermediate step, a neutral isoscalar particle with a mass of $16.70\pm 0.35(\text{stat})\pm 0.5(\text{syst})\mathrm{MeV}/c^2$ and $J^{\pi }=1^{+}$ was created.

Angular correlation spectra of $e^{+}{e}^{-}$ pairs produced in the ${}^3\mathrm{H}(p,e^{+}{e}^{-}){}^4\mathrm{He}$ nuclear reaction have been studied at $E_p=510$, 610, and 900 keV proton energies. The main features of the spectra can be understood by taking into account the internal and external pair creations following the proton capture by ${}^3\mathrm{H}$. However, these processes cannot account for an observed peak around ${115}^{\circ }$ in the angular correlation spectra. This anomalous excess of $e^{+}{e}^{-}$ pairs can be described by the creation and subsequent decay of a light particle during the direct capture process. The derived mass of the particle is $m_{\mathrm{X}}{c}^2=16.94\pm 0.12\left(\mathrm{stat}\right)\pm 0.21\left(\mathrm{syst}\right)$ MeV. According to the mass this is likely the same X17 particle, which we recently suggested [ Phys. Rev. Lett. 116, 042501 (2016)] for describing the anomaly observed in the decay of ${}^8\mathrm{Be}$.

Employing the ${}^{11}\mathrm{B}(p,\gamma ){}^{12}\mathrm{C}$ nuclear reaction, the angular correlation of $e^{+}{e}^{-}$ pairs was investigated in the angular range of ${40}^{\circ }\le \mathrm{\Theta }\le {175}^{\circ }$ for five different proton energies between $E_p=1.50$–2.5 MeV. At small angles ($\mathrm{\Theta }\le {120}^{\circ }$), the results can be well interpreted by the internal pair creation process of electromagnetic radiations with $E1$ and $M1$ multipolarities and by the external pair creation in the target backing. However, at angles greater than ${120}^{\circ }$, additional count excesses and anomalies were observed, which could be well accounted for by the existence of the previously suggested hypothetical X17 particle. Our results suggest that the X17 particle was generated mainly in $E1$ radiation. The derived mass of the particle is $m_{\mathrm{X}}{c}^2=17.03\pm 0.11\left(\mathrm{stat}\right)\pm 0.20\left(\mathrm{syst}\right)$ MeV. According to the mass, and to the derived branching ratio [$B_x=3.6\left(3\right)\times {10}^{-6}$], this is likely the same X17 particle that we recently suggested for describing the anomaly observed in the decay of ${}^8\mathrm{Be}$ and ${}^4\mathrm{He}$.

The photofission cross section of ${}^{238}$U was measured at sub-barrier energies as a function of the $\gamma$-ray energy using a monochromatic, high-brilliance, Compton-backscattered $\gamma$-ray beam. The experiment was performed at the High Intensity $\gamma$-ray Source (HI$\gamma$S) facility at beam energies between $E_{\gamma }=4.7$ MeV and 6.0 MeV and with $\sim 3\%$ energy resolution. Indications of transmission resonances have been observed at $\gamma$-ray beam energies of $E_{\gamma }=5.1$ MeV and 5.6 MeV with moderate amplitudes. The triple-humped fission barrier parameters of ${}^{238}$U have been determined by fitting empire-3.1 nuclear reaction code calculations to the experimental photofission cross section.

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