After the foundations of quantum field theory in the early 1930s, J. Schwinger showed that the spontaneous creation of electron-positron pairs by a constant uniform electric field when its strength is greater than a critical value, E_cr ≈ 1.3 × 10¹⁶ V/cm, which is also referred as neutral vaccum decay. Although many experimental efforts have been made, up to now, no evidence of spontaneous creation of electron-positron pairs is observed via measurements of the pair creation. With the upcoming new large scale scientific facilities, such as HIAF in China, FAiR in Germany, and NICA in Russia, new opportunities to observe the spontaneous e+e- pair creation in on the horizon. As the upgrade of HIAF will provide a merged beam experiment, which will allow two bare ion beams to collide with cm energy of around 6 MeV/u, backgrounds that mainly hindered the observation of e+e- pair creation in previous studies will be greatly suppressed. The latest development of theory shows that the spontaneous pair production can be observed via measurements of the pair-production probabilities for a given set of nuclear trajectories, further, a clear signature of the transition from the subcritical to the supercritical regime is revealed in the positron energy spectra. These new features are of importance to design e+e- spectrometers at HIAF.

Electron-positron pair creation mechanisms. Left, the low-lying energy levels of a H-like ion as functions of the nuclear charge number Z., the 1s vacancy will dive into Dirac sea when the atomic number of united atoms Z_UA is greater than 173. Right, the low-lying energy levels of a quasimolecule formed in collision of two uranium ions (nuclei) as functions of time. The arrows a, b, and c denote different dynamical pair-creation mechanisms and the arrow d indicates the spontaneous pair creation.