Observations have recently been made of ion cyclotron emission (ICE) that originates from the core plasma in the DIII-D [1, 2] and ASDEX-Upgrade [3, 4] tokamaks. The ICE spectral peaks correspond to the local cyclotron harmonic frequencies of fusion-born ions close to the magnetic axis, in contrast to the hitherto usual spatial localisation of the ICE source to the outer midplane edge in tokamak and stellarator plasmas. Core ICE is temporally transient, and may be caused by the rapid onset and increase of local fusion reactivity. This would give rise to a highly non-Maxwellian population of fusion-born ions near their birth energy, forming a thin spherical shell in velocity-space. This shell distribution would be transient; collisional effects acting on longer timescales would drive the fusion-born ions towards a standard slowing-down distribution. For as long as it persists, as pointed out in Ref., the shell might drive the magnetoacoustic cyclotron instability (MCI), which is the excitation process which underlies ICE. Here we present, under core plasma conditions, the first direct numerical simulations of ICE generation by a spherical shell distribution of fusion-born ions in velocity-space. These energetic minority ions are found to relax collectively in particle-in-cell (PIC) computations which follow their self- consistent gyro-orbit-resolved dynamics, together with that of the majority thermal ions and electrons, under the Maxwell-Lorentz system of equations. We relate the computational outputs, which extend into the nonlinearly saturated regime of the MCI, to the analytical theory of the linear MCI for shell-type energetic ion distributions, and to fully nonlinear simulations of related ring-beam energetic ion distributions relaxing under the MCI. We conclude that in future simulations for ICE interpretation, ring- beam distributions may provide an acceptable proxy for shell distributions, while using significantly fewer computational particles and still maintaining a satisfactory signal- to-noise ratio.