  The nuclear reactions in a plasma system with energy distribution deviated
from Maxwellian are proved to have some unique characteristics including those
in their product energy spectrum. Based on this, a new nuclear diagnostic
scheme for measuring the nonthermal ion energy spectrum in high-energy-density
plasmas is proposed, where the effective temperature and spectral peak of the
nonthermal ion energy spectrum are uniquely determined by the spectral width
and spectral peak of the product energy spectrum. Then, taking the laser-driven
magnetic reconnection experiment as an example, according to the
particle-in-cell (PIC) simulation results coupled with the proton-boron (pB)
fusion reaction, the pB reaction rate is increased by 4 orders of magnitude;
meanwhile, the product spectral peak and spectral width are shifted towards
higher energy, deviating from the predictions of the thermal equilibrium
assumption, both of which are on account of the accelerated and heated
suprathermal protons in magnetic reconnection. Furthermore, analyzing a series
of simulation results, the new product spectrum analysis approach suitable for
the parameter range of laser-driven magnetic reconnection is found.
