  In the present paper we treat the second-forbidden non-unique (2nd-nu)
ground-state-to-ground-state $\beta^-$ decay
$^{99}\textrm{Tc}(9/2^+)\to\,^{99}\textrm{Ru}(5/2^+)$, with a 100$\%$ branching
ratio, within the framework of the nuclear shell model (NSM). The energy
spectrum of the electrons emitted in this $\beta$-decay transition
($\beta$-electron spectrum) is sensitive to the wave functions of the involved
initial ($9/2^+$) and final ($5/2^+$) nuclear ground states through the many
involved nuclear matrix elements (NME). The $\beta$-electron spectrum of this
transition is potentially indicative of the effective value, $g_{\rm A}^{\rm
eff}$, of the weak axial coupling, $g_{\rm A}$, of crucial importance for
extraction of information on beyond-the-standard-model physics from the results
of the present and future rare-events experiments. We describe the $\beta$
spectral shape of this decay by using a state-of-the-art $\beta$-decay
formalism and compute the many involved NME using the well established NSM
Hamiltonians $jj45pnb$ and $glekpn$. We have found a strong dependence of the
$\beta$ spectral shape on the value of $g_{\rm A}$ making it a good candidate
for determination of the value of $g_{\rm A}^{\rm eff}$ through comparison with
the corresponding experimental $\beta$ spectral shape. We have also found an
interesting dependence of the $\beta$ spectral shape on the value of the
so-called small relativistic vector NME, sNME, used to match the computed
half-life with the measured one.
