  The effect of electron irradiation with the energy of 2.5 MeV on the
temperature dependences of the resistivity of an optimally doped
$YBa_2Cu_3O_{7-\delta}$ single crystal has been studied. The temperature
dependences of both fluctuation conductivity $\sigma^\prime(T)$ and the
pseudogap $\Delta^*(T)$ on irradiation dose $\phi$ have been calculated within
the local pair model. Here we show that with an increase in $\phi$, the value
of $\rho(300 K)$ increases linearly, while $T_c$ decreases linearly.
Concurrently, the value of $\rho{100 K}$ increases nonlinearly, demonstrating a
feature for $\phi_3$ = 4.3 10$^{18}$ e/$cm^2$, which is also observed in the
number of other dose-dependent parameters.Regardless of the irradiation dose,
in the temperature range from $T_c$ up to $T_{01}$, $\sigma^\prime(T)$ obeys
the classical fluctuation theories of Aslamazov$-$Larkin and Maki$-$Thompson,
demonstrating 3D$-$2D crossover with increasing temperature. The crossover
temperature $T_0$ makes it possible to determine the coherence length along the
$c$-axis, $\xi_c(T)$, which increases by $\approx 3$ times under irradiation.
Furthermore, the range of superconducting fluctuations above $T_c$ also
noticeably increases.At $\phi$ = 0, the dependence $\Delta^*(T)$ typical for
single crystals containing pronounced twin boundaries is observed with a
maximum at $T_{pair}$ ~ 120 K and a distinct minimum at $T = T_{01}$. It was
determined for the first time that at $\phi_3$ = 4.3 10$^{18}$ the shape of
$\Delta^*(T)$ changes strongly and becomes the same as in optimally doped
$YBa_2Cu_3O_{7-\delta}$ single crystals with a very low pseudogap opening
temperature $T^*$ and noticeably reduced $T_{pair}$, while at $T_c(\phi)$ there
are no singularities.
