A combination of London penetration depth and artificial disorder was used to probe the energy gap structure and symmetry of a few members of the iron-based superconductor systems. Information regarding the gap structure and symmetry is an important clue which helps uncover the mechanism behind the unconventional, non-BCS-type superconductors. We used the tunnel-diode resonator method to do London penetration depth measurements with high precision down to 50~mK base temperature. The disorder is introduced by electron irradiation, which was performed at the SIRIUS facility in Ecole Polytechnique (Palaiseau France) to produce point-like disorder in the materials of study. Non-magnetic defects induced by the irradiation influence each material differently depending on its underlying susceptibility to impurity scattering. The response to irradiation provides another key clue about the gap structure and symmetry of iron-based superconductors. This dissertation describes the details of the experimental work on 16 samples from the Ba$_{1-x}$K$_x$Fe$_2$As$_2$ system across the superconducting dome, with the results can be explained coherently with $s_{\pm}$-pairing symmetry. The same gap symmetry was also found in the CaKFe$_4$As$_4$ system. We found that this series is remarkably similar to the Ba$_{1-x}$K$_x$Fe$_2$As$_2$ system in many ways, consistent with other reports in literature. London penetration depth measurements and electron irradiation were also performed on FeSe, which is a unique system in the iron-based superconductor family. Surprisingly, $T_c$ in FeSe was \textit{enhanced} by irradiation which paints a different picture of superconductivity compared to Ba$_{1-x}$K$_x$Fe$_2$As$_2$ and CaK(Fe$_{1-x}$Ni$_x$)$_4$As$_4$. However, the FeSe experimental data could still be explained within the (extended) $s_{\pm}$ paradigm. In conclusion, we found a strong evidence supporting the $s_{\pm}$ pairing symmetry which manifested into different gap structures in several representative systems in the iron-based superconductors family.