reaction cross-section, \(\sigma_{\text{r}}\)

https://doi.org/10.1351/goldbook.R05169
A quantity used in collision theories of reactions in order to interpret calculated or experimental rates. In some collision theories the reaction cross-section is considered to be related to \(b_{\max }\), the maximum value of the impact parameter that allows reaction to occur, by: \[\sigma _{\text{r}}=P_{\text{r}}\ \pi \ b_{\max }^{2}\] where \(P_{\text{r}}\) is the reaction probability.
See: collision cross-section, impact parameter
Source:
PAC, 1996, 68, 149. 'A glossary of terms used in chemical kinetics, including reaction dynamics (IUPAC Recommendations 1996)' on page 183 (https://doi.org/10.1351/pac199668010149)