Efficient production of $^{229m}$Th via nuclear excitation by electron capture

Abstract: The nuclear isomeric state $<sup>{229m}$Th with an exceptionally low excitation energy makes the $<sup>{229}$Th isotope a crucial candidate for nuclear clocks and many other applications. Efficient and controllable production of $<sup>{229m}$Th is essential and still remains a challenge. Here we report a novel approach for efficient production of $<sup>{229m}$Th by the excitation of $<sup>{229}$Th to the above-lying excited state at $29.19$ keV energy via the process of nuclear excitation by electron capture (NEEC). We show theoretically that the production rate of $<sup>{229m}$Th per nucleus with accessible conditions can be six orders of magnitude larger than the value experimentally demonstrated using $29$-keV synchrotron radiation for this indirect excitation. With the efficient production of $<sup>{229m}$Th, our results identify scenarios, as well as the characteristic NEEC signature with which NEEC events could be unambiguously identified, for a clear experimental identification of the long-sought NEEC phenomenon.