Energetic-particle induced soft errors in on-chip cache memories have become a major challenge in designing new generation reliable microprocessors. Uniformly applying conventional protection schemes such as error correcting codes (ECC) to SRAM caches may not be practical where performance, power, and die area are highly constrained, especially for embedded systems. In this paper, we propose to analyze the lifetime behavior of the data cache to identify its temporal vulnerability. For this vulnerability analysis, we develop a new lifetime model. Based on the new lifetime model, we evaluate the effectiveness of several existing schemes in reducing the vulnerability of the data cache. Furthermore, we propose to periodically invalidate clean cache lines to reduce the probability of errors being read in by the CPU. Combined with previously proposed early writeback strategies , our schemes achieve a substantially low vulnerability in the data cache, which indicate the necessity of different protection schemes for data items during various phases in their lifetime.