Acknowledgements

Our study was inspired by the experimental work of Laird et al. [21]. Stimulating discussions with Mark Riley are gratefully acknowledged. The work was supported in part by the U.S. Department of Energy under Contract Nos. DE-FG02-96ER40963 (University of Tennessee), DE-AC05-00OR22725 with UT-Battelle, LLC (Oak Ridge National Laboratory), and DE-FG05-87ER40361 (Joint Institute for Heavy Ion Research), and DE-FG02-07ER41459 (Mississippi State University); by the Latvian Scientific Council (grant No. 05.1724); by the Polish Ministry of Science; by the Academy of Finland and University of Jyväskylä within the FIDIPRO programme; and by the European Union Social Fund and the research program Pythagoras II - EPEAEK II, under project 80861.