PHYSICAL REVIEW C 73, 044316 (2006) Half-life measurements of several states in 95,97 Sr, 97,100,104 Zr, 106 Mo, and 148 Ce J. K. Hwang, 1 A. V. Ramayya, 1 J. H. Hamilton, 1 Y. X. Luo, 1,2 A. V. Daniel, 3 G. M. Ter-Akopian, 3 J. D. Cole, 4 and S. J. Zhu 1,5 1 Physics Department, Vanderbilt University, Nashville,Tennessee 37235, USA 2 Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA 3 Flerov Laboratory for Nuclear Reactions, JINR, Dubna, Russia 4 Idaho National Laboratory, Idaho Falls, Idaho 83415, USA 5 Department of Physics, Tsinghua University, Beijing 100084, Peoples Republic of China (Received 24 August 2005; published 20 April 2006) Half-lives T 1/2 of states in 95,97 Sr, 97,100,104 Zr, 106 Mo, and 148 Ce, which decay by delayed γ transitions, were determined by using a new time-gated triple γ coincidence method. Transition energy dependent effects such as time walks, time jitters, amplitude walks, and possible timing fluctuations of Ge detectors that contribute to the width of the time window were taken into consideration by comparing prompt and delayed cascades with similar transition energies. It is shown that the normalized triple γ coincidence counts of two prompt cascades with similar transition energies are similar. Also, it is observed that the real triple γ coincidence counts in the prompt cascades change systematically with the widths of the coincidence time window and the transition energies. Half-lives of states in 100,104 Zr were measured for the first time. The half-lives of states in the delayed cascades were determined by using the prompt cascades with transition energies similar to those in delayed cascades. The half-life of the 2 + state in 104 Zr is measured to be 2.0(3) ns. The B(E2;2 + → 0 + )(e 2 b 2 ) value and quadrupole deformation β 2 are 0.40(6) (e 2 b 2 ) and 0.47(7), respectively. 104 Zr has the most deformed 2 + state among medium and heavy even-even nuclei, except for 102 Sr. This method is only approximately valid, but it is believed to be generally within 10% of the true value. DOI: 10.1103/PhysRevC.73.044316 PACS number(s): 21.10.Tg, 25.85.Ca, 27.60.+j, 29.30.Kv I. INTRODUCTION Half-life T 1/2 measurements of nuclear states have been a major source of information on nuclear deformations, shell structures, and validity of nuclear models. Previously, half- lives of several states in neutron-rich nuclei were determined by measuring singles γ spectra as a function of time or from γ -γ coincidence spectra with isotopes produced in the fission of 235 U, 239 Pu, 248 Cm, and 252 Cf. Most of the previous results were obtained by using the separated isotopes produced in the fission of these nuclei. Numerous level half-lives were also determined from the β -γ,α-γ , and β -conversion electron coincidences. Recently, using large Ge detector arrays (Gammasphere, Eurogam), many new transitions have been added to the known level schemes, and several other new nuclei have been identified. In previousl works [1,2], we proposed a method for measuring long half-lives of states by using triple γ coincidence data. In the present work, we propose a new method for measuring short half-lives of the order of ns. In this method, triple γ coincidence spectra in which all three transitions are prompt are compared with the triple γ coincidence spectra in which one of the three transitions is delayed (see Fig. 1 and Table I). Table II shows that the normalized triple γ coincidence counts of two prompt cascades with the similar transition energies are similar. Also, it is observed that the real triple γ coincidence counts or N1 values in the prompt cascades vary systematically with the width of the coincidence time window t w and the three transition energies (see Tables II and III; Figs. 2 and 3). The normalized triple γ coincidence counts or N1 values in the prompt cascade increase systematically, not randomly, even at the smaller time windows of 4, 8, and 16 ns. The only difference between the delayed and prompt cascades is that the delayed cascade has one state with a longer half-life. Therefore, the triple γ coincidence counts of a delayed cascade, corrected by normalization factors extracted from the prompt cascade, determine a decay constant λ and half-life by being fitted to the equation N = C (1 − e −λt w ). Here, t w is the coincidence time window and C is a constant which is a measure of the coincidence counts at large time windows. In this method, all of the complicated timing factors, except for the half-life effect in the triple γ coincidence counts of the delayed cascade, are reasonably corrected by the normalization factors obtained from a prompt cascade. Using this techique, we measured the short half-lives of several states in 100,104 Zr, 106 Mo, and 148 Ce. Among the known medium to heavy nuclei, two regions of large deformation have been observed. The nuclei around 78 Sr with Z = 38 and N = 40 [4] and 102 Sr with Z = 38 and N = 64 [5] are known to have large deformation for the 2 + state. Therefore, it is interesting to determine the deformation for 104 Zr with N = 64. From the 2 + state half-life, the quadrupole deformation β 2 is found to be large in 104 Zr. We also remeasured the half-lives of states in 95,97 Sr and 97 Zr in order to prove that the technique works well and to remove existing discrepancies in the previous measurements. The half-life of the first excited state of 104 Zr was, for the first time, measured by using the present technique. In the present spontaneous-fissioning work, the centroid shift techniques measuring time difference spectra between two transitions were not used because background in the γ -γ matrix was too high. 0556-2813/2006/73(4)/044316(12)/$23.00 044316-1 ©2006 The American Physical Society