An isomeric state in 52 Fe has been located at E x ~ 6.83 ±0.25 MeV with the reaction 40 Ca( 14 N,/w) 52 Fe. The state decays by positron emission to the (11 + ) 3.837-MeV state of 52 Mn with T 1 / 2 = 56 ±8 sec. The probable spin and parity of the isomer is 12 + .It is well known that residual interactions acting within the ground-state shell-model configuration can produce long-lived "spin-gap" isomers at high excitation with unusual decay properties. For example, theJ 7r =^" isomers at E x = 3.1 MeV in the mirror nuclei 53 Fe and 53 Co decay by £4, M5, and E6 y-ray transitions 1 (T 1/2 = 2.5 min) and by combined proton-/3 + emission 2 (T 1/2 = 242 msec), respectively. Up to now, however, the only reported example of this phenomenon in an even-even nucleus is the 45-sec a?-decaying state at 2.93 MeV in 212 Po. 3 ' 4 This Letter reports the observation of an isomeric state analogous to that in 212 Po at 6.83 ±0.25 MeV excitation in theiV=Z nucleus 52 Fe. The state decays by /3 + emission (T 1/2 = 56 ±8 sec) and evidently has J" = 12 + , the maximum spin attainable in the (/ 7 / 2 )" 4 configuration. The 52 Fe isomer, as well as the absence of isomerism 5 in the conjugate nucleus 44 Ti, can be accounted for by shell-model calculations based on the (f 7/2 T 2 spectra of 54 Co and 42 Sc, respectively.The 52m Fe activity was first observed in the course of delayed-)3-y coincidence experiments designed to study proton-rich nuclei in the/ 7/2 shell. A natural calcium target ~2 mg/cm 2 thick evaporated onto a gold backing was bombarded with 46-MeV 14 N ions from the Stony Brook FN Van de Graaff accelerator. A slotted rotating wheel periodically interrupted the incident beam and provided a signal to enable the electronics and initiate a linear ramp voltage. Electrons passing through a thin Mylar window were detected in a collimated NE102 scintillator, and y rays were observed with a 50-cm 3 Ge(Li) detector. For each beam-off event the )3-and y-energy signals, the p-y time-to-amplitude signal, and the magnitude of the ramp voltage were converted to digital words and stored sequentially on magnetic tape for later analysis. Figure 1 shows a typical delayed-y-ray coincidence spectrum. In addition to lines from known 6 P activities, y rays at 622, 870, 929, 1416, and 2038 keV are apparent. Recently, a y-ray cascade through high-spin states in 52 Mn has been observed in studies of the reactions 51 V( 3 He, 2n) 52 Mn and 24 Mg( 32 S, 3/>w) 52 Mn by Signorini and Stefanini. 7 The five transitions in their 52 Mn level scheme (Fig. 2) appear to be identical to those seen in Fig. 1, suggesting that the activity observed results from the /3 + decay of a high-spin isomeric level formed in the reaction 40 Ca( 14 N, pn) 52 Fe. The results of our experiments to date can be summarized as follows.The five y rays associated with the new activity have intensities in the delayed-y-ray singles spectra consistent with the 52 Mn cascade, 7 and they decay with a common half-life of 56 ±8 sec. The 42m Sc peaks provide an internal check on th...
