Nanosecond isomers in near-spherical 142,143 Pm Sarmishtha Bhattacharyya* Bhabha Atomic Research Centre, Mumbai 400 085, India Somen Chanda † and Swapan Kumar Basu ‡ Variable Energy Cyclotron Centre, Kolkata 700064, India M. B. Chatterjee Saha Institute of Nuclear Physics, Kolkata 700 064, India G. Mukherjee, § R. Palit,  P. K. Joshi, and H. C. Jain Tata Institute of Fundamental Research, Mumbai 400 005, India Received 7 September 2001; published 7 January 2002 In-beam measurements of nanosecond lifetimes have been undertaken for some of the excited states in near-spherical nuclei 142,143 Pm using the pulsed-beam  coincidence technique and the generalized centroid- shift method of analysis. The 133 Cs( 13 C, xn ) reaction with a 60 MeV pulsed beam of 13 C was used to populate the excited states in the respective nuclei. The measured mean lifetimes have been compared with correspond- ing single-particle estimates. DOI: 10.1103/PhysRevC.65.027301 PACS numbers: 21.10.Tg, 25.70.Gh, 27.60.+j The prediction and discovery of the high-spin yrast traps around 146 Gd in the late 1980s 1 led to the belief that an island of high-spin isomers exists in the region 64Z 71 and N 82. As a result of limitations in experimental ar- rangements, isomers with T 1/2 20 ns could not be observed in the earlier measurements. Very recently, a number of new high-spin isomers have been reported 2 for Eu and Sm nuclei with Z =63,62 and N =82,83 which exhibit interesting systematics in their excitation energies, half-lives, and spins. It is suggestive of the likely extension of the predicted island of isomers below Z =64 and warrants similar measurements in the neighboring Pm nuclei with Z =61. We have recently investigated the high-spin states in 142 Pm 3 and 143 Pm 4 using the 133 Cs ( 13 C, xn ) reaction and extended the level schemes considerably, which depict an irregular yrast se- quence, typical of near-spherical nuclei. It is observed that the intensity balances of feeding and deexciting  rays for some of the states in 143 Pm are not consistent, though their placement could be fixed up from the observed intensities in the relevant gated spectra. This discrepancy may be attrib- uted to the presence of unobserved  rays as well as isomers at moderate and high spin and excitation energies. In order to explore the scenario, we have undertaken in-beam measure- ments of nanosecond lifetimes in 142,143 Pm excited states us- ing 13 C pulsed beam and the reaction 133 Cs( 13 C, xn ) at E =60 MeV. The present experiment was performed at the BARC- TIFR 14UD Pelletron Accelerator at Mumbai. The target was prepared by vacuum evaporation of ultraspecture 99.99% CsNO 3 on gold backing (5 mg/cm 2 ) in a controlled condi- tion 5. The target thickness was 800  g/cm 2 . The beam pulsing system 6 consists of a double-drift harmonic buncher at low energy and a rf sweeper at the high-energy end. These are phase locked with respect to a master oscil- lator using an amplitude and phase control unit. The system provided 13 C pulsed beam with a period of 213 ns and a typical beam width of 2 ns. A prompt time resolution full width at half maximum FWHM of 12 ns, without any energy selection, was obtained in this experiment with a Ge clover detector. The data were acquired in list mode and sorted off line into a 4096512 add-back energy-time ma- trix, from which the background-subtracted time distribu- tions were projected against various  rays of interest, al- ready known from our earlier studies 3,4. The time distributions were analyzed for most of the cases by means of the centroid shift method 7, except for a few cases, where the spectra have substantial delayed component. The half-lives were determined by the slope method for the said cases and compared with the previous measurements in order to test the reliability of the present mesurements. Figure 1 shows a typical delayed time spectrum corresponding to 234.9 keV  ray in 143 Pm, which dexcite the levels at 1898.3 (15/2 + ) keV cf. Fig. 5 of Ref. 4. In the same figure, the prompt time distribution corresponding to 535.6 keV transi- tion, deexciting 5115.9 keV (31/2 - ) level, is shown for com- parison. The half-life extracted for the 1898.3 keV level by the slope method corroborates well within error with the same obtained from earlier measurements and adopted in Nuclear Data Sheets 8. In extracting lifetime information from the first moment of the time distributions, necessary steps, as described in Ref. 7, were followed. Unlike con- ventional  delayed concidence measurements the interpo- *Present address: Health Physics Unit, Variable Energy Cyclotron Center, Kolkata 700 064, India. † Present address: Physics Dept., Fakir Chand College, Diamond Harbour, West Bengal, India. ‡ Contributing author. Electronic address: skb@veccal.ernet.in § Present address: Physics Division, Argonne National Laboratory, Argonne, IL 60439.  Present address: Gesellschaft fu ¨r Schwerionenforschung GSI, Darmstadt, Germany. PHYSICAL REVIEW C, VOLUME 65, 027301 0556-2813/2002/652/0273014/$20.00 ©2002 The American Physical Society 65 027301-1