The Sm149(d,3He) reaction has been used to populate levels in Pm148. Nineteen new excited states have been observed below 1 MeV excitation energy in Pm148. The possible astrophysical implications of these results are discussed

Champagne, A.E.

Lesko, K.T.

Norman, E.B.

Carolina Digital Repository

PHYSICAL REVIEW C VOLUME 37, NUMBER 2Brief ReportsFEBRUARY 1988Brief Reports are short papers which report on completed research or are addenda to papers previously published in the PhysicalReview. A Brief Report may be no longer than 3g printed pages and must be accompanied by an abstractLow-lying levels in ' PmEric B. Norman and Kevin T. LeskoNuclear Science Division, Lawrence Berkeley Laboratory, University of California at Berkeley, Berkeley, California 94720Arthur E. ChampagnePhysics Department, Princeton Uniuersity, Princeton, New Jersey 08544(Received 9 November 1987)The ' Sm(d, 'He) reaction has been used to populate levels in ' 'Pm. Nineteen new excitedstates have been observed below 1 MeV excitation energy in ' Pm. The possible astrophysical im-plications of these results are discussed.The elements heavier than iron are believed to be syn-thesized via neutron-capture reactions that occur instars. In their pioneering work on the origin of the ele-ments, Burbidge, Burbidge, Fowler, and Hoyle' pointedout that two distinct neutron-capture processes were re-quired to explain the observed solar-system elementaland isotopic abundances. In the s (slow) process, theneutron Aux is so low that if a beta-unstable nucleus isproduced, it almost always has time to decay before thenext neutron capture occurs. In the r (rapid) process, onthe other hand, the neutron Aux is so high that manysuccessive neutron captures can occur before a beta de-cay takes place. The site of the s process is generally be-lieved to be the helium-burning zones of red-giant stars.A de6nite site for the r process has yet to be established.While it is generally true that in the s process theneutron-capture rates are low compared to typical beta-decay rates, there are sites along the s process pathwhere the half-lives are sufficiently long that neutroncapture on these unstable nuclei competes favorably withbeta decay. If one knows the relevant isotopic abun-dances, neutron-capture cross sections, and beta-decayhalf-lives, such "branch points" allow one to infer theneutron density during the s process.Through recent measurements of the neutron-capturecross sections of ' ' '" Stn, Winters et al. have shownthat such a branch in the s process path occurs at theodd-odd nucleus ' Pm. However, the usefulness of thisbranch cannot yet be fully exploited because of the lackof information on the nuclear structure of ' Pm. Until600— SIT1 (d, He) 148PEc( —27.7 MeV8 = 20'cU400—0O200— C) (Q ( LA1CGIcuCD C3r200I700400300 500 600 800 g00Channel NumberFIQ. 1. The '" Sm(d, 'He)' Pm spectrum observed at 20 deg with an incident energy of Ed =27.7 MeV.37 860 1988 The American Physical Society37 BRIEF REPORTS 861the present study was begun, only three levels in ' Pmwere known. The ground state, ' Pm, is a J =1 lev-el with a 5.37-day beta-decay half-life. The first excitedstate at 76 keV is a J"=2 level that gamma decays tothe ground state. The second excited state at 137 keV isa J =6 isomer, ' Pm, that decays almost entirely bybeta-minus decay with a 41.3-day half-life. It has beenestimated that during the s process, the ' Pm(n, y) reac-tion produces roughly equal amounts of ' Pm andPm . However, at the high temperatures at whichthe s process is believed to occur, (1—4)X10' K, thequestion arises as to whether this population will bepreserved or whether ' Pmg and ' Pm could comeinto thermal equilibrium. Winters et al. have shownthat the neutron density inferred from this s processbranch point is a factor of 3 larger if equilibrium isreached than if the initial equal mixture of ' Pm andPm is preserved.There are many potential mechanisms by whichPmg™could reach thermal equilibrium during the sprocess. One of the most important is undoubtedly pho-toexcitation. In the hot stellar environment at which thes process is thought to occur, there is an enormous fluxof high energy photons. Thus it is possible that a nu-cleus, initially in the isomeric state, could absorb one ofthese photons and be excited to a higher-lying levelwhich subsequently decays to the ground state. In orderfor the timescale for equilibration under s process condi-tions to be shorter than the half-life of ' Pm, suchmediating levels must lie below approximately 1 MeVexcitation energy. Thus, to decide if this actually hap-pens during the s process, the positions and gamma-decay properties of levels in ' Pm must be known.As a first step toward answering the question ofwhether ' Pm ' reach thermal equilibrium in the s pro-cess, we have performed an experiment to locate low-lying levels in ' Pm. The ' Sm(d, He)' Pm reactionwas performed using a 27.7-MeV deuteron beam fromthe Princeton University cyclotron. The target consistedof 54 pg/cm of metallic samarium enriched to 91.59%Sm evaporated onto a 31 }ug/cm carbon backing.Beam currents of 200—300 nA were used in the presentmeasurements. The reaction He particles were momen-tum analyzed with the Princeton quadrupole-dipole-dipole-dipole (QDDD) spectrometer and were detectedat the focal surface with a 60-cm long position-sensitiveproportional counter in coincidence with a plastic scin-tillator. The spectrometer was calibrated using the(d, He) reaction on targets of ' Sm and ' Sm becausethese reactions have similar Q values to that of theSm(d, He)' Pm reaction and because the levels inPm and ' Pm populated via this reaction are wellknown. ' The measured energy resolution was 16 keVfull width at half maximum (FWHM).Figure 1 illustrates the spectrum of He particles ob-served at a laboratory angle of 20 deg. In addition tothe three previously known levels in ' Pm, many newstates appear. Using our calibration of the spectrometer,we have been able to determine the positions of nineteennew levels below 1-MeV excitation energy with uncer-tainties of +6 keV. From these data, we have also mea-958910859810745715700659644609567553526462413389363305219137 41.3 days7614861 P~875.37 daysFIG. 2. Level scheme of ' Pm based on the results of thepresent study and those of previous investigations.sured the Q value for the' Sm(d, He)' Pm reaction tobe —2.064+0.006 MeV.If the (d, He) reaction proceeds by a direct one-stepmechanism, then the low-lying states in ' Pm we expectto populate should arise from couplings of an odd 2d5&2or Ig7/Q proton with the odd 2f7/2 neutron of the targetnucleus. Such couplings yield six pairs of negative pari-ty states with spins of 1, 2, 3, 4, 5, 6, and single J =0and J =7 negative parity states. A level scheme of thiskind has been observed in a similar study ' of the odd-odd nucleus ' Pm. As a result of the ' Sm target hav-ing J =-,', J =1 ~6 states can all be populated vial. =2 proton transfers. Thus, while the ' Sm(d, He) re-action is not suitable for determining the spins and pari-ties of the states we observe, it is likely that many of thelevels seen in our spectrum do arise from these coupling~of the odd proton and odd neutron. Figure 2 summa-862 BRIEF REPORTS 37rizes what is now known about the level scheme ofPm from the results of the present study and those ofprevious investigations.An example of the type of state that could mediatetransitions between ' Pm and ' Pm is one of theJ =4 levels described above. Such a state should havesizable decay branches to both the J =2 level at 76keV (which decays to the ground state) and to the iso-mer. To determine which, if any, of the levels observedin the present investigation serve as such a mediator dur-ing the s process, the gamma-decay properties of theselevels must be known. Experiments are now under wayat Lawrence Berkeley Laboratory to study electromagnetic transitions in ' Pm in hopes of answering thisquestion.We wish to thank the late W. H. Moore for assistancein operating the cyclotron. This work was supported atLawrence Berkeley Laboratory by the U.S. Departmentof Energy under Contract No. DE-AC03-76SF00098 andat Princeton University by the U.S. National ScienceFoundation.'G. R. Burbidge, E. M. Burbidge, W. A. Fowler, and F. Hoyle,Rev. Mod. Phys. 29, 54 (1957).R. R. Winters et al. , Astrophys. J. 300, 41 (1986).3Table of Isotopes, 7th ed. , edited by C. M. Lederer and V. S.Shirley (Wiley, New York, 1978).4I. S. Lee et al. , Nucl. Phys. A371, 111 (1981).5M. R. Macphail and R. G. Summers-Gill, Nucl. Phys. A237,285 (1975).6M. R. Macphail and R. G. Summers-Gill, Nucl. Phys. A263,12 (1976)~

Low-lying levels in Pm148

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Low-lying levels in Pm148

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