Cosmo-Nuclear Physics Laboratory work report 2007-2008

Isao Tanihata, Hooi Jin OngRCNP, Osaka University

Abstract

Following is the summary of the activities of Cosmo-Nuclear Physics Laboratory in RCNP/Osaka University for the period from October 2007 until September 2008.

The main research aim of the laboratory has been set as “Studies of tensor forces and other hidden interactions in neutron-rich nuclei and neutron-rich nuclear matter”. The tensor forces plays important roles in the pion exchange interactions that provide the most important attraction in nuclear forces. The importance of the tensor forces has been demonstrated in reproducing the properties of nuclear matter as well as in explaining the binding energies of deuteron and alpha particles. Admixture of D wave in those nuclei is one of the clear evidence of the tensor interactions.

In contrast, this interaction has not been included explicitly in nuclear models such as mean field models of heavier nuclei. In the shell model, it is included only as a part of the residual interactions. However recent experimental evidences, the changes of magic numbers and the orders of single-particle orbitals in neutron rich nuclei, have revealed an importance of the tensor forces. In theory, recent ab initio calculations of light nuclei also show essential importance of the tensor forces for binding nuclei up to mass number 12. To take the tensor forces into account explicitly, new theoretical methods, such as tensor optimized shell model and UCOM, are under investigation in places like RCNP and GSI.

We plan to study the tensor interactions in nuclei from three experimental points of view. 1. The Equation of State (EOS) of neutron rich matter.2. The high-momentum (~ 2 fm-1) components in momentum distribution of nucleons in nuclei

through tensor correlations.3. The systematic change of shell orbitals in neutron rich nuclei.

The first viewpoint will be studied though the proton elastic scattering on Ni isotopes at GSI/SIS facility. Details of the proposal may be found at http://www.rcnp.osaka-u.ac.jp/Divisions/cnp/

Studies from the second viewpoint are considered effective to see the basic contribution of the tensor forces in nuclear structure. We consider (p, d), (p, pd), (p, nd) reactions at high momentum transfer would be clean methods to pick up the high-momentum component in nucleon internal motion.

A nucleon and di-nucleon transfer reactions will be studies as the third view point. We would like to improve the radioactive beam facility at RCNP to optimize the condition for transfer reactions at 20-30A MeV where traditional DWBA analysis works best.

For those studies we are working at several radioactive beam facilities, RCNP, TRIUMF, and GSI. The projects in each facility are briefly described below.

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1. RCNP project1.1 High momentum component due to the tensor forcesExperimental evidences other than deuteron and alpha are scarce and indirect so far and thus we

need more direct evidences in heavier nuclei. At RCNP, we plan to look for effects of the tensor interactions in 12C and 16O and other nuclei by deducing and observing the difference in the internal momentum distributions at around 2 fm-1 where the effect of the tensor interaction appears as an enhancement of probability. For example, the s-wave nucleons in 12C and 16O are expected to have largely different contribution of the tensor interactions. Therefore a difference in momentum distributions is expected in between these nuclei.

We consider that a clean method to pick up the high-momentum component in nucleon internal motion would be (p, d), (p, pd), (p, nd) reactions at high momentum transfer. We have proposed the experiment to B-PAC of RCNP at the beginning of 2008. This experiment (E314: Search for Direct Evidence of Tensor Interactions: High Momentum Component in Nuclei” http://www.rcnp.osaka-u.ac.jp/Divisions/cnp/) and approved to have a beam time.

Our present plan is to measure (p,d) and (p,pd) differential cross sections on 12C and 16O using proton beams at 200 MeV, 300 MeV and 392 MeV. The experiment will be performed using the Grand Raiden spectrometer in early spring of 2009.

Collaborators: H. J. Ong, I. Tanihata, H. Okamura, A. Tamii, M. Yosoi, K. Suda, T. Adachi, Y. Tameshige, H. Matsubara, D. Ishikawa (RCNP), S. Sakaguchi (Miyazaki Univ.), K. Matsuta, M. Fukuda, M. Mihara, D. Nishimura (Osaka Univ.), A. Ozawa (Tsukuba Univ.), K. Sekiguchi (RIKEN), K. Ikeda (RIKEN), H. Toki, T. Myo, Y. Ogawa (RCNP)

1.2 Development of RIB experiments at RCNPAfter the improvements of the ion source of the ring

cyclotron at RCNP, intensities of heavy ions have been raised dramatically. Using those high intensity heavy ions of energy up to 100A MeV (K value of the ring cyclotron is 400 that correspond to 3.2 Tm in rigidity of a particle), the EN course would be a powerful RIB facility. The maximum rigidity of the EN course is also 3.2 Tm so that RI beams of energy up to ~95A MeV would be usable. The systematic change of shell orbitals can best be studied by nucleon transfer reactions.

A proposal “RI Beam at RCNP” has been submitted to P-PAC in August. The committee commended the proposal and some of the budget for the related developments has been approved. The details of the proposal may be obtained in http://www.rcnp.osaka-u.ac.jp/Divisions/cnp/

The aim of the proposal is to develop experimental programs using RIB at RCNP. 1. One is to measure the charge changing cross sections of neutron-rich nuclei to determine radii

of proton distributions. We can deduce thickness of neutron skin from these data by combining the radii of nucleon distribution determined by reaction and interaction cross sections. The thickness of neutron skin is considered to be most sensitive method to study the EOS of

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asymmetric nuclear matter. Systematic measurements of the charge-changing cross section are expected to provide such an important tool.

2. The other is a systematic study of nucleon transfer reactions at 20A-30A MeV. This energy range is known to be appropriate to apply DWBA analysis to obtain information on the contributing orbitals. For this purpose, two different types of detector systems have been considered. One is the combined system of detectors, for detecting all particles in the final state and a solid-hydrogen target. The other is the active target chamber. Both of the system aims to observe angular distributions of transfer reactions using a very low intensity RIB. Some description of those systems and their advantages is described in the proposal. http://www.rcnp.osaka-u.ac.jp/Divisions/cnp/

Developments proceeded this year are summarized in the following.1.2.1 Production of RIB in 20A-30A MeV

The fragment separator at RCNP (the EN course) has unique character compared with other fragment separators in the world. It is designed to use a uniform thickness degrader instead of a wedge shaped degrader.1 Not only it is convenient for making a degrader, this method is advantageous for using a thicker degrader. It is due to the fact that the magnification of the RI beam at the achromatic focus after the separation (I call this as F2 in the following) is constant even if the thickness of the degrader varies. It is not the case for the wedge shaped degrader: the magnification becomes larger when a thicker degrader is used. Therefore the intensity of a separated beam becomes much lower when the energy is reduced. In contrast the constant magnification of the EN course is an advantage for making a lower-energy RI beam suitable for two-body reaction studies.

In July 2008, we have tested the concept of making 20A-30A MeV beam of 10Be. Beam of 23A MeV 10Be has been produced from 12C beam of 100A MeV. The production Be target of 2.402 g/cm2 thick and the Al degrader at F1 of 0.918 g/cm2 have been used. Figure 2 shows the scatter plot between the position at the final focus and pulse height in a Si detector. The central cluster of the events is 10Be and the other cluster at top-right side is 11B. This result was exactly the same as expected from the Monte-Carlo simulations.

In the present condition, we found that the beam of Li isotopes can not be produced at low energies because of the limitation of a usable target and degrader thickness. This is not the restriction due to any principle limit but is just due to the geometrical restrictions of present chamber system. From this test experiment we found that some modifications are necessary for the EN course.

Fig. 2. Test data of 23A MeV 11Be production from 100A MeV 12C beam.

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1 A wedge-shaped degrader are usually shaped to keep the momentum dispersion of the beam. With this degrader the dispersion of the first and the second stages of a separator is kept to be the same. With uniform degrader, in contrast, the momentum dispersion become larger after the degrader. The dispersion of the second stage has to be readjusted to obtain achromaticity at F2.

Another interesting rule was found from the simulation of production yield. The produced intensity of RIB in a certain energy bin does not depend on the incident beam energy. Figure 3 shows the simulation of the production yield of 10Be from 12C of very different energy. One may use a thicker target if a higher energy incident beam is used. However as seen in the Fig. 3 the production yields at around 30A MeV are same for 70A and 100A MeV incident energies. Instead the production yield of higher energies increases with beam energy.

Now we understood the optics of the EN course enough to start transfer reaction experiments including the energy loss and the multiple scattering effects in a target as well as a degrader. Charge changing cross sections, reaction cross sections, and other fragmentation reactions at higher energy up to 90A MeV or so would also be available.

1.2.2 Developments of detector systemIn this year, we have constructed the detector

chambers for detection of forward going particles. The scattering angle of particles from nucleon transfer reactions at present energy are mostly at small scattering

angles in the laboratory frame. To cover the largest solid angle, we designed an annular type detector telescope consisted of two annular Si strip detectors and CsI ring array. Three sets of telescopes will be placed along the beam line to cover the forward angles. Each telescopes is

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Fig. 4 Forward detector system for transfer reaction experiments.Fig. * Detector chambers for transfer reaction study

Fig. 4. Detector telescope consisted of two layer of Si detectors and CsI array. Si detector has 1.5 mm size strip in annular direction and 16 sectors. CsI array has 16 CsI crystals.

mounted inside a compact vacuum chamber so that the relative distance between telescopes can be changed to optimize the covering angles for different reactions.

Those detector system has more than 300 channels of readout signals. We are developing the read-out circuit based on the Viking chip. Design for reading out the one set of telescope has been finished and a test module is under construction.

An active target is also under the design stage. The active target consist of gas TPC chamber and Si-CsI telescope wall surrounding the TPC. The gas chamber works as the reaction target and as tracking device. Particle identification can be made for very light and low energy particles but it can not be done for particles that leaves the gas chamber area. The Si-CsI telescope would be used for identifying such particles by ΔE-E method. For gas chamber, we have decided to use µPIC system developed at Kyoto university by Tanimori group. The µPIC for 10 x 10 cm2 read out is readily available. µPIC information may be obtained from http://www.rcnp.osaka-u.ac.jp/~sakemi/mpgdWS/slide/mpgdWS_tanimori.pdf. The whole system is under the designing stage.

Collaborators: H. J. Ong, K. Hirota, T. Suzuki, I. Tanihata, A. Tamii, S. Ajimur, H. Okamura(RCNP), K. Matsuta, M. Fukuda, M. Mihara, D. Nishimura, T. Shimoda, . Odahara (Osaka Univ.), K. Kimura (Nagasaki Inst. of App. Sci.), T. Tanimori (Kyoto Univ.), A. Ozawa (Tsukuba Univ.), S. Ishimoto (KEK), T. Kobayashi (Tohoku Univ.)

2. Project at TRIUMF2.1 p(11Li, 9Li)t reaction studies

The analysis of the first measurement of two-halo neutron transfer reactions has been completed. The data was taken at TRIUMF ISAC-II facility at the beginning of 2007. The result has been published this year in Physical Review Letters and data has been presented in several international conferences as well as national meetings. (see the description of the paper in section 8.

2.2 d(11Be, 12Be)p reaction studiesAccelerated beam 11Be of 5A MeV at ISAC-2 has been used to observe a neutron transfer

reaction. The program is under the collaboration between TRIUMF-St. Mary’s U.-Osaka U. The aim of the experiment is to see the particle configurations (or the single particle spectroscopic factors) of the low lying states in 12Be. The data taking experiment has been done in November 30- December 13 using Tigress facility at TRIUMF. The data analysis is going on and 12Be final states have been clearly identified.

2.3 Proposed experiments at TRIUMFSeveral experiments have been proposed to the EEC (Experimental Evaluation Committee)

of TRIUMF. Among them the largest proposal is S1187 entitled as “Effects of surface excess neutrons on shell structure and pairing correlation: the first program for IRIS at ISAC II”. The IRIS is the Si-array detector facility for transfer reaction studies at low energy. We plan to provide the solid hydrogen target for this facility. This proposal include studies with light

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radioactive nuclei as well as medium heavy nuclei of mass number~132. The beam time has been approved as high-priority for experiment with light nuclei. Physics of heavy nuclei also has been accepted as high-priority but we have to wait for the development of the accelerated radioactive beam of this mass range.

S1147 (Investigation of low-lying resonances) in 11Li, has also been accepted as a high-priority experiment by the EEC. This experiments use the low-energy high-intensity beam of 11Li and aim to confirm the unbound excited state at 1.2 MeV in 11Li. It also can detect the other excited states if they exist. The existence and the excitation energy of the excited state in 11Li, a neutron halo nucleus, are one of the most important information for understanding nuclei at the drip line.

Neither of above experiments has been assigned for the beam time yet but we plan to request the beam time in late 2009.

Collaborators: H. J. Ong, K. Hirota, I. Tanihata (RCNP) A. Shotter, B. Davids, C. Ruiz, L. Buchmann, P. Walden, A. Gallant C. E. Pearson, G. Ball, G. Hackman, J. N. Orce, S. Triambak, S. Williams(TRIUMF), R. Kanungo, H. Al Falou, R. A. E. Austin (St. Mary’s Univ.), A. M. Laird, B. R. Fulton, C. A. Diget, S. P. Fox (Univ. of York), F. Sarazin (Colorado School of Mines), C. Andreoiu, D. Cross, R. Kshetri (Simon Fraser Univ.), C. E. Svensson, P. E. Garrett (Univ. of Guelph), C. Y. Wu (LLNL), F. Delaunay, J. Gibelin, N. L. Achouri, N. Orr (LPC/Caen)

3. Project at GSI3.1 Fragmentation of 24O and O and Mg isotopesFragmentation cross sections of neutron rich isotopes of O and Mg have been measured at SIS/

FRS facility of GSI. The aim of the experiments is to determined the configuration of valence neutrons by observing the momentum distributions of one and two neutron removal reaction at high energy (~900A MeV).

It has been known that N=16 is a closed shell appears in neutron rich nuclei. In particular several evidences show double magic characters of 24O nuclei. It is a case where the gap between 2s1/2 and 1d3/2 is large and thus the valence neutrons in 24O should have a large spectroscopic amplitude of s1/2 wave. An s-wave can easily be identified by the measurement of the momentum distribution of neutron removal reaction at high energy. The momentum distribution as well as the fragmentation cross section of the 23O fragment from 24O has been determined. The on going analysis suggests that the 2s1/2 neutron spectroscopic factor is 1.74±0.19 within eikonal model. It is a large value and suggests the spherical nature of the shell closure and thus consistent with the double closed magic nature. (submitted for publication)

Further analysis of 24O data as well as analyses of other nuclei are going on. 3.2 p-elastic scattering of Ni isotopes and EOS of asymmetric matterSeveral years ago, we have proposed the experiment for determination of the Equation of State

(EOS) of asymmetric nuclear matter to GSI Program Advisory Committee (GSI-PAC). The experiment (S272) proposes to measure the proton elastic scattering cross sections using beam of neutron rich Ni isotopes. It is expected that the nucleon density distributions determined from the elastic scattering is precise enough to see the difference of the EOS predicted by different theories for example a skyrme type model or a relativistic mean field model. The proposal was approved but has not run because a long development time was necessary for the construction of detectors. The detection system has been completed last year and being tested and used for study of proton elastic scattering with lighter elements such as carbon. (ESPRI collaboration)

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At the beginning of 2008 we have asked the GSI-PAC to resume the experiment. The GSI-PAC discussed our report and approved the progress report with high priority. We therefore are planning to run the experiment in summer 2009.

Collaborators: H. J. Ong, K. Hirota, J. Zenihiro, I. Tanihata (RCNP), H. Sakaguchi, Y. Maeda (Miyazaki Univ.), S. Terashima (RIKEN), A. Ozawa (Tsukuba Univ.) T. Kobayashi, Y. Matsuda (Tohoku Univ.), H. Geissel, C. Scheidenberger, C. Nociforo, L. Chulkov, P. Egelhof, H. Simon, K. Summerer, H. Weick (GSI), N. Kalantar-Nayestanaki, M. N. Harakeh, A. Berg, H. Woertche (KVI), R. Kanungo (St. Mary’s Univ.)

4. Presentations4.1 Publications

1. Ong, H. J., N. Imai, D. Suzuki et al., “Lifetime measurements of the first excited states in 16,18C”, Physical Review C 78 (2008) 014308.

The electric quadrupole transition from the first 2+state to the ground 0+ state in 18C was studied through a lifetime measurement by an upgraded recoil shadow method applied to inelastically scattered radioactive 18C nuclei. The measured mean lifetime is 18.9±0.9(stat)±4.4(syst) ps, corresponding to a B(E2;21+0gs+) value of 4.3±0.2±1.0 e2 fm4, or about 1.5 Weisskopf units. The mean lifetime of the first 2+ state in 16C was remeasured to be 18.3±1.4±4.8 ps, about four times shorter than the value reported previously. The discrepancy between the two results was explained by incorporating the γ-ray angular distribution measured in this work into the previous measurement. These transition strengths are hindered compared to the empirical transition strengths, indicating that the anomalous hindrance observed in 16C persists in 18C.

2. Suzuki, D., H. Iwasaki, H. J. Ong et al., “Lifetime measurements of excited states in 17C: Possible interplay between collectivity and halo effects”, Physics Letters B 666 (2008) 222.

Lifetime measurements were performed on low-lying excited states of the neutron-rich isotope View the MathML source using the recoil shadow method. The γ-decay mean lifetimes were determined to be 583±21(stat)±35(syst) ps for the first excited state at 212 keV and 18.9±0.6(stat)±4.7(syst) ps for the second excited state at 333 keV. Based on a comparison with the empirical upper limits for the electromagnetic transition strengths, these decays are concluded to be predominantly M1 transitions. The reduced M1 transition probabilities to the ground state were deduced to be (1.0±0.1)x10-2 µN2 and (8.2+3.2/-1.8) x10-2 µN2, respectively, for the first and second excited states. The strongly hindered M1 strength as well as the lowered excitation energy represents unique nature of the 212-keV state.

3. Iwasaki, H., S. Michimasa, M. Niikura et al., “Persistence of the N=50 shell closure in the neutron-rich isotope 80Ge”, Physical Review C 78 (2008) 021304.

The 2 and 2 states of the neutron-rich isotope 80Ge were studied by intermediate-energy heavy-ion scattering, providing for the first time the experimental value of B(E2;2+ 0) for 80Ge. The experimental data are well reproduced by large-scale shell model calculations, which suggest the important role of the proton particle-hole 2p3/2-1f5/2 excitation in the configuration of the 2+ state.

4. Satou, Y., T. Nakamura, N. Fukuda et al., “Unbound excited states in 19, 17C”, Physics Letters B 660 (2008) 320.

The neutron-rich carbon isotopes 19,17C have been investigated via proton inelastic scattering on a liquid hydrogen target at 70 MeV/nucleon. The invariant mass method in inverse kinematics was employed to reconstruct the energy spectrum, in which fast neutrons and charged fragments were detected in coincidence using a neutron hodoscope and a dipole

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magnet system. A peak has been observed with an excitation energy of 1.46(10) MeV in 19C, while three peaks with energies of 2.20(3), 3.05(3), and 6.13(9) MeV have been observed in 17C. Deduced cross sections are compared with microscopic DWBA calculations based on p-sd shell model wave functions and modern nucleon–nucleus optical potentials. Jπ assignments are made for the four observed states as well as the ground states of both nuclei.

5. Elekes, Z., Z. Dombradi, N. Aoi et al., “The study of shell closures in light neutron-rich nuclei”, Journal of Physics (London) G35 (2008) 014038.

The behavior of the nuclear shell structure far away from the valley of stability is discussed in this paper with an emphasis on the N = 20 magic number. Two experimental examples, namely γ-spectroscopy of 27Ne by 1H(28Ne, 27Ne) reaction and single-particle spectroscopy of 23O by 22O(d, p)23O* → 22O+n process in inverse kinematics, are shown providing indirect indication and a direct evidence for the extreme decrease of the N = 20 shell gap and, consequently, the vanishing N = 20 magicity.

6. Aoi, N., H. Suzuki, E. Takeshita et al., “Shape transition observed in neutron-rich pf-shell isotopes studied via proton inelastic scattering”, Nuclear Physics A 805 (2008) 400c.

The structure of the neutron rich pf-shell nuclei 58Ti36, 60Cr36 and 62Cr38 was studied by proton inelastic scattering in inverse kinematics. Large quadrupole deformation lengths of 1.12(16) fm and 1.36(14) fm were obtained for 60Cr and 62Cr, respectively, which confirm the enhanced collectivity in these nuclei. An excited state was observed in 62Cr at 1180(10) keV with tentative assignment of Jπ=4+. The increase in the Ex(4+)/ Ex(2+) ratio indicates that the nature of collectivity changes from vibrational to rotational in 62Cr and affords evidence for the development of static deformation. The excitation energy of the newly observed 2+ state in 58Ti (1046(17) keV) shows hindered collectivity. The contrast between the structures of Cr and Ti suggests that the contributions of both protons and neutrons are crucial for the large collectivity in Cr close to N=40. From the comparison to shell model calculations, the large collectivity is found to originate from the admixture of the pf- and gd-shells across the N=40 sub-shell gap.

7. Tanihata, I. (2008). "Radioactive beam science, past, present and future." Nuclear Instrument and Method in Physics Research B 266(19-20): 4067-4073.

Since high-energy radioactive nuclei were used for collision measurements, many discoveries in physics and many inventions in technique of producing and delivering radioactive beam have been made. In this paper, firstly, I briefly review developments in radioactive beam science and then show a close relation between development in technology and physics. Based on such consideration, I discuss the probable best scheme of radioactive ion beam production for studies of r-process that is considered to be one of the most important and exciting subjects for the future.

8. Tanihata, I., M. Alcorta, et al. (2008). "Measurement of the Two-Halo Neutron Transfer Reaction 1H(11Li; 9Li)3H at 3A MeV." Physical Review Letters 100: 192502.

The p(11Li,9Li)t reaction has been studied for the first time at an incident energy of 3A MeV at the new ISAC-2 facility at TRIUMF. An active target detector MAYA, built at GANIL, was used for the measurement. The differential cross sections have been determined for transitions to the 9Li ground and first excited states in a wide range of scattering angles. Multistep transfer calculations using different 11Li model wave functions show that wave functions with strong correlations between the halo neutrons are the most successful in reproducing the observation.

9. Mythili, S., B. Davids, et al. (2008). "Lifetimes of states in 19Ne above the 15O+α breakup threshold." Physical Review C 77: 035803.

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The 15O(α, γ )19Ne reaction plays a role in the ignition of type I x-ray bursts on accreting neutron stars. The lifetimes of states in 19Ne above the 15O+α threshold of 3.53 MeV are important inputs to calculations of the astrophysical reaction rate. These levels in 19Ne were populated in the 3He(20Ne, α)19Ne reaction at a 20Ne beam energy of 34 MeV. The lifetimes of six states above the threshold were measured with the Doppler-shift attenuation method. The present measurements agree with previous determinations of the lifetimes of these states and in some cases are considerably more precise.

10. Iwasa, N., T. Motobayashi, et al. (2008). "Large proton contribution to the 2+ excitation in 20Mg studied by intermediate energy inelastic scattering." Physical Review C 78: 024306.

Coulomb excitation of the proton-rich nucleus 20Mg was studied using a radioactive 20Mg beam at 58A MeV impinging on a lead target. The reduced transition probability B(E2;02) was extracted to be 177(32) e2 fm4, which agrees with the theoretical predictions by a cluster model assuming 16O + 2p + 2p structure, a mean-field approach based on the angular momentum projected generator coordinate method, and the USD shell model. The ratio of the neutron-to-proton multipole matrix elements Mn/Mp in the mirror nucleus 20O was deduced to be 2.51(25) with the Mn value evaluated from the measured B(E2) value for 20Mg with the help of isospin symmetry. The results confirm the large Mn/Mp value previously reported in 20O, leading to the dominant role of the four valence nucleons in the 2excitation and persistence of the 16O core in 20O and 20Mg.

11. Fallis, J., J. A. Clark, et al. (2008). "Determination of the proton separation energy of 93Rh from mass measurements." Physical Review C 78: 022801.

The proposed p process, which occurs in the early time proton-rich neutrino winds of core-collapse supernovae, has the potential to resolve the long-standing uncertainty in the production of the light p-nuclei 92Mo and 94Mo. A recent study incorporating this p process has indicated that the proton separation energy Sp of 93Rh is especially important in determining the relative production of these two isotopes. To reproduce the observed solar 92Mo/94Mo abundance ratio of 1.57 a Sp value for 93Rh of 1.64±0.1 MeV is required. The previously unknown masses of 92Ru and 93Rh have been measured with the Canadian Penning Trap mass spectrometer resulting in an experimental value for Sp(93Rh) of 2.007±0.009 MeV. This implies that with our current understanding of the conditions in core-collapse supernova explosions the p process is not solely responsible for the observed solar 92Mo/94Mo abundance ratio

12. Petrascu, M., A. Constantinescu, et al. (2007). "Experimental state of n-n correlation function for Borromean halo nuclei investigation." Nuclear Physics A 790: 235c-240c.

The present experimental and theoretical state of Cnn correlation function for Borromean halo nuclei investigation is reviewed. Some of the consequences of a recently appeared new theory of Cnn, [M.T. Yamashita, T. Frederico, and L. Tomio, Phys. Rev. C 72 (2005) 011601] together with the experimental possibilities to test this theory will be presented in this contribution.

13. Notani, M., H. Sakurai, et al. (2007). "Projectile fragmentation reactions and production of nuclei near the neutron drip-line." Physical Review C 76: 044605.

The reaction mechanism of projectile fragmentation at intermediate energies has been investigated observing the target dependence of the production cross sections of very neutron-rich nuclei. Measurement of longitudinal momentum distributions of projectile-like fragments within a wide range of fragment mass and its charge was performed using a hundred-MeV/n 40Ar beam incident on Be and Ta targets. By measurement of fragment momentum distribution, a parabolic mass dependence of momentum peak shift was

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observed in the results of both targets, and a phenomenon of light-fragment acceleration was found only in the Be-target data. The analysis of production cross sections revealed an obvious enhancement of the target dependence except target size effect when the neutron excess is increased. This result implies the breakdown of factorization (BOF) of production cross sections for very neutron-rich nuclei near the drip line.

14. Frekers, D., J. Dilling, and I. Tanihata (2007). "Electron capture branching ratios for the odd-odd intermediate nuclei in double-beta decay using the TITAN ion trap facility." Canadian Journal of Physics 85(1): 57-75.

We suggest a measurement of the electron capture (EC) branching ratios for the odd–odd intermediate nuclei in double-beta (β– β–) decay using the new ion trap facility TITAN at the TRIUMF radioactive beam facility. The EC branching ratios are important for evaluating the nuclear matrix elements involved in the β– β– -decay for both, the 2ν and the 0ν-decay mode. Especially the neutrinoless (0νββ) mode is presently at the center of attention, as it probes the Majorana character of the neutrino, and if observed unambiguously, knowledge of the nuclear matrix elements are the key for determining the neutrino mass. The EC branches are in most cases suppressed by several orders of magnitude relative to their β– -counterparts owing to much lower decay energies, and are, therefore, either poorly known or not known at all. Here, the traditional methods of producing the radioactive isotope through irradiation of a suitable target and then measuring the K-shell X-rays have reached a limit of sensitivity. In this note, we will describe a novel technique to measure the EC branching ratios, where the TITAN ion traps and the ISAC radioactive beam facility at TRIUMF are the central components. This approach will increase the sensitivity limit because of significantly reduced background levels. Seven cases will be discussed in detail and connections to hadronic charge-exchange reactions will be made. For most of these, the daughter isotopes are β– β– -decay nuclei that are presently under intense experimental investigations.

15. Fang, D. Q., W. Guo, et al. (2007). "Examining the exotic structure of the proton-rich nucleus 23Al." Physical Review C(76): 031601(R).

The longitudinal momentum distribution (P// ) of fragments after one-proton removal from 23Al and reaction cross sections (σR ) for 23,24Al on a carbon target at 74A MeV have been measured. The 23,24 Al ions were produced through projectile fragmentation of 135A MeV 28 Si primary beam using the RIPS fragment separator at RIKEN. P// is measured by a direct time-of-flight (TOF) technique, while σR is determined using a transmission method. An enhancement in σR is observed for 23 Al compared with 24 Al. The P// for 22 Mg fragments from 23 Al breakup has been obtained for the first time. FWHM of the distributions has been determined to be 232 ± 28 MeV/c. The experimental data are discussed by using the Few-Body Glauber model. Analysis of P// demonstrates a dominant d -wave configuration for the valence proton in ground state of 23 Al, indicating that 23 Al is not a proton halo nucleus.

4.2 International conferences and meetings1. “Hirshegg 2008: Modern Aspect in Nuclear Physics” January 13-19, 2008, Hirshegg, Austlia,

Invited talk “Spectroscopy of drip line nuclei with thick target technique”, I. Tanihata2. “Halo 08” Work shop, March 27-28, 2008, TRIUMF, Vancouver, Canada, Invited talk “Nuclear

Halos-Past, Present, and Future”, I. Tanihata3. “CNS-RIKEN Joint International Symposium on Frontier of Gamma-ray Spectroscopy and

Perspectives for Nuclear Structure Studies (Gamma08)”, April 3 – 5, 2008, Wako, Japan. Oral presentation: “Observation/confirmation of hindered E2 strengths in 16,18C”, presented by H. J. Ong, and N. Imai, D. Suzuki et al.

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4. “APS meeting at St. Louise” April 11-15, 2008, Invited talk “Measurement of two-halo neutron transfer p(11Li, 9Li)t reaction at 3A MeV”, I. Tanihata

5. “50th Anniversary Symposium on Nuclear Sizes and Shapes”, June 23-25, 2008, University of Surrey, Guildford, UK, Invited Keynote talk “Radii and Density Distributions of Unstable Nuclei and Halos”, I. Tanihata

6. “The Third China-Japan-Korea Hadron and Nuclear Physics 2008 Symposium (HNP08)”, June 23 – 27, 2008, Lanzhou, China. Oral presentation: “Confirmation/observation of hindered E2 strengths in 16,18C”, Presented by H. J. Ong, and N. Imai, D. Suzuki et al..

7. “The Fifth International Conference on Exotic Nuclei and Atomic Masses (ENAM08)”, September 7 – 13, 2008, Ryn, Poland. Oral+Poster presentation: “Observation/confirmation of hindered E2 strengths in 18C/16C”, Presented by H. J. Ong, and N. Imai, D. Suzuki et al..

4.3 Other meetings1. 「第４回　停止・低速不安定核ビームを用いた核分光研究会」SSRI4　２００７年１２月２０-２１日、東北大学サイクロトロン・ラジオアイソトープセンター、招待講演「Radioactive Beam Science, Past, Present, and Future」谷畑勇夫

2. 2008年国立天文台研究会「rプロセス元素組成の統合的理解-量子ビームでさぐる宇宙進化の理解を目指して-」　２００８年３月１３、１４日、筑波大学・大学会館、招待講演「r-過程原子核の大量生成法はあるのか？」谷畑勇夫

3. 日本物理学会第６３回年次大会　近畿大学本部２００８年３月２２-２６日、原著講演「11Li核ハローの２中性子移行反応の測定」谷畑勇夫、他２４名

4. 日本物理学会第63回年次大会, 平成20年3月22日 - 26日, 近畿大学本部, 大阪　原著講演: 「16,18Cの第一2+励起状態の寿命測定」王　恵仁、他22名

5. 日本物理学会第６３回年次大会　近畿大学本部２００８年３月２２-２６日、実験核物理・理論核物理領域　合同シンポジューム「パイ中間子の役割から見える原子核の新しい描像」、招待講演「テンソル力の効果を見る新しい実験」谷畑勇夫

6. 「第２１回筑波不安定核セミナー」２００８年６月１９日、日本原子力研究開発機構、講演「Transfer Reaction of Two-Halo Neutrons in 11Li and Their Correlation=11Li +p --> 9Li+t measurement with MAYA」谷畑勇夫

7. 京都大学基礎物理学研究所研究会「原子核の分子的構造と低エネルギー核反応」, 平成20年7月2日 ‒ 4日, 京都大学, 京都,　口頭発表: 「16,18Cの第一2+励起状態の寿命測定」王　恵仁、他22名

8. RCNP Workshop on Unstable Nuclei, August 8 - 9, 2008, Osaka University, Osaka, Japan.　Poster presentation: “Observation/confirmation of hindered E2 strengths in 16,18C”, H. J. Ong and N. Imai, D. Suzuki et al.

9. 日本物理学会科学セミナー「越境する科学」２００８年８月２３-２４日、東京大学駒場、招待講演「原子核を知ることは自分の起源を知ること」谷畑勇夫

10.文化講演会「最先端の自然科学にふれる」２００８年９月１３日、西条市総合文化会館、招待講演「二回の大爆発で、私たちは作られた」谷畑勇夫

11

11. Kanto Gakuin University Yokohama Autumn School of Nuclear Physics, October 9 -10, 2008, Yokohama, Japan. Oral presentation: “Anomalously hindered E2 strengths in 16,18C”, H. J. Ong

5. Other activities1. “Review of Super-FRS of FAIR project at GSI.” May 4-7, 2008, GSI, Darmstadt, Germany,

Review committee chairman.2. RCNP Workshop on unstable nucleiの主催 Title:「RCNPにおける不安定核の研究 － RCNPビームラインの可能性を探る」August 8 – 9, 2008, Venue: RCNP lecture room

Registered participants: 64 (including one from abroad) Number of talks: 30 Number of posters: 6

Photos at the workshop

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6. Brief descriptions of selected works

8.1 Measurement of two-halo neutron transfer reaction p(11Li,9Li)t at 3A MeV

(truncated from Physical Review Letters 100 (2008)192502.

I. Tanihata*, M. Alcorta**, D. Bandyopadhyay, R. Bieri, L. Buchmann, B. Davids, N. Galinski,

D. Howell, W. Mills, R. Openshaw, E. Padilla-Rodal, G. Ruprecht, G. Sheffer, A. C. Shotter,

S. Mythili, M. Trinczek, and P. Walden,TRIUMF, 4004 Wesbrook Mall, Vancouver, BC, V6T

2A3, CanadaH. Savajols, T. Roger, M. Caamano, W. Mittig***, and

P. Roussel-Chomaz,GANIL, Bd Henri Becquerel, BP 55027, 14076 Caen

Cedex 05, FranceR. Kanungo and A. Gallant,

Saint Mary’s University, 923 Robie St., Halifax, Nova Scotia B3H 3C3, Canada

M.Notani and G. Savard,ANL, 9700 S. Cass Ave., Argonne, IL 60439, USA

I. J. Thompson,LLNL, L-414, P.O. Box 808, Livermore CA 94551, USA

The neutron-rich Li isotope 11Li has the most pronounced two-neutron halo. Presently the most important question about the halo structure concerns the nature of the interaction and correlation between the two halo neutrons. In a halo, the correlation may be different from that of a pair of neutrons in normal nuclei for several reasons. Halo neutrons are very weakly bound and, therefore, the effect of the continuum becomes important. The wave function of the halo neutrons has an extremely small overlap with that of the protons and, thus, may experience interactions much different from those of neutrons in normal nuclei. The density of halo neutrons is very low compared with normal nuclear density and, thus, may give rise to quite different correlations from that in stable or near-stable nuclei.

So far, there have been several experimental attempts to elucidate the nature of these correlations between the halo neutrons in 11Li. For example, measurements of neutrons and 9Li from the fragmentation of 11Li have been used to determine the momentum correlation between two halo neutrons [1]. However, the contribution of the 10Li resonance, which decays to 9Li+n immediately, made it difficult to reach definitive

conclusions. Later, Zinser et al. [2] studied high-energy stripping reactions of 11Li and 11Be to 10Li, and the analyses of the momentum distributions suggests the necessity of considerable mixing of (1s1/2)2 and (0p1/2)2 configurations in the ground state of 11Li. The importance of the s-wave contribution is also seen in Coulomb dissociation measurements [ 3, 4]. Determinations of such amplitudes have also been attempted from data associated with the beta-decay of 11Li; however, no definite conclusions could be reached.

The newly constructed ISAC-2 accelerator at TRIUMF now provides the highest intensity beam of low-energy 11Li up to 55 MeV. This beam enabled the measurement of the two-neutron transfer reaction of 11Li for the first time. The reaction Q-value of 11Li(p,t)9Li is very large (8.2 MeV) and, thus, the reaction channel is open at such low energies. The beam energy used in this experiment (33 MeV) is not as high as usually used in studies of direct reactions, nevertheless due to the low separation energy of the two halo neutrons (~400 keV compared with about 10 MeV in stable nuclei) and low Coulomb barrier (~0.5 MeV) the reaction is expected to be mainly direct. Momentum matching is also good at this low energy because of the small internal momentum of the halo neutrons.

The beam of 11Li was accelerated to energy of 36.9 MeV. Beam intensity on the target was about 2500 pps on average, and about 5000 pps at maximum. Measurement of the transfer reaction was made possible at this low beam intensity through the use of the MAYA active target detector brought to TRIUMF from GANIL. MAYA has a target-gas detection volume (28 cm long in the beam direction, 25 cm wide, and 20 cm high) for three-dimensional tracking of charged particles, and a detector telescope array at the end of the chamber. Each detector telescope consisted of a 700 µm thick Si detector and a 1 cm thick CsI scintillation counter of 5*5 cm2. The array consists of twenty sets of telescopes. MAYA was operated with isobutane gas first at a gas pressure of 137.4 mbar and then at 91.6 mbar. These two different pressure settings were used to cross check the validity of the analysis by changing the drift speed of ionized electrons and by changing the energy loss density. The coverage of center of mass angles was also

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*present address: RCNP, Osaka University, Mihogaoka, Ibaraki 567-0047, Japan.**present address: Institute de Estructura de la Materia, CSIC, Serrano 113bis, E-28006 Madrid, Spain.***present address: NSCL, MSU East Lansing, MI 48824-1321, USA.[1] I. Tanihata et al., Phys. Letters B 287 (1992) 307.[2] M. Zinser et al., Nucl. Phys. A 619 (1997) 151.

[3] S. Shimoura et al., Phys. Letters B 348 (1995) 29.[4] T. Nakamura et al., Phys. Rev. Letters 96 (2006) 252502.

different under these pressures - as will be discussed later. Reaction events were identified by a coincidence between a parallel plate avalanche chamber (PPAC), which is placed just upstream of MAYA, and the Si array. 11Li ions that did not undergo a reaction were stopped in the blocking material just before the Si array. Details of MAYA can be found in Ref. [5].

Figure 1 shows the determined differential cross sections of transition to the ground state and the first excited state of 9Li. The overall uncertainty in the absolute cross section values is about ±10%. The transition to the first excited state (Ex=2.69 MeV) has been observed. If this state were populated by a direct transfer, it would indicate that a 1+ or 2+ halo component is present in the ground state of 11Li(3/2-) because the spin-parity of the 9Li first excited state is 1/2-. This is new information that has not yet been observed in any of previous investigations. Compound nucleus contribution should be small: at present energy, the angular distribution of compound decay must be essentially isotropic, and hence the deep minima observed in the angular distributions of the ground state and the first excited state exclude the strong contribution. However, before a final conclusion can be made, detailed studies of coupled channels and sequential transfer effects need to be undertaken.

Multistep transfer calculations to determine the differential cross sections to the ground state of 9Li have been made. For these calculations several of the three-body models from Ref.[6], recalculated using the hyperspherical harmonic expansions of Ref.[7], with projection operators to remove the 0s1/2 and 0p3/2 Pauli blocked states, have been used. In particular, the P0, P2 and P3 models from [6], which have percentage (1s1/2)2 components of 3%, 31% and 45%, respectively were used. The corresponding matter radii for 11Li are 3.05, 3.39 and 3.64 fm. For comparison, a simple (p1/2)2 model based on the P0 case, but with no n-n potential to correlate the neutrons, was also investigated. All models here do not include an excitation of 9Li core.

The calculations reported here included the simultaneous transfer of two neutrons from 11Li to 9Li in a one step process, as well as coherently the two-step sequential transfers via 10Li. The simultaneous transfers used a triton wavefuction calculated in the hyperspherical framework with the SSC(C) nucleon-nucleon force [8], and a three-body force to obtain the correct triton binding energy. The sequential transfers passed through both 1/2+ and 1/2- neutron states of 10Li, with spectroscopic factors given by respectively the s- and p-wave occupation probabilities for 11Li models of [11]. The spectroscopic amplitudes for <d|t> and <10Li|11Li> include a factor of

€

2 to describe the doubled probability when either one of the two neutrons can be transferred. S and P wave radial states were used with effective binding energies of 1.0 and 0.10 MeV respectively; this ensured a rms radii of ~ 6 fm, which is the mean n-9Li distance in the 11Li models. The differential cross sections were obtained using the FRESCO.

Curves in Fig. 1 show the results of the calculations. The wave function (p1/2)2 with no n-n correlation gives very small cross sections that are far from the measured values. Also the P0 wave function, with n-n correlation but with a small (s1/2)2 mixing amplitude, gives too small cross sections. The results of the P2 and P3 wave functions fit the forward angle data reasonably well but the fitting near the minimum of the cross section is unsatisfactory. The results may be sensitive to the choice of the optical potentials as well as the selection of the intermediate states of two-step processes. Detailed analysis of such effects should be a subject of future studies.

In summary, we have measured for the first time the differential cross sections for two-halo neutron transfer reactions of the most pronounced halo nucleus 11Li. Transitions were observed to the ground and first excited state of 9Li. Multistep transfer calculations were applied with different wave functions of 11Li. It is seen that wave functions with strong mixing of p and s neutrons which includes three-body correlations, provides the best fit to the data for the magnitude of the reaction cross section. However the fitting to the angular shape is less satisfactory. The population of the first excited state of 9Li suggests a 1+ or 2+ configuration of the halo neutrons. This shows that a two- nucleon transfer reaction as studied here may give a new insight in the halo structure of 11Li. Further studies clearly are necessary to understand the observed cross sections as well as the correlation between the two-halo neutrons.

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[5] C. E. Demonchy et al., Nuc. Instr & Methods A 573 (2007) 145. [6] I.J. Thompson and M.V. Zhukov, Phys. Rev. C 49 (1994) 1904.

[7] I.J. Thompson et al., Phys. Rev. C 61, 24318 (2000).[8] R. de Tourreil and D.W.L. Sprung, Nucl. Phys. A 242 (1975) 445.

8.2 Radioactive beam science, past, present and future.(truncated from Nucl. Instr. and Meth. in Phy. Res. B 266 (2008) 4067-4073.)Isao Tanihata, RCNP/Osaka University, 10-1 Mihogaoka, Ibaraki, Osaka, 567-0047, Japan

Reaction studies with radioactive ion beams (RIB) were firstly invented in the middle 1980th in Berkeley using the only high-energy heavy-ion machine at that time. It was motivated strongly by the discoveries of projectile fragmentations of high-energy heavy ions (~ 1A-2A GeV) colliding on target nuclei. [10] The projectile fragmentations presented two important properties for production of radioactive beams. One is the copious production of nuclei far from the stability line and the other is the narrow momentum distribution of the fragments. The first RIB experiments have been performed using an existing beam line of Bevalac just placing a production target and collimators at relevant places to keep an achromaticity of the beam. A few but extremely important discoveries have been made by those first generation experiments and opened a RIB science.

Since then, various methods of producing RIB have been developed. They are now classified into two categories. One is the in-flight method and the other is the reacceleration method. The in-flight method uses an electromagnetic separator to select and guide RIBs to the reaction target for studies. Not only the projectile fragmentation of high-energy heavy-ions but also other reactions with incident energies from a few MeV to a few GeV per nucleon have been used. Different types of reactions such as transfer reactions, projectile fragmentations, and fission in flight are the main production reactions. The reacceleration method uses the ISOL technique to produce radioactive nuclei, then ionizes and accelerates desired nuclei to energies high enough for collision studies.

Scientific goals of RIB science have been identified and published in many documents for the planning of new facilities such as RIA, SPIRAL2, FAIR, and EURISOL. Among them I consider that the study of r-process is the most appealing for other parts of the scientific society as well as for the general society, needless to say the scientific importance as nuclear and nuclear-astrophysics. Gold, an important element for modern electronics and jewelries, is one of the products of r-process. The R-process is the only process that synthesize elements heavier than Bi. The Uranium that gave us the chance to discover radioactivity and thus nuclei is also a product of r-process. So I feel that the understanding of r-process is an obligation for nuclear scientists.

Let us consider a question, “What would be the best facility for r-process studies?” The r-process goes though extremely neutron rich region of the nuclear chart and thus most of nuclide on the path have not been produced yet. Two important mass regions were identified for understanding r-process. Both are regions with mass numbers just below the peaks of abundance. Presently nuclear models using accepted shell structure underestimate the abundances in these regions. An artificial modification (weakening) of shell structure brings the estimation closer to the observed abundances. [11] So it is important to know the structure of nuclei in these region to conclude whether the weakening of shells are true or some other mechanisms are involved. For the region above A>170 we still don’t know how to produce such nuclei. However, present days facility are getting close to the region below A<150.

What would be the best facility for the study of such nuclei? The most important obviously is the production of the nuclei along the r-process path. Presently, actinide-fission, either in ISOL or in flight, is considered to be the best methods. For example, 132Sn is produced with high intensity even enough for reaction studies. However, it is still difficult to produce nuclei south of 132Sn the region of real interests. Problems exist for both types of facilities. In in-flight method, the primary intensity of U would be limited. In Fig. 2 the expected production rate of 132Sn isotopes are shown for the incidents of 136Xe and 238U. Definitely many more 132Sn is produced from U fission in flight. However this comparison may not be realistic because the incident beam intensities are assumed to be the same. Usually it is much easier to produce Xe ions than U ions and thus this difference may be compensated. In particular our nuclei of interest are the ones with the smaller Z and thus

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Fig. 2. Yield estimation at RIBF.

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References* Present address: RCNP/Osaka University, 10-1 Mihogaoka, Ibaraki, Osaka, 567-0047, Japan[10] A. S. Goldhaber and H. H. Heckman, Ann. Rev. Nucl. Part. Sci. (1978) 16. H. H. Heckman, D. E. Greiner, P. J. Lindstrom, and F. S. Bieser, Phy. Rev. Lett. 28 (1972) 926.[11] B. Pfeiffer, K. –I. Kratzand, K. –K. Thielemann, Zeit. Phys. A 357 (1997) 235.

the difference of productions is even smaller. (See lines in Fig. 3)

An ISOL facility with U target can give extremely high intensity 132Sn beam. For example SPRAL2 expect to have more than 109 /s of accelerated beam. However the intensity for nuclei south of 132Sn is again expected to be weak. By reducing Z by 1, intensity is already below 102 /s. It is due to the fact that refractory elements are difficult to extract from a target. Unfortunately most of them are refractory. A similar situation can be seen near and below the Ni isotopes that belong to another interesting region of nuclei. Therefore present and presently proposed facilities will have basic difficulty for producing important nuclides.

An interesting and feasible possibility exists in a new arrangement of the separation technique and the acceleration. Instead of using the in-flight method for producing various kinds of elements and stop-and-reaccelerate, lets use the methods in reverse order. One has to recall that it is not necessary, at in-flight facility, to accelerate all kind of elements in an driver accelerator to obtain beams of a variety of elements. It needs some of the neutron rich ions to produce neutron-rich nuclei of elemental variety. Some idea of such secondary fragmentation has been discussed in Eurisol facility discussion. [3] The neutron rich isotope 132Sn is such a nucleus that can be used effectively for production of neutron rich nuclei. The production yield of nuclei from the projectile fragmentation of 132Sn is estimated and shown in Fig. 3 in which the intensity of SPRAL2 is used. The intensities of important nuclei are orders of magnitude higher than that expected from other methods. Projectile fragmentation produces many isotopes and thus these enhancements exist for a wide variety of neutron rich nuclei southwest of 132Sn.

Another example is the production of 78Ni and around. Those nuclei are important for the r-process and also important for the understanding of nuclear structure far from the stability line. In this case beam of 91Kr or 92Kr from ISOL would be the most effective. Intensity

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Fig.3. Yield of N=82 isotones by various methods

estimated for 91Kr at SPIRAL2 is 7x1010 /s. Fragmentations of 91Kr of that intensity will give the yield as shown in Fig. 4. More than 103 /s of 78Ni will be available. It is two orders of magnitudes higher than any other proposed facilities. With such an intensity most of the spectroscopic studies can be performed.

This new scheme uses the power of ISOL for the high-intensity production of specific elements and then uses the versatility of projectile fragments.12 It is not necessary to develop a beam of difficult elements in ISOL, instead maximize the intensity of the most favorable elements for extraction. The high intensity RI beam has to be accelerated to an energy high enough for projectile fragmentation, this is about 100 MeV per nucleon.

Important measurements for understanding r-process path are masses, lifetimes, and beta-decay branches including neutron emission amplitudes. Therefore the experiments themselves are not very difficult once such nuclei are produced. It is also important that those data are extremely valuable for understanding the nuclear structure far from the stability line and to extract important interactions that change magic numbers in neutron rich nuclei. Thus refined nuclear model would be used to predict the r-process path where we still do not know how to produce important nuclei.

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[12] This idea has been presented in EURISOL project white paper. “The physics case for EURISOL (December 2003) A14.

8.3 Lifetime measurements of first excited states in 16,18C

(excerpted and modified from Physical Review C78 (2008) 014308)

H. J. Ong, N. Imai, D. Suzuki, H. Iwasaki, H. Sakurai, T. K. Onishi, M. K. Suzuki, S. Ota, S. Takeuchi, T. Nakao, Y. Togano, Y. Kondo, N. Aoi, H. Baba, S. Bishop, Y. Ichikawa, M. Ishihara, T. Kubo, K. Kurita, T. Motobayashi, T. Nakamura, and Y. Yanagisawa

The advent of experimental techniques and accelerators has contributed towards new discoveries such as the discoveries of the neutron halo and skin structure, as well as the modification of the magic numbers.

Recently, we reported another exotic phenomenon of extremely suppressed B(E2) value for the transition between the first 2+ (21+) state to the ground (0gs+) state in neutron-rich 16C. The B(E2) was obtained by measuring the mean lifetime of the 21+ state using a new experimental technique. In general, an even-even atomic nucleus tends to exhibit global behavior of a quantum liquid drop, wherein the B(E2) is inversely proportional to the excitation energy of the 21+ state (E(21+)). However, the measured B(E2) of 16C was found to deviate greatly from the value expected by the empirical formula of the E(21+), indicating a suppressed proton collectivity in 16C.

Contrary to the suppressed proton collectivity, a large neutron collectivity was suggested based on the measurement of the interference between the nuclear and electromagnetic interactions in the excitation from the ground state to the 21+ state. Indeed, a large quadrupole deformation length was observed in a recent work on the proton inelastic scattering, revealing that the neutrons predominantly contribute to the strength of the excitation to the 21+ state whereas the protons seem to be frozen.

The suppressed B(E2) may indicate quenched effective charges and/or the emergence of a new magic number Z=6 in the light neutron-rich carbon isotopes. For neutron-rich nuclei with weakly-bound neutron(s), the core polarization is likely to be weakly induced. This effect together with the effect of large isospin gives rise to quenched core polarization charges, in particular, a small neutron effective charge that reduces the contribution of the valence neutron(s) to the B(E2) value. Indeed, the quenched effective charges have been observed in the neighboring 15,17B nuclei. Moreover, in the case of the closed shell nuclei, the B(E2) value will also be reduced. In this context, we note that the emergence of the proton magic number Z=6 has been suggested by a shell model calculation. To shed light on the exotic phenomenon exhibited by 16C and to scrutinize the claim for the emergence of the Z=6 magic number in the neutron-rich C isotope, more experimental information specifically on the neighboring 18C isotope is awaited.

Here, we report on the first lifetime measurement for the 21+ state in 18C populated via inelastic scattering of a 79-MeV/nucleon 18C beam on a 9Be target. Besides, lifetime measurements were also extended to the 21+ state of 16C, thereby reexamining the result reported previously. In these measurements, the 21+ state was populated through two different

reactions, namely, inelastic scattering of 16C on 9Be at 72 MeV/nucleon and breakup reaction of 18C at 79 MeV/nucleon. Moreover, the γ-ray angular distribution was measured for the 21+ state produced in the 16C inelastic scattering at 40 MeV/nucleon to incorporate the distribution in an improved analysis of the previous data. Measurements were also performed for known lifetimes of the excited 1/2- state in 11Be and 3- state in 16N produced through breakup reaction of 18C to verify the method.

The lifetime measurements were performed by means of the recoil shadow method (RSM), which was first applied to our previous work on 16C. In this method, the lifetime is determined by observing the emission-point distribution of γ rays emitted in flight from excited nuclei produced in inverse-kinematics reactions of incident radioactive projectiles. In the present work, we have upgraded the RSM to enhance both the efficiency and the accuracy of the measurement. The upgraded scheme thus employed involved a large array of NaI(Tl) detectors as well as a novel procedure that enables determination of lifetimes independent of theγ-ray anisotropy, which is to arise from nuclear spin alignment.

The experiment was performed at the RIKEN accelerator research facility. Secondary beams of 16,18C were produced in two separate measurements through projectile fragmentation of an 110-MeV/nucleon 22Ne primary beam, and separated by the RIPS beam line. The 16,18C beams with energies of 72 MeV/nucleon and 79 MeV/nucleon, respectively, were directed at a 370-mg/cm2 9Be target placed at the exit of the RIPS beam line. In the case of the measurement of the γ-ray angular distribution for the reanalysis of the previous data, the energy of the 16C beam was further reduced to 40 MeV/nucleon by using a 6-mm-thick aluminum degrader. Two sets of parallel plate avalanche counters (PPACs) were placed upstream of the target to measure the position and angle of the projectile incident upon the target. Outgoing particles from the target were identified by the ΔE-E-TOF method using a plastic scintillator hodoscope located 3.8 m downstream of the target. Scattering angles were determined by combining the hit position information on the hodoscope with the incident angles and hit positions on the target obtained by the PPACs.

To implement the RSM concept, a thick γ-ray shield was placed around the target. The γ rays from the excited nuclei in-flight were detected by an array of 130 NaI(Tl) detectors. In the present work, we measured the deexcitation γ rays with and without the lead shield and determined the deficiency of the γ-ray yields due to the lead shield. The lifetime was determined by comparison of the measured deficiency with the simulated one. The Monte Carlo simulation was performed using the GEANT code, taking into account the geometry of the experimental setup, the energy and emittance of the projectile, the angular spread due to reaction and multiple scattering, and the energy loss in the target.

The τ(21+) values for 16,18C determined in the present work are 18.9±0.9±4.4 and 18.3±1.4±4.8 ps, which correspond to B(E2) values of 2.6±0.2±0.7 e2fm4 and 4.3±0.2±1.0 e2fm4 for 16,18C, respectively. Although the B(E2) value for 18C is almost twice as large as that of 16C, it is comparable to the B(E2)= 3.7

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e2fm4 of the closed-shell 14C nucleus. Both the energy of the 21+ state and the B(E2) value remain small in 18C, clearly indicating that the phenomenon of hindered E2 strength observed in 16C persists in 18C. The B(E2) values for 16,18C obtained in the present work are comparable to or smaller than those of the singly- or doubly-closed nuclei. In the framework of shell model calculation, the suppressed B(E2) values can be attributed to the small effective charges and the widening of the energy gap between the π(p1/2)-π(p3/2)

orbitals. The strong hindrance of the E2 transition can also be illustrated through comparison with an empirical formula based on a liquid-drop model, where the B(E2) values for 16,18C are seven and five times overestimated. The small B(E2) values for 16,18C, together with the small value for 14C, indicate a possible proton-closed shell in the neutron-rich 14,16,18C nuclei.

7. AcknowledgementThe laboratory has been established by a donation of Dr. and Mrs. A. Suzuki to the Osaka

university. Whole research project become possible under this contribution. We would like to express our greatest acknowledgement to him.

The experimental program has been supported by the grant-in-aid for Scientific Research No. 20244030 from Japan Society for the Promotion of Science (JSPS).

December 2008

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