RIKEN Projectile Fragment Separator Research Articles

Laser spectroscopy of atoms in superfluid helium for the measurement of nuclear spins and electromagnetic moments of radioactive atoms

A new laser spectroscopic method named “OROCHI (Optical RI-atom Observation in Condensed Helium as Ion catcher)” has been developed for deriving the nuclear spins and electromagnetic moments of low-yield exotic nuclei. In this method, we observe atomic Zeeman and hyperfine structures using laser-radio-frequency/microwave double-resonance spectroscopy. In our previous works, double-resonance spectroscopy was performed successfully with laser-sputtered stable atoms including non-alkali Au atoms as well as alkali Rb and Cs atoms. Following these works, measurements with 84−87Rb energetic ion beams were carried out in the RIKEN projectile fragment separator (RIPS). In this paper, we report the present status of OROCHI and discuss its feasibility, especially for low-yield nuclei such as unstable Au isotopes.

Laser–radio-frequency double-resonance spectroscopy of84−87Rbisotopes trapped in superfluid helium

In this paper, we report on a laser spectroscopy measurement of ${}^{84\ensuremath{-}87}\mathrm{Rb}$ isotopes in superfluid helium (He ii) at 1.8 K using laser--radio-frequency double-resonance spectroscopy. Rb ion beams $(g60$ MeV/u) provided by the RIKEN projectile fragment separator (RIPS) were injected and trapped into He ii. The stopping position of atoms in He ii was precisely confirmed by laser spectroscopy. By optically pumping the trapped Rb isotopes, large atomic spin polarization (up to 40%) of each observed isotope in the ground state was achieved. The laser--radio-frequency double-resonance spectra were observed for stable ${}^{85,87}\mathrm{Rb}$ isotopes as well as for unstable isotopes ${}^{84,84m,86}\mathrm{Rb}$ by scanning a weak magnetic field with a fixed-frequency RF field. From the measured Zeeman splitting, nuclear spin values for ${}^{84m,84\ensuremath{-}87}\mathrm{Rb}$ isotopes were determined with reasonable accuracy. The number of ions injected into He ii for the resonance spectra measurement was on the order of $10{}^{4}$ particles per second. This work may open new opportunities for the study of various particles trapped in condensed helium in several fields.

Nuclear reactions of 19,20C on a liquid hydrogen target measured with the superconducting TOF spectrometer

Reaction cross sections with various kinds of breakup channels for neutron-rich carbon isotopes 18–20C and for 9Be impinging on a liquid hydrogen target were investigated at 40 MeV/nucleon. The nuclides of interest were produced via projectile fragmentation from a 63 MeV/nucleon 40Ar beam and were separated in flight at the RIKEN projectile fragment separator (RIPS). The combination of the large-acceptance superconducting TOF spectrometer, TOMBEE (TOF Mass analyzer for exotic BEam Experiment), with a liquid hydrogen target, CRYPTA (CRYogenic ProTon and Alpha target system), enables simultaneous measurements of several reaction channels: the reaction cross sections (σR), individual elemental fragmentation cross sections (σΔZ), charge-changing cross sections (σcc), neutron-removal cross sections (σ−xn), and charge-pickup cross sections (σΔZ+1) for 19,20C; σΔZ, σ−xn, and σΔZ+1 for 18C; and σR for 9Be. The present σR of 9Be on proton, σR=397±23 mb, measured in the inverse kinematics, was consistent with the previous measurements using proton beams at different laboratories. The σR of 19C and 20C on proton were determined to be σR=754±22 mb and σR=791±34 mb, respectively. Taking into account the beam energy and target dependence of σR, the present σR are found to be considerably enhanced compared with those measured at around 1 GeV/nucleon. The σΔZ+1 appears to increase with the mass number of the projectiles, and it significantly contributes to σR in the present energy range. The finite-range optical-limit and few-body Glauber model analyses were performed for σR to study the nuclear matter density distributions and to derive the relative strength of the s-wave components of the valence neutrons in 19C and 20C. A neutron halo structure of 19C is confirmed with an s-wave dominance of the valence neutron when the effect of the charge-pickup reaction is taken into account. The large σ−n of 19C and σ−2n of 20C also support the decoupled structures of 18C +n and 18C+2n, respectively. The σcc of 19C and 20C agree with each other within their experimental uncertainties, which might indicate a similar proton density distribution in 19C and 20C. The σΔZ decreases monotonically without the even–odd effect as the number of removed protons increases.

MEASUREMENT OF THE PROTON-PROTON CORRELATION FUNCTION FROM THE BREAK-UP OF 22Mg AND 20Ne

An experiment of 22 Mg and 20 Ne beams bombarding on a 12 C target at an energy of 60~70 A MeV has been performed at the RIKEN projectile fragment separator (RIPS) in the RIKEN Ring Cyclotron Facility to study the two-proton correlated emission from 22 Mg and 20 Ne excited states. The two-protons momentum correlation functions have been obtained for 22 Mg and 20 Ne , respectively. The trajectories of the 22 Mg decayed products (20 Ne + p + p ) were also measured to get the angular correlations between the two protons in Center of Mass of decaying system by relativistic-kinematics reconstruction. The results exhibit that 22 Mg has the features of 2 He cluster decay mechanism.

Temperature-Dependent Behavior of Impurity Atoms Implanted in Highly Oriented Pyrolytic Graphite –An Application of a New Online TDPAC Method–

The behavior of impurity atoms in graphite is one of the most intriguing subjects in current materials science. It is well known, for graphite intercalation compounds, that the type and concentration of intercalated impurities control their physical and chemical properties. It is thus of great importance to investigate the behavior of intercalants in the two dimensional layers for further understanding of their interacting nature with the graphite matrix. In order to investigate such interactions, it is essential to introduce impurities of interest into graphite; and, for that purpose, the ion-implantation method would be the simplest method. Nuclear techniques with radioactive probes are highly suited to such studies on the behavior of dilute impurities because of their high sensitivity. In our recent study, we developed a new detection apparatus for radioactive impurities: an online time-differential perturbed angular correlation (TDPAC) measurement system. The TDPAC method is a spectroscopy using unstable nuclei as the probe; by observing the time-variant angular correlation of rays successively emitted in the disintegration process of the excited nucleus of the probe, we can obtain direct information on the local field in the vicinity of the probe nucleus through electromagnetic interactions between the probe nucleus and extranuclear field. The newly developed system enables the application of shortlived unstable nuclei to the spectroscopy as the parents of probes; they are implanted in samples of interest at high energy immediately after their production by acceleratorbased nuclear reactions. In our previous paper, we demonstrated the applicability of the present online system and reported on the residence site of F( O) implanted in highly oriented pyrolytic graphite (HOPG). In comparison with the previous data obtained at 18K, we here discuss the displacement of the F( O) nuclei at room temperature on the basis of the damping trend of the TDPAC spectra. The online TDPAC experiment was carried out at the RIKEN Nishina Center. Ionized Ne was accelerated by a two-stage acceleration with the AVF cyclotron and the ring cyclotron up to 110MeV/u at a beam intensity of 150 pnA. A variety of radioactive nuclides were produced at a Be production target by projectile-fragmentation reactions; the secondary beam of interest, O (t1=2 1⁄4 26:9 s), was separated out of these heavy ions at a purity of 98% or higher by the RIKEN projectile-fragment separator (RIPS). A wellfocused pulsed beam of O was implanted deep in an HOPG sheet at a beam intensity of 10 s . The online TDPAC measurements were performed at room temperature on the 1357–197 keV cascade rays from F nuclei, whose 197 keV intermediate state with nuclear spin of I 1⁄4 5=2 has a half-life of 89.3 ns. In the present work, the time evolution of the angular correlations of the cascade rays was observed at =2 and directions. In order to obtain better counting statistics, we used sixteen BaF2 detectors. They were arranged in four independent detector planes for the investigation of the sample-to-detector configuration dependence of perturbation patterns for a single-crystalline sample. All the events were taken in a list mode using a CAMAC system so that we could work on off-line data processing searching for the optimum conditions on energy gates for the detected rays of interest. The experimental details are described elsewhere. The TDPAC spectra of F( O) implanted in the HOPG sheet at room temperature are shown in Fig. 1 together with the previous data obtained at 18K. In the present experiment, we derive the time evolution of the angular correlation as a function of the time interval between the cascade -ray emissions, RðtÞ, from the following simple arithmetic operation: -0.2

MEASUREMENTS OF REACTION CROSS SECTION AND FRAGMENT MOMENTUM DISTRIBUTION FOR N=10 PROTON-RICH ISOTONES

The longitudinal momentum (p//) distributions of fragments after one-proton removal and reaction cross sections (σR) for several N = 10 isotones at intermediate energy (~ 70A MeV ) have been measured simultaneously at the RIken Projectile fragment Separator (RIPS). p// was measured using a direct time-of-flight (TOF) technique, while σR was determined using a transmission method. An enhanced σR is observed for 23 Al comparing with its neighbors. The p// distribution for 22 Mg fragments from 23 Al one-proton removal reaction has been obtained for the first time. The experimental data are discussed within a Few Body Glauber (FBG) framework. Analysis of p// indicates a dominant d-wave component in the ground state of 23 Al . This agrees with recent experimental results. We also suggest the possibility of an enlarged 22 Mg core for proton-rich nucleus 23 Al .

SPECTROSCOPY ON UNSTABLE NUCLEI AT RIKEN

Recent studies on nuclear structure by using radioactive isotope beams available at the RIKEN projectile-fragment separator (RIPS) are introduced. Special emphasis is given to experiments selected from recent programs that highlight studies; anomalous quadrupole natures in 16 C , new results on 28 Ne with respect to the island-of-inversion region. Experimental programs at the new facility called RIBF are also introduced.

Spectroscopy on neutron-rich nuclei at RIKEN

Recent studies on nuclear structure by using radioactive isotope beams available at the RIKEN projectile-fragment separator (RIPS) are introduced. Special emphasis is given to experiments selected from recent programs that highlight studies; the particle stability of very neutron-rich nuclei, 34Ne, 37Na and 43Si, the nuclear structure of 27F, 30Ne and 34Si at N ∼ 20, and the anomalous quadrupole transition in 16C.

RF deflector system for proton-rich RI beams in RIKEN

An RF deflector system was constructed and installed at the RIKEN Projectile-Fragment Separator (RIPS) in order to increase the fraction of proton-rich nuclei of interest in RI beams. This system was successfully commissioned and used in experiments with proton-rich 46Cr, 50Fe, 54Ni, and 23Al beams.

One-neutron halo structure inC15

The one- or two-neutron removal reactions as well as reaction cross sections for C-14,C-15 on carbon target have been studied by using 110A MeV Ne-22 primary beam on Riken Projectile Fragment Separator in RIKEN. The longitudinal momentum distributions of C-13,C-14 fragments from C-15 and C-13 fragments from C-14 breakup have been measured at 83A MeV by means of direct time-of-flight method. Full width at half maximum (FWHM) of the distributions have been determined to be 71+/-9 MeV/c and 223+/-28 MeV/c for C-14 and C-13 from C-15, and 195+/-21 MeV/c for C-13 from C-14. The FWHM for C-13 fragments from C-15 and C-14 breakup are consistent with the Goldhaber model's prediction. While the FWHM of C-14 fragments from C-15 is much smaller, which confirms the experimental results from MSU and GANIL, an anomalous enhancement from its neighbors has been observed in the measured reaction cross section of C-15. The experimental data are discussed in the framework of the Glauber model. The analysis of both the fragment momentum distributions and reaction cross sections indicates a dominant s-wave component in the ground state of C-15.

Slow RI-beams from projectile fragment separators

The projectile fragment separator provides a wide variety of short-lived RI-ions with less restrictions on their chemical property or lifetime limit. The beam energy and quality is, however, not adequate for low-energy beam experiments, in particular for trapping experiments. Recently, one has proposed to obtain a low-energy beam from an energetic RI-beam leaving a projectile fragment separator by using a large gas-catcher and an rf ion-guide system. In off-line and in on-line test experiments, the principle of the rf ion-guide was proven. An overall efficiency of 0.2% for 70 MeV/u 8Li from the RIKEN projectile fragment separator (RIPS) was obtained so far.

Design and construction of a position-sensitive rotating implantation target system with β–γ discrimination capabilities

A rotating implantation target system has been designed for use at the RIKEN projectile fragment separator. While the original design is for the study of short β-decay lifetimes, the versatility of the device is also noted. The ability of the position-sensitive detector system to distinguish β and γ rays is studied, and the correlation of implantation position to beam position for purposes of particle identification is also described. Future improvements and experiments are planned and described briefly. The target detector system represents a simple, inexpensive, yet efficient way of conducting online implantation experiments where particle identification and rapid measurements are desired.

Spectroscopy on very neutron-rich nuclei beyond N = 20

Recent studies on nuclear structure by using radioactive isotope beams available at the RIKEN projectile-fragment separator (RIPS) are introduced. Special emphasis is given to two selected experiments from recent programs that highlight studies on the magicity loss observed for very neutron-rich nuclei beyond N = 20 in the “island-of-inversion” region; the particle stability of 31F, and the low-lying excited states of 34Mg.

Spectroscopy on Exotic Nuclei at RIKEN

Recent studies on nuclear structure by using radioactive isotope beams available at the RIKEN projectile-fragment separator (RIPS) are introduced. Special emphasis is given to selected experiments from recent programs that highlight studies on the magicity loss observed for very neutron-rich nuclei beyond N=20 in the 'island-of-inversion' region; the particle stability of 3 1 F, and the low lying excited states of 3 4 Mg.