IGLIS-NET Newsletter No.1 May 2013

Transcription

1 Introduction: This is the first issue of the IGLIS-NET (In-Gas Laser Ionization and Spectroscopy NETwork) newsletter, which was launched on Dec after a fruitful discussion session during the international workshop on low-energy radioactive isotope beam production by in-gas laser ionization for decay spectroscopy held at RIKEN. The IGLIS-NET is initially constituted by 14 participating research groups and institutes, being the KEK RNB group ( responsible for the activity management. Frequent exchanges of the communications among these participants will be promoted through the website of the IGLIS-NET ( under construction at the moment), and the status of research activities of participating groups will be periodically summarized in a newsletter. The present issue includes five status reports from IGISOL facility of Jyväskylä, IGLIS laboratory at KU Leuven, GALS at JINR, KISS at KEK, and PALIS/SLOWRI at RIKEN. IGLIS-NET News: The construction fund for the SLOWRI facility at RIKEN-RIBF was successfully raised at the end of Jan The fabrication of main components has been started and their installations are planned by the end of March In a recent on-line experiment, an energetic Fe beam from the Riken Ring Cyclotron was injected into the argon gas cell of the KISS setup and, after being stopped, neutralized, transported by argon buffer gas in the gas cell, a laser-ionized Fe beam was successfully produced. The IGISOL-4 facility at Jyväslkylä is under commission, allowing direct access of up to three laser beams to the target area either for in-gas-cell or in-jet ionization. For in-gas-cell and in-jet laser spectroscopy, the effect of pressure broadening was studied in the reference cell, in the gas cell and in the gas-jet by performing Ti:Sapphire laser scans of the 327nm transition in stable copper. Two methods for producing neutron deficient Ag isotopes, with the final goal of reaching 94 Ag and the (21 + ) isomer, have been tested on-line. Further studies will be performed to determine which of the two methods (i.e. hot cavity or gas cell) gives better efficiency, which is essential for a future proposal on the resonance ionization spectroscopy for the isotopes and isomers of Ag. An off-line test bench based on an Ion Guide Quadrupole Mass Spectrometer (IGQMS) system has been recently installed, which will be coupled with the laser ion source in order to provide necessary off-line test and 1

2 development time. A new set-up GALS (GAs-cell based Laser-ionization Setup) for the production and study of heavy neutron-rich nuclei located along the neutron closed shell N = 126 is under construction at JINR, Dubna. The construction of a new IGLIS laboratory at KUL will be completed by June and first off-line test is foreseen in this fall. The developments of a high-efficient laser ionization system especially for in-gas-jet laser spectroscopy will be carried out. The resonance ionization in a supersonic gas jet should allow better spectral resolution than that achieved by in-gas-cell ionization spectroscopy. After full characterization and optimization in off-line tests at the IGLIS laboratory, the laser system of the IGLIS set-up will be relocated at the S3 facility in GANIL. Workshop and Conference: Workshop on low-energy radioactive isotope beam production by in-gas laser ionization for decay spectroscopy at RIKEN, Dec , 2012, RIKEN-Wako. The 11 th IGISOL workshop (Conference on Stopping and Manipulation Ions, SMI-13), June 11-13, 2013, Jyväslkylä ( ). RIBF Collaboration Meeting (KISS, SLOWRI), June 24-25, RIBF User s Meeting, June 26-27, Recent Publications: Please look at IGLIS-NET website ( where the publications are grouped according to the topics defined in the IGLIS-NET framework. Job Opportunities / Personnel Exchanges or Transfers N. Imai (KEK) has returned in March from CERN / ISOLDE after one-and-a-half years stay. Any requests or comments about the IGLIS-NET newsletter are welcome. Please send them via iglis-net@kek.jp. 2

3 Status Report (1) Resonance ionization spectroscopy and laser development at the IGISOL facility, Jyväskylä (Iain D. Moore*, Mikael Reponen, Annika Voss, Volker Sonnenschein, Ilkka Pohjalainen, Enaam Hasan ) I. In-gas-cell/ In-gas-jet laser spectroscopy Resonance photo-ionization with high power pulsed lasers provides in principal isotopically and isomerically pure beams, using the unique atomic fingerprint to selectively excite and finally ionize the atom of interest. Indeed, the laser ion source also provides the possibility for in-gas-cell or in-gas-jet resonance ionization spectroscopy, which at IGISOL was first discussed in 2006 [1]. In-gas-jet spectroscopy may be viewed as an extension of the so-called LIST (Laser Ion Source Trap) technique, in which neutral atoms are selectively ionized upon exit from the gas cell, within the gas jet, and are captured by the rf field of an rf multipole (sextupole) before transport to the mass separator. The gas jet is an attractive environment for spectroscopy due to the substantial reduction in the atomic line broadening caused by Doppler and pressure effects. Figure 1. Ti:Sapphire frequency scans of the 327 nm transition in copper, probed in a reference cell, gas cell and gas jet. The vertical line indicates the centre-of-gravity (CoG) of the reference cell structure. The new IGISOL-4 facility (currently being commissioned) allows direct access of up to 3

4 three laser beams to the target area, either for in-source or in-jet ionization. In December 2012, Ti:sapphire laser scans of the first step nm transition in stable copper were performed, comparing the spectral resolution obtained in a reference cell (vacuum, crossed beams geometry), a gas cell (180 mbar He) and the gas jet. Figure 1 illustrates the hyperfine splitting which is dominated by the ~12 GHz splitting of the atomic ground state. A clear effect of pressure broadening is seen in the gas cell, dramatically reduced in the gas jet. The residual broadening is dominated by the laser linewidth. A frequency shift of ~3 GHz observed in the gas jet resonance translates into a jet velocity of 1040(70) m/s. Near future plans include a continuation of the gas jet studies published in 2011 [2]. Using an off-line ion guide quadrupole mass spectrometer system (IGQMS), we will directly study the differences between the density distribution and velocity profiles of gas jets formed with exit holes and Laval nozzles via laser ionization. This will support the indirect fluorescence measurements of the jets, for example the false colour visualization shown in Fig. 2. This work aims to improve the geometrical atom-laser overlap, directly proportional to the LIST efficiency. In parallel with developments associated with a reduction of the laser linewidth, the optimum spectral resolution for gas jet spectroscopy will be obtained. Figure 2. False colour visualization of the gas jet from a Laval nozzle. In-gas-cell spectroscopy and selective laser ionization for radioactive beam production benefits greatly from the pioneering Leuven gas cell geometry which sees a physical separation of the laser ionization zone from the recoil stopping volume. This not only improves the laser ion survival in the presence of the ionization density associated with the passage of an accelerator beam (and thus increases the effective volume for laser ionization), but also allows the use of ion collector plates to improve the selectivity via the collection of non-neutralized ions. Figure 3 illustrates the dual-chamber concept adapted for use at IGISOL, along with locations of a 223 Ra α-recoil source which was used to determine the ion guide efficiency. Efficiencies of 10 20% were obtained from the ionization volume (positions A1, A2 and A3), dropping by a factor of ten with the source in the stopping chamber (positions B1 and B2). Ions created close to the walls (B2) suffer high losses due to negligible gas flow and diffusion. Such results may be combined with more detailed gas flow simulations in order to optimize the geometry of such an ion guide. The dual-chamber gas cell is planned for use in several experiments at IGISOL-4 in 4

5 conjunction with laser ionization. Immediate physics opportunities include the study of neutron-deficient silver isotopes, multi-quasi-particle isomeric states in bismuth and the search for the low-lying isomeric state in 229 Th. On-going studies related to the role of the buffer gas plasma will be pursued, furthering the knowledge gained at IGISOL-3 [3]. Longer term interests include studies of the refractory elements from tungsten to platinum. [1] I.D. Moore et al., AIP Conf. Proc., 831 (2006) 511. [2] M. Reponen, I.D. Moore, I. Pohjalainen et al., Nucl. Instrum. and Meth. A 635 (2011) 24. [3] I.D. Moore, T. Kessler, T. Sonoda et al., Nucl. Instrum. and Meth. B 268 (2010) 657. Figure 3. Schematic view of the dual-chamber gas cell. 223 Ra α-recoil source locations are labeled A1 to A3 and B1, B2. An extension chamber may be fitted in front of the ion collector for non-resonant ion suppression and transverse laser ionization. II. Towards a study of the (21 + ) isomer in N=Z 94 Ag The radioactive neutron deficient silver isotopes and isomers around the N=Z region have been of considerable interest for several years. In particular, 94 Ag (N=Z) may exhibit the most unique isomer in existence. 94 Ag has been identified as having a spin trap isomer with the highest spin, (21 + ), ever observed for β-decaying nuclei. The isomer s long half-life of ~400ms, high excitation energy and high spin are matched by an unparalleled selection of decay modes including, among others, β decay and one-proton decay. However, the most exotic form of decay that has been claimed to exist in 94m Ag (21 + ) is two-proton emission [1]. Recently, the existence of the two-proton decay mode has been questioned due to the non-observation of states in 92 Rh that were supposedly populated and thus used as evidence for the decay [2]. At JYFLTRAP, a mass measurement of 92 Rh and 94 Pd, the respective two-proton and β-decay daughters of 94 Ag, has deepened the mystery [3]. The 5

6 combination of the mass measurements with the original 1-proton or 2-proton decay data leads to a contradiction in the expected isomeric state excitation energy. Additionally, an experiment performed at LBNL aiming to confirm the two-proton decay failed to observe evidence for the decay channel [4]. Mukha et al. originally proposed that the unexpectedly large probability for the two-proton decay could be due 94 Ag having a strongly-deformed prolate shape, an observation unsupported by large-scale shell-model calculations by Kaneko et al. [5]. To solve the conundrum, direct mass measurements of 93 Pd, 94 Ag and 94m Ag (21 + ) are needed to unambiguously determine the energy of isomer. In addition, a direct measurement of the spectroscopic quadrupole moment using in-source resonance ionization spectroscopy will provide insight into the shape of the isomer. A development program is underway at IGISOL to produce neutron deficient Ag isotopes, with the final goal of reaching 94 Ag and the (21 + ) isomer. Two production methods have been identified. The first utilizes a new inductively-heated hot cavity catcher, developed in collaboration with the JYFL ECR group. This was recently commissioned by implanting a 487 MeV beam of 107 Ag 21+ into the catcher at depths corresponding to the simulated implantation depths expected for the 94 Ag fusion-evaporation recoils. Following diffusion out of the graphite and effusion from the catcher, the silver atoms were resonantly ionized in a three-step scheme (UV-blue-CVL) by perpendicular laser beams. By pulsing the primary beam, the evacuation time could be measured. Depending on the catcher temperature and the implantation depth, this time varied from ~2.5 ms to ~7 ms as presented in Fig. 4. A manuscript discussing these latest results is under preparation. Figure 4. Evacuation times of 107 Ag for different catcher temperatures and implantation depths. The second approach uses the dual chamber gas cell (see Section I). In 2012, a test beam time focused on the 36 Ar( nat Zn,pxn) Ag reaction. This unfortunately failed due to the high intensity, well focused Ar beam damaging the zinc target. Nevertheless, a coating of stable 6

7 silver on the back of the target acted as a source of sputtered atoms allowing the selectivity of the laser ionization process to be probed using helium and argon buffer gases. Figure 5 presents the selectivity as a function of primary beam intensity. In 2013, a proposal was submitted to the JYFL PAC for beam time to study the production of neutron deficient silver isotopes Ag using the reactions 92 Mo( 14 N,2pxn) 104 x Ag and 64,nat Zn( 36 Ar,pxn) Ag. The first aim is to determine which of the two methods, hot cavity or gas cell, provides the highest overall efficiency. Resonance ionization spectroscopy will then be performed to extract nuclear spins and magnetic dipole moments, including those previously measured at the LISOL facility by the Leuven team. This will verify the LISOL results and thus test the reliability of the RIS method. On success, a future proposal will be submitted in collaboration with Leuven to tackle the isotopes and isomers of Ag. On April 18 th, the proposal was accepted. Figure 5. Laser ionization selectivity for silver in helium and argon buffer gases. [1] I. Mukha et al., Nature 439 (2006) 298. [2] O.L. Pechenaya et al., Phys. Rev. C 76 (2007) [3] A. Kankainen et al., Phys. Rev. Lett. 101 (2008) [4] J. Cerny et al., Phys. Rev. Lett. 103 (2009) [5] K. Kaneko et al., Phys. Rev. C 77 (2008) III. Development of narrow-band pulsed Ti:sapphire laser systems for in-jet laser ionization and high resolution spectroscopy The technique of resonance laser ionization has proven to be a very successful method for the production of radioactive ion beams (RIB). In the future, groups are aiming to push the technique to its limits. This includes the pursuit of complete suppression of isobaric interferences, enhancement of isotopic abundance sensitivity and even selection of isomeric 7

8 states. The advantages of in-jet laser ionization are well understood a reduction in the broadening mechanisms (Doppler, pressure) which often limits the ability to separate individual isotopes and isomeric states in conventional hot cavity and gas-cell based ion sources. The currently used pulsed laser systems at facilities such as ISOLDE, ISAC, LISOL and IGISOL provide only a modest resolution of the order of a few GHz. In order to fully exploit the advantages of gas-jet based laser ionization, new high resolution pulsed laser systems are being developed at IGISOL. Dual-etalon Ti:sapphire laser A relatively simple way of reducing the laser linewidth of a Ti:sapphire-based laser system is the insertion of a second etalon into the cavity. For wide range wavelength tuning this approach requires a synchronized movement of both etalons to avoid strong power dependence and mode-hops. The first spectroscopy experiments with such a newly developed dual-etalon Ti:sapphire laser system at JYFL were recently performed using a collimated atomic beam of natural copper. Figure 6 (left) showcases the achieved reduction in linewidth comparing the dual and single etalon configuration in measurements of the hyperfine structure of the 327 nm transition in copper. In this example, the resulting fit FWHM is reduced from 6.6 GHz to 2.0 GHz, a reduction in laser linewidth by a factor of ~3. Figure 6 (right) illustrates a different spectroscopic transition (244nm) which allows the separation of all four expected hyperfine components. Several other measurements have been performed to compare the extracted hyperfine parameters with literature values in order to investigate systematic uncertainties in this newly developed method of frequency scanning. Figure 6. An example of the linewidth reduction achieved using a dual etalon laser configuration compared to the standard single etalon Ti:sapphire resonator (left). Scanning the first resonant step at 244 nm in stable copper reveals the 4 hyperfine peaks (right). Injection-locked Ti:sapphire laser A further reduction in the linewidth of the normal z-shaped Ti:sapphire cavity is not easily 8

9 achieved due to spatial hole-burning and increased losses for intra-cavity etalons with higher finesse. The technique of injection locking in combination with a newly constructed bow-tie ring cavity alleviates these problems and promises narrow-band radiation with a linewidth of less than 20 MHz without introducing additional losses from frequency-selective elements. The planned setup for use at IGISOL is shown in Fig. 7. A continuous wave (cw) master laser (Sirah Matisse TS) is injected via an output coupler into a pulsed pumped Ti:sapphire bow-tie cavity (slave). To lock the slave laser cavity to this master laser a commercial stabilization unit (TEM LaseLock) is fed with a photodiode (PD) signal detecting the leaked light after one of the high reflecting (99.9%) cavity mirrors. By dithering the cavity length using a piezo-actuated mirror and using phase sensitive detection (PSD), a regulating loop can be constructed to keep the cavity in resonance with the Matisse laser. To avoid saturation of the photodiode signal and to keep the regulation loop undisturbed during the pulsed (10 khz) lasing of the slave cavity, a fast switch grounds the photodiode as well as its amplifier for a short period of time after the pump pulse. The tuning range of the bow-tie cavity is limited by the maximal traveling distance of the fast piezo, thus about 5 GHz. To extend this range in order to cover the mode-hop free tuning range of the Matisse laser (about 50 GHz), two Brewster plates, actuated with bending piezos, will be inserted into the cavity. By tilting these plates the effective cavity length can be adjusted without affecting the alignment of the cavity. Figure 7. A schematic showing the principal of how to injection-lock the pulsed Ti:sapphire laser. The choice of a cw Ti:sapphire laser as the master laser allows us to cover the whole gain spectrum of the Ti:sapphire crystal in the slave laser. Earlier variants of this system used an 9

10 external cavity diode laser for seeding, which severely limited the available wavelength range [1]. Currently the system is still in the development phase, however stable locking of the slave cavity to a stabilized He-Ne laser has been demonstrated (shown in Fig. 8). Most recently, the Matisse laser and a Millennia pump laser have been installed, with the output of the Matisse coupled into a 35m optical fiber. The Matisse will also be available for collinear laser spectroscopy at the IGISOL facility. An impressive rms frequency noise of typically 50 khz was achieved on its first day of operation. Figure 8. Left: The PD signal (red) during a triangular ramp of the piezo voltage (black). The oscillations superimposed on the cavity fringes are due to the dithering of the piezo mirror at a frequency of 41 khz. Right: The PD signal while the cavity is locked to the frequency of the He-Ne laser. The black curve shows the error/lock-in signal, again slightly affected by the dithering of the piezo. [1] T. Kessler et al., Laser Physics 18 (2008) 842. IV. Hunting for the illusive 229m Th isomeric state The thorium isotope 229 Th is a fascinating nucleus and truly remarkable in many aspects. This nuclear system supposedly possesses an isomeric state with an extremely low excitation energy of just a few ev above the nuclear ground state. In this often termed nuclear clock isomer, the two states are connected by an M1 transition. The isomer is the only known nuclear candidate which can be accessed by laser systems. Such features render 229m Th ideally placed for a long list of experimental opportunities, notably as a nuclear time standard with a precision predicted to 19 decimal places, as an ideal candidate for the space-time variation tests of fundamental constants and as the most prominent case to study the interplay of atomic electrons and the nucleus. The physics spectrum covered by this single nucleus thus connects a variety of fields from atomic physics, nuclear physics, fundamental interactions, metrology, precision spectroscopy and so on. A large number of well-recognized institutes are working towards exploitation of the isomeric 10

11 state properties, and this was reflected in a 3 day workshop hosted by the ExtreMe Matter Institute (EMMI) at GSI, in September 2012, organized by C. Brandau and I.D. Moore. With over 100 publications, including several Physical Review Letters, the isomer has been under discussion for 30 years. The caveat however is as follows: the direct observation of the transition and even definitive evidence pointing towards the existence of the isomer is still pending. An experimental ansatz to verify the existence of the isomeric state was proposed in Jyväskylä a few years ago, involving a measurement of its unique hyperfine structure (HFS) with respect to the structure of the nuclear ground state [1]. Measurements of the HFS of 229 Th have already been carried out by other groups, though the ground state HFS has only been probed in singly-ionized thorium [2] and triply-ionized thorium [3]. Recently, in collaboration with the University of Mainz, we have performed a measurement of the ground state of atomic 229 Th using high-resolution resonance ionization spectroscopy (RIS). A template for the HFS has been obtained, realizing a milestone towards the goal of possible isomer identification via its HFS [4]. Figure 9 illustrates the template with the light grey shaded area representing the search region of interest. With an expected isomeric state HFS of less than 1GHz, determination of the nuclear moments using in-source RIS will be extremely challenging. However, the possibility of performing in-jet spectroscopy will be explored in the future in combination with the injection-locked Ti:sapphire laser system. Figure 9. A measurement of the HFS of atomic 229 Th. The vertical line represents the CoG of the HFS and the grey band shows the span of the search region of interest for the isomeric state. Production of the isomeric state will be pursued at JYFL using two approaches. The first method will use an alpha decay recoil source of 233 U mounted in a specially designed gas cell [5], from which approximately 2% of the alpha decay is expected to populate the isomeric state. The second method will use the dual-chamber gas cell and the on-line reaction 232 Th(p,p3n) 229 Th at 50 MeV. In addition, approved beam time has been allocated 11

12 for an exploration of the thorium system using collinear laser spectroscopy, in combination with the laser ion source and the production methods discussed here. In order to boost the sensitivity of the collinear beams technique, a new methodology will be developed ion resonance ionization (IRIS) in the rf cooler-buncher. This will promote singly-charged thorium to doubly-charged thorium, and via time-gating techniques, a pure beam of a single isotope will be created. In the near future, all the laser spectroscopic tools available at the IGISOL facility will be combined in order to hunt down the illusive 229m Th isomer. [1] B. Tordoff et al., Hyp. Int. 171 (2007) 197. [2] W. Kälber et al., Z. Phys. A 334 (1989) 103. [3] C. Campbell et al., Phys. Rev. Let. 106 (2011) [4] V. Sonnenschein et al., J. Phys. B 45 (2012) [5] V. Sonnenschein, I.D. Moore et al., Eur. Phys. J. A 48 (2012) 52. V. An off-line Ion Guide Quadrupole Mass Spectrometer (IGQMS) system A new experimental facility is currently under development which will be coupled with the laser ion source in order to provide much needed off-line test and development time which is not always possible at IGISOL due to the on-going commissioning and future experimental programme. This test bench is based on an Ion Guide Quadrupole Mass Spectrometer (IGQMS) system, with the QMS having been loaned from the University of Mainz. This miniature IGISOL facility consists of a gas cell-based ion source (spark source, heated filament or laser ablation), a differential pumping system and a mass analysis stage. A recent photograph of the setup is shown in Fig. 10. The currently on-going and future projects with the off-line system include: Buffer gas impurity studies The suppression of impurities in the helium buffer gas such as water, nitrogen, oxygen and so forth is being studied with the QMS spectrometer, as well as a residual gas analyzer. A separate buffer gas purification system has been built to study the effects of different cryogenic cold traps (activated charcoal, zeolite 13X molecular sieve) and a getter device. Access to calibration gases is also possible. This work is part of the development of a renewal of the buffer gas purification system for IGISOL-4 in order to achieve sub-parts-per-billion impurity conditions during IGISOL operation. In the future we will purchase a cryo-cooler device to continue our development of a cryogenic ion guide for IGISOL [1]. Ion-optical improvements Currently, a simple skimmer electrode separates the ion guide from the ion-optical elements 12

13 Figure 10. Photograph of the Ion Guide Quadrupole Mass Spectrometer system (April 2013). in the differential pumping section. In order to improve the transport efficiency from the ion guide, a squeezer lens system mounted before the skimmer is being investigated. To improve the transmission further, there are plans to replace the skimmer by an RF multipole. Gas jet studies As discussed earlier, the IGQMS system offers easy access to continue the studies of gas jet properties. Resonant laser ionization will be used to directly probe heavy atoms in the gas jet in order to study the effect of a reduced temperature and pressure on atomic linewidths. The off-line system has easy access to mounting translatable stages which can hold optical fibers required for laser transport. Flow velocity and density profiles can be extracted while optimizing the ion guide nozzle shape. Californium-252 fission source In 2013 we expect to purchase a 252 Cf source which will allow access to neutron-rich refractory elements for studies of the elemental extraction efficiency dependence from a gas cell, resonance ionization spectroscopy of refractory elements, to a possible calibration source for the independent yield curves from charged-particle induced fission being studied using JYFLTRAP at IGISOL. We have recently acquired the gas cell used by Leuven for their characterization studies of the LISOL laser ion source using 252 Cf [2] for this project. [1] A. Saastamoinen, I.D. Moore et al., Nucl. Instrum. and Meth. A 685 (2012)70. [2] Yu. Kudryavtsev et al, Nucl. Instrum. and Meth. B 266 (2008)

14 VI. Towards Ion Resonance Ionization Spectroscopy (IRIS) in a RF cooler-buncher During the time of IGISOL-3, the Ti:sapphire laser system which forms the backbone of the FURIOS laser ion source at JYFL was used for optical manipulation of refractory elements in the rf cooler in preparation for collinear laser spectroscopy experiments. By pumping ions from the ground state, population may be moved to an excited, metastable state, from which high-resolution collinear laser spectroscopy may be performed. Such metastable state spectroscopy may be preferable if the ground state transition is either too weak, unfavorable for extracting nuclear parameters or beyond the wavelength capabilities of a CW laser. For example, the nuclear spin cannot be determined if the transition takes place from a ground state spin J=0 J'=1 as there is no ground-state splitting. Likewise, the nuclear spin cannot be determined for J=1/2 J =1/2 and neither the quadrupole moment. The method of manipulating the ionic state population was demonstrated on-line in 2009 using niobium fission fragments [1]. Later, the method was also used for the Y and Mn systems [2, 3]. An extension to this method is ion resonance ionization (IRIS), whereby an ion is ionized to the doubly-charged state by multiple resonant transitions. Applying this method in the cooler-buncher could result in ultra-pure beams. Typically, the cooled ion bunch suffers from isobaric contamination which adds to the background due to laser light scatter in the collinear beams technique. If the ion of interest or the major contaminant were to be ionized into a doubly-charged ion, it could be separated by its time of flight. This methodology is initially planned for the study of the thorium system. In order to transport multiple laser beams to the rf cooler, we are currently developing a system based on multiple high power optical fibers. An initial test has demonstrated a transport efficiency of up to 80% using 1.5 W of laser light at ~420 nm. The immediate goal is to optimize the output coupling from the fiber in order to enable efficient post-fiber harmonic generation. [1] B. Cheal et al., Phys. Rev. Lett. 102 (2009) [2] K. Baczynska et al., J. Phys. G 37 (2010) [3] F. Charlwood et al., Phys. Lett. B 690 (2010)

15 Status Report (2): GALS setup for production and study of heavy neutron rich nuclei at Dubna (V.I. Zagrebaev, S.G. Zemlyanoy*, E.M. Kozulin, Yu. Kudryavtsev (KUL), V. Fedosseev (CERN), R. Bark (NRF, South Africa), H.A. Othman (Menoufiya Univ., Egypt)) Introduction A new setup for production and study heavy neutron-rich nuclei located along the neutron closed shell N=126 is under construction at JINR, Dubna. The present limits of the upper part of the nuclear map are very close to stability, while the unexplored area of heavy neutron-rich nuclides along the neutron closed shell N = 126 is extremely important for nuclear astrophysics investigations and, in particular, for the understanding of the r-process of astrophysical nucleosynthesis. A new way was recently proposed for the production of these nuclei via low-energy multi-nucleon transfer reactions. The estimated yields of neutron-rich nuclei are found to be rather high in such reactions and several tens of new nuclides can be produced, for example, in the near-barrier collision of 136 Xe with 208 Pb. This setup could definitely open a new opportunity in the studies at heavy-ion facilities and will have significant impact on future experiments. Properties of light and medium mass nuclei located far from the stability line have been studied already for many years. As a rule, the light exotic nuclei are produced in fragmentation processes and the medium mass neutron rich nuclei with А~100 in fusion reactions. Production and study of heavier neutron rich nuclei with A>160 encounters both physical and technical problems. Due to the curvature of the stability line (its increasing bending to the neutron axis), fusion reactions of stable nuclei produce only proton rich isotopes of heavy elements. For example, in fusion of rather neutron rich 18 O and 186 W isotopes one may get only the neutron deficient 204 Pb excited compound nucleus, which after evaporation of several neutrons shifts even more to the proton rich side. As a result, on the nuclear map there are, for example, 19 known neutron rich isotopes of cesium (Z = 55) and only 4 of platinum (Z = 78). For elements with Z > 100 only neutron deficient isotopes (located to the left of the stability line) have been synthesized so far. That is the main reason also for the impossibility to reach the center of the island of stability (Z~114 and N~184) in the superheavy mass region, and neutron deficient isotopes of elements with Z>120 (being synthesized in fusion reactions) should have very short half-lives (less than one microsecond), insufficient for their separation and identification. At the same time, the unexplored area of heavy neutron rich nuclei is extremely important for nuclear astrophysics investigations and, in particular, for the understanding of the r-process of astrophysical nucleogenesis (a sequence of neutron-capture and β - decay 15

16 processes). Just in this region the closed neutron shell N=126 is located which is the last waiting point in the r-process (see Fig. 1). The half-lives and other characteristics of these nuclei are extremely important for the r-process scenario of supernovae explosions. Study of the structural properties of nuclei along the neutron shell N = 126 could also contribute to the present discussion of the quenching of shell gaps in nuclei with large neutron excess. Figure 1. Upper part of the nuclear map. r-process of nucleosynthesis is shown schematically. There are three possibilities for the production of such nuclei. These are the multi-nucleon transfer reactions [1], fusion reactions with extremely neutron rich radioactive nuclei and rapid neutron capture processes. Today the two last methods look unrealizable because of low intensity of radioactive beams and low neutron fluxes in existing nuclear reactors. On the contrary, the low-energy multi-nucleon transfer reactions, as well as the quasi-fission processes [2] (which are similar) are quite practical. They can be used for the production of new neutron rich isotopes not only in the region of Z~80 but also in the superheavy mass area [3]. Theoretical estimations show that several tens of new nuclides in the region of N=126 and Z~75 can be produced, for example, in the near-barrier collision of 136 Xe with 208 Pb (see Fig. 2). Neutron rich isotopes of transfermium elements can be also produced in the multi-nucleon transfer reactions at low-energy collisions of actinide nuclei. Production and study of properties of neutron rich fermium isotopes (A > 260) are extremely interesting for several reasons. Firstly, as mentioned above, all known isotopes of fermium (and of more heavy elements) are located to the left side of the beta-stability line (see Fig. 1). Secondly, the well known fermium gap (isotopes Fm with very short half-lives for spontaneous fission) impedes formation of nuclei with Z>100 by the weak neutron fluxes realized in existing nuclear reactors. In this connection, it is extremely interesting to know what is the first 16

17 β - decaying fermium isotope and how long is its half-life. This is important not only for reactor but also for explosive nucleosynthesis in which this fermium gap might be bypassed. Figure 2. Cross sections for formation of heavy nuclei in collisions of 136Xe+208Pb at center-of-mass energy of 450 MeV [1]. Open circles in the right panel correspond to unknown isotopes. Unfortunately, the neutron rich heavy nuclei with Z>70 formed in the multi-nucleon transfer reactions cannot be separated and studied at available setups created quite recently just for studying the products of deep inelastic scattering (such as VAMOS, PRISMA and others). These fragment separators (as well as other setups) cannot separate heavy nuclei with Z>70 by their atomic number (mass separation is more simple with time-of-flight technique, for example). Scheme of setup However during the last several years a combined method of separation has been intensively studied and developed based on stopping nuclei in gas and subsequent resonance laser ionization of them [4-6]. This method was used up to now for separation and study of light exotic nuclei and fission fragments. Such techniques allows one to extract nuclei with a given atomic number, while a separation of the single ionized isotopes over their masses can be done rather easily by a magnetic field. Half-lives of heavy neutron rich nuclei, which we are interested in (as a rule, β - decaying), are much longer than the extraction time of ions at such a setup. The scheme of this setup is shown in Fig. 3. Neutron rich isotopes of heavy elements are produced in multi-nucleon transfer reactions with heavy ions accelerated up to 5 10 MeV/nucleon (depending on projectile target combination). The target, a foil of about 300 μg/sm 2 thickness (or larger), is placed at the window of gas cell (or inside it). Nuclear reaction products, escape the target as multi-charged ions, are decelerated and neutralized due to collisions with the atoms of buffer gas in the gas cell filled with pure argon or helium. Then the desired atoms (with a given Z number) are ionized by means of two or three-step 17

18 resonance laser irradiation and extracted through the supersonic nozzle or skimmer into the vacuum volume as positive charged ions (Q=+1) with slow energies of about 0.2 ev. In vacuum the ions are transported through the sextupole or quadrupole ion guide system (see below), which helps to pump away the residual buffer gas and keep the ions. Figure 3. Schematic view of setup for resonance laser ionization of nuclear reaction products stopped in gas with subsequent mass separation of them and transportation to detecting system. Then the ions are accelerated up to kev and separated by the magnet of the mass separator. Detailed description of the technique of stopping and cooling the ion beams in buffer gas, laser ionization and separation can be found in [4-14]. In this way a low-energy beam of single ionized atoms is produced with an extremely small emittance. Atomic nuclei of this beam have a definite value of charge number and a given (chosen) mass value, there are neither isobars nor isotopes. This allows one to perform subsequent high sensitive analysis of spectroscopic and decay properties of these nuclei, as well as a measurement of spins, dipole magnetic and quadrupole electric moments and charge radii of these nuclei by means of laser spectroscopy. The setup consists of the following elements (units): - laser system, - mass-separator, - front end system including: gas cell, system for extraction of the cooled ion beam and its cleaning from the buffer gas, electrostatic system for final formation and acceleration of the ion beam, - system for delivery and cleaning of the buffer gas and pressure stabilization inside the gas cell, 18

19 - vacuum system, - high voltage and radio frequency units, - diagnostic and control systems for the ion beam. Required beams of accelerated ions The use of the designed setup for separation and study of heavy neutron rich nuclei does not require any specific beams of accelerated ions. The ion beams available at FLNR (JINR, Dubna) are well satisfied our requirements which look as follows. - Ions: 16,18 О, 20,22 Ne, 24,26 Mg, 32,34,36 S, 40 Ar, 40,44,48 Ca, 48,50 Ti, 54 Cr, 58 Fe, 62,64 Ni, 84,86 Kr, 136 Xe, 238 U (i.e., quite different depending on the problem to be solved). - Beam energies: 4,5 9 MeV/nucleon (slightly above the Coulomb barrier) - Beam intensity: not restricted (up to pps). - Beam spot at the target: 3 10 mm in diameter (not very important). - Beam emittance: 20 mm mrad. - Targets: different, including actinides Th, U, Pu, Am, Cm. It is evident that the proposed method allows one to extract not only heavy transfer reaction products but also any other nuclei with half-lives longer than a few tens of milliseconds including neutron rich fission fragments, fusion reaction products and light exotic nuclei. Such studies are already performed at the similar setups, for example, in CERN, Finland and Belgium. Estimated efficiency of such setups is about 10% and depends on half-life of extracted ion. This is quite sufficient for study of nuclear reaction products formed with cross sections of about 1 microbarn at beam intensity of 0.1 pμa. This method allows one to study structure and decay properties of new exotic nuclei as well as spectroscopic properties of the corresponding atoms. At higher beam intensity this setup could be used also for separation and study of neutron rich long living superheavy nuclei produced in multi-nucleon transfer reactions with actinide targets (see above). [1] V. Zagrebaev, W. Greiner, Phys. Rev. Lett. 101 (2008) [2] M.G. Itkis et al., Nucl. Phys. A734 (2004) 136. [3] V.I. Zagrebaev, Yu.Ts. Oganessian, M.G. Itkis, and Walter Greiner, Phys. Rev. С73 (2006) (R). [4] U. Köster et al., Spectrochim. Acta B58 (2003) [5] Yu. Kudryavstev et al., Nucl. Instr. and Meth. Phys. Res. B204 (2003) 336. [6] I.D. Moore et al., J. Phys. G: Nucl. Part. Phys. 31 (2005) S1499. [7] J. Ärje et al., Phys. Rev. Lett. 54 (1985) 99. [8] G. Savard et al., Nucl. Instr. and Meth. Phys. Res. B204 (2003) 582. [9] J. Äystö, Nucl. Phys. A693 (2001)

20 [10] L. Vermeeren et al., Phys. Rev. Lett.73 (1994) [11] Yu. Kudryavtsev et al., Nucl. Instr. and Meth. Phys. Res. B114 (1996) 350. [12] T. Kessler et al., Nucl. Instr. and Meth. Phys. Res. B266 (2008) 681. [13] Yu. Kudryavtsev et al., Nucl. Instr. and Meth. Phys. Res. B267 (2009) [14] H.L. Ravn and B.W. Allardyce: On-Line Mass Separators. In: Treatise on Heavy-Ion Science, Vol. 8 ed. by D. Allan Bromley (Plenum Publishing Corporation, 1989)

21 Status Report (3): In-gas laser ionization and spectroscopy (IGLIS) laboratory at KU Leuven (Yu. Kudryavtsev*, P. Creemers, R. Ferrer, L. Gaffney, C. Granados, M. Huyse, E. Mogilevskiy, P. Van den Bergh, P. Van Duppen (KUL), B. Bastin, D. Boilley, P. Delahaye, N. Lecesne, H. Lu, F. Lutton, J. Piot, H. Savajols, J. C. Thomas, E. Traykov (GANIL), E. Liénard, X. Fléchard (LPC-Caen), S. Franchoo (IPN-Orsay)) Introduction A new laser laboratory to perform in-gas laser ionization and spectroscopy (IGLIS) studies is under construction at the KU Leuven, Belgium. The studies that will be carried out in the IGLIS laboratory will consist in the development of a high-efficient laser ion source based on multi-step resonant laser ionization for the production of radioactive ion beams extracted from a supersonic gas jet and for high-resolution in-gas-jet laser spectroscopy [1]. Implementation of resonance ionization in the supersonic gas jet should allow to increase the spectral resolution by one order of magnitude in comparison with the currently performed in-gas-cell ionization spectroscopy. As soon as the new method is fully characterized in off-line conditions in the IGLIS laboratory and optimal conditions are achieved, the laser system of the IGLIS setup will be relocated at the S 3 facility in GANIL for laser spectroscopy studies along the N=Z line and those in the heavy and super heavy regions [2]. This unique system could also operate as an integrated ion source for the efficient and selective production of a wide variety of radioactive ion beams (RIB). Figure 1. Schematic layout of the new IGLIS laboratory that is under construction at KU Leuven. 21

22 IGLIS laboratory The schematic layout of the IGLIS laboratory is shown in Fig. 1. It consists of two rooms. In the first room, the IGLIS mass separator and the gas jet setup are located. The mass separator will be at 40 kv while the front end is kept at ground potential. The laser system is located in the adjacent room where clean air conditions are provided. Figure 2. Front end of the mass separator for in-gas jet laser resonance ionization and spectroscopy experiments. Dimensions are indicated in mm. The front end of the mass separator, see Fig. 2, consists of a gas cell-, a differential pumping- and an extraction chambers. The supersonic gas jet will be formed in the de Laval nozzle that is attached to the gas cell. In the first experiments, copper atoms seeded in the jet will be ionized in a two-step process in the region defined by the crossing of the first- and the second-step laser beams with the gas jet. The laser- produced ions will be transported 22

23 by the S-shaped RFQ ion guide and the extraction RFQ ion guide in the differential pumping chamber towards the extraction electrode and up to the mass separator. The required vacuum in the setup is provided by a fully-dry pumping system. The pressure in the gas cell chamber P1 is defined by the stagnation pressure in the gas cell and by the desired Mach number of the gas jet. The higher the Mach number is the better spectral resolution can be achieved (Fig.7 in [1]). In the present setup we aim to work with jets of a Mach number between 8 and 14. This should allow us to obtain a spectral resolution in the range MHz for the nm 4s2S1/2 4p2P1/2 transition of copper. Figure 3 shows the required pressure in the gas cell chamber (P1) as a function of the gas jet Mach number for different stagnation pressures in the gas cell together with calculated pressures in the differential pumping chamber (P2) and in the extraction chamber (P3). The effective area of the apertures of about 1 cm2 connecting the chambers enable proper differential pumping while keeping efficient transport of the laser ions through the RFQ structures. The chosen turbo molecular pumps should guarantee a vacuum in the extraction chamber better than 1e-05 mbar for a gas cell pressure of up to 500 mbar. Figure 3. Pressure in the gas cell chamber (P1), in the differential pumping chamber (P2) and in the extraction chamber (P3) as a function of the Mach number of the gas jet for different stagnation pressures in the gas cell. 23

24 The required pressure in the gas cell chamber is provided by a turbo molecular pump and by a screw pump. There are two modes of operation that can be used depending on the stagnation gas cell pressure, the nozzle throat diameter d and the Mach number. In the first mode the screw pump is used as a backing pump for the turbo pump. Since the maximum turbo pump inlet pressure for argon has a limit of about 1.4E-02 mbar this mode of operation can be used only for relative high Mach numbers and has restrictions on the maximum nozzle throat diameter for different gas cell pressures. Those values are shown in Fig. 4 in red. Thus, for a cell pressure of 500 mbar the throat diameter should be smaller than 0.7 mm for a gas jet with a Mach number equal to 14. In the second mode of operation only the screw pump is used. Since this pump has no inlet gas pressure limitation and has almost constant pumping speed in the required pressure range, the maximum Mach number that can be reached depends only on the nozzle throat diameter d. The maximum Mach number is changed from 8 for d=1.5mm up to 12.5 for d=0.5 mm. Max. Mach number Figure 4. Maximum Mach number that can be obtained in the jet as a function of the gas cell pressure (for argon) for two modes of operation and different nozzle throat diameters. In red: with the turbo pump and the screw pump as a backing pump. In black: only the screw pump is in use Laser system d 1.5 mm With turbo pump STP d 1.0 mm d 0.7 mm With screw pump GXS450/4200 d=0.5 mm d=1 mm d=1.5 mm P gc, mbar The layout of the laser system that will be used for the in IGLIS studies is shown in the Fig. 5. The system is located in a clean room ISO8 (< part/m3) and temperature stabilized 24

25 within ± 0.5 o C. It consists of two INNOSLAB Nd:YAG lasers (Edgewave GmbH) with a maximum power of 90 W for the green (532 nm) and 36 W for the UV (355 nm) light. They will be used as pump sources at a maximum pulse repetition rate of 15 khz for two CREDO pulsed dye lasers (Sirah Lasertechnik GmbH) as well as for a pulsed dye amplifier. The latter, will be used in combination with a continuous wave (CW) single mode diode laser Ta-pro (Toptica Photonics AG). A wide tuning-range of the laser system covering the wavelength interval nm will allow the application of two-step schemes to accomplish efficiently the ionization process. Depending on the laser spectroscopy mode, in-gas-cell or in-gas-jet, these lasers can be arranged in different ways. The main difference lies in the choice of lasers utilized in the step used to probe the hyper-fine structure (HFS) when performing spectroscopy studies. A dye laser operating in narrow-band mode will suffice to produce an output light with ideal linewidth (~1.8 GHz) for spectroscopy studies in the gas cell. Figure 5. Layout of the IGLIS laser system for the in-gas cell and for the in-gas jet laser photoionization spectroscopy In the case of in-gas-jet spectroscopy, a narrow-band light is crucial for obtaining the highest resolution. Here, the light resulting from the amplification of the CW single-mode diode laser radiation in the pulsed dye amplifier with subsequent second harmonic generation will be 25

26 employed for the HFS transition to be studied. A fraction of the laser beams will be sent to an atomic beam unit (ABU), where an atomic beam of the element of interest will be produced and atoms will be ionized in a cross beam geometry (90 o ). The laser-produced ions will be analyzed in a time-of-flight mass spectrometer. This will allow monitoring the resonances under minimized Doppler broadening and shift conditions. Gas jet formation An important requirement to obtain the maximum spectral resolution for the in-gas-jet spectroscopy is the extraction of atoms from the cell in a well-collimated gas jet. By employing a specially designed nozzle the spatial overlap with the laser beams will be maximized. To provide these conditions the background pressure in the gas cell chamber should be precisely adjusted to avoid the formation of shocks. Thus, one of the main goals at the IGLIS laboratory is to investigate the occurrence and position of shocks in the gas jet for different nozzle designs in order to obtain experimental parameters that will allow the best spectral resolution together with a high total laser ionization efficiency. Direct gas jet visualization will be carried out by the Planar Laser Induced Fluorescence (PLIF) technique. In PLIF, a probing laser beam formed into a thin sheet is sent at normal incidence to the gas jet (see Fig. 2), where it excites fluorescence of a seeded species. The PLIF image will be recorded by a CCD camera. Rare Elements in Gas Laser Ion Source and Spectroscopy at S 3 The last generation Electron Cyclotron Resonance (ECR) ion source of the SPIRAL2 facility will provide unprecedented high-intensity stable ion beams (reaching 100 pμa intensities for light ions (A<60); A-PHOENIX A/q=6 type). The Super Separator Spectrometer (S 3 ) of the SPIRAL2 facility will use the stable ion beams accelerated by the SPIRAL2 superconducting linear accelerator (LINAC) to investigate radioactive nuclei with low-production cross section (giving access to the sub-nanobarn region) using mainly the fusion-evaporation or multi-nucleon transfer reactions. S 3 is characterised by a enhanced beam rejection power of up to and a mass resolution of about1/400) [3]. The S 3 spectrometer itself has received funding through an Equipex grant for the construction of its main components (ANR-10-EQPX-46-01). The REGLIS 3 (Rare Elements in-gas Laser Ion Source and Spectroscopy at S 3 ) device, based on the in-gas laser ionization and spectroscopy technique developed at the University of Leuven, will be installed at the focal plane of the spectrometer [2]. The latter will be operated in a converging mode, in which the outcoming beam will be focussed onto the 3 x 5 cm 2 entrance window of the gas cell. Owing to REGLIS 3, laser-spectroscopy studies of transactinide nuclei, refractory elements and medium-mass N=Z nuclei will become possible, enabling the determination of ground and isomeric-state properties. Considering that no 26

27 facility other than SPIRAL2-S 3 can efficiently produce these nuclei, it foretells the outstanding character of the experimental results that will come from the installation of REGLIS 3. Four letters of intent to perform laser spectroscopy studies using the REGLIS 3 setup have been already submitted and presented to the SPIRAL2 Scientific Advisory Committee: one on the very-heavy elements (Z ~ ) (to validate nuclear structure theories, essential in the search for and understanding of the superheavy elements and those at the limit of nuclear existence), one on the Sn isotopes (to test the validity of shell-model predictions by measuring spins and magnetic moments around 100 Sn, essential to clarify the role of the single particle orbitals involved in this region of the chart of nuclides), one on 94 Ag (to study high-spin isomerism, beta-delayed p emission, 2-p emission) and one on 80 Zr (to elucidate the role of specific single-particle orbitals in the structure of nuclei along the N=Z line, and also that of the effective interactions used in this region of deformation). Moreover, among the letters of intent submitted for the phase 2 of the SPIRAL2 project, eight of them require beams from S 3 to be delivered to the DESIR facility through the lowenergy beam line, underlying thus the strategic function of the REGLIS 3 device: experimental projects have been proposed on high-accuracy mass measurement of nuclei with Z~104 and towards the N=Z line, on the search of cluster radioactivity around 100 Sn, on the test of the unitarity of the CKM matrix through superallowed beta emitters decay studies, on ground states properties and electron shake-off measurements using the collinear laser spectroscopy technique along the N=Z nuclei and finally on TAS measurements around mass 100 to study the quenching of the GT strength. Conclusion The construction work of the IGLIS laboratory has already been started and will continue through mid June when the clean room for the laser laboratory will be finished. Some of the instrumentation is already available as the dipole magnet, the magnet power supply, the main part of the gas handling system, the single mode diode laser, and the wavelength meter. The vacuum pumps, the Nd-YAG lasers, the dye lasers and amplifier have been ordered and will be delivered in the next months. The atomic beam unit and the time-of-flight mass spectrometer are currently ready to be assembled. First off-line tests foreseen in the fall [1] Yu. Kudryavtsev, R. Ferrer, M. Huyse, P. Van den Bergh, P. Van Duppen, The in-gas-jet laser ion source: Resonance ionization spectroscopy of radioactive atoms in supersonic gas jets, Nuclear Instruments and Methods in Physics Research B 297 (2013) [2] R. Ferrer, Yu. Kudryavtsev, B. Bastin, D. Boilley, P. Creemers, P. Delahaye, E. Lienard, X. Flechard, S. Franchoo, L. Ghysa, M. Huyse, N. Lecesne, H. Lu, F. Lutton, E. Mogilevskiy, 27

28 D. Pauwels, J. Piot, D. Radulov, L. Rens, H. Savajols, J. C. Thomas, E. Traykov, C. Van Beveren, P. Van den Bergh, P. Van Duppen: In Gas Laser Ionization and Spectroscopy experiments at the Superconducting Separator Spectrometer (S3): Conceptual studies and preliminary design, Nuclear Instruments and Methods in Physics Research B, EMIS2012 proceedings, submitted to Nuclear Instruments and Methods in Physics Research B. [3] A. Drouart, A. M. Amthor, D. Boutin, O. Delferrière, M. Duval, S. Manikonda, J. A. Nolen, J. Payet, H. Savajols, M.-H. Stodel, D. Uriot, The Super Separator Spectrometer (S 3 ) for SPIRAL2 stable beams, Nucl. Phys. A 834 (2010) 747c. 28

29 Status Report (4): Status of RIKEN SLOWRI project M. Wada*, T. Sonoda, P. Schury, Y. Ito, S. Arai, I. Katayama, T. Kubo, K. Kusaka, T. Fujinawa, T. Maie, H. Yamasawa (RIKEN), F. Arai, A. Ozawa (Tsukuba), H. Iimura (JAEA), H. Tomita, T. Takatsuka (Nagoya), T. Furukawa (Tokyo Met.), H. Wollnik (Giessen), A. Takamine (Aoyama), K. Okada (Sophia), Y. Matsuo (Hosei), H. Miyatake, S.C. Jeong, H. Ishiyama, I. Imai, Y. Hirayama, Y.X. Watanabe (KEK), T. Wakui, T. Shinozuka (Tohoku) I. Construction status of the SLOWRI facility at RIKEN-RIBF A long awaited project SLOWRI, RIKEN s universal low-energy RI-beam facility was financially granted by the supplementary budget of FY2012. Almost all contracts for the new facility were made by March 2013 and the construction will be completed by the end of March The planned layout of the facility is shown in Fig.1. The present contracts include five major setups as follow (refer to Fig.1): Gas Cell A (RF-carpet ion guide gas cell) A large cryogenic He gas cell with RF-carpet for main beam experiments will be placed at the exit of the D4 magnet of BigRIPS. The RF-carpet will consist of two p arts: a linear curtain structure for transporting thermal ions downstream in the cell and a concentric structure for concentrating ions to a small exit nozzle. Both will operate in ion-surfing mode with traveling waves. Gas Cell B (PALIS) A compact argon gas cell for parasitic production of RI-beams via resonant ionization method will be placed in the vicinity of the F2 slit of BigRIPS. Those nuclei abandoned in the slit can be captured in the cell, quickly neutralized and transported by gas flow towards the exit where resonant laser radiations will selectively re-ionize the radioactive nuclei. ISOL + BTL These two gas catchers will be followed by isotope separators (SD1 and SD2) to provide mass selection. The two beams will be merged into one beam transport line, consisting of hundreds of electrostatic quadrupole singlets, and transported to a low-energy experimental room. Beam Cooler-Buncher and MRTOF-MS Continuous low-energy (< 30 kev) RI-beams will be cooled in a linear ion trap and converted into ion bunches to be delivered to various experimental devices, which will initially include a collinear laser spectroscopy apparatus as well as a multi-reflection time-of-flight mass spectrograph. Lasers for resonant ionization 29

30 Two pulsed dye lasers (CREDO) and a 10 khz pumping Nd:YAG laser (Edgewave) will be installed. A 100 khz Nd:YAG laser (LEE Laser) for pumping Ti:Sa lasers will be also installed. Two gas cells will be fabricated by this fall and offline commissioning will start immediately at B3F and K12 rooms (where one can always access even if accelerator is running). The isotope separator hardware and the beam transport line will be installed by March Online commissioning will start by fall Figure 1. Planned Layout of SLOWRI facility II. Online mass measurement with MRTOF An important experiment at SLOWRI will be direct mass measurement of short-lived nuclei. A multi-reflection time-of-flight mass spectrograph (MRTOF-MS) was developed and tested off- and on-line. Online mass measurements of the short-lived lithium isotope 8 Li (T 1/2 = 840 ms) have been performed in FY2012. A low energy continuous 8 Li + beam from the prototype SLOWRI was accumulated in a tapered linear rf quadrupole ion trap and stored in a flat rf quadrupole ion trap for further cooling and bunching. The ion bunch was injected into the MRTOF-MS and after several hundreds revolutions (a few milliseconds), the ions were ejected and detected 30

31 by a micro-channel plate detector. The total TOF was compared with that of reference ions to determine the mass of 8 Li with a relative uncertainty of 6.6x10-7. We found that a single reference ion is sufficient to determine the mass of unknown ions by the square of the TOF ratio. In the 8 Li measurement, 12 C the atomic mass standard --- was used for the reference. Even though the masses differ by 50%, the systematic uncertainty due to a possible TOF offset of 10 ns is still within the present accuracy. Figure 2. TOF spectra of 8 Li + and 12 C + This new mass spectrograph is best suited for direct mass measurements of very heavy nuclei, such as super heavy elements. We have started a project, SHE-MASS, aiming to comprehensively measure masses of nuclei heavier than uranium at the GARIS facility. For any time-of-flight mass spectrometers, mass references are indispensable. We developed an electro-spray ion source for molecular ions, which can produce a variety of molecular ions in a wide mass range. We used a compact rf-carpet for accumulating molecular ions with low abundant isotopes and found that there are isobaric molecules in many mass numbers. We have also confirmed that the accuracy of mass determination is always within the precision of the mass measurements in tests with isobaric triplets III. Investigation of the ion surfing transport method with a circular rf-carpet RF-carpets will play crucial roles in the SLOWRI facility for efficient transport of thermal ions in gas cells. Originally an ion barrier force is formed by inhomogeneous rf field and a dragging force is formed by a gradient of dc potential superimposed on the rf field. Recently, 31

32 Bollen et al. of MSU developed a so called ion surfing mode rf-carpet and tested with a linear rf-curtain type carpet. We have tested the surfing mode transport with a coaxial circular rf-carpet at RIKEN. In addition to the ion-barrier rf signal, a four-phase audio frequency traveling wave signal was superimposed on the rf-carpet. Depending on the direction of the traveling wave, ions were transported to either the outermost electrode or the Farady cup placed behind the central exit with almost unity efficiency. This new mode can transport ions without need for a dc potential gradient along the carpet, allowing us to achieve a drift velocity of 100 m/s using a large size carpet without discharge problems. Figure 3. (a) Concept of ion surfing with schematic of the applied rf and audio wave signal phase. (b) Sketch of the transport efficiency measurement method. 32

33 Status Report (5): Status of KISS project (Y. Hirayama*, N. Imai, H. Ishiyama, S.C. Jeong, H. Miyatake, M. Oyaizu, Y.X. Watanabe (KEK), Y.H. Kim (Seoul Nat l Univ.), M. Mukai (Tsukuba), Y. Matsuo, T. Sonoda, M. Wada(RIKEN), M. Huyse, Yu. Kudryavtsev, P. Van Duppen(KUL)) Introduction The beta-decay properties of nuclei with N = 126, which act as progenitors in the r-process path forming the third peak (A 195) in the r-abundance element distribution, are considered critical for clearly understanding astrophysical sites for the production of the heavy elements such as gold and platinum [1]. We have constructed the KEK Isotope Separation System (KISS) at RIKEN to produce pure low-energy beams of the neutron-rich isotopes around N = 126 and to study their beta-decay properties that are also of interest from astrophysical viewpoint [2]. We are going to produce and study 200 W, 201 Re, 202 Os and 203 Ir (Z = 74 77, N = 126), which have not been produced at any other facilities, by carrying out the multi-nucleon transfer (MNT) reactions [3] between an energetic stable 136 Xe beam at 10 MeV/nucleon and a 198 Pt target. The recoil energies of the nuclei with N = 126 produced as target-like fragments (TLFs) are as low as 1 MeV/nucleon and have a wide energy distribution. The emission angles calculated by the GRAZING code [4] vary widely, and the average value of the emission angles is found to be 65 in the laboratory frame. The characteristics of the MNT reaction products make it difficult to collect the nuclei with N = 126 by using an in-flight-type electromagnetic spectrometer. Therefore, in the KISS project, we employ a gas catcher to efficiently collect all reaction products, and we adopt the laser resonance ionization technique to select the nuclei with specific atomic numbers Z of collected nuclei and use electromagnetic separator (ISOL) to obtain the nuclei with specific mass numbers A. Multi-nucleon transfer reaction Although the MNT reaction between two heavy ions at the low energy around the Coulomb barrier is considered as a promising tool to produce and investigate neutron-rich nuclei and the reactions have been studied intensely, the mechanism of the reaction is not fully understood. The GRAZING calculation, which is often used for comparison with measurements, does not reproduce the experimental data in terms of absolute cross sections and mass distributions in massive transfer channels. In order to investigate the effectiveness of the reaction system of 136 Xe+ 198 Pt, we have studied collisions of the system by using the large acceptance magnetic spectrometer VAMOS++ [5] and the high efficiency 33

34 gamma detector array EXOGAM [6] at GANIL [7]. Figure 1. Distribution of the PLFs detected by the VAMOS++ Figure 2. Isotopic distributions of the PLFs with the atomic number from 50 to 58 34

35 The isotopic distributions of the detected PLFs are shown in Fig. 2 for each element. The left panel shows the distributions for Z = 54 and the proton stripping channels from 1 to 4 protons (Z =53 50), and the right panel shows the distributions for the proton pick-up channels from 1 to 4 protons (Z =55 58). The abscissa indicates the mass and the ordinate indicates the production cross section. The black points are the experimental data. The red and blue lines indicate the GRAZING calculations without and with the e ects of particle evaporations, respectively. The measured isotopic distributions are almost reproduced by the GRAZING calculations for the proton stripping channels from 1 to 4 protons. Although the measured distributions are shifted to the heavier side than the calculated ones for the proton pick-up channels, they show the greatly enhanced yields than the calculations for the pick-up channels of more than one proton and there seem to be enough neutron stripping events accompanied by the proton pick-up channels. The present results indicate the advantage of using a neutron-rich beam as heavy as 136 Xe for producing neutron-rich TLFs around N=126. The advantage would be made clearer by further analysis of the gamma-rays from the TLFs detected by the EXOGAM array. Figure 3. Schematic layout of KISS 35

36 KISS (KEK Isotope Separation System) Figure 3 shows a schematic layout of the KISS, which has been constructed at the RIBF facility in RIKEN since the beginning of the year It consists of the laser system, mass-separator system, three decay measurement stations and gas-cell system [8]. The decay measurement stations are planned to be installed this summer for the development experiment of lifetime measurement scheduled at the end of September Other equipments have been installed and tested at the off-line [8] and on-line tests using 56 Fe beam. The laser system consists of two frequency-tunable dye lasers (ScanMate and FL3002) pumped by two Excimer (LPX240i, XeCl, 308 nm) lasers and has been installed at the separated room by the concrete shield below the KISS experimental hall. The mass-separator system has QQDQQ configuration, in which Q and D denote the quadrupole and dipole magnets, respectively. Deflection angle and pole gap of the D magnet are 45 degrees and 70 mm, respectively. Measured mass resolving power is 900, which is in good agreement with the expected value [9]. Gas cell system The gas cell system includes an argon gas feeding line with a gas-purification device (MonoTorr Phase II 3000), a gas-catcher cell, and a sextupole ion-guide (SPIG) as shown in Fig. 4-(a). Figure 4. Schematic view of the gas cell system (a), and of the gas cell (b) The vacuum chamber of the gas-cell system is separated into three rooms for the differential pumping. The boundaries of first, second, and third rooms are indicated by the red, blue and black thick lines, respectively. Typical pressure values at the rooms of P1, P2, and P3 are 36