Search

A new multi-reflection time-of-flight mass spectrograph for nuclear mass measurements with high precision has been constructed at the SLOWRI facility at RIKEN-RIBF. We aim to couple this new device to a cryogenic gas cell operated at the BigRIPS facility of RIBF. It is part of the SLOWRI project for slowing down and stopping intense radioactive ion beams provided downstream of RIKEN’s superconducting ring cyclotron accelerator. In particular, this device is dedicated to be located at the end of the BigRIPS beam line to access ions that are presently guided to beam dumps during in-beam experiments. In this article the apparatus is described in detail.

We developed KEK Isotope Separation System (KISS) in RIKEN Nishina center to produce neutron-rich isotopes around N = 126 for the study of β-decay properties from astrophysical interests. The KISS is an argon-gas-cell-based on laser ion source with ISOL, and, therefore, allows us to extract heavy refractory elements and investigate neutron-rich isotopes around N = 126. We extracted the laser-ionized unstable nuclei in the vicinity of 198Pt, and performed β-γ spectroscopy and in-gas-cell laser ionization spectroscopy successfully.

The Optimized Energy Degrading Optics (OEDO) beamline was constructed at RIBF to produce slowed-down, well-focused radioactive-isotope beams at 10–50 MeV/u, and began operation in June 2017. We examined the performance of the OEDO system by decelerating high-energy medium-mass radioactive ions. The main components of the OEDO beamline are two superconducting quadrupole triplet magnets and one radio-frequency deflector. This paper introduces the unique design of the ion optics of the beamline and describes the achieved performance of the beamline in the commissioning and following physics experiments. We will discuss the properties of energy-degraded beams produced by the OEDO beamline by comparison of simulations and the achieved performance.

We developed a multi-segmented proportional gas counter to perform β-decay spectroscopy of neutron-rich nuclei extracted from KEK Isotope Separation System (KISS). The 32 segmented gas counters allow us two-dimensional tracking of the events to identify β-ray events by discriminating background events. The detectors being by low-density material of thin aluminized Mylar foils and P10 gas enabled us to detect low energy electrons down to 100 keV. The absolute detection efficiency was 45% at Q β = 1 MeV, and the measured background event rate was 0.11 cps. The detector was applied to perform β-decay spectroscopy and hyperfine structure measurements of neutron-rich isotopes of Pt, Ir, and Os. We started to develop the gas counter with three-dimensional tracking by replacing the material of anode wire from BeCu to resistive thin carbon to reduce the background event rate below 0.01 cps.

We introduce a new concomitant referencing mode for operating a multi-reflection time-of-flight mass spectrograph (MRTOF-MS), wherein the reference and analyte ions are interleaved on a cycle by cycle bases. Using this mode, we demonstrate an improved technique for performing wide bandwidth mass measurements via MRTOF-MS. This new technique offers a simplified analysis and high-accuracy., 8 pages, 6 figures, follow-up for DOI:10.1016/j.ijms.2013.11.005

A multi-segmented proportional gas counter (MSPGC) with high detection efficiency and low-background event rate has been developed for β -decay spectroscopy. The MSPGC consists of two cylindrically aligned layers of 16 counters (32 counters in total). Each counter has a long active length and small trapezoidal cross-section, and the total solid angle of the 32 counters is 80% of 4 π . β -rays are distinguished from the background events including cosmic-rays by analyzing the hit patterns of independent counters. The deduced intrinsic detection efficiency of each counter was almost 100%. The measured background event rate was 0.11 counts per second using the combination of veto counters for cosmic-rays and lead block shields for background γ -rays. The MSPGC was applied to measure the β -decay half-lives of 198Ir and 199mPt. The evaluated half-lives of T 1 ∕ 2 = 9 . 8 ( 7 ) s and 12.4(7) s for 198Ir and 199mPt, respectively, were in agreement with previously reported values. The estimated absolute detection efficiency of the MSPGC from GEANT4 simulations was consistent with the evaluated efficiency from the analysis of the β - γ spectroscopy of 199Pt, saturating at approximately 60% for Q β > 4 MeV.

We have developed a doughnut-shaped argon gas cell with a rotating target for the KEK Isotope Separation System (KISS). The new gas cell increased extraction yields for target-like fragments (TLFs) produced in multi-nucleon transfer (MNT) reactions. KISS is an on-line isotope separator, which makes use of a laser ion source combined with an argon gas cell as a gas catcher for TLFs. This system has been implemented for nuclear spectroscopy of neutron-rich isotopes in the region of N = 126 . A gas flow simulation was performed to optimize the design of the gas cell for efficient and fast transport of TLFs. To confirm the performance expected at the design stage, we performed off- and on-line experiments by using iridium atoms evaporated from a filament in the gas cell and TLFs produced by a 136 Xe beam and 198 Pt target system respectively. We could increase the extraction yield by more than an order of magnitude by applying a doughnut-shaped gas cell with a rotating target. Thus, we could perform nuclear spectroscopy in this heavy region with the use of KISS.

Various isotopes of Ac, Ra, Fr, and Rn were produced by fusion-evaporation reactions using a 48Ca beam. The energetic ions were stopped in and extracted from a helium gas cell. The extracted ions were identified using a multi-reflection time-of-fight mass spectrograph. In all cases, it was observed that the predominant charge state for the extracted ions, including the alkali Fr, was 2+.

We report on the performance of a recently developed high-efficiency, low-background, three-dimensional tracking multi-segmented proportional gas counter (3DT-MSPGC), intended for β -decay spectroscopy of neutron-rich nuclei in the vicinity of neutron magic number N =126 at the KEK Isotope Separation System (KISS) facility. The 3DT-MSPGC uses two concentric cylindrical layers of proportional gas counters (16 per layer, 32 in total) featuring resistive anode wires (3 k Ω /cm). By independently measuring the charge deposition from both ends of each counter, we constrain β particle trajectories and determine where they intersect with the central axis of the 3DT-MSPGC. New compact charge-sensitive pre-amplifier modules, along with shaping amplifier and fast-timing amplifier modules, were developed to deduce the hit position by precisely measuring the charge deposition and timing. We have studied the performance of the 3DT-MSPGC in on- and off-line experiments. A longitudinal position resolution of ≤ 3.5 mm (FWHM) was achieved for each counter. The determination of the event trajectories allowed for suppression of background events, resulting in the background-event rate reaching 0.020(3) count-per-second (cps) while still maintaining a high-detection efficiency. This reduction of background is shown to improve the signal-to-noise ratio in determination of the half-life of 196Ir by a factor of 2.3(6), resulting in a significant improvement in achievable precision.

In order to investigate nuclei produced via fusion–evaporation reactions, especially super-heavy elements (SHE), we have begun construction of a facility for conversion of fusion–evaporation residues (EVR) to low-energy beams. At the base of this facility is a small cryogenic gas cell utilizing a traveling wave RF-carpet, located directly following the gas-filled recoil ion separator GARIS-II, which will thermalize EVRs to convert them into ion beams amenable to ion trapping. We present here the results of initial studies of this small gas cell.

The technique of laser resonance ionization is employed for an element-selective ionization of multi-nucleon transfer reaction products which are stopped and neutralized in a gas cell filled with argon gas at 50 kPa. We have been searching for efficient laser ionization schemes for refractory elements of Z = 73–78 using a time-of-flight mass-spectrometer (TOF-MS) chamber. To evaluate the isotope shift and ionization efficiency for each candidate of the ionization scheme, isotope separation using the TOF-MS was devised. The TOF-MS was designed to separate the isotopes using two-stage linear acceleration with a mass resolving power M / Δ M of >350. A mass resolving power of 250 was experimentally confirmed by measuring the TOF of laser-ionized tantalum ( Z = 73) ions with mass number 181. We searched for a laser resonance ionization scheme of tantalum using the TOF-MS.

KEK Isotope Separation System (KISS) has been developed at RIKEN to produce neutron rich isotopes with N = 126 to study the β -decay properties for application to astrophysics. The KISS is an element-selective mass-separation system which consists of an argon gas cell-based on laser ion source for atomic number selection and an ISOL mass-separation system. The argon gas cell of KISS is a key component to stop and collect the unstable nuclei produced in a multi-nucleon transfer reaction, where the isotopes of interest will be selectively ionized using laser resonance ionization. We have performed off- and on-line experiments to study the basic properties of the gas cell as well as of the KISS. We successfully extracted the laser-ionized stable 56 Fe (direct implantation of a 56 Fe beam into the gas cell) atoms and 198 Pt (emitted from the 198 Pt target by elastic scattering with a 136 Xe beam) atoms from the KISS during the commissioning on-line experiments. We furthermore extracted laser-ionized unstable 199 Pt atoms and confirmed that the measured half-life was in good agreement with the reported value.

We developed an in situ radiotracer method for diffusion studies in solids using short-lived α-emitting 8Li tracer. In the method, while implanting a pulsed 8Li beam into a solid material of interest, the α particles emitted into the implantation side of the sample surface were detected as a function of time. By changing the implantation depth and the detection angle against the sample surface according to lithium diffusivity (deep implantation and large angle with a large solid angle, or shallow implantation and small angle with a narrow solid angle), the method can be sensitive to a wide range of diffusion length ranging from micrometer scale to nanometer scale per second. The feasibility of the method was demonstrated by measuring the lithium diffusion coefficients to the order of 10−12 cm2/s in lithium ionic conductors.

We have developed a laser ion source for the target-like fragments (TLFs) produced in multi-nucleon transfer (MNT) reactions. The operation principle of the source is based on the in-gas laser ionization and spectroscopy (IGLIS) approach. In the source TLFs are thermalized and neutralized in high pressure and high purity argon gas, and are extracted after being selectively re-ionized in a multi-step laser resonance ionization process. The laser ion source has been implemented at the KEK Isotope Separation System (KISS) for β-decay spectroscopy of neutron-rich isotopes with N = 126 of nuclear astrophysical interest. The simulations of gas flow and ion-beam optics have been performed to optimize the gas cell for efficient thermalization and fast transporting the TLFs, and the mass-separator for efficient transport with high mass-resolving power, respectively. To confirm the performances expected at the design stage, off-line experiments have been performed by using 56Fe atoms evaporated from a filament in the gas cell. The gas-transport time of 230 ms in the argon cell and the measured KISS mass-resolving power of 900 are consistent with the designed values. The high purity of the gas-cell system, which is extremely important for efficient and highly-selective production of laser ions, was achieved and confirmed from the mass distribution of the extracted ions. After the off-line tests, on-line experiments were conducted by directly injecting energetic 56Fe beam into the gas cell. After thermalization of the injected 56Fe beam, laser-produced singly-charged 56Fe+ ions were extracted. The extraction efficiency and selectivity of the gas cell in the presence of plasma induced by 56Fe beam injection as well as the time profile of the extracted ions were investigated; extraction efficiency of 0.25%, a beam purity of >99% and an extraction time of 270 ms. It has been confirmed that the performance of the KISS laser ion source is satisfactory to start the measurements of lifetimes of the β-decayed nuclei with N = 126 .

We have developed an in situ and nanoscale Li diffusion measurement method in Li battery materials using an α-emitting radioactive 8Li tracer. In this method, while implanting a low-energy (8 keV) 8Li beam, the α particles emitted at a small angle (10°) relative to the sample surface were detected as a function of time. Measurement for Li diffusion coefficients in a spinel phase LiMn2O4 (LMO) thin film has been started, which is used as an electrode in a Li ion secondary battery. An obvious Li diffusion effect in LMO was observed at the sample temperature of 623 K, and the further measurement is underway.

The KEK Isotope Separation System has been developed to study the β -decay properties of the neutron-rich nuclei around the neutron magic number N = 126. These properties are essential for understanding the origin of the third peak in the r-process element abundance pattern. The detector system for β -decay spectroscopy at the KEK Isotope Separation System should have high detection efficiency for low-energy β -rays, and should be operated under a low-background environment. The detector system of the KEK Isotope Separation System consists of β -ray telescopes and a tape transport system. The solid angle covered by the β -ray telescopes is as large as 75% of 4 π in total. The Q β -value dependence of the detection efficiency was estimated by Geant4 simulation. The background rate was 0.09 cps using a veto counter system and Pb shields. This background rate allows us to measure the lifetime of 202Os.

The tilted-foil method for producing spin-polarized radioactive isotope beams has been studied using the re-accelerated radioactive 8 Li and 123 In beams produced at Tokai Radioactive Ion Accelerator Complex (TRIAC) facility. We successfully produced polarization in a 8 Li beam of 7.3(5)% using thin polystyrene foils (4.2 μg/cm 2 ). The systematic study of the nuclear polarization as a function of the number of foils and beam energy has been performed, confirming the features of the tilted-foil technique experimentally. After the study, a spin-polarized radioactive 123 In beam, which is the heaviest ever polarized in its ground state by this method, has been successfully generated by the tilted-foil method, for the nuclear spectroscopy around the doubly magic nucleus 132 Sn.

We are developing in situ diffusion measurements in solids using short-lived radioactive ion beams provided from an isotope separator on-line (ISOL). We examined the feasibility of a new in situ nanoscale diffusion measurement method using a radioactive 8 Li tracer by computer simulations. Under moderate experimental conditions, we have found that the detection limit of lithium diffusion coefficient can be improved to a low value of 1 × 10 - 12 cm 2 / s . Also, in situ sodium diffusion measurement in Na battery materials can be applicable using a radioactive 20 Na tracer. We found Na diffusion coefficients can be measured, ranging from 10 - 6 to 10 - 10 cm 2 / s by computer simulations.

Accurate measurement of the cross-section of the 12C(α, γ)16O reaction is very important in nuclear astrophysics. Enriched 12C targets prepared with the high energy vibrationl powder plating [HIVIPP] method for this purpose have been used successfully. Initially, however, the 12C deposits for studying this small cross-section partially peeled from their Au backings when irradiated with beam currents≥2 times the 7 μA pulsed α-beam used previously. This problem was solved when the amorphous 12C powders were converted to graphite by heat-treatment in a high-temperature furnace. We investigated powder heat treatments at temperatures of 2600–3500 °C to reveal, which conditions resulted in uniform and highly adhesive depositions on Au backings.

We have evaluated and compared the performances of a 50 ‐ μ m -thick CERN GEM and a 400 ‐ μ m -thick Thick GEM in low-pressure He/CO2 (90%/10%) mixed gas. In particular, the empirical gas gains of the standard CERN GEM and the Thick GEM have been measured and investigated.

The diffusion coefficients of Li in the NaTl-type Li intermetallic compound of β-LiGa have been measured by using a short-lived radioactive diffusion tracer. As the tracer, the α-emitting radioisotope of 8Li delivered as the energetic and pulsed beam from Tokai Radioactive Ion Accelerator Complex (TRIAC) was implanted into the β-LiGa compounds with the composition in the range of about 43 to 54 at.% Li. By analyzing the time-dependent yields of the α-particles measured according to the repetition cycle of the beam, the tracer diffusion coefficients were extracted over the wide range of Li composition. Abnormal composition-dependence of Li diffusion coefficients in β-LiGa was observed; the stoichiometric β-LiGa showed the highest diffusivity of Li. By referring to the composition-dependent diffusivity of Li in the iso-structural β-LiAl and β-LiIn, we could identify the abnormal diffusion of Li in very Li-poor composition of β-LiGa. The anomaly has been discussed qualitatively in terms of the formation of defect complex and the interaction between the constitutional defects.

The KEKCB is an electron cyclotron resonance ion source (ECRIS) operating at the microwave frequency of 18 GHz and has successfully operated as the charge breeder at Tokai Radioactive Ion Accelerator Complex (TRIAC). Following an overview of the TRIAC, the recent operating results of the KEKCB have been demonstrated. Special attention was paid on the element-dependent charge breeding efficiency and the beam impurity originating from the ECR plasma of the KEKCB.

The excitation function of the 8 Li(d, t) 7 Li reaction was directly measured using 8 Li beam irradiating a deuterated polyethylene target at center-of-mass system beam energies ( E cm ) between 0.3 and 1.2 MeV. This is the first cross section measurement of the above reaction below E cm = 1.5 MeV , covering Gamow peaks for 1 – 3 × 10 9 K of astrophysical interest. A broad resonance-like structure was observed at around E cm = 0.8 MeV . The astrophysical reaction rate deduced from the present result is far larger than the previously reported values estimated from extrapolation of data observed at higher energies, roughly by one order of magnitude at about 1 × 10 9 K .

Low-energy (1–2 MeV/nucleon) radioactive nuclear beams (RNBs) of 8Li, 12B and 16N have been produced through nuclear transfer reactions using a recoil mass separator (RMS) at Japan Atomic Energy Research Institute (JAERI). The contamination of the primary beam to the RNB was investigated quantitatively and was well suppressed. The typical beam intensities of 8Li-, 12B- and 16N-RNB were 1.4 × 10 4 , 7.8 × 10 3 and 4.7 × 10 3 pps , respectively. Their purities were 99 % , 98 % and 98.5 % , respectively.

A gated multiple-sampling and tracking proportional chamber (GMSTPC) has been made for the study of ( α , n) and (p, n) type reactions of astrophysical interest induced by low-energy radioactive nuclear beam. It consists of a drift-space region, a proportional-counter region divided into 24 segments and a gating grid. The GMSTPC is placed in such a way that the beam enters directly into its drift space, so that the chamber gas also acts reaction targets. The gating grid allows the gas multiplication of electrons only when it is triggered by neutron signals. Under the operation of this gating grid, almost no pulse-height defects have been observed up to 40 kHz beam-injection rates. Characteristics and performance of the GMSTPC are reported. We also describe some details of a whole detector system including a time-of-flight counter, a neutron-counter wall consisting of 28 plastic-scintillator bars and the GMSTPC, with which three-dimensional trajectories and energies of both charged particles and neutrons can be measured event by event. The measured efficiency of the detector system as well as the analysis efficiency in the case of the Li 8 ( α , n ) reaction are also given.

With using low-energy RI beams from an in-flight RIB separator, CRIB, the most crucial stellar reaction 14 O( α ,p) 17 F for the onset of the high-temperature rp-process was directly investigated and the transitions through the states at around 6.2 MeV in 18 Ne were first observed, confirming the importance predicted before. The ( α ,p) reaction leading to the first excited state in 17 F was also found to have a large contribution. A proton resonance search experiment of 23 Mg + p was also discussed, which is a part of our series of resonance search studies relevant to the early stage of the rp-process.

The excited states in the light neutron-rich nucleus 11Be have been studied through the β-delayed neutron- and γ-decays from spin-polarized 11Lig.s.. The level scheme and decay scheme of 11Be∗ are established from the β–γ, β–n and β–n–γ coincidences, and the spins and parities for 7 levels in 11Be are firmly assigned from the β-decay asymmetries measured in coincidence with the delayed radiations. The neutron-spectroscopic factors of the levels in 11Be are also determined. Some of the levels show good accord with predictions by the anti-symmetrized molecular dynamics (AMD) theory, which predicts various types of 2α-cluster states in the rotational bands and a single α-cluster state.

As an effort to effectively use the short-lived radioactive ion beams to be available in near future at Tokai Radioactive Ion Accelerator Complex, we have developed a radiotracer method for diffusion studies in solids. A test experiment has been performed to measure the diffusion coefficients of Li in the sample of LiAl compound, by using the α-emitting radiotracer of 8Li (T1/2 = 0.84 s). The yields of α-particles emitted from diffusing 8Li primarily implanted in the sample have been measured as a function of time under a recurrent time sequence; the implantation of 8Li for 1.5 s (beam-on) and subsequent diffusion for 4.5 s (beam-off). We have found that the time-dependent yields of the α-particles could be used as a measure of the diffusivity of the tracer in a none-destructive way. By taking into account the experimental conditions, we have simulated the time-dependent α-particle yields for various diffusion coefficients and compared with the experimental ones in order to extract the diffusion coefficients of Li in the sample. The process of the simulations and the comparisons is discussed.

The detailed level scheme of 11 Be, including spin-parity assignments, has been established from a β -delayed decay spectroscopy of spin-polarized 11 Li ( Li g.s. 11 → β Be * 11 → n Be * 10 → γ Be g.s. 10 ). From the decay scheme of 11 Be, neutron spectroscopic factors of the levels in 11 Be have been determined. The present results have been compared with the predictions by the Anti-symmetrized Molecular Dynamics (AMD) theory, where various types of α -cluster states have been predicted for the excited states both in 11 Be and 10 Be. Some of the levels in 11 Be show good accord with the 2 α -cluster states in the rotational bands and with a single α -cluster state.

An accurate exclusive measurement of the 8 Li( α , n) 11 B reaction cross sections has been carried out successfully by using a highly efficient detector system and high-purity low energy 8 Li-beam. The excitation function obtained with high statistics in the energy region from 0.9 to 2.7 MeV in the center-of-mass system is reported. The present cross sections show that the averaged ratio of total cross section to the inverted neutron capture cross section is 4 ∼ 5. Some details of the experimental method are also described.

An ISOL based radioactive nuclear beam (RNB) facility in Japan, TRIAC (Tokai Radioactive Ion Accelerator Complex), will be able to provide various short-lived radioactive isotopes for experiments with an energy ranging from 0.1 to 1.1 MeV/nucleon. As an interdisciplinary application of the RNB for materials science, two experiments have been proposed; diffusion studies in super ionic conductors, and β-NMR and PAC studies by implantation of polarized RNB. The present status of the experimental and instrumental development for the experiments is discussed.

We have studied, by a computer simulation, the feasibility of the radiotracer method for diffusion studies using short-lived radioactive nuclear beams as tracers. We have considered two typical examples of radiotracers that will be available in KEK-JAERI RNB facility, 18F (half-life: 1.8 h, β+-emitter) and 8Li (half-life: 0.83 s, α-emitter). The sample is set on a given temperature and irradiated by the tracer beams for a time-duration. In the case of 18F, the serial sectioning technique by the ion-beam-sputtering device is assumed to obtain the concentration-depth profile of the tracer diffused during the irradiation time. In the case of 8Li, the simulation reveals that the time-dependent yields of α particles can be used as a measure of the diffusivity of the tracer in a non-destructive way. The present method could be applied to measure rather large diffusion coefficients of various radiotracers with a short measuring time.

A new ISOL-based radioactive nuclear beam (RNB) facility has been constructed as a joint project of the High Energy Accelerator Research Organization (KEK) and the Japan Atomic Energy Research Institute (JAERI) at the Tandem accelerator of the Tokai site of JAERI. It uses heavy-ion linacs and related equipment of the presently closed KEK-RNB facility. Fission fragments with energy from 0.1 to 8 MeV/u will be available at this facility.

An ohmic-heating laser ion source with a thin ionizer of thickness of 30 μm has been developed for the TIARA-ISOL. It can form an electric field of 4–5 V/cm inside the ionizer. The properties of the laser ion source were tested on- and off-line with aluminum isotopes. The FWHM of the time distribution of the bunched photoions from the ion source was about 4 μs for 27Al. In on-line experiment, a photoionization efficiency of about 0.1% for 25Al was obtained.

For the construction of an electron cyclotron resonance (ECR) charge breeder (CB) at KEK-JAERI radioactive nuclear beam facility, we have made a pilot charge breeding system consisting of two compact-sized ECR ion sources. In order to find the necessary conditions for an ECRCB with high efficiency, the charge breeding efficiency for Ar ions in nitrogen plasma has been measured and compared with the results from the simulation of the trajectories of the ions in the plasma. By explicitly taking into account the cumulative small angle scattering of charged ions as well as the confinement magnetic field of the pilot CB, the simulation predicts the stopping efficiency. This is defined as a probability that the injected ions are still within the plasma volume when their initial directional motions have become randomized. Incorporating the ionization efficiency for the ions, the overall breeding efficiency has been estimated. Finally, the characteristics of the ECRCB recently installed for tests at KEK is described.

The performance of the RNB facility at KEK-Tanashi, which is a pilot facility for the E-arena in the KEK–JAERI joint project, is presented. The muonic X-ray spectroscopy of unstable nuclei by combining the RNB with muon-beam from the M-arena in the joint project is introduced.

For the radioactive nuclear beam facility at KEK Tanashi based on an isotope separator on-line (ISOL), two different types of ion sources are currently employed: a single stage 6.4 GHz ECR and a surface ionization-type ion sources. Several proton-rich radioactive nuclear beams were developed for astrophysical interests. LiF powder target in a water-cooled Cu container is used for the production of Ne isotopes and CaF 2 mixed with fine graphite powder for Na isotopes. The ion beams are bunched at the exit of the ion source in order to reduce the beam loss in post acceleration.

We have developed an active target type gas detector, multi-sampling tracking proportional chamber (MSTPC) for studies of low-energy nuclear reactions such as (α, n) and (α, p). For higher rate capability, a gas electron multiplier (GEM) foil was adopted, instead of a multi-wire proportional counter, as a component of the MSTPC. We studied the basic properties of gas gain in a 400 μm thick GEM (THGEM), which was compared to those of standard CERN GEMs. The THGEM was examined for gain instability and positive-ion feedback, especially with low-pressure He-base mixed gas. The THGEM was found to be applicable as a proportional counter using our gas target and He as the operating gas.

The magnetic moment and electric quadrupole moment of a neutron-rich nucleus 18 N have been determined by the β-NMR method, taking advantage of the fragment spin polarization induced in the intermediate-energy projectile fragmentation reaction. To assure the existence of a sufficient polarization prior to the resonance search, a scheme of polarization measurement by means of an adiabatic field rotation has been developed and successfully incorporated in the β-NMR experiment. From the observed resonance in a Pt stopper the magnetic moment for the 18 N ground state has been determined to be |μ|=0.3279±0.0013μN. The 18 N fragments have also been implanted in a single crystal Mg stopper, and from the observed quadrupole frequency |eqQ/h|=73.2±1.4 kHz, the electric quadrupole moment |Q|=12.3±1.2 mb has been deduced. The obtained μ and Q are compared with shell model calculations.

A project to acquire directly nuclear cross sections of (α, n) and (p, n) reactions using low energy light neutron-rich radioactive nuclear beams (RNB) is in progress at JAERI-Tandem facility. The method of the production of neutron-rich RNB, an improvement of a gas counter (MSTPC) in order to work effectively under the high RNB-injection rate, and the feasibility of our experiments are discussed.

In order to study the starting point of the r-process, the 16 N(α, n) cross section has been measured directly at the energy region of E cm = 1.5 – 4.0 MeV, corresponding to the Gamow peak energy T 9 = 2 – 6.

The Gamow-Teller matrix elements are calculated for the 0 f 7 2 shell nuclei on the basis of the shell model assuming 40Ca as an inert core and taking into account particle excitations from the 0 f 7 2 orbit to the upper fp-shell, i.e., the 1 p 3 2 , 0 f 5 2 and 1 p 1 2 orbits. For some of the nuclei with A ⩽ 46 the maximum number of the excited particles, Njump, of up to 6 is considered. From a comparison with experiment it is shown that the model with Njump = 1 and 2 gives a better agreement than the one with Njump = 0, i.e., the (0 f 7 2 ) n model, and that an introduction of effective single-particle matrix elements with a renormalization factor ϱ = 0.75 further improves the agreement between theory and experiment.

The characteristics and performance of a small sized gas-filled recoil isotope separator recently made at INS are described. The total efficiency and the ΔBρ / Bρ values have been measured using low velocity 16 O, 40 Ar and 68 As ions and found to be 10 and 5%, respectively. The Z -dependence of the mean charge is discussed.

An ion-guide isotope separator on-line (IGISOL) has been constructed at INS. The overall efficiency of the IGISOL has been measured to be about 2%. An effect of the plasma formed along the path of the cyclotron beam was investigated by using the 27 Al(α, 2pn) 28 Al, 19 F( 12 C, 2pn) 28 Al and 12 C( 19 F, 2pn) 28 Al reactions. Decrease of the relative efficiency with the increase of the beam intensity has been observed, indicating evidence for the plasma effect for the first time.

A gamma-ray detector system of large NaI(Tl) crystals has been constructed to investigated heavy ion reaction mechanisms and highly excited nuclear structure with heavy ion beams. The NaI(Tl) crystals have as a whole a cross section of hexagonal shape of 30 cm between the parallel sides and a length of 20 cm, and are composed of seven substructures based on a honeycomb shape. This is designed for detection of high energy gamma rays with high resolution, and also for use as an anti-Compton spectrometer for a Germanium detector. A good energy resolution of 2.7 (3.2)% fwhm was obtained for 22.3 MeV gamma rays with 18 (39)% detection effeciency of the full-energy-loss peak with anti-Compton mode (sum mode) operation of the trapezoidal NaI(Tl) crystals. Compton scattering background in a spectrum by a Ge detector was successfully reduced by about 60% in applying the trapezoidal NaI(Tl) crystals for Compton suppression. The performance of the spectrometer is discussed.