RIKEN Accelerator Research Facility Research Articles

DECAY PATTERN OF PYGMY STATES OBSERVED IN NEUTRON-RICH26Ne

Coulomb excitation of the exotic neutron-rich nucleus26Ne on a208Pb target was measured at 58 A.MeV in order to search for low-lying E1 strength above the neutron emission threshold. The radioactive beam was produced by fragmentation of a 95 A.MeV Ar beam delivered by the RIKEN Accelerator Research Facility. The set-up included a NaI ( Tl ) gamma-ray array, a charged fragment hodoscope and a neutron wall. Using the invariant mass method in the25Ne +n channel, we observe by multipole decomposition a sizable amount of E1 strength between 6 and 10 MeV excitation energy. For the first time, the decay pattern of low-lying strength in a neutron-rich nucleus is measured.

Three-nucleon force effects in theH1(d→,p→p)nreaction at 135 MeV/nucleon

The deuteron to proton polarization transfer coefficients ${K}_{\mathit{yy}}^{{y}^{'}}$ and ${K}_{y}^{{y}^{'}}$ together with the deuteron analyzing powers are measured in three coplanar configurations of the deuteron-proton breakup reaction with a 135-MeV/nucleon polarized deuteron beam at the RIKEN Accelerator Research Facility. The data are compared with theoretical predictions based on exact solutions of the three-nucleon ($3N$) Faddeev equations with high-precision nucleon-nucleon ($\mathit{NN}$) forces, alone or combined with two $3N$ force (3NF) models, the $2\ensuremath{\pi}$-exchange Tucson-Melbourne${}^{'}(99)$ (${\mathrm{TM}}^{'}99$) and Urbana IX. Large 3NF effects have been found for all the measured observables. Predicted effects are supported by the data, with the exception of the vector analyzing power ${A}_{y}^{d}$. For this observable, theory based on only $\mathit{NN}$ forces is sufficient to explain the data. The behavior of the breakup analyzing powers is found to be different from the corresponding observables in elastic nucleon-deuteron scattering.

Decay Pattern of Pygmy States Observed in Neutron-RichNe26

Coulomb excitation of the exotic neutron-rich nucleus (26)Ne on a (208)Pb target was measured at 58 MeV/u in order to search for low-lying E1 strength above the neutron emission threshold. This radioactive beam experiment was carried out at the RIKEN Accelerator Research Facility. Using the invariant mass method in the 25Ne+n channel, we observe a sizable amount of E1 strength between 6 and 10 MeV excitation energy. By performing a multipole decomposition of the differential cross section, a reduced dipole transition probability of B(E1)=0.49+/-0.16e(2) fm(2) is deduced, corresponding to 4.9+/-1.6% of the Thomas-Reiche-Kuhn sum rule. For the first time, the decay pattern of low-lying strength in a neutron-rich nucleus is measured. The extracted decay pattern is not consistent with several mean-field theory descriptions of the pygmy states.

Study of spin parts of three nucleon forces via [formula omitted] breakup reactions at intermediate energies

Measurements of deuteron–proton breakup reactions at 135 MeV/A have been made at RIKEN Accelerator Research Facility, focusing on the specific coplanar configurations. The obtained results are compared with the Faddeev calculations based on the modern nucleon–nucleon forces together with the Tucson-Melbourne99, and UrbanaIX three nucleon forces.

Measurement of the 2H(d,pn) reaction at 0 degrees at 270 MeV

The energy spectrum of the 2 H(d,pn) reaction at 0° at 270 MeV was obtained at RIKEN Accelerator Research Facility. The spectrum in the kinematical region where the resulting pn pair has the relative energy smaller than ∼2 MeV was analyzed by plane-wave impulse approximation calculations assuming the configurations of final state interaction with s-wave.

Performance of neutron polarimeter SMART-NPOL

The neutron polarimeter SMART-NPOL has been constructed at the RIKEN Accelerator Research Facility for measuring polarization correlations of proton–neutron systems. The SMART-NPOL system consists of 12 parallel neutron counter planes of two dimensionally position-sensitive plastic scintillators with a size of 60 × 60 × 3.0 cm 3 . Polarimetry measurements were made using the analyzing power of the H 1 ( n ⇒ , n ) H 1 reaction occurring in the plastic scintillators. The effective analyzing power of SMART-NPOL was measured with polarized neutrons from the zero-degree Li 6 ( d ⇒ , n ⇒ ) reaction with an incident deuteron energy of 135 MeV / A . The effective analyzing power thus obtained was 0.26 ± 0 . 01 stat ± 0 . 03 syst and the double scattering efficiency was 1.1 × 10 - 3 .

Nuclear structure and astrophysics studies at RIKEN

At the RIKEN Accelerator Research Facility, various nuclear structure and astrophysics studies have been made using fast exotic beams. Most of the studies are performed in the so-called reversed kinematics paradigm by measuring the deexcitation γ rays or particle decays of unstable nuclei from their excited levels. Several examples of such studies are presented. The RIKEN RI Beam Factory, which will have its first beam ready by the end of 2006, will greatly extend the region of study to more exotic and heavier nuclei.

Tensor analyzing power T 20 of the dd → 3Hen and dd → 3Hp reactions at zero angle for energies 140, 200, and 270 MeV

RIKEN Accelerator Research Facility data on the tensor analyzing power T{sub 20} of the dd {sup {yields}} {sup 3}Hen and dd {sup {yields}} {sup 3}Hp reactions at zero angle for deuteron kinetic energies of 140, 200, and 270 MeV are reported. The observed positive sign of T{sub 20} clearly demonstrates the sensitivity to the D/S-wave ratio in the {sup 3}He and {sup 3}H wave functions in the energy range of the experiment. Data on T{sub 20} for the {sup 3}Hen channel are in agreement with those for the {sup 3}Hp channel within the experimental errors.

Nuclear structure studies far from stability using intermediate-energy RI beams

At the RIKEN Accelerator Research Facility, various spectroscopic studies have been made using fast exotic beams. Most of the studies are performed in the reversed-kinematics by measuring de-excitation γ rays or particle decays from excited levels of unstable nuclei. Several examples of spectroscopic studies of unstable nuclei are presented. The RIKEN RI Beam Factory, which will have the first beam at the end of 2006, will greatly extend the region of study to more exotic and heavier nuclei.

Low-energy radioisotope beam separator CRIB

A low-energy in-flight type RI beam separator, called CRIB, has been installed for nuclear physics and nuclear astrophysics by the Center for Nuclear Study, the University of Tokyo in the RIKEN Accelerator Research Facility. It consists of a double achromatic system and an Wien filter. It is capable of providing RI beams at 5–10 MeV/nucleon. Since CRIB has been developed in the end of 2000, many proton-rich RI beams were successfully produced via the ( p, n), ( d, t), and ( 3 He , n) reactions in inverse kinematics. In this paper, the design of the separator and its performance are discussed including the method of RI-beam production at low energies.

Measurement of the tensor analyzing power [formula omitted] in the [formula omitted] and [formula omitted] at intermediate energies and at zero degree

The data on the tensor analyzing power T20 in the dd→He3n and dd→H3p reactions obtained at 140–270 and 140–200 MeV of the deuteron kinetic energy, respectively, and at zero degree were measured at RIKEN Accelerator Research Facility. The observed positive sign of T20 clearly demonstrates the sensitivity to the D/S wave ratios in the 3He and 3H in the energy domain of the measurements. The T20 data for the 3He–n and 3H–p channels are in agreement within experimental accuracy.

Polarization transfer measurement forH1(d⃗,p⃗)H2elastic scattering at135MeV∕nucleonand three-nucleon force effects

The deuteron to proton polarization transfer coefficients for the $d$--$p$ elastic scattering were precisely measured with an incoming deuteron energy of 135 MeV/u at the RIKEN Accelerator Research Facility. The data are compared to theoretical predictions based on exact solutions of three-nucleon Faddeev equations with high--precision nucleon--nucleon forces combined with different three-nucleon forces (3NFs), representing the current, most popular models: the $2\pi$-exchange Tucson-Melbourne model, a modification thereof closer to chiral symmetry TM'(99), and the Urbana IX 3NF. Theory predicts large 3NF effects, especially in the angular range around the cross section minimum, but the present data only partially concurs, predominantly for $K_{xx}^{y'}-K_{yy}^{y'}$ ($K_{xx}^{y'}$, $K_{yy}^{y'}$). For the induced polarization, $P^{y'}$, the TM$'$(99) and Urbana IX 3NFs reproduce the data, but the Tucson-Melbourne 3NF fails to describe the data. For the polarization transfer coefficients, $K_{y}^{y'}$ and $K_{xz}^{y'}$, the predicted 3NF ffects are in drastic conflict to the data. These facts clearly reveal the defects of the 3NF models currently used.

Radiation Effects on Cryogenic Temperature Sensors of Cernox, CGR and PtCo

Irradiation tests have been carried out in view of selecting the cryogenic temperature sensors for use in the RIKEN Superconducting Ring Cyclotron (SRC). About 300 temperature sensors will be installed, and most of them will be exposed to high radiation doses during the operation of accelerator. Those cryogenic temperature sensors, Cernox/spl trade/, CGR and PtCo, at room temperature were bombarded by the intense 210 MeV proton beam at the RIKEN Accelerator Research Facility (RARF) of RIKEN.

Polarization transfer measurement for d-p scattering and three nucleon force effects

Precise measurements of the deuteron to proton polarization transfer coefficients for the d-p elastic scattering has been made at 135 MeV/u at RIKEN Accelerator Research Facility. The obtained results are compared with the Faddeev calculations based on modern nucleon-nucleon forces together with Tucson-Melbourne, Tucson-Melbourne' and Urbana-Argonne type of three nucleon forces.

POLARIZATION TRANSFER MEASUREMENT FOR d-p ELASTIC SCATTERING: TO SEARCH FOR THREE NUCLEON FORCE EFFECTS

Precise measurements of the deuteron to proton polarization transfer coefficients for the d-p elastic scattering have been made at 135 MeV/u at RIKEN Accelerator Research Facility. The obtained results are compared with the Faddeev calculations based on modern nucleon-nucleon forces together with Tucson-Melbourne Tucson-Melbourne' and Urbana–Argonne type of three nucleon forces.

Development of a Detector System for Transient Magnetic Field Experiments with Radioactive-Isotope Beams and a Test Experiment

For g-factor measurements of excited states of unstable nuclei, we have been developing a setup for the transient field experiment at RIKEN Accelerator Research Facility. For the preparation of ferromagnetic material, thick Gd foils have been annealed. In a test experiment using a 22Ne beam, the projectile Coulomb excitation to the first excited state has been observed.

RIKEN RI-Beam Factory project

The RIKEN Accelerator Research Facility has proposed the `RI-Beam Factory' project. In the 1997 fiscal year, the construction budget for the first phase of this project has been formally approved. The factory is aimed at providing the world's most intense radio-isotope beams at energies of several hundred MeV/nucleon over the whole range of atomic masses. For that purpose, a cascade of a ring cyclotron and a superconducting ring cyclotron will boost the energy of heavy-ion beams from the existing ring cyclotron up to 400 MeV/nucleon for light ions and more than 100 MeV/nucleon for very heavy ions. Multi-use experimental storage rings (MUSES) consisting of an accumulator-cooler ring, a booster synchrotron ring and double storage rings will also be used as a new generation of experimental installation. The MUSES will enable us to conduct various types of unique colliding experiments.

Detector system of the first focal plane of the spectrometer SMART at RIKEN

A detector system of the first focal plane of SMART, the 135 MeV/u high-resolution spectrometer at RIKEN Accelerator Research Facility, is described. It consists of a pair of multi-wire drift chambers and a trigger scintillator hodoscope contained in a He-filled detector box. A major subject using this system is the measurement of the (d, 2He) reaction making the most of its large angular and momentum acceptances. Without seriously sacrificing the detection efficiency, reasonably good energy and angular resolutions for 2He, 460 keV and 9 mrad (FWHM), respectively, have been achieved after optimizing the optics property of the spectrometer.

Atomic physics at the planned new RIKEN facility

RIKEN accelerator research facility (RARF) has started a project to extend the present heavy ion accelerator complex to produce high-energy and high-intensity radioactive ion beams. This report reviews the present status of RARF and describes the new project, RIKEN RI Beam Factory. Some proposed experiments and possibilities in atomic physics are also described.

Polarized 3He project at RIKEN

A new approach to study the high momentum component of deuteron D-state wave function by measuring the polarization correlation coefficient, C ∥, for the 3 H e( d , p) 4He reaction has been proposed. In order to carry out the experiment, a spin exchange type polarized 3He target was constructed at RIKEN Accelerator Research Facility (RARF). The density of the target was 2.2 × 10 20 cm −3 (8.2 amagats), and the polarization was about 10%. The spin flip system worked well. The experiment has been carried out in November, 1996, at RARF, using a polarized deuteron beam of 270 MeV.