Y. Fujita

Mass measurements of exotic nuclei at the ESR

Abstract We plan to measure masses of exotic nuclei far from the valley of β-stability using the Experimental Storage Ring (ESR) at GSI, Darmstadt, as a time-of-flight (TOF), multi-turn mass spectrometer. For this purpose the ring must be tuned to an isochronous mode, so that the flight time per turn of an ion depends only on its mass-to-charge ratio and not on its velocity spread. For these measurements the timing detector is planned to be placed directly in the ESR to record each circulating ion.

High resolution beam line for the Grand Raiden spectrometer.

Abstract We have designed and constructed a new beam line which can accomplish both lateral and angular dispersion matching with the Grand Raiden spectrometer. In dispersive mode, lateral and angular dispersions of the beam line are b 16 =37.1 m and b 26 =−20.0 rad , respectively, to satisfy matching conditions for Grand Raiden. In achromatic mode, the beam line satisfies the double achromatic condition of b16=b26=0. The magnifications of the beam line are (Mx,My)=(−0.98,0.89) and (−1.00,−0.99) for dispersive and achromatic modes, respectively. In the commissioning experiments, we have succeeded to separate the first excited 2+ state of 168Er with E x =79.8 keV clearly from the ground state in the (p,p′) reaction. We achieved energy resolutions of ΔE=13.0±0.3 and 16.7±0.3 keV in full width at half-maximum for 295 and 392 MeV protons, respectively. These energy resolutions agree with the resolving power of Grand Raiden for an object size of about 1 mm .

Realization of matching conditions for high-resolution spectrometers

For precise measurements of nuclear-reaction spectra using a beam from an accelerator, a high-resolution magnetic spectrometer is a powerful tool. The full capability of a magnetic spectrometer, however, can be achieved only if the characteristics of the beam coming from the accelerator are matched to those required by the spectrometer by properly adjusting the beam line conditions. The matching methods, lateral dispersion matching, focus matching and also the kinematic correction compensate the spectrum line-broadening effects caused by the beam momentum spread and reaction kinematics. In addition, angular dispersion matching should be performed if good resolution of the scattering angle is important. Diagnostic methods developed to realize these matching conditions for the spectrometers K600 at IUCF and Grand Raiden at RCNP are presented.

Better-resolution measurement of vertical scattering angle in a new ion-optical mode of spectrometer ''Grand Raiden''

Demand for near-zero-degree measurements using a magnetic spectrometer is increasing with the growing interest in the study of L ¼ 0 nuclear excitation modes. For the precise determination of scattering angles near 08, both horizontal and vertical scattering angle components have to be measured with good accuracy. It is, however, not easy to realize a good vertical angle resolution using modern high-resolution magnetic spectrometers with small vertical angle magnifications, like ‘‘Grand Raiden’’ at RCNP or ‘‘K600’’ at IUCF. A new ion-optical mode which enables the precise determination of vertical components of scattering angles from vertical position measurements in the focal plane has been developed for these spectrometers. For the wide range of spatial as well as momentum acceptances, vertical angle resolution better than 8 mrad is realized for Grand Raiden at RCNP. # 2001 Elsevier Science B.V. All rights reserved.

Charge-Exchange Reactions with a Secondary Triton Beam

A secondary triton beam from fragmentation of 560-MeV alpha-particles has been used in a high-resolution (t,He-3) charge-exchange experiment at intermediate bombarding energies. The experiment was carried out at the National Superconducting Cyclotron Laboratory using a He-4 beam from the K1200 cyclotron. The radioactive triton beam of (0.5-1.0)x 10(6) particles/s with a mean energy of 350 MeV was produced in a production target of the A1200 fragment separator and transported to the target position of the S800 magnetic spectrometer. Ray-tracing and dispersion matching techniques were employed to detect He-3 particles from the C-12(t,He-3)B-12 reaction near 0 degrees. An energy resolution of Delta E approximate to 160 keV or Delta E/E approximate to 4.6 x 10(-4) (FWHM) was achieved. This is an improvement over our previous results and opens the possibility for studying high-resolution (n,p)-type reactions at intermediate bombarding energies

Spin-isospin resonances in nuclei

Spin-isospin excitations in nuclei have been investigated via the (He-3,t) reaction at 450 MeV. The volume integrals of the effective interactions J(sigma tau) and J(tau) for the (He-3,t) reaction at 450 MeV have been empirically determined to be 172+/-17 MeV-fm(3) and 53+/-5 MeV-fm(3), respectively. These values are consistent with those obtained for the (p,n) reaction at the same bombarding energy per nucleon. The Gamow-Teller strength can be deduced with the same reliability as the (p,n) reaction at the bombarding energy above 100 MeV. The neutrino absorption cross sections by Ga-71 is obtained on basis of the present (He-3,t) measurement. The new SNU value obtained for Ga-71 is 130.3+/-1.6(stat.)+/-16.8(syst.). The spin-isospin responses for the Sn isotopes, Cd-116 and Mo-100 are presented and are discussed in relation with double beta-decay. A measurement of decay protons from the spin-isospin resonances in N-13 is presented. A new possibility of studying the spin-isospin responses on the beta(+) side via the (t,He-3) reaction using a dispersion matching technique is discussed.

TOF-Mass-Measurements of exotic nuclei at the ESR of the GSI

Abstract We plan to measure masses of very shortlived exotic nuclei far from β-stability by using the Experimental Storage Ring (ESR) of the GSI as an energy isochronous Time-of-Flight (TOF) mass-spectrometer which requires a special ion-optical setting of the quadrupoles in the ESR. Via projectile fragmentation a large variety of exotic nuclei can be produced. which can be separated in flight by the Fragment Separator (FRS) prior to being injected into the ESR. A special time-pick-off-detector has been developed, which uses secondary electron emission from a thin foil, which is penetrated by the ions under investigation. This detector has been designed for use in the ESR-vacuum of 10 −11 mbar and has an intrinsic time-resolution of ≈ 85ps. Also a Time Recording System (TRS) with a time-jitter of only ≈ 50ps has been build. The TRS is capable of recording time-marks for a few ions over a large number (> 500) of turns.

Mass measurements of stored and cooled exotic nuclei at GSI

Abstract Recent results of mass measurements of exotic heavy nuclei using Schottky mass spectrometry are reported. The investigated nuclei were produced and separated by the fragment separator FRS, injected, stored and cooled in the experimental storage ring ESR. Their masses were determined from the frequency analysis of the beam noise. In the present experiment the masses of more than 150 proton-rich nuclei in the mass region A ∼- 200 have been determined for the first time. Representative results are given for proton-separation energies and shell corrections. For some elements the proton drip line can be determined.

Direct mass measurements of proton-rich nuclei in the region from tellurium to polonium

Abstract In recent experiments using the FRS-ESR facilities at GSI we have measured the masses of 225 proton-rich nuclei in the range of 135≤A≤209. Applying the new technique of Schottky-Mass Spectrometry an accuracy of about 100 keV and a resolving power of 4 · 10 5 was achieved. Masses for cooled projectile fragments with half-lives larger than a few seconds were determined by their revolution frequencies in the ESR. Reliable predictions for the proton-drip line can be made for elements from bismuth to protactinium using the precisely measured Q α -values from literature and our new mass values for the isotopes at the ends of the corresponding α-chains.