B. M. Sherrill

The GSI projectile fragment separator (FRS): A Versatile magnetic system for relativistic heavy ions

The projectile fragment separator FRS designed for research and applied studies with relativistic heavy ions was installed at GSI as a part of the new high-energy SIS/ESR accelerator facility. This high-resolution forward spectrometer has been successfully used in first atomic and nuclear physics experiments using neon, argon, krypton, xenon, and gold beams in the energy range from 500 to 2000 MeV/u. For the first time relativistic xenon and gold fragments have been isotopically separated. In this contribution we describe first experiments characterizing the performance of this spectrometer.

The super-FRS project at GSI

The GSI projectile fragment separator FRS has demonstrated with many pioneering experiments the research potential of in-flight separators at relativistic energies. Although the present facility has contributed much to the progress in the field of nuclear structure physics, major improvements are desirable in the future. The characteristics of the proposed next-generation facility at GSI, the Super-FRS, will be presented and compared to other projects. The Super-FRS is a large-acceptance superconducting fragment separator followed by different experimental branches including a combination with a new storage-cooler ring system. This system consists of a collector ring (CR) and a new experimental storage ring (NESR) which allow precision mass and lifetime measurements as well as in-ring reaction studies. The NESR can be operated in combination with an electron ring to measure electron scattering with exotic nuclei. This electron heavy-ion collider will open up new fields for nuclear structure research.

Commissioning the A1900 projectile fragment separator

An important part of the recent upgrade of the NSCL facility is the replacement of the A1200 fragment separator with a new high acceptance device called the A1900. The design of the A1900 device represents a third generation projectile fragment separator (relative to the early work at LBL) as it is situated immediately after the primary accelerator, has a very large acceptance, a bending power significantly larger than that of the cyclotron and is constructed from large superconducting magnets (quadrupoles with 20 and 40 cm diameter warm bores). The A1900 can accept over 90% of a large range of projectile fragmentation products produced at the NSCL, leading to large gains in the intensity of the secondary beams. The results of initial tests of the system with a restricted momentum acceptance (±0.5%) indicate that the A1900 is performing up to specifications. Further large gains in the intensities of primary beams, typically two or three orders of magnitude, will be possible as the many facets of high current extraction from the ion sources, acceleration of intense, low charge-state ions in the K500 cyclotron, transfer and stripping injection in K1200 cyclotron are optimized. A liquid-lithium cooled beryllium target system is being constructed to use with the high power beams (up to ≈5 kW) that will be available from the coupled-cyclotron facility. An overview of the design, construction and commissioning studies of the A1900 device will be presented along with some of the results from the initial exotic isotope production studies.

Identification of 45 New Neutron-Rich Isotopes Produced by In-Flight Fission of a 238U Beam at 345 MeV/nucleon

A search for new isotopes using in-flight fission of a 345 MeV/nucleon 238 U beam has been carried out at the RI Beam Factory at the RIKEN Nishina Center. Fission fragments were analyzed and identi...

Coulomb dissociation of 11Li.

Kinematically complete measurements for Coulomb dissociation of [sup 11]Li into [sup 9]Li+2[ital n] were made at 28 MeV/nucleon. The [ital n]-[ital n] correlation function suggests a large source size for the two-neutron emission. The electromagnetic excitation spectrum of [sup 11]Li has a peak, as anticipated in low-energy dipole resonance models, but a large post-breakup Coulomb acceleration of the [sup 9]Li fragment is observed, indicating a very short lifetime of the excited state and favoring direct breakup as the dissociation mechanism.

81Kr in the Great Artesian Basin, Australia: a new method for dating very old groundwater

Abstract The measurement of cosmogenic 81Kr (t1/2=(2.29±0.11)×105 yr) has been proposed for many years as a reliable tool for groundwater dating in the range from 105 to 106 yr. In this paper, we report on the first use of 81Kr to determine the age of groundwater from four wells in the Great Artesian Basin in Australia. As the concentration of 81Kr in old groundwater is only a few hundred atoms per liter, krypton was extracted from large (16 000 l) groundwater samples and was analyzed for the isotopic abundance of 81Kr by accelerator mass spectrometry (AMS) with a cyclotron. 81Kr/Kr isotope ratios of (1.54±0.22)×10−13, (1.78±0.26)×10−13, (2.19±0.28)×10−13 and (2.63±0.32)×10−13, respectively, were measured for these samples. It is reasonable to assume that krypton dissolved in surface water in contact with the atmosphere has the known atmospheric 81Kr/Kr ratio of (5.20±0.40)×10−13. The observed reduction of isotope ratios in the groundwater samples can then be interpreted as being due to radioactive decay since recharge. This results in respective groundwater ages of: (4.02±0.51)×105 yr, (3.54±0.50)×105 yr, (2.87±0.38)×105 yr and (2.25±0.42)×105 yr. The main emphasis of this paper lies on the description of the analytic procedure to extract a reliable 81Kr signal from large groundwater samples. Although the uncertainties are still relatively large (primarily due to counting statistics caused by the low cyclotron AMS efficiency), the new technique enabled for the first time a definite determination of residence times for old groundwater with 81Kr. It thus confirms the hope that this radionuclide may become a very valuable tool for groundwater dating.

Discovery of 40Mg and 42Al suggests neutron drip-line slant towards heavier isotopes.

A fundamental question in nuclear physics is what combinations of neutrons and protons can make up a nucleus. Many hundreds of exotic neutron-rich isotopes have never been observed; the limit of how many neutrons a given number of protons can bind is unknown for all but the lightest elements, owing to the delicate interplay between single particle and collective quantum effects in the nucleus. This limit, known as the neutron drip line, provides a benchmark for models of the atomic nucleus. Here we report a significant advance in the determination of this limit: the discovery of two new neutron-rich isotopes—40Mg and 42Al—that are predicted to be drip-line nuclei. In the past, several attempts to observe 40Mg were unsuccessful; moreover, the observation of 42Al provides an experimental indication that the neutron drip line may be located further towards heavier isotopes in this mass region than is currently believed. In stable nuclei, attractive pairing forces enhance the stability of isotopes with even numbers of protons and neutrons. In contrast, the present work shows that nuclei at the drip line gain stability from an unpaired proton, which narrows the shell gaps and provides the opportunity to bind many more neutrons.

Identification of New Isotopes 125 Pd and 126 Pd produced by In-flight Fission of 345 MeV/nucleon 238 U: First Results from the RIKEN RI Beam Factory

A search for new isotopes using in-flight fission of a 345 MeV/nucleon 238 U beam has been carried out in the commissioning experiment of the next-generation in-flight radioactive isotope beam separator BigRIPS at the RI Beam Factory at the RIKEN Nishina Center. Two neutron-rich palladium isotopes 125 Pd and 126 Pd were observed for the first time, which demonstrates the great potential of the RIKEN RI beam factory.

Observation of a proton halo in8B

The longitudinal momentum distribution of7Be was measured after the break-up reaction of8B in C, Al, and Pb targets at 1471 A·MeV. We observed a narrow distribution with a FWHM of (81±6) MeV/c in all targets. The experimental results indicate an extended spatial distribution of the loosely bound proton in8B, and agree with QRPA calculations.

The A1200 projectile fragment separator

Abstract A beam analysis device, the A1200, has been constructed at the National Superconducting Cyclotron Laboratory (NSCL) for routine cyclotron beam analysis. This device can also be used to separate radioactive beams produced by projectile fragmentation. Since the A1200 begins the K1200 cyclotron beam lines, radioactive ions can be delivered to any experimental device. The details of the mechanical and optical designs are presented. In addition some of the planned experiments with the separated radioactive beams are discussed.