R. L. Gill

Retardation of B(E2; 01+ → 21+) rates in 90-96Sr and strong subshell closure effects in the A ~ 100 region

Abstract Lifetimes of low-lying levels in 90, 92, 94, 96Sr were measured using a recently developed βγγ fast timing method. The deduced B(E2; 01+ → 21+) rates which fill the N = 52–58 gap in the known B(E2) strengths for 78–100Sr, are exceptionally low, ≈8 W.u., and provide evidence for a strong Z = 38 subshell effect extending from N = 50 to N = 58. Furthermore, these rates establish a close similarity between 88–96Sr and 90–98Zr nuclei, which form a region of the lowest B(E2) values (for nuclei with A > 56) second only to 204–210Pb. The SrB(E2) values are compared with predictions based on various global, regional and local systematics as well as the results of laser spectroscopic measurements. The discrepancy with the latter is explained as due to octupole vibrational collectivity. Unique to the Q0 systematics in the Sr region is a sudden shape change from spherical to deformed at N = 60. This transformation is so remarkably large and abrupt for Sr nuclei that it suggests a “phase change” in Sr collectivity.

Observation of 0+ states in 118Cd and the systematics of intruder states

Abstract The structure of 118 Cd was studied through the decay of 118 Ag by angular correlation, γ-γ coincidence, and γ-multiscaling measurements. The results showed the existence and E2 decay properties of two new 0 + states at 1285.8 and 1615.0 keV. They were interpreted in terms of a schematic model calculation of all the even Cd isotopes which is based on simple properties of a quadrupole interaction and which involves the mixing of normal vibration-like and intruder rotation-like 2p-4h configurations.

Deformation and shape coexistence of 0+ states in 98Sr and 100Zr

Abstract Half-lives of 21+ levels [2.80(8) ns in98Sr, 0.55(2) ns in100Zr] and the 98Sr 41+ level [80(6) ps] were obtained using a β-γ-γ triple coincidence system. Analysis of B(E2) and B(E0) rates indicates that 98Sr and100Zr have very similar structures. Both have a highly deformed (β∼0.4) yrast band and a moderately deformed (|β|∼0.2) low-lying band. The highly deformed bands in 98Sr and 100Zr are nearly identical to the yrast bands in 100Sr and 102Zr.

First Measurement of Proton-Proton Elastic Scattering at RHIC

Abstract The first result of the pp2pp experiment at RHIC on elastic scattering of polarized protons at s =200 GeV is reported here. The exponential slope parameter b of the diffractive peak of the elastic cross section in the t range 0.010⩽|t|⩽0.019 (GeV/c)2 was measured to be b=16.3±1.6(stat.)±0.9(syst.) (GeV/c)−2.

Measurement of the analyzing power A[N] in pp elastic scattering in the CNI region with a polarized atomic hydrogen gas jet target

The analyzing power AN for pp elastic scattering is expected to reach a peak value of 0.045 in the Coulomb Nuclear Interference (CNI) region at a momentum transfer −t of 0.003 (GeV/c). During the 2004 RHIC Run, we completed a measurement of AN in the CNI region by detecting the recoil protons from pp elastic scattering using a polarized atomic hydrogen gas jet target and the 100 GeV RHIC proton beam. We report the first measurements of the AN absolute value and shape in the −t range from 0.0015 to 0.010 (GeV/c) with a precision better than 0.005 for each AN data point. The recoil protons were detected with two arrays of Si detectors. The absolute target polarization as monitored by a Breit-Rabi polarimeter was stable at 0.924±0.018. This result allows us to further investigate the spin dependence of elastic pp scattering in the very low −t region.

A new TRISTAN thermal ion source

Abstract A new thermal ion source with an integrated target which is heated by electron bombardment to a temperature of 2500°C has been developed for the TRISTAN on-line fission product mass separator at Brookhaven National Laboratory. Initial on-line tests demonstrated that this ion source can extend the range of accessible elements to the rare-earth region. Yields are presented for isotopes of Ce, Pr, Pm, Sm, Eu, Gd, Tb, Dy, ho, Ga, Ge, As, Y, Zr, Nb, In, Sn and Sb.

A versatile data acquisition system and the ISOL facility TRISTAN

Abstract The on-line mass separator, TRISTAN, is located at Brookhavens High Flux Beam Reactor. A Nielsen-type ion source, which can contain up to 8g of 235 U in an external beam with a flux of ∼2 × 10 9 n/cm 2 /s is used to generate short-lived fission products. Experiments have been constructed on two of the five available beam lines to measure β-γ and γ-γ coincidences, γ-γ angular correlations and delayed neutron emissions. A Users Group has been formed to coordinate research between University groups and BNL. Developments planned for TRISTAN include FEBIAD, surface ionization and negative-surface ionization-type ion sources, and a He-jet system as well as construction of new experimental facilities. An off-line separator, ISTU, is available for the development program. We have constructed a versatile, modular data acquisition system to service experiments on TRISTAN and other nuclear research facilities at the HFBR using CAMAC interfacing. Standard, commercially-available electronic instruments and computer programs, such as FORTRAN and system routines, are used throughout. Simple interfaces have been built to adapt non-CAMAC equipment to CAMAC input registers. Up to eight different experiments can be multiplexed on the branch highway by a fast microprogrammed branch driver with a 4096 word memory. The branch driver delivers pre-processed data to a bus which links devices such as a central processor, 1 megaword core memory, tape drives, discs, display processor and terminal. The following features are offered: two 8192 channel pulse height analyzers, a 3-parameter coincidence unit, 4 multiscalers, a timed sequence of delayed γ-ray spectra (33 spectra of 4096 channels each), a 2-parameter (pulse height versus time-of-flight) analyzer, 16 scalers and 24 experimental interlocks. Up to 100 different spectra are available to users for display during an experiment. Event mode recording on magnetic tape and direct sorting into the 1 megaword array are used as appropriate. A master program, almost entirely written in FORTRAN, allows users to control and monitor each experiment with simple mnemonic codes. Analysis of the data is carried out on a companion computer devoted to data processing, allowing rapid sorting, plotting, listing and analysis of data.

A Four-detector System For γ - γ Angular Correlation Studies

Abstract A multiple detector system for γ - γ coincidence and angular correlation measurements is described. The system consists of four large coaxial Ge detectors set at fixed positions, enabling the measurement of γ - γ coincidences between any pair. Thus, six distinct angles are measured simultaneously. The performance of the system was studied with a 152 Eu source. An application for γ - γ correlation studies in 146 Ce is described. Systematic errors, if present, are shown to be less than 0.03 and 0.05 for A 2 and A 4 , respectively. The extension of this system to more than four detectors is straightforward.

IBA calculations near the Z = 64 subshell

Abstract It has recently been shown that the effects of the Z = 64 subshell are important for nuclides with N ⩽ 88 but disappear as N approaches 90, due to the influence of neutron-proton interactions. IBA-2 calculations show excellent agreement with experimental energy systematics if the effective reduction in valence bosons caused by the Z = 64 subshell closure is explicitly included. The deduced parameters then behave as predicted by the microscopic basis of the IBA-2 model, in contrast with previous studies.

Comparison between solid graphite and graphite cloth as a target backing material for an Isol system

Abstract A high temperature target-ion source system developed for the TRISTAN on-line isotope separator at Brookhaven National Laboratory is described. The system consists of a surface ionization chamber with a Re surface and 235U in a graphite target matrix. Both solid graphite and graphite cloth targets were tested. The overall separation efficiencies for isotopes of Rb, Cs, Sr, Ba, In, Ce,Pr, Nd and Pm which were extracted directly from the ion source were measured. Production rates were measured as a function of temperature up to 2000°C and the ion source performance was monitored over long periods of time. The performance of solid graphite targets was found to give superior stable operation with no deterioration in production rates for operation times of up to 2000 h. In contrast, graphite cloth targets exhibited deterioration in efficiency with time for most elements with differing rates. The production rates of Ce and Pr isotopes increases significantly during the first 400 h of operation, followed by a slow decrease.