John A Becker

First-forbidden beta decay near A = 40

Shell model calculations in the full sdpf model space are carried out for the known and possible first-forbidden beta decays for 34 <A < 44. The main purpose is to study the strong quenching of the unique (ΔJ = 2) decays, which results from the repulsive nature of the T = 1 particle-hole interaction, and the strong meson-exchange enhancement of the ΔJ = 0 decays. Procedures for calculating the relevant matrix elements and combining them to form the decay rate are described in detail. Varous approximations designed to display more clearly the dependence of the rates on the contributing matrix elements are presented and the associated errors assessed. The results are compared to experiment and conclusions are drawn regarding our present understanding of first-forbidden beta decays.

Magnetic rotation in 197Pb and 198Pb

Abstract High-spin states in 197Pb and 198Pb were populated in the 186W(18O ,x n) reactions. In-beam γ -ray coincidences were measured in two experiments using the Gammasphere and the Eurogam II spectrometer arrays, respectively. In both nuclei new bands of enhanced magnetic dipole transitions were found and the known cascades were partly reordered and extended to higher spins. In most cases, γ -ray transitions connecting the magnetic rotational bands to lower-lying states have been identified. Configuration assignments are suggested for the bands. The systematic behavior confirms the shears mechanism. An effective interaction between the main high-spin proton and neutron orbitals is derived.

Simultaneous ballistic deficit immunity and resilience to parallel noise sources: A new pulse shaping technique

A new and different time variant pulse processing system has been developed based on a simple CR-RC filter and two analog switches. The new pulse processing technique combines both ballistic deficit immunity and resilience to parallel noise without a significant compromise to the low energy resolution, generally considered a mutually exclusive requirement. The filter is realized by combining two different pulse-shaping techniques. One of the techniques creates a low rate of curvature at the pulse peak, which reduces ballistic deficit, while the second technique increases the tolerance to low frequency noise by modifying the noise history. Several experimental measurements are presented, including tests on a co-planar grid CdZnTe detector. Improvements on both the resolution and line shape are shown for the 662 keV line of /sup 137/Cs.

Low-energy M1 excitation mode in 172Yb

Two-step-cascade spectra in 172-Yb have been measured after thermal neutron capture. They are compared to calculations based on experimental values of the level density and radiative strength function (RSF) obtained from the 173-Yb(3-He,alpha gamma)172-Yb reaction. The multipolarity of a 6.5(15) mu_N^2 resonance at E_gamma=3.3(1) MeV in the RSF is determined to be M1 by this comparison.

Gamma‐Ray Spectroscopy at TRIUMF‐ISAC: the New Frontier of Radioactive Ion Beam Research

High‐resolution gamma‐ray spectroscopy is essential to fully exploit the unique scientific opportunities at the next generation radioactive ion beam facilities such as the TRIUMF Isotope Separator and Accelerator (ISAC). At ISAC the 8π spectrometer and its associated auxiliary detectors is optimize for β‐decay studies while TIGRESS an array of segmented clover HPGe detectors has been designed for studies with accelerated beams. This paper gives a brief overview of these facilities and also presents recent examples of the diverse experimental program carried out at the 8π spectrometer.In this article we prove the bivariate uniqueness property for a particular “max-type” recursive distributional equation (RDE). Using the general theory developed in [5] we then show that the corresponding recursive tree process (RTP) has no external randomness, more preciously, the RTP is endogenous. The RDE we consider is so called the Logistic RDE, which appears in the proof of the ζ(2)-limit of the random assignment problem [4] using the local weak convergence method. Thus this work provides a non-trivial application of the general theory developed in [5]. AMS 2000 subject classification : 60E05, 60J80, 60K35, 62E10, 82B43.

Coulomb excitation of the 242mAm isomer

The 242mAm isomer, a well-known candidate for photodepopulation research, has been studied in this first ever Coulomb excitation of a nearly pure (≈98%) isomer target. Thirty new states, including a new rotational band built on a Kπ = 6− state, have been identified. Strong K-mixing results in nearly equal populations of the Kπ=5− and 6− states. Newly identified states have been assigned to the Kπ=3− rotational band, the lowest states of which are known to decay into the ground-state band. Implications regarding K-mixing and Coulomb excitation paths to the ground state are discussed.

Absolute Partial γ-ray Cross Sections in 238U(n, xny) Reactions

Measurements of discrete γ-ray spectra have been carried out as a function of incident-neutron energy for nuclei populated in 238U(n, xnγ (x ≤ 4) reactions. The GEANIE spectrometer, comprised of 26 Ge detectors (11 planar and 15 coaxial), was used to detect γ-rays. Neutrons were provided by the “white” neutron source of the Los Alamos Neutron Science Center’s WNR facility. The energy of the incident neutrons was determined using the time-of-flight technique. Absolute cross sections were determined for emission of several γ-rays from low-lying states of 235-238U isotopes (spin up to 10ħ and excitation energy up to ~1 MeV) as a function of incident-neutron energy (1 MeV < En 100 MeV). Uncertainties associated with the spectroscopic analysis of the data are discussed. Predictions of partial γ-ray cross sections from GNASH calculations up to neutron energy En = 30 MeV are compared to the experimental results and are generally in good agreement. Combining the experimental results with the predictions of the nuclear reaction modeling provides a measurement of the 238U(n, n′) reaction cross section and validate this technique for determining reaction cross sections using γ-ray spectroscopy.

Intermediate-Energy Neutron-Induced Fission of Uranium: Product Yields and Isomer Studies

An experimental program for studying the properties of fission induced by intermediate energy neutrons has been started at the Los Alamos Neutron Science Center (LANSCE). Experiments using the GEANIE high-resolution gamma-ray spectrometer were performed to extract excitation functions for fission products obtained from a 238U target. Results of the singles data for xenon isotopes are presented to illustrate the method, and show that the rotational ground-state-band transitions are suited for the extraction of production cross-sections of even-even fission products. Coincidence data are analyzed to produce isotopic distributions of barium for three neutron energy groups. Also, an example of the evolution with incident neutron energy of the relative intensities of coincident transitions is compared to data of spontaneous fission. The result may indicate the persistence in the fission products (prior to gamma-ray decay) of residual angular momentum from the compound nucleus at neutron energies above 50 MeV.

Fission neutron spectra measurements at LANSCE—status and plans

A program to measure fission neutron spectra from neutron‐induced fission of actinides is underway at the Los Alamos Neutron Science Center (LANSCE) in a collaboration among the CEA laboratory at Bruyeres‐le‐Châtel, Lawrence Livermore National Laboratory and Los Alamos National Laboratory. The spallation source of fast neutrons at LANSCE is used to provide incident neutron energies from less than 1 MeV to 100 MeV or higher. The fission events take place in a gas‐ionization fission chamber, and the time of flight from the neutron source to that chamber gives the energy of the incident neutron. Outgoing neutrons are detected by an array of organic liquid scintillator neutron detectors, and their energies are deduced from the time of flight from the fission chamber to the neutron detector. Measurements have been made of the fission neutrons from fission of 235U, 238U, 237Np and 239Pu. The range of outgoing energies measured so far is from 0.7 MeV to approximately 8 MeV. These partial spectra and average fiss...