R.-M. Larimer

Influence of physical and chemical environments on the decay rates of 7Be and 40K

We measured the electron-capture decay rate of 7Be implanted into hosts of graphite, boron nitride, tantalum, and gold. We have found that this decay rate varies by as much as 0.38% from one host to another. We also measured the electron-capture decay rate of 40K in four different chemical compounds and as both a solid and dissolved in solution. To within our measurement precision of ±1%, we have found no influence of the environment on the 40K decay rate. The implications of these results for the solar neutrino problem and for potassium/argon dating are discussed.

Search for Strange Matter by Heavy Ion Activation

We present the results of an experimental search for stable strange matter using the heavy ion technique, based on properties of strangelets. We studied samples of a meteorite, terrestrial nickel ore, and lunar soil. Our search improved by 2 to 3 orders of magnitude the existing experimental limit on the strange matter content in normal matter and enabled probing the flux of low mass strangelets on the lunar surface for the first time.p {copyright} {ital 1998} {ital The American Physical Society}

An improved Test of the Exponential Decay Law

Abstract We have compared the decay rate of freshly prepared 40K to that of 40K that is >4.5 × 109 years old. We find that to within the experimental uncertainly of ± 11%, the decay rates are the same. This indicates that the exponential nature of the decay law is valid down to time-scales of approximately 10 −10 t 1 2 .

Polarization in proton-deuteron scattering at 50 MeV☆

Abstract The analyzing power for 50 MeV polarized proton scattering from deuterons has been measured for center-of-mass angles 10 to 160 degrees. Relative uncertainties are generally less than 0.01.

N = 90 region: The decays of Eu 152 m , g to Sm 152

The decays of {152m,g}Eu to 152Sm have been studied by gamma-ray spectroscopy using the 8pi Spectrometer, an array of 20 Compton-suppressed Ge detectors. Very weak gamma-decay branches in 152Sm were investigated through gamma-gamma coincidence spectroscopy. All possible E2 transitions between states below 1550 keV with transition energies > 130 keV are observed, including the previously unobserved 2^+_3 to 0^+_2 401 keV transition. The results, combined with existing lifetime data, provide a number of new or revised E2 transition strengths which are critical for clarifying the collective structure of 152Sm and the N=90 isotones.

Measurement of 107Ag(α,γ)111In Cross Sections

Cross sections have been measured for the 107Ag(α,γ)111In reaction at several α‐particle energies between 7.8 MeV and 11.9 MeV. This reaction is of interest because it can provide a check on calculations of low‐energy (α,γ) cross sections required for stellar nucleosynthesis predictions. Stacks of natural Ag foils of 1 μm thickness and 99.97% purity were bombarded with 4He+ beams. Following irradiation, the yields of the 171‐keV and 245‐keV photons produced in the 2.805‐day electron‐capture decay of the 111In product nucleus were measured off‐line. The Ag foils were interleaved with 99.6% purity, 6‐μm‐thick natural Ti foils so that known cross sections for the 48Ti(α,n) reaction could be used to check the accuracy of the beam current integration. For any given beam energy, beam energy degradation in the foils resulted in lower effective bombarding energies for successive foils in the stack, enabling measurements to be made for several energies per irradiation. The measured cross sections are compared with...

Production of cosmogenic nuclides in thick targets by alpha bombardment. Part I — short-lived radioisotopes

Abstract Production of short-lived cosmogenic nuclides in planetary surfaces and remote spacecraft detectors was simulated by bombarding “thick” C, Mg, Al, Si, SiO2, Fe, Ni, and Ge targets with 60, 90 and 120 MeV alpha particles. Gold foils were used to monitor alpha particle fluence; product nuclides were measured by gamma ray spectroscopy. The results were used to calculate production yields for each alpha energy, as well as cross sections averaged over the energy ranges 60–90 and 90–120 MeV.

Evidence for the emission of a massive neutrino in nuclear beta decay

The authors have studied the beta -spectrum of 14C using a germanium detector containing a crystal with 14C dissolved in it. They find a feature in the beta -spectrum 17 keV below the endpoint which can be explained by the hypothesis that there is a heavy neutrino emitted in the beta -decay of 14C with a mass of 17+or-2 keV and an emission probability of 1.40+or-0.45%. In addition, they have studied the inner bremsstrahlung spectrum of 55Fe and also find indications of the emission of a approximately 17 keV neutrino.

BEARS: A radioactive ion beam initiative at LBNL

BEARS is an initiative to develop a radioactive ion-beam capability at Lawrence Berkeley National Laboratory. The aim is to produce isotopes at an existing medical cyclotron and to accelerate them at the 88″ Cyclotron. To overcome the 300-meter physical separation of these two accelerators, a carrier-gas transport system will be used. At the terminus of the capillary, the carrier gas will be separated and the isotopes will be injected into the 88″ Cyclotron’s Advanced Electron Cyclotron Resonance ion source. The first radioactive beams to be developed will include 20-min 11C and 70-sec 14O, produced by (p, n) and (p, α) reactions on low-Z targets. Tests at the 88″ Cyclotron lead to projections of initial 11C beams of 2×108 ions/sec 14O beams of 1×106 ions/sec. Construction of BEARS is expected to be completed in the spring of 1999.

The evidence for 17‐keV neutrinos

We have studied the β‐spectrum of 14C using a germanium detector containing a crystal with 14C dissolved in it. We find a feature in the β‐spectrum 17 keV below the endpoint which can be explained by the hypothesis that there is a heavy neutrino emitted in the β‐decay of 14C with a mass of 17.1±0.5 keV and an emission probability of 1.3±0.3%. These results are consistent with observations of similar anomalies in the β‐decays of 3H and 35S, and the internal bremsstrahlung decays of 55Fe and 71Ge.