Session B8 - Nuclear Astrophysics and Neutrinos.
ORAL session, Saturday morning, April 28
Room Congressional B, Renaissance Hotel$

[B8.001] The ratio of S-factors for (p,\gamma) reactions on ^12C and ^13C at astrophysically relevant energies

We have measured the ratio of the (p,\gamma) reaction yields on ^12C and ^13C with low energy protons (E_p \leq 160 keV). At equilibrium in the CNO cycle, the abundances of ^12C and ^13C are inversely proportional to the (p,\gamma) reaction rates on these nuclei. The (p,\gamma) experiments were performed by stopping a 60 \muA proton beam in a frozen mixture of ^12C and ^13C benzenes. The \gamma-rays from both reactions were detected by a 128% HPGe detector. The preliminary experimentally determined ratio of S-factors for ^12C(p,\gamma)/^13C(p,\gamma) is 0.25 \pm 0.05 at an effective proton energy of 143 keV. A detailed description of the procedures used to determine this ratio and its error will be presented. The results will be compared to the ratio obtained from previous direct measurements of the two cross sections.

[B8.002] Cross section for the astrophysical ^14C(n,\gamma )^15C reaction via the inverse reaction

The chain of reactions in primordial nucleosynthesis in the neutron-rich environment of an asymmetrical big bang involves the reaction ^14C(n,\gamma )^15C (T. Kajino et al., ApJ. 364, 7 (1990)). An E1 transition cannot follow s-wave neutron capture because the ground state of ^15C has J^p = 1/2^+ and the only bound state has J^p = 5/2^+. While p-wave capture, which will have E1 transitions to both states, will be weak at thermal energies, it has been estimated to have a cross section of 0.1 mb at 30 keV, and another calculation (M. Wiesher et al., ApJ. 363, 340 (1990)) gave 4 \mub. With a Maxwellian neutron spectrum characterized by kT = 23.3 keV, a measurement of the cross section gave (1.72 \pm 0.43) \mub (H. Beer et at., ApJ. 387, 258 (1992)). We have used a beam of ^15C ions at E/A = 35 MeV to measure cross sections for 15C breakup on targets of C, Al, Zn, Sn and Pb. As a function of decay energy the Coulomb part of the breakup cross section was determined. By detailed balance the cross section for the ^14C(n,\gamma )^15C reaction was determined as function of neutron energy up to \sim 0.5 MeV.

[B8.003] Pair Bremstrahlung in Neutrino-Nucleus Collisions

Bremstrahlung of charged lepton-antilepton pairs by neutrinos in the electromagnetic field of nuclei is of potential importance for neutrino physics in high Z environments and representative of a class of processes connecting leptonic electroweak sectors to real or virtual photons. Standard Model cross sections and distributions for first generation flavor production by both electron and muon neutrinos will be presented and can be used to estimate backgrounds to neutrino experiments, including flux normalization. The energy and angular distributions will be discussed via analytic approximations to the matrix elements. Event signatures and possible roles in stellar and laboratory neutrino physics will be addressed.

[B8.004] Inclusive Neutrino and Antineutrino Reactions in ^56Fe

Cross sections for the inclusive neutrino and antineutrino reactions ,\nu_e + ^56Fe\longrightarrow e^- + X,\nu_\mu + ^56Fe\longrightarrow \mu^- + X,\bar\nu_e + ^56Fe \longrightarrow e^+ + X, and \bar\nu_\mu + ^56Fe \longrightarrow \mu^+ + X, are obtained from threshold to 300 MeV. For the electron neutrino reaction the Michel spectrum averaged cross section is calculated and for the muon neutrino reaction the cross section averaged over the LSND muon neutrino spectrum is given. A phenomenological treatment making use of total muon capture data is used in this calculation. The importance of these reactions both as background for existing experiments as well as for planned experiments on iron targets is discussed. The results are compared to those of previous calculations and to limited experimental results currently available.

[B8.005] Considerations on Using ORELA for RIB Production

The ORELA (Oak Ridge Electron Linear Accelerator) has been suggested as a high-intensity source of radioactive ions via 150 MeV bremmstrahlung-induced photo-fission on a Uranium target (W.T. Diamond, Nucl. Instruments and Methods A 432 (1999) 471.). We have studied the use of an aerosol-loaded He gas jet to transport fission products from an ORELA target to the Holifield Radioactive Ion Beam Facility. Yields measured at very low power levels have been consistent with those expected. The geometries of suitable target designs have been evaluated by using a Cf-252 source to simulate the photo-fission target, transporting the fission products by means of a He jet to a detector array for low-background counting using coincident CLARION Ge clover elements. The potential yields from realistic high-power (50kW) targets have been estimated.

[B8.006] CLEAN - A Technique for the Detection of Low Energy Solar Neutrinos

An idea for detecting low energy solar neutrinos is presented. In the CLEAN approach, liquid helium or liquid neon is used as a scintillation medium for the detection of neutrino-electron scattering. The extreme ultraviolet scintillation light is wavelength shifted for detection by photomultiplier tubes. Radioactive isotopes are removed from the scintillator through cryogenic distillation and by freezing out impurities onto cold surfaces. External sources of background are minimized through the use of a self-shielding detector.

[B8.007] The ^8Li Electron Calibration Source for the Sudbury Neutrino Observatory (SNO)

SNO has developed an electron source for calibrating its detector response to the Cerenkov radiation produced when ^8B neutrinos from the Sun interact within the SNO D_2O volume. The electron is from the beta-decay of ^8Li which has an end-point similar to that of the ^8B neutrinos and a 0.84 second half-life. The ^8Li is produced by the (n,\alpha) reaction on ^11B using 14-MeV neutrons from a commercial deuterium-tritium (DT) generator, located in a shielding pit approximately 70 meters from the center of the SNO detector. The ^8Li produced is transported via a small diameter capillary in a

helium gas/aerosol stream to a decay chamber that can be placed within the SNO detector water volumes. The ^8Li decay betas are tagged by detection of coincident alphas from the breakup up of the excited daughter nucleus, ^8Be*. A detailed description of the system will be given along with a presentation of initial ^8Li measurements in SNO.

[B8.008] Nanosecond Timing with Digital Electronics over Kilometer Distances.

Large volume (km3) neutrino detectors require nanosecond timing over long (km) distances. A fully digital system, in which photomultiplier waveforms are digitized in the remote sensing unit (Digital Optical Module), offers many advantages provided the timing precision can be achieved. The method we have devised is termed Reciprocal Active Pulsing and works as follows: A sharp time signal is generated at one end of the long cable connecting the DOM (at depth) with the surface electronics. The attenuated and dispersed signal received at the DOM is digitized. Following a delay measured by a local clock, an identical sharp signal is generated in the DOM and sent back to surface where it is digitized. Provided the signals are generated, received, and their digitized waveforms processed identically at each end, the timing delay (effective length of the long cable) is exactly 1/2 the round trip time (measured by a master clock at surface) minus 1/2 the known delay in the DOM. The period of the DOM clock is measured by repeating this process at known time intervals. Results from laboratory measurements and from field tests of a string of 41 DOMs at the South Pole will be presented.

Supported by the NSF and by U.S. Department of Energy under Contract No. DE-AC03-76SF00098.

[B8.009] Background Underground at WIPP

Recent interest to establish a dedicated underground laboratory in the United States prompted an experimental program at to quantify the enviromental backgrounds underground at the Waste Isolation Pilot Plant (WIPP) in Carlsbad, New Mexico. An outline of this program is provided along with recent experimental data on the cosmic ray muon flux at the 650 meter level of WIPP. The implications of the cosmic ray muon and fast neutron background at WIPP will be discussed in the context of new generation, low background experiments envisioned in the future.

[B8.010] A Simulation of the Performance of the Fine-Grain Modular Neutrino Detector at ORLaND

A novel design of a fine-grain modular detector (FGMD) and its performance using simulations by GEANT 3.21 are described. The 50 tons detector could be housed in the Oak Ridge Laboratory for Neutrino Detectors (ORLaND) adjacent to the Spallation Neutron Source. The detector is composed of alternating planes of plastic scintillator and X-Y proportional drift tubes (PDT). Each scintillator module and PDT has dimensions of 2\times2\times400 cm^3 and are stacked up to a height of 4 m. There are eleven uniformly spaced target inserts for the study of neutrino-nucleus interactions for nuclei of interest. Our simulations (1) of the Michel spectrum, (2) of efficiencies for the trackable electrons, muons, protons, and neutrons, (3) for the angular resolution of direction, and (4) for the fitting of the tracks show that the FGMD is an ideal detector to perform many different neutrino measurements at ORLaND.

Part B of program listing