Session B3: Nuclear Physics

Radiative Capture Rates from DRAGON: Nova and Supernova Diagnostics

The DRAGON recoil separator facility at TRIUMF measures radiative alpha and proton capture reactions of astrophysical importance in inverse kinematics. This is done using radioactive and stable ion beams produced and accelerated using the ISAC (Isotope Separator and ACcelerator) facility. Over the last few years, the DRAGON collaboration has embarked on a program to measure a variety of reactions considered vital to the understanding of various astrophysical scenarios. In particular, we have tried to focus partly on those reactions involved in the creation and destruction of important astrophysical gamma-emitters such as $^{22}$Na, $^{26}$Al and $^{44}$Ti. Such radionuclides are crucial to the understanding of novae and supernovae through their potential for detection with space-based gamma-ray observatories enabling the validation of stellar models. With particular emphasis on the recent $^{26g}$Al(p,$\gamma )^{27}$Si measurement, we will outline the methodology behind making such measurements with DRAGON, discuss the results obtained in recent experiments, and put forward the interpretations for the possible implications these results have on the understanding of certain astrophysical scenarios.   

Nuclear Physics with Lattice QCD

I will discuss recent progress in using lattice QCD to compute quantities of interest in nuclear physics.   

Unitarity of the CKM mass-mixing matrix as a test of the Standard Model: status and future prospects

The Cabibbo-Kobayashi-Maskawa (CKM) matrix parameterizes the rotation between the weak and mass eigenstates of the quark families. If the Standard Model of electroweak interactions is complete, then unitarity requires that the sum of the square of the top row of elements is unity, \emph{i.e.}\ $V_{ud}^2+V_{us} ^2+V_{ub}^2=1$. Currently, this test fails at the $2.4\,\sigma$ level, a provocative discrepancy which may be an indication of new physics not contained within the Standard Model framework. By far the largest element, $V_{ud}$, is known from measurements of the comparative half-lives, or $ft$-values, of the nine precisely measured $0^{+}\!\rightarrow\!0^+$ superallowed $\beta^+$ decays. The 2$^{\mathrm{nd}}$ largest, $V_{us}$, is measured from kaon decays. Recently, the values of both of these elements as adopted by the Particle Data Group have been called into question; it remains to be seen whether unitarity will be satisfied in the end or if the discrepancy is even more pronounced. Within the nuclear physics community, many groups around the world have research programs aimed at improving the measurement of $V_{ud}$. Some are extending the number of cases of precisely measured superallowed decays; others are testing the theoretical corrections needed to extract $V_{ud}$ from the $0^{+}\!\rightarrow\!0^+$ decays. The neutron represents another exciting opportunity to measure $V_{ud}$ because the theoretical corrections are simpler to calculate in this three-quark system. I will review the physics behind the CKM matrix and discuss some of the experiments in progress which will improve the precision of the unitarity test. As examples, I will discuss the UCNA experiment using ultra-cold neutrons and an experiment involving $^{\mathrm{32}}$Ar as a test of the theoretical corrections applied to the superallowed $\beta$ decays.   

Neutron Multiplicities in the Actinides

Accurate measurements of prompt, scission, and pre-equilibrium neutron multiplicities from the nuclear fission process are of great importance to nuclear technology. The variation of these quantities with mass division, the excitation energy of the fissioning system E*, and the kinetic energy release to the fragments sheds light on the partition of energy during the large scale collective motion of the scission process. While some systems have been characterized very well, many others remain unknown. Instead of using the (n,f) reaction, we are using the surrogate (d,pf) reaction. Measurements performed recently at the CENPA yielded information on the $^{237-239}$U, $^{236-239}$Np, and $^{240}$Pu fissioning systems. Analysis is in progress.   

Neutron Capture Cross Sections of Tellurium Isotopes

Neutron capture by the stable even-mass Te isotopes (A = 120 to 130) produces in the neighboring odd-neutron isotopes a low-spin ground state (1/2 or 3/2) and a high-spin (11/2) isomeric state. By irradiating samples of natural isotopic Te in our reactor, we have measured the neutron capture cross sections for all of the odd-mass radioactive ground states and isomers produced in the capture process. By using Cd-shielded and unshielded irradiations, we have been able to obtain both the effective thermal cross sections and the resonance integrals. Comparison with similar neutron capture processes in Sn isotopes leads to interesting systematic effects, especially among the thermal cross sections of the low-spin and high-spin states.   

Characterization of Solid State Ultracold Neutron Detectors

The reflective properties of ultracold neutrons (UCN) enable easy transport and bottling but make neutron detection a technical challenge. Typically, UCN are allowed to accelerate in the Earth's gravitational field to sufficient velocity to penetrate an aluminum entrance window of a $^{3}$He proportional counter. Here we describe the construction and characterization at the ILL of two kinds of prototype solid-state detectors which can be used to monitor the UCN density inside the UCNA spectrometer at LANL without gravitational acceleration, and perhaps more critically, without the danger of $^{3}$He leaks. The first type consists of 300 $\mu$g/cm$^2$ of LiF evaporated onto 200 nm thick Ni foils. The second type consists of $\sim$ 10$^{18}$ $^{10}$B ions implanted in a 200 nm thick V layer, also evaporated onto Ni foils. From monte carlo simulations, we find that LiF has a critical velocity nearly equal to that of aluminum, whereas the boron foils do indeed have a lower cutoff. Because of these cutoffs and the small size of the detectors, our solid-state detectors, thus, equal (for LiF) or outperform (for Boron) aluminum window proportional counters for {\it{in-situ}} density measurements.   

Polarized neutron $\beta$-decay: proton asymmetry and recoil-order currents

We present an analytic calculation of the proton asymmetry from polarized neutron $\beta$-decay, including recoil-order effects. The differential decay rate in terms of electron energy and proton direction follows, parametrized in terms of the most general Lorentz-invariant hadron current coupled to a left-handed lepton current. Implications for experimental efforts to measure recoil-order currents are discussed.   

$^{31}{\rm P}(p,\gamma)$ and the isobaric multiplet mass equation

We present results of a recent $^{31}{\rm P}(p,\gamma)$ experiment to test the isobaric multiplet mass equation (IMME). The energies of the de-excitation $\gamma$ rays were measured with high precision to obtain the excitation energy of lowest $T=2$ state of $^{32}{\rm S}$. Our result, together with a recent measurement of the $^{32}\rm Ar$ mass, makes the $A = 32$ multiplet the most precisely measured $T=2$ quintet provides the most stringent test of the isobaric multiplet mass equation.   

Will Lanthanum Halide Scintillators Make NaI(Tl) Obsolete?

The commercial availability of lanthanum halide scintillators (LaCl$_{3}$:Ce and LaBr$_{3}$:Ce) has been much anticipated due to their significantly better resolution (3-4{\%} at 662 keV) relative to NaI(Tl). Unfortunately, our initial investigation of these scintillators revealed significant alpha contamination quite apparent in background spectra. Using measurements of the detector in coincidence with a HPGe detector, we identified the alpha-contaminant as Ac-227. Since this time, the alpha contamination has been substantially reduced so that a second contaminant, La-138 (a beta, gamma, and x-ray source) has become the dominant contaminant in the material. Commercially-available sizes of lanthanum halide scintillators have now reached sizes suitable for handheld Radioactive Isotope Identification Devices (RIIDs). To study the potential of this new material for RIIDs we performed a series of measurements comparing a 1.5'' x 1.5'' LaBr$_{3}$ detector with an Exploranium GR-135 RIID, which contains a 1.5'' x 2.2'' NaI(Tl) detector. Measurements were taken for short timeframes of seconds to minutes, as typifies RIID usage. Measurements included examples of naturally occurring radioactive material (NORM) typically found in cargo. Of particular interest was the extent to which interference between the La-138 contaminant and K-40, a radioisotope commonly found in NORM, compromise the lanthanum halide performance. Example spectra, detector comparisons and results will be shown.   

Nucleon-Nucleon Interaction above 1 GeV

A summary of the work by Eyser, Machleidt, and Scobel [Eur. Phys. J. A 22, 105 (2004)] on nucleon-nucleon (NN) scattering above 1 GeV will be presented. Motivated by the recent measurements of proton-proton spin-correlation parameters up to 2.5 GeV laboratory energy at COSY (Juelich, Germany), Eyser et al. investigated models for NN scattering above 1 GeV. Signatures for a gradual failure of the traditional meson model with increasing energy were clearly identified. Since spin effects are large up to tens of GeV, perturbative QCD cannot be invoked to fix the problems. Various theoretical scenarios are discussed, however, the conclusion is that, at this time, we have neither a phenomenological nor a theoretical understanding of the spin dependence of the NN interaction above 1 GeV.   

The Nuclear Equation of State and Its Applications to Neutron Stars

One of the most challenging problems in both theoretical and experimental nuclear physics is to understand the nature of matter under extreme conditions of density and pressure. Observations of neutron star properties impose important constraints on the equation of state of dense matter, as the latter is the basic input quantity that enters the structure equations of these compact objects. Continuing with our systematic study of the effective nucleon-nucleon interactions in dense and isospin-asymmetric hadronic environment, we will present predictions of neutron star masses and radii obtained from our relativistic equation of state. We use realistic nucleon-nucleon potentials defined in the framework of the meson-exchange potential model and the Dirac-Brueckner approach. We will provide an overview of theoretical predictions and recent observational data. This broad outlook will help us gauge the quality of our tools and determine the importance of mechanisms beyond the present model.   

Nuclear Physics and Cancer Treatment: Recent Advances

Exactly 100 years ago, the ``Bragg Peak'' was discovered. The ionization caused by alpha particles in matter shows a characteristic peak at the end of the particles' trajectories. In 1946, the nuclear physicist Wilson suggested to use beams of heavy charged particles for the treatment of localized tumors. The Bragg peak can be focused on the tumor and, thus, little damage is done to the surrounding healthy tissue. In this way, the occurence of negative side effects from a radiation treatment is substantially reduced as compared to conventional radiation therapy which uses X-ray. Today, we have 40 years of experience in the treatment of cancer by beams of protons and heavy ions. A recently published 10-year follow-up study has proven that the cure rate of proton therapy is as good (or even better) as with `conventional' treatment protocolls but the rate of side effects is by up to an order of magnitude smaller. This makes proton (and heavy ion) therapy clearly suprior to other forms of radiation therapy. Unfortunately, this fact is little known even by some oncologists.   

Koide's Mass Formula for Neutrinos

We derive Koide's mass formula as an eigenvector equation. We show that to within current experimental error, the square roots of the masses of the charged leptons follow the simple equation $(m^-_n)^{0.5} = \mu_1(1 + \sqrt{2}\cos(\delta_1 + 2n\pi/3))$ where $\delta_1$ is the interesting number $.22222204717(48)$ and $\mu_1$ is a constant. Next we generalize the Koide formula to the neutrinos by assuming that the square root of the mass of the smallest neutrino must be taken to be negative. Then masses of the simple form $(m^0_n)^{0.5} = \mu_0(1 + \sqrt{2}\cos (\delta_1 + \pi/12 + 2n\pi/3))$ where $3\;\mu_0 = 3^{12} \;\mu_1,$ satisfy recent neutrino oscillation measurements close to the centers of the error bars. Finally, we discuss the preon model for the fermions that led to the above discovery.