We present the first study of the β decay of 23 Al undertaken with pure samples. The study was motivated by nuclear astrophysics questions. Pure samples of 23 Al were obtained from the momentum achromat recoil separator (MARS) of Texas A&M University, collected on a fast tape-transport system, and moved to a shielded location where β and β-γ coincidence measurements were made. We deduced β branching ratios and log ft values for transitions to states in 23 Mg, and from them determined unambiguously the spin and parity of the 23 Al ground state to be J π = 5/2 + . We discuss how this excludes the large increases in the radiative proton capture cross section for the reaction 22 Mg(p, γ ) 23 Al at astrophysical energies, which were implied by claims that the spin and parity is J π = 1/2 + . The log ft for the Fermi transition to its isobaric analog state (IAS) in 23 Mg is also determined for the first time. This IAS and a state 16 keV below it are observed, well separated in the same experiment for the first time. We can now solve a number of inconsistencies in the literature, exclude strong isospin mixing claimed before, and obtain a new determination of the resonance strength. Na(p, γ ) 23 Mg have both been suggested as possible candidates for diverting some of the flux in oxygen-neon novae explosions from the A = 22 into the A = 23 mass chain.

Obtaining reliable data for nuclear reactions on unstable isotopes remains an extremely important task and a formidable challenge. Neutron capture cross sections-crucial ingredients for models of astrophysical processes, national security applications, and simulations of nuclear energy generation-are particularly elusive, as both projectile and target in the reaction are unstable. We demonstrate a new method for determining cross sections for neutron capture on unstable isotopes, using ^{87}Y(n,γ) as a prototype. To validate the method, a benchmark experiment is carried out to obtain the known ^{90}Zr(n,γ) cross section analogously. Our approach, which employs an indirect ("surrogate") measurement combined with theory, can be generalized to a larger class of nuclear reactions. It can be used both with traditional stable-beam experiments and in inverse kinematics at rare-isotope facilities.

We report the first branching-ratio measurement of the superallowed 0+→0+β transition from Ca38. The result, 0.7728(16), leads to an ft value of 3062.3(68) s with a relative precision of ±0.2%. This makes possible a high-precision comparison of the ft values for the mirror superallowed transitions, Ca38→38mK and K38m→Ar38, which sensitively tests the isospin symmetry-breaking corrections required to extract Vud, the up-down quark-mixing element of the Cabibbo-Kobayashi-Maskawa (CKM) matrix, from superallowed β decay. The result supports the corrections currently used and points the way to even tighter constraints on CKM unitarity.

Background: Models to calculate small isospin-symmetry-breaking effects in superallowed Fermi decays have been placed under scrutiny in recent years. A stringent test of these models is to measure transitions for which the correction is predicted to be large. The decay of 32 Cl decay provides such a test case.

The effect of the production mechanism on the decay of a compound nucleus is investigated. The nucleus 90 Zr was produced by three different reactions, namely 90 Zr(p, p ) 90 Zr, 91 Zr(p, d) 90 Zr, and 92 Zr(p, t) 90 Zr, which served as surrogate reactions for 89 Zr(n, γ). The spin-parity (J π ) distributions of the states populated by these reactions were studied to investigate the surrogate reaction approach, which aims at indirectly determining cross sections for compound-nuclear reactions involving unstable targets such as 89 Zr. Discrete γ-rays, associated with transitions in 90 Zr and 89 Zr, were measured in coincidence with light ions for scattering angles of 25-60 • and 90 Zr excitation energies extending above the neutron separation energy. The measured transition systematics were used to gain insights into the J π distributions of 90 Zr. The 90 Zr(p, p ) reaction was found to produce fewer γ-rays associated with transitions involving high spin states (J = 6-8 ) than the other two reactions, suggesting that inelastic scattering preferentially populates states in 90 Zr that have lower spins than those populated in the transfer reactions investigated. The γ-ray production was also observed to vary by factors of 2-3 with the angle at which the outgoing particle was detected. These findings are relevant to the application of the surrogate reaction approach.

We measured the half-life of the superallowed 0 + → 0 + β + emitter 26 Si to be 2245.3(7) ms. We used pure sources of 26 Si and employed a high-efficiency gas counter, which was sensitive to positrons from both this nuclide and its daughter 26 Al m . The data were analyzed as a linked parent-daughter decay. To contribute meaningfully to any test of the unitarity of the Cabibbo-Kobayashi-Maskawa (CKM) matrix, the f t value of a superallowed transition must be determined to a precision of 0.1% or better. With a precision of 0.03%, the present result is more than sufficient to be compatible with that requirement. Only the branching ratio now remains to be measured precisely before a ±0.1% f t value can be obtained for the superallowed transition from 26 Si.

No abstract

The half-life of 37 K has been measured to be 1.23651(94) s, a value nearly an order of magnitude more precise than the best previously reported. The β + decay of 37 K occurs mainly via a superallowed branch to the ground-state of its T = 1/2 mirror, 37 Ar. This transition has been used recently, together with similar transitions from four other nuclei, as an alternative test of CVC and method for determining V ud , but the precision of its f t value was limited by the relatively large half-life uncertainty. Our result corrects that situation. Another motivation for improving the f t value was to determine the standard-model prediction for the β-decay correlation parameters, which will be compared to those currently being measured by the Trinat collaboration at Triumf. The new f t value, 4605(8) s, is now limited in precision by the 97.99(14)% ground-state branching ratio.