Background: Classical novae are cataclysmic nuclear explosions occurring when a white dwarf in a binary system accretes hydrogen-rich material from its companion star. Novae are partially responsible for the galactic synthesis of a variety of nuclides up to the calcium (A ∼ 40) region of the nuclear chart. Although the structure and dynamics of novae are thought to be relatively well understood, the predicted abundances of elements near the nucleosynthesis endpoint, in particular Ar and Ca, appear to sometimes be in disagreement with astronomical observations of the spectra of nova ejecta. Purpose: One possible source of the discrepancies between model predictions and astronomical observations is nuclear reaction data. Most reaction rates near the nova endpoint are estimated only from statistical model calculations, which carry large uncertainties. For certain key reactions, these rate uncertainties translate into large uncertainties in nucleosynthesis predictions. In particular, the 38 K(p,γ ) 39 Ca reaction has been identified as having a significant influence on Ar, K, and Ca production. In order to constrain the rate of this reaction, we have performed a direct measurement of the strengths of three candidate = 0 resonances within the Gamow window for nova burning, at 386 ± 10 keV, 515 ± 10 keV, and 689 ± 10 keV. Method: The experiment was performed in inverse kinematics using a beam of unstable 38 K impinged on a windowless hydrogen gas target. The 39 Ca recoils and prompt γ rays from 38 K(p,γ ) 39 Ca reactions were detected in coincidence using a recoil mass separator and a bismuth-germanate scintillator array, respectively. Results: For the 689 keV resonance, we observed a clear recoil-γ coincidence signal and extracted resonance strength and energy values of 120 +50 −30 (stat.) +20 −60 (sys.) meV and 679 +2 −1 (stat.)±1(sys.) keV, respectively. We also performed a singles analysis of the recoil data alone, extracting a resonance strength of 120 ± 20(stat.)±15(sys.) meV, consistent with the coincidence result. For the 386 keV and 515 keV resonances, we extract 90% confidence level upper limits of 2.54 meV and 18.4 meV, respectively. Conclusions: We have established a new recommended 38 K(p,γ ) 39 Ca rate based on experimental information, which reduces overall uncertainties near the peak temperatures of nova burning by a factor of ∼250. Using the rate obtained in this work in model calculations of the hottest oxygen-neon novae reduces overall uncertainties on Ar, K, and Ca synthesis to factors of 15 or less in all cases.

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Angular distributions and spectroscopic strengths from the 2' Th(d, t)22 Th reaction provide new independent support for the rotational band assignments which led to the proposal of an excited state at~5 eV in ' Th.From a careful study of the y transitions accompanying the a decay of 'U, Kroger and Reich' reported that the ground and first excited states of 2 Th are separated bỹ 0.1 keV. They classified these, respectively, as the [633] and -, ' [631] Nilsson states. Very recently Reich and Helmer, 2 using state-of-the-art germanium detectors and more accurately known calibration energies, have reported a more stringent limit of -1~4 eV for the energy difference.A long-lived nucleus with such a closely spaced pair of levels affords a special situation where excitation may be achieved by "non-nuclear" methods; for example, optical or thermal processes. Accordingly, it is timely to report additional evidence supporting the existence and location of the -' , [633] and -' , [631] rotational bands in 2 Th.In connection with investigations of nuclear octupole deformation, we have recently been studying the levels of both "9Th and 23'Th by using (d, t) particle spectroscopy. The only known previous (d, t) study of 29Th has not been published and is cited in the Nuclear Data Sheets only as a private communication.It was performed with 12 MeV deuterons and spectra were recorded at only two angles. To allow reliable assignments of I-transfer values we have measured triton spectra at 17 angles between 5' and SO' with a deuteron beam energy of 17 MeV. This Rapid Communication reports our evidence for rotational bands based on the -, ' [633] and -' , [631] levels in Th.As with the y-ray studies, our technique does not permit the resolution of the two bandheads; however, the new data provide additional strong support for two very closely spaced levels in 9Th.The Th target was prepared by vacuum evaporation of natural thorium metal on a 20-iug/cm carbon foil. TheTh target (Ttg2-75000 years, isotopic purity )99. 9/o) was prepared by direct deposition from an isotope separator onto a thicker carbon foil and is the same target that was used in an earlier study of the Th(d, p) reaction. The thorium thicknesses, determined from elastic scattering count rates in a silicon surface-barrier monitor counter at 8=30 during the experiments, were -30 and -40 pg/cm for Th and Th, respectively.Beams of 17 MeV deuterons from the McMaster University Tandem Accelerator were used; the reaction products were analyzed with the Enge split-pole magnetic spectrograph and detected with photographic plates. Figure 1 shows the low-energy portions of the 2' Th(d, t) Th and Th(d, t) 3'Th spectra. The resolution for the~3 tTh spectra was typically 5.5-6.0 keV full width at half maximum (FWHM) while for the 2 9Th spectra it was -7 keV FWHM in the best cases. The larger peak widths for the 229Th data are believed due to the greater thickness of carbon supporting the radioactive 23 Th target. The Nilsson interpretations shown in Fig. 1