Sean W. Finch

In support of Los Alamos National Laboratory’s double shell program and the radiochemical diagnostic of mix, a study of the coincidence γ rays was carried out at the tandem accelerator of the Triangle Universities Nuclear Laboratory. A tungsten foil was irradiated with 13 MeV deuterium beams, and coincidence γ rays with multiplicity up to five were observed with a dual high-purity germanium detector system. The level schemes of 182W and 184W were constructed from the identified coincidence γ rays. Many of these γ rays were confirmed by coincidence counting of two solid radiochemical collection diagnostic foils from a recent tungsten-capsule double shell shot N260124 at the National Ignition Facility. Measuring the decay curve of the tungsten characteristic γ rays revealed a new model-independent method for estimating the effect of mix.

The dipole response of ^{102}Ru has been investigated for the first time using nuclear resonance fluorescence, providing comprehensive and detailed information about the nucleus. The M1 strength, concentrated in the 3-4 MeV region, is consistent with a scissors resonance. As a surprising new feature, a pronounced E1 resonancelike structure at 4.5-5.5 MeV has been found. It is interpreted as an octupole resonance and provides evidence for low-spin octupole correlations in this excitation energy regime. The experimental results are compared with quasiparticle random phase approximation calculations, which reproduce the gross features of the cumulative strength. These observations support the coexistence of M1 scissors and octupole-induced E1 resonances as a general feature of deformed nuclei with octupole softness.

Magnetic dipole, M1, strengths were studied in semimagic ^{50}Ti up to the neutron-separation threshold by combining data from (d,p) one-neutron transfer, (γ,γ^{'}) real-photon scattering, (e,e^{'}) inelastic scattering at extreme backward angles, and (p,p^{'}) at E_{p}=210 MeV and extreme forward angles. The combination of all probes provided unique access to the neutron spin-flip contribution and the possibility to evaluate its role in generating the spin-flip M1 strengths. The small contribution of the neutron (1f_{7/2})^{-1}(1f_{5/2})^{+1} spin-flip transitions, which were probed with the (d,p) reaction, to the overall strength in ^{50}Ti questions the standard picture for the microscopic origin of spin-flip strength in the fp shell. For ^{50}Ti, this Letter shows that J^{π}=1^{+} states with larger neutron (1f_{7/2})^{-1}(1f_{5/2})^{+1} spectroscopic factors do not correspond to the ones with the largest B(M1;0_{1}^{+}→1_{i}^{+}) strengths.

The carbon oxygen ratio (C/O) at the end of stellar helium burning is a crucial nuclear input to stellar evolution theory. Knowledge of the C/O ratio with sufficient accuracy has eluded measurement over the past five decades. It is determined by the rate of oxygen formation in the fusion of helium with ¹² C, denoted as ¹² C( α , γ ) ¹⁶ O. Even though recent methods employing a time projection chamber can measure the time-reverse photo-dissociation reaction, the results still do not show unambiguous agreement with the predictions of quantum scattering theory. Here, we improve this method using a N ₂ O gas target. This improvement allows us to eliminate the background caused by ¹² C photo-dissociation events, obtain complete angular distributions (0 ^∘ −180 ^∘ ), and measure the cross sections over the 1 ⁻ resonance in ¹⁶ O at E _cm ~ 2.4 MeV. These measurements resolve the discrepancy that was previously observed between the measured E 1− E 2 mixing phase angle ( ϕ ₁₂ ) of ¹² C( α , γ ) ¹⁶ O and the predictions of quantum scattering theory. This newfound agreement demonstrates the viability of our method for conducting measurements at lower energies.

Nuclear activation is a well-established technique for inferring neutron yields in laser direct-drive deuterium–tritium (DT) and deuterium–deuterium (D2) implosions at the OMEGA Laser Facility. Zirconium has long been considered an excellent candidate for measuring DT neutron fusion yields by observing decays of the 90Zr(n,2n)89Zr ground-state reaction. As it has a higher energy threshold than present activation detectors utilizing copper, zirconium provides a means to infer primary neutron yields that are less susceptible to being skewed due to neutron scattering within the experimental environment. However, with a 78.41-h half-life, it is not operationally practical to utilize this reaction for OMEGA experiments, which have a 1-h shot cycle. Zirconium’s 90Zr(n,2n)89mZr reaction presents itself as a viable candidate to infer neutron yields within a shot cycle, given its half-life of 4.16 min. We present an overview of the approach and methodology, utilizing first principles techniques, to infer the primary neutron yields from OMEGA DT fusion experiments by using both the isomeric and the ground-state reaction. Yields inferred from both reactions are compared, which are in good agreement between the two.

The carbon oxygen ratio (C/O) at the end of stellar helium burning is a crucial nuclear input to stellar evolution theory. It is determined by the rate of oxygen formation in the fusion of helium with ¹² C, denoted as ¹² C(α,γ) ¹⁶ O. Knowledge of the C/O ratio with sufficient accuracy has eluded measurements over the past 51 years. We previously demonstrated the feasibility of a new method using gamma beams to measure the time-reverse photo-dissociation reaction, ¹⁶ O(γ,α) ¹² C, inside a Time Projection Chamber detector. Here we report the first significant ``Physics Result'' obtained using our new method. We used an N ₂ O gas target that, unlike our previous measurement, allowed us to eliminate ¹² C, photo-dissociation events - a significant background that can lead to systematic errors. In this current, essentially background-free measurement, we obtained detailed complete angular distributions (0°-180°) with higher statistics. Cross sections were measured over the 1 ^- resonance in ¹⁶ O at E ^cm ≈2.4 MeV, where previous measurements of ¹² C(α,γ) ¹⁶ O exhibit a disturbing disagreement of the measured E1-E2 mixing phase angle (φ ¹² ) with an elementary prediction of quantum mechanics. The agreement presented in this work demonstrates the viability of our new method and provides a strong impetus for conducting measurements at lower energies.