Neutron-upscattering enhancement of the triple-alpha process

The neutron inelastic scattering of carbon-12, populating the Hoyle state, is a reaction of interest for the triple-alpha process. The inverse process (neutron upscattering) can enhance the Hoyle state’s decay rate to the bound states of ¹²C, effectively increasing the overall triple-alpha reaction rate. The cross section of this reaction is impossible to measure experimentally but has been determined here at astrophysically-relevant energies using detailed balance. Using a highly-collimated monoenergetic beam, here we measure neutrons incident on the Texas Active Target Time Projection Chamber (TexAT TPC) filled with CO₂ gas, we measure the 3α-particles (arising from the decay of the Hoyle state following inelastic scattering) and a cross section is extracted. Here we show the neutron-upscattering enhancement is observed to be much smaller than previously expected. The importance of the neutron-upscattering enhancement may therefore not be significant aside from in very particular astrophysical sites (e.g. neutron star mergers).

The triple-alpha process plays a role in nucleosynthesis, in the formation of ¹²C. Here, the authors discuss the rate and role of the neutron upscattering phenomenon on the triple-alpha process in a multi-step process.

Strong Neutron Pairing in core+4n Nuclei

The emission of neutron pairs from the neutron-rich N=12 isotones ^{18}C and ^{20}O has been studied by high-energy nucleon knockout from ^{19}N and ^{21}O secondary beams, populating unbound states of the two isotones up to 15 MeV above their two-neutron emission thresholds. The analysis of triple fragment-n-n correlations shows that the decay ^{19}N(-1p)^{18}C^{*}→^{16}C+n+n is clearly dominated by direct pair emission. The two-neutron correlation strength, the largest ever observed, suggests the predominance of a ^{14}C core surrounded by four valence neutrons arranged in strongly correlated pairs. On the other hand, a significant competition of a sequential branch is found in the decay ^{21}O(-1n)^{20}O^{*}→^{18}O+n+n, attributed to its formation through the knockout of a deeply bound neutron that breaks the ^{16}O core and reduces the number of pairs.

Quasifree (p, 2p) Reactions on Oxygen Isotopes: Observation of Isospin Independence of the Reduced Single-Particle Strength

Quasifree one-proton knockout reactions have been employed in inverse kinematics for a systematic study of the structure of stable and exotic oxygen isotopes at the R^{3}B/LAND setup with incident beam energies in the range of 300-450  MeV/u. The oxygen isotopic chain offers a large variation of separation energies that allows for a quantitative understanding of single-particle strength with changing isospin asymmetry. Quasifree knockout reactions provide a complementary approach to intermediate-energy one-nucleon removal reactions. Inclusive cross sections for quasifree knockout reactions of the type ^{A}O(p,2p)^{A-1}N have been determined and compared to calculations based on the eikonal reaction theory. The reduction factors for the single-particle strength with respect to the independent-particle model were obtained and compared to state-of-the-art ab initio predictions. The results do not show any significant dependence on proton-neutron asymmetry.

New Measurement of the Direct 3α Decay from the ^{12}C Hoyle State

Excited states in certain atomic nuclei possess an unusual structure, where the dominant degrees of freedom are those of α clusters rather than individual nucleons. It has been proposed that the diffuse 3α system of the ^{12}C Hoyle state may behave like a Bose-Einstein condensate, where the α clusters maintain their bosonic identities. By measuring the decay of the Hoyle state into three α particles, we obtained an upper limit for the rare direct 3α decay branch of 0.047%. This value is now at a level comparable with theoretical predictions and could be a sensitive probe of the structure of this state.

Evidence for triangular D3h symmetry in 12C

We report a measurement of a new high spin Jπ=5- state at 22.4(2) MeV in 12C which fits very well to the predicted (ground state) rotational band of an oblate equilateral triangular spinning top with a D3h symmetry characterized by the sequence 0+, 2+, 3-, 4±, 5- with almost degenerate 4+ and 4- (parity doublet) states. Such a D3h symmetry was observed in triatomic molecules, and it is observed here for the first time in nuclear physics. We discuss a classification of other rotation-vibration bands in 12C such as the (0+) Hoyle band and the (1-) bending mode band and suggest measurements in search of the predicted ("missing") states that may shed new light on clustering in 12C and light nuclei. In particular, the observation (or nonobservation) of the predicted ("missing") states in the Hoyle band will allow us to conclude the geometrical arrangement of the three alpha particles composing the Hoyle state at 7.654 MeV in 12C.

Prevention of HPV-related oral cancer: assessing dentists' readiness

OBJECTIVES

Epidemiological research indicates an association between the Human Papillomavirus (HPV) with a subset of oral cancers (OC). Dentists may play a role in primary prevention of HPV-related OC by discussing the HPV vaccine with patients. This study assessed dentists' readiness to discuss the HPV vaccine with female patients.

STUDY DESIGN

Cross-sectional web-based survey.

METHODS

A web-based survey based on the Transtheoretical Model was administered among Florida dentists (n = 210).

RESULTS

The majority of participants (97%) fell into the precontemplation and contemplation stages of readiness to discuss the HPV vaccine with patients. Perceived role and liability were determined to be predictive of dentists in contemplation stage as opposed to those in precontemplation (P < 0.05).

CONCLUSIONS

Findings suggest liability and perceived role as processes of change necessary to guide dentists to primary prevention of HPV-related OC despite high levels of knowledge. As public awareness of HPV-related OC increases, dentists may become more involved in primary prevention. Results of the current study may assist in developing intervention strategies for engaging dentists in discussing the HPV vaccine with patients.

Signatures for multi-alpha-condensed states

An experimental way of testing Bose-Einstein condensation of alpha clusters in the atomic nucleus is reported. The enhancement of cluster emission and the multiplicity partition of possible emitted clusters could be direct signatures for the condensed states. The barrier for the emission of clusters, such as (8)Be and (12)C(O(+)(2)), is calculated and compared with the barrier for the sequential emission of 2 or 3alpha particles from the compound nucleus. For the calculations, a simple approach using a folded Woods-Saxon potential is used.

Multiphonon vibrations at high angular momentum in 182 Os

Evidence is presented for multiphonon excitations based on a high-spin (25 Planck) intrinsic state in the deformed nucleus 182 Os. Angular momentum generation by this mode competes with collective rotation. The experimental data are compared with tilted-axis cranking calculations, supporting the vibrational interpretation. However, the lower experimental energies provide evidence that more complex interactions of states are playing a role.