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Register a new userExclusive decay study of the 16.62,MeV (2$^-$, T=1) resonance in $^{12}$C
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The 3$alpha$ decay of the 16.62,MeV (2$^-$, T=1) resonance in $^{12}$C has been studied for nearly a century starting with one of the first nuclear reaction studies at the Cavendish Laboratory in the 1930s. In the hitherto latest study published a decade ago a model based on earlier work from the 1960s was found to give a good account of a set of inclusive data. This model describes the decay as an l=3 $alpha$-particle populating the 2$^+$ state of $^8$Be. Here we provide new exclusive data on the 3$alpha$ decay of the 16.62,MeV resonance, and demonstrate that the decay is best described by a model with predominantly l=1 emission with an admixture of l=3.
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The reaction $^{11}textrm{B}+p$ has been used to populate the $(J^pi,T) = (2^+,1)$ state at an excitation energy of 16.11 MeV in $^{12}$C. $gamma$-decay to unbound states in $^{12}$C are identified from analysis of the decay of the populated daughter states. Due to a new technique, $gamma$-decay to the 10.8 MeV 1$^-$ state is observed for the first time, and transitions to the 9.64 MeV (3$^-$) and 12.71 MeV (1$^+$) are confirmed. Unresolved transitions to natural parity strength at 10 MeV and 11.5-13 MeV are also observed. For all transitions partial widths are deduced
Multiple alpha coincidence and correlations are studied in the reaction $^{12}$C+$^{12}$C at 95 MeV for fusion-evaporation events completely detected in charge. Two specific channels with Carbon and Oxygen residues in coincidence with $alpha$-particles are addressed, which are associated with anomalously high branching ratios with respect the predictions by Hauser-Feshbach calculations. Triple alpha emission appears kinematically compatible with a sequential emission from a highly excited Mg. The phase space distribution of $alpha$-$alpha$ coincidences suggests a correlated emission from a Mg compound, leaving an Oxygen residue excited above the threshold for neutron decay. These observations indicate a preferential $alpha$ emission of $^{24}$Mg at excitation energies well above the threshold for $6-alpha$ decay.
The reaction $^{11}B+p$ has been used to populate the $(J^{pi},T)=(2^+,1)$ state at an excitation energy of 16.11 MeV in $^{12}$C, and the breakup of the state into three $alpha$ particles has been studied in complete kinematics. A two-step breakup model which includes interference effects is found to provide the most accurate description of the experimental data. The branching ratio to the ground state of $^8$Be is determined to be 5.1(5)% in agreement with previous findings, but more precise by a factor of two, while the decay to the first-excited state in $^8$Be is found to be dominated by $d$-wave emission.
The neutron yield in $^{12}$C(d,n)$^{13}$N and the proton yield in $^{12}C(d,p)^{13}$C have been measured by deuteron beam from 0.6 MeV to 3 MeV which is delivered from a 4-MeV electro static accelerator bombarding on the thick carbon target. The neutrons are detected at $0degree$, $24degree$, $48degree$ and the protons at $135degree$ in the lab frame. The ratios of the neutron yield to the proton one have been calculated and can be used as an effective probe to pin down the resonances. The resonances are found at 1.4 MeV, 1.7 MeV, 2.5 MeV in $^{12}C(d,p)^{13}$C and at 1.6 MeV, 2.7 MeV in $^{12}$C(d,n)$^{13}$N. This method provides a way to reduce the systematic uncertainty and helps to confirm more resonances in compound nuclei.
Passive detection of special nuclear material (SNM) is challenging due to its inherently low rate of spontaneous emission of penetrating radiation, the relative ease of shielding, and the fluctuating and frequently overwhelming background. Active interrogation (AI), the use of external radiation to increase the emission rate of characteristic radiation from SNM, has long been considered to be a promising method to overcome those challenges. Current AI systems that incorporate radiography tend to use bremsstrahlung beams, which can deliver high radiation doses. Low-energy ion-driven nuclear reactions that produce multiple monoenergetic photons may be used as an alternative. The $^{12}$C(p,p)$^{12}$C is one such reaction that could produce large gamma-ray yields of highly penetrating 4.4- and 15.1-MeV gamma rays. This reaction does not directly produce neutrons below the $sim$19.7-MeV threshold, and the 15.1-MeV gamma-ray line is well matched to the photofission cross-section of $^{235}$U and $^{238}$U. We report the measurements of thick-target gamma-ray yields at 4.4 and 15.1 MeV from the $^{12}$C(p,p)$^{12}$C at proton energies of 19.5, 25, and 30 MeV. Measurements were made with two 3 EJ309 cylindrical liquid scintillation detectors and thermoluminescent dosimeters placed at 0 and 90 degrees. We estimate the highest yields of the 4.4- and 15.1-MeV gamma rays of 1.65$times10^{10}$ sr$^{-1}mu$ C$^{-1}$ and 4.47$times10^8$ sr$^{-1}mu$ C$^{-1}$ at a proton energy of 30 MeV, respectively. The yield of 4.4 and 15.1 MeV gamma rays in all experimental configurations is greater than a comparable deuteron-driven reaction that produces the same gamma-ray energies- $^{11}$B(d,n$gamma$)$^{12}$C. However, a two orders of magnitude increase of the neutron radiation dose is observed when the proton energy increases from 19.5 to 30 MeV.
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