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Register a new userGamma-rays from Muon Capture in $^{14}$N
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Many new $gamma$-rays have been observed, following muon capture on $^{14}$N. One had been reported before, and the low yield is confirmed, indicating that the nuclear structure of $^{14}$N is still not understood. Gamma-rays from $^{13}$C resulting from the reaction $^{14}$N($mu^{-}$,$ u$n)$^{13}$C compare favourably with states observed in the reaction $^{14}$N($gamma$,p)$^{13}$C. More precise energies are also given for the 7017 and 6730 keV $gamma$-rays in $^{14}$C.
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We report on a new measurement of $^{14}$N(p,$gamma$)$^{15}$O for the ground state capture transition at $E_p$ = 360, 380 and 400 keV, using the 400 kV LUNA accelerator. The true coincidence summing effect --the major source of error in the ground state capture determination-- has been significantly reduced by using a Clover--type gamma detector.
We survey a new generation of precision muon lifetime experiments. The goal of the MuCap experiment is a determination of the rate for muon capture on the free proton to 1 percent, from which the induced pseudoscalar form factor $g_P$ of the nucleon can be derived with 7 percent precision. A measurement of the related $mu$d capture process with similar precision would provide unique information on the axial current in the two nucleon system, relevant for fundamental neutrino reactions on deuterium. The MuLan experiment aims to measure the positive muon lifetime with 20 fold improved precision compared to present knowledge in order to determine the Fermi Coupling Constant $G_F$ to better than 1 ppm.
The singlet capture rate $Lambda_S$ for the semileptonic weak process $mu+p to n+ u_mu$ has been measured in the MuCap experiment. The novel experimental technique is based on stopping muons in an active target, consisting of a time projection chamber operating with ultra-pure hydrogen. This allows the unambiguous determination of the pseudoscalar form factor $g_P$ of the charged electroweak current of the nucleon. Our first result $g_P(q^2=-0.88 m^2_mu) = 7.3 pm 1.1 $ is consistent with accurate theoretical predictions and constitutes an important test of QCD symmetries. Additional data are being collected with the aim of a three-fold reduction of the experimental uncertainties. Building on the developed advanced techniques, the new MuSun experiment is being planned to measure the muon capture rate on the deuteron to 1.5% precision. This would provide the by far most accurate experimental information on the axial current interacting with the two-nucleon system and determine the low energy constant $L_{1A}$ relevant for solar neutrino reactions. Muon induced atomic and molecular processes represent challenges as well as opportunities for this science program, and their interplay with the main nuclear and weak-interaction physics aspects will be discussed.
Using the Double Chooz detector, designed to measure the neutrino mixing angle $theta_{13}$, the products of $mu^-$ capture on $^{12}$C, $^{13}$C, $^{14}$N and $^{16}$O have been measured. Over a period of 489.5 days, $2.3times10^6$ stopping cosmic $mu^-$ have been collected, of which $1.8times10^5$ captured on carbon, nitrogen, or oxygen nuclei in the inner detector scintillator or acrylic vessels. The resulting isotopes were tagged using prompt neutron emission (when applicable), the subsequent beta decays, and, in some cases, $beta$-delayed neutrons. The most precise measurement of the rate of $^{12}mathrm C(mu^-, u)^{12}mathrm B$ to date is reported: $6.57^{+0.11}_{-0.21}times10^{3},mathrm s^{-1}$, or $(17.35^{+0.35}_{-0.59})%$ of nuclear captures. By tagging excited states emitting gammas, the ground state transition rate to $^{12}$B has been determined to be $5.68^{+0.14}_{-0.23}times10^3,mathrm s^{-1}$. The heretofore unobserved reactions $^{12}mathrm C(mu^-, ualpha)^{8}mathrm{Li}$, $^{13}mathrm C(mu^-, umathrm nalpha)^{8}mathrm{Li}$, and $^{13}mathrm C(mu^-, umathrm n)^{12}mathrm B$ are measured. Further, a population of $beta$n decays following stopping muons is identified with $5.5sigma$ significance. Statistics limit our ability to identify these decays definitively. Assuming negligible production of $^{8}$He, the reaction $^{13}mathrm C(mu^-, ualpha)^{9}mathrm{Li}$ is found to be present at the $2.7sigma$ level. Limits are set on a variety of other processes.
Neutron energy-dependent angular distributions were observed for individual $gamma$-rays from the 0.74 eV p-wave resonance of $^{139}$La+$n$ to several lower excited states of $^{140}$La. The $gamma$-ray signals were analyzed in a two dimensional histogram of the $gamma$-ray energy, measured with distributed germanium detectors, and neutron energy, determined with the time-of-flight of pulsed neutrons, to identify the neutron energy dependence of the angular distribution for each individual $gamma$-rays. The angular distribution was also found for a photopeak accompanied with a faint p-wave resonance component in the neutron energy spectrum. Our results can be interpreted as interference between s- and p-wave amplitudes which may be used to study discrete symmetries of fundamental interactions.
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