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Register a new userUltra-sensitive in-beam gamma-ray spectroscopy for nuclear astrophysics at LUNA
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Ultra-sensitive in-beam gamma-ray spectroscopy studies for nuclear astrophysics are performed at the LUNA (Laboratory for Underground Nuclear Astrophysics) 400 kV accelerator, deep underground in Italys Gran Sasso laboratory. By virtue of a specially constructed passive shield, the laboratory gamma-ray background for E_gamma < 3 MeV at LUNA has been reduced to levels comparable to those experienced in dedicated offline underground gamma-counting setups. The gamma-ray background induced by an incident alpha-beam has been studied. The data are used to evaluate the feasibility of sensitive in-beam experiments at LUNA and, by extension, at similar proposed facilities.
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An escape-suppressed, composite high-purity germanium detector of the Clover type has been installed at the Laboratory for Underground Nuclear Astrophysics (LUNA) facility, deep underground in the Gran Sasso Laboratory, Italy. The laboratory gamma-ray background of the Clover detector has been studied underground at LUNA and, for comparison, also in an overground laboratory. Spectra have been recorded both for the single segments and for the virtual detector formed by online addition of all four segments. The effect of the escape-suppression shield has been studied as well. Despite their generally higher intrinsic background, escape-suppressed detectors are found to be well suited for underground nuclear astrophysics studies. As an example for the advantage of using a composite detector deep underground, the weak ground state branching of the Ep = 223 keV resonance in the 24Mg(p,gamma)25Al reaction is determined with improved precision.
We report on the first in-beam $gamma$-ray spectroscopy of uc{23}{Al} using two different reactions at intermediate beam energies: inelastic scattering off uc{9}{Be} and heavy-ion induced one-proton pickup, uc{9}{Be}( uc{22}{Mg}, uc{23}{Al}$+gamma$)X, at 75.1 MeV/nucleon. A $gamma$-ray transition at 1616(8) keV -- exceeding the proton separation energy by 1494 keV -- was observed in both reactions. From shell model and proton decay calculations we argue that this $gamma$-ray decay proceeds from the core-excited $7/2^+$ state to the $5/2^+$ ground state of uc{23}{Al}. The proposed nature of this state, $[ uc{22}{Mg}(2^+_1) otimes pi d_{5/2}]_{7/2+}$, is consistent with the presence of a gamma-branch and with the population of this state in the two reactions.
Excited states in the neutron-rich N=38,36 nuclei uc{60}{Ti} and uc{58}{Ti} were populated in nucleon-removal reactions from uc{61}{V} projectiles at 90~MeV/nucleon. The gamma-ray transitions from such states in these Ti isotopes were detected with the advanced gamma-ray tracking array GRETINA and were corrected event-by-event for large Doppler shifts (v/c sim 0.4) using the gamma-ray interaction points deduced from online signal decomposition. The new data indicate that a steep decrease in quadrupole collectivity occurs when moving from neutron-rich N=36,38 Fe and Cr toward the Ti and Ca isotones. In fact, uc{58,60}{Ti} provide some of the most neutron-rich benchmarks accessible today for calculations attempting to determine the structure of the potentially doubly-magic nucleus uc{60}{Ca}.
We report on the first in-beam $gamma$-ray spectroscopy of the proton-dripline nucleus $^{40}$Sc using two-nucleon pickup onto an intermediate-energy rare-isotope beam of $^{38}$Ca. The $^{9}$Be($^{38}$Ca,$^{40}$Sc$+gamma$)X reaction at 60.9 MeV/nucleon mid-target energy selectively populates states in $^{40}$Sc for which the transferred proton and neutron couple to high orbital angular momentum. In turn, due to angular-momentum selection rules in proton emission and the nuclear structure and energetics of $^{39}$Ca, such states in $^{40}$Sc then exhibit $gamma$-decay branches although they are well above the proton separation energy. This work uniquely complements results from particle spectroscopy following charge-exchange reactions on $^{40}$Ca as well as $^{40}$Ti EC/$beta^+$ decay which both display very different selectivities. The population and $gamma$-ray decay of the previously known first $(5^-)$ state at 892 keV and the observation of a new level at 2744 keV are discussed in comparison to the mirror nucleus and shell-model calculations. On the experimental side, this work shows that high-resolution in-beam $gamma$-ray spectroscopy is possible with new generation Ge arrays for reactions induced by rare-isotope beams on the level of a few $mu$b of cross section.
A dedicated setup for the in-beam measurement of absolute cross sections of astrophysically relevant charged-particle induced reactions is presented. These, usually very low, cross sections at energies of astrophysical interest are important to improve the modeling of the nucleosynthesis processes of heavy nuclei. Particular emphasis is put on the production of the $p$ nuclei during the astrophysical $gamma$ process. The recently developed setup utilizes the high-efficiency $gamma$-ray spectrometer HORUS, which is located at the 10 MV FN tandem ion accelerator of the Institute for Nuclear Physics in Cologne. The design of this setup will be presented and results of the recently measured $^{89}$Y(p,$gamma$)$^{90}$Zr reaction will be discussed. The excellent agreement with existing data shows, that the HORUS spectrometer is a powerful tool to determine total and partial cross sections using the in-beam method with high-purity germanium detectors.
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