Search

The PADME experiment, at the Laboratori Nazionali di Frascati (LNF), in Italy, will search for invisible decays of the hypothetical dark photon via the process $e^+e^-\rightarrow \gamma A'$, where the $A'$ escapes detection. The dark photon mass range sensitivity in a first phase will be 1 to 24 MeV. We report here on measurement and simulation studies of the performance of the Small-Angle Calorimeter, a component of PADME's detector dedicated to rejecting 2- and 3-gamma backgrounds. The crucial requirement is a timing resolution of less than 200 ps, which is satisfied by the choice of PbF$_2$ crystals and the newly released Hamamatsu R13478UV photomultiplier tubes (PMTs). We find a timing resolution of 81 ps (with double-peak separation resolution of 1.8 ns) and a single-crystal energy resolution of 5.7%/$\sqrt{E}$ with light yield of 2.07 photo-electrons per MeV, using 100 to 400 MeV electrons at the Beam Test Facility of LNF. We also propose the investigation of a two-PMT solution coupled to a single PbF$_2$ crystal for higher-energy applications, which has potentially attractive features., Comment: 12 pages, 19 figures. v2: added section on radiation damage studies

Electron-positron angular correlations were measured for the isovector magnetic dipole 17.6 MeV state ($J^\pi=1^+$, $T=1$) $\rightarrow$ ground state ($J^\pi=0^+$, $T=0$) and the isoscalar magnetic dipole 18.15 MeV ($J^\pi=1^+$, $T=0$) state $\rightarrow$ ground state transitions in $^{8}$Be. Significant deviation from the internal pair creation was observed at large angles in the angular correlation for the isoscalar transition with a confidence level of $> 5\sigma$. This observation might indicate that, in an intermediate step, a neutral isoscalar particle with a mass of 16.70$\pm0.35 $ (stat)$\pm 0.5 $ (sys) MeV$/c^2$ and $J^\pi = 1^+$ was created., Comment: 5 pages, 5 figures

A multi-detector array has been designed and constructed for the simultaneous measurement of energy- and angular correlations of electron-positron pairs. Experimental results are obtained over a wide angular range for high-energy transitions in 16O, 12C and 8Be. A comparison with GEANT simulations demonstrates that angular correlations between 50 and 180 degrees of the electron-positron pairs in the energy range between 6 and 18 MeV can be determined with sufficient resolution and efficiency., Comment: 7 pages, 7 figures. arXiv admin note: text overlap with arXiv:nucl-ex/0311002 by other authors

The fission probability of 232Pa was measured as a function of the excitation energy in order to search for hyperdeformed (HD) transmission resonances using the (d,pf) transfer reaction on a radioactive 231Pa target. The experiment was performed at the Tandem accelerator of the Maier-Leibnitz Laboratory (MLL) at Garching using the 231Pa(d,pf) reaction at a bombarding energy of E=12 MeV and with an energy resolution of dE=5.5 keV. Two groups of transmission resonances have been observed at excitation energies of E=5.7 and 5.9 MeV. The fine structure of the resonance group at E=5.7 MeV could be interpreted as overlapping rotational bands with a rotational parameter characteristic to a HD nuclear shape. The fission barrier parameters of 232Pa have been determined by fitting TALYS 1.2 nuclear reaction code calculations to the overall structure of the fission probability. From the average level spacing of the J=4 states, the excitation energy of the ground state of the 3rd minimum has been deduced to be E(III)=5.05 MeV., Comment: 6 pages, 8 figures

Proton decay from the 3$\hbar\omega$ isoscalar giant dipole resonance (ISGDR) in $^{58}$Ni has been measured using the ($\alpha,\alpha'p$) reaction at a bombarding energy of 386 MeV to investigate its decay properties. We have extracted the ISGDR strength under the coincidence condition between inelastically scattered $\alpha$ particles at forward angles and decay protons emitted at backward angles. Branching ratios for proton decay to low-lying states of $^{57}$Co have been determined, and the results compared to predictions of recent continuum-RPA calculations. The final-state spectra of protons decaying to the low-lying states in $^{57}$Co were analyzed for a more detailed understanding of the structure of the ISGDR. It is found that there are differences in the structure of the ISGDR as a function of excitation energy., Comment: Minor changes after review. Accepted for publication in Phys. Rev. C. 19 pages; 7 figures

The excitation and subsequent proton decay of the isoscalar giant dipole resonance (ISGDR) in $^{208}$Pb have been investigated via the $^{208}$Pb($\alpha, \alpha^{\prime}p)^{207}$Tl reaction at 400 MeV. Excitation of the ISGDR has been identified by the difference-of-spectra method. The enhancement of the ISGDR strength at high excitation energies observed in the multipole-decomposition-analysis of the singles $^{208}$Pb($\alpha,\alpha^{\prime}$) spectra is not present in the excitation energy spectrum obtained in coincidence measurement. The partial branching ratios for direct proton decay of ISGDR to low-lying states of $^{207}$Tl have been determined and the results are compared with predictions of continuum random-phase-approximation (CRPA) calculations., Comment: 12 pages, 4 figures; accepted for publication in Physics Letters B

Electron-positron pairs have been observed in the 10.95-MeV $0^-\to0^+$ decay in $^{16}$O. The branching ratio of the e$^+$e$^-$ pairs compared to the 3.84-MeV $0^-\to2^+$ $\gamma$ decay of the level is deduced to be $20(5)\times10^{-5}$. This magnetic monopole (M0) transition cannot proceed by $\gamma$-ray decay and is, to first order, forbidden for internal pair creation. However, the transition may also proceed by the emission of a light neutral $0^{-}$ or $1^{+}$ boson. Indeed, we do observe a sharp peak in the $e^{+}e^{-}$ angular correlation with all the characteristics belonging to the intermediate emission of such a boson with an invariant mass of 8.5(5) MeV/c$^2$. It may play a role in the current quest for light dark matter in the universe., Comment: 6 pages

We investigated the feasibility of quantum-correlation measurements in nuclear physics experiments. In a first approach, we measured spin correlations of singlet-spin (1S0) proton pairs, which were generated in 1H(d,2He) and 12C(d,2He) nuclear charge-exchange reactions. The experiment was optimized for a clean preparation of the 2He singlet state and offered a 2pi detection geometry for both protons in the exit channel. Our results confirm the effectiveness of the setup for theses studies, despite limitations of a small data sample recorded during the feasibility studies., Comment: 13 pages, 7 figures

Populating states in 22Mg via the (p,t) reaction in inverse kinematics with a 55 MeV/nucleon 24Mg beam, we have measured the proton-decay branching ratios of the levels at 5.96 MeV and 6.05 MeV and obtained an experimental upper limit on the branching ratio of the 5.71 MeV state. On the basis of the present and previous measurements, we assign spins and parities to the 5.96 MeV and 6.05 MeV states. We combine our branching ratios with independent measurements of the lifetimes of these states or their 22Ne analogs to compute the resonance strengths and thereby the astrophysical rate of the 21Na(p,g)22Mg reaction. We perform hydrodynamic calculations of nova outbursts with this new rate and analyze its impact on 22Na yields., Comment: 8 pages, 6 figures, 1 table, submitted to PRC

A recoil coincidence technique has been applied to measure the alpha-decay branching ratios of near-threshold states in 19Ne. Populating these states using the (p,t) reaction in inverse kinematics, we detected the recoils and their decay products with 100% geometric efficiency using a magnetic spectrometer. Combining our branching ratio measurements with independent determinations of the radiative widths of these states, we calculate the astrophysical rate of 15O(alpha,gamma)19Ne. Using this reaction rate, we perform hydrodynamic calculations of nova outbursts and conclude that no significant breakout from the hot CNO cycles into the rp process occurs in novae via 15O(alpha,gamma)19Ne., Comment: submitted to PRC

The 15O(alpha,gamma)19Ne reaction is one of two routes for breakout from the hot CNO cycles into the rp process in accreting neutron stars. Its astrophysical rate depends critically on the decay properties of excited states in 19Ne lying just above the 15O + alpha threshold. We have measured the alpha-decay branching ratios for these states using the p(21Ne,t)19Ne reaction at 43 MeV/u. Combining our measurements with previous determinations of the radiative widths of these states, we conclude that no significant breakout from the hot CNO cycle into the rp process in novae is possible via 15O(alpha,gamma)19Ne, assuming current models accurately represent their temperature and density conditions.