Enhanced γ-Ray Emission from Neutron Unbound States Populated in ß Decay.

Total absorption spectroscopy is used to investigate the ß-decay intensity to states above the neutron separation energy followed by γ-ray emission in (87,88)Br and (94)Rb. Accurate results are obtained thanks to a careful control of systematic errors. An unexpectedly large γ intensity is observed in all three cases extending well beyond the excitation energy region where neutron penetration is hindered by low neutron energy. The γ branching as a function of excitation energy is compared to Hauser-Feshbach model calculations. For (87)Br and (88)Br the γ branching reaches 57% and 20%, respectively, and could be explained as a nuclear structure effect. Some of the states populated in the daughter can only decay through the emission of a large orbital angular momentum neutron with a strongly reduced barrier penetrability. In the case of neutron-rich (94)Rb the observed 4.5% branching is much larger than the calculations performed with standard nuclear statistical model parameters, even after proper correction for fluctuation effects on individual transition widths. The difference can be reconciled by introducing an enhancement of 1 order of magnitude in the photon strength to neutron strength ratio. An increase in the photon strength function of such magnitude for very neutron-rich nuclei, if it proves to be correct, leads to a similar increase in the (n,γ) cross section that would have an impact on r process abundance calculations.

The radioisotope complex project "RIC-80" at the Petersburg Nuclear Physics Institute.

The high current cyclotron C-80 capable of producing 40-80 MeV proton beams with a current of up to 200 µA has been constructed at Petersburg Nuclear Physics Institute. One of the main goals of the C-80 is the production of a wide spectrum of medical radionuclides for diagnostics and therapy. The project development of the radioisotope complex RIC-80 (radioisotopes at the cyclotron C-80) at the beam of C-80 has been completed. The RIC-80 complex is briefly discussed in this paper. The combination of the mass-separator with the target-ion source device, available at one of the new target stations for on-line or semi on-line production of a high purity separated radioisotopes, is explored in greater detail. The results of target and ion source tests for a mass-separator method for the production of high purity radioisotopes (82)Sr and (223,224)Ra are also presented.

Reactor decay heat in 239Pu: solving the γ discrepancy in the 4-3000-s cooling period.

The ß feeding probability of (102,104,105,106,107)Tc, 105Mo, and 101Nb nuclei, which are important contributors to the decay heat in nuclear reactors, has been measured using the total absorption technique. We have coupled for the first time a total absorption spectrometer to a Penning trap in order to obtain sources of very high isobaric purity. Our results solve a significant part of a long-standing discrepancy in the γ component of the decay heat for 239Pu in the 4-3000 s range.

Quenching of the SnSbTe cycle in the rp process.

The nuclides 104-108Sn, 106-110Sb, 108,109Te, and 111I at the expected endpoint of the astrophysical rp process have been produced in 58Ni+natNi fusion-evaporation reactions at IGISOL and their mass values were precisely measured with the JYFLTRAP Penning trap mass spectrometer. For 106Sb, 108Sb, and 110Sb these are the first direct experimental mass results obtained. The related one-proton separation energies have been derived and the value for 106Sb, Sp=424(8) keV, shows that the branching into the closed SnSbTe cycle in the astrophysical rp process is weaker than expected.

Mass measurements and implications for the energy of the high-spin isomer in 94Ag.

Nuclides in the vicinity of 94Ag have been studied with the Penning trap mass spectrometer JYFLTRAP at the Ion-Guide Isotope Separator On-Line. The masses of the two-proton-decay daughter 92Rh and the beta-decay daughter 94Pd of the high-spin isomer in 94Ag have been measured, and the masses of 93Pd and 94Ag have been deduced. When combined with the data from the one-proton- or two-proton-decay experiments, the results lead to contradictory mass excess values for the high-spin isomer in 94Ag, -46 370(170) or -44 970(100) keV, corresponding to excitation energies of 6960(400) or 8360(370) keV, respectively.

Measurement of the beta+ and orbital electron-capture decay rates in fully ionized, hydrogenlike, and heliumlike 140Pr ions.

We report on the first measurement of the beta+ and orbital electron-capture decay rates of 140Pr nuclei with the simplest electron configurations: bare nuclei, hydrogenlike, and heliumlike ions. The measured electron-capture decay constant of hydrogenlike 140Pr58+ ions is about 50% larger than that of heliumlike 140Pr57+ ions. Moreover, 140Pr ions with one bound electron decay faster than neutral 140Pr0+ atoms with 59 electrons. To explain this peculiar observation one has to take into account the conservation of the total angular momentum, since only particular spin orientations of the nucleus and of the captured electron can contribute to the allowed decay.

Observation of proton radioactivity of the (21+) high-spin isomer in 94Ag.

We have observed direct one-proton decay of the (21+) isomer in the N=Z nuclide 94Ag into high-spin states in 93Pd by detecting protons in coincidence with gamma-gamma correlations and applying gamma gates based on known 93Pd levels. Two decay branches have been identified, with proton energies of 0.79(3) and 1.01(3) MeV and branching ratios of 1.9(5)% and 2.2(4)%, respectively. The corresponding partial half-life values are 21(6) and 18(4) s. The Q value of the direct proton decay of the (21+) isomer was found to be 5.78(3) MeV. The very small reduced widths of the observed proton decays might reflect dominating collective configurations in the (21+) isomer, and the fine structure of the proton spectrum might indicate a strong deformation of this state.