Direct proton decay of 0.56-s147Tm and search for this decay mode among very neutron-deficient isotopes with 53?Z?67

Abstract

The earlier preliminary assignment of a 1,055 +_6 keV proton line to direct proton decay of a47Tm is supported by cross bombardment measurements and by a negative result from a p0sitron-proton coincidence experiment. The half-life was remeasured to be 0.56 +0.04 s. For two types of thermal ion sources, overall efficiencies were estimated for on-line mass separation of known short-lived isotopes of promethium, europium, terbium, and holmium. Direct proton decay was searched for among very neutron-deficient isotopes of these elements, and of iodine and caesium. No evidence for direct proton decay was found. Based on estimated overall efficiencies, on calculated cross-sections, and on predictions from the gross-theory of fl decay, half-life limits for direct proton decay were deduced.

Proton spectroscopy beyond the drip line nearA=150

“…Figure 4 shows the implantation-decay time difference spectra for the 1.11-MeV peak and the 1.05-MeV peak from which new independent half-life measurements have been made for each of the two lines. The resulting half-life values, deduced from least-squares fits to the data, are t t&2 = 360+40 ps for the excited-state transition (previous value t ized =360+80 ps [8]) and t, &2=580+70 ms for the ground-state transition (previous value t t&z =560+40 ms [15]) which are in excellent agreement with the previous measurements. Tentative evidence has also been discovered for a previously unidentified weakly populated (cr (100 nb) proton decay peak in the A =147 mass gated spectrum with an energy of 947.4+5.0 keV and a half-life in the range 100 ps -10 ms. On the basis of Q~-value arguments the most likely source of this activity is ' Tm, since for the remaining A =147 residues typical Q~v alues are -0.4, -2.…”

“…Our assignment of the d3/2 state as the first excited level in ' Tm therefore casts doubt on these published absolute excitation energies of the single-particle states in ' 'Tm. [15]. 'b~deduced using P-decay half-life estimates from [31].…”

Groundstate proton radioactivity of nuclei in the tin region

“…Plochocki et al [9] deduced from experimental mass values that the odd proton in 114Cs, 11~ and 1~ is only very weakly bound, suggesting that these nuclei constitute the lightest isotopes of the corresponding elements which are stable against proton emission. On the other hand, the negative results of an experimental search for proton radioactivity in the tin region by D'Auria et al at the CERN Isolde facility [10] and by Schardt et al [7] and Larsson et al [11] at the GSI online mass separator were consistent with halflives of the nuclei ~ 13Cs and 1 ~ shorter than about 1 ms, indicating that they decay by a fast transition unobservable in these experiments.…”

“…One has to take into account that the fl+-decay branch of these nuclei is not negligible. A 20% proton decay branch was estimated for the decay of 147Tm by Larsson et al [11], resulting in a partial halflife for proton decay of about 3 s and for fl+-decay of 0.7 s. For 151Lu a similar/?+-decay halflife between 0.5 s and 1.0 s was assumed yielding a branching ratio for proton decay of (88 • 5)%. The results of the calculations are listed in Table 1 together with the experimental halflives.…”

“…For comparison, the halflives of the known proton emitters in the rare earth region, 151Lu [3] and 147Tm ground-state [5,11] and 14VTm excited low spin state [20], were also calculated. For these nuclei only an h11/2 ground-state, which is also expected from shell model systematies, is compatible with the experimental data.…”