New neutron-deficient isotopes fromKr78fragmentation

Abstract

In an experiment with the BigRIPS separator at the RIKEN Nishina Center, the fragmentation of a 78 Kr beam allowed the observation of new neutron-deficient isotopes at the proton drip-line. Clean identification spectra could be produced and 63 Se, 67 Kr, and 68 Kr were identified for the first time. In addition, 59 Ge was also observed. Three of these isotopes, 59 Ge, 63 Se, and 67 Kr, are potential candidates for ground-state two-proton radioactivity. In addition, the isotopes 58 Ge, 62 Se, and 66 Kr were also sought but without success. The present experiment also allowed the determination of production cross sections for some of the most exotic isotopes. These measurements confirm the trend already observed that the empirical parameterization of fragmentation cross sections, EPAX, significantly overestimates experimental cross sections in this mass region.

Discovery of 68Br in secondary reactions of radioactive beams

“…The shorter values are consistent with the non-observation after the flight time through the A1900 fragment separator of ≈ 440 ns [16]. Also the most recent search for new neutrondeficient isotopes, carried out at the RIBF, identified the N = 29, 31, 32 isotopes 63 Se and 67,68 Kr, but no event of 68 Br was observed [20]. As the fragment separator setting of BigRIPS [21] was centered on the N = 30 isotopes 65 Br and 64 Se, the large acceptance allowed only for partial transmission of N = 33 isotones, and no limit on the lifetime of 68 Br could be established.…”

“…Both effects may explain the non-observation of 68 Br in the secondary beams of earlier searches for new isotopes. With an estimated lifetime around 50 ns, 68 Br could nevertheless have been observed also at the BigRIPS separator [20] or at the NSCL [16]. In the present experiment, a conservative upper limit for the experimental yield of 68 Br after the BigRIPS separator amounted to 200 counts, which is more than two orders of magnitude lower than for 70 Kr.…”

“…With the EPAX3 parametrization [27], the calculated cross section for the production of 68 Br amounts to σ = 1.29 · 10 −5 mb, a factor of two higher than for 70 Kr. This suggests that states of 68 Br produced in the fragmentation of the primary beam are short-lived, with lifetimes much shorter than the flight time through the separator, in line with the non-observation of 68 Br in earlier experiments [16,20]. The present experiment was not optimized for 68 Br and therefore the transmission of this isotope is not well determined.…”

Two-Proton Radioactivity ofKr67

“…The fragments of interest were identified by the BigRIPS standard detection setup consisting of a series of plastic scintillators, multisampling ionization chambers, and parallel-plate avalanche counters. Details of the identification procedure can be found in a recent paper [26].…”

“…In the present experiment, BigRIPS was optimized for four different settings relevant to the present Letter: (i) a setting on 51 Ni for WAS3ABi calibration, (ii) a setting on 65 Br to produce proton-rich nuclei, notably 59 Ge, 63 Se, and 67 Kr, (iii) a setting on 64 Se to study its decay characteristics, and (iv) a setting on 62 Se to search for the new isotopes 58 Ge and 62 Se (see [26] for details).…”