First determination of $\beta$-delayed multiple neutron emission beyond $A=100$ through direct neutron measurement: The $P_{2n}$ value of ${}^{136}\mathrm{Sb}$

Background: $\beta$-delayed multiple neutron emission has been observed for some nuclei with $A\le 100$, being the ${}^{100}\mathrm{Rb}$ the heaviest $\beta 2n$ emitter measured to date. So far, only $25P_{2n}$ values have been determined for the $\approx 300$ nuclei that may decay in this way. Accordingly, it is of interest to measure $P_{2n}$ values for the other possible multiple neutron emitters throughout the chart of the nuclides. It is of particular interest to make such a measurement for nuclei with $A>100$ to test the predictions of theoretical models and simulation tools for the decays of heavy nuclei in the region of very neutron-rich nuclei. In addition, the decay properties of these nuclei are fundamental for the understanding of astrophysical nucleosynthesis processes, such as the $r$-process, and safety inputs for nuclear reactors.

Purpose: To determine for the first time the two-neutron branching ratio, the $P_{2n}$ value, for ${}^{136}\mathrm{Sb}$ through a direct neutron measurement and to provide precise $P_{1n}$ values for ${}^{136}\mathrm{Sb}$ and ${}^{136}\mathrm{Te}$.

Method: A pure beam of each isotope of interest was provided by the JYFLTRAP Penning trap at the Ion Guide Isotope Separator On-Line (IGISOL) facility of the University of Jyväskylä, Finland. The purified ions were implanted into a moving tape at the end of the beam line. The detection setup consisted of a plastic scintillator placed right behind the implantation point after the tape to register the $\beta$ decays and the BELEN detector, based on neutron counters embedded in a polyethylene matrix. The analysis was based on the study of the $\beta$- and neutron-growth-and-decay curves and the $\beta$-one-neutron and $\beta$-two-neutron time correlations, which allowed us the determination of the neutron-branching ratios.

Results: The $P_{2n}$ value of ${}^{136}\mathrm{Sb}$ was found to be 0.14(3)% and the measured $P_{1n}$ values for ${}^{136}\mathrm{Sb}$ and ${}^{136}\mathrm{Te}$ were found to be 32.2(15)% and 1.47(6)%, respectively.

Conclusions: The measured $P_{2n}$ value is a factor 44 smaller than predicted by the finite-range droplet model plus the quasiparticle random-phase approximation (FRDM+QRPA) model used for $r$-process calculations.

Figure 1

Aluminum tube linking JYFLTRAP and the end of the beamline. The rear side of the moving tape at the implantation position can be seen. The plastic scintillator detector used as a $\beta$ decay counter, shown in the bottom-right, was placed 6 mm behind the moving implantation tape at the end of the aluminum tube.

Figure 2

The BELEN detector embedded in the HDPE matrix and shielding, and the data-acquisition system.

Figure 3

BELEN detector efficiency from the geant4 simulation: total efficiency and for each one of the three rings of ${}^3\mathrm{He}$ proportional counters (dashed lines) [38] and neutron spectra of the measured isotopes (from ensdf [39]) as a function of the neutron energy (colored lines). See the text and Table 1 for details.

Figure 4

BELEN energy spectrum acquired with the 48 ${}^3\mathrm{He}$ tubes for the ${}^{136}\mathrm{Sb}$ measurement.

Figure 5

The mcnpx simulation of the $\beta$-counter efficiency (dashed line) and electron end-point energies from the neutron energy spectra of the measured isotopes (colored lines) [39]. The lower-$\beta$ efficiency at low energies affects the detection of correlated $\beta n$ events for those isotopes with a low-$Q_{\beta }$ value (see the text for details).

Figure 6

Analysis of the $\beta$ and neutron events accumulated during the implantation-and-decay cycles in the growth-and-decay curves for each isotope measured. In the analysis of (b) ${}^{137}\mathrm{I}$ the fit function also includes a parameter to reflect the loss of the xenon isotopes in the chain. As xenon is a noble gas, a fraction of the nuclei rapidly escape from the implantation tape. Details of this effect are presented in Ref. [20]. In the case of (a) ${}^{95}\mathrm{Rb}$ the trap purification cycle can be identified. In all cases the ions were extracted from the trap in bunches, once in every 181 ms according to the length of this cycle. This was repeated as many times as needed during the “beam on” time. So the beam was not continuous but rather quasicontinuous.

Figure 7

$\beta 1n$-correlation events for the ${}^{136}\mathrm{Te}$ analysis. The 2630 forward and 548 backward events were registered during the experiment. According to the $\beta n$-background level on the left side, the neutron moderation time in polyethylene for this experiment was determined to be $500\mu \mathrm{s}$.

Figure 8

Background level evaluation of double-neutron events $N_{2n}$ for the ${}^{136}\mathrm{Sb}$ setting. As can be observed, the number of events in a background measurement is of the same order as in the setting of interest. See the text for details.

Figure 9

${}^{136}\mathrm{Sb}\beta 2n$-correlation events. In blue are shown the first correlated neutrons after a $\beta$ decay, and in red are shown the second neutrons in the correlation. A total of 55 forward events were recorded in which are included several background contributions (see the text for details).

Figure 10

$\beta 1n$ correlation for the ${}^{136}\mathrm{Sb}$ analysis. A total of 9494 forward and 168 backward events were registered. Beyond the calculation to determine the number of $\beta 1n$ net events, the background rate in this $\beta 1n$ channel is relevant in the determination of the truly $\beta 2n$ events. (See the text for details).