$\beta$-delayed neutron emission studies of ${}^{137,138}\mathrm{I}$ and ${}^{144,145}\mathrm{Cs}$ performed with trapped ions

A detailed study of the $\beta$-delayed neutron emission properties of ${}^{137,138}\mathrm{I}$ and ${}^{144,145}\mathrm{Cs}$ has been performed by confining ions in the Beta-decay Paul Trap. The daughter ions following $\beta$ decay emerge from the trapped-ion cloud with negligible scattering allowing reconstruction of the recoil-ion energy from the time of flight. From this information, the neutron-emission branching ratios and neutron-energy spectra were deduced. The results for the ${}^{137}\mathrm{I}$ and ${}^{144,145}\mathrm{Cs}$ decays are in agreement with previous results performed using direct neutron-detection techniques. In the case of ${}^{138}\mathrm{I}$, a branching ratio of 6.18(50)% is obtained, yielding a value consistent with the more recent results, which are a factor of two larger than measurements made prior to 1978.

Figure 1

Schematic of the cross-sectional end view of the BPT experimental setup (not to scale). The dashed arrows represent an example of decay-product trajectories when a neutron is emitted.

Figure 2

Recoil-ion TOF spectra following the $\beta$ decay of (a) ${}^{137}\mathrm{I}$, (b) ${}^{138}\mathrm{I}$, (c) ${}^{144}\mathrm{Cs}$, and (d) ${}^{145}\mathrm{Cs}$. Each spectrum shows the sum of the four $\mathrm{\Delta }E$-MCP detector combinations after the subtraction of random coincidences. The coincidences between 0.3 and $1.6\mu \mathrm{s}$ are due to recoil ions having emitted a neutron, whereas the vast majority of the distribution beyond $2\mu \mathrm{s}$ is due to decays without neutron emission. For ${}^{137}\mathrm{I}$ decays, the TOF distribution beyond $2\mu \mathrm{s}$ is narrower than the others primarily because the large transition strength to the ground state and the correlation between the leptons lead to the recoil-ion momenta distribution being more focused opposite the $\beta$-particle momentum.

Figure 3

The PHD for ${}^{137}\mathrm{Xe}$ ions following the $\beta$ decay of ${}^{137}\mathrm{I}$ striking the (a) top and (b) right MCP detectors with impact energies of 5 keV. The fit [shown as smooth line in yellow (light-gray)] is a sum of Gaussian distributions, which take into account the observed spatial variation in gain across the face of the MCP detector. This gain variation imparts a visible skew to the PHD. The spectra for the other isotopes are similar.

Figure 4

The calculated fraction of MCP detector pulses above the discriminator threshold as a function of ion-impact velocity. The recoil ions from $\beta$ decays to the ground state or accompanied by $\gamma$-ray emission have impact velocities around 0.08 mm/ns, whereas recoil ions following $\beta \mathrm{n}$ emission have a range of energies that extend well above 0.10 mm/ns.

Figure 5

The coincident $\beta$-ion detection efficiency (normalized to the product of the corresponding detector solid angles, and hence expressed as a fraction) for the $\beta \mathrm{n}$ decay of ${}^{137}\mathrm{I}$ for each $\mathrm{\Delta }E$-MCP detector pair. The ${180}^{\circ }$ detector combinations have higher efficiencies primarily due to the additional contribution from neutrons triggering the $\mathrm{\Delta }E$ detector in coincidence with the recoiling ion. For the highest neutron energies, little energy remains for the leptons, and therefore the fraction of $\beta$ particles detected drops rapidly; although this effect is large for decays populating excited states within 500 keV of $Q_{\beta }-S_n$, a negligible fraction of the neutron spectrum is typically found there due to the highly suppressed phase space for these decays. The lower efficiency of the top MCP detector is also evident. The detection-efficiency curves for the other isotopes show similar features and also only begin a rapid drop within 500 keV of $Q_{\beta }-S_n$.

Figure 6

Neutron-energy spectra determined from $\beta$-ion coincidences compared to previous results from direct neutron-detection methods: Shalev 1977 [45], Greenwood 1997 [46], and Greenwood 1985 [19]. In the region filled with hatch lines, the $\beta \mathrm{n}$ decays cannot be resolved from decays without neutron emission.

Figure 7

Comparison of the BPT results ($\beta \mathrm{r}{P}_n$ values) to the recommended IAEA values [14] obtained from previous direct measurements.

Figure 8

Comparison of ${}^{138}\mathrm{I}{P}_n$ values from Aron 64 [21], Asghar 1975 [22], Kratz 1978 [52], Reeder 1980 [18], Rudstam 1993 [17], and Gomez 2011 [20] with the results from this work (using the $\beta \mathrm{r}{P}_n$ value).