Proposal for Mid-Shell Actinide Mass Measurements

In preliminary work towards No and Lr mass measurements (NP1306-LINAC07) we made precision mass measurements of \(^{205,206}\)Fr and \(^{201}\)At and rough mass measurements of \(^{201}\)Bi, \(^{201, 205}\)Po, \(^{205, 206}\)At and \(^{205, 206}\)Rn (al. 2015). These measurements were achieved with only 6 hours of machine time, during which the system efficiency was \(\approx\)0.5% for \(^{205}\)Fr.  As shown in Fig. \ref{figIsobars}, we have demonstrated an ability to measure multiple isobaric chains simultaneously. This will enable a very efficient survey of the general mass landscape in this region. Here we propose a separate and independent experiment to study this region in detail.

Experimental goals

We have three goals for the proposed experiment. Firstly, and overall survey of the mass landscape will contribute to the general knowledge of the field. Secondly, we wish to verify whether or not the isomeric states in the region are as numerous as currently understood. And finally, we hope to measure the production ratios for the various isomeric states as a means to better understand the nuclear structure in the region.

Survey of mass landscape

Particularly for Ac, Th, and Pa isotopes in this region the mass landscape is not well-studied. The region has very few directly measured mass values, and where masses are directly measured they tend to have been done at ESR (Litvinov 2008) and could benefit from a cross-check by an alternate method. As reported in (al. 2015), we found deviations from ESR values for \(^{201}\)As and \(^{201}\)Po.

Verification of isomeric existence

In the region we desire to study there are 23 nuclei with isomeric states of \(T_{1/2}\gg\)1 ms. Among these only six isomeric states have been identified by detection of \(\gamma\)-rays from internal transition. In 15 cases, the isomeric states have been inferred from detection of multiple \(\alpha\)-decays energies. In the remaining cases, the isomer is presumed to exist due to largely differeing half-life measurements. Among the inferred isomers, there are six cases where the measured half-lives of ground state and isomeric state are within 2\(\sigma\) of eachother. We believe that there is a reasonable possibility that in at least some cases, the isomeric state may be incorrectly inferred from decay to an excited state in the daughter nucleus. By directly observing these isomeric states in a mass spectrum, we could unambiguously confirm their existence.

Determination of isomeric production ratios

The large number isomeric states known to exist in this region provide us with a special means to investigate the nuclear structure in this region. It is generally understood that direct production (e.g. by complete fusion) of nuclei can be naïvely understood, to first-order, to populate isomeric states with a probability proportional to \((2J_m+1)/(2J_g+1)\) (Kulko 2007), where \(J_m\) and \(J_g\) are the spin of the isomer and ground state, respectively. Historically, studies of isomeric ratios (Bowry 2013, Jong 1997, Kulko 2007) have measured the decay of isomeric and ground states to infer the relative populations. While such studies are the only means to address short-lived (\(T_{1/2}\ll\)1 ms) isomeric states, they require corrections for detector efficiency energy dependence, decay branching ratios, et cetera. By direct mass measurements, we will determine the production ratios without need for such corrections. This will allow us to probe the spins of ground and isomeric states in these nuclei in a new and independent method.

Experimental methods

The proposed experiment will make use of fusion-evaporation reactions. The reaction products will be separated from the projectile beam using the gas-filled recoil separator GARIS-II. A gas cell after GARIS-II will be used to thermalize and quickly extract the fusion-evaporation products. A tandem pair of RF trap triplets will accumulate and prepare ions before they are injected into the MRTOF-MS.