Request for modification of NP1306-LINAC07

We request to extend the length and modify the contents of our previously approved experiment NP1306-LINAC07. Based on the new availability of \(^{50}\)Ti and \(^{54}\)Cr beams, and in light of the results of the commissioning portion of NP1306-LINAC07, we believe it is reasonable to extend the campaign to include isotopes of Rf, Db and Sg. We presently have 1.5 days of previously allocated machine time remaining, and wish to request an additional 20 days of machine time to complete the original proposed measurements of Fm, Md, No, and Lr isotopes and an additional 7 days to measure Rf and Db isotopes which can be produced using the new \(^{50}\)Ti beam. We are further requesting 7 days of conditional machine time for measurements of Sg when a sufficiently intense \(^{54}\)Cr beam becomes available.

Goal of the proposed experiment

The goal of the proposed experiment is to provide mass anchor-points for super heavy element (SHE) nuclei. At present, masses have been directly measured for only six trans-Uranium nuclei – \(^{252-255}\)No and \(^{255-256}\)Lr (Block 2010, Dworschak 2010, Block 2013, Block 2015) – with the SHIPTRAP Penning trap (Block 2007) at GSI. While Penning trap mass spectroscopy (PTMS) can achieve tremendous mass resolving power for long-lived and light nuclei, it has some drawbacks for low-yield, short-lived and heavy nuclei. For short-lived heavy nuclei, the maximum revolving power achievable by PTMS is limited by the lifetime of the nucleus to be studied. For example, in the case of \(^{260}\)No (T\(_{1/2}\)=102 ms), SHIPTRAP would be limited to \(R\)\(_m\)\(\approx\)82,000 in the case of doubly charged ions (even less for singly charged ions). Furthermore, in PTMS, a measurement requires sufficiently populating a time-of-flight ion cyclotron resonance curve; typically N\(\sim\)100 ions are required to do so.

While the drawbacks of PTMS are not extremely detrimental to the feasibility for mass measurements of No and Lr, for which production cross-sections are large and half-lives are generally greater than one second, it will become a strong hindrance for systematic measurements of even Rf isotopes, whose typical half-lives and production cross-sections are both much smaller. To bypass these drawbacks, we are implementing a multi-reflection time-of-flight mass spectrograph (MRTOF). The MRTOF has a limited mass resolving power, currently \(R_m\)\(\lesssim\)175,000. However, it can achieve that resolving power in only a few milliseconds, thereby being able to access even the shortest-lived SHE isotopes. Furthermore, as a true spectrograph, it can provide a mass measurement even for extremely low-yield species, as the detection of every individual ion can be viewed as an independent mass measurement.

With the gas cell and transport line commissioning successfully completed, we now wish perform a systematic study of trans-Uranium isotopes of Fm through Sg. Many of these are easily produced and some have been measured at SHIPTRAP, allowing us to verify accuracy and benchmark the system against SHIPTRAP. Performing such a systematic study will greatly increase the number of directly measured masses of trans-Uranium nuclei, which could have an impact on theoretical predictions of the “Island of Stability”. By extending the original proposal to include Rf, Db and Sg isotopes in the region, we can also shed more light on the behavior of the possible \(N\)=152 (sub)shell closure (Block 2015).

We further propose to make an initial mass measurement to anchor the SHE nucleus \(^{278}\)113 (Morita 2012). This would allow the use of \(\alpha\)-decay chains previously measured from \(^{278}\)113 to estimate the mass, making it the heaviest nuclei with directly or indirectly determined mass value.

Experimental setup

A sketch of the system is shown in Fig. \ref{figSystem}. Primary beam form RILAC impinges on a thin target and produces radioactive isotopes by complete fusion. The excited compound nucleus de-excites by evaporation particles, most typically \(x\)n or p\(x\)n. These fusion-evaporation products are then separated from the primary beam by the gas-filled recoil separator GARIS-II (not shown in Fig. \ref{figSystem}).

The fusion-evaporation product ions pass through a 0.5 \(\mu\)m mylar window upon exiting GARIS-II. Their energy is degraded by a mylar degrader (typical thickness \(\sim\)\(\mu\)m) that can be rotated up to 45\(\circ\) to increase the effective thickness, and further degraded by the gas cell’s \(\sim\)2.5 \(\mu\)m thick mylar entrance window. The ions then lose energy in collisions with the He gas, typical pressure \(P\)=100 mbar, eventually coming to rest inside the gas cell.

A DC gradient produced by an array of ring electrodes push the ions towards an rf-carpet structure at the extraction end of the gas cell. The rf-carpet rapidly extracts the ions, which then enter a resistive rf-sextupole ion guide to drag them through an intermediate pressure region and deliver them to the first set of rf-quadrupole ion traps. This trap consists of a pair of large 4-rod Paul traps and a “

After ejection from the flat trap, the ions are accelerated to 2 keV by a pulsed drift tube and transported \(\sim\)2 m before being decelerated by a second pulsed drift tube. Following the deceleration, the ions are recaptured in a second trap system which is identical to the one after the gas cell. The second trap system accumulated RI in one Paul trap and in the other Paul trap \(^{133}\)Cs\(^+\) ions from a thermal ion source are accumulated. Ions from the two Paul traps are alternatively transfered to the flat trap, cooled, and ejected into the MRTOF. The ions reflect within the MRTOF for a predetermined amount of time, corresponding to a specific number of reflections at which the maximum resolving power is achieved, before being released to a multichannel plate ion detector.

At multiple points in the system there are silicon PIN diodes that can be inserted to observe \(\alpha\)-decay. These detectors have allowed us to determine the efficiencies of the various components.