The JAEA TANDEM Recoil Mass Separator

Figure 1. A picture of the JAEA Recoil Mass Separator

Despite constant improvements in the production of radioactive ion beams, fusion evaporation reactions using intense primary stable beams onto stable targets still constitute a valuable method to access exotic proton-rich nuclei. In such reactions, beam and target nuclei fuse into an excited compound nucleus which often de-excites by evaporating light particles, such as neutrons, protons, alphas, etc. The evaporation is a statistical process usually resulting in the production of several different isotopes. In sufficiently heavy compound nuclei, fission becomes the dominant de-excitation pathway. The main experimental challenge to overcome in this type of reactions is that the nuclei of interest are usually produced with a cross-section many orders of magnitude smaller than other evaporation residues. The probability of producing the wanted isotope may be less than 1 in 100 million. The Recoil Mass Separator (RMS) at the JAEA-TANDEM laboratory is used to study find this nuclear needle in the haystack, the very-low yield evaporation residues (sub-nb cross cross section) produced via fusion evaporation. The RMS separates in-flight the isotopes of interest from the high-flux of the primary beam and from most of the other residues [1,2]. The separation is electromagnetic: the RMS comprises mainly two 25o electric dipoles (ED1 and ED2), and one -50o magnetic dipole (MD), which disperse the reaction products by their mass-to-charge (m/q) ratio. Quadrupole doublets (Q1,Q2 and Q3,Q4) allow to focus the recoils irrespective of their angular spread. The electric and magnetic components are mounted on a rotating platform (-5o to 40o), 9.4 m in length. A schematic diagram is shown in Figure 2.

Figure 2. Schematic drawing of the JAEA RMS. From [2]

The RMS was designed especially to reduce the background originating from the beams scattered from the anode surface of the ED1. For this purpose, the ED1 anode is vertically split into two parts separated by 1 cm, so that the primary beam can pass through this gap without hitting the anode surface. The slit anode also allow to monitor the beam current during the experiment, using a Faraday cup located behind the ED1 anode.

Figure 3. A picture of ED1 taken during the installation at the JAEA-TANDEM, showing the split anode.

The RMS was constructed and installed in 1995 [3]. At an early stage of the experiments, it was used to separate new neutron deficient isotopes in order to study the decay properties of short-lived α-emitting isotopes near the proton drip line in the light actinide region [4,5]. Subsequently, it was mostly employed in the study of the reaction mechanism of sub-barrier fusion [6,7].
Energy acceptance±12%
Mass Acceptance±4%
Mass Resolution300
Mass dispersion1.5cm/%(variable)
Solid Angle21 msr(variable)
Mx Magnification1.66(variable)
My magnification1.17(variable)

Table 1. Main ion-optical properties of the JAEA RMS

Recently, a new research programme was initiated at the RMS to study the decay properties of short-lived alpha and proton emitters in the 100Sn region, in collaboration with colleagues from UTK-Knoxville, Oak Ridge National Laboratory and the University of York. ASRC funding under the REIMEI program was obtained in particular to attempt the measurement of the fastest-predicted "superallowed" alpha decay 104Te→100Sn [8-10]. The RMS resolving power and beam suppression, the use of highly stripped Si detectors, and the powerful digital data acquisition and analysis software developed at UT-ORNL [11], will hopefully permit to reach such ambitious goal.

Figure 4. RMS Focal plane showing the new decay station used to investigated the 100Sn region.


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