| 研究実績の概要 |
According to the plan, during FY26, the sensitivity of the detection setup was considerably increased, making our setup one of the best in the world to achieve the desired goal, i.e. the measurement of the superallowed alpha decay 104Te->100Sn.
At the JAEA Tandem accelerator, exotic nuclei close to the N=Z line can be produced via fusion evaporation using stable beams and targets. The isotopes of interest are selected by the Recoil Mass Separator (RMS), and reach our detection setup, comprising a Multi Wire Proportional Counter (MWPC) to gate the recoiling isotopes, and a Segmented Si detector setup where the ions are implanted. Since the RMS permits only mass selection, the desired isotopes are only a small fraction of the total number of same-mass nuclei.
Major improvements were introduced during FY26. The focal plane chamber is now surrounded by a scintillator detector array, mounted on a purposely-built support. The main aim is to veto beta-delayed proton events, originating from contaminant isotopes of the same mass, gating on 511-keV gamma rays from positron annihilation. Also, to maximize the implantation rate, all other masses on the focal plane were blocked by newly-built mechanical slits before the MWPC.
The sensitivity was tested by studying the 113Ba to 101Sn triple-alpha-decay chain. Two events were identified, very likely originating from the 113Ba decay. The analysis is still ongoing, but the sensitivity was about 2-3nb for 9 days of beam time. In the approved 30-day experiment, we can hope to see events produced with sub-nanobarn cross section.
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| 現在までの達成度 (区分) |
現在までの達成度 (区分)
2: おおむね順調に進展している
理由
The desired modifications (new slits, scintillator array surrounding the focal plane chamber) were successfully implemented. The 113Ba test experiment, which we performed in collaboration with colleagues from Univ. of Tennessee, Oak Ridge National Laboratory and Univ. of York, was necessary to verify the sensitivity before attempting the measurement of the superallowed 104Te decay. The extremely calculated cross-section for the production of 112Ba or 108Xe, either of which decays into 104Te, required several preliminary tests, which started to be carried out in FY25.
The current setup is currently the best which could be achieved with the allocated funds and within the time span. Additional minor modifications will be introduced to further improve the vacuum and the signal to noise ratio (new window on the MWPC, new valves, etc.). Thirty days of beamtime have been allocated by the JAEA TANDEM PAC to run the desired experiment, which will be carried out within a few months.
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| 今後の研究の推進方策 |
Thanks to the detector and chamber developments, and to the successful tests, 30 day of beam time have been allocated by the JAEA TANDEM PAC for our planned experiment to measure the superallowed alpha decay of 104Te. The isotope of interest will be studied by searching the triple alpha chain 112Ba->108Xe->104Te->100Sn.
Very likely, we will run two weeks in the autumn of 2015 and two weeks at the beginning of 2016. Although the analysis of these data usually is rather time consuming, the presence of a few candidate 108Xe->104Te->100Sn alpha decays should be visible offline shortly after the run.
If the cross-section however will prove to be even lower than suggested by calculations and interpolations from mass measurements, a detector capable of withstanding higher rates on the focal plane but with worse resolution may need to be employed in a follow-up experiment.
A successful measurement would almost certainly result in publications in high-profile journals like Nature or Physical Review Letters, as well as presentations at international conferences. Furthermore, the achieved sensitivity has opened the path to additional studies of neighboring proton-emitting and alpha-decaying isotopes, such as 112Cs, 93Ag or 89Rh. A new proposal will be submitted to the TANDEM PAC for follow up experiments.
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