Alpha and beta decays properties of the heaviest N = Z nuclei

Krzysztof Rykaczewski1, Robert Grzywacz2, Katsuhisa Nishio3, Andrei Andreyev4, Carl Gross1,Stan Paulauskas2, Shintaro Go2, Y. Xiao2, Nathan Brewer1, Robert Wadsworth4, DavidJenkins4, Charles Barton4, Michael Bentley4, Riccardo Orlandi3, Hiroyuki Makii3, Kentaro
Hirose3
1Oak Ridge National Laboratory, Oak Ridge, Tennessee, U.S.A.
2Department of Physics, University of Tennessee, Knoxville, Tennessee, U.S.A.
3JAEA, Advanced Science Research Center, Tokai, Ibaraki, Japan
4The University of York, Heslington, United Kindom

Abstract
We performed the spectroscopic studies of exotic nuclei in the vicinity of doubly-magic N=Z=50
100Sn by means of Recoil Mass Separator at the JAEA Tandem Laboratory. We obtained anexperimental evidence for beta decay of new N=Z+1 isotope. This isotope is an alpha precursorof the super allowed alpha decay chain 109Xe-105Te-101Sn observed earlier at JAEA Recoil MassSeparator. However, its decay is dominated by beta transitions and following proton emission.
The identification of 113Ba was enabled by a new detection technique for fast charge particle
decays which has been developed at the University of Tennessee and commissioned at the JAEA
Recoil Mass Separator. This technique is based on fast-response, radiation-hard and position
sensitive YAP scintillator detector allowing to identify alpha-activities with half-lives down to
100 ns level. It was designed for the experiment aiming in a discovery of new N=Z nuclei 108Xeand 104Te located near the doubly magic 100Sn and in the investigation of their super allowed
alpha decays. Coupling the new detector to an array of LaBr3(Ce) scintillators will allow fasttiming measurements for the excited states populated by alpha transitions in odd-mass nuclei
near 100Sn. This research program got slowed down by the troubles with high energy 58Ni beamof sufficient intensity at Tandem accelerator in December 2016, but it will continue after theTandem operation will restart.
1. Research Objectives
The overarching goal of our studies is the investigation of nuclear structure of proton-rich nuclei
around the doubly-magic nucleus 100Sn having the same neutron (N) and proton numbers (Z).
N=Z=50. 100Sn is the heaviest N=Z nucleus discovered so far. The fact that protons and neutrons
occupy the same orbitals may reveal a presence of a new type of pairing in nuclei – the protonneutron (p-n) isoscalar pairing, which could be different from the well-known pairing between
like nucleons (n-n and p-p), leading to a new type of superfluidity. An experimental program,which aims to identify heaviest N=Z nuclei is ongoing at the JAEA Tandem Laboratory. Thisproject focuses on the discovery of the isotopes 112Ba, 113Ba, 108Xe and 104Te produced in the fusion-

evaporation reactions. The main goal is to identify the so-called “superallowed” α-transition
occurring in 104Te→100Sn decay. Specifically, a unique type of enhancement of the α-particle
emission from nuclei in the vicinity of 100Sn was predicted [1], made possible by the special
properties of N=Z nuclei and the shell structure near 100Sn. This project can achieve this goal by
discovering two out of the three heaviest N=Z isotopes, present in the 108Xe→104Te→100Sn
superallowed α-decay chain. The alpha decay of 108Xe generates also new isotope 104Te (N=Z=52),
the latter one decaying by the ultimate superallowed transition 104Te→100Sn. The production of108Xe is based on fusion-evaporation reaction; excited compound nucleus 112Xe* is formed in thefusion reaction between 58Ni and 54Fe, and the 108Xe is generated by evaporating 4 neutrons from
112Xe*. This research is a continuation of the program initiated at the HRIBF (ORNL,USA)

which resulted in numerous technical development [2,3], and enabled discovery experiments in
the 100Sn region.
Production of 108Xe itself is already a big challenge in a nuclear physics community. Attempt at
RIKEN-RIBF facility to produce N=Z nuclei beyond 100Sn using fragmentation technique wasdone, but no clear decay data was reported so far. A future plan of Spiral2 at GANIL (France)
also intends to produce these nuclei. Our objectives can be realized at the JAEA-Tandem using
our new developed detection setup, combined to the JAEA Recoil Mass Separator [4]. The new
N=Z isotopes of interest can be produced using high-energy (~250 MeV), intense (>30 pnA),
stable Tandem beams or by a temporary re-operation of the Booster Linac.
Due to unfavourable conditions of the Tandem accelerator, neither scenario seemed realistic
within FY2016. Hence we decided to reinvestigate the α-decay chain 109Xe→105Te→101Sn, shownin Fig. 1, in which excited states in 105Te and 101Sn [5,6] are populated. The decay of these excitedstates, usually in the picosecond range, is expected to be slowed down to the hundreds ofpicosecond range by the change in two units of orbital angular momentum when the valence
neutron moves from the d5/2 to the g7/2 orbit (L-forbidden M1 competing with E2 transition). For
this transition to occur, the wavefunction of the valence neutron needs to mix with excitations ofthe 100Sn core. These decays would constitute the first study of electromagnetic transition
probability so close to this N=Z closed shell nucleus, and assess the magic nature of 100Sn.

Fig 1：Modified from [6]. Decay chain of 109Xe, where the α decay of 105Te(109Xe)
populates excited states in 101Sn(105Te). The neutron single-particle orbits
involved in this M1 transition are shown in the inset on the left

2. Research ContentsOur experimental program to eventually measure the 104Te α decay is still ongoing at the JAEA
Tandem. The rare evaporation of 4 neutrons from excited 112Xe create α-radioactive 108Xe product.
Since the cross section for 108Xe corresponds roughly to only 1 part in a billion (1/109), theseparation of reaction products is a critical issue. The JAEA Recoil Mass Separator (RMS) has
been proven to be capable to isolate mass 108 isotopes from the other produced nuclei. Theimplantation of 108Xe ions and subsequent α decay can be detected in a position sensitive detector
at the focal plane of the separator. The selectivity required to identify the decays of these exotic
isotopes can be achieved through the observation of the characteristic complete or partial αdecay chains. The 108Xe activity will leave a unique double-alpha decay pattern allowing us toidentify this rare nucleus and its decay products 104Te and 100Sn. This pile-up signal will result
from two subsequent alpha decays within few tens of nanoseconds. The feasibility of thetechnique was proved in test experiments on the known 109Xe α-decay chain and in the discovery
experiment of 113Ba [7]. Despite a much smaller than expected production cross-section, evidenceof the first production of 113Ba was deduced from the collected data and it constitutes the topic
of the Ph.D. thesis of Y. Xiao at University of Tennessee, Knoxville. The worsening of the Tandem
accelerator conditions delayed our main goal and, for FY2016, we developed an alternative
program which could be achieved even with a lower Tandem terminal voltage (16 MV), i.e. thelifetime measurement of the 172-keV state in 101Sn. If successful, this would be the firstmeasurement of electro-magnetic transition probability neighboring 100Sn.

Fig 2：Schematic diagram of new timing setup. Isotopes produced in the 58Ni+54Fe
reaction at 220 MeV, separated according to the A/q by the RMS reach the focal plane.
The new YAP scintillator detector, where mass 109 ions are implanted and decay, issurrounded by a compact array of LaBr3(Ce) scintillators, for an absolute photo-peakefficiency of ∼25% at 172 keV.
The signature of the production of 101Sn is given by the unique pile-up signal corresponding tothe double α decay 109Xe→105Te→101Sn. 109Xe can be produced via the 58Ni+54Fe fusionevaporation reaction at 220 MeV. The RMS separate the evaporation residues (ERs) according
to their mass-to-charge ratio, and focus them onto a compact decay station, drawn schematicallyin Fig. 2. The setup comprises mechanical slits to select only the mass of interest, a Multi Wire

Proportional Counter (MWPC) to monitor the trajectories of ERs and scattered beam
background, and a newly developed YAP scintillator detector where 109Xe implant and decay.
The YAP detector, constructed at UTK/ORNL, is a 100-µm-thin YAP scintillator crystal, mounted
on a segmented light guide (24x24 pixels) and a position-sensitive photomultiplier tube to obtainhigh pixilation to compensate for the high implantation rate. Time-correlations between MWPC
and YAP permit to discriminate charged-particle decays from implants of residues and fromscattered beam. Eight LaBr3(Ce) scintillators, with a timing response of 200 ps resolution,
provided by the UTK collaborators, surrounding the target chamber, enable particle-γcoincidences. The delayed coincidence between decay α particles detected in YAP and γ–raysdetected in the LaBr3 permit to determine lifetime of the first excited states in 101Sn and 105Te.
According to systematics for the L-forbidden transition, these lifetimes are expected to be in the500-1000 ps range. A two-week experiment was scheduled to run in January 2017, but it had tobe canceled due to a temporary closure of the accelerator due to a vacuum issue occurred in late
2016. The experiment will be carried out after the Tandem restart.
3. Research results

Fig 3: (from left to right) 2-D map of implant positions, time response and in-beam decayenergy spectra obtained with the new YAP detector developed at UTK-ORNL. Itsradiation hardness permits to expose this detector to intense particle bombardment forextended period of time without any appreciable loss in time resolution (~200 ps) orenergy resolution, measured to be ∼9% for an α energy of 3.44 MeV.
The temporary unavailability of intense 58Ni beam at the JAEA Tandem resulted in a setback of
our anticipated completion time and of our publications. Nonetheless, the JAEA Tandem is theonly facility, in addition to ORNL, where the 109Xe-105Te-101Sn double alpha chain was observed,and probably the only facility worldwide to have gathered evidence for the production of 113Ba.
This work resulted in the doctorate thesis work of Y. Xiao at the University of Tennessee, and apublication summarizing the results is in preparation [8]. Furthermore, the development andcommissioning of the YAP scintillator, of which representative spectra are shown in Fig. 3 andits employment to detect fast consecutive α decays, is the topic of a technical publication to besubmitted to the journal Nuclear Instruments and Methods in Physics Research [9].
4. Conclusion
The achievements of this research programme, driven by the desire of measuring thesuperallowed α decays in 104Te, brought us very close to carry out a measurment which largescale facilities like RIKEN or GANIL are still uncapable of carrying out. Our newly-developeddetection setup makes this ambitious project more likely to succeed at the JAEA-Tandem. In ourexperiments at the JAEA-Tandem RMS, performed with detection system combing UTK, ORNL
and JAEA detectors and state-of-art digital data acquisition from UTK/ORNL, the decay chain
109Xe-105Te-101Sn was observed and the evidence for a decay of new isotope 113Ba was obtained.

On our side, all is ready to carry out our proposed measurements, and we sincerely hope that allthese efforts will be repaid by some successful experimental runs when the Tandem accelerator
facility will become again capable of providing the intense, high-energy beams required by thisproject. When the production capabilities at JAEA Tandem will be recovered, preferably with athe Booster Linac being a part of the acceleration scheme, the discovery of the new isotopes 108Xeand 104Te may be achieved at JAEA laboratory.
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[8] Y. Xiao et al., To be submitted.

[9] R. Grzywacz et al., To be submitted.