Degeneracy at 1871 keV inCd112and implications for neutrinoless double electron capture

Abstract

High-statistics β-decay measurements of112 Ag and 112 In were performed to study the structure of the 112 Cd nucleus. The precise energies of the doublet of levels at 1871 keV, for which the 0 + member has been suggested as a possible daughter state following neutrinoless double electron capture of 112 Sn, were determined to be 1871.137 (72) One of the most pressing issues in subatomic physics today is the question of the nature of the neutrino. The observation of neutrino oscillations [1][2][3] has revealed neutrino mixing and that neutrinos are massive particles or, more precisely, that at least two of the mass eigenstates are nonzero. However, it is still unknown if neutrinos are their own antiparticles (Majorana) or distinct from their antiparticles (Dirac). Furthermore, the oscillation experiments yield information on ( m) 2 , and thus the masses and their orderings remain unknown. Observation of neutrinoless double-β-decay, ββ(0ν), would reveal the Majorana nature of neutrinos, and a measure of the decay rate, λ 0ν , would provide information on the neutrino masses viaThe G 0ν (Q ββ ,Z) factor is the phase-space integral including the Fermi function, M 0ν is the nuclear matrix element, and For ββ(0ν) searches, the typical signature is a peak at Q(ββ) in the sum-β-energy spectrum. However, such searches must strongly suppress the backgrounds from natural radioactivity and cosmic rays. A new class of ββ(0ν) experiments will attempt to surmount these problems; however, the neutrinoless double electron capture, ECEC(0ν), process offers a potentially attractive alternative.The basic physics of the ECEC process was outlined some time ago [4] and included an estimate of the radiative ECEC(0ν) process, which was further refined in Ref. [5]. Following the formation of a virtual capture state with two electron holes in the 1s shell, an internal bremsstrahlung (IB) photon is emitted accompanied by an electron transition from the 2p to the 1s shell. The IB photon is emitted at an energy E IB ofwhere m is the difference in the initial and final atomic masses, E ex is the excitation energy in the daughter nucleus, and E(e i ) is the binding energy of the resulting electron hole in the final state. The normal IB process involves a transition of one of the electrons to an intermediate state from which it is captured, and this process dominates for large Q values. However, for small Q values, where both electrons may be captured leading to a virtual two-electron-hole atom, the process has a resonant enhancement when Q ≡ m − E(1S) − E(2P ) − E ex = Q res ≡ E(2P ) − E(1S); i.e., the radiative IB photon energy matches the K α hypersatellite x-ray energy [6]. In this case,and for favorable cases where Q = Q res , λ 0ν may be enhanced by several orders of magnitude [6].

On the robustness of surface vibrational modes: case studies in the Cd region

“…Figure 4 shows the low-lying excited states in 112 Cd grouped on the basis of B(E2) values [6,7]. Evidently, the pattern does not match that shown in figure 3.…”

“…In particular, at the two-phonon level, the B(E2) value for decay from the 2 + state is a factor of 4 weaker than expected, and that of the 0 + state is extraordinarily small. At the three-phonon level, the 0 + state has no observed decay to the two-phonon 2 + state [7,8], and the 2 + member has an enhanced decay to the 0 + member of the two-phonon triplet only. This disagreement is in marked contrast to the similarities between figure 1 and figure 2.…”

Shape coexistence at low spin in theZ= 50 region and its spectroscopic signatures

“…An intensive program of high-statistics β-decay studies of the Cd isotopes, 116 Sn, and extending into the Xe isotopes has been initiated at TRIUMF-ISAC. Experiments performed to date include the β-decay of 110 In [47,48,85], 112 Ag/In [86,87], 116 In, and 122,124,126 Cs [88,89] studied with the p 8 spectrometer. A further study of 110 Cd via the decay of a long-lived source 110 Ag has been performed at Oak Ridge [90], and studies of the neutron-rich Cd isotopes 116,120,124,126 Cd [91][92][93] have been performed.…”

Double beta decays of 124Xe investigated in the QRPA framework

“…Almost all of these calculations considered transitions to the final ground states only. Q-value measurements and nuclear-structure calculations associated with the R0νECEC processes were performed for 112 Sn in [22,23], for 74 Se in [24], for 136 Ce in [25], for 106 Cd in [26,27], for 96 Ru in [28,29] and for 152 Gd in [30]. Nuclear-structure calculations for the R0νECEC decays to the final ground state of some of the heavy nuclei were performed in [31].…”