Study of the Neutron-rich Isotope $^{46}$Ar Through Intermediate Energy Coulomb Excitation

Abstract

For certain combinations of protons and neutrons, there is a theoretical expectation that the shape of nuclei can assume octupole deformation, which would give rise to reflection asymmetry or a "pear shape" in the intrinsic frame, either dynamically (octupole vibrations) or statically (permanent octupole deformation). In this paper, I will briefly review the historic evidence for reflection asymmetry in nuclei, describe how recent experiments carried out at REX-ISOLDE are constraining nuclear theory and how they contribute to tests of extensions of the Standard Model, and look at future prospects for this field.

Mass Measurements Demonstrate a StrongN=28Shell Gap in Argon

“…The existence of the N = 28 shell gap for argon is a matter of some controversy. Several previous experimental studies have assessed the shell structure of neutronrich argon [19][20][21][22][23][24][25][26][27][28][29]. Investigation of the energy of the lowest excited states of 45 18 Ar 27 via β-decay spectroscopy of 45 17 Cl 28 suggested a weakened, but still present, N = 28 shell closure for argon [21].…”

Spectroscopy ofAr46by the(t,p)two-neutron transfer reaction

“…The low 3/2 − 1 state has likely a complicated structure, involving also proton excitations [20] and can therefore not be regarded as a sign of a reduced shell gap. The B(E2; 2 + 1 → 0 + gs ) as determined by intermediate beam energy Coulomb excitation is rather small [4][5][6], a result in disagreement with the shell model calculations [11,12,21] as well as calculations using the generator coordinate method with the Gogny D1S interaction [22]. The latter calculations predict a coexistence of spherical and deformed states at low excitation energy.…”

“…The B(E2; 2 + 1 → 0 + gs ) value for 46 Ar (Z = 18) has been measured using Coulomb excitation at intermediate energies [4][5][6] as well as extracted from the measured lifetime [7] giving conflicting results. The value determined in the Coulomb excitation experiments (B(E2; 2 + 1 → 0 + gs ) = 39(8) e 2 fm 4 [4], 44(6) e 2 fm 4 [5], and 54(5) e 2 fm 4 [6]) points to a moderate deformation and collectivity in 46 Ar consistent with a the expectation for a semi-magic nucleus. This is supported by timedependent Hartree-Fock-Bogoliubov calculations [8] that link the increase in collectivity with respect to 48 Ca to a quenching of the N = 28 shell gap.…”

Single-particle structure atN=29: The structure ofAr47and first spectroscopy ofSGade1, Tostevin2, Bader3 et al. 2016Phys. Rev. C15050View full textAdd to dashboardCite

“…While some observables, for example those re-lated to the masses of the Ar isotopes around N = 28 [13], are well described, calculations appear to over-predict the B(E2) excitation strength to the first 2 + state [2,[14][15][16]. This result is not without controversy, on the experimental side, where an excited-state lifetime measurement yields higher collectivity, in agreement with the shell model [17], but is at odds with several Coulomb excitation measurements that consistently yield a lower B(E2) value [2,14,16]. Using an inverse kinematics 46 Ar(d, p) reaction, states in 47 Ar with significant singleneutron spectroscopic strength were measured [18] and were used in the development of the SDPF-U SM effective interaction [11].…”