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Pandemonium vs structure: Beyond-neutron-threshold spectroscopy at ALTO

David Verney (Institut de Physique Nucléaire d'Orsay, France)

11h00 GANIL seminar room (105)

a coffee will be served 15mn before


The possible existence of low-lying structures in the β-strength function above the neutron-separation energy (Sn) had given rise to—sometimes bitter—debates in the past, opposing those who believed in a completely statistical nature for the decay of states located above the threshold (the “nuclear Pandemonium”) and those who, on the contrary, believed that structure effects should drive the observed particle and γ emissions.

There has been recently growing evidence [1,2], from β-delayed gamma and neutron spectroscopy measurements on neutron-rich nuclei, seemingly pointing towards the existence of specific regions of the daughter nucleus excitation spectrum that clearly concentrate larger fractions of the β-strength. These regions significantly overlap with regions where highly collective excitation modes (typically E1) of the nucleus are expected to lie, and the question of the nature of unbound states populated by high-Qβ radioactivity was raised again.

We have recently launched an experimental program at ALTO to investigate the structure of the neutron-threshold region. This endeavor was further encouraged by two remarkable serendipities. The first one concerns the unexpected observation of “ultra”-high-energy γ-rays (8-9 MeV) [3] in the β-delayed emission products of 83Ga (Z=31 ; N=52 ; T1/2=312 ms ; Qb=11.7 MeV) sources collected at the BEDO station [4]. The second one concerns β-delayed neutron-emission probability (Pn) measurements of the 82,83,84Ga (N=51,52,53) precursors [5] using the neutron counter TETRA [6] : quite unexpectedly, after a steep increase of the Pn values from N=51 to 52, the Pn falls down again at N=53 by a factor ~2.

These results will be presented and discussed. It will be shown that they clearly point towards the existence of structures in the threshold region of the daughter-nucleus excitation spectrum, governing the decay properties in the 78Ni region. Some of these structures might be related to the hypothetical emergence of a neutron skin beyond N=50. Current scenarios in several nuclear applications and astrophysics [7] can be substantially affected. In particular, the relative importance of first-forbidden transition in the 78Ni region may have long been overestimated.

Perspectives for further investigation of these questions at ALTO using the PARIS and MONSTER γ and neutron spectrometers coupled to the BEDO station will also be presented.


[1] M. Scheck, S. Mishev, V. Yu. Ponomarev et al. PRL 116, 132501 (2016)

[2] M. Madurga, S. V. Paulauskas, R. Grzywacz et al. PRL 117, 092502 (2016)

[3] A. Gottardo, D. Verney, I. Deloncle et al. PLB 772, 359 (2017)

[4] A. Etilé, D. Verney, N. N. Arsenyev et al. PRC 91, 064317 (2015)

[5] D. Verney, D. Testov, F. Ibrahim et al., PRC 95, 054320 (2017)

[6] D. Testov, D. Verney, B. Roussière et al., NIM A815, 96 (2016)

[7] T. Marketin, L. Huther and G. Martinez-Pinedo PRC 93, 025805 (2016)

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