A Problem in the Statistical Description of Beta-Delayed Neutron Emission Oliver Gorton12Calvin Johnson1 and Jutta Escher2 1San Diego State University California USA

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A Problem in the Statistical Description of Beta-Delayed Neutron Emission

Oliver Gorton1,2,∗,Calvin Johnson1,∗∗ , and Jutta Escher2,∗∗∗

1San Diego State University, California, USA

2Lawrence Livermore National Laboratory, California, USA

Abstract. Reaction measurements on ﬁssion products are being planned at both Argonne National Lab and

at the Facility for Rare Isotope Beams. These indirect experiments produce speciﬁc short-lived nuclei via beta

decay, and the subsequent neutron and gamma emission are studied. Some initial experiments found a surprising

overabundance of gamma emission, which theory has yet to explain. To remedy this, we are developing an

integrated nuclear data workﬂow that connects advanced nuclear shell model codes for describing the beta

decay with a contemporary nuclear reaction model code.

Beta decay is the mechanism for element transmuta-

tion towards stability, and plays an important role in com-

petition with neutron-capture in the formation of heavy

elements: Understanding this is one of the central tasks

of the nuclear theory community and the new Facility for

Rare Isotope Beams (FRIB) [1, 2]. A less common, yet

important [3] process is the emission of one or more neu-

trons immediately following beta decay in a process called

beta-delayed neutron emission (BDNE), depicted in Fig-

ure 1.

Figure 1. Schematic depiction of beta-delayed neutron emission

for the case of 94Rb. The distribution of states populated by beta

decay can be predicted with a microscopic structure model like

quasiparticle random-phase approximation or shell model. The

decay of the neutron and gamma emitting nucleus (94Sr) is his-

torically described by a statistical model, assuming compound

nuclear decay.

∗e-mail: ogorton@sdsu.edu

∗∗ e-mail: cjohnson@sdsu.edu

∗∗∗ e-mail: escher1@llnl.gov

1 Statistical description of BDNE does not

explain experiments

Theoretical description of BDNE has been an ongoing

challenge since the late 1970’s [4]. The idea of selectiv-

ity[4] of states populated by beta decay, leading to non-

statistical neutron emission, was proposed to explain peak-

features seen in delayed neutron spectra. These eﬀects

were almost explained away by the Pandemonium eﬀect

[5], which indicated that such peaks were artifacts due to

detector limitations. Others showed that in some cases,

(a) statistical models of BDNE could reproduce the peaks

if excited states in the residual nucleus were included

[6], and (b) that states populated in beta decay could be

strongly connected to excited states in the ﬁnal nucleus

[7], supporting selectivity. At the close of the decade, the

nuances were better understood, including the importance

beta-decay strength function shape and nuclear level den-

sities [8, 9].

With the development of total absorption gamma-ray

spectroscopy (TAGS) [10, 11] and its application to the

study of BDNE [12–14], the Pandemonium eﬀect can be

avoided. Even so, the statistical description in some cases

signiﬁcantly under-predicts the intensity of emitted pho-

tons from TAGS experiments, e.g. in [13]. Thus, BDNE

remains an important and unresolved modeling challenge.

There are three hypotheses which may explain this.

Hyp. 1: Photon-decay strength function is stronger

than assumed. This is the simplest explanation: if the

photon-strength function is enhanced, the nucleus formed

by beta decay will de-excite below the neutron separa-

tion energy before delayed neutrons are emitted. If this

is the explanation, then many neutron capture rates will

also have to be re-evaluated, especially for astrophysical

interests.

Hyp. 2: Forbidden beta decay is stronger than as-

sumed, blocking high-l neutron emission. If forbidden

transitions play a signiﬁcant role in these neutron-rich

arXiv:2210.05904v2 [nucl-th] 22 Oct 2022

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AProblemintheStatisticalDescriptionofBeta-DelayedNeutronEmissionOliverGorton1;2;,CalvinJohnson1;,andJuttaEscher2;1SanDiegoStateUniversity,California,USA2LawrenceLivermoreNationalLaboratory,California,USAAbstract.ReactionmeasurementsonssionproductsarebeingplannedatbothArgonneNationalLabandatth...

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A Problem in the Statistical Description of Beta-Delayed Neutron Emission Oliver Gorton12Calvin Johnson1 and Jutta Escher2 1San Diego State University California USA

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