In the framework of the INFN NUCL-Ex experiment, we extended the investigation on the decay of light nuclei at excitation energies above particle emission thresholds, by performing exclusive fusion-evaporation measurements. The

16O + 12C reaction was investigated at three different bombarding energies, 90.5, 110 and 130 MeV. For complete fusion, such reactions lead to a fused 28Si∗ compound

nucleus respectively at 55, 63 and 72 MeV excitation energy. By investigating this autoconjugate system we put into evidence the role of non-statistical effects, also

clearly observed in our previous studies in lighter systems

Bruno, M.

Camaiani, A.

D'Agostino, M.

Frosin, C.

Morelli, L.

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DOI 10.1393/ncc/i2019-19099-4Colloquia: EuNPC 2018IL NUOVO CIMENTO 42 C (2019) 99Deviations from Hauser-Feshbach behaviour in evaporation chainsin light heavy-ion collisionsC. Frosin(1)(2), L. Morelli(3)(4)(12), M. Bruno(3)(4), M. D’Agostino(3)(4),A. Camaiani(1)(2), G. Casini(1), S. Barlini(1)(2), S. Piantelli(1), G. Baiocco(8),A. Buccola(1)(2), M.Bini (1)(2), C. Ciampi(1)(2), M. Cinausero(5),M. Cicerchia(5)(6), D. Fabris(7), F. Gramegna(5), F. Gulminelli(9),G. Mantovani(5)(6), T. Marchi(5), I. Lombardo(10), P. Ottanelli(1)(2),G. Pasquali(1)(2), G. Poggi(1)(2), S. Valdrè(2) and G. Verde(10)(11)(1) INFN, Sezione di Firenze - Firenze, Italy(2) Dipartimento di Fisica, Università di Firenze - Firenze, Italy(3) INFN, Sezione di Bologna - Bologna, Italy(4) Dipartimento di Fisica, Università di Bologna - Bologna, Italy(5) INFN, Laboratori Nazionali di Legnaro - Padova, Italy(6) Dipartimento di Fisica, Università di Padova - Padova, Italy(7) INFN, Sezione di Padova - Padova, Italy(8) Dipartimento di Fisica, Università di Pavia - Pavia, Italy(9) LPC (IN2P3-CNRS/Ensicaen et Université) - F-14076 Caen Cedex, Caen, France(10) INFN, Sezione di Catania - Catania, Italy(11) Dipartimento di Fisica, Università di Catania - Catania, Italy(12) GANIL, CEA/DSM-CNRS/IN2P3 - F14076 Caen Cedex, Francereceived 5 February 2019Summary. — In the framework of the INFN NUCL-Ex experiment, we extendedthe investigation on the decay of light nuclei at excitation energies above particleemission thresholds, by performing exclusive fusion-evaporation measurements. The16O + 12C reaction was investigated at three different bombarding energies, 90.5, 110and 130 MeV. For complete fusion, such reactions lead to a fused 28Si∗ compoundnucleus respectively at 55, 63 and 72 MeV excitation energy. By investigating thisautoconjugate system we put into evidence the role of non-statistical effects, alsoclearly observed in our previous studies in lighter systems.1. – IntroductionThe statistical decay model, based on the Hauser-Feshbach formalism [1], is a wellknown theory used for describing the fusion-evaporation mechanism of the compoundnucleus formed in central collisions. The detailed output and the decay channels ofCreative Commons Attribution 4.0 License (http://creativecommons.org/licenses/by/4.0) 12 C. FROSIN et al.the reaction are predicted based on the knowledge of the level densities and trasmissioncoefficients. Nevertheless, nuclear structure signatures are especially evident in lightnuclei, even at high excitation energy. In particular, according to the Ikeda diagrams [2],α-clustered excited states are expected at excitation energies close to the multi-alphadecay threshold in even-even N = Z nuclei.By using an exclusive channel selection and a highly constrained statistical code, it ispossible to put into evidence deviations from the statistical behaviour in the decay of thehot fused source formed in the collision. For this purpose we employed two statisticaldecay Monte-Carlo codes. The first is Hauser-Feshbach light (HF) [3], a dedicatedcode designed by the collaboration where the statistical ingredients are optimized for thedescription of light nuclei (A < 40). The code explicitly includes all the experimentallymeasured excited levels from the archive NUDAT2. The second one is Gemini++ [4], astandard code employed in nuclear physics for describing the decay of hot nuclei.2. – The experimentThe experiment was performed at the LNL (Laboratori Nazionali di Legnaro) usinga pulsed beam of 16O delivered by the XTU TANDEM accelerator at 90.5, 110 and130 MeV. The employed apparatus is composed of the coupling of two detectors, namelyGARFIELD and RingCounter (RCo). Hereafter, we will recall a few main features of theapparatus while a detailed description is given in [5]. GARFIELD is a two stage detectorwhich covers the polar region 30◦ < θ < 170◦. It is made of an ionization drift chamberand CsI(Tl) scintillators. Light reaction particles can be identified in charge and masswith a threshold around 1 AMeV and the energy can be determined up to a few percentaccuracy [5]. The RCo is an array of three telescopes made by an ionization chamber(IC), silicon strips (Si) and CsI(Tl) scintillators which covers the most forward angles(5◦ < θ < 17◦). The charged particle and fragment identification is performed throughthe ΔE-E technique in IC-Si with a threshold of around 0.8÷1 AMeV. Mass identificationfor the light particles in also possible through either ΔE-E in Si-CsI and/or pulse shapeanalysis in the CsI with a threshold of 6 AMeV [5]. The combination of the two devicesallows a nearly-4π coverage (around 70%), which permits to perform complete chargedetection of the events and to discriminate the different reaction mechanisms.3. – ResultsExperimentally, the selection of the fusion-evaporation mechanism is based on thecoincidence between light charged particles (LCP) and a fragment (evaporation residue)detected at forward angles by RCo. Moreover, we imposed charge and longitudinalmomentum conservation which characterizes the complete detection of fusion events. Wecompared the predictions of the Monte-Carlo codes, filtered through a software replicaof the apparatus, to the experimental data. In the first part of the analysis we focusedon inclusive observable which describe the general behaviour of the reaction and thenwe proceeded to investigate more exclusive observables by selecting the various decaychannels. The statistical models are generally able to reproduce globally the experimentaldata but clear differences can be noticed for more exclusive observables such as the α-particle energy spectra detected in coincidence with even-Z residues shown in fig.1. Onecan observe for example the Zres = 10 (Neon) case where the energy tails for α-particlesare not reproduced by the models for all three energy cases and also for Zres = 8 (Oxygen)at 130 MeV. In order to understand better these differences, we discriminated further theDEVIATIONS FROM HAUSER-FESHBACH BEHAVIOUR IN EVAPORATION CHAINS ETC. 350 100Eα(MeV)410310210110Yield (arb. units)410310210110Yield (arb. units)410310210110Yield (arb. units) =7resZ50 100Eα(MeV)=8resZ50 100Eα(MeV)=9resZ50 100Eα(MeV)=10resZ50 100Eα(MeV)=11resZ50 100Eα(MeV)=12resZFig. 1. – Laboratory energy spectra for α-particles detected in coincidence with a residue ofcharge Zres, indicated in each pad, for 90.5 (top), 110 (middle) and 130 (bottom) MeV . Blackdots represent the experimental data while the red and green line are the predictions of HFand Gemini++, respectively. In each panel the distributions are normalized to unitary area fora better shape comparison.different channels which contribute to each Zres spectra. Table I contains the branchingratio for the channels containing the maximum number of allowed α-particles for theselected Zres. Since the excitation energy of the CN is large compared to the neutron/semission threshold, the decay channels are shown with the possible number “xn” ofemitted neutrons. The obtained values are in agreement with the statistical predictionsfor odd-Z residues, with the exception of 23−xnNa+α+p+xn channel at 130 MeV, whileeven-Z are clearly underestimated by both models for most of the studied energies andTable I. – For Zres from Nitrogen to Magnesium, the branching ratio for the channels containingthe maximum number of allowed α-particles are reported. Measured values are compared, foreach bombarding energy, to model calculations (HF and Gemini++). Errors on the experimentalvalues (about 5%) take into account both statistical error and the systematics due to the uncorrectparticle identification.90.5 MeV(%) 110 MeV(%) 130 MeV(%)Zres Channels Exp HF Gem Exp HF Gem Exp HF Gem7 15−xnN+xn+p+3α 100 100 94 98 99 96 95 97 958 16−xnO+xn+3α 100 99 91 99 87 63 88 43 319 19−xnF+xn+p+2α 99 99 99 93 93 96 88 89 8310 20−xnNe+xn+2α 74 17 13 45 6 9 29 2 411 23−xnNa+xn+p+α 95 95 91 93 87 85 88 55 6112 24−xnMg+xn+α 53 11 18 35 5 8 28 3 34 C. FROSIN et al.20 40 60 80 100Eα(MeV)410310210110Yield (arb. units)90.5 MeV20 40 60 80 100Eα(MeV)110 MeV20 40 60 80 100Eα(MeV)130 MeVFig. 2. – Laboratory energy spectra for α-particles detected in the 20−xnNe + 2α + xn channel.Black empty dots represent the experimental data while the red and green line are the predictionsof HF and Gemini++, respectively. In each panel the distributions are normalized to unitaryarea for a better shape comparison.Zres. For these cases, in principle one can think of the shape differences in fig.1 as dueto the different evaluation of the branching ratios in the experimental data and by thestatistical decay models. In fact, one can look at the single α-particles energy spectrafor the specific channels of table I. In fig.2, the case of 20−xnNe + 2α + xn is shownas an example at the three energies. One can notice that the shape still presents cleardifferences with respect to the statistical models which are more evident at the highestenergy. This means that the kinematics of the decay, for these specific channels, needsto be further studied in order to determine if non-statistical correlations are involved.4. – ConclusionsWe have compared experimental data for the 16O+12C reaction at 90.5, 110 and 130MeV with the results of two Hauser-Feshbach statistical calculations for the decay of the28Si∗ compound nucleus. The measured data are compatible with the expected behaviorof a complete fusion-evaporation reaction. For specific channels, corresponding to theemission of α-particles in coincidence with an even-Z residue, a higher branching ratiowas measured with respect to the statistical decay models. These preliminary results, inanalogy to the findings of [6,7], can suggest that alpha correlations can play a role in thereaction stage or in the further evaporative decay.REFERENCES[1] Hauser W. and Feshbach H., Phys. Rev., 87 (1952) 366;[2] Horiuchi H. and Ikeda .K, Prog. Theor. Phys., 40 (1968) 277;[3] Baiocco G. et al., Phys. Rev. C, 87 (2013) 054614;[4] Charity R. J. et al., J. Phys. Rev. C, 82 (2010) 014610;[5] Bruno M. et al., Eur. Phys. J. A, 49 (2003) 128;[6] Morelli L. et al. , J. Phys. G: Nucl. Part. Phys., 41 (2014) 075107; 41 (2014) 075108.[7] Camaiani A. et al. , Phys. Rev. C, 97 (2018) 044607;

Deviations from Hauser-Feshbach behaviour in evaporation chains in light heavy-ion collisions

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Deviations from Hauser-Feshbach behaviour in evaporation chains in light heavy-ion collisions

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