application/xmlEvidences for resonance states in 5HM.S GolovkovYu.Ts OganessianD.D BogdanovA.S FomichevA.M RodinS.I SidorchukR.S SlepnevS.V StepantsovG.M Ter-AkopianR WolskiV.A GorshkovM.L ChelnokovM.G ItkisE.M KozulinA.A BogatchevN.A KondratievI.V KorzyukovA.A YukhimchukV.V PerevozchikovYu.I VinogradovS.K GrishechkinA.M DeminS.V ZlatoustovskiyA.V KuryakinS.V Fil'chaginR.I Il'kayevF HanappeT MaternaL StuttgeA.H NinaneA.A KorsheninnikovE.Yu NikolskiiI TanihataP Roussel-ChomazW MittigN AlamanosV LapouxE.C PollaccoL NalpasTritium targetTritium beamTwo-neutron transfer reactionPhysics Letters B 566 (2003) 70-75. doi:10.1016/S0370-2693(03)00803-7journalPhysics Letters BCopyright © unknown. Published by Elsevier B.V.Elsevier B.V.0370-26935661-224 July 20032003-07-2470-75707510.1016/S0370-2693(03)00803-7http://dx.doi.org/10.1016/S0370-2693(03)00803-7doi:10.1016/S0370-2693(03)00803-7http://vtw.elsevier.com/data/voc/oa/OpenAccessStatus#Full2014-01-01T00:14:32ZSCOAP3 - Sponsoring Consortium for Open Access Publishing in Particle Physicshttp://vtw.elsevier.com/data/voc/oa/SponsorType#FundingBodyhttp://creativecommons.org/licenses/by/3.0/JournalsS300.3PLB19936S0370-2693(03)00803-710.1016/S0370-2693(03)00803-7ExperimentsFig. 1Two-dimensional plot of proton–triton coincidences from the t(t,pt) reaction presented as the 5H energy versus the triton energy in the rest frame of the 5H system. The events associated with the t+t→p+t+2n reaction are confined in the triangle area shown in the picture. The projection of the area marked by the triangle contour is shown as inset.Fig. 2Missing mass energy spectrum of the 5H nucleus from the t(t,ptn) reaction. The thick solid line corresponds to the best fit with two narrow 5H resonance states. Curves 1, 2 and 3 show, respectively, the simulation of the three-particle phase space, n–n final state interaction and t–n final state interaction. The dotted curve shows the detection efficiency, calculated for the triple, p+t+n coincidence events, as a function of the 5H decay energy (see the text).Evidences for resonance states in 5HM.SGolovkovaYu.TsOganessianaD.DBogdanova1A.SFomichevaA.MRodinaS.ISidorchukaR.SSlepnevaS.VStepantsovaG.MTer-Akopiana∗gurgen.terakopian@jinr.ruRWolskiaV.AGorshkovaM.LChelnokovaM.GItkisaE.MKozulinaA.ABogatchevaN.AKondratievaI.VKorzyukovaA.AYukhimchukbV.VPerevozchikovbYu.IVinogradovbS.KGrishechkinbA.MDeminbS.VZlatoustovskiybA.VKuryakinbS.VFil'chaginbR.IIl'kayevbFHanappecTMaternacLStuttgedA.HNinaneeA.AKorsheninnikovfgE.YuNikolskiifgITanihatafPRoussel-ChomazhWMittighNAlamanosiVLapouxiE.CPollaccoiLNalpasiaFlerov Laboratory of Nuclear Reactions, JINR, 141980 Dubna, RussiabRussian Nuclear Federal Center, All-Russian Research Institute of Experimental Physics, 607190 Sarov, Nizhni Novgorod Region, RussiacUniversite Libre de Bruxelles, B-1050 Bruxelles, BelgiumdInstitut de Recherches Subatomiques, F-67037 Strasbourg Cedex, FranceeUniversity of Louvain, Nuclear Physics Department Ch. du cyclotron, 2-1348 Louvain-la-Neuve, BelgiumfRIKEN, Hirosawa 2-1, Wako, Saitama 351-0198, JapangOn leave from the Kurchatov Institute, Kurchatov sq. 1, 123182 Moscow, RussiahGANIL, BP 5027, F-14076 Caen Cedex 5, FranceiDSM/DAPNIA/SPhN, CEA Saclay, F-91191 Gif-sur-Yvette Cedex, France∗Corresponding author.1Deceased.Editor: V. MetagAbstractResonance states of 5H were investigated through the two-neutron transfer reaction t(t,p)5H. A triton beam at 57.5 MeV and a cryogenic liquid tritium target were used. The 5H missing mass spectrum in triple coincidence, proton+triton+neutron, shows a resonance at 1.8±0.1 MeV above the t+2n decay threshold. This energy is in good agreement with the result reported in Phys. Rev. Lett. 87 (2001) 092501. The resonance width, Γintr⩽0.5 MeV, is surprisingly small and difficult to reconcile with theory predictions.PACS27.10.+h25.60.JeKeywordsTritium targetTritium beamTwo-neutron transfer reactionInterest in the hydrogen isotopes heavier than tritium has been continual over the last forty years. This is more than mere part of the quest of neutron drip-line nuclei: indeed, the super-heavy isotopes of hydrogen are also the nuclear systems with the most extreme excess of neutrons ever attained for a direct study. The progress in experimental technique including the use of secondary beams of radioactive nuclei opens favorable conditions for the study of such exotic nuclear systems lying beyond the neutron drip line. The result of the first experiment devoted to the search for the 5H nucleus in the p(6He,2He) transfer reaction [1] was basically different from earlier experimental observations made with stable nuclear beams [2] and slow pions [3]. The information about a resonance state of 5H observed in [1] at 1.7±0.3 MeV above the t+2n decay threshold revived interest in this nucleus in theory and experiment.Earlier experimental results about 5H were compiled in Ref. [4], where the resonance position (ER=7.4 MeV) and width (Γ∼8 MeV) from Ref. [3] were recommended. Recently these authors reported on new measurements made for the 9Be(π−,pt) reaction with improved resolution and better statistics and claimed the observation of four resonances in 5H at 5, 10, 18 and 26 MeV above the t+2n threshold [5]. The latest compilation of experimental data concerning 5H was presented in Ref. [6]. More recently 5H was studied by the invariant mass method, in proton removal reaction of relativistic 6He on a carbon target [7]. The authors reported on the observation of a broad (Γ∼5 MeV) peak centered at ER∼3 MeV.Recent theoretical estimations made in three-body models in the framework of the hyperspherical harmonics expansion [8] and the generator coordinate method [9] predict the position of the 5H ground state at 2.5–3 MeV, with the resonance width varying between 1 and 4 MeV. Calculations employing the hyperspherical functions method for the 5H nucleus treated as a five-body system [10] predict a lower energy for the resonance state, 1.14 MeV above the t+2n threshold.In the present study, we apply the missing mass method to search for the 5H resonance states populated in the t(t,p)5H reaction. The choice of this reaction was based on several reasons. In the p(6He,2He)5H reaction, the pickup of one proton from 6He should selectively populate the 1/2+ ground state in 5H [1]. The t(t,p)5H reaction should populate excited states in 5H as well as the ground state. Furthermore, a better resolution for the resonance width and a larger range for the measured 5H excitation energy, as compared to the p(6He,2He) reaction, were expected from the Monte-Carlo simulations of our experimental conditions.First attempt to obtain 5H resonance states using the t+t reaction had been made using a 22.25 MeV triton beam [11]. The authors reported on a peak in the 5H missing mass spectrum obtained from inclusive proton data. They tried to fit this spectrum assuming either a 5H resonance state occurring at 1.8 MeV above the t+2n threshold (Γ∼1.2 MeV) or a phase-space curve. Their conclusion was that the difference between the phase space and resonance curves was “not exceedingly great” and that additional data obtained at higher triton bombarding energy were needed.The present experiment was performed with a 57.5 MeV triton beam, with triple coincidences of protons and decay products of 5H, i.e., tritons and neutrons, instead of proton singles in [11].The experiment was performed at the U-400M cyclotron of FLNR JINR (Dubna, Russia). The 58 MeV triton beam delivered by the cyclotron was transported by the modified beam line of the ACCULINNA separator [12] to the scattering chamber containing the tritium target and charged particle detectors. This beam line was also used to reduce the angular and energy spreads of the primary triton beam to 7 mrad and 0.3 MeV (FWHM), respectively. Finally, the triton beam with a typical intensity of 3×107 s−1 was focused in a 5 mm spot on the environmentally safe liquid tritium target [13].The 0.4 mm-thick target cell was hermetically sealed by 12.5 μm-thick stainless steel entrance and exit windows. The target cell was embedded in a small protective volume with windows of the same thickness, acting as the second security barrier. The target gas composition was 92% tritium, with a contamination of 7% deuterium and 1% hydrogen. Being tightly fixed on the cold finger of a cryo-cooler the target cell was surrounded by a thermal screen. The working temperature of the target cell was close to 20 K.Charged particles were detected by two telescopes. The first telescope, installed on one side of the beam axis, was dedicated to the detection of the protons of energy below 40 MeV. It was a four-stage telescope of one 400 μm and two 1 mm-thick Si strip detectors followed by a 6 mm thick Si(Li) detector, covering forward angles between 18° and 32°. On the opposite side, the second telescope for the triton detection covered angles between 15° and 39°. It consisted of a 70 μm, a 400 μm and two 1 mm-thick Si strip detectors, and was optimized to detect relatively slow tritons originating from the 5H decay. Neutrons were detected by 41 scintillation modules of the time-of-flight neutron spectrometer DEMON [14]. The DEMON modules were installed behind the triton telescope at a distance of 2.5 m from the target and covered an angular range of θlab=37°±19°. Data acquisition was triggered when time correlated signals came either from both telescopes or from the proton telescope and one DEMON module. The experimental resolution was estimated to be about 400 keV for the 5H resonance states lying up to 5.0 MeV above the t+2n threshold.In the missing mass method, the detection of proton singles is in principle sufficient to determine the excitation energy of the 5H nucleus produced in the t(t,p)5H reaction. However, the background from target windows and other reaction channels of the t+t→p+X type left no chance to see any structure in the inclusive proton spectrum. This background in the spectra was reduced by the coincidence of protons with tritons resulting from the decay of 5H. A separate experiment made with an empty target cell confirmed that the background produced by the target windows in the proton–triton coincidence spectrum was negligible. In Fig. 1 a two-dimensional plot is presented, showing the triton kinetic energy in the 5H rest frame (Et) versus the missing mass energy of the 5H nucleus (E5H). Since the E5H energy is equal to the energy released in the 5H→t+2n decay, the kinetic energy of tritons associated with 5H should satisfy the condition Et⩽25E5H. In other words, all events associated with the t+t interaction should be confined in the triangle area, marked in Fig. 1. The most intense spot in the upper left corner of the plot corresponds to the elastic scattering of tritons on the hydrogen that is present as admixture in the tritium target and in water molecules deposited on the heat screen of the target cell. A separate experiment made with the target cell filled with pure deuterium showed that the other events observed in Fig. 1 outside the triangle area originate from the interaction of incident tritons with the deuterium impurity in the target. The projected spectrum obtained for the area confined in the marked triangle is shown as inset in Fig. 1. No prominent structure can be seen in this spectrum.Our analysis showed that the quasi-free scattering (QFS) of projectile tritons from protons bound in the tritium target nuclei gives the main contribution to the events in the triangle of Fig. 1. It was shown (see, e.g., Ref. [15]) that the QFS process is important in nuclear collisions occurring at Ec.m.∼10–50 MeV. In the present case, the two spectator neutrons confined in the tritium target nucleus acquire low energy in the QFS process. This implies that the contribution of QFS can be reduced by selecting high-energy neutrons.Taking into account this consideration, the analysis for triple p+t+n coincidence events was made with the selection of neutrons having energy higher than 2.5 MeV. The missing mass energy spectrum obtained from these events for the 5H system is shown in Fig. 2.This spectrum is qualitatively different from the missing mass energy spectrum derived for 5H from the p–t coincidence events (see inset in Fig. 1). The elimination of the QFS process from the spectrum presented in Fig. 2 is one reason for this difference. Another reason is the dependence of the detection efficiency on the 5H decay energy. This dependence was estimated by a Monte-Carlo simulation for the p–t–n coincidences, with a 2.5 MeV energy threshold for the neutron detection. An isotropic c.m. angular distribution was taken for the t(t,p)5H reaction, as indicated by the data, and phase space distribution was assumed for the 5H decay products. The sensitivity to the shape of this distribution is low due to the large angular acceptance. The dotted curve in Fig. 2 shows the resulting detection efficiency, which presents a smooth pattern. In spite of the relatively low statistics, due to the triple coincidence condition, a narrow peak is seen at 1.8 MeV in the data shown in Fig. 2. The steep rise in the spectrum observed between 2 and 3 MeV is a certain indication for a second resonance at 2.7±0.1 MeV.Data shown in Fig. 2 were analyzed by a complete Monte-Carlo simulation, taking into account the instrumental resolution and detection efficiency. The continuum in the spectrum was supposed to originate from the three-body (t+n+n) phase space, as well as the n+n and t+n final state interactions (FSI). The n+n FSI was modelled using the known scattering length, ann=18.5 fm, and the t+n FSI with the resonance parameters of 4H taken from Ref. [16]. The analysis showed that these processes explain the bulk of the spectrum shown in Fig. 2, the main contribution arising from the one-neutron transfer reaction t(t,pn)4H. However, it is not possible to get a pronounced peak in the spectrum without including a sharp 5H resonance.The thick solid line drawn in Fig. 2 shows the fitting result obtained with the assumption that two peaks positioned at 1.8 MeV and 2.7 MeV are present in the spectrum, both peaks having a width equal to the instrumental resolution (FWHM=400 keV). From this result we conclude that we have found evidence for a resonance state of 5H at 1.8±0.1 MeV above the t+2n decay threshold, with a statistical significance of about 2 standard deviations. A striking feature of this resonance state is its small width governed by the instrumental resolution. Lack of statistics allows to give only an upper limit of 0.5 MeV for the true width of the resonance.The position of the peak obtained at 1.8±0.1 MeV is in a good agreement with the 5H ground-state resonance reported at 1.7±0.3 MeV in Ref. [1]. Our present estimation of the 0.5 MeV upper limit for the resonance width may be compatible with the value Γobs=1.9±0.4 MeV reported by the authors of Ref. [1], if their 1.3 MeV instrumental resolution is somewhat underestimated. We cannot exclude that in our present experiment we observe a narrower structure due to interference with the next excited state of 5H. This last question will be the subject of future investigation.The peak at 2.7 MeV is a candidate for the 5/2+ state, member of the 5/2+–3/2+ doublet of excited states, which can be anticipated for 5H by analogy with the neighbor neutron-halo 6He nucleus (see Ref. [8]). One must say however that the presence of this peak in the spectrum shown in Fig. 2 is ascertained with a low statistical significance (it is less than two standard deviations).The cross sections of the two-neutron transfer reactions populating the ground (E5H=1.8 MeV) and excited (E5H=2.7 MeV) 5H states are 18+20−10 μb/sr and 35+20−20 μb/sr, respectively. These estimated cross sections are based on the data averaged over the angular range covered by our measurements. The main contribution in the errors comes from the uncertainty in the neutron detection efficiency. These cross sections are one order of magnitude lower than the predictions of DWBA calculations.The two reactions, p(6He,2He)5H and t(t,p)5H, should be different with respect to the population probability obtained for different resonance states in the 5H nucleus. As discussed above, the simplest mechanism in the case of the first reaction is the pick-up of a 1s proton from the ground state of 6He. The experimental evidence confirming this mechanism was presented in Ref. [1]. Hence, the 1/2+ state of 5H should be preferentially populated in the reaction p(6He,2He)5H. Various theoretical estimations have shown that this is the ground state of 5H. DWBA calculations give cross section values of the order of a few mb/sr for this reaction at forward angles. The reaction t(t,p)5H involves the transfer of the two neutrons. There is no preference in this reaction for the population of the ground state in 5H. The predicted (5/2+,3/2+) doublet of excited states should also be populated. DWBA calculations made for the one step transfer of a dineutron in singlet state give a few-hundreds μb/sr cross section for the 5H ground state population. Similar calculations made for the two-neutron transfer reaction p(6He,α)t were in reasonable agreement with experimental data [17].For both reactions, p(6He,2He)5H and t(t,p)5H, the measured absolute cross section values are at least by a factor of 20 less than the DWBA predictions. Two reasons can explain this discrepancy for the case of the p(6He,2He)5H reaction: (i) the incoming and outgoing particles (the proton and the 2He proton pair, respectively) may interact with halo neutrons of the 6He nucleus thus reducing the 5H survival probability, (ii) the ground state of the 5H nucleus may have a specific structure, e.g., being very extended in space. Generally speaking, it may have a structure different from t+2n. This would result in a rather small spectroscopic factor for the one proton pick-up reaction p(6He,2He)5H and could be the origin of the cross section reduction for the t(t,p)5H reaction.The small widths of the two 5H resonance states obtained in the present work is striking because the theoretical models treating this nucleus as a three-body system [8,9] predict that the resonance widths should not be less than ∼1 MeV. However, the results obtained with the algebraic version of the resonating-group method [18] show that values approaching few hundreds keV are reasonable for the 5H resonance states lying below 2 MeV. One should notice, however, that similar estimations for 6Be considerably underestimate the width of this system [18]. Therefore, the question concerning the widths of 5H resonances remains a challenge both for theory and experiment.In summary, we have searched for 5H resonance states in the two-neutron transfer reaction t(t,p)5H at 57.5 MeV. We observed, with a confidence level of 2 standard deviations, a peak at 1.8±0.1 MeV above the t+2n decay threshold in the missing mass spectrum obtained in the triple p+t+n coincidence events. The peak position is in agreement with the previous observation [1]. The better resolution obtained in the present measurement allows us to set an upper limit of 0.5 MeV for the intrinsic width of this resonance state. 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