exotic nuclei structure and reaction noyaux exotiques ... - IPN - IN2P3

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First fast-timing timing experiment at ALTO: nuclear-state half-life life measurements in 137,139 Cs and 138 Ba. IPNO Participation: B. Roussière and the DA-ALTO group Collaboration : CSNSM-Orsay, Tandar CNEA-Buenos Aires, INRNE BAS-Sofia Le dispositif de ‘’fast-timing’’ développé pour mesurer, auprès d’ALTO, les vies moyennes des états excités des noyaux dans la gamme de quelques dizaines de ps à quelques ns a été testé en ligne. Les mesures ont été réalisées sur les niveaux excités de 137,139 Cs et 138 - Ba obtenus lors de la décroissance des noyaux 137-139 Xe. Les noyaux de Xénon riches en neutrons étaient produits par la photo-fission de 238 U induite en bombardant une cible épaisse de carbure d’uranium par un faisceau d’électrons (E = 50 MeV, I ≤ 500 nA). Les résultats obtenus sur la masse 138 ont permis, puisque les vies moyennes dans 138 Ba sont bien connues, de caractériser très précisément le dispositif expérimental et de valider les techniques d’analyse. Les vies moyennes des cinq premiers états excités de 139 Cs ainsi que celle du premier état excité de 137 Cs ont été mesurées pour la première fois. Introduction Nuclear-state half-lives, giving access to transition probabilities, provide direct insight into the structure of the nucleus and offer one of the most stringent test of the nuclear models. The fast-timing technique is well adapted to measure half-lives in the sub-nanosecond range. Developed at first with BaF 2 crystals [1], this method knows nowadays a revival of interest due to the advent of LaBr 3 scintillators [2] that have good spectroscopy properties (high scintillation light yield, fast emission and the best energy resolution among all scintillators). In the frame of our lanthanide nuclei study program at ALTO [3], we have developed a fast-timing experimental set-up and optimized, by off-line tests with radioactive sources, its efficiency for measuring half-lives in the [100 ps - some ns] range by the slope method [4]. To test on-line this fast-timing system, we have performed measurements in 137,139 Cs and 138 Ba nuclei obtained from -decay of 137,138,139 Xe that are well produced at PARRNe, either using the TANDEM deuteron beam or the low-intensity ALTO electron-beam. Experimental procedure 137-139 Xe were produced by photo-fission in a thick uranium carbide target using a 100 to 500 nA, 50 MeV electron beam. The target, composed of 122 UC x disks of 14 mm diameter and 1 mm thick, was associated with a MK5 hot plasma ion source. The ions were extracted under 30 kV, mass-separated and collected on a mylar-aluminium tape. The collected radioactive sources were then transported in front of the detection device consisting of one Ge detector and three scintillators (one PilotU plastic for detection, one BaF 2 and one LaBr 3 for detection, all three with 25.4 mm diameter and 3 mm, 30 mm and 10 mm length respectively). The yield of 139 Xe was measured to be 2×10 5 ions/ s. The measurement cycles were chosen in order to optimize the counting rates: for A = 139 t collection = t counting = 40 s, for A = 137 t collection = t counting = 4 m and for A = 138 t collection = 7.5 m and t counting = 30 m. Experimental results In 138 Ba, except the level located at 1898.7 keV (T 1/2 = 2.164(11) ns), the excited states most fed – by the decay of 138g Cs have very short half-lives [5]. Therefore, in view of the time properties of the detectors used, the transitions de-exciting these levels can be considered as prompt transitions and the 138 mass provides an ideal candidate for calibration purpose. Thus measurements on mass 138 allowed us i) to validate our experimental setup for the determination of half-lives in the ns range since, for the level located at 1898.7 keV, we have measured T 1/2 = 2.11 (5) ns, in excellent agreement with the value reported in the literature; and ii) to precisely characterize our fast-timing set-up, in particular by the determination of the promptcurve properties (position and width). Figure 1 shows the displacement of the center of gravity of the prompt curve as a function of the -ray energy when time measurement is performed between the and LaBr 3 or BaF 2 detectors or between the LaBr 3 and BaF 2 detectors; one can note in this case the excellent agreement between the values observed experimentally (red dots) and those extrapolated from - measurements (yellow grid), which points out the consistency of measurements Figure 1 19
Figure 2 a n d analysis. Figure 2 shows the intrinsic time resolution obtained for the BaF 2 and LaBr 3 detectors. In the 137 Xe→ 137 Cs decay, the most fed levels are the 7/2 + ground state (I = 67%) and the 5/2 + first excited state located at 455.5 keV (I = 31%). Only an upper limit (T 1/2 ≤ 0.1 ns) was known for the half -life of the 5/2 + state [6]. The good time resolution of our fast-timing system allowed us to determine precisely its value: T 1/2 = 59 (5) ps (see figure 3). In 139 Cs, we have determined, for the first time, the half-lives of the first five excited levels (see figure 4). For this nucleus, it was essential to use the triple coincidence technique where the timing information is obtained from the delayed coincidence between fast-timing detectors while the desired decay cascade is selected by a coincident -ray in the Ge detector. One can see in figure 4 that two cascades involving -rays of very close energy, 225-289 keV (drawn in red) and 296-218 keV (drawn in green), exist in the level scheme. With scintillators, despite the good energy resolution of LaBr 3 , the 218-225 keV and 289-296 keV doublets cannot be separated. Therefore, because of the relative intensities of the transitions, the delayed coincidence between the BaF 2 and LaBr 3 detectors gives access only to the half-life of the 218 keV level. However, using the technique, we could, by setting with the Ge detector a gate on the 225 keV or 218 keV transition, i) select the 225 289 keV cascade, which leads to the measurement of the sum of the half-lives of the Figure 4 Figure 5 Figure 6 515 and 289 keV levels (figures 4 and 5) or ii) select the 296 218 keV cascade, which leads to the half-life of the 515 keV level (figures 4 and 6). Figure 3 References [1] H. Mach et al., Nucl. Instr. and Meth. A 280 (1989) 49. [2] E.V.D Van Loef et al., Applied Physics Letter 77 (2000) 1467. [3] M.A. Cardona et al., 2006 Proposal to the Tandem-ALTO PAC. [4] B. Roussière et al., IPNO Activity report 2006-2007, p. 29. 20
Page 1 and 2: EXOTIC NUCLEI STRUCTURE AND REACTIO
Page 3 and 4: in the spectra by analyzing their t
Page 5 and 6: were taken from ref. [5]. Concernin
Page 7 and 8: keV -ray with the particles and the
Page 9: the 605.9 keV line of the 74 Zn 2+
Page 13 and 14: ing particles was thus obtained by
Page 15 and 16: First p-p p collisions in the ALICE
Page 17 and 18: Quarkonia physics with ALICE IPNO P
Page 19 and 20: NA50 updated puzzle IPNO Participat
Page 21 and 22: Status of the HADES experiment (I):
Page 23 and 24: Status of the HADES experiments (II
Page 25 and 26: Status of the HADES experiment (III
Page 27 and 28: Hadron physics in pbar-p p annihila
Page 29 and 30: Hadron Electrodynamics IPNO Partici
Page 31 and 32: G0 experiment at Jefferson Lab. IPN
Page 33 and 34: GEp-III and GEp-2 at Jefferson Lab
Page 35 and 36: Exotic low mass narrow baryons from
Page 37 and 38: The PVA4 parity violation experimen
Page 39 and 40: Etude des Distributions de Partons
Page 41 and 42: Latest Results from GRAAL IPNO Part
Page 43 and 44: Persistence of the Polarization in
Page 45 and 46: The northern site of the Pierre Aug
Page 47 and 48: The Pierre Auger southern Observato
Page 49 and 50: In figure 4 we plot the differentia
Page 51 and 52: THEORETICAL PHYSICS Research in the
Page 53 and 54: transfer reaction, the 34 Si(d, 3 H
Page 55 and 56: Collective excitations with SKYRME
Page 57 and 58: Pairing and nuclear incompressibili
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Evaporation-residue residue cross s
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Extending the VMI model: normal and
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Alpha particle condensation in nucl
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Collective Modes in Ultracold Trapp
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Exploring key unknown neutrino prop
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Relic astrophysical and cosmologica
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After a term by term Borel resummat
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Chiral expansions of the π 0 lifet
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IPNO Participation: H. Sazdjian Eff
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fer is used to extract f + (0). We
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cleon. The spectrum becomes strongl
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A pseudo Coulombian potential in D=
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The many-body problem with an energ
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The rotational spectrum and the att
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Z-identification of very heavy nucl
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The Pegase project, a new solid sur
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But at 80 keV total energy ( 200 eV
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Isospin effects on fragment and par
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Radial collective energy and fragme
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Latent heat of the phase transition
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Alpha-particle particle condensatio
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Fluo X: Deexcitation time measureme
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ENERGY AND ENVIRONMENT The increasi
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toe/cap/y which energy access inequ
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nides. A global comparison can be p
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3D coupling of Monte-Carlo neutroni
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Fission cross sections of actinides
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Studies and synthesis of specific m
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Chemistry and Electrochemistry of T
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function of temperature. The recent
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global electron number of the redox
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(D 0 /D)-1 ln complexes of Pa(V). I
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