In order to investigate the evolution of nuclear deformation in the region of the chart of nuclides around mass numbers A ' 110 and A ' 150, an experiment was performed at the Argonne National Laboratory where the gamma-decay radiation emitted from the fission fragments of 252Cf was measured using 51 Gammasphere detectors coupled with 25 LaBr3(Ce) detectors. In this work, a short description of the experimental setup is presented together with some preliminary results from the fast-Timing analysis of the 4+ state of the nucleus 100Zr. A lifetime value of τ = 50(28) ps was obtained using the Generalized Centroid Shift Method. This result agrees with the literature value of τ = 53(4) ps within one standard deviation

Anderson, J.T.

Ayangeakaa, A.D.

Berry, T.A.

Bottoni, S.

Bruce, A.M.

Burrows, I.

Carpenter, M.P.

Carrol, R.J.

Copp, P.

Cullen, D.M.

Daniel, T.

Gallardo, M.C.

Gamba, E.R.

Greene, J.P.

Gurgi, L.A.

Hartley, D.J.

Ilieva, R.

Ilieva, S.

Janssens, R.V.F.

Kondev, F.G.

Kröll, T.

Lalkovski, S.

Lane, G.J.

Lauritsen, T.

Lazarus, I.

Lotay, G.

Martínez, G.F.

Nia, C.R.

Podolyák, Z.

Pucknell, V.

Reed, M.

Regan, P.H.

Rohrer, J.

Rudigier, M.

Sethi, J.

Seweryniak, D.

Shand, C.M.

Simpson, J.

Smolen, M.

Stefanova, E.A.

Vedia, V.

Yordanov, O.

Zhu, S.

Carolina Digital Repository

Vol. 49 (2018) ACTA PHYSICA POLONICA B No 3FAST-TIMING MEASUREMENTS IN 100Zr USINGLaBr3(Ce) DETECTORS COUPLED WITHGAMMASPHERE∗E.R. Gambaa,†, A.M. Brucea, M. Rudigierb, S. Lalkovskib,cS. Bottonid, M.P. Carpenterd, S. Zhud, A.D. AyangeakaadJ.T. Andersond, T.A. Berryb, I. Burrowse, R.J. CarrolbP. Coppf , M. Carmona Gallardog, D.M. Cullenh, T. DanielbG. Fernández Martínezi, J.P. Greened, L.A. GurgibD.J. Hartleyj, R. Ilievab, S. Ilievai, R.V.F. JanssensdF.G. Kondevk, T. Krölli, G.J. Lanel, T. Lauritsend, I. LazaruseG. Lotayb, C.R. Niţăm, Zs. Podolyákb, V. Pucknelle, M. ReedlP.H. Reganb,n, J. Rohrerd, J. Sethid, D. SeweryniakdC.M. Shandb, J. Simpsone, M. Smoleńo, V. VediagE.A. Stefanovap, O. YordanovpaUniversity of Brighton, Brighton, UKbUniversity of Surrey, Guildford, UKcUniversity of Sofia “St. Kl. Ohridski”, BulgariadPhysics Division, Argonne National Laboratory, Argonne, USAeSTFC Daresbury Laboratory, Daresbury, UKfUniversity of Massachusetts Lowell, Lowell, Massachusetts, USAgUniversidad Complutense, Madrid, SpainhUniversity of Manchester, Manchester, UKiTechnische Universität Darmstadt, Darmstadt, GermanyjDepartment of Physics, U.S. Naval Academy, Annapolis, USAkNuclear Engineering Division, Argonne National Laboratory, Argonne, USAlAustralian National University, Canberra, AustraliamHoria Hulubei National Institute of Physics and Nuclear EngineeringBucharest-Măgurele, RomanianAIR Division, NPL, Teddington, UKoUniversity of the West of Scotland, Paisley, UKpInstitute for Nuclear Research and Nuclear Energy, Sofia, Bulgaria(Received December 13, 2017)In order to investigate the evolution of nuclear deformation in the re-gion of the chart of nuclides around mass numbers A ' 110 and A ' 150,∗ Presented at the XXXV Mazurian Lakes Conference on Physics, Piaski, Poland,September 3–9, 2017.† Corresponding author: E.Gamba@brighton.ac.uk(555)556 E.R. Gamba et al.an experiment was performed at the Argonne National Laboratory wherethe gamma-decay radiation emitted from the fission fragments of 252Cf wasmeasured using 51 Gammasphere detectors coupled with 25 LaBr3(Ce) de-tectors. In this work, a short description of the experimental setup is pre-sented together with some preliminary results from the fast-timing analysisof the 4+ state of the nucleus 100Zr. A lifetime value of τ = 50(28) ps wasobtained using the Generalized Centroid Shift Method. This result agreeswith the literature value of τ = 53(4) ps within one standard deviation.DOI:10.5506/APhysPolB.49.5551. IntroductionImportant information about the evolution of the deformation acrossthe nuclear chart can be obtained from lifetime measurements of low-lyingexcited states in nuclei. The lifetime of the first excited states is an essentialingredient in the calculation of the reduced transition probability B(E2)which gives, in turn, the quadrupole moment related to the deformationparameter β2. In this experiment, a source of 252Cf was used in order topopulate the regions around mass numbers A ' 110 and A ' 150. Thissource allowed to produce fission fragments with the highest yields in thesedeformed regions with respect to other fission sources [1]. Gamma raysemitted from the fission fragments were detected using a hybrid detectorarray made of 51 HPGe detectors from Gammasphere and 25 LaBr3(Ce)scintillator detectors. These scintillator detectors were used in the past tomeasure lifetimes in the sub-nanosecond range [2, 3] and they have been usedin many successful experiments, for example during the EXILL-FATIMAcampaign [3] and at RIKEN [4]. The 25 scintillator detectors are part of theUK NuSTAR Collaboration [5, 6] and in this experiment they were coupledfor the first time to a fully digital acquisition system.2. The setupThe two hemispheres of the hybrid array were arranged in a 4π geom-etry around the 252Cf fission source which was sandwiched between twoplatinum disks. The LaBr3(Ce) detectors consisted of a 1.5′′ × 2′′ cylin-drical crystal coupled with a R9779 Hamamatsu PMT. In order to reducethe Compton background and also the number of cross-talk events betweenneighbouring detectors, each detector was equipped with a lead shield. Dur-ing the experiment, a typical LaBr3(Ce) count rate for one single detectorwas 2.7 kHz. Each array had its own acquisition chain which worked in-dependently. A detailed description of the two acquisition systems can befound in Refs. [7, 8] for the digital Gammasphere acquisition (DGS), and inRef. [9] for the LaBr3(Ce) coupled with DGS. The stand-alone DGS collectedFast-timing Measurements in 100Zr Using LaBr3(Ce) Detectors Coupled . . . 5573-fold events within a coincidence window of 2 µs for spectroscopic purposes.A second data set contained the fast-timing information which consisted of2-fold events in the LaBr3(Ce) array, in coincidence with at least one gammaray detected in Gammasphere. The time coincidence window between Gam-masphere and the LaBr3(Ce) array events was set to 500 ns and the windowbetween any two coincident events in the LaBr3(Ce) detectors was 200 ns.3. Preliminary resultsA preliminary measurement of the lifetime of the 4+ state in 100Zr ispresented in this section. In order to isolate the 4+ state at 564 keV, a totalof 16 different pairs of energy gates has been applied to the data set. Thegates have been selected by using the independent DGS dataset to checkfor clean combinations of gates on the yrast band in 100Zr (excluding the6+ → 4+ and 4+ → 2+ transitions) and on the even-A fission partners146,148,150Ce. This procedure gave the spectrum shown in Fig. 1 (left) wheretransitions from 100Zr and its fission partners are labelled. In Fig. 1 (right),the DGS energy spectrum with no gates applied is shown for comparison.4+ 2+(100Zr)6+ 4+(100Zr)511 keVPt K 2+ 0+(100Zr)8+ 6+(150Ce)2+ 0+(146Ce)Pb K+Pt kβPt KPb K+Pt k2+ 0+(100Zr)5112+ 0+(98Zr)2+ 0+(110Ru)Fig. 1. Left: Sum of 16 double-gated DGS energy spectra. Right: Total DGSenergy spectrum with no gates applied.The LaBr3(Ce) γ–γ matrix shown in Fig. 2 (left) is produced by applyingthe same set of gates used for the DGS spectrum in Fig. 1 (left). The peakencircled in solid black/blue represents the coincidence between the 6+ → 4+and 4+ → 2+ transitions, while the black dotted lines represent the graphicalcuts used to estimate the contribution to the time distribution from theCompton background. In Fig. 2 (right), a DGS γ–γ matrix shows the sameenergy regions as (left) after the gating procedure. The same graphical cutsare superimposed on this matrix to show that the background gates arerelatively free from any contaminants. The histograms shown in Fig. 3 (left)represent the delayed and anti-delayed time distributions for the 4+ state.The delayed curve (lighter/blue) has been obtained from the case where558 E.R. Gamba et al.:Ce [keV]3Energy LaBr330 340 350 360 370:Ce [keV]3Energy LaBr400450500550600210Energy HPGe [keV]330 340 350 360 370Energy HPGe [keV]400450500550600-110110Fig. 2. (Colour on-line) Left: γ–γ matrix for the LaBr3(Ce) array, obtained usingthe same gates as in Fig. 1 (left). With black/blue solid line is represented the cuton the coincidence peak for the cascade 6+ → 4+ → 2+, while the black dottedlines are the background cuts. Right: DGS γ–γ matrix for comparison with (left).the feeding transition 6+ → 4+ is used as a start and the decay transition4+ → 2+ as the stop, while the anti-delayed distribution (darker/red) isobtained in the opposite case. The Generalized Centroid Difference method(GCD), described in Ref. [10], allows the determination of the lifetime ofthe state by measuring the centroid difference (∆C) between these two timedistributions. The ∆C between these curves was measured as 184(16) ps,but has to be corrected for the value of the Prompt Response Distribution(PRD) curve and also for the time delay given by the background eventsunderlying the full energy peak (∆CBg). The PRD curve of this setup givesΔCP=0.30497 keVFig. 3. (Colour on-line) Left: Centroid difference between the delayed (lighter/blue)and anti-delayed (darker/red) time distributions, obtained from the cascade6+ → 4+ → 2+ in 100Zr. Right: DGS-gated LaBr3(Ce) energy spectrum in theregion around the 497-keV peak. The black dotted lines marks the cut limits usedto obtain the time distributions. The descendant/blue line show the fitted lin-ear background, and a peak-to-background ratio of 0.30 was determined for thechannels inside the black dotted lines.Fast-timing Measurements in 100Zr Using LaBr3(Ce) Detectors Coupled . . . 559a value of 124(7) ps for the energy combination at 497 keV and 352 keV, andthe background time correction is evaluated using the method described inRef. [3]. The lifetime value is then calculated using the equation in Ref. [3]τ =12(∆C +∆C −∆CBgP− PRD), (1)where P is the peak-to-background ratio of the energy transition at 497 keV.In Fig. 3 (left), the Gaussian fit for the peak at 497 keV (6+ →4+), ob-tained from the LaBr3(Ce) array after applying the DGS gates, is shown(darker/red curve), together with the estimated linear background (lighter/blue line). The black dotted lines are the gate limits used to obtain the timedistributions shown in Fig. 3 (right), taken to the left-hand side of the peakin order to minimize the contribution of the peak at 511 keV. A peak-to-background ratio of 0.30 has been measured for the region inside the dottedlines. In Fig. 4, the ∆C values for the background contributions are shownby the lighter/blue dots. The background energies at which the points areplotted are the centroids of the graphical cuts in Fig. 2 (left) and thesepoints were then fitted with the straight/red line shown in Fig. 4, using a χ2minimization algorithm. ∆CBg is extracted from the background fit at theenergy of 497 keV and its value is 174(14) ps. A similar analysis has beencarried out for the peak at 352 keV and a ∆CBg value of 168(20) ps wasobtained, together with a P value of 0.28. The weighted average of the twolifetime values τ = 65(47) ps and τ = 45(36) ps, obtained by applying thisprocedure to both transitions, gives a lifetime value of τ = 50(28) ps, consis-tent within one standard deviation with the literature value of τ = 53(4) ps[3, 11, 12]. The large experimental error associated with this measurementPRD curveCCbgPRD value at 497 keVFig. 4. ∆C values for the background at different energies around the peak at497 keV (lighter/blue dots). The straight/red line is the linear fit of these values,obtained via a weighted χ2 minimization algorithm. The darker/red dot representsthe ∆C value for the 6+ → 4+ → 2+ cascade. ∆CBg was calculated to be 174 psat the energy of 497 keV. The PRD curve is represented by the solid/blue line.560 E.R. Gamba et al.arises from the small P value which consequently enlarges the statisticalerrors (see Eq. (1)). An uncertainty of 1σ was considered for both the PRDand the background curves.4. ConclusionsTogether with the results presented in Ref. [9], this work shows that it ispossible to measure level lifetimes as short as a few tens of picoseconds usingthe GCD method on this dataset. Both works show results consistent withthe literature. The background selection procedure suggested in Ref. [3] hasbeen successfully applied to these data for the first time.UK authors acknowledge the support of the Science and Technology Fa-cilities Council (STFC). E. Stefanova and O. Yordanov acknowledge supportfrom BNSF under contract DFNI-E02/6.REFERENCES[1] A.C. Wahl, Symposium on Physics and Chemistry of Fission, IAEA, 1965.[2] N. Mărginean et al., Eur. Phys. J. A 46, 329 (2010).[3] J.M. Régis et al., Nucl. Instrum. Methods Phys. Res. A 763, 210 (2014).[4] F. Browne et al., Phys. Lett. B 750, 448 (2015).[5] Zs. Podolyak, Rad. Phys. Chem. 95, 14 (2014).[6] S. Lalkovski et al., Acta Phys. Pol. B 47, 637 (2016).[7] I-Y. Lee et al., Nucl. Phys. A 520, 641c (1990).[8] J.T. Anderson et al., IEEE NSS/MIC 2012, 1536 (2012).[9] M. Rudigier et al., Acta Phys. Pol. B 48, 351 (2017).[10] J.-M. Régis et al., Nucl. Instrum. Methods Phys. Res. A 726, 191 (2013).[11] H. Ohm et al., Z. Phys A 334, 519 (1989).[12] A.G. Smith et al., J. Phys G: Nucl. Part. Phys. 28, 2307 (2002).

Fast-Timing measurements in 100zr using labr3(ce) detectors coupled with gammasphere

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Fast-Timing measurements in 100zr using labr3(ce) detectors coupled with gammasphere

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