Production cross section and decay study of 243Es and 249Md

This is a self-archived version of an original article. This version may differ from the original in pagination and typographic details. Author(s): Title: Year: Version: Copyright: Rights: Rights url: Please cite the original version: CC BY 4.0 https://creativecommons.org/licenses/by/4.0/ Production cross section and decay study of 243Es and 249Md © 2018 the Authors Published version Briselet, R.; Theisen, Ch.; Vandebrouck, M.; Marchix, A.; Airiau, M.; Auranen, Kalle; Badran, Hussam; Boilley, D.; Calverley, Tom; Cox, Daniel; Déchery, F.; Bisso, F. Defranchi; Drouart, A.; Gall, B.; Goigoux, T.; Grahn, Tuomas; Greenlees, Paul; Hauschild, K.; Herzan, Andrej; Herzberg, R. D.; Jakobsson, Ulrika; Julin, Rauno; Juutinen, Sakari; Konki, Joonas; Leino, Matti; Lightfoot, A.; Lopez-Martens, A.; Mistry, A.; Nieminen, Päivi; Pakarinen, Janne; Papadakis, Philippos; Partanen, Jari; Peura, Pauli; Rahkila, Panu; Rubert, J.; Ruotsalainen, Panu; Sandzelius, Mikael; Sarén, Jan; Scholey, Catherine; Sorri, Juha; Stolze, Sanna; Sulignano, B.; Uusitalo, Juha; Ward, A.; Zielinska, M. Briselet, R., Theisen, Ch., Vandebrouck, M., Marchix, A., Airiau, M., Auranen, K., Badran, H., Boilley, D., Calverley, T., Cox, D., Déchery, F., Bisso, F. D., Drouart, A., Gall, B., Goigoux, T., Grahn, T., Greenlees, P., Hauschild, K., Herzan, A., . . . Zielinska, M. (2019). Production cross section and decay study of 243Es and 249Md. Physical Review C, 99(2), Article 024614. https://doi.org/10.1103/physrevc.99.024614 2019 PHYSICAL REVIEW C 99, 024614 (2019) Production cross section and decay study of 243Es and 249Md R. Briselet,1Ch. Theisen,1,*M. Vandebrouck,1A. Marchix,1M. Airiau,1K. Auranen,2,†H. Badran,2D. Boilley,3,4 T. Calverley,2,5D. Cox,2,5,‡F. Déchery,1,6F. Defranchi Bisso,2A. Drouart,1B. Gall,6T. Goigoux,1T. Grahn,2P. T. Greenlees,2 K. Hauschild,7A. Herzan,2,§R. D. Herzberg,5U. Jakobsson,2,R. Julin,2S. Juutinen,2J. Konki,2,¶M. Leino,2A. Lightfoot,2 A. Lopez-Martens,7A. Mistry,5,#P. Nieminen,2,** J. Pakarinen,2P. Papadakis,2,5J. Partanen,2P. Peura,2P. Rahkila,2 J. Rubert,6P. Ruotsalainen,2M. Sandzelius,2J. Saren,2C. Scholey,2J. Sorri,2,†† S. Stolze,2,†B. Sulignano,1J. Uusitalo,2 A. Ward,5and M. Zieli´ nska1 1Irfu, CEA, Université Paris-Saclay, F-91191 Gif-sur-Yvette, France 2Department of Physics, University of Jyvaskyla, P.O. Box 35, FI-40014 Jyvaskyla, Finland 3Grand Accélérateur National d’Ions Lourds (GANIL), CEA/DSM - CNRS/IN2P3, Bd Henri Becquerel, BP 55027, F-14076 Caen Cedex 5, France 4Normandie Université, UNICAEN, Caen, France 5Department of Physics, University of Liverpool, Oliver Lodge Laboratory, Liverpool L69 7ZE, United Kingdom 6Institut Pluridisciplinaire Hubert Curien, F-67037 Strasbourg, France 7CSNSM, IN2P3-CNRS, F-91405 Orsay Campus, France (Received 28 September 2018; published 14 February 2019) In the study of the odd-Z,even-Nnuclei 243Es and 249Md, performed at the University of Jyväskylä, the fusion-evaporation reactions 197Au(48Ca,2n)243Es and 203Tl(48Ca,2n)249Md have been used for the first time. Fusion-evaporation residues were selected and detected using a gas-filled separator coupled with its focal-plane spectrometer. For 243Es, the recoil decay correlation analysis yielded a half-life of 24 ±3 s and a maximum production cross section of 37 ±10 nb. In the same way, a half-life of 26 ±1s,anα-branching ratio of 75 ±5%, and a maximum production cross section of 300 ±80 nb were determined for 249Md. The decay properties of 245Es, the daughter of 249Md, were also measured: an α-branching ratio of 54 ±7% and a half-life of 65 ±6s. Experimental cross sections were compared to the results of calculations performed using the KEWPIE2 statistical fusion-evaporation code. DOI: 10.1103/PhysRevC.99.024614 I. INTRODUCTION Determining the boundaries of the nuclear chart, particularly, in the region of superheavy nuclei (SHN), is one *[email protected] †Present address: Physics Division, Argonne National Laboratory, 9700 South Cass Avenue, Lemont, Illinois 60439, USA. ‡Present address: University of Lund, Box 118, 221 00 Lund, Sweden. §Present address: Institute of Physics, Slovak Academy of Sciences, SK-84511 Bratislava, Slovakia. Present address: Department of Chemistry, Laboratory of Radiochemistry, University of Helsinki, P.O. Box 55, FI-00014 Finland. ¶Present address: CERN, CH-1211 Geneva 23, Switzerland. #Present address: GSI Helmholtzzentrum für Schwerionenforschung GmbH, 64291 Darmstadt, Germany. **Present address: Fortum Oyj, Power Division, P.O. Box 100, 00048 Fortum, Finland. ††Present address: Sodankylä Geophysical Observatory, University of Oulu, 90014 Oulu, Finland. Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. of the key questions driving fundamental nuclear physics. The SHN owe their existence to shell effects as without them the Coulomb repulsion would make the nuclei beyond Z=104 unstable against fission [1]. In this context, detailed spectroscopy of very heavy nuclei (VHN) and SHN is of paramount importance to provide information on the nuclear landscape close to the high-Alimit of the nuclear chart as well as on the nature of the predicted island of stability. The challenge of these experiments is related to low production cross sections and, in odd-mass nuclei, to the complexity of spectra where various collective and single-particle excitations may lie close in energy. On the other hand, the studies of odd-mass nuclei are rewarded by the wealth of information regarding single-particle states, exceeding what can be obtained for even-even nuclei [2]. Regarding the known excited states of single-particle or collective nature, little data is available for Es (Z=99) and Md (Z=101) isotopes [2,3]. Before in-beam spectroscopy of these odd-Znuclei can be attempted, feasibility studies are a prerequisite, in particular, measurements of production cross sections. Such measurements also help to improve the description of the fusion-evaporation reaction mechanism, providing new constraints for the models. In this paper, the production cross sections for 243Es and 249Md populated directly in the fusion-evaporation reactions 2469-9985/2019/99(2)/024614(9) 024614-1 Published by the American Physical Society R. BRISELET et al. PHYSICAL REVIEW C 99, 024614 (2019) 197Au(48Ca,2n)243Es and 203Tl(48Ca,2n)249Md are reported. The targets and projectiles were chosen as a compromise between the predicted production cross sections and the transmission in the separator. In particular, very asymmetric reactions using actinide targets were not considered as in such cases: (i) The large angular dispersion due to the low recoil velocity and neutron emission results in a poor transmission, (ii) the low recoil energy reduces the detection efficiency at the focal plane, both effects being not fully compensated by enhanced cross sections. The present paper also allowed the half-lives and decay properties of these nuclei to be updated as well as those of 245Es, populated by the αdecay of 249Md. It should be noted that α-decay branching ratios and, to a lesser extent, half-lives are needed to deduce production cross sections. Finally, the measured production cross sections for 243Es and 249Md are discussed in the context of the Z≃100 region and compared to the predictions of the KEWPIE2 statistical fusion-evaporation code [4]. II. EXPERIMENTAL SETUP The experiments were performed at the Accelerator Laboratory of the University of Jyväskylä (JYFL). The fusionevaporation residues, including 243Es and 249Md, were separated from the fission fragments, the primary 48Ca beam and the beamand target-like reaction products using the recoil ion transport unit (RITU) gas-filled separator [5,6], which was operated at a He pressure of 0.4–0.6 mbars. The RITU transmission is estimated to be approximately 30% for the reactions considered here. The beam current was measured at regular intervals using a Faraday cup and monitored using the detectors counting rate, thus, allowing the beam dose to be deduced with an uncertainty of 20%. At the focal plane of RITU, the separated fusionevaporation residues were first detected in a positionsensitive multiwire proportional counter (MWPC) and then implanted in two adjacent double-sided silicon strip detectors (DSSDs), both detectors being part of the γrecoil electron α-tagging (GREAT) spectrometer [7]. The MWPC provided a time-of-flight (ToF) and energy loss (E) measurement, allowing: (i) selection of the fusionevaporation residues using a ToF-Eidentification matrix and (ii) correlations with the DSSD, which enable the recoiling residues (coincidence) to be discriminated from the decay products (anticoincidence). Each DSSD is 300-μm thick and consists of 60 ×40 strips with a 1-mm strip pitch. The Yside of the DSSD was calibrated using an external mixed 239Pu,241Am, and 244Cm-αsource. An energy offset is applied to account for the energy loss of the αparticle in the detector entrance window (in the case of an external source) and for the daughter nucleus recoil (decay from the detector after implantation) so that the resulting energy corresponds to the literature value for the nuclei studied in the present paper. The Xside was amplified with a higher gain to measure low-energy conversion electrons and calibrated using an external 133Ba source. Signals from all detectors were processed by a triggerless acquisition system known as the total data readout [8]. The recoil decay correlation [keV] DSSD E 7000 8000 9000 Counts/10 keV 0 5 10 15 correlationsαRecoilEs 243 FIG. 1. α-particle energy spectrum of 243Es measured in the DSSD resulting from recoil-αcorrelations using a maximum search time of 268 s. analysis was performed using the software package GRAIN [9]: After the first selection using the ToF-Eidentification matrix, the fusion-evaporation residues (recoils) were identified using the energy of the αparticles registered in the same pixel of the DSSD subsequent to the implantation of a recoil. The SAGE array [10] surrounded the target for the prompt γand conversion-electron detection, however, data from this detector were not used in the present work. III. 243Es DECAY PROPERTIES AND PRODUCTION CROSS SECTION A. Decay and half-life measurement The 243Es isotope was discovered in the 1970s by Eskola et al. using the 233U(15N,5n)243Es reaction [11], and later revisited in the 1990 s by Hatsukawa et al. using the 233U(14N,4n)243Es reaction [12]. A more recent study, performed with the SHIP separator at GSI by Antalic et al. [13], has shown that 243Es decays to its daughter via an αparticle with an energy of 7893 ±10 keV with a half-life of T1/2= 23 ±3 s and an α-decay branching ratio of 61 ±6%. An α-particle fine-structure was tentatively observed with peaks at 7745 ±20 and 7850 ±20 keV. In the work of Antalic et al. [13], 243Es was populated in the decay of the mother nucleus 247Md, whereas in the present paper, it was directly produced in the 197Au(48Ca,2n)243Es reaction, with a 21-pnA 48Ca beam at ∼210-MeV energy impinging on a 197Au target. The 48Ca +197Au reaction has already been studied in the 1990s by Gäggeler et al. [14], however, few spectroscopic data were available at that time, preventing the discrimination of fusion-evaporation residues from 2nand 3nchannels. Figure 1presents the α-particle energy spectrum measured in the DSSD resulting from recoil-αcorrelations with the decay of 243Es clearly visible. The time distribution (T)oftheαdecay with respect to the implantation, selecting the 243Es α-decay energy, is presented in Fig. 2. In the inset, the time distribution is drawn as a function of ln(T) using a maximum search time of 10 h. The peak at ln(T)=10.5 corresponds to the 243Es decay, whereas that around ln(T)=16 is related to random 024614-2 PRODUCTION CROSS SECTION AND DECAY STUDY OF … PHYSICAL REVIEW C 99, 024614 (2019) T [s] 0 50 100 150 200 250 300 350 Counts/8 s 1 10 3 sEs) = 24 243 ( 1/2 T ln( T[ms]) 0 2 4 6 8 101214161820 Counts 0 2 4 6 8 10 12 14 Es 243 random FIG. 2. Time distribution of αdecays with respect to the 243Es fusion-evaporation residue implantation. The inset shows the same data as a function of ln(T) with Texpressed in milliseconds. It should be noted that the range is different for the two spectra: 350 s for the main panel and 135 h for the inset. The fit using a twocomponent decay curve (real and random) is shown with a solid line. correlations occurring at an average time interval of ≈5000 s. The spectrum in the main panel can be fitted using the function [15], f(T)=Ae−(λ+r)T+Be−rT,(1) where λis the decay constant of the nucleus of interest and r is the random correlation rate. Similarly, the spectrum in the inset can be fitted following the method described in Ref. [16]. As expected, both procedures give the same result, yielding the half-life of T1/2=24 ±3 s, in agreement with the results of the experiment performed at SHIP [13]. The inset of Fig. 2demonstrates that the 243Es decay events can be well separated from the background in the defined range of ln(T)<12.5, which corresponds to a time window of 268 s after the recoil implantation. This search time is used in the next section in order to determine the number of events corresponding to the αdecay of 243Es. The recoil-α-αcorrelations were used to search for the decay of 239Bk following the 243Es decay. The negative outcome of this search is again consistent with the results of the measurement at SHIP [13]. The decay properties of nuclei studied in the present paper are summarized in Table I. B. Production cross section In order to study the production cross section for 243Es using the fusion-evaporation reaction 197Au(48Ca,2n)243Es, two different beam energies were used. The target used for this measurement was a 270 ±13-μgcm−2-thick 197Au selfsupporting foil. The cyclotron delivered a 213.0±1.0-MeV beam first passing through the 100-μgcm−2carbon window of the SAGE electron spectrometer. The first part of the study was performed with a beam energy in the middle of the target (MoT) estimated to be 210.0±1.0 MeV. Then, a carbon degrader foil of 100 μgcm−2was placed upstream to reduce the incident energy (MoT) to 208.0±1.0 MeV. The spectrum presented in Fig. 1corresponds to the total statistics, namely, with and without the degrader. TABLE I. Summary of decay properties obtained in the present paper compared to the literature values. Nucleus Half-life (s) α-decay branching ratio (%) Reference 243Es 24 ±3239Bk not observed This paper 23 ±361±6[13] 245Es 65 ±654±7 This paper 40 ±10 [17] 80+96 −28 80+20 −50 [18] 66 ±6[12] 55+12 −8.4[19] 249Md 26 ±175±5 This paper 25+14 −7>60 [18] 19+3 −2[20] 23.8+3.8 −2.9[19] 23 ±3[21] 75 [22] The number of counts attributed to the 243Es αdecay was obtained using a maximum search time of 268 s. The contribution from random correlations was estimated by integrating the random correlations component [second term in Eq. (1) in the case λr] using this time window. After subtracting this background, the number of αparticles stemming from 243Es was determined to be 50 ±7(32±6) without (with) the carbon degrader foil. The uncertainties were evaluated following the method described in Ref. [23]. In the present work, the statistics is large enough to consider standard normal distributions, therefore, symmetric uncertainties are adopted. During the acquisition time without and with the degrader, the number of 48Ca nuclei that impinged on the 197Au target was equal to (1.6±0.3) ×1016 and (1.2±0.2) ×1016, respectively. Taking into account the 197Au target thickness, the α-decay branching ratio of 61 ±6% [13], the α-detection efficiency of 55%, and assuming a RITU transmission of 30%, a production cross section σ(243Es) =37 ±10 nb was deduced for a beam energy of 210.0±1.0 MeV (without the degrader), and σ(243Es) =32 ±9 nb for a beam energy of 208.0±1.0 MeV (with the degrader). Only statistical uncertainties corresponding to the beam dose, number of α particles, and α-decay branching ratio are given. The RITU transmission of 30% is actually a transmission ×detection efficiency including the transmission through the separator, the time-of-flight, and the DSSD detection efficiencies. The results are presented in Table II. TABLE II. Production cross sections for 243Es using the fusionevaporation reaction 197Au(48Ca,2n)243Es measured for two different 48Ca beam energies (Ebeam corresponds to the middle of target). Nαis the number of observed αdecays after background subtraction. Ebeam (MeV) 48Ca dose Nασ(nb) 210.0±1.0(1.6±0.3) ×1016 50 ±737±10 208.0±1.0(1.2±0.2) ×1016 32 ±632±9 024614-3 R. BRISELET et al. PHYSICAL REVIEW C 99, 024614 (2019) [keV] DSSD E 6500 7000 7500 8000 8500 9000 0 10 20 30 correlationsα-αRecoilDaughter (c) 10 20 30 40 correlationsα-αRecoilMother (b) 20 40 60 80 correlationsαRecoil- (a) Fm 249 Es 245 Md 249 Counts/10 keV FIG. 3. α-particle energy spectra of 249Md,249Fm and 245Es resulting from (a) recoil-αand (b), (c) recoil-α-αcorrelations using a maximum search time of 10 min. IV. 249Md DECAY PROPERTIES AND PRODUCTION CROSS SECTION The odd-Znucleus 249Md was populated using the fusionevaporation reaction 203Tl(48Ca,2n)249Md in three different irradiation campaigns. The first campaign was focused on cross-section measurements at two different bombarding energies of 214.3±1.1 and 212.7±1.1 MeV. The results are reported in Sec. IV B. The two subsequent campaigns aimed principally at the in-beam and decay spectroscopy of 249Md, results of which will be reported in a forthcoming paper. The data collected in the three campaigns were used to derive the 245Es and 249Md half-lives and α-decay-branching ratios as presented in the following section. A. 249Md and 245Es decay and half-life measurement The α-particle energy spectra obtained using recoil-αand recoil-α-αcorrelations with the statistics of the three campaigns summed together are presented in Fig. 3. A maximum search time of 10 min after the identification of an implanted recoiling nucleus was used. 249Md features an electron capture (EC)/β+-decay branch feeding 249Fm. The αdecay of the latter is observed using recoil-αcorrelations since the detection system is insensitive to the β+particle [see panel (a)ofFig.3]. The 245Es αdecay observed using recoil-α correlations corresponds to the events when the αparticle emitted from 249Md escapes from the DSSD without being detected. The αdecay of 249Fm is more clearly visible in Fig. 4, which represents the α-decay time on a logarithmic scale as a function of the α-particle energy. Using recoil-α-α correlations allows the mother 249Md and daughter 245Es α decays to be isolated as shown in the (b) and (c) panels of Fig. 3. From the literature, the α-particle energies are as follows: Eα(249Md) =8026 ±10 keV [21], and Eα(245Es) = 7730 ±1keV [12]. The satellite peaks in the αdecay of 249Md at 7956 and 8087 keV, suggested in Ref. [21], are also tentatively observed in the present paper. [keV] DSSD E 7200 7400 7600 7800 8000 8200 8400 T[ms])Δln( 4 6 8 10 12 14 Fm 249 Es 245 Md 249 FIG. 4. α-decay time distribution on a logarithmic scale (from ∼7msto∼20 min) as a function of the decay energy. Figure 5shows the time distribution of the 249Md αdecay with respect to the implantation time. The distribution plotted as a function of ln(T) for a maximum search time of 24 h is shown in the inset. As shown in this plot, the random correlations are negligible, therefore, the time distribution displayed in the main panel can be fitted with a single exponential function. A half-life of T1/2=26 ±1 s is obtained using a maximum search time of 300 s. This value can be compared with previously measured half-lives. The 249Md decay has been studied at SHIP by Heßberger and co-workers following the αdecay of 257Db(→253Lr →249Md) [18,20] T [s]Δ 0 50 100 150 200 250 300 Counts/s 1 10 T[ms])Δln( 0 2 4 6 8 10 12 14 16 Counts 0 5 10 15 20 1 s±Md) = 26 249 ( 1/2 T FIG. 5. Time distribution of αdecays with respect to the 249Md fusion-evaporation residue implantation. The inset shows the same data as a function of ln(T) with Texpressed in milliseconds. It should be noted that the range is different for the two spectra: 300 s for the main panel and 18.2 h for the inset. The fit using a onecomponent decay curve is shown with a solid line. 024614-4 PRODUCTION CROSS SECTION AND DECAY STUDY OF … PHYSICAL REVIEW C 99, 024614 (2019) T [s]Δ 0 100 200 300 400 500 600 Counts/10 s 1 10 T[s])Δln( 0123456789 Counts 0 2 4 6 8 6 s±Es) = 65 245 ( 1/2 T FIG. 6. Time distribution of the αdecay of 245Es with respect to the 249Md αdecay. The inset shows the same data as a function of ln(T) with Texpressed in seconds. It should be noted that the range is different for the two spectra: 600 s for the main panel and 2.25 h for the inset. The fit using a one-component decay curve is shown with a solid line. and the αdecay of 253Lr [21] and by Gates et al. using the Berkeley gas-filled separator following the αdecay of 257Db [19]. Our revised half-life of 249Md obtained via direct production and with higher statistics is compatible with the values obtained in these works: 25+14 −7s[18], 19+3 −2s[20], 23 ±3s[21], 23.8+3.8 −2.9s[19]; see also Table I. Similarly, Fig. 6presents the time distribution of the 245Es αdecay with respect to that of 249Md, the time represented in linear and as a function of ln(T) scales. Again, the background is found to be negligible. The distribution was then fitted with a single component. The halflife T1/2(243Es) =65 ±6 s was extracted, a value compatible with those obtained by Heßberger et al. following the αdecay of 257Db(→253Lr →249Md →245Es): 80+96 −28 s [18], by Hatsukawa et al. after direct synthesis using the fusion-evaporation reactions 238U(14N,7n)245Es and 237Np(12C,4n)245Es: 66 ±6s[12], and by Gates et al. following the αdecay of 257Db: 55+12 −8.4s[19]; see also Table I. The α-decay-branching ratio of 249Md is defined as the ratio of the α-decay branch to 245Es, to the total decay strength, including the EC/β+branch to 249Fm. The latter is evaluated using the number of events attributed to the 249Fm α decay from Figs. 3and 4, corrected for the 249Fm α-decaybranching ratio. A correction is also applied to take into account the fraction of 249Fm nuclei that decay during the search time of 600 s. The 249Fm half-life of 2.6±0.7 min is taken from the evaluated data [24]. The 249Fm α-decay branching ratio of 15.6±1.0% is taken from Heßberger et al. [25], which is more recent than the evaluation of Ref. [24].1The resulting α-decay-branching ratio deduced in the present 1It should be noted that, in Ref. [25], the half-life of 249Fm has not been remeasured. The value adopted in this reference is actually that of the evaluation of Ref. [24], i.e., 2.6±0.7 min. In the most recent NUBASE2016 evaluation [26], the α-decay-branching ratio of 249Fm work is bα(249Md) =75 ±5%. The evaluated value of bα(249Md) >60% [24] corresponds to the measurement of Heßberger et al., which has been obtained in the study of the 257Db decay chain [18]. A more recent value of bα(249Md) = 75%, quoted without uncertainty in the Ph.D. thesis of Streicher [22], is in perfect agreement with our measurement; see also Table I. The α-decay-branching ratio of 245Es can be extracted in two distinct ways. The first possibility is to derive it as the ratio of the number of events corresponding to 249Md obtained using recoil-α-αand recoil-αcorrelations, corrected for the DSSD efficiency for a full-energy measurement α= 55% under the condition that the recoil-α-αcorrelations are obtained by gating on the full-energy peaks only, bα(245Es) =Nrecoil-α-α(249Md) Nrecoil-α(249Md) 1 α .(2) The second option is to obtain it as the ratio of counts corresponding to 245Es and 249Md in the total α-particle spectrum. Both methods lead to the same value of bα(245Es) =54 ± 7%. For comparison, the previously reported values were bα(245Es) =40 ±10% (Eskola [17]), bα(245Es) =80+20 −50% (Heßberger et al. [18]). The decay properties of 249Md and 245Es are summarized in Table I. B. Production cross section The fusion-evaporation reaction 203Tl(48Ca,2n)249Md was studied at two different bombarding energies. The cyclotron delivered a 218-MeV beam first passing through the 100-μgcm−2carbon window of the SAGE electron spectrometer. The 203Tl target having a thickness of 318 ±16 μgcm−2 was evaporated on a carbon foil of 20 μgcm−2and covered by a 10-μgcm−2carbon protection layer. The resulting energy in the middle of the 203Tl target was estimated to be 214.3± 1.1 MeV. Using in addition an 80-μgcm−2carbon degrader foil resulted in an energy of 212.7±1.1MeVMoT. The spectra were obtained using a search time of 207 s, i.e., eight 249Md half-lives. Contrary to the 243Es case, the background was found to be negligible. The total number of 48Ca particles that impinged on the target was (1.8±0.4) ×1015 [(1.5±0.3) ×1015]forthe measurement without (with) the carbon degrader foil. Using a203Tl target thickness of 318 ±16 μgcm−2,anα-branching ratio of 75 ±5%, a RITU transmission ×detection efficiency of 30% and a full-energy α-detection efficiency of 55%, cross sections σ(249Md) of 300 ±80 and 70 ±40 nb are deduced for the incident energies of 214.3 and 212.7 MeV, respectively. Again, only statistical uncertainties are given. The results are summarized in Table III. V. DISCUSSION In this section, we discuss the new cross-section measurements for 243Es and 249Md. These results are placed in the is taken from Ref. [24](33±9%), whereas for the half-life only the value from Ref. [27](96±6 s) is selected. 024614-5 R. BRISELET et al. PHYSICAL REVIEW C 99, 024614 (2019) TABLE III. Production cross sections for 249Md using the fusionevaporation reaction 203Tl(48Ca,2n)249Md measured for two different 48Ca beam energies (Ebeam corresponds to the middle of the target). Ebeam (MeV) 48Ca dose Nασ(nb) 214.3±1.1(1.8±0.4) ×1015 68 ±8 300 ±80 212.7±1.1(1.5±0.3) ×1015 12 ±470±40 context of experimental cross sections for cold fusionevaporation reactions and the 2nchannel for Z≈100, presented in Fig. 7and compared to new reactions dynamics calculations using the statistical fusion-evaporation code KEWPIE2[4]. A. 2nchannel fusion-evaporation systematics It is generally acknowledged that the fusion-evaporation reactions can be described as three subsequent independent processes: capture, compound-nucleus formation, and survival of the residual nucleus. The description of the capture step is rather well controlled in terms of barrier penetration with no rapid evolution as a function of mass and charge when using similar projectiles and targets. The formation step results in a sharp decrease in the cross section for projectiletarget combinations with ZpZt1600–1800, known as the fusion hindrance, which prevents the formation of a compound nucleus by leading the dinuclear composite towards the quasifission route. This effect starts to act in the region considZ residue 98 100 102 104 106 108 Cross section [nb] -3 10 -2 10 -1 10 1 10 2 10 3 10 Ca beam, this work 48 Ca beam 48 other beam 250 252 253 254 249 251 244 246 250 243 244 238 240 241 255 256 257 259 260 261 264 FIG. 7. Systematics of the fusion-evaporation cross sections in the 2nchannel as a function of Zof the residual nucleus. The filled red squares correspond to reactions induced by a 48Ca beam, whereas the blue circles correspond to those using other beams. The new 243Es and 249Md measurements are denoted by empty square symbols. The mass number Aof the residual nucleus is given to the right of each symbol. Data are taken from Refs. [28]( 238,240,241Cf), [29]( 244Cf), this paper (243Es,249Md), [30]( 244,246Fm), [31]( 250Fm), [32]( 251Md), [33]( 250No), [34]( 252,253No), [14]( 254No,255Lr), [35] (256Rf), [20]( 257Db), [36]( 259Sg), [21]( 260Sg), [37]( 261Bh), [38] (264Hs). ered here, and it can account for the exponential decrease in the cross sections observed for larger Zvalues in Fig. 7. Consequently, only the survival step can account for the decrease in cross sections below Z≈102. The global trend displayed by the cross sections presented in Fig. 7may be explained by a combination of two effects. First, the fourfold magic character of the 48Ca +208Pb →256No∗reaction leads to a low-Qvalue and, therefore, a higher survival probability in the evaporation and de-excitation processes. This enhancement is observed for 254No and neighboring residual nuclei. Second, the semimagicity at Z=100,N=152 leads to higher shell corrections (higher fission barrier) and, therefore, a higher survival probability around 252Fm. Note that, if the cross sections are plotted as a function of the mass or neutron number, they also display a bell-shaped behavior. B. Cross-section calculations In the following, the fusion-evaporation cross sections illustrated with the new experimental results for 243Es and 249Md are discussed in terms of survival from the compound to the residual nucleus with an emphasis on the effect of the fission barrier. The present measurements are performed in a mass region where the fusion hindrance is not yet significant. Consequently, the fusion process is modelled in the KEWPIE2 code by considering only the capture phase, which is computed using a proximity potential and the Wentzel-KramersBrillouin (WKB) approximation, see Ref. [4] for details. The KEWPIE2 code [4] treats the competition between light-particle evaporation and fission, which occurs within an excited compound nucleus using the statistical formalisms of Weisskopf [41] and Bohr-Wheeler [42], respectively. The entire set of default parameters used in the KEWPIE2 code is presented in Ref. [4]. In the following, we will only focus on a few parameters, which are not well defined either theoretically or experimentally in this mass region [43]. These parameters are the reduced friction parameter β, the shell-damping energy Ed, and the shell corrections Esh. These parameters are related to the viscosity of nuclear matter, the stability of shell corrections with temperature, and the fission-barrier height, respectively, following Eq. (3) for the latter: Bf=BLDM −Esh,(3) where Bfis the fission-barrier height and BLDM is the liquiddrop fission barrier. The default values used in the KEWPIE2 code are β=2×1021 s−1,Ed=19 MeV, whereas the finite-range droplet model (FRDM) Esh shell corrections are taken from Ref. [40]. It should be stressed that those parameters mainly affect the fission process that is known to be dominant for heavy and superheavy nuclei. Indeed, a small variation of the fission parameters, such as the strength of the dissipation or the fission-barrier heights, leads to a significant modification of the survival probability and, consequently, the related observables, in particular, the production cross sections. Figure 8presents the experimental results for the production cross sections for 243Es and 249Md (Tables II and III) compared to the calculations performed with the KEWPIE2 code using the default parameters. For 249Md, the 024614-6 PRODUCTION CROSS SECTION AND DECAY STUDY OF … PHYSICAL REVIEW C 99, 024614 (2019) [MeV] beam E 195 200 205 210 215 220 225 [nb]σ -3 10 -2 10 -1 10 1 10 2 10 3 10 Es Exp. 243 Es, KEWPIE2 244 Es, KEWPIE2 243 Es, KEWPIE2 242 (a) [MeV] beam E 195 200 205 210 215 220 225 [nb]σ -3 10 -2 10 -1 10 1 10 2 10 3 10 Md Exp. 249 Md, KEWPIE2 250 Md, KEWPIE2 249 Md, KEWPIE2 248 (b) FIG. 8. Comparison between the experimental production cross sections for (a) 243Es obtained in the present paper and the calculations of the 1n,2n,and3ncross sections performed with the KEWPIE2 code using the default parameters (macroscopic part described by the Thomas-Fermi parametrization as proposed by Myers- ´ Swi ˛atecki [39] and the microscopic part based on the FRDM shell corrections [40]). (b) The same for 249Md. calculation reproduces the measured production cross sections well, whereas it underestimates them by a factor of 5 for the 243Es case. The discrepancy for this latter case cannot be explained by a failure of the fusion model. Indeed, for a beam energy corresponding to the present measurement (Ecm ≈ 169 MeV), the fusion model provides a fusion cross section σfus =55 mb in good agreement with the measurement σfus = 42 mb of Ref. [44]. Moreover, a discussion of the fusion cross section for the 48Ca +208Pb reaction for which the WKB approximation provides a good description without fusion hindrance considerations can be found in Ref. [4]. In Fig. 9, the fission-barrier heights or the reduced friction parameters have been increased in order to reproduce the measurements for the 2nevaporation channel. Concerning the fission-barrier heights, it is necessary to add 500 keV to the absolute value of the shell corrections [with the liquid-drop fission barrier kept unchanged, see Eq. (3)] to obtain good agreement between the calculations and the data. Furthermore, the reduced friction parameter has to be increased by a factor of 3, i.e., to β=6×1021 s−1in order to obtain the same agreement. [MeV] beam E 195 200 205 210 215 220 225 [nb]σ -3 10 -2 10 -1 10 1 10 2 10 3 10 Es Exp. 243 + 500 keV sh EΔEs, 243 + 500 keV sh EΔEs, 242 3×βEs, 243 3×βEs, 242 FIG. 9. Comparison between the experimental production cross sections for 243Es extracted in the present paper and the calculations performed with the KEWPIE2 code considering either an adjustment of +500 keV of the barrier heights or an adjustment of the reduced friction parameter to β=6×1021 s−1. It should be stressed that these adjustments remain within the uncertainty intervals for these parameters as discussed in Refs. [4,43]. Moreover, no theoretical model can presently predict the fission-barrier heights with an accuracy better than 0.5–1 MeV [45–47]. In the SHN region, differences between the models can be as large as 4 MeV [48]. Consequently, we cannot attribute the discrepancy observed for 243Es (Fig. 8) to any specific parameters used in the KEWPIE2 code nor to any inputs from other nuclear models, in particular, those related to the fission process. Hence, the measured production cross sections for the 243Es and 249Md isotopes can be fully explained within the uncertainties in nuclear models and phenomenological parametrizations implemented in the KEWPIE2 code. A way to provide constraints on the parameters used in the KEWPIE2 code would be to use more precise measurements in the VHN and SHN mass regions for a whole set of different evaporation channels, including a large scan in excitation energy for each of them. Indeed, using relevant data can help to fix and/or eliminate the impact of a specific parameter. Such an approach based on the Bayesian inference is discussed in Refs. [4,49]. VI. CONCLUSION The odd-Z243Es and 249Md were produced in the 197Au(48Ca,2n)243Es and 203Tl(48Ca,2n)249Md fusion-evaporation reactions, respectively. The half-life of 243Es,249Md and its daughter 245Es were measured, and the results were found compatible with those obtained in previous measurements following the αdecay of heavier nuclei. The precisions of the half-lives of 249Md and 245Es were increased as well as those of the α-decay-branching ratios for those nuclei. Production cross sections of 243Es and 249Md have been measured for the first time using 48Ca-induced reactions and compared to the calculations performed with the KEWPIE2 code [4]. 024614-7 R. BRISELET et al. PHYSICAL REVIEW C 99, 024614 (2019) ACKNOWLEDGMENTS We acknowledge the accelerator staff at the University of Jyväskylä for delivering a high-quality beam during the experiments. The Au targets were kindly provided by GSI. Support has been provided by the EU 7th Framework Programme “Integrating Activities-Transnational Access” Project No. 262010 (ENSAR) and by the Academy of Finland under the Finnish Centre of Excellence Programme (Nuclear and Accelerator Based Physics Programme at JYFL Contract No. 213503). [1] D. Ackermann and C. Theisen, Phys. Scr. 92,083002 (2017). [2] M. Asai, F. Heßberger, and A. Lopez-Martens, Nucl. Phys. A 944,308 (2015). [3] C. Theisen, P. T. Greenlees, T.-L. Khoo, P. Chowdhury, and T. Ishii, Nucl. Phys. A 944,333 (2015). [4] H. Lü, A. Marchix, Y. Abe, and D. Boilley, Comput. Phys. Commun. 200,381 (2016). [5] M. Leino, J. Äystö, T. Enqvist, P. Heikkinen, A. Jokinen, M. Nurmia,A.Ostrowski,W.Trzaska,J.Uusitalo,K.Eskola,P. Armbruster, and V. Ninov, Nucl. Instrum. Methods Phys. Res., Sect. B 99,653 (1995). [6] J. Sarén, J. Uusitalo, M. Leino, and J. Sorri, Nucl. Instrum. Methods Phys. Res., Sect. A 654,508 (2011). [7] R. D. Page, A. N. Andreyev, D. E. Appelbe, P. A. Butler, S. J. Freeman, P. T. Greenlees, R.-D. Herzberg, D. G. Jenkins, G. D. Jones, P. Jones et al.,Nucl. Instrum. Methods Phys. Res., Sect. B204,634 (2003). [8] I. H. Lazarus, D. E. Appelbe, P. A. Butler, P. J. Coleman-Smith, J. R. Cresswell, S. J. Freeman, R. D. Herzberg, I. Hibbert, D. T. Joss, S. C. Letts et al.,IEEE Trans. Nucl. Sci. 48,567 (2001). [9] P. Rahkila, Nucl. Instrum. Methods Phys. Res., Sect. A 595,637 (2008). [10] J. Pakarinen, P. Papadakis, J. Sorri, R. D. Herzberg, P. T. Greenlees,P.A.Butler,P.J.Coleman-Smith,D.M.Cox,J.R. Cresswell, P. Jones et al.,Eur. Phys. J. A 50,53 (2014). [11] P. Eskola, K. Eskola, M. Nurmia, and A. Ghiorso, Phys. Fenn. 8, 357 (1973). [12] Y. Hatsukawa, T. Ohtsuki, K. Sueki, H. Nakahara, I. Kohno, M. Magara, N. Shinohara, H. L. Hall, R. A. Henderson, C. M. Gannet et al.,Nucl. Phys. A 500,90 (1989). [13] S. Antalic, F. P. Heßberger, S. Hofmann, D. Ackermann, S. Heinz, B. Kindler, I. Kojouharov, P. Kuusiniemi, M. Leino, B. Lommel, R. Mann, and Š. Šáro, Eur.Phys.J.A43,35 (2010). [14] H. W. Gäggeler, D. T. Jost, A. Türler, P. Armbruster, W. Brüchle, H. Folger, F. P. Heßberger, S. Hofmann, G. Münzenberg, V. Ninov et al.,Nucl. Phys. A 502,561 (1989). [15] M. E. Leino, S. Yashita, and A. Ghiorso, Phys. Rev. C 24,2370 (1981). [16] K. H. Schmidt, Eur.Phys.J.A8,141 (2000). [17] P. Eskola, Phys.Rev.C7,280 (1973). [18] F. P. Heßberger, G. Münzenberg, S. Hofmann, Y. K. Agarwal, K. Poppensieker, W. Reisdorf, K. H. Schmidt, J. R. H. Schneider,W.F.W.Schneider,H.J.Schöttet al.,Z. Phys. A 322,557 (1985). [19] J. M. Gates, S. L. Nelson, K. E. Gregorich, I. Dragojevi´ c, C. E. Düllmann, P. A. Ellison, C. M. Folden III, M. A. Garcia, L. Stavsetra, R. Sudowe, D. C. Hoffman, and H. Nitsche, Phys.Rev.C 78,034604 (2008). [20] F. P. Heßberger, S. Hofmann, D. Ackermann, V. Ninov, M. Leino, G. Münzenberg, S. Saro, A. Lavrentev, A. G. Popeko, A. V. Yeremin et al.,Eur.Phys.J.A12,57 (2001). [21] F. Heßberger, S. Hofmann, B. Streicher, B. Sulignano, S. Antalic, D. Ackermann, S. Heinz, B. Kindler, I. Kojouharov, P. Kuusiniemi et al.,Eur. Phys. J. A 41,145 (2009). [22] B. Streicher, Synthesis and spectroscopic properties of Synthesis and spectroscopic properties of transfermium isotopes with Z =105, 106 and 107, Ph.D. thesis, Cornelius University, Bratislava, 2006. [23] W. Brüchle, Radiochim. Acta 91,71 (2003). [24] K. Abusaleem, Nucl. Data Sheets 112,2129 (2011). [25] F. P. Heßberger, S. Antalic, D. Ackermann, Z. Kalaninová, S. Heinz, S. Hofmann, B. Streicher, B. Kindler, I. Kojouharov, P. Kuusiniemi et al.,Eur.Phys.J.A48,75 (2012). [26] G. Audi, F. G. Kondev, M. Wang, W. Huang, and S. Naimi, Chin. Phys. C 41,030001 (2017). [27] F. P. Heßberger, S. Hofmann, D. Ackermann, P. Cagarda, R.-D. Herzberg, I. Kojouharov, P. Kuusiniemi, M. Leino, and R. Mann, Eur.Phys.J.A22,417 (2004). [28] J. Khuyagbaatar, F. P. Heßberger, S. Hofmann, D. Ackermann, V. S. Comas, S. Heinz, J. A. Heredia, B. Kindler, I. Kojouharov, B. Lommel et al.,Eur.Phys.J.A46,59 (2010). [29] J. Konki, B. Sulignano, P. T. Greenlees, C. Theisen, K. Auranen, H. Badran, R. Briselet, D. M. Cox, F. Defranchi Bisso, J. Dobaczewski et al.,Phys.Rev.C97,024306 (2018). [30] H. Gäggeler, T. Sikkeland, G. Wirth, W. Brüchle, W. Bögl, G. Franz, G. Herrmann, J. V. Kratz, M. Schädel, K. Sümmerer, and W. Weber, Z. Phys. A 316,291 (1984). [31] J. E. Bastin, R.-D. Herzberg, P. A. Butler, G. D. Jones, R. D. Page, D. G. Jenkins, N. Amzal, P. M. T. Brew, N. J. Hammond, R. D. Humphreys et al.,Phys.Rev.C73,024308 (2006). [32] A. Chatillon, C. Theisen, E. Bouchez, P. A. Butler, E. Clément, O. Dorvaux, S. Eeckhaudt, B. J. P. Gall, A. Görgen, T. Grahn et al.,Phys.Rev.Lett.98,132503 (2007). [33] D. Peterson, B. B. Back, R. V. F. Janssens, T. L. Khoo, C. J. Lister, D. Seweryniak, I. Ahmad, M. P. Carpenter, C. N. Davids, A. A. Hecht et al.,Phys.Rev.C74,014316 (2006). [34] Y. T. Oganessian, V. K. Utyonkov, Y. V. Lobanov, F. S. Abdullin, A. N. Polyakov, I. V. Shirokovsky, Y. S. Tsyganov, A. N. Mezentsev, S. Iliev, V. G. Subbotin et al.,Phys.Rev.C 64,054606 (2001). [35] I. Dragojevi´ c, K. E. Gregorich, C. E. Düllmann, M. A. Garcia, J. M. Gates, S. L. Nelson, L. Stavsetra, R. Sudowe, and H. Nitsche, Phys.Rev.C78,024605 (2008). [36] G. Münzenberg, S. Hofmann, H. Folger, F. P. Heßberger, J. Keller, K. Poppensieker, B. Quint, W. Reisdorf, K. H. Schmidt, H. J. Schött et al.,Z. Phys. A 322,227 (1985). [37] G. Münzenberg, P. Armbruster, S. Hofmann, F. P. Heßberger, H. Folger, J. G. Keller, V. Ninov, K. Poppensieker, A. B. Quint, W. Reisdorf et al.,Z. Phys. A 333,163 (1989). [38] N. Sato, H. Haba, T. Ichikawa, D. Kaji, Y. Kudou, K. Morimoto, K. Morita, K. Ozeki, T. Sumita, A. Yoneda et al.,J. Phys. Soc. Jpn. 80,094201 (2011). [39] W. D. Myers and W. J. ´ Swi ˛atecki, Phys.Rev.C60,014606 (1999). 024614-8