Status of super heavy element research using GARIS at RIKEN

ELSEVIER

Nuclear Physics A738 (2004) 129-135 www.elsevier.com/locate/npe

Status of super heavy element research using GARIS at

RIKEN

K. Morinioto K. Morita D. Kaji S. Goto H. Haba E. Ideguchi R. Kanungo K. Katori H. Kouraad H. Kudo T. Ohnishi A. Ozawaae J.C. Peterf T . Suda K. Sueki I. Tanihata F. Tokanaig H. Xuh A.V. Yerernin A. Yoneda A. Yoshida T.-L. Zhad T. Zhengk RIKEN (The Institute of Physical and Chemical research), Wako, Sait,ama 351-0198, Japan Center

for Nuclear Science, University of Tokyo, Wako-shi, Sait,ania 351-0198, Japa.n

Depatment of Chemistry, Niigata University, Ikarashi, Niigata 950-2181, Japan dAdvanced Research Institute for Science a,nd Engineering, Waseda University, Okubo 3-4- 1, Shirijuku- ku Tokyo 169-8555, Ja,pan ~

University of Tsukuba, Tsukuba, Ibaraki 305-8577, Japan fLaboratoire de Physique Corpusculaire de Caen; LPC/ISMRA 6: Boulevard du hlarechal Juin 14050 CAEN cedex, France gDepartnient of Physics; Yamagata University, Shirakawa, Yamagat,a 990-8560. Japan hInstit,ute of Modern Physics, Chinese Academy of Science, Lanzhou 730000, China Flerov Laboratory of Nuclear Reactions, Joint Institute of Nuclear Research, RU-141 980 Dubria JInstitut,eof High Energy Physics, Chinese Academy of Science, Beijing 100039, China kSchool of Physics, Peking University, Beijing 100871, China

A gas-filled recoil separator GARIS for heavy element research was installed at, an experimental hall of the RIKEN linear Accelerator (RILAC) facility. One of the interesting applications of the separator is t.he discovery of nuclei of superlieavy elements whose at,omic number are grater then 110. We performed experimenk t o study productions and decays of heavy nuclei, 271Dsand 111, which were produced wit,h 208Pb+ Ni 271Ds n and 20yBi Ni + 111 n reactions. Fourteen atoms of 271Ds a,nd t,he same number of 272111 were detect,ed and identified. Exisknce of an isomeric state in 271Ds was confirnied. In the decay chains of 272111, spontaneous fission decays were observed in decay of 264Bhand 260Db.

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0375-9474/$ see front matter 0 2004 Elsevier B.V All rights reserved. doi:10.1016/j.nuclphysa.2004.04.021 -

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1. Introduction

To search for heavicst elements is one of thc interestiiig sul-)ject,sill cxperiinentirl niiclear physics. Kew elenieni s n.hose atomic iiunibcw were greater than 10 I were synthcsizcd by heavy-ion induced fiisioii reactions. Becansc? t,he fissility of a heavy . ten1 iiicrca.ses with ons of tlie heavy system tend increase of t,he atomic: iiumber, tlie proc1uct)ion cross to decrexc in exponentially with the increase of the iic number. Main difficulty in searching of the heaviest elenleiits arises fi-om the small productioii cross seciioiis. The small cross section is 1irnit)ingfurther research of nuclei with greatcr atomic nunibers. To overcoinc the difficiiky more int,cnse primary beams as well as rapid and efficient, separation methods assuming low rates of background particles (scatt,ercd particles or target nuclei. products froiri other reactions than coniplete fusion) miist h applied. T7roin thc beginning of yea.r 2002 RILAC (R.IKEN Liiicar ACcelcrator) facility sthrted the operation in higher energy (up to 5.8AMeV) aftmcirinstallatjion of additioiial accelc (Charge Stiite Llultiplier) [1]. The accelerat,or complcx of the R I I A C facility (18GHz ECR ion source [ 2 ] .RFQ linac [ 3 ] , original RILAC [4].and the CSLI) provides the suitable beam for studying very heavy systems. A gas-fillrti recoil separator GARlS [5] was installed iri the experimental hall of RILAC f;icilit,y for stiidies of 1ic~;wyelemen1,s ut ilizirig the full a.tlvantage of t,he accelerator complcx. As a first, kttt,empt, wc studied production aiid decay of 271Ds. Tlic isotope ivas produccd by the 208Pb G4Ki+ 27iDs n reaction. The prestxt work has confirnietl tlie experinicntal results obtained a t GSI; reported by S. Hofmann et al., [G 81. of the isotope ""Ds was confirnicd consecliicritly. In hcavy elenicril. research, ion exprxiirient doric by differcrit groups or devices t o confirm oiit result is ’71Ds[7]%iiiid 270Ds[10]were important,. As far a s clement 110. product,ioiis of 26’Ds[9]~ rrported 1)y Hofiimriri ot al.. GSi in Ger iiy, anti prothiction of "’Ds[ll] was rc~port,cd by L a z a r w et al.,FLNR of JIYR in R L ~ 1 ; :and 2"Ds[12] by Ghiorso et al., LBNL in 1JSA. But, there are almost no cxperirnents t o confirm same nuclick: by different groups. Recently. LBKL groiip reported the confirmation of ’71Ds[13] by observing t {yo decay chairis st,;arted from the isotopc. Present work providccl a concrctc confirniat.ion of a. product ion of "IDS iisiiig the simt’ reaction, but, a diffcwnt type of cxpcrirr1t:rit irl device tliari used in the p ci decay of 272 1 11 using T < i 61Ki -- ’i2 111 + ort,cd hy Hofnianri c:t a1.[14.7] iising the result, is the first clear confirrriation for . ""Bh and ’"8i\lt. reported previously by a GSI group. Kmv iriformatiori on tlicir half-lives arid decaJ- ciicrgies as ~v-clla s the excitation fiinct,ion is presented.

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2. Experimental sct-up

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The experimental sc’tiips for thc studies of Ds a r i d 2721 11 u-erc almost t,lic siiriie each other except for tlie targets. Thtx tletailes of t h e expeririitmtal setup is given in AIorita ct al. [ 151.

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K. Morinzofo et al. /Nuclear Physics A738 (2004) 129-135

3 . R.esults and discussion 3.1. "’’Pb(04Ni,ln) reaction Wr observed tot& fourteeii ticca,y chains identified to the decays of 27’Ds from tlir aiialysis of generic. correlations. Totally 58 a -tlecays were registered. Thirty t h e e of 58 decays (57’2)wc~c’detected oiily by PSD.and 25 decays (43%) n-ere detcctcd both PSI) i~iitlSSD.For scv(m of fourteeii chains; 6-fold ( E R - a l - a 2 - ~ 3 - a 4 - ~corrcl 5 ) at ions ’ werti ohst:rved where Eft denotes the (:vaporation rwidue. For f a i r chairis, 5-fold correlations wcrc observed. For t,wo chains, 4-fold correlnt,ions were obscrved. For 011~’ cliain, 2-foltl corrrhtion was o 1 ~ : i ~ e t i . ICt performed the measiiyemeiit at four heairi energies. Tlie energies froiii the accelrrator are listed in TABLE 1. togcther wit’i thc energies at, t,lie half-depth of the target, ~ g target e thickriess iised, iiiiniber of obenergy spreads in the target, beam dose, a served events: ant1 tlie corresporitling cross sections. Errors in the cross sections in tlic t a b k are only statist,ical one in tcrrris of 10 coiifidence level.

Talk 1 Surrirriary of the cx;it,atiori fiinction measurements. E,,:pririiary beam energy. E,:energy at tlic half deptli of t,he target. A E: energy loss of the beam in tlie targct. T,,:average targct thickness. ":errors in t h cross section and upper limit are given iii ternis of I n confidence level. Obsrrvcd d’ T,, E, AE Dose E,,, McV MeV MeV x1018 11, g/cm2 cvcrits pb

31011 31,1+1 316i1 32011

308 311 314 318

il 3 il 2 fl 2 fl 1

100 095 1.07 101

230 230 220 210

1

t 9 0

18:;;

8 0+6 0 -4 0

17’; <37

The beam encrgy giving tlie maximum cross section was 316 MeV. Corresponding ceiit,er-of mass encrgy at t>hehalf-depth of tlic target is 240 MeV. The FWHICI value 01 the cxcitation function is deduced to be 4 N e V assuming that the shape of the curvc is tl-ir. Gaussian. D w a y tiine distributions in logarithmic scale and decay triergy spectra for each decay genrrations are shown in Fig. 1. The measured tiecay energies of 01s detected just, after the iriiplantatioris of 2 7 1 Dinto ~ the PSD range fi-om 9.9 to 10.8 hIeV shown in the figure. The energy centers of tlir peak in the spectra. were deduced to be 10.45 LIeV and 10.73 \IeV. The decaj, energy of 271Ds reported by GSI group [6,8] was 10.73 LleV. The value agrees ~ w l lwith t,lic: higher one of tlie present result. Measured decihy times of cul group, whicli were the tiriir: diffiwnce between the moment of implantation of the isotopcs and the first N emissions. are 7.54. 2.20. 46.0.87.1. 4.63, 238.7. 1.74:1.36. 1.31: 0.057.6.59. 3.80: 1.69. and 0.92 iiis iri the order of the observation. They coiiltl be divided into tv-o groups. one of decay

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K. Moiimoto et al. /Nuclear PlqJsicsA738 (2004) 129-135

E a (MeV)

Figure 1. a: Decay time dist,ributions in logarithmic scale. A t denotes either A t ( E R a l ) or At(a, - a,,+,). Histograms are separated by decay generatioiis al,a2, 0 3 , 04; and 05. Curws in t,he figure are decay curves wit,h mean livcs ( 7 ) indicated in the figure. Mean lives were obtained in the present work. b: Decay energy spectra for earh decay generations.

t,inies sliorter than 10 nis (11 events), and another of decay tirnps longer than 10 ms (3 nis and that of events). The inearl life of tlie first groiip was calculated to be 2.9 ms with errors of 1 c ~corifiderice level. The irieari life t,iines of ’"Ds second was 124 reported by Hofniarin et al.. [ 6 3 ] were 1.8 ?:: ms (11 events) a i d 65 ?iionis (2 everits): respectively. These values obtained in the two irideperiderit expcrinicnts coincicled well both tlie short decay and long decay groups wit,hiii la statistical errors. Number ratios of short and long dccay time groups, 11/3 by RIKEN group and 11/2 by GSI group, also agrws well. Based on tlie comparison of the experimeiit,al results of thc prescmt stiidy and the one reported by GSI group meiitioned above, tlie production of ari isotope 271DsTYAS confirmed. Furthermore, it became clear that thc isotope 271Dshas ; ~ least t two sta.tcs with different mean lives. Then we could conibine the result to gct thc values of lifetinies with bet,t,er counting statistics. The averaged mean life of the shorter one is 2.35?: ms rns. The values correspond to half-lives (T1/2) of anti that of the longer one is 100 1.63 ?:,$: ms, and 69 T:y nis, respectively.

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3.2. ’09Bi(7oZn,lri) react ion Iri the first, cxperimental run, measurement,s were carried oiit, at the iricidcnt @’Nibe.a.ni energy of 323 MeV. Three correlated events iderit,ified to the decay chains of 27’111 isotopes. But during the irradiation the target detcrioration was indicated on tlie brim monitor spectrum. Because the actual beam energy in target is thus not vie11 ltnowri in this case, the first t,hrre decay chains are exrluded from thc ana.lysis of exc~itationfiinction given below. The irradiations mere performed at t,liree differmt bexn energies. M’e assiimed all decay chains from chain 4 to chain 14 ticlong to 272111decxys. To deduce the

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K. Moriiiioto et al. /Nuclear Physics A738 (2004) 129-135

ci-oss section, the collection c.fficiency of GARIS was assumed t,o be SO%,. The rcsults arc suinmarizetl iii TABLE 2.

Ta.ble 2 Surrirnary of the excitation fimction rneasurenients. E,,L:primarybeani criergy. E,:energy at t,he half depth of the target. T,,:averagetarget thickness. o T,, dose Observed E2n E,. MeV MeV / L g/cm2 xl0ls events pb 2.02 252 320.0 317.5 3 2.st;; 4.94 2x5 323.0 320.3 8 2.5:; 2.50 326.0 323.2 298 0 O.O-t:,.:,

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’I’heatt,aclied errors in the cross sect,ions are only statjisticalo~ies at 68% confidence level and those iri energy werc calciilated from the energy loss at the t,arget. At the highest ericrgy. Ec = 323.2 MeV. no event Tniils observed and an upper limit was calculated to be 1.1 pb. From a. Gaussian fit assinning thc: widt,h (FWHIbI) of 4 MeV which is t,he same value ~ L Sin the case of 271Dsdescribed in the previous section it was found (.hat the maximum cross sect,ion was at Ec = 318.9 MeV which corresponds to the center-of-miss energy of 244.1 MeV. Decay eriugies arid decay tinies observed in 2"O"Bi("Ni,ln) reaction are plotted in Fig. 2. We assigned the 01. 0 2 ; 0 3 , 014, and a 5 to the decay of 272111."’Lit . "’Bli. ’GoDb; and ?"Lr from the analysis of generic correlat,ions. We observed 8 decays for cu5. Thrcc of 14 decay chains were ended by fission. Two of them were in a 3 (204Bh)group and one was in (24 (26"Dl))group as described below. Three decays were missing in 05 group. Observed dccay energies (8.35 - 8.65 MeV) and decay times (T1/2 = 18 5:’s) of 05 decay arc in good agreement with the literature valucs for decay of 256J,r (Ea = 8.32 - 8.64 MeV, T1/2 = 28 s)[16]. We observed 12 decays for 04 groiip. Eleven decays were detected as alpha decays and one was detected as spontaneous fission decay. Observed decay energies (8.35- 9.40 MeV) a.nd decay times (T1/2 = 5.7 52.: s) of a 4 decay are in agreenicnt with thc 1it)eraturevalues for decay of 26"Db (Ea = 9.04 - 9.12 MeV, T1/2 = 1.5 s)[16]. 260Dbis known to dccay by spontaneous fission with branching ratio of !1.6%,[16].Our observation of fission evcnt for t u l group agrees very well with the literature. value. Since the 014 and a5 tlecays are assigned to 2"0Db and 256Lr,al, 0 2 ; and a 3 dccays arc unambiguously assigned to the decay of 272111.ant1 268LIt, arid 26’1Bh. Decay energies imd decay times arc suniniarized in Table 3 together with thc values rtported by Hofniann et al., in ref. [14]. Errors in the T1/2 are 1 (T confidence level. For 272111,(14 n -decays were observed) the T1/2 is calculated to 3.8 Ins. The prcsent valiie is morc than two tinies longer than t,he value reported iii ref. [14]. Observed decay energy ranges from 10.20 t,o 11.56 MeV showing broad tiist,ribut,ion, althoiigh t,he ’ peak ’ is located a t 11.0 UeV. For 268Mt, (14 a -decays wrre observed) the T1/2 is ciLlculated to 21 rns. Thc present, valiie is sliorter than the valrie report,od in ref. [14]. Observed clecay e1ic:rg.y rarlgea fi-on1

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K. Moriinoto et al. /Nuclear Physics A738 (2004) 129-135

134

5 0

5

E--J

70 0

- O ._r 7.1.3s

:LIzl 8

loops

1oms

1s

100s

A T (sec)

10000s

9

10

11

12

E, I MeV

Figure 2. a: Decay time distributions in logarithmic scale. At, denotes either At(ER c y I ) or At(a, - a,+,). Curves in the figure are decay curves with mean lives ( 7 ) indicated in the figure. b: Decay energy spectra for each decay generations.

9.40to 10.77 MeV showing broad distribut,ion, alt,hongh the ’ peak ’ is located at 10.4 MeV. For 2G4Bh3 (12 a-decays were observed) the T1/2 is calculated to 0.9 2:; s. The present value is in good agreement with the value reported in ref. [14]. Observed decay energy ranges from 8.86 to 9.83 MeV showing broad distribution, although t’he ’peak’ is located at 9.7 MeV. We observed 2 fission events in the decay of 264Bhcorresponding to a branching ratio of 14%. Decay times for the fission event were 1.0 s and 4.9 s, respectively. In the calculation of T1/2 these values are also included. This is the first observation of spontaneous fission decay in Bh isotopes. This fission event may come from EC decay product 264Sgwhich is unknown nucleus up to now because our set’up was not sensitive to distinguish the EC decay.

4. Acknowledgement

Many thanks are due to all accelerator st,aff nienibers for their excellent operation for long period of time. Authors would like to thank professor T. Chihara and Dr. N. Suzuki for synthesizing material nikkeloceiie for ion-source. Authors would like to thank greatly professors Y. Yano arid M.Ishihara for their continuous support cricouragement and useful suggestions. We would like t,o thank also Dr. M.Kase for his all support and arrangement about the beamtime. ~

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