Decay and in-beam studies of neutron-deficient Po and Ra isotopes at JYFL

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/ Decay and in-beam studies of neutron-deficient Po and Ra isotopes at JYFL © Authors, 1999 Published version Leino, M; Allatt, RG; Andreyev, AN; Cocks, JFC; Dorvaux, O; Enqvist, T; Eskola, K; Helariutta, K; Huyse, M; Jones, PM; Julin, R; Juutinen, S; Kankaanpaa, H; Keenan, A; Kettunen, H; Kuusiniemi, P; Muikku, M; Page, RD; Rahkila, P; Savelius, A; Trzaska, WH; Uusitalo, J; Van Duppen, P Leino, M., Allatt, R., Andreyev, A., Cocks, J., Dorvaux, O., Enqvist, T., Eskola, K., Helariutta, K., Huyse, M., Jones, P., Julin, R., Juutinen, S., Kankaanpaa, H., Keenan, A., Kettunen, H., Kuusiniemi, P., Muikku, M., Page, R., Rahkila, P., . . . Van Duppen, P. (1999). Decay and in-beam studies of neutron-deficient Po and Ra isotopes at JYFL. Acta Physica Polonica B, 30(5), 12451254. https://www.actaphys.uj.edu.pl/fulltext?series=Reg&vol=30&page=1245 1999 Vol. 30 (1999) ACTA PHYSICA POLONICA B No 5 DECAY AND IN-BEAM STUDIES OF NEUTRON-DEFICIENT Po AND Ra ISOTOPES AT JYFL  M. Leino a , R.G. Allatt b , A.N. Andreyev  ; d , J.F.C. Coks a , O. Dorvaux a , T. Enqvist a ; e ; K. Eskola f , K. Helariutta a , M. Huyse  , P.M. Jones a , R. Julin a , S. Juutinen a , H. Kankaanpää a , A. Keenan b , H. Kettunen a , P. Kuusiniemi a , M. Muikku a , R.D. Page b , P. Rahkila a , A. Savelius a , W.H. Trzaska a , J. Uusitalo a ; g , P. Van Duppen  a University of Jyväskylä, Department of Physis P.O. Box 35, FIN-40351 Jyväskylä, Finland b University of Liverp o ol, Department of Physis, Liverp o ol, U. K.  University of Leuven, Instituut vo or Kernen Stralingsfysia, Leuven, Belgium d Permanent address: FLNP, JINR, Dubna, Russia e Present address: GSI, Darmstadt, Germany f University of Helsinki, Department of Physis, Helsinki, Finland g Present address: Argonne National Laboratory, Argonne, Illinois, USA (Reeived Deember 8, 1998) An extensive program to study the pro dution, deay prop erties, and nulear struture of very neutron-deient polonium and radium nulei is underway at the Department of Physis, University of Jyväskylä, Finland (JYFL). The main to ols used in these studies are the gas-lled reoil separator RITU and various germanium gamma-ray arrays. In the ourse of these studies, among others the following new isotop es have been pro dued: 204 Ra, 203 Ra, and 202 Ra. Isomeri alpha deaying states have b een disovered in 203 Ra and 191 Po. Fine struture in the deay of 192 Po to the oblate and prolate band heads in 188 Pb has b een observed. In-b eam gamma-ray sp etra have b een, for the rst time, measured for 192 Po, 206 Ra, 208 Ra, and 210 Ra. Development of olletivity in nulei in the Po-Ra region and the systematis of redued alpha widths will b e disussed. PACS numbers: 23.20.Lv, 25.70.z, 23.60.+e  Presented at the International Conferene Nulear Physis Close to the Barrier, Warszawa, Poland, June 30July 4, 1998. (1245) 1246 M. Leino et al. 1. Intro dution The availability of large esap e suppressed Ge detetor arrays [1℄ has during the past few years made it p ossible to study the development of olletive phenomena in nulei far from stability. The established metho d of pro duing these nulei is heavy ion indued fusion. In the region of heavy elements where ssion of the omp ound system strongly dominates over partile evap oration, eient ltering is needed to extrat in-b eam gamma-rays originating from weak evap oration hannels. This an b e ahieved by ombining the Ge array with a reoil separator. Only those gamma-rays will b e aepted whih are in oinidene with separated evap oration residues. Due to the small pro dution ross setions of these neutron-deient nu- lei, a high-transmission separator is desirable. Suh devies do not always provide a unique identiation of the separated nulei. This drawbak an b e overome by using the Reoil Deay Tagging (RDT) metho d [2℄. The separated nuleus is identied through its harateristi deay in a p osition sensitive fo al plane silion detetor. Most often, alpha deay is used for identiation although proton radioativity an also b e used. An example of the p ower of the RDT metho d will b e shown in Set. 6 in onnetion with the disussion of the 206 Ra in-b eam study. Due to the seletivity of the pro ess through whih nulear levels are p opulated by the emission of gamma-rays following the pro dution of evaporation residues, imp ortant non-yrast low lying levels are not always aessible to study using in-b eam metho ds. A omplementary metho d is then provided by radioative alpha or b eta deay. An example is the observation of deformed 2 p - 2 h intruder 0 + states in the losed shell nulei 192  198 Pb from b eta deay of Bi isotop es [3℄. The determination of hindrane fators in alpha deay may also provide information on nulear struture [4℄. The region of neutron-deient nulei around Pb with the losed Z = 82 proton shell is partiularly interesting for the study of shap e o existene phenomena and the related intruder states. The losed proton shell favours spherial shap e but around midshell ( N = 104 ) the interation of 2 p - 2 h and 4 p - 4 h and higher exitations aross the shell with the large numb er of ative valene neutrons leads to a deliate omp etition b etween spherial and deformed states at low spin and exitation energy [5,6℄. Another way of lo oking at the deformation is based on deformation driving high-j neutron orbitals [7℄. Here we will base the disussion on the np-nh exitation onept. In this pap er, we will present results from studies of neutron-deient Po and Ra nulei p erformed at the Department of Physis, University of Jyväskylä, or JYFL. In all the exp eriments, the gas-lled heavy ion reoil separator RITU [8℄ was used to separate and identify the nulei. Heavy ions were aelerated using the K = 130 MeV JYFL ylotron. Both in-b eam gamma-ray studies and alpha deay studies will b e disussed. Deay and in-Beam Studies of Neutron-Deient... 1247 2. Observation of exited states in 192 Po The Po isotop es present an illuminating example of the development of olletive phenomena far from stability [3,7,9℄. A wide variety of states an b e studied through in-b eam metho ds as well as through alpha and b eta deay. On the other hand, alpha deay of Po isotop es provides an eient omplementary metho d to study the struture of Pb isotop es. This will b e disussed in Set. 3. Prior to our work on 192 Po, exited states had b een known down to 194 Po whih was studied using the Fragment Mass Analyzer (FMA) at Argonne [10℄. In that work, a ontinuation of the steep deline of level energies of p ositive parity yrast levels up to 10 + as ompared with heavier Po isotop es was observed. It was of interest to extend these studies and to approah the neutron midshell at N = 104 ( 188 Po). This exp eriment was p erformed using 178 MeV 36 Ar ions to b ombard a 70% enrihed 500  g/m 2 thik 160 Dy target. The results have b een published in Ref. [11℄ and rep eated in Ref. [12℄. Prompt gamma-rays from the target were deteted by the DORIS array onsisting of nine TESSA typ e [13℄ esap e suppressed Ge detetors and having an eieny of 0.6% at 1.3 MeV. The ground state band from 192 Po was observed up to the 8 + level. The tentative spin and parity assignments are based on transition intensities, level systematis and on  -  oinidene measurements. Unambiguous assignment to 192 Po was based on the use of the RDT metho d. The low-energy systematis for even-even Po isotop es is shown in Fig. 1. The data are from Refs. [7,911,14℄. 190 192 194 196 198 200 202 204 206 208 210 212 500 1000 1500 2000 0 + (4p-2h) 4 + 2 + 2+ 4+ 6+ 8+ 6+ 4+ 2+ Excitation energy (keV) Mass number Fig. 1. Systematis of low-lying level energies of even-even Po isotop es. 1248 M. Leino et al. 3. Fine struture in the  deay of 192 Po Prolate strutures in the neutron-deient 184 Pb, 186 Pb and 188 Pb nulei were disovered reently in in-b eam exp eriments [1517℄. It was onluded [16℄ that the struture of the observed bands is dierent from the 2 p - 2 h proton intruder states in 190  208 Pb mentioned ab ove. A omparison with the well established prolate bands observed in the 184 ; 186 Hg isotones indiated that the observed strutures are indeed asso iated with the prolate minimum predited by the p otential energy surfae alulations (see for example [6℄). Due to deay out of the bands, the 0 + band heads were not observed in these exp eriments. In another reent work [18℄, on ne struture in the  deay of 192 Po, an exited 0 + state was observed in 188 Pb. It was onluded that this was the oblately deformed 2 p - 2 h proton intruder state observed in heavier Pb isotop es. In the work desrib ed here [19℄, the standard p osition-sensitive fo al plane detetor was ombined with an auxiliary detetor system for observing alpha partiles and onversion eletrons esaping from the stop detetor at bakward angles. The reation used to study ne struture in the alpha deay of 192 Po was 36 Ar + 160 Dy. The b ombarding energy was varied b etween 172 and 184 MeV and the target thikness was 500  g/m 2 . When alpha deay takes plae from the ground state of 192 Po to low-lying exited 0 + states in 188 Pb, the exitation energy in the daughter nuleus is exp eted to b e removed primarily through onversion eletrons from E0 transitions. Thus, by demanding a oinidene b etween alpha deays in the stop detetor and onversion eletrons in the auxiliary detetor, it was p ossible to observe alpha deay ne struture to two low-lying 0 + states in 188 Pb. One of these, at an exitation energy of (568  4) keV, was identied as the oblate band head thus onrming the result of Ref. [18℄. The other state has an exitation energy of (767  12) keV and was identied as the prolate 0 + band head. It is of interest to dedue the mixing amplitude of the oblate 4p-2h state in the ground state of 192 Po. This an b e ahieved through exp erimental alpha deay hindrane fators and mixing amplitudes for the exited 0 + states in 188 Pb [4℄. The hindrane fators as determined relative to alpha deay to the ground state of 188 Pb were dedued to b e 0.67  0.10 (deay to the oblate 0 + state) and 0.22  0.08 (deay to the prolate 0 + state). The mixing amplitudes were determined following the pro edure of Draoulis [20℄ by tting to the exp erimental levels from this work and from Ref. [16℄. Then, by representing the ground state of 192 Po as a ombination of 2 p - 0 h and 4 p - 2 h states, it was p ossible to determine the admixture of  63% for the intruder 4 p - 2 h onguration in the ground state of 192 Po. The unmixed Deay and in-Beam Studies of Neutron-Deient... 1249 prolate 0 + level energy of  730 keV in 188 Pb agrees well with the value of  710 keV estimated from the energies of higher memb ers of the band [16℄. 4. Alpha deay studies of neutron-deient Po isotop es The main interest b ehind  deay studies of neutron-deient Pb and Po nulei lies in the determination of redued alpha deay widths [21℄ and in the nulear struture information to b e dedued from these. An early measurement using a gas-lled separator at Lawrene Berkeley Lab oratory of the half-life of 192 Po [22℄ gave the surprising result that the redued width of 192 Po is lower than that of its heavier Po neighb ours. This b ehaviour diers from that of heavier elements for whih the redued alpha width inreases as the neutron numb er dereases. This result was onrmed by the ndings in the ne struture studies [18,19℄. This work was extended to the isotop e 190 Po for whih a half-life of (1.9 +0 : 6  0 : 4 ) ms was measured at JYFL [23℄. It was found that the saturation trend i. e. the near-onstany of redued widths p ersisted for 190 Po. In later work using RITU at JYFL, the half-life value for this nuleus was improved to (2.53  0.33) ms [24℄. One should note that the saturation of the Po  deay widths an b e removed by summing up the widths of the deay to the even-even Pb ground state and to the oblate exited 2 p - 2 h 0 + state [23℄. In Fig. 2 we show the systematis of the redued  deay widths for even-even nulei from Po to Th. The data are from Refs. [19,2429℄. Conerning new data on the alpha deay of the o dd isotop e 191 Po, we refer to Ref. [30℄. 102 104 106 108 110 112 114 116 118 120 122 124 126 128 130 132 10 100 Th Ra Rn Po Po 102 104 106 108 110 112 114 116 118 120 122 124 126 128 130 132 10 100 102 104 106 108 110 112 114 116 118 120 122 124 126 128 130 132 10 100 ( ) ( ) ( ) 102 104 106 108 110 112 114 116 118 120 122 124 126 128 130 132 10 100 Reduced width (keV) Neutron number Fig. 2. Redued alpha deay widths of even-even Po, Rn, Ra, and Th isotop es. The round symb ols for Po represent sums of widths for deays to the Pb ground state and to the oblate 0 + exited state. The values for 196 Rn, 202 Ra, and 210 Th (in brakets) are based on the observation of only three, one and two deay hains, resp etively and have rather large asymmetri error bars. 1250 M. Leino et al. 5. Alpha deay studies of neutron-deient Ra isotop es During the last few years, the three new radium isotop es 202 ; 203 ; 204 Ra were disovered using RITU [25℄. The results onerning 202 Ra are tentative sine they are based on the observation of only one deay hain orresp onding to a ross setion of 2 nb in the reation 36 Ar + 170 Yb. These data give information on the nulear mass surfae around the proton drip line as well as on redued widths. They are also of interest due to the predited app earane of nulear ground state deformation in the viinity of 200 Rn [31℄. The predited ground state deformation for 200 Rn, ab out 0.2, would orresp ond to an exitation energy of  140 keV for the rst 2 + state. Suh a low energy was exluded in our work [25℄ sine no deays to the 2 + state were observed. This result has later b een onrmed by in-b eam RDT data [32℄ where the 2 + state was found to b e at 433 keV exitation energy. In 203 Ra, two alpha deaying isomeri states were found. They were observed to deay to orresp onding states with I  = 3/2  and 13/2 + , resp etively, in 201 Rn. The hindrane fators determined for the two transitions were 1.6 +1 : 2  0 : 4 (13/2 + ! 13/2 + ) and  0.04 (3/2  ! 3/2  ). The rst of these is based on the observation of seven deay hains and is the basis for the assumption that the alpha deay is unhindered and onnets states with idential spin and parity. The latter value is quite unertain sine it is based on the observation of only one deay hain. It would b e desirable to ollet more data on the deay of the orresp onding 3/2  isomer. The redued  deay widths of even-even neutron-deient Ra isotop es are shown in Fig. 2. 6. In-b eam studies of 206 ; 208 ; 210 Ra Radium nulei have six protons outside the losed Z = 82 shell. When also the numb er of neutron holes inreases as one moves away from the losed N = 126 shell towards the proton drip line it is exp eted that a new region of ground state deformation will b e enountered. As disussed ab ove, suh a region has b een predited to emerge rather suddenly around 200 Rn and 204 Ra [31℄. The rapid drop of pro dution ross setions as one moves towards more neutron-deient nulei makes it diult to study these nulei. In addition, the large numb er of valene partiles do es not allow for a straightforward interpretation of the dedued level shemes. Nevertheless, an attempt was made to study these nulei in-b eam using the JUROSPHERE array. JUROSPHERE onsisted of 11 TESSA-typ e [13℄ and 14 Eurogam phase I typ e [33℄ Compton suppressed Ge detetors in this partiular exp eriment. It had a total photop eak eieny of 1.6% at 1.3 MeV. The Eurogam Ge detetors were provided by the Frenh/UK Loan Po ol. Prior to our studies, the lightest Ra isotop e for whih transitions were known was 212 Ra [34℄. Due to the exp eted presene of isomers in these nulei, additional Ge detetors were Deay and in-Beam Studies of Neutron-Deient... 1251 plaed at the fo al plane of RITU. For a more detailed desription of the exp erimental setup see Ref. [35℄. 0 100 200 300 400 500 600 700 800 Energy (keV) 0 10 20 30 40 50 Counts 0 5 10 15 20 x 103 Counts 710.6 474.3 577.8 247.4 X-rays Any Recoil-Gamma Coincidence Ra 206 Fig. 3. Gamma-ray sp etra measured in-b eam in the reation 40 Ar + 170 Yb. Upp er panel: gamma-rays in oinidene with any fusion pro dut observed at the fo al plane of the separator. Lower panel: Gamma-rays in oinidene with 206 Ra nulei, identied on the basis of their harateristi alpha deay. The dashed lines have b een drawn to guide the eye. 0+ (2+) (4+) (6+) (8+) 0+ (2+) (4+) (6+) (8+) (4+) (6+) 0+ (2+) (4+) (6+)(8+) (4+) 0+ 2+ 4+ 6+ 8+ 10+11– 8+ 0+ 2+ 4+ 6+8+ 8+ 11– A E (MeV) 1 2 206 208 210 212 214 πh9/2 ns ns <1 ns270 300 ns850 2.24µ 10.9µ67µ sss 3 6 Fig. 4. Systematis of low-lying level energies for neutron-deient even-even Ra isotop es. 1252 M. Leino et al. To demonstrate the p ower of the RDT metho d, we show in Fig. 3 a omparison of in-b eam gamma-ray sp etra measured for 206 Ra with and without the RDT-based identiation of the separated nulei. The pro dution ross setion of 206 Ra in the reation employed, 180 MeV 40 Ar + 170 Yb, was only  5  b and the reoil-gated sp etrum is ompletely dominated by the muh more abundant Fr and Rn nulei pro dued in harged partile evap oration hannels. The RDT sp etrum, on the other hand, shows no p eaks from ontamination ativities. In Fig. 4, a ompilation of the level shemes dedued for 206 ; 208 ; 210 Ra from the present work and for 212 ; 214 Ra from earlier work [34,36,37℄ is shown. 7. Disussion Our new data on the level struture of 192 Po show that the trend of dereasing level energies p ersists as one pro eeds still farther towards the proton drip line. However, the steepness of the drop is signiantly redued. Similarly to the even-mass Pt nulei, this attening of energy systematis an b e interpreted as evidene for a ground state intruder onguration. An interesting example of the kind of omplementary information one an dedue from deay exp eriments is our value for the admixture of the oblate 4p-2h omp onent in the ground state of 192 Po as dedued on the basis of alpha deay hindrane fators,  63% [18,19℄. Moreover, on the basis of NilssonStrutinsky typ e of alulations it was suggested by May et al. [5℄ that in 192 Po an oblate deformed minimum b eomes the ground state. The most obvious feature of the systematis of level energies of Ra isotop es shown in Fig. 4 is the smo oth and gradual derease of the 2 + energies. There is no evidene for an abrupt inrease in ground state deformation. The exitation energies of the 6 + and 8 + memb ers of the  h 6 9 = 2 multiplet inrease with dereasing neutron numb er until at 206 Ra where a drop in the exitation energy of the 8 + state is observed. Furthermore, the 8 + state deays to the (lowest) 6 + state through a fast transition in 206 Ra while in 208 Ra this transition is signiantly hindered. Assuming that the lowest 6 + states in 206 Ra and 208 Ra at very nearly idential exitation energy have the same onguration, p erhaps  h 9 = 2 5 f 7 = 2 , this would imply that the 8 + states in 206 Ra and 208 Ra have dierent origin. Conerning redued widths for alpha deay, the summing up of widths of b oth ground state to ground state and ground state to exited 2 p - 2 h 0 + state transitions for Po nulei removes the anomalous saturation b ehaviour of Po nulei. To study the width systematis in more detail, it would b e of interest to p erform preision measurements on the half-life of 196 ; 198 Rn and of 202 ; 204 Ra. The values for 196 Rn and 202 Ra are based on the observation of only three deay hains [26℄ and one deay hain [25℄, resp etively. Further-