High K Bands in Mid-Supershell Nuclei
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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/ High K Bands in Mid-Supershell Nuclei © 2003 Jagellonian University Published version Butler, P. A.; Humphreys, R. D.; Greenlees, P. T.; Herzberg, R.-D.; Jenkins, D. G.; Jones, G. D.; Kankaanpää, H.; Kettunen, H.; Rahkila, P.; Scholey, C.; Uusitalo, J.; Amzal, N.; Andreoiu, C.; Andreyev, A; Appelbe, D.; Bastin, J. E.; Brew, P. M. T.; Eskola, K.; Freeman, S. J.; Gerl, J.; Hammond, N. J.; Hauschild, K.; Helariutta, K.; Hessberger, F.-P.; Huerstel, A.; Ikin, P. J. C.; Jones, P. M.; Joss, D. T.; Julin, R.; Juutinen, S.; Keenan, A.; Khoo, T.-L.; Korten, W.; Kuusiniemi, P.; Le Coz, Y.; Leino, M.; Leppänen, A.-P.; Muikku, M.; Nieminen, P.; Odegard, S. W.; Page, R. D.; Page, T.; Pakarinen, J.; Reiter, P.; Simpson, J.; Sletten, G.; Theisen, Ch.; Varley, B. J.; Wollersheim, H.-J. Butler, P. A., Humphreys, R. D., Greenlees, P. T., Herzberg, R.-D., Jenkins, D. G., Jones, G. D., Kankaanpää, H., Kettunen, H., Rahkila, P., Scholey, C., Uusitalo, J., Amzal, N., Andreoiu, C., Andreyev, A., Appelbe, D., Bastin, J. E., Brew, P. M. T., Eskola, K., Freeman, S. J., . . . Wollersheim, H.-J. (2003). High K Bands in Mid-Supershell Nuclei. Acta Physica Polonica B, 34(4), 2107-2117. https://www.actaphys.uj.edu.pl/R/34/4/2107 2003
Vol. 34 (2003) ACTA PHYSICA POLONICA B No 4 HIGH K BANDS IN MID-SUPERSHELL NUCLEI P.A. Butler a , R.D. Humphreys a , P.T. Greenlees b R.-D. Herzberg a , D.G. Jenkins a , G.D. Jones a , H. Kankaanpää b H. Kettunen b , P. Rahkila b , C. Sholey a ; b , J. Uusitalo b N. Amzal a , C. Andreoiu a , A. Andreyev a , D. Appelbe j J.E. Bastin a , P.M.T. Brew a , K. Eskola , S.J. Freeman k , J. Gerl d N.J. Hammond a , K. Haushild e , K. Helariutta d F.-P. Heÿberger d , A. Hürstel e , P.J.C. Ikin a , P.M. Jones b D.T. Joss j , R. Julin b , S. Juutinen b , A. Keenan b , T.-L. Khoo f W. Korten e , P. Kuusiniemi b , Y. Le Coz e , M. Leino b A.-P. Leppänen b , M. Muikku b , P. Nieminen b , S.W. Ødegård g R.D. Page a , T. Page a , J. Pakarinen b , P. Reiter h , J. Simpson j G. Sletten i , Ch. Theisen e , B.J. Varley k , and H.-J. Wollersheim d a Oliver Lo dge Lab oratory, University of Liverp o ol, Liverp o ol L69 7ZE, U.K. b Department of Physis, University of Jyväskylä, FIN-40014 Jyväskylä, Finland Dept. of Phys. Si., University of Helsinki, FIN-00014 Helsinki, Finland d GSI, D-64291 Darmstadt, Germany e DAPNIA/SPhN CEA-Salay, F-91191 Gif-sur-Yvette, Frane f Argonne National Lab oratory, Argonne, Illinois 60439, USA g Department of Physis, University of Oslo, N-0316 Oslo, Norway h Ludwig Maximilians Universität, D-85748 Garhing, Germany i Niels Bohr Institute, 2100 Cop enhagen, Denmark j CLRC Daresbury Lab oratory, Warrington WA4 4AD, U.K. k Shuster Lab oratory, University of Manhester, Manhester M13 9PL (Reeived November 12, 2002) The sp etrum of prompt onversion eletrons emitted by exited 254 No nulei has b een measured, revealing disrete lines arising from transitions within the ground state band. A striking feature is a broad distribution that p eaks near 100 keV and omprises high multipliity eletron asades, probably originating from M 1 transitions within rotational bands built on high K states. Evidene for the existene of isomeri states in 254 No is presented. PACS numb ers: 21.10.k, 23.20.Lv, 23.20.Nx, 23.60.+e Presented at the XXXVI I Zakopane Sho ol of Physis Trends in Nulear Physis, Zakopane, Poland, September 310, 2002. (2107)
2108 P.A. Butler et al. 1. Intro dution The understanding of the struture of the heaviest elements, in partiular sup erheavy elements (SHE), is essential for the development of mean eld theories that are used to predit nulear prop erties far from stability (for reviews see [1, 2℄). Exp erimental insight into the struture of sup erheavy nulei an b e obtained by diret measurement of the ground state prop erties of nulei (for review, see [3℄). Attempts to reah the spherial SHE have b een rep orted reently in whih the observation of -deay from nulei with Z = 114 , 116 and N = 174 , 176 with lifetimes of the order of seonds has b een laimed [4, 5℄. Equally imp ortant information an ome from the study of mid-shell deformed nulei, sine seleted single partile orbitals that lie lose to the spherial shell gap in SHE are lose to the Fermi level in nulei having large quadrup ole deformation. Suh information an ome from radioative deay sp etrosopy or from in-b eam sp etrosopy. In the latter tehnique the prompt deay pro ess is tagged by detetion of the reoiling nuleus or by alpha deay from the reoil (RDT), using eletromagneti separators. In this manner, in-b eam -ray sp etrosopy has enabled the rotational b ehavior of the even-even nulei 252 No [6℄ and 254 No [79℄ to b e studied up to spin 20 ~ . In these exp eriments the reation pro duts, although p opulated with small ross setions ( 3 barn), have b een separated from the dominant ssion bakground. The measurements onrmed that nulei having Z 100 and N 150 have ground state deformation of 0 : 3 . It is exp eted that these nulei are the homologues of neutron rih Er-Hf in the lower osillator shell where rotational bands built on high K states are observed. The quantiation of the struture of these states will b e imp ortant for xing the parameters of mo dels used to predit SHE prop erties. We rep ort here a new exp erimental metho d that an reveal information on heavy nulei additional to that obtained from gamma-ray sp etrosopy. Our tehnique allows the diret detetion of internal onversion eletrons, emitted at the target, in a broad-range, high eieny eletron sp etrometer. The eletrons an b e tagged by reoil detetion or using RDT. The sensitivity of the tehnique is demonstrated here by applying it to the measurement of the low-lying transitions in the ground state band of 226 U, whih are easily observed in a relatively short running time, and to a study of onverted transitions in 254 No, for whih the 4 + ! 2 + up to the 10 + ! 8 + transitions were observed. These studies reveal a striking dierene in the bakground of unresolved transitions for the two reations. We hyp othesize that this arises from the exp eted presene of highK multi-quasipartile states near the Fermi surfae in 254 No.
High K Bands in Mid-Supershel l Nulei 2109 2. In-b eam eletron sp etrosopy: SACRED The eletron sp etrometer, SACRED [10℄, employs a single Si PIN wafer, 500 mirons thik, segmented into 25 pixels onneted to individual ampli- ation and timing hannels. The geometry is irular, with 6 annuli, divided into quadrants, surrounding the entral element. The outer diameter of the detetor is 28 mm. Eletrons are transp orted from the target to the detetor using a solenoidal magneti eld of maximum ux density 0.3 T generated by 4 separated, normal onduting oils. The target-detetor distane is 540 mm. The b eam axis is at an angle of 2.5 Æ to the eld axis, interseting at the target p osition. This arrangement has the advantage of having an approximately ollinear geometry, while ensuring that the b eam is displaed by 25 mm from the eld axis at the (upstream) detetor p osition. The resp onse of the detetor to onversion eletrons emitted in the deay of a 133 Ba soure is shown in gure 1. The absolute eieny is ab out 10% for energies < 350 keV. For in-b eam measurements the delta eletron bakground is redued to an aeptable level by an eletrostati barrier plaed b etween the target and the detetor. Further details of the mehanial onstrution are given in Ref. [11℄. 0 50 100 150 200 250 300 10 3 10 4 10 5 10 6 counts/2keV test X Axis Title electron energy (keV) Fig. 1. Sp etrum of 133 Ba soure, showing the detetor resp onse on a logarithmi sale. The ollinear geometry, while oering the advantage of reduing b oth Doppler broadening of the eletron lineshap e and the delta eletron ux in the bakward diretion, enabled the eletron sp etrometer to b e oupled to the gas-lled reoil separator RITU [12℄. The reoil pro duts were transp orted in RITU to a 16-fold segmented and resistive silion pad detetor at its fo al plane. This detetor is divided into approximately 200 pixels.
2110 P.A. Butler et al. The magnet volume of RITU and the setion of SACRED ontaining the target are lled with 0.3 and 0.7 mbar helium gas for reations indued by A 20 and A 50 pro jetiles resp etively. This volume is separated from the remaining volume of SACRED by two 60 g/m 2 arb on foils of 15 mm radius and 170 mm separation with pump ed intermediate volume. In this way the pressure of the region ontaining the barrier and the detetor was maintained at ab out 10 6 torr, thus reduing the bakground from aelerated eletrons pro dued following ionization of the residual gas moleules by the b eam. The energy loss of 50150 keV eletrons in the target and foils is 11.5 keV. 3. Conversion eletron measurements in 226 U These exp eriments were arried out at the aelerator lab oratory of the University of Jyväskylä. In the rst exp eriment a 10 partile nA b eam of 111 MeV 22 Ne (orreting for energy loss in the arb on foils) b ombarded a 208 Pb target of thikness 200 g/m 2 for approximately 25 h. In this exp eriment the p otential of the eletrostati barrier was 35 kV with resp et to target and detetor. The onversion eletron sp etrum of 226 U, pro dued in the 6 b 4 n hannel, is shown in gure 2(a). It was obtained by requiring that the detetion of any eletron at the target using SACRED b e aompanied by the detetion of reoils in the fo al plane detetor of RITU within a time window of 50 ns of their arrival at the fo al plane. The reoils are identied by requiring that there is an alpha deay of energy range 7.497.63 MeV within 800 ms in the same pixel of the implantation detetor. There were 1280 tagged reoils deteted in this exp eriment. Figure 2(b) shows a simulated sp etrum obtained using the Monte Carlo program desrib ed in Ref. [10℄, orresp onding to the same numb er of reoils as observed exp erimentally. The simulation assumes that the observed onversion eletrons only arise from transitions previously observed in an array of -ray sp etrometers and identied using the RDT tehnique [13℄. More details of the onversion eletron exp eriment and its results, in partiular the rst rm assignment of the energy of the 2-0 transition, are given in Ref. [14℄. It is evident from omparison of the exp erimental and simulated sp etrum that the exess bakground observed over that a- ounted for by the simulation is quite small. The simulation takes into aount the transp ort of the eletrons in the magneti and eletri elds, sattering in or from the detetor, energy sharing at the pixel b oundaries, and threshold eets.
High K Bands in Mid-Supershel l Nulei 2111 0 50 100 150 200 250 300 0 20 40 Y Axis Title electron energy (keV) b) 0 20 2 0 42 L M L M counts/2keV X Axis Title a) Fig. 2. (a): Exp erimental onversion eletron sp etrum (orreted for the ontribution from random oinidenes) tagged by the harateristi -deay of 226 U. In this sp etrum the energies are not orreted for Doppler shift. Transitions in the ground state band are identied. (b): Simulated sp etrum for 226 U as desrib ed in the text. 4. Measurement of high multipliity ontinuum in 254 No In a seond exp eriment a b eam of 219 MeV 48 Ca was employed. In this ase the average b eam energy in the enter of the 208 Pb target (216 MeV) orresp onds to the maximum of the yield of the reation 208 Pb( 48 Ca, 2 n ) 254 No. Targets of thiknesses 250 g/m 2 and 400 g/m 2 of enrihment 98% 208 Pb were eah b ombarded by a b eam of 1.53 partile nA for approximately 110 h eah. The p otential of the eletrostati barrier was 40 kV. Figure 3(a) shows the total eletron sp etrum tagged by the detetion of fusion pro duts. In this ase it is not neessary to verify that the reoils are 254 No by measuring their alpha deay as there are no other omp eting omp ound nuleus hannels: the ombined p opulation of 253 ; 255 No is 1 % of that of 254 No [15, 16℄. The eletron sp etrum whih is tagged by the subsequent deay of
2112 P.A. Butler et al. 0 50 100 150 200 250 300 0 50 100 electron energy (keV) d) 0 10 20 c) g 0 25 50 counts/2keV b) 0 25 50 L M 10 8 8 6 L M 6 4 L M 4 2 L M a) Fig. 3. (a): The exp erimental onversion eletron sp etrum tagged by 254 No re- oils, orresp onding to any number of eletrons deteted in SACRED (solid b old line). In this sp etrum the energies are not orreted for Doppler shift. The lowest transitions in the ground state band are identied. Also shown are the simulated sp etra as desrib ed in the text for g K = 0 (hashed area with solid line b order) and for g K g R = 0 (dashed line). (b): as (a) exept that exp eriment and simulation orresp ond to the detetion of a single eletron. (): Spetrum of seond eletron in oinidene with a seleted region (labelled `g') of energy for the rst eletron, tagged by reoils (solid b old line). Also shown is the exp erimental spe- trum tagged by -deay for any numb er of eletrons deteted (dashed line). (d): eletron sp etrum taken in random oinidene.
High K Bands in Mid-Supershel l Nulei 2113 alpha partiles in the energy range 8.04 to 8.15 MeV, with a searh time of 600 s (see dashed line in gure 3()), has approximately half the total ounts, as exp eted. We were able to distinguish evap oration residues, target-like re- oils, sattered b eam and radioative deay pro duts by employing a parallel plate prop ortional ounter in front of the silion implantation detetor. Two features are apparent from gure 3(a). First, the onversion ele- trons orresp onding to transitions in the ground state rotational band up to spin 10 in 254 No are identiable (the struture observed at 65 and 105 keV arises from hitherto unidentied transitions in 254 No). The details of these measurements, in partiular the rst diret measurement of the energy of the 42 transition, are given in Ref. [14℄. The seond notieable feature is the pronouned bakground, entered at around 100 keV. The intensity of this bakground is muh larger than the bakground observed in the sp etrum orresp onding to 226 U transitions (gure 2(a)). The most interesting prop erty of the bakground is that it has a muh higher eletron multipliity than that of the disrete transitions. This is demonstrated by demanding that the sp etrum is only inremented if no other eletrons are deteted in any of the SACRED pixels within 100 ns of the detetion of the rst eletron. The resulting sp etrum is shown in gure 3(b) in whih the p eak to bakground is signiantly improved. The simulation o de an b e used to roughly estimate the mean eletron multipliity, as it provides an aurate mo del of the resp onse of the SACRED sp etrometer. It remains to mo del the mehanism by whih the entry states in 254 No dep opulate and emit onversion eletrons. The simplest mo del is to assume that only the ground state band is p opulated, so that there is no other soure of onversion eletrons. In this ase the relative intensities of the transitions feeding the 4 + state are taken from the gamma-ray measurements [8, 9℄. The present measurements indiate that the intensity of the 64 and the 42 transitions are the same [14℄, and these are assumed to b e the same as that of the 20 transition. The measured yield of the 64 L transition in the ground state band is 126 18 for a single eletron deteted, 160 30 for any numb er of eletrons deteted (ratio R (6 4) = 0 : 78 0 : 16 ), for a total of 7150 re- oils. In the simulation, arried out for four times the numb er of reoils to that reorded in the measurement, the values for the yield of the 64 transition are resp etively 1280 and 1530 ( R (6 4) = 0 : 84 ). The measurements are onsistent with the simulation if ab out 40% of the p opulation of 254 No passes through the lowest transitions. On average 3.7 eletrons are emitted simultaneously in this deay path, inluding the undeteted 20 transition. In order to mo del the bakground, we assumed that this arises from de- ays within a single rotational band built on a K = 8 isomeri bandhead and p opulated with the same entry spin distribution as that measured for all states by Reiter et al. [9℄. The value of g K is taken to b e 0, with g R = 0 : 3 .
2114 P.A. Butler et al. For this band we assumed a onstant moment of inertia of 100 ~ 2 MeV 1 , slightly larger than that of the highest transitions in the ground state band (this allows a b etter t to the low energy part of the sp etrum). The ele- tron energies are then randomized by applying a gaussian distribution of = 10 keV, to aount for the presene of many suh bands having differing moments of inertia. The alulated sp etrum, also arried out for 28,600 reoils as for the previous simulation and then renormalised by a fator of 0.10, is shown also in gure 3(a). The measured intensity of the bakground mathes the alulated intensity, implying that approximately 40% of all deteted reoils pro eed through these paths. The exp eted values of g K for the lowest 2 quasi-partile K = 8 bands in 254 No are 0 : 3 ( 9 2 [734℄ 7 2 + [613℄ , Ref. [17℄), 0 ( 9 2 [734℄ 7 2 + [624℄ , Ref. [18℄) and 1 ( 9 2 + [624℄ 7 2 [514℄ , Refs. [17, 18℄). For this range of values of g K the alulated ratio of integrated ounts for single and any eletron deteted ( R bkg ) is 0.570.59, similar to the measured value of 0 : 58 0 : 03 . The orresp onding value of the mean eletron multipliity for these paths is in the range 78. In the extreme ase when ( g K g R ) = 0 (purely eletri transitions) the al- ulated value of R bkg is 0.64, orresp onding to an eletron multipliity of 4 distributed evenly b etween I ! I 1 and I ! I 2 transitions. In this ase the alulated sp etrum overestimates the high energy part of the sp etrum (see dashed line in gure 3(a),(b)). If it is assumed that the bandhead is not isomeri, so that the K = 8 state deays immediately to the 8 + memb er of the ground state band, then the value of R bkg for g K = 0 is 0.47 (multipliity 10). This feeding pattern annot b e signiant, as demonstrated in gure 3() whih shows the sp etrum of eletrons in oinidene with a seleted region of the bakground. Finally, we have to onsider the p ossibility that the bakground is largely atomi in origin and that it arises from the atomi ollisions of the reoiling nob elium atoms with the Pb atoms in the target. We b elieve that this p ossibility is unlikely as the bakground has a very dierent shap e to that arising from the delta bakground pro dued by ollisions of b eam partiles with the target. Figure 3(d) shows the eletron sp etrum taken in random oinidene, whih is idential in sp etral shap e to the singles sp etrum. It p eaks in intensity at an energy that is lose to the barrier voltage. In addition, we ompared the integral bakground yield p er nob elium reoil deteted for when the target thikness was 250 g/m 2 (4710 reoils) and when the target thikness was 400 g/m 2 (2440 reoils). The measured values were very similar, 0 : 177 0 : 008 and 0 : 170 0 : 010 , resp etively. If the bakground arose from No+Pb atomi ollisions it might b e exp eted that the yield would sale with the target thikness.