The α decays of the two neutron-deficient nuclei 172Hg and 173Hg were observed for the first time using the 78Kr(96Ru,2n) and 80Kr(96Ru,3n) reactions, respectively. The reaction products were dispersed according to their mass-to-charge state ratios in the Argonne Fragment Mass Analyzer and implanted in a double-sided silicon strip detector, where their subsequent decays were studied using spatial and time correlations between implants and decays. A half-life of 250(+350-90) μs and an energy of 7350(12) keV were deduced for the α decay of 172Hg. In 173Hg the half-life was measured to be 0.93(+0.57-0.26) ms and the corresponding energy is 7211(11) keV. In addition, the half-life and energy of the α decay of 174Hg were measured more precisely. The reduced widths deduced for these Hg isotopes indicate that the observed decays correspond to unhindered Δl = 0 transitions. The α-decay Q values are compared with the values calculated using mass tables by Möller and Nix, and by Liran and Zeldes. The latter mass tables show better agreement with the data

Bingham, C.R.

Brown, L.T.

Carpenter, M.P.

Conticchio, L.

Davids, C.N.

Henderson, D.J.

Janssens, R.V.F.

Nisius, D.

Seweryniak, D.

Uusitalo, J.

Walters, W.B.

Wauters, J.

Woods, P.J.

Carolina Digital Repository

RAPID COMMUNICATIONSPHYSICAL REVIEW C, VOLUME 60, 031304Decay properties of the new isotopes172Hg and 173HgD. Seweryniak,1,2 J. Uusitalo,1 M. P. Carpenter,1 D. Nisius,1 C. N. Davids,1 C. R. Bingham,3 L. T. Brown,1 L. Conticchio,2D. J. Henderson,1 R. V. F. Janssens,1 W. B. Walters,2 J. Wauters,3 and P. J. Woods41Argonne National Laboratory, Argonne, Illinois 604392University of Maryland, College Park, Maryland 207423University of Tennessee, Knoxville, Tennessee 379964University of Edinburgh, Edinburgh, United Kingdom~Received 2 June 1999; published 10 August 1999!Thea decays of the two neutron-deficient nuclei172Hg and173Hg were observed for the first time using the78Kr( 96Ru,2n) and 80Kr( 96Ru,3n) reactions, respectively. The reaction products were dispersed according totheir mass-to-charge state ratios in the Argonne Fragment Mass Analyzer and implanted in a double-sidedsilicon strip detector, where their subsequent decays were studied using spatial and time correlations betweenimplants and decays. A half-life of 250(2901350) ms and an energy of 7350~12! keV were deduced for theadecay of 172Hg. In 173Hg the half-life was measured to be 0.93(20.2610.57) ms and the corresponding energy is7211~11! keV. In addition, the half-life and energy of thea decay of174Hg were measured more precisely. Thereduced widths deduced for these Hg isotopes indicate that the observed decays correspond to unhinderedD l 50 transitions. Thea-decayQ values are compared with the values calculated using mass tables by Mo¨llerand Nix, and by Liran and Zeldes. The latter mass tables show better agreement with the data.@S0556-2813~99!50409-3#PACS number~s!: 23.60.1e, 21.10.Dr, 21.10.Tg, 27.70.1qonaetoorleeatoneafi-ofeeeHutstateat-o-bo-ag--on,ofI.tedrdingWithin the last several years, a wealth of new information nuclei located at and beyond the proton drip line hbecome available in the region of the nuclidic chart betwethe closed proton shellsZ550 and Z582. Much of thiswork has been directed at the identification of new proemitters in odd-Z nuclei ranging from 131Eu (Z563) to185Bi (Z583) @1#. In addition, studies of thea-decay prop-erties of proton-rich nuclei has yielded complementary infmation on nuclei lying at the edges of stability. For exampmeasurements ofa-decay energies and lifetimes far from thline of stability represent one of the means to discriminbetween different mass formulas. The growing uncertaintydifferent models when moving towards the proton-drip liis reflected in an increasing spread of predicted massesa-decayQ values.In this Rapid Communication, we report on the identication of the two new isotopes172Hg and 173Hg by theirrespectivea decays. In addition, the half-life and energythe a decay of 174Hg have been measured with higher prcision. Prior to this work174Hg was the lightest known Hgisotope@2#. It should also be noted that there has also brenewed interest in the study of excited states in the lightisotopes. Recent measurements have examined the evol0556-2813/99/60~3!/031304~4!/$15.00 60 0313snn-,efnd-ngionof shape-coexistence between the near-spherical groundand the excited well-deformed prolate structure in176,178Hg@3,4#. In addition, Nilsson-Strutinsky calculations predict ththe light Hg isotopes (N<98) should exhibit an excited superdeformed minimum withb250.520.56 @5# which couldbe populated at high spins.The light isotopes of Hg studied in this work were prduced using beams of78Kr and 80Kr, from the ATLAS su-perconducting linear accelerator at Argonne National Laratory, impinging on a 0.4 mg/cm2 thick 96Ru isotopicallyenriched target. The 96Ru target was placed on0.7 mg/cm2 thick Al backing, which faced the beam durinthe experiment. The78Kr( 96Ru,2n) reaction at a beam energy of 375 MeV was used to produce172Hg, while 173Hgwas populated in the80Kr( 96Ru,3n) reaction at a beam energy of 400 MeV. Using the same beam-target combinatibut a lower beam energy of 375 MeV,174Hg was populatedvia the two-neutron evaporation channel. The summarythe reactions used in the present work is shown in TableEvaporation residues from the fusion reactions lisabove were separated from the beam and dispersed accoto their mass-to-charge state ratio (M /Q) using the ArgonnewereducedofTABLE I. The properties of the light Hg isotopes studied in the present work. The cross sectionsestimated assuming a 10% FMA efficiency. A screening correction of 29.7 keV was added to the dea-decayQ values. The half-lives (t1/2Rasm) and reduced widths (d2) were calculated using the prescriptionRasmussen@12#. Further details are given in the text.Reaction ECN* s Ea Qa t1/2 t1/2Rasm d2@MeV# @nb# @keV# @keV# @ms# @ms# @keV#96Ru(80Kr,2n)174Hg 37 330 7066(8) 7262(8) 1.920.310.4 2.4 92~17!96Ru(80Kr,3n)173Hg 50 15 7211(11) 7411(11) 0.9320.2610.57 0.86 67~25!96Ru(78Kr,2n)172Hg 36 9 7350(12) 7555(12) 0.2520.0910.35 0.33 96~55!©1999 The American Physical Society04-1coalnidtieiiref-aenuywgigwidtionksi-sd-thescayere-ae re-nedhteringnotD DRAPID COMMUNICATIONSD. SEWERYNIAK et al. PHYSICAL REVIEW C 60 031304Fragment Mass Analyzer~FMA! @6#. After passing through aposition sensitive parallel grid avalanche counter~PGAC!placed at the focal plane of the FMA for theM /Q measure-ment, the residues were implanted into a double-sided silistrip detector~DSSD! placed about 40 cm behind the focplane. The front and back surfaces of the 43 cm2, 60 mmthick DSSD were each divided into 40 strips. The front aback strips were orthogonal to each other, effectively diving the detector into 1600 independent pixels. Using spaand time correlations, the implants were linked with thsubsequenta or proton decays and with decays of thedaughter and granddaughter nuclei. The DSSD strips wgain-matched using a244Cm-240Pu a source and the sum oall strips was also calibrated using knowna activities pro-duced in the 78Kr196Ru reaction, namely,167Os, 168Os,171Ir, 172Pt, 171Pt, 171Au, and 172Au. The energies of thesealines were adopted from Ref.@7# and references therein, except for 171Au where the data were taken from Ref.@8#. Inorder to measure lifetimes every event was time stampedhalf-lives were fitted to the distribution of times betwecorrelated implant and decay events using the maximlikelihood method@9#.Figure 1~a! presents the energy spectrum of all decadetected in the DSSD using the78Kr196Ru reaction at anenergy of 375 MeV. In Fig. 1~b! only decays correlatedwithin 40 ms with implanted mass 172 residues are shoThe spectrum is dominated by strong lines correspondinthea decay of172Au and 172Pt ~corresponding to 1p1n and2p channels, respectively!. In addition, an isolated groupconsisting of three events is visible above 7 MeV in F1~b!. These three events are also present in Fig. 1~c! whichcontains decays followed within 100 ms bya particles cor-responding to the decay of168Pt @10# (Ea'6.83 MeV), andthus are assigned to the decay of the previously unknoisotope 172Hg. Based on these three events an energy7350~12! keV and a half-life of 250(2901350) ms were deducedfor the a decay of 172Hg.The decay spectrum measured for the80Kr196Ru reac-tion at 400 MeV is shown in Fig. 2~a!. The significant back-FIG. 1. Energy spectra ofa particles measured in the DSSusing the96Ru178Kr reaction at 375 MeV:~a! all decays;~b! de-cays following mass 172 residues within 40 ms;~c! decays followedby 6.83 MeV 168Pt a particles within 100 ms.03130nd-alrrendmsn.to.nofground visible at higher energies in Figure 2~a! is associatedwith implant events for which the focal plane detector dnot register a heavy residue, leading to the misinterpretaof this event as a decay. Fig. 2~b! presents decays which tooplace within 40 ms after the implantation of mass 173 redues. The spectrum is dominated by twoa lines correspond-ing to 173Au and 173Pt ~corresponding top2n and 2pn chan-nels, respectively!. A group of a particles with energies inexcess of 7 MeV can be seen as well in Fig. 2~b!. Sevenof these events were found to be followed within 100 mby a particles with energies of about 6.7 MeV, corresponing to the knowna decay of 169Pt @11,7# @see Fig. 2~c!#.This leads to an unambiguous assignment of this group todecay of a new isotope173Hg. Based on the propertieof these seven events, an energy of 7211~1 ! keV and half-life of 0.93(20.2610.57) ms were extracted for thea decay of173Hg.The results of the80Kr196Ru experiment at 375 MeV areshown in Fig. 3. The topmost panel shows the total despectrum whereas Fig. 3~b! contains only decays followingmass 174 implants within 40 ms. The decays which wfollowed within 100 ms by 6.55-MeVa particles, corre-sponding to the decay of170Pt @7#, are shown in the bottompanel. The line at'7 MeV visible in all three spectra corresponds to thea decay of 174Hg. From the present datadecay energy of 7066~8! keV and half-life of 1.9(20.310.4) mswere deduced, based on the 31 observed events. Thessults agree well with the previous measurement of 7069~11!keV and 2.1(20.711.8) ms reported in Ref.@2#. However, thestatistical errors are smaller since only four events assigto 174Hg were observed in Ref.@2#.It should be noted that the number of observed daugPt a particles correlated with the decay of172Hg, 173Hg, and174Hg, respectively, is consistent with a 100%a-decaybranch in 168Pt, 169Pt, and 170Pt. Table I summarizes theexperimental results obtained for light Hg isotopes, includthe estimated cross sections for172,173,174Hg. The FMA effi-ciency necessary to estimate cross sections wasFIG. 2. Energy spectra ofa particles measured in the DSSusing the96Ru180Kr reaction at 400 MeV:~a! all decays;~b! de-cays following mass 173 residues within 40 ms;~c! decays followedby 6.7 MeV 169Pt a particles within 100 ms.4-2ivcle9tinefeas-lcualI.hadfosesnedicnll,-ue-ndex-pesf ain-forons ineddandMoronis.S.er4,Dg.i-byRAPID COMMUNICATIONSDECAY PROPERTIES OF THE NEW ISOTOPES172Hg . . . PHYSICAL REVIEW C 60 031304measured, but an assumed value of 10% should gcross sections accurate to within a factor of 2. The nu172Hg and 173Hg were produced with cross sections ofnb and 15 nb, respectively. The cross section for popula174Hg is 330 nb, i.e., one order of magnitude highthan for 172Hg and 173Hg. It is also about one order omagnitude higher than the cross section reported in R@2#, where 174Hg was produced as a 6n evaporationchannel.The a-decay half-lives can be calculated using the Rmussen method@12#. In this method, the decay constantl isfactorized into the reduced widthd2 and the barrier penetration factorP according to the formula:l5d2•P/h, wherehis Planck’s constant. The barrier penetration factor is calated using the WKB approximation to extractd2. Alterna-tively, 212Po reduced width can be used to calculate the hlife ( t1/2Rasm). Calculated half-lives and reduceda-decaywidths for the light Hg isotopes are shown in TableThe partialb decay half-lives predicted for172,173,174Hg are0.2 s, 0.24 s, and 0.35 s, respectively@13#. Since thesevalues are at least two orders of magnitude longer tthe measured half-lives,b-decay branches were neglecteThe reduceda-decay widths deduced for172,173,174Hg arevery close to the reference value of 70 keV obtained212Po and other even-A Hg isotopes. This indicates that thea decays can be associated withD l 50 unhindered transi-tions.Figure 4 compares theQ values for the light Hg isotopein the mass range 172–180, including the values obtaifor 172,173,174Hg in the present work, with those calculateusing mass tables by Liran and Zeldes@14#, obtained usinga semiempirical shell-model mass formula, and by Mo¨llerand Nix @15#, obtained using macroscopic-microscopcalculations. As can be seen from Fig. 4, the Liran aZeldes Qa values fit the experimental data rather weeven though they overpredict somewhat theQa valuesfor the lightest Hg isotopes (N592,93). In contrast, the calculations by Mo¨ller and Nix overpredict theQa value of180Hg by about 400 keV, approach the experimental valFIG. 3. Energy spectra ofa particles measured in the DSSusing the96Ru180Kr reaction at 375 MeV:~a! all decays;~b! de-cays following mass 174 residues within 40 ms;~c! decays followedby 6.55 MeV 170Pt a particles within 100 ms.03130eigrf.--f-n.rddsat N596, and significantly underpredict theQa values forthe lightest known Hg isotopes~by about 600 keV for172Hg).In Ref. @8#, a mass excess of216300(33) keV was de-duced for170Pt, by following the decay chain down to150Ho,a nucleus with a measured mass@16#. Based on the assumption that the observeda decay of 174Hg connects to theground state of 170Pt, the mass excess of174Hg is26607(34) keV. In the recent mass evaluation of Audi aWapstra @17# mass excesses of211150(360) and212650(310) keV were estimated for168Pt and 169Pt, re-spectively. These values combined with thea-decayQ val-ues deduced in the present work give estimated masscesses of21170~360! keV and22810~310! keV for 172Hgand 173Hg, respectively.Finally, the cross sections for producing these Hg isotoare quite small. While the predictions by Nazarewicz onew superdeformed region in the light Hg isotopes aretriguing, the population cross sections are too low172–174Hg to allow for a proper search for superdeformatiin these nuclei using even the most powerful existingg-rayarrays such as Gammasphere. However, low spin state174Hg are within reach.In summary, thea decays of two new isotopes172Hg and173Hg were observed for the first time, and thea decay of174Hg was studied with improved accuracy. The reduca-decay widths indicate aD l 50 character for the observetransitions. Thea-decayQ values deduced for172,173,174Hgare reproduced very well using the mass tables of LiranZeldes, whereas values calculated using mass tables by¨l-ler and Nix deviate from the data with decreasing neutnumber.The excellent support of the ATLAS operating staffgreatly appreciated. This work was supported by the UDepartment of Energy, Nuclear Physics Division, undContract Nos. W-31-109-ENG-38, DE-FG02-94-ER4083and DE-FG02-96-ER40983.FIG. 4. a-decayQ values of neutron-deficient isotopes of HThe open squares represent data, including thea-decayQ valuesfor 172,173,174Hg deduced in this work. The filled circles and trangles representa-decayQ values calculated using mass tablesLiran and Zeldes and by Mo¨ller and Nix, respectively.4-3.inennil-.terSP.aesNCm,o,, R.al,.B.P..RAPID COMMUNICATIONSD. SEWERYNIAK et al. PHYSICAL REVIEW C 60 031304@1# P. J. Woods and C. N. Davids, Annu. Rev. Nucl. Part. Sci.47,541 ~1997!.@2# J. Uusitalo, M. Leino, R. G. Allat, T. Enqvist, K. Eskola, P. 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Decay properties of the new isotopes 172Hg and 173Hg

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Decay properties of the new isotopes 172Hg and 173Hg

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