C - ALPHA-DECAY SYSTEMATICS FOR ELEMENTS WITH 50 < Z < 83 K. S. Toth ':'i Oak Ridge National Laboratory* Oak Ridge, Tennessee 37830, U.S.A. INTRODUCTION From observing the slope of the mass-defect curve, one notes that most isotopes whose mass numbers are 2. 140 are unstable toward a-emission. However, with the exception of naturally occurring l^Sm, o-decay was not observed for elements below bismuth until 30 years ago. This was due to the fact that the rate for a-decay is a very sensitive exponential function of the decay energy. The energy available for decay increases rapidly with mass, so that in the region above lead a-decay becomes a dominant decay mode. It was also known that, if one could produce nuclides sufficiently far to the neutron-deficient side of the e-stability line, then with nuclei Z < 83 would undergo a-particle emission. This was shown to be true experimentally in 1949 when Thompson, Ghiorso, Rasmussen and Seaborgl reported the discovery of a-radioactivity in proton- rich isotopes of gold, mercury and the rare earths. Since that time, with the availability of new accelerators, the number of known a-active nuclides below bismuth has steadily increased. In this paper, we will review recent data reported on a-emitting isotopes in this mass region, compare o-decay energies with predictions of mass formulae, and discuss a-decay rates of even-even nuclei. Operated by Union Carbide Corporation under Contract No. W-7405- eng-26 with the U.S. Department of Energy. or TO* DOCUMENT tt By acceptance of this article, the publisher or racipiant acknowledges the U.S. Government's right to retain a nonexclusive, royally-free license in and to any copyright covering the article. NEW ALPHA-EMITTERS The first big impetus for the study of a-decaying nuclides in the medium-weight mass region came in the early 1960's, when several groups at the Berkeley HILAC began using heavy-ion beams and the helium gas-jet technique. Since then, experimentalists at other laboratories have taken up these studies and a large number of a- emitters with 50 < Z < 83 are now known. The most recent compila- tion summarizing information for isotopes in this general mass region was published in 1975 by Gauvin et a]_.^ Interest in the field has not abated. In addition to improving the quality of data, particularly with regard to a-decay rates, investigators^-iO have identified more than 30 new o-emitters during the past few years. Table I summarizes their half-lives and a-decay energies. Most of the isotopes listed in Table I were produced in bom- bardments with nickel and krypton ions accelerated"at UN1LAC: ll4 3 1)110-112!f 112,llcSxe, H*Cs(or Ba), Roeckl et al. and Kirchner 4 et a]_., with the use of an isotope separator; Tj T^Hf, 160ws 6 157-161Ta> 161-164Re> Hofmann et al_., with the use of the velocity filter SHIP; and 3) 166-168R6j 169,170Ir> Schrewe et a],.,? with the use of a gas-jet system. At the accelerator ALICE, Cabot et al.8 utilized copper beams and the gas-jet technique to produce 168,169Rej 165-168oSj anc| 168-170Ir The isotOpe separator facility at Oak M 10 Ridge, U ISOR, was used to identify 184-187T1# Finally, the ISOLDE collaboration in their investigations of rare earth and mercury isotopes reported two new a-emitters, ^°Yb (Ref. 5) and l88Hg (Ref. 9) The experimental situation extant in the region from neodymium to lead is shown graphically in Fig. 1. The figure shows isotopes and their a-decay energies as a function of neutron and proton numbers. For clarity, even-Z nuclides are indicated by bars while odd-Z nuclei are represented by dots. It is seen that a-decay energies increase both with increasing Z (and A) and with decreasing N (as one gets further away from the valley of stability). EXPERIMENTAL AND PREDICTED Qa VALUES Because the characterization of an a-emitter involves combining a half-life with a specific a-particle group, a-decay provides us with a convenient means of discovering new isotopes. Their identi- fication opens the way for further, more extensive studies. In addition, a-decay energies in many instances can be used to deter- mine energy differences between the parent and daughter ground states. Such measurements have therefore been used not only to obtain estimates of masses for nuclei far from stability but also for comparisons with mass formulae predictions. TABLE I New a-Emitting Isotopes Nuclide Tl/2 (sec) Ea (MeV) References 110! 0.69 (4) 3.424 (15) 3 (4) 1111 2.5 (2) 3.150 (30) 3 (4) H2l 3.42 (11) 2.866 (50) 3 }}2Xe 2.8 (2) 3.185 (30) 3 113Xe 2.8 (2) 2.990 (30) 3 114Cs (or 114 Ba) 0.57 (2) 3.226 (30) 3 158yb 99 (12) 4.069 (10) 5 156Hf 0.025 (4) 5.878 (10) 6 157Ta 0.0053 (18) 6.219 (10) 6 158Ta 0.0368 (16) 6.051 (6) 6 159Ta 0.57 (18) 5.601 (6) 6 160ja 5.413 (5) 6 16lTa 5.148 (5) 6 160^ 5.920 (10) 6 16lRe 0.010 (+1|) 6.279 (10) 6 162Re 0.10 (3)"° 6.419 (6) 6 163Re 0.26 (4) 5.918 (6) 6 lb4Re 0.9 (7) 5.778 (10) 6 166Re 2.2 (4) 5.495 (10) 7 167Re 2.0 (3) 5.33 (1) 7 168Re 2.9 (3) 5.14 (1) 7 5.5 (5) 5.26 (1) 8 169Re 5.05 (1) 8 1650s 6.20 (2) 8 166QS 0.3 (1) 6.00 (2) 8 igOs 0.65 (15) 5.84 (1) 8 1680s 2.0 (4) 5.66 (1) 8 (7) 168ir 6.22 (2) 8 }^Ir 0.4 (1) 6.11 (1) 8 (7) 170Ir 1.1 (2) 6.01 (1) 8 (7) 188Hg 4.61 (2) 9 184T1 11 (1) 6.162 (5) 10 11 (1) 5.988 (5) 185T1 1.7 (2) 5.975 (5) 10 186T1 -25 -5.76 10 187T1 18 (3) 5.51 (2) 10 ORNL-OWG 73-I0043R2 • 2 a-EMITTING ISOMERS Fig. 1. Known a-Emitters in the Region from Neodymium to Lead. We have compared experimental decay energies for o-emitters with atomic numbers between 50 and 83 with values taken from four sets of mass predictions, i.e., the shell-model formula of Li ran and Zeldes,11 the formula of Myers and Swiatecki12 which is based on the liquid-drop model with shell corrections, the subsequent mass formula developed by Myers*-* which uses the droplet model, and finally, the Garvey-Kelson mass relations as updated by Janecke.14 A detailed comparison cannot be shown in this short presenta- tion. Instead, we have summarized in Table II the average dif- ference between the experimental Q-values and the four sets of predictions for all 145 isotopes considered. In addition, average deviations were determined for isotopes of each element (or group of elements). The largest and smallest of these deviations are also listed in the table. The Liran and Zeldes formula*! agrees best with data, an average difference of 152 keV, compared with 252, 373, and 630 keV for the predictions of Refs. 12, 13, and 14, respectively. Their formula also shows the least spread in dif- ferences ranging from 312 keV for isotopes in the tellurium region to 75 keV for the iridium nuclei. It is interesting to note that the droplet model 13 yields a larger deviation than the conventional liquid-drop model." However, if the lead, thallium, and mercury nuclides are omitted, then the newer predictions are slightly better, a deviation of 226 keV versus 252 keV for the older liquid- drop model. In Table II the updated Garvey-Kelson predictions have also been broken up into three groups of elements. One sees that for Ref. 14 the deviation is greatest, 872 keV, for the middle group of elements (ytterbium ->• gold). The discrepancies for the remaining two groups, 137 keV (tellurium •+ thulium), and 300 keV (mercury-»- lead) are comparable with those deduced from the other three sets of predictions. ALPHA-DECAY RATES In ot-decay, half-lives for transitions between ground states of doubly-even nuclei are taken to represent unhindered decays. The reduced widths of these s-wave transitions are considered to be standard. A rather regular behavior as a function of both neutron and atomic number is observed for s-wave a-decays. Their reduced widths are largest for nuclei two or four particles beyond a closed shell (with sharp minima occurring at the closed shell), followed by a decrease as one approaches the next closure. These trends can be understood in terms of single-particle models which have shown that the extremely sharp break at N = 126 is essentially a shell structure effect. Figure 2 shows s-wave reduced widths for a-emitting nuclei with Z from 52 to 88 plotted as a function of N. In calculating TABLE II COMPARISON OF EXPERIMENTAL AND PREDICTED Qo's Mass Formula Qexp - Qpred (keV) Liran and Zeldes 152 312 (Te region) [145]* 75 (Iridium) Myers and Swiatecki 252 479 (Gadolinium) [145]* 39 (Osmium) Myers (Droplet Model) 373 1503 (Lead) [145]* 57 (Erbium) 431 (Gold) Myers 226 57 (Erbium) (Without Pb, Tl, Hg) [121]* Garvey-Kelson (Updated) 630 1702 (Tantalum) [145]* 67 (Samarium 259 (Thulium) Garvey-Kelson 137 67 (Samarium) (Te - Tin) [40]* 1702 (Tantalum) Garvey-Kelson 972 500 (Gold) (Yb -*Au) [81]* 300 365 (Mercury) Garvey-Kel son 156 (Thallium) .