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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 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 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 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 , 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.

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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 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 and 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 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 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 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 , 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 region to 75 keV for the 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, , 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 ( ->• gold). The discrepancies for the remaining two groups, 137 keV (tellurium •+ ), 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 () [145]* 39 () Myers (Droplet Model) 373 1503 (Lead) [145]* 57 () 431 (Gold) Myers 226 57 (Erbium) (Without Pb, Tl, Hg) [121]* Garvey-Kelson (Updated) 630 1702 () [145]* 67 ( 259 (Thulium) Garvey-Kelson 137 67 (Samarium) (Te - ) [40]* 1702 (Tantalum) Garvey-Kelson 972 500 (Gold) (Yb -*Au) [81]* 300 365 (Mercury) Garvey-Kel son 156 (Thallium) . (Hg * Pb) [24]* •Number of isotopes included.

these widths we have utilized Rasmussen's formalism,15 wherein the width, 62, is defined by the equation: A = 52P/h, where X is the decay constant, h is Planck's constant, and P is the penetrability factor for the a-particle to tunnel through a barrier. One sees in Fig. 2 that, with the exception of the lead isotopes (to be dis- cussed below in more detail), the trends mentioned above do manifest themselves. Following the sharp drop at N = 126, the widths increase as the neutron number decreases with a maximum at N = 86 due to the influence of the N = 82 closed shell. There are only two points presently available in the tin-tellurium region so that no general pattern can be discerned. However, the 10°Te and 112Xe widths are not inconsistent with the 5^ values for N > 82 nuclei. ORNL-DWG 79-15643 I

2 2 h REDUCED WIDTHS*(8 ) FOR .T-WAVE a TRANSITIONS'

10" 192 o NUCLEAR PHYSICS A230 . 365 (1974) •PHYSICALREVIEW C 19, 2399 (1979) P (08Te

RQ(86) "Xe Pt (78) Rn(88) Po (84) £ W(7.4) Q O Q UJ o: 10

Pb(82)

10 ± 50 80 90 . 100 110 120 130 ! NEUTRON NUMBER Fig. 2. Reduced widths for s-wave a transitions plotted as a function of N. Open and closed points for 192Pb are deduced from Ref. 18 and the present study, respectively. Proton shell effects can also be noted; e.g., the widths decrease from to to as Z = 82 is approached. Another consequence of systematic oc-decay-rate studies, the result of recent investigations in the rare earth region, has been mount- ing evidence for a subshell closure at Z = 64 where the gy/2 and d5/2 proton orbitals are filled. The subshell was first proposed when a discontinuity in the progression of o-decay energies for N = 84 nuclides was noted at Z = 64. Macfarlane et^ a]_.^ made cal- culations using a BCS treatment for the proton system of 82-neutron nuclei and were able to reproduce a discontinuity in theoretical binding energies at that atomic number. In addition, they calcu- lated a-decay transition probabilities for the N = 84 even-A nuclei. The theoretical reduced widths indicated a significant dip at Z = 64. Contrastingly, widths determined^ from then available data showed a general constancy in value except for a dramatic reduction for **%y, i.e., at Z = 66.

Newer data, however, lead to a result which agrees with theory, i.e., the minimum is at Z = 64 (see Fig. 2). This point is illus- trated more fully in Fig. 3, where we have plotted reduced widths for N = 84 even-even nuclei as a function of Z. Included in the figure are the calculations of Macfarlane et _aj_.16 Both the data and the calculations indicate a minimum at ISOoy. The biggest difference between the earlier^ and the newer sets of experimental data has to do with the 150Dy o-branch. The new value of 0.36 + 0.03, deter- mined after investigating the nuclide's electron-capture decay scheme,I? is a factor of two greater than the branch of 0.18 ± 0.02 (deduced from gross r-ray counting) reported earlier. The result is a significantly larger reduced width which eliminates the dip at Z = 66. The o-decay rates for the even-even lead isotopes, from pt> to l"2pb, have been reported^ not to follow the pattern described above. The reduced width of 192Pb is unexpectedly large. Also, the widths increase from N = 104 (186pb) to N = 110 (192pt>) by a factor of about 30. The expectation is that the values should decrease as one approaches N = 126. In Ref. 18, the E.C./e+ strengths were determined from K x-ray intensities. Such deter- minations are subject to a number of corrections. A more precise method involves a known decay scheme. With the use of the UNISOR on-line separator facility, we undertook the investigation of the E.C./e+ decay properties of these lead isotopes, our purpose being to determine new values for their a-decay branches. At this time, the *92pb study is complete, while some of the data for 190 are still being analyzed.

The open and closed points for 192pb -jn p-jg, 2 represent the widths deduced from the data of Ref. 18 and the present study, respectively. Our value is less, by a factor of two, due primarily ORNL-DWG 79-15642 1 I I I I I I REDUCED WIDTHS (S2) FOR N = 84 NUCLEI I f I 10- i I i h Q o a UJ CALCULATIONS cr PHYSICAL REVIEW CM 134. BH96H9641 2 --•

i-2 60 62 64 66 68 70 72 PARENT Z NUMBER Fig. 3. Reduced widths for N = 84 even-A a-emitters. Calculations are taken from Ref. 16. to the fact that the half-life of 192Pb is 3.5 and not 2.2 min (see Table III). Nevertheless, the 192Pb reduced width is still too large; it should be smaller by another factor of three to place it below the *°°Pt value (calculated from data*" recently acquired at UNISOR).

In Table III we have also listed our 190pb results and compared them with the earlier data*° for the same nuclide. Our o-decay branch, as shown in the table, is a preliminary number. Note, however, that it is about 5 times greater than the value published in Ref. 18. The resultant reduced width is 0.098 MeV, which places the 190Pb point in Fig. 2 above, rather than below, the curve joining the isotopes.

In contrast to Ref. 18, our results indicate that the lead reduced widths may very well follow the pattern with neutron number observed for other elements. To see if this is indeed so, we plan to remeasure the l"Opb branching ratio and to extend our study to include 188Pb, TABLE III a-Decay Properties of192 Pb and 190Pb

Present Study P. Hornshoj et al.

Ea(keV) 5112 (5) 5110 192DK Half-life (min) 3.5 (1) o Branch 5.7 x 10"5 (10) 6.9 x 10"5 52(MeV) 0.050 (12) 0.094

Ea(keV) 5577 (5) 5590 190PK Half-life (min) 1.2 (1) 1.1 rD a Branch 2 1.0 x 10-2 (0.4)a 2.1 x 10-3 6 (MeV) 0.098 (39)a 0.021 apreliminary value.

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