THE SWEDISH RESEARCH COUNCILS" LABORATORY Studsvik, Fack „

S-61101 Nyköping 1 LF.75 Sweden 1977

TOTAL B-DECAY ENERGIES AND MASSES OF . . AND IN THE VICINITY OF

E Lund. K Aleklett*. and G Rudstan. The Swedish Research Councils' Laboratory. Studsvik. Fack. S-611 01 Nyköping. Sweden

*) and Chalmers University of Technology. Göteborg. Sweden TOTAL .-DECAY ENERGIES AND MASSES OF TIN. ANTIMONY AND TELLURIUM 1 ISOTOPES IN THE VICINITY OF ^Sno,

By E Lund. K Aleklett. and G Rudstam. The Swedish Research Councils' Laboratory. Studsvik. Fack. S-611 01 Nyköping. Sweden

Abstract: Experimental >-decay energies for short-lived . antimony and tellurium are presented. Mass-separated sources were produced at the on-line separator OSIRIS. By applying -y coincidence methods total -decay energies have been determined 127 131 for the following nuclides: " Sn. 128.130.131.13^ and 134.135Je The excess has been derived for these nuclei, and com- parisons are made with mass formula predictions.

127 131 Key-word: RADIOACTIVITY " Sn, ^,130,131,13^ and M .135^ measured i-spectra, deduced total (-decay energies, and mass excesses. Comparison with mass formula predictions. Mass-separated fission products.

*) and Chalmers University of Technology, Göteborg, Sweden - 1 -

INTRODUCTION

The shape of the nuclear mass surface far away from the region of -stability is of considerable interest. Mass data on extremely neutron-rich nuclei are important for the theories of nucleo- synthesis and also for predictions about superheavy nuclei In the present work attention is paid to the shape of the mass surface in the vicinity of doubly-rlosed Sn. This is the only nuclide far from stability with closure of two major shells presently within ex- perimental reach» something which gives special importance to the nuclear region covered.

During the last ten years much work has been devoted to highly unstable nuclei and mass-laws. The mass laws are based on experimen- tal mass data for nuclides rather close to the .-stability line, and extensive extrapolations are required to reach the nuclear region of interest for calculations on nucleosynthesis via rapid neutron capture pro<-c--,Sf e.g. twenty to thirty mass units away from the line of sta- bi . / Experimentally, this region is beyond reach but ISOL*techniques r. ;J

A comparison of some nuclear mass formulae with experimental s ises by Wing shi ws that the nuc'ear shell effects were originally adequately treated in the mass predictions. In spite of different • ;proaches to the problem all formulae show almost the same deviation rom empirical masses even close to stability. Therefore, experimental results in the region around the doubly-closed shell nucleus Sn 3 e very important for the construction of a nuclear mass formula.

Using ISOL techniques total .-decay energies have been reported for gaseous fission products and their daughters covering the mass range A = 88 - 95 and A = 138 - 142 . Total decay energies of short-lived neutron-deficient isotopes of and cesium and neutron-rich isotopes of and have also been published These experiments as well as the present investigations use the indi- rect method to deduce new masses by determining the mass differences from nuclear decay energies (Q -values) and by adding these mass diffe- rences to known masses. Important new data have also been obtained on the masses of short-lived light nuclei by means of direct mass-spectro- 8 9) metric measurements ' and recently such measurements hav)ve tbeen per- 10) formed using ISOLDE produced samples of rubidium isotopes

*) ISOL = Isotope Separator On-Li ne - 2 -

The isotope-separator facility OSIRIS connected to a reactor at Studsvik produces isotopes of some 20 fission elements . It is well suited for a systematic study of the decay energies of these nuclides and hence for mapping the mass surface on the neutron-rich side of stability. The Q -measurements are of a survey character: the emphasis has been to measure a large number of cases with a moderate precision rather than aiming at the highest possible accuracy. So far about 50 nuclides with half-lives between 1 s and 20 min have been studied.

This article reports Q -values for 11 nuclides in the mass range A = 127 - 135. Other members of articles on nuclear Q - values measured at OSIRIS report results for , » » an isotopes » isotopes » and the 132-mass chain • In the near future the series will continue with Q -determinations of the and isotopes in the mass range A = 113 - 124. the isotopes from A = 86 - 89 and the isotopes from A = 136 - 139.

The experimental technique is briefly outlined in Section 2. More detailed descriptions are given in refs. ' ' • Section 3 con- tains details of the Q -value measurements for each nuclide including comparisons with other published results. Finally» experimental Q -values and deduced mass excesses are compared with predictions from different mass formulae in Section 4.

2. EXPERIMENTAL TECHNIQUE

In the present work total i-decay energies have been deter- mined using two somewhat different methods:

I) A Si(Li) detector system in coincidence with two Nal(Tl)- detectors. The samples were transported by means of a magnetic tape system from the collector chamber to the spectrometer position with a transportation time of 3 s

II) A Si(Li) detector system in coincidence with one Ge(Li)- detector placed close to the collecting position with a tape transportation time of only 0.3 s - 3 -

In the original experiments Method I was used. A set of >- peaks were chosen to gate the —particle spectra- Generallyt the highest observable peaks were used, presumably corresponding to the highest levels with appreciable .-feed, and the Q -value is obtained Dy addinq the gate energy to the end-point energy. In some cases, however, the .-branches to individual high energy levels are very weak, and it is necessary to complete the measurements with low-energy ,-gates. Since the separator has a very poor element selection the mass chains often contain more than one isobar, and many nuclides have rather complicated ,-spectra. Then, the resolving power of the Nal-detectors is unsufficient. and the -spectra coincident with the .-gates chosen will be composed of several components. In such cases Method II was used if the activity was sufficient for the somewhat lower efficiency of the ,-detector.

Good knowledge of the level structure of the daughter is always essential for unambiguous Q -determinations.

3. EXPERIMENTAL RESULTS

The decay of the 2.2 h ground state and the A.13 min isomeric 127 17-21) state of Sn have been investigated by several authors . The spin and parity of rm$n are assigned to be 3/2+ ' . and this isomeric staie has been placed 5 keV above the ground state . Kauranen observed that the decay of mSn is followed by the emission of a 491 keV ,-line. and he also determined the end-point energy of the .-branch coincident with this y -line to be 2.7 • 0.1 MeV. which gives a total decay energy of 3.2 • 0.1 MeV. The coincidence spectrum was taken with very low statistics, however, and the determi- nation was based on the low-energy branch of a gross i-spectrum (e g a i-spectrum obtained without any coincidence requirements). In the present work the half-life of the isomer was determined to be 3.5 0.5 min by multiscaling the 491 keV ,-line. This accuracy is sufficient for establishing that the ,-line belongs to m5n. - 4 -

With a collection time of 1 min the short-lived activity is considerably enhanced compared to the 2.2 h activity. By far the strongest ^-transition in the spectrum is the one at 491 keV. This line was used to gate the o-spectrum which was found to have an end- point energy of 2.718 • 0.040 keV. The end-point energy of a gross B-spectrum was determined to be 2.714 » 0.029 MeV. No i-particles were seen above this energy, which indicates negligible interference from the 2.2 h activity. A mean value of the two determinations there- fore gives the Q -value of the low-spin isomer

Q .= 3.206 • 0.024 MeV

in agreement with earlier experiments but with improved accuracy. A Fermi-Kurie (FK) plot of the -spectrum coincident with the ,-tran- sition at 491 keV is shown in Fig 1.

As the ground state lies 5 keV below the isomeric state 127g the total .-decay energy for Sn is

Q = 3.201 • 0.024 MeV

VPWF 10

8

491

_ 127Sn 491 keV gate

to 2.0 E(MeV)

Fig 1. Fermi-Kurie plot of the [.-spectrum coincident with the y-transition at 491 keV in the decay of 127Sn. A partial decay scheme is shown as an insert. - 5 -

128 128 3.2 The_nucMdes____Sn and Sb

The component of the isobaric chain at mass number 128 are:

128Sn (59 min)_>128mSb (10-0 min)

* 128Te (stable)

128gSb '9.1 h)

A partial le»el scheme containing the information pertinent to the present investigation is given in Fig 2. It is based on the re- , 24.25.26) ferences Using Method II three ,-transitions depopulating the levels at 635 and 833 keV of 128mSb were chosen as gates for the —spectra. The results of the FK-plots are given in Table 1.

The mean value of the -decay energy to Sb is 1.29 • 0.04 ..eV in agreement with ref » but at variance with the end-point energies obtained by •• --srint i Ilat ion spectrometry which seem to be . . .28) too high

The decay energy given above corresponds to feeding to the iso- meric state of Sb. According to systematics the difference between 29) the isomeric and ground states is expected to be less than 20 keV The total -decay energy of 128Sn may therefore be given as

For the study of the decay of omSb »-gates were chosen at 314. 753, and 788 keV. The first two transitions are members of a cascade 314-754-743 keV between the 1811 keV level and the ground-state (cf. Fig 2). The r-spectra coincident with the 314 and 754 keV lines look similar, and they consist both mainly of particles to the 1811 keV level, thus showing that the ,-feed to the 1498 keV level is neg- ligible. Only the end-point energy of the ;>-spectrum coincident with the

upper transition 314 keV is used for the Qf,-value determination, however. The y-transition of energy 788 keV is the first member of a cascade from the 2599 keV level and thus suitable as gate.

The results of the FK-plots are collected in Table 1. The mean value of the decay energy of Sb is determined to be 4.39 » 0.04 keV. Taking into account the energy o* the isomeric transition the total decay 128 energy of Gn may be given as

+0 04 Q =4.39 - Mev. - 6 -

59.3 min 128

833

80 /o 635

153 78 46 0 10.0 min 9.1 h

2599

80% 1811 I 1497

743

stable 0+ 128 Te

Fig 2. Partial decay scheme of 128Sn and 128mSb. - 7 -

Table 1

The end-point energies of the v-spectra coincident with 128 128 different >-gates in Sn and Sb

max Nuclide Gate .--feeding to Range of fit Q -value level energy (MeV) (MeV) (MeV) (keV) (keV/

i28Sn 482 635 0.46 - 0.62 0.65 0.04 1.28-0.04 635 0.40 - 0.62 0.68'0.15 1.32-0.15 (T1/2 557 59 min) 680 833 C.27 - 0.46 0.49-0.09 1.32-0.09

Mean value 1.29 0.04

128Sb Gross , 2.6 - 4.3 4.62 0.41 181ia) 1.4 - 2.5 2.58 0.06 1811 0.9 - 2.5 2.580.04 4.390.04 (T1/2 3H (1811) 1.2 - 2.5 2.58 0.07 10.0 min) ° 788 2599 0.7 - 1.6 1.82 0.22 4.42 0.22

Mean v£.lue 4.390.04 a) Second component

129 3.2 The nuclide SC n 129 The half-lives of two isomers of Sn have been determined Dy ;-counting at this laboratory to be 2.230.03 min and 8.9-0.6 min» respectively, in agreement with other determinations.

The decay of 129Sn has been very little investigated so tar. 129 Only intensities and energies of some .-transitions in Sb are available A fragmentary level scheme, based on a study of the reaction Te(d» He) Sb is shown

In the present work eight ,-peaks were chosen as gates for the

t-spectra using Method I. The predominant i-branch has an energy of 3.35-0.12 KeV. It is very intense in coincidence with the 642 keV

(-ray and extremely weak in coincidence with gates in the energy range 1130 - 1198 keV and at 1460 keV (in these cases interpreted as due to accidental coincidences). The odd antimony isotopes show - 8 great similarity to each others and we suggest that the low-spin isomei son r of Sn mainly feeds the (3/2*. 5/2+) level at 642 keV in 129, Sb. in analogy with the situation at mass 127.

The resulting Q -value is for the 3/2+ isomer of 129Sn Q = 4.000.12 HeV

A FK-plot of the .-spectrum corresponding to the .-gate around 642 keV is shown in Fig 3.

129 From studies of the decay of In performed at this laboratory it is known that the 11/2 isomer of >C7Sn lies 3J keV above the 3/2 ground state A 2100 keV transition belonging to the 2.23 min decay was found to be in coincidence with a -branch of end-point energy 1.940.17 MeV. The addition of the gate energy gives 4.04 0.17 MeV, in agreement with the Q,-value. This strongly suggests the presence of a level at 2100 keV.

2.23 min ivg-^f 8,9 mm j£_ 129 2100 Sn

1450 25

7/2- ,?646 20 129 Sb

15 129 Fig 3. Sn Fermi-Kurie plot of 646 keV gate the B-spec t "urn co- incident with the y- transition at 646 keV 10 in the decay of 129sn A fragmentary level scheme is shown as an insert. The level energy 646 teV is ta- ken from reaction 305 studies » decay 3.34*0.10 MeV measurement indica- tes a transition energy at 642 keV. U) 1.5 2.0 2.5 3J0 E(MeV) - 9 -

The decays of Sn and Sb have been thoroughly studied by Kerek et al.33'3A). The excited states in the chain 130Sn -430Sb —l|°Te have also been investigated by Kristyak et a. . The nuclides belong to the following isobaric chain:

130.InCO.5 ,„ „3 s),_ 130g^ Sn(3.c ,-,Qo mm. ). _ 130-_^Sb(6., ,, ,6 mm. .) J30Te (stable) 130m i'"! T liUmSn(1.7

The half-lives given above are those obtained by Kerek et al.J by mult i scaling ,-lines.

In the present work the total decay energies of the two isomers 130 130 of Sr. and the one of the 6.6 min isotner of Sb have been investi- gated. Method II was used because of the complicated ,-spectrum from three different nuclides collected at the same time. Partial decay schemes for the two isomers of tin are given in Fig 4. The /-transitions of energies 780 and 229 keV. depopulating a level at 1042 keV, and a transition of energy 435 keV depopulating a level at 697 keV (cf. Fig 4) were chosen as gates. The results of the corresponding FK-plots are given in Table 2. The Q ,-value of the ground state of Sn was found to be

Q = 2.19 • 0.03 MeV

As an example a FK-plot corresponding to the 780 keV gate is shown in Fig 5.

According to systematics the 1.7 min isomer is expected to lie about 1800 keV above the ground state of Sn .A ,-transition at 145 keV has been found to be. in coincidence with (-particles from the

1.7 min isomer and also coincident with r-lines of energies. 544 and 899 keV . These /-transitions are not coincident with any of the tran- sitions connecting even parity states of Sb. It has been con- cluded that mSn only feeds the 40 min isomer of Sb.

Three >-1 i nes belonging to the decay of mSn were chosen as gates# i e those of energies 145, 311» and 899 keV. Only the 899 keV gate gave a inspect rum useful for the determination of the decay energy» however. The position of the 311 keV transition in the decay scheme is not established. The i -spectrum coincident with the 145 keV line - 10 -

2119 1992 1800 1.7 min

1217

130 Sn

1044

r 1 688

t 145 85 6.6 min 5+ 0 40min

3413

stable Öf-

Fig 4. Partial level scheme showing the decay of the two isomers of 1^Osn and the 6.6 min isomer 1™Sb. - 11 -

1.163 i 0.051 2-

I I 0.4 V E(MeV) Fig 5. Ferm—Kurie plot of the -spectrum coincident with the ,-transition at 780 keV in the decay of

\/JL VPWF

< i l

| ' I 2 -t

i

i 1

130mo T bn f ^v^. 2.96^0.31 ~" 899 keV gate

I i u 1 3 E(MeV)

Fig 6. Fermi-Kurie plot of the : -spectrum coincident with the ,-transition at 899 keV in the decay of 13OmSn. - 12 -

Table 2 The end-point energies of the .-spectra coincident w'th ,-gates in 13°9'13OmSn and 130Sb

Nuclide Gate ; -feeding to Range of .max Q -value energy level fit (keV) (keV) (MeV) (MeV> (MeV) 1042 0.6 - 1.1 1.14 0.5 2.18*0.05 130Sn 229

(T1/2 3.8 435 697 0.9 - 1.5 1.49 0.09 2.190.09 min) 780 1042 0-7 - 1.1 1.160-05 2.20 0.05

Mean value 2.190.03

130Sn 145 1044.(145) 1.5 - 2.8 2.97 0.27

(T1/2 1.7 311 1.6 - 2.6 3.18 0-57 min) 899 1044 1-3 - 2.5 2.960.31 4.000.31 Mean vajLue 4.000.31

130Sb 816 2449 1.0 - 2.0 2.54 0-35 4.99 0.35

(T1/26.6 1018 2833 0.7 - 2.1 2.18 0.11 5.01 0.11 min) 1142 1981 0.4 - 2.0 2.64 0.45 4.62 0.45 1598 3413 0.7 - 1.7 1.83 0.29 5.24 0.10

Mean value 5.02 0.10

is complex with branches to both the 1044 keV level and to the one at 145 keV (this branch was found to be very weak).

As a result of the measurement the decay energy of 1.7 min 130m Sn was found to be

Q(, = 4.00 • 0.31 MeV.

This places the isomeric state 1.79 0.31 MeV above the ground state# in accordance with systematics. This conclusion presumes that the energy difference between the antimony isomers is negligible. Its size is as yet unknown. - 13 -

A FK-pLot corresponding to the 899 keV gate is shown in Fig 6.

With a collection time of 2 minutes only a small amount of the 40 min isomer of " Sb was present. No accurate determination of its Q -value has been made.

The 6.6 min isomer (5 ) is supposed to decay to several levels above the 4 level at 1633 keV in Te. The .--spectra corresponding to four gates were used for a determination of the total decay energy. The results of the measurements are given in Table 5» yielding as a mean value Q = 5 02 • 0.08 MeV.

This value is considerably lower than the value 5.9 MeV obtained for the 40 min isomer by Kerek et al.

The nycl^des§[]?CdSb

The half-life of Sn i s known to be 55 s from (--counting Very little is known about the level structure of Sbr however. Only energies and intensities of some ,-lines have been determined

Method II was used in the present experiment. A y-line of energy 798 keV is the dominating one in the .-spectrum The coinci- 131 dent ;-branch is very strong indicating that the decay of Sn is 127 129 similar to the decays of Sn and Sn. In the latter cases the 3/2 -state decays with mainly one i-branch to levels at 491 keV in Sb and 646 keV in Sb. We suggest t ha; 7'98 keV is a ground state transition. Thus, the Q ,-value for Sn is obtained by adding the end-point energy of the ,-spectrum 3.79 0.20 MeV to the gate energy 798 keV. This gives the following total i-decay energy:

Q = 4.59 • 0.20 MeV.

In 127 Sn and 129Sn two isomers are present» with spins and parities 3/2 and 11/2 . respectively. For the case Sn the 3/2 - 129 state lies about 5 keV above the 11/2 -state, but for Sn and Sn the 3/2+-states are probably the ground states - 14 -

The decay scheme of Sb (T = 23 min) is rather well known 37) » but the Q -value has not been experimentally determined before. The r-strength is spread out over more than twenty levels in Te- the strongest branch (27 Z of the imparticles) feeding a level at 1876 keV. The end-point energies of the i -spectra corresponding to the ,-gates chosen are collected in Table 3.

The Q -value obtained for Sb is Q = 3.19 • 0.07 MeV

Table 3

Summary of experimental results for sb

Nuclide Gate E ,-feeding to Range of max Q -value energy (keV) fit (keV) (MeV) (MeV) (MeV]i

131Sb 933 1876 0.7 - 1..1 1 .30 0. 10 3.18 0.10 943 943 1.4 - 2 .0 2 .15 0. 62 3.09 0.62 1876a) 0.7 - 1 .1 1.52 •0. 12 3.20 •0.12 1123 2067 0.5 - 1 .0 1.14 '0. 25 3.21 0.25

Mean value 3.19 0.07 a) Second component

3.6 The nuclides 13ASb and.134Te

The half-life of the 10.3 isomer of 134Sb has been determined both from >-multianalysis and total .-counting at this laboratory . In addition, a 0.85 s isomer has been discovered by multiscaling high 38) energy K-particles . Finally, delayed neutron counting has revealed the 134Sn-isobar of half-life 1.04 s39).

The decays of the two isomers of Sb have been thoroughly investigated earlier with the following results . A strong ; -branch from the 0.85 s 0 -state is expected to feed the ground state of Te, and the end-point energy has been determined to be 8.4 • 0.3 MeV. The gross h-spectrum in the 10.3 s decay has also been measured and found to contain two branches with end-point energies ' 6 MeV and 6.8'0.3 MeV, - 15 -

respectively. The latter oresumably feeding a level at 1691 keV. The resulting Q,_,-value is 8.4 • 0.3 MeV. It has not been possible to determine which of the two isomeric states of antimony is the ground state.

A partial decay scheme ot 10.3 s Sb is shown in Fig 7. A r-spectrum was measured in coincidence with the 706 keV ,-line de- populating the 2398 keV level. The 1691 keV level is also expected to be fed by a rather strong r-branch. This level has a half-life of 162 nst however» and no coincidences were detected with the resolving time used in the present coincidence experiment (30 ns).

In an experiment using Method I the end-point energy was determined to be 5.85 ' 0.27 MeV. Because of low counting rates this method was preferred although there is a slight possibility of contamination from Te which has a strong ,-transition of energy 713 keV. A second experiment was therefore performed using Method II in order to check the degree of contamination of that part of the .-spectrum used for the end-point energy determination. In fact, no 134 contamination is expected since the Q -value of Te is several 134 MeV lower than that of Sb. The second experiment yielded the end- point energy 5.81 0.56 MeV» and the mean Q -value of the two deter- minations is

Q, = 8.24 ' 0.24 MeV, in agreement with the determination b/ Kerek et al.38)

A FK-plot of the ;-spectrum in coincidence with the 707 keV gate is shown in Fig 7.

The level scheme of I is well known» but until now no i;-y coincidence measurement has been carried out to determine an experimen- 134 tal Q -value for Te. We therefore measured the i-spectra in co- incidence with ,-oates at energies 278 and 767 keV, transitions which depopulate the 923 keV and 847 keV levels, respectively (cf. Fig 7). Method II was used in this case. The end-point energy for the ,-branch to the 923 keV level was found to be 0.61 0.16 MeV, and the branch to the 847 keV level had an Ema* of 0.73 > 0.11 MeV. The resulting Q -value, calculated as a weighted mean, is Q = 1 .56 0.09 MeV. - 16 -

_ 10.3»

2 398

(E MeV)

Fig 7. Fermi-Kurie plot of the ,-spectrum coincident with the .-transition at 706 keV in the decay of 134sb as measured using Method II.

18 l

870 603 135 Te

870 keV gate

E(MeV)

Fig 8. Fermi-Kurie plot of the (.-spectrum coincident with the ^-transition at 870 keV in the decay of 135Te. - 17 -

Th? The half-life of Te has been determined to be 19.2 > 0.2 s by means of total ^-counting at this laboratory . in agreement with other observations. The decay of Te has been studied by Borg et al. . A partial level scheme of I from this reference is in- serted in Fig 8. The ,-gates were set at energies 870 and 603 keV. The p-spectrum corresponding to the latter transition is composed of >-branches to both the 870 keV level and the 603 keV level» however» and the determination of the E will be uncertain, therefore» only the 3-spectrum coincident with 870 keV ,-rays was used tor the Q - value determination. The end-poir-i was determined to have the energy 5.08 * 0.24 MeV. By adding the level energy we get the following result:

Q = 5.95 0.24 MeV.

4. DISCUSSION

The Q -values obtained in the present work are compiled in Table 4. For comparison, the predictions from six different mass formulae are included. The mass formulae chosen for this purpose are the droplet models by Myers / Groote et al. . and Seeger- 42c) Howard < the semiempirical she' -model formula by Liran and Zel- des i and the empirical mass relations by Comay and Kelson 43) and Garvey et al.

The average uncertainty of the experimental Q -values ob- tained in the present work is 0.10 MeV. This can be compared to <. iQ i »exp - 0 i »pred. i> for the different mass formulae. In this mass region the best Q(-value predictions are given by the droplet foi— mula by Myers^2a and by the mass relations^'4 . - 18 -

Table 4

Summary of experimental Q,-values obtained in this work and 42a-e) 45) ref 14) compared with different mass formulas ref ana

Nuclide Qo-value Q - Q (MeV) b . .exp >red (MeV) a b c d e f

127Sn 3.201l0.024 0.34 0.06 -0.13 0.16 0.06 0.12

128Sn 1 29+0-06 -0.07 -0.57 -0.19 -0.04 -0.12 -0.01 '• -0.04 129Sn 4.00±0.12 0.21 -0.12 -0.10 0.04 0.07 0.15

130Sn 2.19'0.03 -0.08 -0.65 -0.24 -0.08 0.02 -0.06

131Sn 4.59±0.20 -0.07 -0.47 -0.43 -0.29 0.06 -0.04

132Sn 3.08-0.04 -0.08 -0.70 -0.32 -0.29 -0.16 -0.24

128Sb 0.39 0-35 0.11 -0.11 -0.16 -0.15

130Sb 5.02-+0.10 0.12 0.03 -0.19 -0.36 -0.32 -0.28

131Sb 3.19:'0.07 -0.20 -0.53 -0.35 -0.44 -0.39 -0.49

132Sb 5.53f0.07 -0.24 -0.40 -0.58 -0.75 -0.41 -0.55

134Sb 8.24+0.24 -0.42 -0.25 -0.55 -0.96 -0.30 -0.45 13 Se 1.56-(0.09 0.04 -0.51 0.18 0.04 -0.11 -0.16

l35Te 5.95+0.24 0.06 0.04 0.27 -0.28 0.38 0.31

- Cl > 0.18 0.36 0.28 0.30 0.20 0.23

a) ref 42a d) ref 42d

b) ref 42b e) ref 42e

c) ref 42c f) ref 43 - 19 - Table 5

Compilation of experimental mass excesses for neutron-rich iso- topes of Sn. Sb. and Te in the mass region around Sn

Nuclide Mass excess B-decay Q, Nuclide Mass excess P (MeV) (MeV) (MeV)

127 127Sb -86.704+0.007a 127Sn >127Sb 3.201+0.024 Sn -83.503+0.025

128 +0.04 128 -88.991 +0-004a Sb •128Te 123 -84.60 Te 4-39-^6 Sb 128 J28msb i28 128mSb -84.60 +0.04 Sn 1.29*0.04 Sn -83.31 ±0.06

a 129 129 129 129Sb -84.628+0.022 Sn > Sb 4.00*0.12 Sn -80.63 ±0.12

130Te -87.347 *0.O05a 130Sb >130Te 5.02+0.10 130Sb -82.33 ±0.10

130 130Sb -82.33 ±0.10 130Sn >130Sb 2.19*^0.03 Sn -80.14 ±0.10

131Te -85.200 t0.005a 131Sb >131Te 3.19*0.07 131Sb -82.01 ±0.07 131-, 131Sb -82.01 '0.07 131Sn Sb 4.59-0.20 131Sn -77.42 ±0.?1

132Te -85.201 ±0.025a 132Sb >132Te 5.53*0.07 132Sb -79.67 *0.07

132Sb -79.67 +0.07 132Sn •132Sb 3-08+0.04 132Sn -76.59 +0.08

134 a 134 134 134 I -83.97C±0.060 Te > I 1.56*0.09 Te -82.41 + 0.11 13 134Te -82.41 +0.11 134Sb ^ Se 8-24±0.24 134Sb -74.17 +0.26

135 a 135 135 135 I -83.795 ±0.029 Te , I 5.95i0.24 Te -77.85 *.»

42f) a) ref i*.2 Masses Starting from that member of an isobarich chain» whose mass is known» the masses of isobars further away from the i -stability line can be determined by adding the measured total -decay energies (or the mass differences). Thus» the mass excesses for the tin. antimony, and tellurium isotopes are calculated and collected in Table 5. These mass excesses are then compared with predictions from the mass formulae mentioned above. The average uncertainty of the mass excesses in Table 6 is 0.11 MeV and the deviations between the predicted values and the experi mental ones lie between 0-3 and 1.0 MeV. The situation is the same as for Q -values: the formula by Myers 42a) seems to give the best 132 predictions in this region around r^Sn,,-,. Note that in this com- 132 pilation the results for Sn and i:52Sb from ref 14 have been in- cluded- A general conclusion (cf. Table 6) is that all masses are predicted to be less bound than experimentally found. 4.3 P^ans for_the future This study of nuclear masses in the region around the doubly- 132 closed shell nuclide Sn has to be completed in some cases. Above all. the 133-mass chain should be studied. In addition, we plan to measure also some more short-lived species such as Sn.

No attempt has been made to carry out a comprehensive com- parison with various mass formulae in order to pick out the most precise ones. Thos work has to be postponed until the extensive study of all nuclides within experimental reach at OSIRIS is finished. - 21 -

Table 6

Summary of experimental mass excesses obtained in the present work and comparisons with different mass formula predictions

NucI i de Mass excess Exp mass excess - predicted mass excess (MeV) CMeV)

127 Sn -83.503±0.025 0.79 0.22 -0.6 0.00 -0.15 0.03 128 Sn -83.31 -0.06 0.51 -0.36 -0.7 -0.18 -0.34 -0.06 129 Sn -80.65 +0.12 0.45 -0.39 -0.9 -0.25 -0.59 -0.13 130 Sn -80.14 0.10 0.19 -1.00 -1.1 -0.44 -0.73 -0.30 131 Sn -77.42 0.22 -0.10 -1.29 -1-4 -0.78 -0.89 -0.63 132 Sn -76.59 0.08 -0.31 -1.88 -1.6 -1.13 -1.10 -0.90

123 + Sb -84.60 ?-^ 0.58 0.21 -0.5 -0.14 -0.22 -0.05 ~\j • u6 130 Sb 0.27 -0.3b -0.8 -0.36 -0.75 -0.24 -82.33 0.10 131 Sb -0.03 -0.82 -1.1 -0.49 -0.95 -0.?9 -82.01 0.07 132 Sb -0.23 -1.18 -1.3 -0.84 -0.94 -0.66 -79.67 0.07 Sb -0.47 -1.43 -1-3 -1.15 -0.71 -0-76 -74.17 0.26 134 Te -0.05 -1.18 -0.7 -0.19 -0.41 -0.31 -82.41 0.11 135 Te -0.18 -0.96 -0.6 -0.48 -0.14 +0.04 -77.85 '0.24

0.32 ' exp pred 0.87 1.0 0.49 0.61 0.36

a) ref 42a d) ref 42d b) ref 42b e) ref 42e c) ref 42c f) ref 43 - 22 -

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