Yrast isomers in 119Sn, 121Sn and 123Sn have been identified among products of heavy ion collisions with 124Sn targets 10-15% above the Coloumb barrier. Isomeric decay schemes are reported, and further evidence for half-filling of the νh11 2 subshell at N = 73 is presented. For (νh11 2nseniority-3 states, observed level energies agree well with results of fractional parentage calculations

Ahmad, I.

Bearden, I.G.

Bhattacharyya, P.

Blomqvist, J.

Broda, R.

Carpenter, M.P.

Daly, P.J.

Fornal, B.

Grabowski, Z.W.

Henry, R.G.

Janssens, R.V.F.

Khoo, T.L.

Lauritsen, T.

Liang, Y.

Mayer, R.H.

Nisius, D.T.

Richter, L.

Sferrazza, M.

Carolina Digital Repository

ELSEVIER 29 September 1994 Physics Letters B 336 (1994) 308-312 PHYSICS LETTERS B Gamma-ray studies of 119'121'123Sn isomers formed in deep inelastic heavy ion collisions * R.H. Mayer a, D.T. Nisius a, I.G. Bearden a, E Bhattacharyya a, L. Richter a, M. Sferrazza a, Z.W. Grabowski a, EJ. Daly a, R. Broda b, B. Fornal b, I. Ahmad c, M.E Carpenter c, R.G. Henry c, R.V.F. Janssens c, T.L. Khoo c, T. Lauritsen c, y. Liang c, j. Blomqvist d a Physics and Chemistry Depts., Purdue University, W. Lafayette, IN 47907, USA b Institute of Nuclear Physics, PL-31342 Cracow, Poland e Physics Division, Argonne National Laboratory, Argonne, IL 60439, USA d Physics Department Frescati, Royal Institute of Technology, S-10405 Stockholm, Sweden Received 6 June 1994 Editor: R.H. Siemssen Abstract  Yrast isomers in ll9Sn, 12lSn and 1238n have been identified among products of heavy ion collisions with lZaSn targets 10-15% above the Coulomb barrier. Isomeric decay schemes are reported, and further evidence for half-tilting of the/-'hll/2 subshell at N = 73 is presented. For (/..,hll/2) n seniority-3 states, observed level energies agree well with results of tractional parentage calculations. The neutron hll/2 subshell is being filled in A = 116-131 tin nuclei, and it should be possible to study (Vhll/2) n excitations through this complete series of  isotopes. At the start of  the present investigations, (~,hll/2) n, seniority v = 2, 10 + isomers and their de- cays had been well studied in ll6Sn, ll8Sn and 12°Sn at one end of  the series [ 1-3] ,  and in the fission products 128Sn and 13°Sn at the other [4] ,  but there was no information about analogous 10 + isomers in 122Sn, 124Sn and 126Sn, which were expected to have long half-lives. Since tin nuclei with A > 120 are not accessible by fusion-evaporation reactions, we searched for the missing isomers among the products * Work supported by the U.S. Department of Energy under con- tracts nos. DE-FG02-87ER40346 and W-31-109-ENG-38, and by the Polish Scientific Committee under grant no. 224319203. Elsevier Science B.V. SSDI 0370-2693 (94) 01039-0 of  deep-inelastic heavy ion reactions on 124Sn and 1225n, 10-15% above the Coulomb barrier. The y y  coincidence data recorded during our first experiment using a pulsed beam of 325 MeV 76Ge ions on thick 122'1248n targets were found to include new informa- tion about yrast states of  moderately high spins (up to I ,-~ 14) in more than 50 neutron-rich nuclei. Specifi- cally, the sought-for (Uhl l /2 )  n l0  + isomers in 122Sn and 124Sn were identified with half-lives of  62(3)  and 45(5)  /zs, respectively [5] ,  but little 1268n was produced. The B(E2;  10 + --+ 8 +) values for 122Sn and 124Sn, together with the other values determined earlier, located the half-filling of  the neutron hll/2 subshell close to N = 73 (whereas the 7rh11/2 half- filling in N = 82 isotones occurs just below Z = 71 [6] ). The marked difference between neutrons and protons could be traced [5] to the relative sl/2, d3/2 R.H. Mayer et al. /Physics Letters B 336 (1994) 308-312 309 and h11/2 single particle energies, which are altered by the Coulomb potential; the s1/2 and d3/2 energies for protons are raised more than the h11/2 energy. The spectra recorded in the 122'124Sn + 76Ge ex- periments showed in the 800-1250 keV range several strong unknown y-rays, decaying with 5-10/zs  half- lives, that we suspected might be due to isomerism in odd-A Sn nuclei. The decay of a 1.75/zs 19/2 + iso- mer in ll7Sn had previously been studied in some de- tail [7], but nothing was known about yrast isomers in the heavier odd-A isotopes. In follow-up experi- ments already briefly reported [8], 1 mg/cm 2 1228n a n d  124Sn targets, each enriched to >96% and backed with lead, were bombarded with 344 MeV 8°Se ions from the ATLAS accelerator in a 10/zs beam-on, 50 /zs beam-off pulsing configuration. Twelve Compton- suppressed Ge detectors of the Argonne-Notre Dame BGO y-ray facility measured off-beam spectra in two- parameter E z, versus time mode, as well as 7y  coinci- dence events. The data analysis identified three pairs of coincident y-rays following separate isomers: 818 and 1220 keV transitions in decay of a 10/zs ll9Sn isomer; 841 and 1151 keV transitions in decay of a 5 /zs 121Sn isomer; and 838 and 1107 keV transitions in decay o f a  7/zs 123Sn isomer. As detailed in Ref. [8], isotopic assignments of the y-ray pairs were based on evidence from these and other in-beam reaction studies and on energy systematics arguments. Like the known 19/2 + isomer in t]7Sn, the three new isomers were interpreted as 19/2 + states of/-'hll/2 X 5 -  ( o r  t/h~l/2 sl/2) character, decaying to 15 /2-  and 1 l / 2 -  states by cascades of  M2 and E2 transitions. The 15/2-  and 19/2 + level energies follow closely the highly regular systematics of the 2 + and 5-  energies in the neighbor- ing even-A Sn nuclei, and the B(M2) values for the 19/2 + ~ 15 /2 -  isomeric transitions in the four odd- A tin isotopes including l lVSn are equal within a factor of two. A weak E3 isomeric decay branch 19/2 + --~ 13 /2-  ~ 11 /2 -  was established [7] in the 117Sn nu- cleus; analogous but even weaker branches were also observed in the decays of the 119'121'1238n 19/2 + iso- mers. Identification of the 1220, 1151 and 1107 keV 15/2-  ~ 11 /2-  transitions in 119'121'1238n w a s  a sig- nificant first step towards further studies of (ph 11/2) n v = 3 yrast states in these nuclei. By setting coin- cidence gates on the 1220 and 1151 keV transitions t a)' Gate'122'O keV . . . .  119S '  n t 5 0 f  r-- = co co t 100 b) Gate 1151 keV ~ 121°no x G0 J ro c) Prompt coincidence 1 2 3 Q n  6O gate 1107 keV 20 ~" Z 0 " 0 10 ~ d) Prompt coincidence 0 gate 170 keY 5 0  ~ o  C~ r-- © _ [ o l o  ~ e) Delayed c o i n c i d e n c e  g j- ~ ] N gate 1107 keY 4oF~tI~ o  ot, l? 0 . _a--,.J . . . . .  i ~  - L =  _1. . . . . . . . . .  f ) ~ e l a y e d  coincidence o gate208 keY co 15 ¢.0 ,~. 0 O) aO O , ,  ~ ,  ~ , - - ' . . " " i " ' -  " I 200 400 600 800 1000 1200 E~, (keV) Fig. 1. a), b) Off-beam y y  prompt coincidence data for the 124Sn + 344 MeV 8°Se system showing y-ray spectra coincident with 15 /2 -  ----, 11 /2-  transitions in H9Sn and 121Sn. Labelled y-rays are assigned as transitions occurring in the 19/2 + or 2 7 / 2 -  isomeric decays in these nuclei. Peaks marked X are 86Sr y-rays resulting from 8°Se reactions on carbon or oxygen target impurities. c ) - f )  show ]23Sn y y  coincidence data measured 1-30/zs off-beam for the system 124Sn + 665 MeV 136Xe. The prompt spectra c) and d) were recorded with the timing condition Tyy < 300 ns, and the delayed coincidence spectra e) and f) with the condition 0.I /zs < Tyy < 30 #s. (Figs. 1 a) and b) ) ,  feeding y-ray cascades of 175, 513 and 1106 keV in ]19Sn and of 176, 471 and 1030 keV in 121Sn were identified and assigned as 2 7 / 2 -  23 /2 -  --~ 19/2-  --~ 15 /2-  sequences deexciting 27 /2 -  isomers of dominant (uh] 1/2) n character (Fig. 2). Although no transition multipolarities were deter- mined, these configuration assignments could be made with confidence because the experimental 2 3 / 2 -  and 27 /2 -  level energies were found to be in close agree- ment with those predicted for ( P h l l / 2 )  n u = 3 states by fractional parentage calculations that will be de- scribed later in this article. Half-lives for the 2 7 / 2 -  310 R.H. Mayer et al. / Physics Letters B 336 (1994) 308-312 34 ns ~ 27/2- 23/2- 0.17/2s 97/9- 34/zs - 1 - 1 9 / z  2~z 6.s J ,,/,-2~ I '"2" 8'"s ~ I'FI"" 13/2- T. ̂~'15/2- 13/2-~ 119Sn 121Sn 123Sn Fig. 2. Isomeric decay schemes for 119Sn, 121Sn and 123Sn. Widths of transition arrows are proportional to the observed intensities, with internal conversion contributions unshaded. isomers in l l9Sn and 121Sn were determined from the 124Sn + 8°Se data to be 34(10) ns and 167(25) ns, respectively, and the corresponding transition proba- bilities for the isomeric E2 transitions are: B(E2; 2 7 / 2 -  ---+ 2 3 / 2 - ,  lt9Sn) = 82(24) e2fm 4 and B(E2; 2 7 / 2 -  ~ 2 3 / 2 - ,  121Sn) = 16(2) e2fm 4 These values are in good accord with those previously determined for the 10 + ~ 8 + transitions in the neigh- boring even-A tin nuclei. A search through the 124Sn -t- 8°Se data for a y-ray cascade deexciting a X23Sn 2 7 / 2 -  isomer was not fruitful; neither could any yrast isomers in A > 124 tin isotopes be identified in these data. For this reaction system, the observed favored direction of neutron flow in deep inelastic processes was from target to projectile. We recently extended our investigations by studying the system 124Sn q- 665 MeV 136Xe, expecting on the basis of N / Z  equilibration considerations [ 9 ] that the 136Xe beam would favor production of neutron-rich species (and perhaps 123Sn) more than the 76Ge and 8°Se beams used earlier. The first phase of the exper- iment was devoted to isomeric decay measurements, again using the Argonne-Notre Dame y-ray facility, but with special attention devoted to detection of yy  coincidence events across/zs isomers. The overall pat- tern of reaction yields in the Z = 50 region was def- initely shifted towards higher neutron numbers, and the missing 126Sn 10 + isomer with tl/2 "~ 6.5/xs was finally found [ 10 ]. More important in the present con- text were the high quality prompt and delayed y-ray coincidence data obtained for 123Sn, as illustrated in Fig. 1 c ) - f ) .  They identified, in coincidence with the 1107 keV 15/2-  ~ 11 /2-  123Sn transition, y-ray de- cay paths crossing 6/zs and 8/zs intermediate isomers as well as prompt y-ray cascades, which together es- tablished an interesting 123Sn yrast level scheme up to a 34 ~s isomeric level at 2713 keV (Fig. 2). This uppermost isomer and the level 170 keV below it are, without doubt, the (Uhll/2) n v = 3 2 7 / 2 -  and 2 3 / 2 -  states; their excitation energies agree with predictions within a few keV. The tiny E2 transition probability, B(E2; 2 7 / 2 -  ~ 2 3 / 2 - ,  123Sn) = 0.06 e z fm 4, which is less than 2 × 10 -3 Weisskopf units, manifests anew the half-filling of the ~'hll/2 subshell at N = 73. In consequence, the configuration-forbidden 560 keV M2 transition is seen to be competitive with the 170 keV E2 in decay of the 34 ~s  isomer. The thousand- fold increase in half-life between the l l9Sn and 123Sn 27 /2 -  isomers is a striking illustration of the impact of subshell occupation on transition probabilities. The 23/2 + isomer located in 123Sn is an obvious counter- part of the well-established (uhl 1/2d3/2) 7 -  isomers R.H. Mayer et al. /Physics Letters B 336 (1994) 308-312 311 in even-A tin isotopes, but we could not firmly de- cide from the available data whether there are simi- lar 23/2 + microsecond isomers in 119Sn and 121Sn as well. The 2 7 / 2 -  and 2 3 / 2 -  levels in 119,121'123Sn nuclei are expected to be rather pure (vh11/2) n v = 3 states. Such shell model yrast states with single dominant configurations comprising a few high-j valence nucle- ons can generally be decomposed into simpler config- urations with fewer valence particles, which may cor- respond to known yrast levels in neighboring nuclei. In an earlier extensive analysis [ 11 ] of high-spin states in the N ,-~ 82 region above 146Gd, where hi 1/2 valence protons play a dominant role in forming yrast excita- tions, ground state masses for many shell model nuclei located far from stability were estimated by exploit- ing fractional parentage relationships between specific few valence nucleon yrast states, whose excitation en- ergies were known from y-ray studies. The situation in A = 116-124 tin nuclei is more favorable, since the masses are accurately known; thus, comparisons of  calculated (uhl 1/2) n energies with experiment pro- vide unusually exacting tests of the configuration as- signments. For example, the 2 7 / 2 -  aligned (uhll/2) 3 state in 123Sn can be broken up by a fractional parent- age decomposition into simpler states with two, one, and zero h11/2 neutrons outside a 12°Sn 0 + core [ 12]. Expressing the total energy of this 2 7 / 2 -  state as a sum of 123Sn ground state mass M123 and excitation energy E ( 2 7 / 2 - )  gives the relation: M123 + E ( 2 7 / 2 - )  = 3M122 + 3(c.f.p.) E(8 +, 10 +) - 3M12! - 3 E ( 1 1 / 2 - )  + M120, where E(8+,10 +) are energies of known (z, h11/2) 2 8 + and 10 + states in 122Sn, here weighted by appro- priate coefficients of  fractional parentage (c.f.p.), and E ( l l / 2 - )  in 121Sn is known to be 6 keV. The indi- vidual masses may be combined in a mass "window", W, and evaluated using modern mass tables [ 13]. In the present case, W = M120 - 3M12! + 3M122 - M123 = -5512(5 )  keV and E ( 2 7 / 2 - )  = W + 3(c.f .p .)E(8 +, 10 +) - 3 E ( 1 1 / 2 - )  is calculated to be 2710(5) keV, com- pared to the experimental 2713 keV. For the 123Sn 2 3 / 2 -  state, with the known 6 + energy in 122Sn added to the input data, the calculated energy 2540(5) keV is also in near-perfect agreement with experiment. Results of such fractional parentage calculations of 27 /2 -  and 2 3 / 2 -  energies for the three nuclei ll9Sn, 121Sn and 123Sn are summarized in Table 1, where the input energies used and the values of the three mass windows are also specified. The excellent agreement with experiment in all three cases is remarkable, con- sidering that the core nuclei ll6Sn, llSSn and J2°Sn are only singly magic, and it provides vital support for interpretation of the observed levels as (phi 1/2)n v = 3 states. For the isomeric transitions between (~,h~l/z) n states in the tin isotopes, the dependence of the E2 transition probability on the phi 1/2 subshell occupa- tion number n, and on the neutron effective charge eeff, may be expressed [ 14], taking the radial matrix < r 2 > for/Jhll/2 to be 32 fm 2, by the equations: 2 B(E2; 10 + --~ 8 +) = 1.17 (6 - n) 2 eel f and B(E2; 2 7 / 2 -  ~ 2 3 / 2 - )  = 4.41 (6 - n) 2 ee2ff. Following Ref. [5], it is convenient to plot the E2 transition amplitudes (or square roots of the measured B(E2) values) versus A, knowing that the E2 matrix element must have opposite signs in the bottom and top halves of the Vhll/2 subshell, and that it becomes zero at the point of half-filling, n = 6, when particle and hole contributions exactly cancel one another. All the E2 amplitudes determined for the Sn isotopes are displayed together in Fig. 3, with the x / - B ~  values for the odd-A nuclei multiplied by 0.514 (i.e. X/1.17/4.41) to compensate for the different geo- metrical factors entering the v = 2 and v = 3 B(E2) equations. The results for the odd-A nuclei clearly match up excellently with those previously obtained for the even-A tin nuclei, and they reinforce earlier conclusions about the uht l /2  subshell filling and about the enhancement of the neutron effective charge to- wards the middle of the N = 50-82 major shell. Not all features of the 119"121'123Sn level spectra are satis- factorily understood, but further discussion must be postponed to a later paper. Regarding the points still missing in Fig. 3, it may well be possible by similar methods to identify and characterize the decay of the 27 /2 -  isomer expected in 125Sn, but a search for cor- responding isomers in 127Sn and 1298n would appear to offer a more difficult experimental challenge. 312 R.H. Mayer et al. / Physics Letters B 336 (1994) 308-312 Table 1 Summary of the fractional parentage calculations of (vhn/2)  3 energies in ll9Sn, 121Sn and 123Sn. Input for calculation of the 2 3 / 2 -  and 2 7 / 2 -  energies in the odd-A nucleus ASn consisted of the 11 /2-  level energy in the A-ZSn nucleus, the 6 +, 8 + and 10 + energies in the a--lSn nucleus, and the mass window W, evaluated using neutron separation energies given in Ref. [ 13]. For comparison, experimental energies are listed in the right hand columns. Input level energies Mass window Output energies Experiment (keV) W (keV) (keV) (keV) 117Sn llSSn 119Sn l]9Sn 11 /2 -  6 + 8 + 10 + 2 3 / 2 -  2 7 / 2 -  2 3 / 2 -  2 7 / 2 -  --5223(3) 315 2879 3052 3108 2926(3) 3118(3) 2928 3103 ll9Sn 120Sn 121Sn 121Sn 11 /2 -  6 + 8 + 10 + 2 3 / 2 -  2 7 / 2 -  2 3 / 2 -  2 7 / 2 -  -5558(5)  90 2697 2837 2903 2666(5) 2834(5) 2659 2835 121Sn 122Sn 123Sn 123Sn 11 /2 -  6 + 8 + 10 + 2 3 / 2 -  2 7 / 2 -  2 3 / 2 -  2 7 / 2 -  -5512(5)  6 2555 2690 2765 2540(5) 2710(5) 2543 2713 8 II) ! 4 oO 0 u - - - -  -2 -4 I I l , l , l , l , l , i  , o •  Sn Isotopes o even-A k,,ck, • odd-A N _  f--, t'Xl 1, - -  0 "~" I , I i I i I i I , I , I , l I 0 116 118 120 122 124 126 128 130 A Fig. 3. E2 transition amplitudes for (Vh l l /2 )  n 10 + --* 8 + and 2 7 / 2 -  ~ 2 3 / 2 -  transitions in even-A and odd-A Sn isotopes. To facilitate intercomparison of the data points, values for the odd-A nuclei have been multiplied by a factor 0.514 (see text). Where error bars are not shown, they lie within the plotted points. A line is drawn through the data points to guide the eye. References [ 1 ] M. Ishihara, R. Broda and B. Herskind, Proc. Int. Conf. Nucl. Phys. 1 (1973) 256. [2] A. van Poelgeest et al., Nucl. Phys. A 346 (1980) 70. [3] S. Lunardi et al., Z. Phys. A 328 (1987) 487. [4] B. Fogelberg et al., Nucl. Phys. A 352 (1981) 157. [51 R. Broda et al., Phys. Rev. Lett. 68 (1992) 1671. [6] J.H. McNeill et al., Phys. Rev. Lett. 63 (1989) 860. [7] O. Hashimoto et al., Nucl. Phys. A 318 (1979) 145. [8] R.H. Mayer et al., Z. Phys. A 342 (1992) 247. [9] See e .g .H .  Freiesleben and J.V. Kratz, Phys. Rep. 106 (1984) 1. [ 10] R.H. Mayer et al., to be published. [11] J. Blomqvist, P. Kleinheinz and P.J. Daly, Z. Phys. A 312 (1983) 27. [12] The analogous decomposition of 149Ho (~h11/2) 3 states into substructures known in lighter N = 82 nuclei has been discussed in Ref. I 11 ]. [13] G. Audi and A.H. Wapstra, Nucl. Phys. A 565 (1993) 1. [14] J. Blomqvist, International Review of Nuclear Physics (World Scientific, Singapore, 1984) Vol. 2, pp. 1-32. T h e  a u t h o r s  t h a n k  t h e  A T L A S  a c c e l e r a t o r  s t a f f  fo r  t h e i r  e f f o r t s  in  p r o v i d i n g  t h e  b e a m s  f r o m  t h e  n e w  p o s -  i t ive  i on  in j ec to r .  

Gamma-ray studies of 119, 121, 123Sn isomers formed in deep inelastic heavy ion collisions

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Gamma-ray studies of 119, 121, 123Sn isomers formed in deep inelastic heavy ion collisions

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