For the 92Mo+255 MeV60Ni system, inelastic and few-nucleon transfer events populating non-collective states of moderately high spin have been studied by γγ coincidence measurements. Besides the strong inelastic scattering channel, twelve transfer processes were identified, ranging from 1 n to 2α transfer; typically, cross coincidences between the γ-rays from both products were observed. Potential spectroscopic applications are indicated

Broda, R.

Daly, P.J.

Drigert, M.W.

Janssens, R.V.F.

Khoo, T.L.

Ma, W.C.

Quader, M.A.

Carolina Digital Repository

Volume 251, number 2 PHYSICS LETTERS B 15 November 1990 Inelastic and transfer reactions in 92Mo-I-255 MeV 6°Ni collisions studied by y), coincidences R. Broda ~, M.A. Quader 2, p.j. Daly Chemistry Department, Purdue University, West Lafayette, IN 47907, USA R.V.F. Janssens, T.L. Khoo, W.C. Ma 3 Physics Division, Argonne National Laboratory. Argonne. IL 60439, USA and M.W. Drigert Idaho National Engineering Laboratory, Idaho Falls. ID 83415, USA Received 27 June 1990 For the 92Mo+ 255 MeV 6°Ni system, inelastic and few-nucleon transfer events populating non-collective states of moderately high spin have been studied by Y't' coincidence measurements. Besides the strong inelastic scattering channel, twelve transfer processes were identified, ranging from I n to 2u transfer; typically, cross coincidences between the ),-rays from both products were observed. Potential spectroscopic applications are indicated. Transfer  reactions induced by heavy ions with in- cident  energies close to the Coulomb barr ier  have been intensively investigated using both charged par- ticle spectroscopy and pa r t i c l e -gamma coincidence methods.  The good energy resolution available in T- ray spectroscopy allows measurement  of  discrete state populat ions,  and is part icularly impor tan t  when the target nuclei are deformed and Coulomb excitat ion in both entrance and exit channels leads to popula-  t ion o f  high-spin states in the heavier  product .  Parti-  cle-T-ray coincidence techniques were used as long ago as 1973 to study such processes in ~60-induced reactions [1] ,  and recent exper iments  employing much heavier  beams have demonst ra ted  [2 -4 ]  the * Work supported by the US Department of Energy under con- tracts Nos. DE-FG02-87ER40346 and W-3 I- 109-ENG-38. Permanent address: Institute of Nuclear Physics, PL-313 42 Cracow, Poland. 2 Present address: Oncology Center, University of Rochester, Rochester, NY 14642, USA. 3 Present address: Tsinghua University, Beijing, P.R. China. populat ion of  states with spins up to a round I =  20. The use o f  pan ic le  detect ion improves  channel selec- t ivi ty and yields valuable informat ion about  the re- action kinematics.  A significant practical  d isadvan-  tage is the necessity to compromise  between pan ic le  detection and T-ray yield requirements by using rather thin ( < 0 . 5  m g / c m  2) se l f -supponing targets; Dop-  pler shift correct ions must  then be appl ied  to restore the y-ray energy resolution. A different approach is to forgo part icle detect ion and to use YT coincidence techniques to study inelas- t ic / t ransfer  processes that populate  states of  moder-  ately high spin in the product  nuclei. Here, the emis- sion of  known y-rays identify the final nuclei; an especially impor tan t  feature is that  the mutual  exci- ta t ion of  both reaction products  may be directly ob- served through coincidences of  y-rays emit ted  from reaction partners.  The advent  o fmul t ide tec to r  arrays o fCompton-suppressed  Ge spectrometers,  providing greatly improved  sensit ivity and selectivity in y)' co- 0370-2693/90/$ 03.50 © 1990 - Elsevier Science Publishers B.V. ( North-Holland ) 245 Volume 251, number 2 PHYSICS LETTERS B 15 November 1990 incidence spectrometry, has made this method of  study particularly attractive. In a recent 92Mo-F 255 MeV ~Ni  experiment, which initially had as its main objective the elucidation of  high-spin level structures in the fusion--evaporation products ~49Ho and ~5°Er, the excellent 77 coinci- dence data acquired were found to include many events arising from inelastic and transfer reaction products. Here we present the results of  a thorough analysis o f  these events. The target consisted of  99% enriched 92Mo of  0.8 m g / c m  ~ thickness with a thick Pb backing to stop both recoils and beam. It was lo- cated at the center of  the Argonne-Notre  Dame BGO ~'-ray facility (which consisted, at that time, of  eight Compton-suppressed Ge detectors and a central ball of  14 BGO hexagons), and it was bombarded with 255 MeV 6°Ni ions from the ATLAS accelerator. The experimental conditions were optimized for obser- vation of  high-lying cascades in the fusion-evapora-  tion products, and only those coincidence events were stored in which at least two Ge detectors and two BGO elements fired within a 200 ns interval. This multiplicity condition effectively suppressed Cou- lomb excitation events, and it also selected those ine- lastic/transfer events in which states of  at least mod- erately high spin were populated. The BGO coincidence pulses werc used to define the time of  reaction, and, in the latter comprehensive sorting of  the Ge coincidence data, delayed and prompt events could be cleanly separated. Both the A ~ 150 fusion evaporation products and the A ~ 92 transfer prod- ucts are spherical nuclei having many isomers, and in both cases the separation of  the delayed events greatly improved the ~t detection sensitivity. Examples o f  the y-ray coincidence spectra ob- tained are shown in fig. 1. For four different reac- tions, we show spectra with separate gates set on the lowest-lying transitions in both product nuclei; cross coincidences arising from simultaneous mutual ex- citation o f  both products are evident, and they pro- vide unambiguous identification o f  the reaction. For most 7-rays no appreciable Doppler broadening was observed, indicating that the feeding and state life- times involved are longer than the stopping time of  the reaction products. In addition to the strong inelastic channel, twelve transfer processes were identified, ranging from I n to 2ct transfer, and involving excitation to high-spin (up 2 0 0 ~  I§- ~ 4+ x'5'Tm 4 + o92Mo eg'Mo I I 3 + o + 7 v6ONi v59Ni I 0 0 ] -  ~.  ,~ o I 2 Igote:1332keV 60Ni_ iooo|1o5- INELASTIC I gore: 775 keY 92Mo- 2 + 500  ,r  §* .~+.,_ 9- .,. 7 -  • o d ,,v ~o~ o ~:~ o 4 2+ tl O' v'~ I 0 V-- - V ..L- I 0 J.2l i__~+~.l  . . . .  . I ,. '~ . ,  O93M0 e 9'2Mo V59Ni v58Ni o o 5 0  ,e j . .  , I o '~ ~,7 g o i ' e :  3 : 5 9  keY 59Ni -  o 0 ~ , 5 / 2 - 1 3 / 2 *  I n - T R A N S F E R  I r,'~ I o gate: 1476 keV93Mo 5O (13 ~/~- 11/2 + 7/2- o ~ v I-- o, /  Z ,..[ L, JL.~.,,,-._ ~,  ~j ..,.L. JJC., ,_.;.:_Ur::, T . . . . . . . . .  D 0 --~r,,.t-,-, ,....-.. - .... 0 2 + ;4  + o94M' 0 e95M 0 0 4+ ~ v58N i 20 I r g°teq454 keV 58Ni 4+ 2 n - T R A N S F E R  2 0 0  _ , ~ 1  + gol"e:871 keY 94Mo I 0 0  0 ,v , v, IOOl- 9-1°4 * 92 + o 96Ru egSRu l =*~ I " 56Fe v55Fe ~ L  I°~1 . gate:847 keV56Fe " v r  ? i2+l~'f-8+lio + 5- 4+6 + I . . . . . .  o 0 ~ ; , ' ' , ' , . , . - F ' , . ; .  " ," ;"r  ~ ,--":":-"~==" ::~-"- 'o4+ ! 9 T  e - T R A N S F E R  2 0 0 L  ~ ÷* gate: 833 keY 96Ru r. ,2o.]+-07+,oO+ 05 ,+, I . T. . . j . ,  ?1011 l l i , . ,  o " I V, , , r '  0 --u# J:. ~ =,'-'~4~++,- ,,=~ ~- 5 0 0  I 0 0 0  1500 E T ( k e V )  Fig. 1. Sample 7-ray coincidence data for the four reactions spec- ified, showing y-ray spectra coincident with known low-lying transitions in both product nuclei. For individual transiUons, the isotopic assignment and the spin-parity of the parent level are generally indicated. The unlabelled peaks in the 339 and 871 keV gates are identified as transitions coincident with strong '~°Er 7- rays in the gating regions. 246 Volume 251, number 2 PHYSICS LETTERS B 15 November 1990 Table 1 List of reactions identified and their Qu values. Relative coincidence yields (including prompt and off-beam events and taking account of detector efficiencies) are given for the specified grating transitions. Reaction products Qu Heavy product Light product (MeV) gating observed relative observed relative y-ray coincidence y coincidence coincidence y coincidence ( keV ) (keV) yield (keV) yield inelastic 92M0 + 6°Ni 0 773 1509 93 ( 5 ) 1332 34 (4) + I n 93Mo + ~gNi - 3.3 1476 684 9.3 (8) 339 7.2 (6) + In 91Mo-I- 61Ni --4.9 1414 654 18(2) unobserved - - Ip 93Tc q- ~9Co -5.4 750 1434 5.3(8) 1190 1.8(6) + lp 9JNb+6tCu -2.7 2291 819 2.5(6) unobserved - -2n  94Mo + 5SNi -2.6 871 703 33(2) 1454 13( I ) + 2n 9°Mo + 62Ni - 4.4 948 unobserved l 173 2 ( l ) - 2p 94Ru + SSFe - 6.6 1428 755 1.8 (6) unobserved - +2p 9°Zr + 62Zn - 1.3 1129 141 0.6(3) 954 <0.3 - a  96Ru + S6Fe -4.6 833 685 33(2) 847 15( I ) +a SSZr+~Zn - 1.7 1057 1082 11.5(1.6) 992 5( I ) - 2ct l°°Pd + ~2Cr - 10.6 665 750 3.8 ( 5 ) 1434 ~ 1 +2n 84St+raGe -3.7 974 793 7.3(5) 1016 2.3(4) CN* US°Er+2p 1203 "> 490 100 CN* 149Ho + 3p 1560 ") 727 145 ( 8 ) • ) The 1203 keV y-ray is the strongest transition feeding the 2.5 p.s 10 + isomer in tS°Er [6], and 1560 keV is the strongest transition feeding the 11/2- ground state in 149Ho [7]. to I =  12) and modera te ly  high energy. In table 1, the observed processes are listed along with relative yield est imates for both react ion products  which were ob- tained by quant i ta t ive  analysis of  the coincidence in- tensities in spectra gated on the lowest (or  next to lowest) transit ion in each heavy product  nucleus. For  each process, the gating T-ray and the two coincident  T-rays used to est imate the yields of  the heavy and light products  are specified in the table (typically,  T cross coincidences between both products  were ob- served in more than 30% of  the events) .  Yields for the two strongest fus ion-evapora t ion  products  149Ho and ~5°Er are also given for comparison.  In view of  the selection imposed by the coincidence condit ions,  the yields given in table 1 should reflect the relative high-spin populat ions  resulting from the different processes. For  products  having isomers, the delayed events were included in these estimates,  but  when the isomeric half-lives exceeded a few hundred  nanose- conds only a fraction of  the isomer popula t ion  was detected. [ For  example, the 9°Zr produced by 2p pick- up was especially difficult  to observe because of  its long-lived 5 -  isomer and the high-energy ( ~ 2 . 2  MeV) of  the lowest transition. ] Nevertheless, in most cases the listed yield should fairly represent  the high- spin part  of  the react ion cross section, with low mul- t ipl ici ty events including those arising from Cou- lomb excitat ion essentially excluded. The observed yields of  the inelastic scattering product  nuclei are comparable  to those of  the fusion evapora t ion  products ,  and the data  obta ined are con- sequently o f  highest quality. Fig. 2 shows the level schemes o f  both colliding nuclei, with the populat ion of  various states indicated by t ransi t ion arrows hav- ing widths propor t ional  to the measured T-ray inten- sities. The 6°Ni populat ion was normal ized  to reflect the T-ray intensit ies observed in the 92Mo 2 +--*0 + gate. In the 92Mo target nucleus, states with excitat ion energies up to 6 MeV, and spins up to 1 1, were sig- nif icantly populated,  including rather  strong popula-  t ion of  known 8 ÷ and 1 1- isomers. The dominan t  configurations of  these isomers are (ng2/2)8+, sen- iori ty v=2 ,  and (ng93/2pl/2)1 1- ,  v=4 ,  and it is clear that  Coulomb excitat ion plays little part  in their  pop- ulation, which must  instead occur through a nuclear exci tat ion process during overlap of  the nuclear  mat-  ter o f  the two coll iding species. The 6°Ni projecti le  is 247 Volume 25 I, number 2 PHYSICS LETTERS B 15 November 1990 MeV 6. 5- 4- 3- 2- I 8• 7-  11- o+ ,90.. ~1 ~ 4 -  ~ 1  ~+ 147 0 53 480_ g 4 + ~  ~ 4 + 5_ 7 7 3  G + ~  0 ÷ 92Mo5o 1083 1758 ~ 1332 60 28NI32 Fig. 2. The 92Mo and 6°Ni level schemes. The widths of the tran- sition arrows are proportional to the measured "/-ray intensities and they illustrate the relative population of individual levels by inelastic scattering. excited up to comparable  energies, and states up to the 7 -  level at 5.35 MeV could be observed.  In both nuclei, non-yrast  states were also popula ted  as indi- cated in the schemes o f  fig. 2. To answer the interest- ing quest ion of  how much excitat ion energy could be transferred to the coll iding nuclei would require higher statistics, together with detai led examina t ion  of  the y-ray spectra, possibly including the quasicon- t inuum, up to much higher energies than were en- compassed  in the present  measurements .  However,  a significant result is that  exci tat ions to energies ex- ceeding the neutron separat ion energy clearly occur. The three lowest t ransi t ions o f  92M0 were found to coincide with a 339.5 keV "/-ray which was not seen in any gates set on 6°Ni transit ions.  Such a t ransi t ion does not occur in the well-known 92M0 level scheme but it is instead ident if ied as the known ~- _ 3 -  tran- sition to ground in the S9Ni nucleus. F rom the ob- served intensities,  we deduce that  ~ 5% of  the events in which 92M0 is excited results in the ejection o f  a neutron from the projecti le  yielding a 5gNi product  nucleus rather  than 6°Ni. It appears  further that  the mir ror  process in which the excited 92M0 nucleus emits  a neutron happens  even more frequently. In ta- ble 1, the process labelled In-pickup and showing rather  high coincidence intensit ies for 9tMo )'-rays, reveals no trace of  6~Ni )'-rays. Instead, the 1332 keV ground state t ransi t ion in 6°Ni was observed with low intensi ty in coincidence with the 9~Mo ),-rays. Al- though other  mechanisms cannot  be excluded, it seems likely that  the 9~Mo yield comes mainly  from inelastic collisions followed by one neutron emission from the excited 92M0. Similar  features were ob- served for the process designated I p pick-up, but with significantly lower yield. A compar ison  of  the yields of  various transfer re- act ions given in table I, bearing in mind  the selection imposed by the exper imental  mult ipl ic i ty  condi t ion,  leads to the following general conclusions. In the 92M0+255 MeV 6°Ni collisions, t ransfer  reactions contr ibute  a substantial  fraction o f  the reaction cross section, with part icular ly strong contr ibut ions  from 2n- and a-str ipping.  Transfer  of  part icles from pro- ject i le  to target generally dominates ,  but no clearcut correlat ion between the yields and the ground state Q-values is apparent .  For  neutron transfers, the Q~ values are fairly s imilar  but  the yields are quite dis- s imilar  e.g. merely a trace of  2n pickup was observed,  whereas 2n str ipping,  with a less than 2 MeV more favorable Q-value, is one of  the dominan t  processes. Cons idera t ions  of  the Q-matching model  [5]  are consistent  with the observed larger cross section for a-s t r ipping relative to a-pickup,  but for 2a transfers the trend is opposite.  Clearly, the observed processes are quite different from the gentle quasi-elastic trans- fers s tudied with kinemat ic  selection around the grazing angle region. MeV 6 6 0  7- 3 703 5- 2 lO7C 1 6 + 1303 4 + 12 + 601 ' ~ 1 0  + 867 8 ÷ soo - - 6  531 8133685 4+ 2  - -  _ _ 0  ÷ 1238 847 2 + 0 0 + 256Fe 30 Fig. 3. The 96Ru and 56Fe level schemes, illustrating the relative population of individual levels in the a-transfer process. 248 . Volume 251, number 2 PHYSICS LETTERS B 15 November 1990 150 I00 50 GO t -  0 Z 30C 0 20O 6 + 5 -  i 92Mo goJte = 773 keV 2 + 9- 7- i 21/2 + 91Mo 38 n j . ~  m=~_n~17/2;gore: 654 ._tr.~_ 1271;22 + keV : 654 1312 + 1414 13/2+ 10C 17/2- 9/2+ [ i 0 ' ' ' " 4 0  94Mo 8+ 4 + 2+ 98 ns /8~ + gate,850 keV 532S~------~'~; J850 " _ ~ 4"" 20  1703 2 + 6~ ~ 871 0+ 0 ~' II"J'L" J~' "[ . . . . . .  L J, ,,~ . . . . . . . . . . . . .  I . . . . . . . . . . . .  i i 500 tO00 1500 E? , (keV)  Fig. 4. Delayed coincidence spectra for three reaction products having known high-spin isomers. Transitions are labelled by their parent level I"  values. Isomeric decay schemes for 9~Mo and ~Mo are displayed inset, while the two 92Mo isomeric levels have al- ready been shown in fig. 2. A part icular ly striking example,  the mutual  exci- ta t ion of  the product  nuclei from a-stripping,  is illus- t ra ted in fig. 3. The 96Ru states up to 12 + could be easily identif ied,  and the ~6Fe excitat ion extends to its 6 ÷ state. In the coincidence spectrum gated on the 56Fe 847 keV 2 + --,0 ÷ transi t ion,  in addi t ion  to 56Fe and 96Ru y-rays, several weak y-rays from the deex- ci ta t ion o f  the 8 ns ~ +  isomer in 95Ru were observed. These arise through one neutron emission from highly excited 96Ru nuclei produced in a-transfer.  Similarly, the spectrum gated on the 833 keV 96Ru 2+--,0 + t ransi t ion shows coincident  55Fe y-rays. Other trans- fer processes also gave indicat ions of  the deposi t ion o f  excitat ion energy sufficient to allow subsequent neutron emission. For  example,  the spectrum coin- c ident  with the 1476 keV transi t ion from the In- s tr ipping product  93Mo showed 5SNi y-rays as well as much stronger 93M0 and 59Ni "/-rays. With  higher sta- tistics, such processes could be s tudied in more  sat- isfactory detail .  We show in fig. 4 examples  o f  de- layed coincidence spectra for three nuclei in which isomers are populated rather  strongly. They demon-  strate part icularly clearly that  these types o f  excita- t ion processes should be useful in spectroscopic stud- ies, especially for investigations o f  non-collective nuclear excitations.  In summary,  by 77 coincidence measurements  we have identif ied inelastic and transfer  react ion prod-  ucts from collisions 0f255 MeV 6°Ni with 92M0. Mu- tual excitat ion of  both reaction products  was typi-  cally observed, providing clearcut identification of  the reaction involved. Populat ion o f  high-spin (up to 1 = 12) non-collective states was seen, along with high excitat ion energy transfer  which somet imes  led to subsequent neutron emission.  The results i l lustrate the potent ial  of  T~' coincidence techniques for exam- ining at high resolution some aspects o f  ine las t ic /  transfer processes, such as correlated energy and an- gular momen tum transfers into both product  nuclei. Specific applications may include the spectroscopy o f  certain neutron-excessive nuclei that  cannot  be reached by fus ion-evapora t ion  reactions. The pros- pect of  much larger "/-ray detector  arrays in the near  future enhances the at t ract iveness o f  these lines of  investigation. For  fruitful discussions, we thank K.E. Rehm, W. Henning, J. Wilczynski  and K. Siwek-Wilczynska.  References [I] R. Broda et al., Proc. Intern. Conf. on Nuclear physics (Munich, 1973) p. 470. [2] A.O. Machiavelli et al., Nucl. Phys. A 432 (1985) 436. [3] M.W. Guidry et al., Phys. Lett. B 163 (1985) 79; C.Y. Wu el al., Phys. Lett. B 188 ( 1987 ) 25; S. Juutinen et al., Phys. Lett. B 192 (1987) 307. [ 4 ] F.W.N. de Boer et al., Z. Phys. A 325 (1986) 457. [ 5 ] P.J.A. Buttle and L.J.B. Goldfarb, Nucl. Phys. A 176 ( 1971 ) 299. [6] Y.H. Chung et al., Phys. Rev. C 29 (1984) 2153. [7] J. Wilson et al., Z. Phys. A 296 (1980) 185. 249 

Inelastic and transfer reactions in 92Mo+255 MeV 60Ni collisions studied by γγ coincidences

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Inelastic and transfer reactions in 92Mo+255 MeV 60Ni collisions studied by γγ coincidences

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