Beck, F.

Byrski, T.

Gehringer, C.

Merdinger, J.C.

Schutz, Y.

Vivien, J.

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GAMMA-RAY TRANSITION ENERGYCORRELATIONS IN 156Er AND 160YbJ. Vivien, Y. Schutz, F. Beck, T. Byrski, C. Gehringer, J.C. MerdingerTo cite this version:J. Vivien, Y. Schutz, F. Beck, T. Byrski, C. Gehringer, et al.. GAMMA-RAY TRANSITIONENERGY CORRELATIONS IN 156Er AND 160Yb. International Conference on NuclearBehaviour at High Angular Momentum, 1980, Strasbourg, France. 41 (C10), pp.C10-135-C10-138, 1980, <10.1051/jphyscol:19801013>. <jpa-00220632>HAL Id: jpa-00220632https://hal.archives-ouvertes.fr/jpa-00220632Submitted on 1 Jan 1980HAL is a multi-disciplinary open accessarchive for the deposit and dissemination of sci-entific research documents, whether they are pub-lished or not. The documents may come fromteaching and research institutions in France orabroad, or from public or private research centers.L’archive ouverte pluridisciplinaire HAL, estdestine´e au de´poˆt et a` la diffusion de documentsscientifiques de niveau recherche, publie´s ou non,e´manant des e´tablissements d’enseignement et derecherche franc¸ais ou e´trangers, des laboratoirespublics ou prive´s.JOURNAL DE PHYSIQUE CoZloque CIO,  suppZgment au n012, Tome 41, de'cembre 1980, page C10-135 GAMMA-RAY TRANSITION ENERGY CORRELATIONS IN l S 6 ~ r  AND 160yb J.P. Vivien, Y .  Schutz, F . A .  Beck, T .  Byrski, C .  Gehringer and J .C.  Merdinger. Centre de Recherches NucZe'aires 67037 Strasbourg Cedez, France. Extensive s tud ies ly2  of the rotational nucleus 160yb have shown the presence of two backbendings in the yras t  l ine ,  as well as the existence of irregulari-  t i e s  in the side bands. The interpretation of the level scheme i n  terms of bandcrossing2 has been very 3 successful. The nucleus 1 5 6 ~ r ,  whose yras t  band i s  very similar to  that  of 160yb up to  spin 24-5, has 4 been shown to  exhibit a sharp second backbending. This second i r regular i ty  has been interpreted as being most l ike ly  due to  a neutrons ef fec t .  In order to complement the studies of the discrete l ires,  we looked a t  the y-continuum via the Y - Y  energy cor- 5 relat ion technique described by B. Herskind . The two nuclei 160yb and 1 5 6 ~ r  were respectively produced i n  the heavy ion fusion reactions 14'sm + 160 a t  Einc = 105 MeV and l4 'pr + "F a t  Einc = 100 MeV. In both reactions the 4n channel i s  prominent w i t h  84 % of the total  cross section fo r  the production of 160yb and - 70 % f O r 1 5 6 ~ r  ; the percentage fo r  the 3 and 5n ex i t  channels are respec- t ive ly  = 16 % fo r  l6'yb and 15 % f o r  155y157~r.  The maximum angular momentum brought into the compound nucleus calculated according the formula deduced by 6 Baas f o r  a liquid drop i s  52% and 51 f i  respectively. To record the y - y  coincidence matrix, a se t  of three Ge(Li) detectors was used. The detectors were care- fu l ly  chosen with nearly the same response function and thei r  gain properly matched t o  be able to  sum a l l  the three possible +-y combinations. In the resulting coincidence spectra, on1 y a small percen- tage of the to ta l  number of events i s  due to  photo- peak X photopeak coincidences, the r e s t  being due to  Compton scattered events. In order to  remove the Compton background from the spectra two methods can be applied. One consists  of unfolding the two dimen- tional Ge(Li) X Ge(Li) spectra, a f t e r  a scrupulous study of the response function of each detector in s i tu .  This procedure i s  now operating but i s  very time consuming for  the computer. A second method 5 calculates the deviation aNi from the average coincident ra te .    his method does not yield the absolute counts value, but enables the structure of they -y coincidence f i e ld  to  be reproduced quite easily.  I t  has been applied in the present study. In f ig .  1 and 2 ,  t o  enhance the gross structure observed in the two dimentional correlation spectra, the energy resolution of the detectors has been a r t i f i c i a l l y  deteriorated from 2 keV to  20 keV. This procedure gives more evident spectra, but one loses information f o r  the f ine  structure. For tha t  reason, the resul ts  obtained for  d i f ferent  s l i ces  perpendicular t o  the 45O diagonal are  taken from the real correlation spectra. The valley along the diagonal observed i n  these spec- t r a  i s  the signature of the rotational behaviour of the nuclei. The width W of the valley i s  d i rec t ly  related to  the "collective" moment of iner t ia  3c, tha t  i s  the moment of iner t ia  which corresponds to J 2 a rotational band, by the relat ion W = 16 -h /2?c. F.S. stephens e t  a17 have shown that  fo r  intermedia- t e  nuclear deformation (0-0.21 and for  one or  a few par t ic les  i n  a high - j shel l ,  a coupling scheme where j i s  quantized along the rotation axis i s  valid (Rotational A1 ignment) . The following energy spec- 10 Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19801013JOURNAL DE PHYSIQUE E ll 5,- 5, Fdwte 1 : P o s X v e  com&aAion npecttLurd A f l ~ j  dot " " ~ b  and coWaponding phoject.ionn penpendicdan. .to .the diagond. Fotr each nfice the  mean enengy v d u e  ( E y l  + Ey21 /2 -in indicated. %I  and EUZ, hs and?iW6' ahe Rhe backbending dhequenciu i n  the  yhdcl.t band and side ban& h e , 5 p ~ c f i ~ & L j .  E 1, Figme 2 : dame an i n  F.igwie l Soh 7 5 6 ~ h .  ?i 2 2 trum i s  then obtained E = e j a  + - ( I - j a )  where ja represents the a l igned  angular momentum along t i le  r o t a t i o n  a x i s  and e j a  i s  the  s i n g l e  p a r t i c l e  energy. The t r a n s i t i o n  energy w i t h i n  a band i s  : - " 4(1- ja )  = 2 h w  (1). From the  cor re la -  E~ -q t'ion spectra one can deduce an average value o f  3c, corresponding t o  several bands w i t h  d i f f e r e n t  ja. Figures l and 2 d i s p l a y  t h e  bidimensional co r re la -  t e d  spectra obta ined f o r  160yb and 156~r, together  w i t h  d i f f e r e n t  s l i c e s  perpendicular t o  the  45' d i a g e  na l .  The v a l l e y  can be fo l lowed up t o  870 keV f o r  160yb and t o  920 keV f o r  156~r. For low energy t r a n s i t i o n s ,  up t o  - 500 keV, the observed narrowing o f  t h e  v a l l e y  f o r  160yb w i t h  increasing energy r e f l e c t s  the  behaviour o f  the ground band moment o f  i n e r t i a  below the  f i r s t  backbending. Beyond the  f i r s t  backbending up t o  750 keV, the  width o f  the  v a l l e y  remairsalmost constant.  The r i d g e  which i s  narrow below the f i r s t  backbending becomes wider, i nd ica t ing  a spread i n  the moment o f  i n e r t i a .  One can then r e -  present the  y r a s t  l i k e  decay by several c o l l e c t i v e  bands w i t h  d i f f e r e n t  moments o f  i n e r t i a  correspon- d ing t o  var ious degrees o f  alignment o f  t h e  s i n g l e  p a r t i c l e s .  The d i s c o n t i n u i t i e s  observed i n  the d i f -  f e r e n t  bands o f  the  d i s c r e t e  spectra, shows up i n  the y-y c o r r e l a t i o n  spectra as a f i l l i n g  o f  the  va l ley .  I n  both nuc le i ,  the f i r s t  backbending i s  very sharp and there fo re  does n o t  cause any f i l l i n g  a t  the proper frequency. Instead one observes a f i l l  in r l  o f  the  v a l l e y  i n  160yb a t  a p o i n t  corresponding t o  the  t r a n s i t i o n s  : 589 keV (8' + 6') and a 586 keV (12' + 10') and i n  156~r  t o  the  t r a n s i t i o n s  : At  % W  = 0.40 MeV i n  160yb and 0.38 MeV i n  5 6 ~ r  thev a l l e y  i s  t o t a l l y  f i l l e d  up. A t  h igher  energy the  v a l l e y  appears again and vanishes around 870 keV and 020 keV respec t i ve ly .  The f a c t  t h a t  these s t ruc tu res  i n  the  v a l l e y  are very pronounced i s  understood i n  terms o f  mult i -band crossing, i .e .  t h e  f a c t  t h a t  many bands are expected t o  experience crossings a t  the same frequencies as the  i r r e g u l a r i t i e s  observed i n  t h e  d i s c r e t e  spectra. I n  order  t o  reproduce the  e f f e c t  o f  a mult i -band crossing, we have constructed a computer simulated y y  spectrum assuming t h a t  the d i f f e r e n t  bands experience a cross ing around t h e  same frequency hw = 0.39 MeV. I n  t h i s  s imu la t ion  we have used r e a l i s t i c  detector  response func t ions  t o  cons t ruc t  the  y-y two dimensional coincidence spectrum. Then we have app l ied  the  data reduc t ion  technique discussed before t o  e x t r a c t  the corres-  ponding cor re la ted  spectrum. F igure  3 d isp lays  the  c o r r e l a t i o n  spectrum r e s u l t i n g  from such a simulation f o r  160yb. 1000 682 keV (1 2' -+ 10') and 710 keV (20' -P 18') (see E,,(KeV) p o i n t  noted a i n  the f i gu res ) .  I n  160yb, two st rong F i g m e  3 : ShimLLeated pob.itive cohheRdtion bridges appear %U,') and correspond t o  the  b p ~ o t * w t ~  ANij don I6'yb. f requencies o f  the  d i s c o n t i n u i t i e s  observed i n  the s ide bands. The br idge marked b i n  156~r corresponds The d i s c r e t e  l i n e s  have been inc luded w i t h  t h e i r  t o  t h e  coincidence between two t r a n s i t i o n s  643 keV arrj proper i n t e n s i t y  as deduced from t h e  experimental 639 keV observed i n  one o f  t h e  s ide  bands. data. Some o f  t h e  bands used i n  t h e  c a l c u l a t i o n  a re  c10-138 JOURNAL DE PHYSIQUE shown i n  a backbending p l o t  j n  the upper l e f t  p a r t  2. L.L. Riedinger, 0. Andersen, S. Frauendorf, o f  f i g u r e  3. J.D. Garret,  J.J.Gaardh&je, G.B. Hagemann, The observed end p o i n t  o f  t t ie  v a l l e y  (Fig. l ,2) i s  M. Guttormsen, B. Herskind, Y.V.Makovetzky, P.O. r e l a t e d  t o  the energy o f  t r a n s i t i o n s  de-exc i t i ng  Tjom and J.C. Waddington, Phys. Rev. L e t t .  3 t h e  h ighest  sp in  c o l l e c t i v e  s ta tes  fed  i n  the (1980) 568. react ion.  The corresponding moments o f  i n e r t i a  3. A.W. Sunyar, E. de Mateossian, O.C. K is tner ,  deduced from the  width'  are 136 * 18 ( M ~ v ) - '  fo r  A. Johnson, A.H. Lumpkin and P. Thieberger, 160yb and 111 22 (M~v)- ,?  fo r  156~r. The maximum Phys. L e t t .  625 (1976) 283 angular momentum a t  the  Cop o f  the y r a s t  cascade i s  4. F.A. Beck, T. Byrsk i ,  C. Gehringer, J.C.Merdinger, evaluated t o  be I = 44 TI fo r  both nuc le i .  We assumed Y .Schutz and J.P.Vivien, Con t r ibu t ion  t o  t h i s  t h a t  each neutron takes away 1 . 5 5  and t h a t  4 s ta-  Conference. t i s t i c  y-rays do-exc i te  the n u c l e i  towards the yrast  reg ion each t a k i n g  away 0.5 3 8. Using r e l a t i o n  l, 5. B. Herskind, Con t r ibu t ion  t o  t h i s  conference. the  corresponding s i n g l e  p a r t i c l e  a l igned angular 6. R. Baas, Phys. Rev. Let t . ,  39 (1977) 265. momentum i s  found t o  be = 15 -Fi f o r  y t te rb ium and 7. F.S.Stephens, R.M. Diamond and S.G.Nilsson, = 17 R f o r  erbium. These values are i n  good agree- Phys. L e t t .  44 B (1973) 429. ment w i t h  the a l igned  angular momentum deduced from 8. J.P. V iv ien,  Y. Schutz, F.A. Beck, T. Byrski,  the d i s c r e t e  spectra beyond the second backbending : C .  Gehringer and J.C. Merdinger, Phys. L e t t .  i t  has been evaluated t o  be 17 6 i n  156~r  and a t  85B (1979) 325. l e a s t  1 5 6  i n  160yb. References l .  F.A. Beck, E. Bozek, T. Byrsk i ,  C.  Gehringer, J .C. Merdinger, Y . Schutz, J . Styczen and J.P. Vivien, Phys. Rev. L e t t .  42 (1979) 493 and Proc. Symp. on High Spin Phenomena i n  Nucle i ,  Argonne (Marcbl079) 

GAMMA-RAY TRANSITION ENERGY CORRELATIONS IN 156Er AND 160Yb

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