Evaporation residue cross sections and neutron multiplicity distributions have been measured for the 12C + 144Sm and 64Ni + 92Zr reactions leading to the same compound nucleous 156Er. Statistical model calculations can account for the data in the 12C-induced reaction. In contrast, the inhibition of neutron emission with respect to statistical model predictions seen in 64Ni + 92Zr cannot be explained even with the inclusion of the broad angular momentum distributions required to describe the fusion cross section data

Henning, W.

Holzmann, R.

Janssens, R.V.F.

Khoo, T.L.

Kühn, W.

Lesko, K.T.

Radford, D.C.

Ronningen, R.M.

Stephans, G.S.F.

Van Den Berg, A.M.

Carolina Digital Repository

Volume 181, number 1,2 PHYSICS LETTERS B 27 November 1986 E V A P O R A T I O N  R E S I D U E  C R O S S  S E C T I O N S  A N D  A V E R A G E  N E U T R O N  M U L T I P L I C I T I E S  IN THE 64Ni + 92Zr AND 12C + t44Sm REACTIONS LEADING TO t~6Er R.V.F.  J A N S S E N S ,  R. H O L Z M A N N ,  W. H E N N I N G  1, T.L. K H O O ,  K.T. L E S K O  2 G.S.F.  S T E P H A N S  3, D.C. R A D F O R D  4, A.M.  V A N  D E N  B E R G  s Argonne National Laboratory, Argonne, IL 60439, USA 6 W. K O H N  11. Physikalisches Instit~t, Universiti~t Giessen, Giessen, Fed. Rep. Germany and R.M. R O N N I N G E N  NSCL, Michigan State University, East Lansing, MI  48824, USA Received 15 July 1986 Evaporation residue cross sections and neutron multiplicity distributions have been measured for the 12C + 144Sm and 64Ni + 92Zr reactions leading to the same compound nucteous lS6Er. Statistical model calculations can account for the data in the X2C-induced reaction. In contrast, the inhibition of neutron emission with respect to statistical model predic- tions seen in 64Ni + 92Zr cannot be explained even with the inclusion of the broad angular momentum distributions re- quired to describe the fusion cross section data. Significant deviations between experiment and statistical model  predictions have been observed for the number of  nucleons emitted following certain heavy-ion induced fusion reactions [ 1 - 3 ] .  At  present, there is no unambiguous explanation for the apparent inhibition of  particle emission and more experimental  data are needed. In this letter the formation and sub- sequent decay of  the compound nucleous 156Er, where such effects have been observed [1], is studied in the 12C + 144Sm and 64Ni + 92Zr reactions. Fusion 1 Present address: GSI, D-6100 Darmstadt 11, Fed. Rep. Germany. 2 Present address: LBL, Berkeley, CA 94720, USA. 3 Present address: MIT, Cambridge, MA 02139, USA. 4 Present address: AECL, Chalk River, Ontario, Canada. s Present address: Fysisch Laboratorium, Rijksuniversiteit Utrecht, Utrecht, The Netherlands. 6 Work supported by the US Department of Energy, Nuclear Physics Division, under Contract W-31-109-ENG-38. 16 cross sections were measured through direct detect ion of  evaporation residues (ER), and multiplici ty distri- butions for neutrons (n) emitted from the compound nucleus were obtained from a separate experiment using gamma-ray techniques. We find that the inhibition of  n-emission reported previously for the 233 MeV 64Ni + 92Zr reaction [1] persists over a wide range o f  excitat ion energies. In contrast, good agreement with statistical model  calcu- lations is found for the 12C + 144Sm case. It has been suggested [4,5] that  the explanation may lie in the presence of  high/-tails in the angular momentum dis- tr ibution for the Ni-induced reaction since the com- pound nucleus excitat ion energy above the yrast line would then be reduced, thereby resulting in a lower n-multiplicity.  This high/-tai l  originates from zero- point fluctuations or coupled-channels effects which are thought to be responsible for the observed enhance- ment of  fusion cross sections below the barrier [6]. 0370-2693/86/$ 03.50 © Elsevier Science Publishers  B.V. (Nor th -Hol land  Physics Publishing Div i s ion)  Volume 181, number 1,2 PHYSICS LETTERS B 27 November 1986 The present cross section data extend to energies below the barrier and differences in behavior for the two reactions in the barrier region can be studied. Further- more, the data can be used in conjunction with data from the Darmstadt-Heidelberg crystal ball [7] where the/-distributions for the two reactions were deduced from the measured 3,-ray multiplicity distributions. The two sets of data are complementary and can be used to characterize the/-distributions in greater detail. Using the method outlined in ref. [8] the fusion cross sections were measured for 200-300 MeV 64Ni and 50-90  MeV 12C beams incident on 180/ag/cm 2 92Zr and 144Sm targets, respectively. An electrostat- ic deflector separated the relatively low-energy and highly-charged evaporation residues from the beam. These residues were then identified by their differ- ential energy loss in A E - E  detector : assembly con- sisting of a common AE gas ionization chamber and four silicon detectors covering the angular range from 0 ° to 4 °. For the extremely low-velocity evapora- tion residues from the 12C + 144Sm reaction (Erecoil < 6 MeV) unambiguous identification through time- of-flight was achieved with a slow beam-sweeper (410 ns on/2.5/as off). The data presented below were corrected for the efficiency of the deflector and the detector geometry following Monte Carlo ray-tracing calculations (see ref. [8] for details). The uncertainties associated with this procedure are included in the 15% systematic error of the cross sections. Neutron-multiplicities were deduced from yields of individual xn-channels, obtained from separate 3'- ray measurements in which 55-75 MeV 12C and 225-295 MeV 64Ni beams bombarded 1 mg/cm 2 targets. Effective beam energies were obtained follow- ing the procedure described in ref. [1]. The relevant spectra were measured with a Ge detector positioned at 0 ° with respect to the beam direction. Singles spectras as well as spectra in coincidence with one or more elements of an array of eight NaI detectors surrounding the target were used to identify the dif- ferent xn-evaporation channels. The relevant level schemes, 3'-ray anisotropies and detector efficiencies were taken into account, and the associated uncer- tainties were included in the systematic error. Minor corrections for the isotopic composition of the targets (isotopic enrichment ~97% in all cases) were also ap- plied. Fig. 1 a presents the measured ER cross sections IOOC IOC b 40 ' ' ' i ' ' ' I ' ' ' I ' ' ' i ' ' ' ¥ i ,i i 12 c + 144Sm i i i I i , . J I . , , I . . , I , , , I , , , 50 60 70 80 90 I ' ' ' I ' ' ' q ' ' ' i ' IOOC (b) / ~ " E a - - ~  -~ ..... I00 I ~ 1 / I  _64_Ni - i I "s b I0 + 92Zr L ! ÷ ,  . , I , , ,  i , , ,  I , 120 140 160 180 E (MeV) c.m. Fig. 1. Evaporation residue cross sections for the 12C + 144Sm (a) and the 64Ni + 92Zr (b) reactions as a function of the beam energy. An overall 15% systematic uncertainty in the cross section is not included in the figure. The curves refer to calculations discussed in the text; i.e. the dashed curve is a calculation with the model by Swiatecki [9], the solid line includes the effect of barrier fluctuations [10] and the dotted line represents a(ER) calculated from the fusion cross sec- tions (solid line). The energies given in the figure have been corrected for target thickness. The data in (a) have been ob- tained by scaling the highest-energy data points to the calcu- lation (see text). This procedure does not apply to (b) where absolute cross sections are presented. for the 12C + 144Sm reaction. Statistical model cal- culations discussed below indicate that the fission probability is essentially zero at the beam energies dis- cussed here and, hence, the measurements represent the total fusion yield. In this case the residues are characterized by small recoil velocities and by a rather broad angular distribution. Both characteristics severe- ly affect the calculation of the absolute transmission of the deflector system. While this does not affect the relative values of the cross sections, precise absolute values are difficult to deduce. For this reason, the 17 Volume 181, number 1,2 PHYSICS LETTERS B 27 November 1986 data presented in fig. la have been obtained by scaling the highest-energy data points to a curve obtained from a calculation based on the dynamical fusion model by Swiatecki et al. [9]. Calculations performed with other models such as those proposed by Bass [11] or Kailas and Gupta [12] yield equivalent results. As can be seen from fig. la the trend of  the data is very well reproduced over the entire energy range under study. Fig. lb presents ER cross sections for the 64Ni + 92Zr reaction. Here the velocities of  the recoiling ER are large and absolute cross sections are derived from the measurements directly. The dashed line re- presents a calculation similar to the one given in fig. la. Clearly, this calculation does not agree with the data: at energies below the barrier (Ecru = 132.7 MeV) the cross section is much larger than predicted, and above the barrier the measurements fall below the computed values. Thus subbarrier enhancement of  the fusion cross section is observed for this reaction, a phenom- enon found in many reactions with heavy ions ,1. The deviations at higher energies can be attributed to the onset of  fission, a process which escapes detection in the present measurement. The solid line in fig. lb represents a fusion cross section calculation based on the dynamical fusion model by Swiatecki et al. [9] but modified to include the fluctuations in barrier height based on the model of  Esbensen [ 10]. Within this model the parameter o n = 0.61 fm needed to fit the data was larger than expected from the known B (E3) and B(E2) values of  the lowest strongly excited inelastic states (o x = 0.42 fm) but probably not unreasonable if one accounts for the fact that other inelastic (and transfer) channels should be included. The parameters within the dynam- ical fusion model (extrapush) were chosen from the recent systematic study of fusion and fusion-fission in the 58,64Ni +ASn systems [14]. In order to obtain the ER and fission cross sections, the calculated fusion cross sections (solid line fig. lb)  have been introduced into statistical calculations with the code CASCADE [15]. This approach is similar to the one presented by Lesko et al. [14]. As in ref. [14],  use was made of  the Sierk fission barriers [16], where the surface of  the rotating liquid drop modelis smeared out by an exponential folded into a Yukawa shape. * For a recent revew see for example ref. [13]. The partial wave distribution of  the compound nu- cleus reflected the coupling of  the quasi-elastic chan- nels discussed above through the value used for the diffuseness parameter d of the entrance channel trans- mission coefficients of  CASCADE ( T  l = 1 / (  1 + exp [(l - lo)[d ] ) ,  o l = 7rx2(2l + 1) Tl).  This code was also modified to include enhancement of  collective E2 transitions parallel to the yrast line and E 1 deexci- tation through the giant dipole resonance [1]. The experimentally established yrast lines are used as well. The level density parameter used was a = A/12,  which gives agreement with the neutron spectra measured for the decay of  this particular compound nucleus [1 ]. The sensitivity of  the cross section calculations to the value of this parameter was found to be small (calculations with the more commonly used a = A / 8  value for example render essentially the same result). The dotted line of  fig. lb  illustrates the agreement be- tween the measured and calculated ER cross sections. The calculations were performed with d = 4~, a value derived from the measured angular momentum distri- bution [7] for this reaction at Ecm = 139 MeV. Cal- culations with d = 7.5~, the value deduced in ref. [14] for 58,64Ni +ASn reactions yields similar results. The same statistical model calculations discussed above were used to obtain the average n-multiplicities at different beam energies and the excitation functions for the various xn channels. Fig. 2 presents a com- parison between measured and calculated n-multipli- cities as a function of  the compound nucleus excita- tion energy for the two reactions under discussion. For the 12C + 144Sm reaction the fusion cross sections were used as input; i.e. the/-distribution of  the form given above was used with d = 27~ and l 0 was adjusted within CASCADE to reproduce the fusion cross sec- tions (solid line in fig. la). Typical values o f l  0 are given in table 1. The agreement between theory and experiment of  the n-multiplicities is very good (fig. 2a). Furthermore, the excitation functions for the differ- ent xn-channels (not shown) are also well reproduced. Predictions for reaction channels involving the emis- sion of  charged particles could not be tested in detail since the required spectroscopic information on the final nuclei is in many cases unavailable. In those few cases where it is possible good agreement was found. Thus, it is concluded that statistical model calculations are able to describe the decay of  the 156Er nucleus formed in the 12C-induced reaction. 18 Volume 181, number 1,2 PHYSICS LETTERS B 27 November 1986 >.- I- (..) _.1 Z o n-" I-- LI.J Z I • I I I I i2 :c  + i 4 4 S m  ( 0 )  j 3! 2 I , I , I , I , I 4 - (b )  3 64Ni + 9 2 Z r  I , I , I , I , I 30 40 50 60 70 E'X'( 156E r) [MeV] Fig. 2. Average neutron multiplicities for the 1 2 C  + 144Sm and 64Ni + 92Zr reactions as a function of compound nucleus excitation enezgy. Only the 1 -  4 n-channel yields were used to compute the theoretical and experimental n-multiplicities since the level schemes for channels corresponding to higher n-evaporation are not known. The solid lines represent the results of statistical model calculations. In contrast to the agreement noted for 12C + 144Sm, a strong discrepancy is seen for the 64Ni-in- duced reaction (fig. 2). The calculations were performed with d = 4fi and with the fitted fusion cross sections (solid line in fig. lb). The measured n-multiplicities are lower than the predicted values by 0.35 neutrons on the average. Furthermore, the general shape of the measured excitation functions for the xn-channels (not shown) are not well reproduced. Thus, the inhi- bition of neutron emission presented in ref. [ 1 ] for a single beam energy is shown to persist over a wide Table 1 Typical l o values derived from the calculated fusion cross sec- tions with the expressions given in the text. E *  (MeV) 64Ni + 9 2 Z r  1 2 C  + 144Sm Ecm (MeV) I 0 (~/) Ecm (MeV) l 0 (~) 50 141.8 51.5 68.1 29.5 47 138.8 46.0 65.2 27.0 45 136.8 41.0 63.1 25.5 40 132.1 29.0 58.2 21.0 range of excitation energies and angular momenta. This discrepancy is of the same order as the one re- ported in refs. [2,3] and the effect is seen here for a compound nucleus with neutron number in excess of 82, in line with the dependence suggested by Stwertka et al. [2]. It should be emphasized that the calculations pre- sented in fig. 2b for the 64Ni + 92Zr reaction incor- porate the broad angular momentum distributions (d = 4/0 discussed above. As a result the discrepancies between the measurements and the calculations at the lower excitation energies are somewhat reduced with respect to the results of ref. [1]. As in refs. [1,2] yariations within acceptable limits of other parameters in the statistical calculations, such as the E1 and E2 strength or the level density parameter, do not yield satisfactory agreement with the data. For example, variations of the level density parameter from A / 8  to A l l 4  or changes in the diffuseness d from 4~ to 7.5// in CASCADE calculations (corresponding to an ex- citation energy of 49 MeV) would change the n-multi- plicity by less than 5%. Moreover, the E 1, E2 strengths are constrained by the giant dipole resonance energy- weighted sum rule and by experimentally measured ?- ray yields. These findings rule out the suggestion [4,5] that the inhibition of n-emission is due to large dis- tortions of the angular momentum distribution. It should be noted that the presence of very high partial waves in the fusion cross section would not automat- ically imply reduced n-multiplicities since CASCADE Calculations indicate that they contribute mainly to fission and not to the ER's (the fission cross section increases rapidly over the ER cross section for l > 50~). Finally, the ER cross sections [o(ER)] reported here and the spin distributions derived from ?-multi- plicity measurements [7] provide two data sets which are not only complementary but also allow a cross- check between the sets. The data can be combined to give the absolute ER cross section for each partial wave (Ol) through the constraint: sum(o/) = o(ER). Thus o t can be immediately obtained from the spin distribution results (ref. [7]). In principle, a measure- ment of o(ER) by itself does not yield a 1. However, by adopting the form given above for T l with the dif- fuseness given by the spin distribution measurement, o l can be computed. For a compound nucleus excita- tion energy of 47 MeV data are available to compare 19 Volume 181, number 1,2 PHYSICS LETTERS B 27 November 1986 o I derived by  the two methods for the two reactions under discussion. For  the 12C case the agreement is excellent [17], with l 0 = 27 -+ 3h obtained with both methods.  For the 64Ni case there is agreement within the errors: l 0 values of  38 + 4 and 46 + 4h are derived from the spin distribution and a (ER)  measurements, respectively [17]. In summary, we have observed that inhibition of  neutron emission persists over a wide range of  excita- tion energies and angular momenta  for the rather mass symmetric 64Ni + 92Zr fusion reaction, while it  is ab- sent in the more asymmetric 12C + 144Sm reaction. For  the latter reaction a consistent picture of  fusion with subsequent statistical decay of the compound nucleus accounts for the measured data (cross sections and n-multiplicities). For  the former reaction, the presence of  coupled-channel effects and/or zero-point fluctuations manifests itself through the enhancement of  the fusion cross section below the barrier and the presence of large fission cross sections at higher ener- gies. Both features can be accounted for in model  cal- culations which include tails towards high values in the angular momentum distribution. However, the inclu- sion of  these high/-tails is unable to account for the suppression of  neutron emission. All the data now available on the decay of  156Er rule out  all the ob- vious explanations for this suppression, i.e. high/-tai ls ,  anomalously large neutron energies, very large gamma decay widths and uncertainties in the location of  the yrast line. It is concluded that the original suggestion of  trapping in a superdeformed minimum made in ref. [1 ] should be pursued further. Experiments along these lines are in progress. The authors wish to thank B.B. Back, H. Esbensen, S. Landowne and S. Pieper for their computer  codes and for fruitful discussions. References  [1] W. Kllhn et al., Phys. Rev. Lett. 51 (1983) 1858. [2] P.M. Stwertka et al., Phys. Lett. B 150 (1985) 91. [3] C. Cabot et al., Z. Phys. A 322 (1985) 393. [4] B. Haas et al., Phys. Rev. Lett. 54 (1985) 398. [5] P.J. Nolan et al., Phys. Rev. Lett. 54 (1985) 2211. [6] S. Landowne and C.H. Dasso, Phys. Lett. B 138 (1984) 32. [7] A. Ruckelshausen et al., Phys. Rev. Lett. 56 (1986) 2356. [8] W.S. Freeman et al., Phys. Rev. Lett. 50 (1983) 1563; Phys. Rev. C 28 (1983) 919. [9] W.J. Swiatecki, Phys. Scr. 24 (1981) 113; Nucl. Phys. A 376 (1982) 275; S. Bj~rnh~blm and W.J. Swiatecki, Nucl. Phys. A 319 (1982) 471; B.B. Back et al., Phys. Rev. C32 (1985) 195. [10] H. Esbensen, Nucl. Phys. A 352 (1981) 147; H. Esbensen et al., Nucl. Phys. A 411 (1983) 275. [11] R. Bass, Nucl. Phys. A 231 (1974) 45. [12] S. Kailas and S.K. Gupta, Z. Phys. A 302 (1982) 355. [13] M. Beckerman, Proc. Symp. on the Many facets of heavy-ion fusion reactions (Argonne, 1986), to be published. [ 14 ] K.T. Lesko et al., Phys. Rev. Lett. 55 (1985) 803; Phys. Rev. C, to be published. [15] F. Piihlhofer, Nucl. Phys. A 280 (1977) 267. [16] A.J. Krappe et al., Phys. Rev. C 20 (1979) 992; A. Sierk, private communication. [17] T.L. Khoo et al., to be published. 20 

Evaporation residue cross sections and average neutron multiplicities in the 64Ni+92Zr and 12C+144Sm reactions leading to 156Er

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Evaporation residue cross sections and average neutron multiplicities in the 64Ni+92Zr and 12C+144Sm reactions leading to 156Er

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