Sub-coulomb barrier fusion of O+Sn

Volume .'.'.75, number 3

PHYSICS LETTERS B

7 August !986

S U B - C O U L O M B BARRIER F U S I O N OF O + Sn

P. JACOBS, Z. F R A E N K E L , Go M A M A N E and L T S E R R U Y A Departmem of NucIear PSysics, Weizmap.n lnst#u~e of Science, Rehovol 76100, A'rael Received 29 April 1986

Evaporation residue excitation functions have been mea,~rared for ~6,1~,taO + A S h (X = i12, i16, i17, 1!8, t19, !20, 122, !24) from 30 MeV above to 6 MeV below t~e Coulomb barrier. Re&.~eed excitatio~ functioa~, scaled to remove the effects of smoothly varyim~gbarrier pacameters, do ~ot show any strong dependence o.~ the target or projectile isotope. The rela~ve changes a e less tha~ a factor two, and ace not correlated with positive Q-vaIues of neutron transl," c.harmeis.

SuboCoulemb barrier fusion of heavy ions has attracted much recent attention [ 1] because of the observed strong enhancement of the subbarrier fusion cross section compared to or.e-dimensional barrier penetration models. This enhancement has been obse_wed i'n a variety of systems (eat. terse [2-41) and generaliy appears as a strong dependence of tl~e cross section on the projectile or target isotope. The enha~.cements in the systems 160 +Sm [2], 40At +Sm [3] and 40At + Sn [3] baize been understood in a coupbd channels picture as being due to the modification of the interaction barrier resaiting from strong causing to low-lying rotations1 or vibrational excitations af the target nucleus [5]. Cress section enha~'~cemer.t in the subbarrier fusion of 58Ni + 64N~ [4], an the other hand, has been interpreted as a kinematic effect [6] due to the positive Qwatue of the ground state ta ground state 2n transfer channel for g°_at system. This picture has led to severaI additional studies [7-!'01. Strong correlation was obso,wed between subbarrier fusion cross section enhance.. merit and positive Qovalues for 2n transfer chamae::s in the system~ 32,36 S + Me, Ru, Rh and Pd [71 , but enhancement correlated with positive Q-values for in transfer chaqr.,els was not observed ira the systems 33S ~ was ob+ Ru, Pd [71. In t e l [8] ~trang enl~ncemen "° so,wed for only same of the ~ystems w i ~ positive Qvalue for neutron tran,Ker, and it was suggested that cro~s section enh~cement occurs only if flze transfer channel Q-values are both positive and apNoximately equal to the optimum Q-vaiue. In ref. [9], cross ~ction 0370-2693/86/$ 03.50 © Elsevier Science Pub!ishers B.V° (North-Holland Physics Publ.ishing Division)

enhancement for the systems 4°Ca + 4°,4G48Ca was found to be corre!ated with positNe Q~va]ues for 2o stripping (40Ca + 44Ca) or i p ai~d 2p stripping and 2n pickup (40Ca + 48Ca). Neutron trans%r cross sections were measured [10] for the nickel isotopes considered in ref. [4], and it was found that for the system 5SNi + 64Ni the 2n pickup cross seotion (Q "-~3.9 MeV) around the barrier is weak compared to fusion, whereas the ]n pickup cross section (~ ~ - 0 . 9 MeV) axe coeds the fiJsion crass sectbn. For the 5gNi + 58Ni system, all trans%r channels are weak compared to fusion. Possibly the enhancement ;for 5aNi +64Ni is due to strong coupling to the I n transfer channet rather than to the positive Q-value for the 2n transfer channei. At present, the dynamics of transfers.mediated subbarrier fusion are not ur.derstood. In this 1otter we present results of a complete study of subbarrier %sion cross sections of A O isotopes (A = 16, ~7, i8) and ASn isotopes (A = 112, 1 i6, ~,,7 ~ 18, 1 i9,120, t22 and I24). A more detailed report will be published elsewhere. The main aim was to investigate farther the influence of positive Q-values for transfer channels on the subbarr!er fusion cross section. These systems give a wide range of Q-values of neutron transfer, including strongly positive ones. Measurements of 2n stripping cross sections for 180 on even Sn isotopes show strongly enhanced ground state to ground state transitions [! 1]. corresponding to the high!y
Voiume 175, number. 3

PHYStCS LETTERS B

simiIar collectivity [12I. Because of these features, we expect that in a comparitive study of the systems the effect of inelastic excitations on the s,abbarrbr fusion cross section can be factored out, and, based on the argument of ref. [61, that the kinematics of the transfer channels wiil be evident. We have measured evaporated residues from ELA B 5 =--. 0 MeV (i.e. 6 MeV below to 30 MeV above the Coulomb barrier), corresponding to a range of cross section of 100~zb to i b. Pulsed beams of oxygep, {i6,17,.~ O ) w e r e obtained from the 14UD Pelletron accelerator at me Weizmann Institute of Science. The targets were made of 50 ~ag/cm 2 isotopically enriched ~va~o.~te,~ onto a 2 ggg/cm 2 carbon backing. Bean,, currents were below 0 . i pnA. Fission is not expected to occur for such light systems, so that the fusion cross section is identified witl~ the evaporation residue cross section. Evaporation residues were detected i~. a 10 X 8 cm 2 mu]iistep low-pressure muRiwire proportional counter (MWPC) [ ! 3], and the unambiguous identification of the evaporation residues was made by their time of Qight measured against the pulsed beam over a flight path of approximately 30 cm. The MWPC covers the angular range 8 = 2 0 - 8 ° with 2rr azimuthal efficiency; i.e, approximately 85% of the evaporation residue angular "q + : , *~ was direct]y measurable ,:~s,r:bu~,o~ in a singb run. T ~n ,° *,o,a.~ ~ evaporation residue yie!d was obtained by extrapolating the angular distribution for < 2 ° a,.,u . ~ ~ > 8 °, The errors associated with this ex-. trapo!ation are estL,~-ated to be tess than 3%. Cros's sections were d e t e r m b e d by normalization to the e]a~tic yield in two monitors placed symmetrically relative to be beam at -+ 15 °. The incident energy has been corrected for energy ioss in the target, and the cross sections have been corrected for isotopic impurities 0.n all cases other Sn isotopes). ~qe uncertainty in the absoo lute incident energy is estimated to be 100 keV. The detector and the experimental technique are d~sc~eea in more a~t~,~ ~ ~:~ in ref. [14]. To measure cross sections beiow i mb a second, identical MWPC was placed behind the first, and the tkme of ~_ ~i g~ht between the two counters was also measured. Fig. 1 shows fusion excitation. ~hnct,ions for I 8 0 + 112,1!8, i24sn as a function of laboratory energy, The errors shown are statistical. The Coulomb barrier for i80 + ii2sn iS indicated by the arrow, it is seen that the cross s~,m,,..s have a smooth dependence on energy, increase systematically with .*,a,ge.~t mass, and are 272

7 August !986

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+ I80+'d2S,q

~+ ~ d 10.~i ._hE

• 1001

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1

~ 18C,+!18Sn

~+

× !80+!24Sr ]

60

70

80

90

'M

Eta 8 i~ ,eV) Fig, I. Evaporation residue c~-osssection ~ER versus laborator~ bombarding energy ELA B for the systems 180 + ~-~2,~~8'124Sn Errors shown are stati~tica!. The arrow indicates the Couiomb barrier for i 80 + i ~2Sn extracted from the potential de scribed in the text.

nearly paralle] down to the lowest energy measured. The same behavieur is observed for all other combinations o f O and Sn isotopes. In order to show structural and kinematic effects when comparing the excitation functions it is necessary to scale the ordinate and abscissa in a manner that wiii remove effects due to the smooth!y-varying Coulomb barrier height and radius. Such reduced excitation functions are shown in figs. 2 and 3, using barrier parameters extracted from a poten. tial whose n u c b a r part is of the proximity potential form. The central radii have been calculated from the droNe~ model parameters of ref, [15], and the i'nteraction strength has been set at 60MeV/fm. As in ref. [31, this potential reproduces weil ~..e fusion cross section above the barrier but severely underestimates it below the barrier. Fig. 2 shows reduced excitation functions for all targets and for each oxygen projectile separately, tt can be seen that alt cross sections tie on one ~ine, with a spread of at most a factor two ir~ the vertical d!rec,. tion;i.e, that there ia no strong dependence on the target isotope. For the pr~ectile 160 there are no neu° tron trangfer channels with positive Q-value, but for i70 and 180 there is a large range o f positive ground state to ground state Q-values, vary;rag from 1.4 MeV ( ! n stripping) for 170 + t24an'~ to 5 °0 MeV ( l n strip,. ping) for 170 + 117Sn, and 1.4 MeV (2n stripping) for

Volume i75, number 3

PHYSICS LETTERS B

I02 C !0~

~g~

o

' ' i

....

I0~ ~

!60+As,n

J

f

7 August 1986

7>

$

°

100

A0+1175~

,~

Ca/

z

I0-1[-

.

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104

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-5

0

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10

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QM-VB(HeV) Fig. 2. Red,cod excitation fune~ons for a~ Sn targets with the ~ame 0 projectile Co~alombbarrier heights VB and radiiR B were extracted from. ~he potently! described in the text.

180 + 124Sn to 5,5 MeV (2n stripping) for 180 +1!2Sn. Fig. 2 ~hows n o cross section enhancement correlated with positive Q-values. I~ order to iDastrate the influence of the O isotopes, we present in fig° 3 reduced excitation functions for ali prqectfies with llTSn and t24Sn, tn addition to the positive neutron transfer Q-values listed above, I 7 0 + i17Sn has Q = i.3 MeV ( l n pickup), and 180 + ilTSn has O =3°3 MeV and 1.I.MeV for 2n and in stripping, respectively. The systems ! 6 0 + t17,i24sn have no positive Q-va!ues for n transfer channels. It can be seen that there is no enhancement o f the cross sections for 17,180 re!atNe to those for 160. it is interesting to note that this observation precludes observable eno hancement due to coupling to excitations m 180. This mecFanism has been invoked [ 16I to expIain differ° e'.,~ces in the excitation functions for fusion of ! 6 0 + !6'180 [ I 7 ] . We conclude that the subbarrier fusion cross sections for the systems studied here do not show any strong

~? I ,, -5

, , i ~ , , L I

....

0 5 EcH- VB(HeV)

I

I0

15

Fig. 3. Reduced exckaNon functions for £.'IO projectiles with the same Sn target. Cou!omb barrier heights g B and radii R B were extracted from the potential described Ln the text.

dependence on the target or the pr~ecti!e isotope. The relative variations are less than a factor two, and these modest variations are not correlated with the positive Q-values of neutron transfer channels. These resu!ts are markedly different from those obtaLned from sew oral other systems, in particular the Ni + Ni systems. This conclusion is surprising considering the favou> ab!e structure of the targets and the large positive Qvalues availabie ~mthe present case. The optimum Q~ value for neutron transfer in the reaction i t g s n ( 1 8 0 , 160) 120Sn at subbarrier energies has been measured [ I8] to be s!ightiy negative ( - 2 MeV at ESA B = 55 MeV), whereas the 2n ground state to ground state transfer channei has a Q~value of 3.1 MeV. In terms of the suggestion in ref. [8I, this channel should not make a substantial contribution to the subbarrier fusion cross section because it is kinematically mismatched. Our data are consistent with this argument, although at present its validity is unclear. Recentty an interesting correlation was found in ':he systems 58Ni + 58Ni and 58Ni + 64Ni between fusion cross section enhancement and the total transfer strength [ i 0I, which in these systems is dominated by the negative Q-vNue in pickup channel, To our knowl273

Vola.~v.e 175, number 3

PHYSICS LETTERS B

edge thi~ is the oniy case in w N c h a correlation between fusio~ e:qhancement and transfer strength has been studied. Whether or not this corre!ation holds generaliy can only be determined by fi~rther c o m b i n e d subbarfier transfer az-d fusion measurements. In this way the role of transfer in the subbarrier fus~or~ process may be clarified. We thank A. BresM=, R. Chechik and N. Zwang for essential h d p with the detectors, L. Super for expert manufacturing of the targets, and U. Smflansky and R.G. Stokstad for user's1 advice and help during some of the expergments. R eferen c e~

[ t~ S, Steadman, ed~ Prec. Conf. on Fusion reactions below the Co~xlomb barrier (M~T, Cambridge, MA, June !984), Lecture Notes in Physics, Voi. 2I 9 (Springer, Berlir._, 1985}. [2] R.G. Stokstad e* al,, Phys. Roy. Lett. 41 (1978) 465; Phys, Rev. C21 (1980) 2427° [3] W, Rei~erf eta!., Phys. Rev. Lett. 49 (1982) t81 I; NucL Phys, A438 (1985) 2i2, [4] M. Beekerma~ et ~A.,Phys, Roy. LetL 45 (1980) 1472; Phys. Roy. C23 (1981) t581;C25 (1982) 837. [5} P.M~ Jacebs and U. Sm/I.ansky, Phys. Lett. B127 (1983) 313;

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7 August t 986

C.H. Dasse, S. Landowne and A. Wint1~er, NucL Phys. A405 (1983) 381; H. Esbensen, J.~Q. Wu and G.F. Bertsch, Nucl. Phys. A411 (t983) 275; R° LLndsay and N. Rowiey, J. Phys. (MY) GI0 (1984) 805. :~6} R.A. Broglia etaL, Phys. Rev. C27 (i983) 2433. [7] R, Pengo et N., N~cl. Phys. A41t (i983) 255; W.K° Schomburg et aL, in: Prec. L~tern. CoI~. o.n Fus~on reactions beiow the Cou.'.'omb ba.rrie~ (M,rT, Cambfidge~ MA, June 1984), ode S. Steadman, Lec'fure Notes in Phys ics, Voi. 219 (Spriv.ger, Berlin, 1985) p. 178. {8! A.M. Stefan!rfi eta!., P~ys. Roy. C30 (1984) 2088. [91 H.A. Aljuwair etal., Phys. Rev. C30 (i984) 1223. [!01 K.E. Rehm et al., Phys. Rev~ Left. 55 (1985) 280. [1!1 H.G. Boh!en e* al., Z. Phys. A273 (i975) 21 !; H. Spieier e~ al., Z. Phys. A278 (1976) 241. [t2} P.M. Endt, Ate Data Nucl. Data Tables 26 (I98!) 48. [13] A. B~:eskin etaL, NucL Iv.strum, Methods 22t (1984) 363~ [14] I. Tserruya etaL, in: Prec. Intern. Conf. on Fusion reaeo fions below the Coulomb barrier (M~T, Cambridge, MA, June 1984), ed. S. S~eadman, Lecture Notes in Physics, VoL 219 (Springer, Berlin, 1985) p. 325. [t51 W.D. Myers amd K.-H. Schmidt, NucL Phys. A4!0 (1983) 61. [i6] J.-Q° Wu, G. Ber~sch and A.B. Balentekin, Phys. Roy. C32 (i985) 143Z [171 J. Thomas et g., Phys. Roy. C31 (1985) 1980. [t 8] W. ~,~ennLngetal., Phys Rev. C17 (1978) 2245.