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SPIN POLARIZATION OF LIGHT FRAGMENTS FROM 16O + 58Ni REACTIONS
W. Trautmann, W. Dahme, W. Dünnweber, W. Hering, C. Lauterbach, H.
Puchta, W. Kühn, J. Wurm
To cite this version:
W. Trautmann, W. Dahme, W. Dünnweber, W. Hering, C. Lauterbach, et al.. SPIN POLARIZATION
OF LIGHT FRAGMENTS FROM 16O + 58Ni REACTIONS. Journal de Physique Colloques, 1980,
41 (C10), pp.C10-249-C10-252. �10.1051/jphyscol:19801028�. �jpa-00220647�
JOURNAL DE PHYSIQUE CoZZoque C I O , suppldment au n012, Tome 4 1 , de'cembre 1980, page Cl0-249
W. Trautmann, W , Dahme, W. Diinnweber, W. Hering, C. L a u t e r b a c h , H. Puchta, W. ~ i i h n * a n d J.P. ~ u r m * .Sektion Physik, Universitiit Miinchen, 0-8046 Garching, R.F.A.
Max-PZanck-Institute fiir Kemphysik, 0-6900 Heide Zberg, R. F. A .
1 . Introduction
In the y-ray spectra measured in coin- cidence with projectile-like fragments from
60 induced reactions on light and inter- mediate target nuclei the discrete lines due to ejectile excitation are clearly seen superimposed on the exponentially decreasing y-ray continuum (Fig. 1 ) . Using the trans- mission method we have measured the circu- 1 lar polarization of these y-ray lines as a function of Q for the reaction 1 6 0 + 5 8 ~ i at 100 MeV. This system has been the subject of y-ray multiplicity2 and circular polari- zation3 stydtes which have determined amount and orientation of the spin transferred to
between the two fragments and, more gene- rally, the mechanism of projectile excita- tion in heavy-ion reactions leading to continuum spectra. The data also yield in- formation on the polarization of the statis- tical y-ray continuum deexciting high-spin states in the target-like fragment.
2: Experimental
The transmission method permits the measurement of the circular polarization of y-rays as a function of Y-ray energy. As in former circular polarization experiments 4 a double-symmetric detector setup was used consisting of two heavy-ion telescopes at the target-like reaction product. From the Olab=350 and of two y-ray polarimeters per- present investigation we obtain the spin pendicular to the reaction plane (Fig. 2).
polarization of the light fragment which Gamma rays which had passed through the allows to study polarization correlations iron core of the polarimeter magnets were
GAMMA-RAY ENERGY IMeV1
Fig. 1: Gamma-ray spectra recorded with a 27 cm x 3 3 cm NaI detector in coincidence with light fragments from 100-MeV 160 induced reactions.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19801028
JOURNAL DE PHYSIQUE
T , . , , . . . .
0=-35MeV
a
MAGNET COLLIMATOR
I
Fig. 2: Detector arrangement. The beam direc-
Z = 8
1
I ' , ' ' '
5 s N 1
tion points into the drawing plane. 8 -
2--2+
1 6 0 3 - - 0 ' -a = - 7 ~ e v 3.0MeV / 6.13MeV
accepted at angles 8 5 2 0 ' relative to the *--
reaction normal and recorded, in coincidence with light fragments, by two 27 cm x 34 cm
NaI (T1) detectors. The polarization sensi- 0 PO W 60 CH.lNNEL W ID0 $20 $ 4 0 360
tivity A for photopeak events varies very Fig. 3: Recorded y-ray spectra- An electro- nic threshold was set at E = 2 MeV.
little in the range of interest, 2 MeVSE < Y
Y -
8 MeV (Ref. I ) , and was determined assuming linearly related to the nuclear spin polari- that the iron core is magnetized parallel zation, PZ, of the excited ejectiles (Fig. 5).
to the y-ray flight path on a length of For the 60, 3-+0+, 6.13-MeV transition 7 cm + . 7 cm. Taking into account the we find large polarizations with values of measured core magnetization we obtained jpyl up to 100% and the same sign as for A=2.1%+.2%. This introduces an overall the target-like fragments. The correlation systematic uncertainty of +lo% which is not of the polarizations of the two fragments included in the errors given for the is seen by comparing with the polarization measured polarizations.
In the recorded y-ray spectra (Fig. 3 ) the photopeak regions of discrete lines due to ejectile excitation, and the continuum region were defined, and their respective polarization P determined from the
Y
measured count rate asymmetries P A . Y '
Corrections were applied for the continuum part underlying the discrete,lines.
3. Polarization of excited ejectiles
In Fig. 4 the results are given for the two strongest reaction channels with ejectile atomic numbers Z=6 and Z=8. The circular polarization, Py, of the discrete lines is, under fairly general conditionsr nearly
of the statistical y-ray continuum due to the heavy-fragment decay (Fig. 4d) which is consistent with the results obtained for the energy-integrated y-ray spectra 3
.
The situation is different in the (160, 2 ~ ) transfer channel. The polarization of the.l2cI 2+-+0+, 4.44-MeV transition is generally small for Q>-40 MeV and not corre- lated with the polarization of the target-
1 2
like fragment. The decrease of P ( C 2+) Y
with increasing energy loss in the vicinity of Qopt (Qopt=-24 MeV in the semiclassical model 5 ) is familiar from measurements of the 1
.
3 anisotropy of 1 2 ~ ejectiles from (x,'*B) transfer reactions6 and seems to be a gene- ral phenomenon associated with the quasi-
elastic transfer. The increase of P towards Y
positive values with further increasing energy loss is not connected, however, with the transition from dominantly positive to negative scattering angles which occurs near Q=-25 MeV (Fig. 4d). In fact, Py is still small near the maximum of the deep-in- elastic component at Q=-37 MeV. Apparently, in the transfer channel the light-fragment polarization is not in agreement with the prediction of friction models and is not a measure of the sign of the scattering angle.
A possible explanation for the measured P ( 12 ~~2') versus Q can be found if Brinkls
Y
kinematical constraints5 are also applied to the deep-inelastic region. A picture which assumes the transfer of an a-particle after the 1 6 ~ + 5 8 ~ i system has reached a sticking configuration predicts, for negative scatter- ing angles, P =O at the most probable Q=-39
Y
MeV and P >>0 (P <<O) at Q=-45 MeV (Q=-33
Y Y
MeV). This was calculated according to Q=
stick
+ A ~ A v / R ~ with QStick=-33 MeV, X I = 2, v=.046 c in the sticking limit, and R 1 = R('~c)=~ fm. For the direct a transfer, and positive scattering angles, the familiar formula Q=AV -mv2/2+h fiv/~ with v=.088 cC 1 1
at the barrier yields P =O at Qopt=-24 MeV Y
and P >>0 (P ((0) at Q=-12 MeV (Q=-36 MeV).
Y Y
The comparison with the data shows that matching conditions characteristic of one- step processes may play an important role, irrespective of the fact that the target- like fragment may have been excited in a mul.ti-step process.
This interpretation is consistent with the results found in the inelastic channel.
Here we expect large polarizations of the 60(3-) ejectiles in the direction of the
Fig. 4: (a) Free particle spectra,
(b) spectra of particles coincident with y-rays of E >2 MeV,
Y
(c) circular polarization of the 4.44 MeV (I2c,2+0) and 6.13 MeV
( 16 0,3+0) transition on the left and right, respectively,
(d) polarization of the continuum with E >3 MeV
Y
Fig. 5: Gamma-ray circular polarization, pv, vs. nuclear spin polarization,
Pz' assuming gaussian or exponential substate populations for the initial state.
incoming orbital angular momentum + !Lit i.e.
in the direction of the heavy fragment pola- rization, if the energy of the 6.13-MeV ex- citation has to be provided by the relative motion of the reaction partners, i.e. for Q>Qstick =-33 MeV. This follpws from the re-
C10-252 JOURNAL DE PHYSIQUE
quirements of a continuous projectile trajec- tory and of angular momentum conservation 5
.
4. Polarization of Continuum Gamma-rays The P values given for the Y-ray con-
Y 2 L 6 8
tinuum (Fig. 4d) represent lower limits
Ey(MeVl since the continuum spectra also contain
Fig. 6: Polarization of continuum y-rays events which have undergone Compton scatter-
emitted in coincidence with light ing in the magnetized iron of the polarime- fragments (522'10) from deep-inelas- ter, and for which, consequently, the pola- tic collisions.
rization sensitivity is of the opposite sign? result is consistent with the assumption of In the case of the continuum polarization a statistical decay since, for a rigid-body measured in coincidence with deep-inelastic moment-of -inertia J = 13 M~v-' , o2 = JT<13
events (Fig. 61, we estimate that a correc- corresponds to a nuclear temperature T<1 MeV, tion of this effect would lead to P 270% in i.e. to excitation energies of a few MeV
Y
the range 2 MeV<E 54 MeV. At higher y-ray above the yrast line. Within the statistical Y
energies the effect is less important because model, the decrease of P with increasing y- Y
of the exponentially decreasing intensity of ray energy indicates that the high-energetic the spectra. The large y-ray circular polari- quanta are emitted at increasingly higher zations reflect the high degree of nuclear excitation energies.
spin polarization of target-like fragments
from deep-inelastic collisions, and, on the This work was supported by the Bundesminis- other hand, show that spin is carried away terium fur Forschung und Technologie.
by the continuum y-rays on a rate of -0.5 h
(g1.4 h) per transition if we assume PY=70% References
and pure dipole (quadrupole) radiation. I H . Schopper, Nucl. Instr.
2 ,
158 (1958) 2 R. Albrecht et al., Phys. Rev. Lett.34
In a purely statistical picture, the
1400 (1975)
transition probabilities are proportional to 3 C. Lauterbach et al., Phys. Rev. Lett.
41
the density of available final states, and 1774 (1978)
4 W. Trautmann et al., Phys. Rev. Lett.
39
the y-ray circular polarization is, therefore,
1062 (1977)
a measure of the spin dependence of the nu- 5 D. M. Brink, Phys. Lett.
e,
37 (1972)clear level density p(1). In the vicinity of K' Sugimoto et Phys. Rev. Lett.
39,
323 (1977) I=12 which represents the spin region popula-
H. Puchta et al., Phys. Rev. Lett.
43,
ted in the case of large energy loss2 17, P = 623 (1979) Y
70% corresponds to p(I)/p(I+1)=2.5 from which, according to p (I) = (21+1) exp [-I (1 +I) /
202], a2=13 is obtained. The fact that the initial nuclear spin polarization is slightly smaller than unity7 leads to aa <13. This
