Photofission and Photon Scattering Experiments, Are Emphasized and Described in More Detail in This Report on Low Energy Photonuclear Reactions

Brazilian hurnal of Physics, vol. 24, no. 2, June, 1994

Low Energy P hotonuclear Reactions: P hotofission and Photon Scattering Experiments* Ulrich Kneissl Instítut fur Strahlenphysík, Universitat Stuttgart 0-70569Stuttgart, Germany

Received September 12, 1993 Two topics, photofission and photon scattering experiments, are emphasized and described in more detail in this report on low energy photonuclear reactions. The essential experimental progress achieved during the last years is summarized. It is mainly based on the advent of modern high duty cycle and high current electron accelerators and improved detector systems (4a fragment detectors, high resolution 7-spectrometers and polarimeters). Nove1 results are presented. Fragment angular distributions in (e, elf)-coincidence experiments allowed to disentangle a11 six low lying transitions states in 238U.By simultaneous measurements of both fragment mass and angular distributions, the coupling of the Bohr transition states to diverse fission modes (Brosa modes) could be studied and a mass dependence of fission fragment angular distributions could be established. Furthermore, these measurements of fragment angular distributions for separated fragment masses allow to detect contributions of odd harmonics in the angular distributions due to E1/E2 interferences. Linearly polarized photons were used as a new to01 to search for M1 contributions in the near barrier photofission. In first photofission experiments using monochromatic tagged photons the second chance fission of 23" was investigated. The results provided spin and I<-number assignments to low lying transition states in the unstable nucleus 237U.Systematic low energy photon scattering experiments (nuclear resonance fluorescence experiments) provided unique informations on low lying magnetic and electric dipole excitations in heavy nuclei. Precise excitation energies, transition widths, spins, K-numbers and parities of numerous states in heavy nuclei could be determined. The systematics and strength fragmentation of the orbital M1 mode, the so called "Scissors Mode" in deformed rare earth and actinide nuclei could be established. The linear increase of the total M1 strength with the square of the deformation parameter P2 could be confirmed. For the first time the Scis- sors Mode excitation could be detected in an odd mass nucleus (163Dy). On the other hand, low lying electric AI< = O--transitions of remarkable strengths have been observed in a11 investigated even deformed nuclei near 1.5 MeV. These 1- states are discussed in terms of a I< = O rotational band based on an octupole vibration. The surprising nove1 result of the recent systematic polarization studies, however, was the first observation of :nhanced electric dipole excitations in deformed nuclei at excitation energies near 2.5 MeV. The transition energies and the enhanced B(E1)f strengths of 3-5.10-3 e2fm 2 may suggest sn interpretation in terms of the predicted new type of collective electric dipole excitations n deformed nuclei due to reflection asymmetric shapes like octupole deformations and/or ,:luster configurations. Furthermore, a11 these states systematically exhibit decay branching .atios Rerp = B(1- 4 2i)/B(1- -i O:), which hint at I<-mixing. Another very tempting nterpretation of these strong E1 excitations is the explanation as two phonon excitations due ,,o the coupling of octupole (J = 3-, I< = 1) and quadrupole-7-vibrations (J = 2+, I< = 2). The results can be explained in the framework of the sdf-IBA model and the Dynamic Col- lective Model describing octupole vibrations and their coupling to quadrupole vibrations in deformed nuclei. First results for a spherical, odd mass nucleus are presented. In the even V=82 isotones strong E1 excitations are known which can be interpreted as transitions to i,he J" = 1- members of 2+@ 3- multiplets. The coupling of an additional neutron to :,hese two-phonon excitations has been studied in 143Nd. In this nucleus recent experiments imcceeded in the first identification of dipole excitations to a 2+ @I 3-8 particle multiplet.

'Supportec by the Deutsche Forschungsgemeinschaft under contract Kn 154-21 and SFB 201 Mainz of this accelerator mainly should be devoted to experiments with real photons (y-quanta). Two topics out of a'he use ofelectromagnetic probes sucli as real pho- this field are emphasized in this report which make use tons (y-quanta) or virtual photons (inelastic electron of the higli spin selectivity of electromagnetic probes scattering) in nuclear structure research offers some and whicli document the progress achieved in the last attractive features and fundamental advantages. Tlie years due to the advent of CW electron accelerators and excitation mechanism is well known and leads to tlie sophisticated detector and spectrometer set-ups: population of states with low spins[l]. In real photon absorption the transfer of momentum q is equal to the Low energy photofission: Fragment angular dis- excitation energy w (in units of h = c = 1); it is low and trjbu tions near the fission barrier fized. Therefore, mainly electric dipole excitations (El) e Low energy photon scattering: Magnetic and elec- occur, and to much less extent electric quadrupole (E2) tric dipole excitations in heavy nuclei or magnetic dipole (Ml)excitations are induced. The use of iinearly polarized photons or the measurement Both classes of experiments have the common fea- of particle polarizations enable to distinguish between ture to investigate collective excitations of nuclei at electric and magnetic e~citations[~].On the other hand, large deformations. Measurements of fragment angu- in electron scattering experiments (exchange of virtual lar distributions at excitation energies near the barrier photons) the transfer of momentum q can be varied enable to pin down the spectrum of transition states on independently of the excitation energy w by changing top of the fission barrier and, hence, to study low-lying, the kinematics (incident electron momentum ko and/or collective excitations at high deformations (more than scattering angle O,)[']. Therefore, in principie, differ- twice the groundstate def~rmation)[~~~l.By measuring ent multipoles can be excited rather specifically. Tlie simultaneously fission fragment mass distributions the measured form factors provide unique information oii influente of the excitation and of the specific nuclear transition den~ities[~l~].IIowever, at excitation ener- structure of tlie excited state on the fission path can be gies above the particle thresholds the full information studied for the first timeL7-'1. This allows to investigate is only accessible when performing (e, e'X)-coincidence tlie coupling of tlie socalled Bohr fission ~hannels['~]to experiments f3l4].Unfortunately, these experiments had the diverse fission modes ("Brosa medes")["] resulting to wait for the construction of modern high duty cy- in different fission fragment mass distributions . cle electron accelerators. Furthermore, these machiiies Low energy photon scattering off bound states, allow to produce monochromatic tagged photons witli nuclear resonance fluorescente (NRF), represents a reasonable Auxes and to perform single arm counting highly selective and sensitive to01 to investigate low- experiments at high counting rates since the pile-up lying more or less collective dipole excitations in problem is drastically reduced as compared to experi- lieavy nuclei. Tlie discovery of a new class of en- ments using pulsed accelerators. The availability of tlie hanced magnetic excitations in lieavy deformed nu- new @W electron accelerators together witli improved clei in high resolution electron scattering experiments particle detection techniques led to a new generation of by Richter and coworker~[~~Iinitiated numerous elec- photonuclear experiments. troii and photon scattering e~~eriments['~-'~Ito study In the light of the new CW 30 MeV race track mi- tlie fragmentation and systematics of this mode of- crotron under construction at the University of São ten referred to as "Scissors Mode". On the other Paulo I want to restrict myself in this report to the hand, these NRF-experiments also provided evidence discussion of typical low energy pliotonuclear reaction for enlianced, low-lying electric dipole excitations in studies. The design energy of tliis machine (30 MeV) is deformed n~clei~~~-~~].The structure of the corre- too low to achieve in (e, etX)-experiments the momen- sponding JT=l-states are discussed in terms differ- tum transfers q needed to excite multipoles higher than ent collective excitation modes or two phonon excita- L = 1 (dipole). Therefore, the favoured application tions. For odd mass nuclei there are interestings topics Brazilian Journal of Physics, vol.' 24; no. 2, June, 1994

like the search for the first detection of the "Scissors barrier the fragment angular distributions can be ex- ~ode"[~']or the investigation of dipole excitations to plained within A.Bohr's fission channel con~e~t[~~].At two-phonon-particle multiplets in spherical nuclei near these excitation energies most óf the available energy closed shell~[~~]. has been expended into deformation energy, when the Both kincls of experiments discussed in this re- fissioning nucleus reaches the saddle point. Therefore, port make great demands upon the required acceler- the system should be thermodynamically rather "cold" ators and de ;ector systems. (e, elf)-coincidence and at this highly deformed configuration and exhibits a dis- tagged photon experiments are only feasible at high crete spectrum of low-lying collective excitations similar duty cycle elrctron accelerators. The corresponding to that at the groundstate deformation. Within Bohr's experiments14 ''1 reported were performed at the second hypothesis it is assumed that the near barrier fission stage of the Mainz microtr~n[~~]and at the supercon- takes place through these discrete, so called transition ductive micrctron in urbana[']. The photofission ex- states. With the assumptions that the fission process periments usi ig linearly polarized brernsstrahl~n~[~~~~~proceeds along the symmetry axis of the deformed nu- ask for high intensity machines in order to produce cleus and that the projection I< of the spin J of the a reasonable lux of polarized photons. These experi- excited state onto the symmetry axis is conserved dur- ments were performed at the Giessen ~inac['~],which is ing the path from the saddle to the scission point, the now shut down. However, such experiments are highly fragment angular distributions are determined by the desirable to be carried out using polarized monochro- quantum numbers J and I< of the transition states in- matic tagged photons. For the simultaneous measure- volved. ment of both flssion fragment angular distributions and mass distribu~ions, 4~-arrangementsof fragment detectors with electronical read out are neededLZ6]. Dif- ferent set-ups mainly of parallel plate avalanche coun- ters (PPAC's)['~] or multiwire proportional chambers (MWPC's)['" have been recently used. The photon scattering experiments require a high -L+ intensity, GW bremsstrahlung beam to succeed in get- -3+ -? (e, e'f } -I+ - ting a high detrction sensitivity even when using a small K- 1 amount of neccssarily enriched target material (1-3 g of total mass). The high duty cycle is obligatory to oper- ate the y-~peci~rometersat high counting rates without pile-up. High resolution Ge-y-spectrometers of good efficiency have to be used. For parity assignments Ge- Cornpton polarimeters are needed[29>30].The experi- Figure 1: The schematic presentation of the spectrum of ments reported were performed at the photon scatter- transition states in the case of even-even actinide n~clei[~]. Transition states which have been investigated so far by ing fa~ilit~[l~~~~]installed at the high current Stuttgart virtual (e,e'f) and real (y, f) photon experiments are indi- Dynamitron (I:,,,= 4.3 MeV; I;,,= 4 mA) cated.

11. Low ener gy photofission: fragment angular distributions Fig.1 shows schematically the expected spectrum of 11.1 The trarisition state concept transition states for an even-even nucleus, which rep- The measurement of fission fragment angular distri- resents the most favourable case, since the collective butions represents an important to01 to investigate the bands lie well below the intrinsic, single particle excita- 'properties of tlie fission barriers. At energies near the tions (due to the pairing gap). The relative positions of Uirich Kneissi

Excita t ion Energy ( MeV) Figure 2: Separated fission cross sections for partial fission channels (Jn,h') of 238U:(a) total E1 cross section and its separation into the (1-,O) and (1-,1) fission channels, (b) cross sections for the separated (2+,0), (2+,1), and (2+,2) fission channels, and (c) total E2fEO strength distribution (from multipole decomposition), measured at qi = 0.20 fm-I and its separation into the sum of ali (2+,K) fission channels and the (ot,O) fission channel (from Ref. [22]).

the collective bands (groundstate rotational band, oc- suring the three coefficients a, b, c. Therefore, the anal- tupole bands, y-vibration etc.) strongly depend on the ysis of photofission angular distributions has to be re- nuclear shapes at the saddle. Therefore, fragment angu- stricted to the discussion of the three channels assumed lar distributions, which allow to identify specific tran- to be the lowest in excitation energy (J" = I-, I< = sition states, reveal a direct information on the nuclear 0,l and J" = 2+, I< = O). On the other hand, the pos- shape at the extreme deformation of the saddle point. sible q-variation in exclusive (e, elf)-coincidence experiments allows to disentangle 6 transition states in even- 11.2 F'ragment angular distributions in (e, elf)- even nuclei ((J" , I<) = (O+, O), (I-, O), (I-, I), (2+, O), coincidente experiments (2+, l), and (2+, 2)) by measuring at at least two different momentum transfers. This has been recently Using real photons to excite an even-even nucleus demonstrated by (e, eff)-coincidence experiments [4J21 and with the reasonable restriction to E1 and E2 ex- performed at the 185 MeV stage of the Mainz mi- citations five transition states (J" = l-, I< = 0, l and crotron. J" = 2+, I< = 0,1,2) can contribute to the nuclear fission process. The photofission fragment angular dis- The results are plotted in Fig.2: for E1 and E2 tribution can be parametrized as[5161 the deduced partial strength distributions s(J" , K;w)

2 are converted into the respective partial photofission W(0) = a i- b - sin 0 + c.sin2(20) . (1) cross sections u,,~(J ", I<;w). Since for E0 no photofis- The coefficients a and b arise from both dipole and sion cross section exists, the respective strength dis- quadrupole contributions, whereas c comes entirely tribution is plotted. From the increase of the partial from quadrupole fission. In general, the 5 above men- fission cross sections the effective barriers of the re- tioned fission channels cannot be disentangled by mea- spective fission channels (J", I-)can be estimatedL4J2]. Brazilian Joirrnal of Physics, vol. 24, no. 2, June, 1994

The fission barriers of the already known (1-,O), (lW,1), the bending octupole band (I( = I), respectively. The and (2+,0) channels are in reasonable agreement with (2+,2) transition state can be ascribed to the head of the results o-' previous (y, f)[32-341 and inclusive (e, f) the y-vibrational band whereas the first (2+,1) state is e~~eriments[~'~].Also for the (0+,0) channel the data found within an octupole band resulting from a combi- from direct reaction ~tudies[~~]are not in contradic- nation of mass asymmetry and bending, both at slightly tion with tl-e barrier height in the present experi- higher excitation energies. ment which can be estimated only rather roughly. Mi- croscopic QItPA calc~lations[~~]and recent (a, a'f) 11.3 Mass dependence of photofission fragment e~~eriments[:'~]corroborate the rather negligible E0 angular distributions cross section in the excitation energy region up to As already discussed the fragment angular distri- 8 MeV. Furt lermore, for the first time, for the (2+,1) bution in photofission of even-even nuclei is given by and (2+ ,2) cliannels an estimate of the effective fission equ.(l). The coefficients a and b contain both, dipole barriers can I,e given[4*221. and quadrupole contributions; whereas c comes entirely Comparir g the results for 23" with the theoreti- from quadrupole fission. Tabulations of the coefficients cally expected spectrum of low-lying collective transi- a, b, cfor the different fission channels J, Ii can be found tion states oZan even-even nucleus (see Fig.1) a sur- in the literat~re[~?~].The analysis of photofission angu- prisingly gocd agreement can be found. The lowest lar distributions has to be restricted to the three chan- (0+,0) and (2+,0) states occur within the rotational nels lowest in excitation energy (J" = 1-, I( = 0,l groundstate Sand which is reasonable to be the low- and J" = 2+, I< = O). For pure dipole fission (J = 1) est band in ercitation energy. The 1- transition states or the excitation of the lowest I<= O states the quanti- are found at higher excitation energies as band heads ties b/a and c/b are related to the partia1 cross sections of the mass isymmetric octupole band (I( = 0) and U-,,f (J", I<):

U-,J (J" = I-, I< = 0) 5 U7,f (J" =2$,Iir=O) b/a = - 112 c/b = -. O-,,f (J" = I-, I-= 1) 4 U7,f (J" = 1-,I<= O)

In the case of sufficiently low photon energies, where only states with I< = O should be excited, the ratio c/b directly measiires the relative quadrupole contribution. The ~ystemat~icsof the ratio of dipole to quadrupole photofission has been studied already in the pioneering work by Rabcltnov and cow~rkers[~~].This quantity c/b is very sensitive to the relative heights of the Iowest 1- and 2+ transition states. In the picture of the double humped fission barrier with a mass asymmetric outer MASS M* (amu) barrier the dipole to quadrupole photofission provides Figure 3: Relative quadrupole contribution c/b as a function of a variable mass M* which divides the mass distribution direct informittion on the relative heights of the inner into mass regions M < M* and M > M*, respectively. and outer barriers as pointed out by ~andenbosch[~~]. The data were observed for 236~(y,f) at a bremsstrahlung endpoint energy of Ess ~5.7M~v[']. In a recent experimentL71 for the first time sion channel model[ll]. the fission fragment angular and mass distributions have been measured simultaneously for the rei 11.4 Photofission using linearly polarized pho- action 236U(y,f), using the 4.~-arrangementof position tons sensitive PPAC's[~']. The aim of this experiment was to The 1+ transition state within a i< = 1 octupole search for a possible correlation between the multipolar- band (see Fig.l), corresponding to a M1 excitation, ity of the excitation (spin and IC-value of the transition cannot be investigated in (e, e'f)-coincidence experi- states involved) and the fragment mass asymmetry. ments since transverse excitations can only be stud- The obtained results clearly establish a correlation be- ied at backward scattering angles, where, unfortunately, tween the fragment mass asymmetry and and the an- the cross sections are very small. An alternative repre- gular distributions given by the quantum numbers of sents the use of linearly polarized photons. Then the the transition states involved. In Fig. 3 the ratio c/b fragment angular distribution for dipole fission of an is plotted as a function of a variable mass M* which even-even nucleus is given by divides the mass distributions into masses M 5 M* and M > M*. For M 5 M* the ratio c/b is almost W(O) = a + b.sin20.{1 + ~.P~.cos(2.Q>~)).(3) constant as a function of M*. However, for M 2 M* a Here Qif is tlie azimuthal angle between the polariza- distinct increase of c/b can be stated, starting at around tion plane, defined by the electric field vector I? and the M* N 140. This observed increase for a "far asymmet- beam direction, and the reaction pIane. The factor w is ric" massplit (M* 2 140) gives rise to the assumption -1 for magnetic and +1 for electric excitations, respec- that a quadrupole photoabsorption at excitation ener- tively. The experimental asymmetry E, the product of gies near the barrier leads to a stronger coupling to the the degree of polarization P, and the analyzing power standard I1 fission channel within Brosa's multi exit fis- C is given by

- Multipole I( b/a C (O = ~/2). E 1 o +o0 -1 E 1 1 -112 +l The full formalism is described else~here[~1~~1. The e~~eriments[~]have been performed at 'Eable 1.: Angular correlation coefficients and ana- the Giessen facility >or polarized off-axis brems- lyzing powers for dipole photofission using polarized strahl~n~[~~].The average degree of polarization P, photons[6~24]. measured via the photodisintegration of deuterium amounted to about 15-20%. The fragments were de- The analyzing power is maximal under O = a/2 tected by the Giessen 4a-arrangement of position sen- and changes its sign for electric and magnetic excita- sitive PPAC's[~" enabling the simultaneoiis measure- tions (see Table 1). Therefore, the measurement of E ments of both fragment angular and mass distributions. enables model independent parity assignments. In the The isotope 232~hhas been chosen, since Th-nuclei are case of dipole excitations C is simply given by known to have a mass asymmetric outer barrier with Brazilian Jormal of Physics, vol. 24, no. 2, June, 1994 575

negative and positive parity bandsl4l] and, furthermore, (M* = 117) the asymmetries E (O = ~/2)showed neg- only negligible E2 contributions have been observed in ative values of the order of % -10%. Within the error photofission work using unpolarized photons[39]. bars these values are in agreement with the assumption that only electric dipole excitations take place. Further- Three mezsurements had been carried out on 232~h more, the measured data are in agreement with an for- at bremstrahlung endpoint energies of Ess= 12.0,11.0, mer experiment also performed at the Giessen oR-axis and 10.5 Me'? with polarized off-axis bremsstrahlung. bremsstrahlung facility, however with modest angular In Fig.4 the results for the ratio bla and E (O = n/2) are resolution and without mass ~e~aration[~~I plotted as a function of the mass parameter M*, already The mass dependence of E (O = n/2) changes from introduced. li value of M*= 117 means that fragment higher to lower bremsstrahlung endpoint energies. If masses of the whole mass distribution were considered we asssume that only dipole excitations contribute and in the analysis. With an increasing value of M* only that the relative contributions of-the (I+,1) and (I-,1) fission events with a heavy mass Mh > M* have been fission channels will not depend on fragment masses, taken into account. In a11 three measurements the ra- the asymmetry E (O = n/2) can be calculated from the tio b/a increases for more and more asymmetric fission measured b/a val~es[~~].The dashed areas in the right events (M* 2: 140). Therefore, it is of interest to con- part of Fig. 4 shows the results. At least for the low- sider the b/a systematics in the framework of Brosa's est energy Ess= 10.5 M~Va discrepancy between the "multi exit cliannel" model[ll]. The mass asymmetric the calculated and measured azimuthal asymmetry can so called star dard I1 mode contributes' exclusively for be stated. A possible enhancéQ rnagnetic dipole con- values M* 2 145. Neglecting contributions of the mass tribution would contribute with a negative-value and symmetric "siiperlong" mode the measured angular dis- would increase the asymmetry to higher absolute val- tribution at M* = 117,is determined by a weighted ues. Therefore, the observed decreased absolute value superposition of the standard I and standard 11 intrin- must be regarded as a hint for quadrupole excitations sic angular di:hributions. The corresponding weights of which become important. Only the (2f, 2) channel con- both modes could be deduced from the measured fission tributes with a positive sign at 8 = n/2. The cor- fragment mass distributions. With these input data it responding I( = 2 transition state is the head of the was possible to calculate bla as a function of M*. The y-vibrational band, which seems to be lowered at the dashed lines !n Fig.4 represent the results. The gen- outer barrier in the case of a far asymmetric mass split. eral trend of i,he measured data could be qualitatively reproduced. In the case of 236U(7,f) one measurement at Ess= The ratio b/a also measures directly the ratio 9.0 MeV was carried outIg]. This endpoint energy was of the partia1 cross sections for fission through the still to high in order to òbseive strong quadrupole con- I( = O arid I( = 1 transition states (bja = tributions. The measured values of the azimuthal asym- {u-,, (I-, O) / c-,,(I-, 1)) - 112). An increasing value metry without mass discrimination amounted to about of b/a for fai asymmetric fission events points to a -6%, that means half the value observed in the Th mea- lowered (J" = I-, I< = 0) barrier relative to the surements. However, that is what is expected from the (J" = I-, I( = 1) barrier. This seems to be under- fission barrier structure of 23'% (inner and outer barrier standable since the collective (I-, 0) excitation be- are nearly of the same heights), leading to reduced b/a longs to a pear like deformation and the outer bar- ratios and hence to lower asymmetries E. The bla ratio rier is known from theoretical calculations to be en- was observed to increase for far asymmetric fission as ergetically reduced when taking into account reflection it was seen in the Th measurements. The value of the asymmetric di:formations of the nuc~eus[~~].Without azimuthal asymmetry remained nearly constant within discriminating between different fragment mass regions the statistical errors as a function of M*. EBç = 12.0 MeV - .$+C- h ,./ + o -.i0 t t ttttt 0, , , ,_...-. ++ Y j ,..'(**++4 .-:. j 3 -.I5 *,~..,.-..++-+q.'i.* i

Figure 4: Left Part: The dependence of bss / aes on M* and EB~deduced from experiments on 232Th. Right part: The dependence of css(6 = 90") on M' and EBS deduced from experiments on 232Th (note that the data points for different values of M* are not statistically independent from each other). The dashed iines are the limitations of the expected values ess(í9 = 90") as described in the text (from Ref.[9]).

11.5 Second chance photofission using tagged second chance fission threshold (237Uin the case of this photons experiment).

First measurements of the angular and mass distri- The surprising nove1 result of the experiments was butions of fragments from photofission using monochro- the first observation of anisotropic angular distributions maiic tagged photons have been performed by a in the energy range of the second chance fission thresh- Giessen-Urbana collaboration[" at the tagged photon old (N 12 MeV in 23w).At these energies electric dipole facility of the University of Illinois. The fragments were excitations dominate and E2 contributions can be ne- detected by the Giessen 4n-PPAC arrangement12". Un- glected. A combined analysis of the present (y, nf)- fortunately, the photofission cross section near the bar- data and results of recent (e, e'n f)-e~~eriments[~]and rier is very small (a few mb), so a Iarge amount of beam a comparison with previous 239P~(y,f)-dat~[*~] (239P~ time is required for measurements of angular distribu- has the same groundstate spin of 1/2+ as 237~)en - tions, which need excellent statistics. In the energy abled a consistent spin and I<-number assignment to range of the second-chance fission threshold, Bnf, the two low-lying transition states in 237U (E, m 11.3 cross sections are significantly higher. Therefore, one MeV: J" = 312-, I< = 312; E, x 12.6 MeV: J" = motivation for this experiment was to take advantage 5/2+, I( = 5/2). Furthermore, a clear relationship be- of the higher cross section in this region and investi- tween the anisotropies and the fragment mass asymme- gate the angular distributions of fission fragments from try has also been established. This correlation, together 23"~after neutron evaporation in order to obtain in- with the energy dependence of the angular distribution formation on the low-lying transition states in 237~. parameters, points to a ~ossibleinterpretation of the It is also worth mentioning that target nuclei that are results in terms of a recent theoretical model incorpo- unstable become accessible by studying fission at the rating multiple exit channels in fission[ll]. Brazilian Journal of Physics, vol. 24, no. 2, June, 1994 577

11.6 Contrihtions of odd harmonics to photofis- E, 5 MeV the ratio RIP (see equations 7 and 8) is sion fragment angular distributions assumed to be considerable because the amplitudes Ar, of fission induced by a photon of multipolarity E1 or E2 The formitlism to describe fission fragment angular are of the same strength (AEi R A~~)[~~].In contrast distributions which was introduced so far does not dis- to neutron induced fission no intensive monochromatic tinguish between the emission directions of light and sources exist for photons which allow to perform pre- heavy fragments and the expected fission fragment an- cise measurements of photofission angular distributions gular distributions show a symmetry with respect to at such low energies. The study of P-odd asymmetries O = a/2. 11' one can distinguish e~~erimentallybe - by nonmonochromatic photon beams causes a drastic tween iight and heavy fragments, the investigation of reduction of the measurable quantity RIP. the angular correlation q .pl (where q is the momentum of the photon and pl is the momentum of the light frag- I'"'I'"~ ment) allows í,o study a possible appearance of odd har- 0.50 1O' a11 masses monics in thc fragment angular distributions. This is expected beccuse of interference effects between transition states of spposite parity and was already discussed by Hittmair, Wheeler and ~lambaiim[~~-~~].Transition states with different values of the quantum number I( do not interfe -e since they correspond to different inter- na1 states of t ie nucleus. If one takes into consideration dipole and quadrupole excitations and with the restriction to photofission induced by unpolarized photons the polar angular distribution has to be extended by an ad-

ditional term m ~os(O)[~~].It takes the form Figure 5: The ratio R/P(O = 0°), which measures the relative contributions of odd harmonics to the angular distributions, as a function of EBS. Open symbols illustrate the results without distinguishing between fragment masses; the with the abbreviations other data points represent the angular distributions of light (qepi) or heavy (q.ph) fragments (see text) (from Ref.191).

The measured angular distributions for light (heavy) fragments were analyzed to investigate the angular correlation q . pi (q . ph) and to deduce the ra- The ratio R/ P measures the relative contribution of tio R/P (O, EBs). The data points in Fig. 5 repre- odd harmonics in the angular distributions. sent the experimental values of RIP (O = 0°, EBS) = The asyminetry in fission fragment angular distri- f/a (EBs). For a comparison, f/a values of angu- butions with respect to O = 7r/2 was explored by Bau- lar distributions obtained from fission fragments of the wmannet a~.[~~'for neutron induced fission of 2303232Th. whok mass distribution were plotted. Within the error A slight but significant asymmetry was observed in the bars the ratio f/a vanishes if one does not distinguish case of 230~h(1,f).In a quite recent experiment Steiper between heavy and light fragment masses; in such cases et al.['I tried to detect this effect in the case of the the data points show a small negative value. This could 236~(y,f) process at bremsstrahlung endpoint energies be a hint for a small misalignment of the orientation of between 5.5 IdeV and 6.25 MeV where considerable the fission detector relative to the direction of the pho- contributions of the ((J",I() = 2+,0) transition are ton beam./ Because the angular distributions of heavy involved in the fission process[71. At photon energies and light fragments are correlated, nonvanishing arhpli- 578 Ulri ch Kn eissl

tudes of odd harmonics should have opposite signes but 111. Low energy photon scattering: dipole exci- similar absolute values. The data points representing tations in heavy nuclei the angular correlations q .pl and q .ph showed this ex- 111.1 Motivation pected behaviour when taking into account a possible misalignement of the experimental setup. Nevertheless Over the last years, an increased interest has been the size of the error bars made it impossible to obtain shown in the study of low-lying, enhanced dipole exci- clear and definite evidences for the existence of odd tations in deformed nuclei both of magnetic and elec- harmonics. A X2 test carried out by fitting the data tric multipolarity. There have been hints from ex- according equation 6 with and without terms o: cos(O) perimental work and theoretical calculations for the showed that only the data obtained at Ess = 6.0 MeV occurrence of enhanced electric dipole transitions in can be influenced by the contribution of odd harmon- deformed, heavy nuclei due to reflection asymmetric ics in the angular distributions (with terms cx cos(C3): shapes[47)48].Very recently Butler and ~azarewicz[~'] XS = 1.074, Xi= 0.888; without terms oc cos(Q): discussed and explained intrinsic electric Pipole mo- XS = 1.923, Xi= 1.639). Figure 6 explicitly shows ments in nuclei of the Ra-Th and Ba-Sm region deduced from studies of alternating parity bands with enhanced the measured angular distributions akd the adapted E1 and E3 transitions and parity doublets. As dis- parameterization curves. In Ref. [45] it was also men- cussed 6y ~achello[~~]specifialy for the deformed rare tioned that R/P might depend on the particular frag- earth nucleus 150Nd, rather collective E1 groundstate ment mass regions if the fission amplitudes AL in differ- transitions are expected from states near 3 MeV ex- e& fission channels are different. Basically, the detector citation energy due to u.-cluster configurations and/or arrangement ~sed[~"allowed the measurements of such octupole deformations. On the other hand, it is well dependences but the limited statistics at low photon known that in the same excitation energy range around energies made it impossible to deduce unambigous in- 3 MeV strong orbital magnetic dipole excitations occur formation. Obviously, higher statistics combined with in deformed nuclei, often referred to as the "Scissors the use of monochromatic photons would give a more Mode". This magnetic mode was discovered in high clear result . resolution electron scattering experiments by Richter and coworkers in 1984[121. Meanwhile it has been in- a11 masses , E,=6.0 MeV 1.2 vestigated in numerous electron and photon scattering e~~eriments[l~-~~]These studies showed that the M1 strength can be rather fragmented. Therefore, parity determinations are imperative for photon scattering experiments when searching for the above mentioned new electric dipole modes in the same energy region as the orbital magnetic dipole excitations.

111.2 ~x~erimentaltechniques

111.2.1 The nuclear resonance fluorescence method

Nuclear resonance fluorescence (NRF) experiments (photon scattering off bound states) represent an out- Figure 6: The fragment angular distributions measured in standing to01 to investigate these low-lying dipole exci- photofission of 236Uinduced by a bremsstrahlung spectrum with an endpoint energy of 6.0 MeV. The solid lines are the tations and to provide detailed spectroscopic informa- results of fits according to equation (6) (from Ref.[9]). tions. The low transfer of momentum q of real pho- Brazilian Yo~malof Physics, vol. 24, no. 2, June, 1994 579 tons makes ~hotonscattering highly selective in excit- photons using Compton polarimeters. The parity infor- ing low spin states. In particular, only El, M1, and, mation is obtained from the measured azimuthal asym- to a much Icsser extent, E2 transitions are induced. metry c In addition, modern y-spectroscopy offers highly effi- cient Ge-detectors with an excellent energy resolution, with which tlie NRF method can achieve a high sensitivity. This sensitivity is essential for detailed stud- where NL and NI1 represent the rates of Compton scat- ies of the fr xgmentation of the strengths of specific tered events perpendicular and parallel to the NRF collective modes. Furthermore, the use of continuous scattering plane defined by the directions of the pho- bremsstrahluig radiation enables a11 states with suffi- ton beam and the scattered photons, respectively. The cient ground state decay widths to be excited simul- asymmetry E is given by the product of the polariza- taneously. Tiere are no limitations as in experiments tion sensitivity Q of the polarimeter and the degree of using monoeiiergetic photons from capture reactions, polarization Py of the scattered photons. At a scatter- where a chance overlap of the incident photon energy ing angle of O = 90° to the beam axis the polarization and the energy of the excited nuclear state is necessary. P-, amounts to -1 or +1 for pure E1 and M1 exci- The fundamental advantage of the photon scatter- tations, respectively (0-1-0 spin sequences). Therefore, ing technique is the well-understood mechanism of ex- obviously the sign of the asymmetry E determines the citation and deexcitation via the electromagnetic in- parity. teraction. T!ierefore, the following quantities can be extracted in a completely model independent way[25] 111.2.2 The Stuttgart photon scattering facility

e the excitation energies. The experiments have been performed at the bremsstrahlung facility installed at the e the ratio r;/r, (roand r: ground state and total Stuttgart Dynamitron accelerator (Eo -

decay widths, respectively). 4.3 MeV; Itypiol% 0.8 mA (CW)). In total, mostly three detector set-ups were operated simultaneously at e the spins of the excited states. the photon beam: the branching ratios for the decay to excited states. a 3 Ge-detector arrangement to measure angular distributions and scattering cross sections in NRF e the parities of the excited states. experiments (without polarization sensitivity) as So far, in most of the previous systematic photon described int3l]. scattering ex~~eriments,parity assignments came from e a five crystal Compton polarimeter with a cen- a comparison with electron scattering form factor~[~~I tral Ge-detector and four peripheral Ge or Ge(Li)- or by applying the Alaga r~les[~~].Within their va- detectors located below the scattering target[''l. lidity the measured decay branching ratios to the first excited 2+ state and to the O+ ground state (Rezp = e a sectored true coaxial Ge(HP)-polarimeter in- B(l -, 21) / ~3(1-, O?)) enable in even-even deformed stalled in a horizontal scattering plane at a scat- nuclei to determine the I'-quantum numbers of the ex- tering angle O = 95'. The characteristics of this cited states. I\; egative parities can be assigned to K = O new polarimeter are described in more detail in levels, whereari for I( = 1 states either negative or pos- the following section. itive parities are possible. Parities can be determined model indeper dently in photon scattering experiments The scattering targets consisted of cylindrical discs by measuring the linear polarization of the scattered of enriched isotopes with diameters of 2 cm and masses 580 Uirich Kneissl of a 3 - 10g. Plates of aluminum sandwiched the tar- mitron. The polarization sensitivity Q amounts to gets. The lifetimes and decay properties of three 27~1 20% at 0.5 MeV and remains z 9.5% at 4.4 M~v[~']. levels below 4 NeV are precisely kn~wn[~~].There- The overall detection sensitivity of the polarimeter will fore, the numbers of photons resonantly scattered off be considerably increased in the near future by improv- these levels were used for the absolute calibration of ing its response function using a BGO anti-Compton the incoming photon flux. Furthermore, the 2.981 MeV ~hield[~~]. line produced by resonant scattering off 27Al is known to be effectively unpolarized (P, = 1.5%) and served as online test of the performances of the polarimeters. The photon scattering technique, the experimental set- ups and procedure are described in detail in preceding papers[15~311, Figure 7: Layout of the fourfold sectored Compton po- 111.2.3 A sectored Germanium single crystal Iarimeter (dimensions in mm), the numbering of the four sectors, and the coincidence requirements defining horizon- compton polarimeter tal (H) and vertical (V) Compton events (see refs.[16,30]). This new polarimeter consists of a true coaxial p- 111.3 Results for M1 excitations in even-even nu- type Ge(WP) crystal with its outer n-type contact split- clei ted into four electrically insulated surfaces resulting in four electrically separated sectors. The outer surface of The first report on a new magnetic dipole mode each sector and the inner p-type core have electric con- in deformed nuclei by Richter and co~orkers[~~]in tacts connected to separated preamplifiers. The signal 1984 stimulated a huge number of both experimen- from the central core directly provides the total energy tal and theoretical work (for an overview of references of the incident photon, whereas the signals from the sec- see[13114]).This predominantly orbital excitation seems tors are used to define different coincidence conditions, to be a rather general phenomenon. Today it is es- permitting to use the sectored detector as a Compton tablished by numerous systematic electron and photon polarimeter. The dimensions of the crystal and the co- scattering e~~eriments[l~-~~], not only in deformed nu- incidente configurations are shown in Fig. 7. In spite of clei of the rare earth mass region, but it has been ob- the lower polarization sensitivity Q of the single crys- served in actinide as well. Intuitive terms tal polarimeter as compared to that of a five detector Iike "Scissors Mode", aNuclear Wobble", and "Giant set-up the high coincidence efficiency 6, makes the sec- Angle Dapole" have been attributed to these low lying tored polarimeter a competitive instrument. Further- isovector M1excitations which originally have been dis- more, this type of polarimeter needs no energy sum- cussed in the framework of the Two Rotor ~odelf~~]and mation to determine the total energy of the incident the IBM-2 version of the Interacting Boson ~odel[~~]. photons. This feature is of particular advantage in NRF On the other hand, to explain the underlying micro- experiments using continuous bremsstrahlung where an scopic structure of these low-lying 1+ - states severa1 excellent energy resolution and long time stability are groups performed different quasiparticle random phase of crucial importance. approximation (QRPA) calc~lations[~"~~]. The characteristics of the fourfold sectored Ge(HP)- The measurements of the linear polarization of the Compton polarimeter are summarized in Table 2. The scattered photons in the present experiments enabled polarization sensitivity of the device has been deter- for the first time unambiguous parity assignments in a mined in the energy range up to 4.4 MeV study- completely model independent way also to weaker exci- ing (p,pty)-reactions on 12C, 24Mg, 2ãSi and 56Fe at tations, and hence to study in detail the fragmentation the Cologne Tandem facility and the Stuttgart Dyna- of the orbital M1-strength. As an example the results Brazilian Journal of Physics, vol. 24, no. 2, June, 1994

Energy Resolution AE7 2.2 keV at 1332 keV Total Detection Efficiency &total 25 % at 1332 keV Coincidence Efficiency 6, = (H + V)/Total = 25 % at 3 MeV Apparative Asymmetry -< 1 % at 2-4 MeV Polarization Sensitivity Q G 20 % at 0.5 MeV G 9.5 % at 4.4 MeV

l'able 2.: Characteristics of the fourfold sectored single crystal Ge-Compton polarimeter.

of the photo~scattering experiments on stable even- attributed to the orbital M1 mode which exhibits a even Gd and Nd are depicted in Fig.8. rather pronounced fragmentation. The same conclu- It shows a comparison of AI< = 1 transitions in the sion holds for the nucleus 150Nd, where our previous well deformed Gadolinium isotopes and those observed polarization data[29~69]for the three strongest transi- in the nuclei of the Neodymium isotopic chain where tions could be confirmed and, additionally, now the for the first lime the transition from spherical to de- positive parities of two further weaker transitions have formed nuclear shapes could be be ~tudied[~~].In the been determined[l". Gadolinium the M1 transition strengths are concentrated in a narrow group arround 3 MeV and follow the E, = 66. S. A-'/~ MeV rule to be expected for the orbital M1 mode. The same systematic trend has been observed for the Dy is~to~es[~~].On the other hand, in the Xeodymium nuclei only for the deformed nucleus 150Nd, the cluster of states at 3 MeV is in agreement with this behaviour. But for the transitional nuclei 146914a~d,the levels decaying predominantly to the groundstate are spread over the full energy range from 2 to 4 MeV. Fiirthermore, the transition strength to levels with K = 1 is much stronger in the well deformed Gadolinium isotopes.

The positive parities for the group of states near 3 MeV in 156Gd observed in a combined analysis of electron and photon scattering e~~eriments[~~Icould be assigned from tlie corresponding (e,el)-form factor mea- Energy [keV] surements. U lfortunately, in lSaGd only the positive Figure 8: Strength distributions of dipole transitions in parities of the strongest transitions, the narrow doublet the Nd isotopes with decay branching ratios R,, < 1 in (3.192 and 3.201 MeV) not to be resolved in electron comparison with AK = 1 transitions in Gd isotopes at- scattering, coiild be established from the form factor tributed to the orbital M1 mode (from Refs.[31,65,68,71]. For marked transitions positive parities have been estab- behavi~uri~~].However, for 160Gd the present polar- lished (full dots: electron scattering data; crosses: photon ization meas~rements[~"allowed to determine the par- polarization data). ities of a11 states around = 3.2 MeV marked by crosses. This result dernonstrates that most of the states in the Recently the Darmstadt gro~p[70]succeeded in a cluster have tlie same, positive parity and should be systematic NRF study on the Sm isotopes to show that the total orbital M1 strength increases propor- line. Therefore, the new Nd-results represent a reliable tional to the square of the deformation parameter S. independent confirmation of the proposed S2 la~[~Ol. The same general trend is also evident in our previ- This S2-dependeme of the orbital M1 strength origi- ous Nd For this isotopic chain the polariza- nally was predicted by macroscopic models[721731and is tion experirnents were presently completed by measure- now explained in rnicroscopic calculations, too[74-761. ments on the transitional nucleus 146~d[711to establish safe parity assignments for a11 Nd isotopes. The observed strong dipole excitations in the transitional nucleus 146Ndhave decay branchings which are in surpris- ingIy good agreement with predictions from the Alaga rules valid in the rotational limit. Therefore, in Fig.9 for comparison the strengths of a11 excitations are added up which show a decay branching Rexp< 1 corresponding to AI< = 1 transitions in rotational nuclei, including weak transitions where no parity assignments could be achieved. Fig.9 shows the total strengths Figure 9: Total M1 strength observed in the even Nd iso- added up in the energy range 2-4 MeV observed in topes in the energy range 2-4 MeV as a function of the the Nd-isotopes (142,146,1487150~d).Full syrnbols cor- square of the deformation parameter b2. Full symbols: Only respond to M1 strengths of transitions with reliable transitions with known parity have been included. Open symbols: Summed strengths of a11 transitions from spin 1 parity assignments from our linear polarization mea- states with decay branching ratios Rezp 5 1 corresponding surements or from literature; open symbols correspond to h'=l states within the validity of the Alaga rules. The full line represents a fit to the data with determined parities to total strengths obtained by adding up the strengths (fuil symbols, including the 148Ndpoint) (from Ref.[71]). of a11 dipole transitions in the energy range of inter- For comparison the dashed straight line shows the deforma- est (E, = 2 - 4 MeV) showing a Rexp5 1 decay tion de~endenceobserved by the Darmstadt group for the even Sm isot~~es[~~I. branching. The full line represents a fit to the data with determined parities (full symbols, including the 14"Nd point). The fact that the full line does not in- 111.4 Results for E1 excitations in even-even nu- tersect zero is due to the inclusion of the 4095 keV lf dei state in 142~dwhich has the same structure as the 3.97 The systernatics of K = 0, J = 1- states in MeV leve1 in the isotone 144Sm. It should be empha- rare earth nuclei observed in our previous system- sized that the slope of the linear dependence fitted to atic photon scattering experiments[171 shows that the the experimental data is strongly influenced by the ex- K' = 0- strength is mainly concentrated in one or perimental detection sensitivity (number of added up two transitions near 1.5 MeV with summed strengths weak transitions of unknown parities). Consideration 2 of C B(E1) 1 N 20 . 10-~e fm2 (corresponding to a of a11 AI< = 1 transitions in the energy range of in- rather high value of N 4.10-3 Weisskopf units), whereas terest (open symbols), which seems to be reasonable the strength at higher energies is rather fragmented. in particular in the case of the well deformed nucleus These low lying 1- states are discussed in terms of 150Nd, obviously would increase the slope and lead to a K = O rotational bands based on an octupole vibration remarkable overall agreement with the Darmst,adt Sm- as suggested by Donner and reiner ri^^]. This explana- results. For comparison the linear dependence on /32 tion is supported by the observed linear correlation of of the total M1 strengths observed for the Sm-isotopes the energies of the K = O , J = 1; states with the en- in the Darmstadt e~~eriments[~~]is plotted as dashed ergies of closely lying J = 3- states[l71. The strengths Brazilian Joiirnal of Physics, vol. 24, no. 2, June, 1994

of these K == O E1 excitations could be explained by investigated. The results are shown in Fig.10. For an admixture of the Giant Dipole Resonance (GDR) the first time positive parities could be established for to these low lying 1- state~[~~].The same descrip- groups of states in the nuclei 150Nd, 160Gd, 162~164~Y. tion already :iad been successfully applied for the inter- Most of these states are concentrated near 3 MeV and

pretation of low lying 1- states in 4"i, 164~y,232Th should be attributed to orbital M1 excitations ( "Scis- and 238U observed in (e, e')-e~~eriments[~~I.Recently sors Mode" ). Additionally, enhanced electric dipole

Soloviev et el. ["O] described these excitations micro- excitations in tlie same deformed nuclei were observed scopically within a quasiparticle-phonon model. Von at excitation energies of 2.414, 2.471, 2.520 MeV, and Brentano et al.["] explained on a quantitative scale 2.670 MeV, respectively. The transition energies and both the trar sition widths ro and the decay branching the enhanced B(E1) strengths of 3 to 5.10-~e 2 fm2 ratios of thesc: 1-dates using an improved E1 operator may suggest an interpretation in terms of the predicted within the IEiA-sdf-model. new type of collective electric dipole excitations in de- On the other hand, in 150Nd, 160Gdl and formed nuclei due to reflection asymmetric shapes like 162,164DY [18*131821 strong transitions were observed at octupole deformations and/or cluster configurations. higher energ es near 2.5 MeV, which exhibit decay Table 3 summarizes the results and gives a compari- branching ratios of the corresponding levels indicat- son with theoretical estimations, assuming reasonable ing I(-mixing. The original aim of the present polar- deformation parameters and cluster admixtures. Both ization measiirements was to determine the parities of the cluster and the octupole shape models are able to these states to search for the proposed enhanced elec- explain at least the right order of magnitude of the tric dipole excitations due to an octupole deformation observed E1 strengths. However, on the basis of the and/or an a-zlustering and to look for a pdssible rela- present (y,yl)-results it is not possible to distinguish tion to K-mi cing. between the different excitation mechanisms proposed.

2300 2400 2500 2800 2700 2800

~27'(i -) + (2+) &V) Energy [ke~] Figure 11: Experimental excitation energies E~F'(1 -) of Figure 10: Resilts for 160Gd (y, 7') and 162~Y(y, 7'): Parts states attributed to a two phonon excitation versus the sum (a): dipoIe stiength distributions. Parts (b): azimuthal of the K = 1 octupole and the y-vibrational excitations asymmetries E measured by the 5 Ge-detector polarimeter. (~i:f'(I-) + ~;'=~(2+)).The full line corresponds to the Parts (c): azirmthal asymmetries E measured by the sec- exact fulfilment of eq.10. In the case of 162Dy (fuU sym- tored Ge-polarimeter. The dashed lines represent the ex- bol) all needed energies are experimentally known (from pected asymmíitries for M1 (positive values) and E1 tran- Ref.[82]). sitions (negatile values), respectively (from Ref.[1 SI). Another very tempting interpretation is to explain After the first polarization measurements on these states as two phonon excitations in strongly 150~d[651the r eighbouring nuclei 160Gdand 162~ywere deformed nuclei caused by the coupling of octupole Ulrich Kneissl

F Nucleus E, r0 %rp B(E1)T (10-~e2fm2) (MeV) (meV) a-Clust. 0ct.-Def. Exp. 150Nd 2.414 14.9f2.0 0.86f0.09 1.29 2.9 3.0f 0.4

Table 3.: Comparison of experimental results (excitation energies E,, ground state widths ro, decay branching ratios Rexpand reduced transition probabilities B(E1) T) for enhanced E1 groundstate transitions in deformed rare earth nuclei with model estimations (assuming a cluster admixture of =0.001 and deformation parameters of ,B2 =0.25 and p3 =0.1, respectively).

vibrations to the I(" = 2+, y-vibration. Such gated so far by polarization measurements the lowest two phonon excitations were theoretically already ex- J=1 states above the octupole vibrational bandheads plicitely treated by Donner and ~reiner[~qwithin the exhibiting an uncommon decay branching ratio have Dynamic Collective Model in 1966, but up to now such been taken. An excellent agreement between the exci- states could not be detected experimentally. The result- tation energies ~&l(l-) of the assumed two phonon ing 1- states can be excited by dipole transitions from excitations and the sum of the Ii' = 1 octupole and the the ground state as a result of the coupling of the giant 7-vibrational excitations (according to eq. 10) can be electric dipole resonance to the octupole vibrationi7q. stated. Fig.11 illustrates this fact in graphical form. In the following the 1- states near 2.5 MeV are discussed as possible candidates for such two phonon excitations due to the coupling of quadrupole-y-vibrations (J = 2+, I< = 2) and octupole vibrations (J = 3-, Ii' = 1). It is obviously impossible to generate states with J=l by coupling the y-vibration to octupole excitations with (J = 3-, IC = O). Octupole vibrations in deformed nuclei and their strong coupling to the deformed quadrupole spheroid were treated in detail by Donner and reiner ri^^]. In their work it is shown that 0.5 1.0 1.5 2.0 the energy of the two phonon excitation is simply given by the sum of the phonon energies in the examined case Branching Ratio R,, where K=1 and Js=l (J3= third component of the oc- Figure 12: Frequency distribution of experimental decay tupole angular momentum): branching ratios Re,, = B(l -) 2:)/B(1 -+ 0:) [14,83]. The decay branching ratios for the four JT = 1- states (enhanced El-excitations) in lS0Nd, 160Gd, and 162r164DY discussed in the text are indicated by arrows. The informations on the position of the octupole vibrational bandheads with K=1 are rather sparse. In order to gain more information about the ob- Therefore one has to assume in most cases that the served 1- states calculations in the framework of the first JT=l-state following the Ii'=0 octupole vibra- sdf-IBA model have been performed["], which support tional bandhead is a I(=l state. As possible candidates the interpretation as two phonon excitations. There- for the two phonon excitation besides the 1- states in fore, the observed strong E1 excitations near 2.5 MeV the four isotopes 150Nd,160Gd, and 1629164Dyinvesti - in deformed nuclei exhibiting an uncommon decay Brazilian J9urnal of Physics, vol. 24, no. 2, June, 1994

branching may be attributed to a two phonon exci- B(M1) f 2 0.1& could be detected in the energy tation caused by the coupling of the octupole and y- range near 3 M~v[~~]. quadrupole vibrations. This conclusion is based on the For the first nuclear resonance fluorescence ex- nearly quarititative agreement of the experimental exci- periment on an odd mass nucleus performed at tation energies with the sum of the 1' = 1 octupole and the Stuttgart facility 163Dy was chosen as a first I( = 2 y-vibration as suggested by the collective model candidatef2'], since the neighbouring even-even nuclei and on the results of sdf-IBA calculations which repro- 162Dyand 164~yare well inve~ti~ated[~~I.In both iso- duce the experimental energies and the structure of the topes the orbital M1 strength is concentrated within states. However, the sdf-IBA fails to account for the two or three strong transitions, in 164~ythe largest M1 enhanced B (El)values. To obtain more information on strength in a11 rare earth nuclei was observed. Further- the structu::e of these states it is important to get ad- more, detailed spectroscopic information from (n, y), ditional exjberimental data on the octupole vibrational (n, n'y), (d,p), and (d, t) reaction ~tudies[~~]is avail - bands as wt:ll as on higher-lying dipole excitations. able for this isotope. In addition, orbital M1 excita- It should be emphasized that the J" = 1- states tions should be more pronounced in neutron-odd iso- corresponding to the enhanced E1 excitations near 2.5 topes following theoretical considerations["l.

MeV in thc neighbouring nuclei 15'Nd, l6'Gd 9 l'j2Dy, and 164~yas discussed above exhibit a decay branching ratio R,,, tleviating from the expected values for pure K = O and I' = 1 states and therefore hint to a possible 1'-mixirig["]. This fact is demonstrated in Fig. 12 where the experimental branching ratios R,,, of about 200 transitions in well deformed nuclei are compared with the prc dictions of the Alage rules["~~~].One notes two maxima corresponding to R,,, = 0.5 for K = 1 and R,,, = 2 for I' = O. However, some branching ratios lie in between the expectation values and suggest a possible I--inixing. It is of interest to study in future in more detail the occurrence of I(-mixing in these strong E1 transitians.

111.5 Resullts for odd-A nuclei

111.5.1 Sci:ssors mode in deformed, odd-A nuclei?

As discussed above there exists detailed informa- Energy [keV] tion on the clistribution of low-lying electric and in par-

in' com- ticular of mignetic dipole strength in heavy deformed Figure 13: Dipoie strength distribution in 163Dy [20] parison with that in even-even Dy isotopes obtained in pre- even-even niiclei. The M1 strength concentrated near vious NRF mea~urements[~']. 3 MeV exci;ation energy is predominantly of orbital character and is attributed to the so called "Scissors The resulting dipole strength distribution in the Mode". Thc: Darmstadt group very recently reported energy range of the orbital M1 mode is shown in on a first search for low-lying M1strength in the heavy, Fig.13 together with the data for the neighbouring odd nucleus L65H~.However, no strong transition with even-even n~clei[~~].The M1 character of the excitations in 1623164Dyis known from (e,e')[871 and (y, q) photon scattering experiments only states with spins e~~eriments['"~~].Unfortunately, the spins of the ex- 5/2, 712, and 9/2 can be excited from the groundstate cited levels in 163Dy cannot be determined unambigu- (in total 15 levels). It is clear that one needs an exper- ously from this measurement because of the nearly imental method which is selective in strength and spin isotropic angular distributions for excitations from the to detect these fragmented two-phonon particle levels in J" = 512- ground state in 163Dy. Therefore, the prod- the energy range around 3 MeV where the overall level uct of ground state decay width and spin factor densitty is already high. The photon scattering method g= (25 + 1)/(2Jo + 1) (in this case 2/3, 1 or 413) is fulfils these requirements. plotted. The observed concentration of dipole strength in 163Dy around 3 MeV fits very well in the sytematics of the scissors mode in the even Dy isotpes. Both the excitation energy as well as the summed strength (assuming an average spin factor g=l) follow the trends expected for the orbital M1 mode. The strength distribution is in good agreement with recent calculations performed in the framework of the IBFM-approach (Interacting- ~oson-Fermion-~odel)[~~I.The different decay branchings predicted by the IBFM-calculations for severa1 states can be used for tentative spin assignments. Energy [MeV]

111.5.2 Dipole excitations in Odd- A, spherical Figure 14: Experimental (upper part) and theoreticd (lower part) dipole strength distribution in 143~dbetween 2.8 nuclei and 3.6 M~v[~'].If the transitions have an E1 character one finds for the plotted quantity i(2J + 1) . c . Multiphonon excitations have been studied inten- rhed = B(E1;J -t Jo) 1 (with r,Ped= I'(J -+ Jo). Ey3 sively in the 1st years in many experimental and theo- and c = 0.9553.10-~e 2 fm2 MeV3/meV). The level marked retical investigations. In nuclei with a spherical shape a by an asterisk shows a strong branching to the 312- level at 742 keV and is not expected to be reproduced by the multiplet of two phonon states with J" = I-, 2- .... 5- theoretical calculation~[~~~. should arise due to a coupling of a 3- octupole and a 2+ quadrupole vibration. In photon scattering experi- The measured strength distributi~n[~~Iis shown in ments the 1- member of the 2+@3- multiplet has been Fig.14 (upper part). The assumption of electric dipole found in various N = 82 isotone~["~~~~~~~~~].In 14'Nd an character for these transitions is highly favourable from isolated E1 transition from a 1- state at 3425 keV was the known dipole excitations in the neighbouring nuclei. detected in photon scattering e~~eriments[~~*~~].This The total B(E1) 1 strength observed in 143Nd (deexci- energy is close to the sum of the 2+ and 3- vibra- tation of J=5/2, 7/2, and 9/2 states) amounts to about tions at 1576 and 2084 keV, respectively. In a recent three times the strength of the strong 3452 keV transi- e~~eriment[~lIperformed at the bremsstrahlung facil- tion in the even core nucleus 142Nd. The fact that the ity of the Stuttgart Dynamitron accelerator the cou- sum rule is fulfilled proofs the two-phonon @ particle pling of an additional neutron to the 2+ @ 3- muItiplet structure for the excitations in 143~d. has been investigated. In the case of 143~dthe addi- In the lower part the experimental results are com-

tional neutron occupies the f7/2 subshell. Because of pared with calculations. These model calc~lations['~] the coupling of the odd neutron the five levels of the consists of two parts. First the prominent collective 2+ @ 3- multiplet in the core nucleus split into 31 lev- features of the core nucleus (energetic positions of the els in 143~d.Out of these, due to selection rules, in 2 - .... 5- members of the multiplet) are described Brazilian Joiirnal of Physics, vol. 24, no. 2, June, 1994 587

in the framework of the sdf-IBM. Then the coupling tions. Therefore, simultaneous measurements of both, of the additional neutron is calculated using the code angular and mass distributions seem to be indispens- COUPLIN[~~].The calculated distribution (lower part able. of Fig.14) is quite similar to the experimental one. The The photon scattering method (NRF experiments) agreement bc tween the experimental data and the core using intense CW bremsstrahlung beams has proven coupling calculations can be considered to be a strong its outstanding capability to investigate low lying mag- argument in favour of the proposed 2+ 8 3-8 particle netic and electric dipole excitations in Iieavy nuclei. structure of the observed states near 3 MeV. However, it should be noted that parity assignments are of crucial importance for the interpretation of the N. Summary and outlook data. Parities can be determined as discussed above in In this r~porttwo topics, low energy photofis- time consuming measurements of the linear polarization sion and pho ,on scattering experiments, are discussed. of the scattered photons using Compton polarimeters. Fragment angular distributions reveal, as well known, For higher excitation energies (E 2 5 MeV), like for direct information on the spectrum of the transition the search for M1 spin flip strength, the use of polar- states and hence on collective excitations at the large ized photons in the entrance channel is more favourable. deformations of the fission barriers. Essential progress Therefore, high flux sources of linearly polarized pho- could be ach eved in the last years. The most excit- tons are highly desirable. There are many open ques- ing, nove1 result of recent experiments was the obser- tions in nuclear structure physics, partially raised by vation of a rriass dependence of fission fragment an- the recent experiments, which can be tackled by future gular distribiitions. First (e, el f)-coincidence experi- NRF experiments. However, it should be emphasized ments allowetl to disentangle all six transition channels that a deeper insight into the underlying structure of in 23" and demonstrated the power of this method the states excited by real photons comes from the com- using monocEromatic virtual photons. Unfortunately, parison with results obtained by different probes. In data exist on y for this isotope and, furthermore, the particular (e, e')-form factor measurements and (p, p1)- statistics in these experiments did not allow to make experiments are of importance in this respect. diverse mass cuts. Monochromatic, tagged real pho- As stated in the beginning, powerful low energy CW tons, for intensity reasons, up to now only could be electron accelerators are necessary to provide the pho- applied for the investigation of fragment angular dis- ton sources needed in the experiments discussed here. tributions in ,he second chance fission of 23", where After setting to work the 855 MeV stage of the Mainz the higher croiis sections compensate for the low photon Microtron MAMI the 185 MeV electron beam flux. In principle, the use of linearly polarized photons is no more available for further (e, elf)-coincidence ex- offers considerable advantages. However, only continu- periments. A suitable facility for such experiments, ous polarized off-axis bremsstrahlung was available in however, exist at the new 130 MeV superconductive first experiments. Therefore, for future photofission ex- S - DALINAC at ~armstadt[~~~~~].(including a mod- periments there is a strong demand for first investiga- ern large solid angle electron spectrometer). At the tions with mc~nochromatic,if possible polarized pho- injector section of the S - DALINAC (10 MeV) NRF tons. As already pointed out a11 effects observed so experiments very successfully have been performed[961. far in bremssti ahlung experiments will be considerably Due to the limited achievable current of x 20 ,uA'and enhanced in nieasurements using monochromatic pho- the narrow geometry this experimental set up is not well tons. A centra subject of future experiments will be the suited to produce polarized off-axis bremsstrahlung. A study of the ccupling of the Bohr transition states, set- new polarized off-axis bremstrahlung facility recently tling the angular distributions, to diverse fission modes, has been built at the 15 MeV high current e--linac at determining tlie fragment mass and energy distribu- Gent (7 z 2 m~)[~~].The duty cycle of the Gent ma- 588 UIri ch Kn eissl

chine is limited to about 2%, nevertheless, this new 6. E. Jacobs and U. Kneissl, Photon- and Electron- facility devoted to NRF experiments brought a con- Induced Fission in The Nuclear Fission Process, siderable progress compared to the old Giessen set up edited by C. Wagemans (CRC PRESS INC, Boca (duty cycle 0.12%)[~~].The Stuttgart Dynamitron as Raton, 1991). an electrostatic accelerator delivers very high electron 7. W. Wilke et al., Phys. Lett. 207B, 385 (1988). CW currents (up to 4 mA), however, is limited in its 8. W. Wilke et al., Phys.Rev. C42, 2148 (1990). maximum energy to 4.3 MeV. At the present time there 9. F. Steiper et al., Nucl.Phys. A563, 282 (1993). is, at least to my knowledge, no low energy tagged, po- 10. A. Bohr, Proc. of lStUN Int. Conf. on Peace- larized photon facility in operation. Therefore, I see fui Uses of At. Energy, Vol.1, p.151, New York good perspectives for the new 30 MeV microtron under (1956). construction at the University of São Paulo. 11. U. Brosa et al., Physics Reports 197, 167 (1990). 12. D. Bohle et al., Phys.Lett. 137B, 27 (1984). Acknowledgements 13. A. Richter, Nucl.Phys. A507, 99c (1990). 14. A. Richter, Nucl.Phys. A522, 139c (1991). The experiments reported on have been performed 15. U. Kneissl, Prog.Part.Nucl.Phys. 24, 41 (1990). during the last years by our Stuttgart and former 16. U. Kneissl, Prog.Part.Nucl.Phys. 28, 331 (1992). Giessen photonuclear group in collaborations with the 17. A. Zilges et al., Z.Phys. A 340, 155 (1991). groups of Prof's P. von Brentano (Koln), L.S.Cardman 18. H. Friedrichs et al., Phys.Rev. C45, R892 (1992). (Urbana), K.T.Knopfle (Heidelberg), A.M.Nathan (Ur- 19. H. Friedrichs et al., NucLPhys. A553, 553c bana), A.Richter (Darmstadt), and M.Schumacher (1993). (Gottingen). It is a pleasure for me to thank a11 col- 20. I. Bauske et al., Phys.Rev.Lett. 71, 975 (1993). leagues for the very fruitful and stimulating collabora- 21. A. Zilges et al., Phys.Rev.Lett. 70, 2880 (1993). tion and for their really essential contributions to re- 22. Th. Weber et al., Phys.Lett. 215B, 469 (1988). sults presented in this report. Special thanks are due 23. H. Herminghaus, Nucl.Instr. Meth. 138, 1 to my longterm Stuttgart and Giessen collaborators (1976). R.D.Hei1, J.Margraf, H.H.Pitz, F.Steiper, Th.Weber, 24. R. Ratzek et al., Z.Phys. A308, 63 (1982). C.Wesselborg, W.Wilke, and A.Zilges, R.-D.Herzberg 25. U .E.P. Berg and U. Kneissl, Ann.Rev.Nucl.Part .Sci. (Koln) for their outstanding engagement and enthu- 37, 33 (1987). siasm which enabled the realization of the various 26. F. Gonnenwein, Nucl.Phys. A502, 159c (1989). projects. The extensive and continuous financia1 sup- 27. J. Drexler, J. et al., Nucl.Instr. Meth. Phys. Res. port by the Deutsche Forschungsgemeinschaft is grate- 220, 409 (1984). fully acknowledged. 28. W. Wilke et al., NucI.Instr. Meth. Phys. Res. A272, 785 (1988). 29. R.D. Hei1 et al., Nucl.Phys. A506, 223 (1990) 30. B. Schlitt et al., Nucl.Instr. Meth. Phys. Res. 1. H. Überall, Electron Scattering from Complex Nu- clei (Academic, New York, 1971) . A337, 416 (1994). 2. L. W. Fagg and S. S. Hanna, Rev. Mod. Phys. 31. H.H. Pitz et al., Nucl.Phys. A492, 441 (1989). 32. P.A. Dickey and P. Axel, Phys.Rev.Lett. 35, 501 31, 711 (1957). (1975). 3.T. W. Donnelly and J. D. Walecka, 33. L.J. Lindgren et al., Nucl.Phys. A298, 43 (1978). Ann.Rev.Nucl.Sci. 25, 329 (1975). 34. L.J. Lindgren et al., Z.Phys. A285, 415 (1978). 4. Th. Weber et aI., Nucl.Phys. A510, 1 (1990). 5. R. Vandenbosch and J.R. Huizenga, Nuclear Fis- 35. J.D.T. Arruda-Neto et al., Nucl.Phys. A389,378, sion (Academic, New York, 1973). 1982). Brazilian Jo~rnalof Physics, vol. 24, no. 2, June, 1994

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