Dipole excitations in 48Ti studied by nuclear resonance fluorescence

Nuclear Physics North-Holland

A513 (1990) 29-42

DIPOLE

EXCITATIONS

IN 4’?I STUDIED

RESONANCE

BY NUCLEAR

FLUORESCENCE*

A. DEGENER, C. BLASING’, R.D. HEIL, A. JUNG, U. KNEISSL, PITZ, H. SCHACHT, S. SCHENNACH, R. STOCK and C. WESSELBORG’

H.H.

Institut fiir Kernphysik, Universitiit Giessen, D-6300

Giessen,

Fed. Rep. Germany

Received 15 November 1989 (Revised 2 February 1990) Abstract: Nuclear resonance fluorescence experiments have been performed on the nucleus 48Ti. Unpolarized bremsstrahlung of 4.1 and 11 MeV and partially linearly polarized bremsstrahlung of 12 and 14 MeV endpoint energy, respectively, served as the photon sources. The scattered photons were detected by high-resolution Ge y-spectrometers. Precise excitation energies, decay branching ratios, and ground-state decay widths of numerous previously unknown spin-l states have been extracted. Model-independent parity assignments could be achieved for the strongest transitions. The results are compared with data from previous (y, y’) work, from recent (e, e’) and (p, p’) experiments, and with theoretical predictions.

E

NUCLEAR REACTIONS: 48Ti ( y, y’), bremsstrahlung E, = E yx = 4.1 and 11 MeV (unpolarized); 12 and 14 MeV (partially linearly polarized); enriched target; measured: E,, I,., angular distributions, azimuthal asymmetries; deduced: levels, J, n, F,,, reduced transition probabilities.

1. Motivation The search for low-lying dipole excitations in fp shell nuclei, in particular for Ml strength, is of recent interest. As well known, the discovery of a new low-lying magnetic dipole mode as observed in 1984 by Richter and coworkers ‘) in highresolution electron scattering experiments on ls6Gd stimulated a large number of both theoretical and experimental work (see e.g. ref. ‘)). Since then, this predominantly orbital Ml excitation, commonly called “scissors mode” following the intuitive picture given by the two-rotor model 3*4), has been observed in numerous electron scattering (see e.g. ref. ‘)) and photon scattering experiments (see e.g. ref. ‘)) not only in the mass region of deformed rare-earth nuclei but also in deformed actinide isotopes “). A discussion of theoretical descriptions, schematic and microscopic models, and their relationships can be found in a recent publication ‘). l

Supported

by the Deutsche Forschungsgemeinschaft. Wild-Leitz GmbH, Wetzlar, Fed. Rep. Germany. address: Brookhaven National Laboratory, Upton, NY, USA.

’ Present address: * Present

0375-9474/90/$03.50 (North-Holland)

0

Elsevier

Science

Publishers

B.V.

A, Degener et al. / Dipole excitations

30

Already in 1985 Zamick “) pointed out that low-lying Ml excitations, at excitation energies near 3-4 MeV and of strengths in the order of B(Ml)t = lpi may be a more general phenomenon. In his single shell (f,,J calculations for the even-even Ti isotopes he predicted corresponding l+ states, however, with about equal orbital and spin cont~butions to the total strength “). Similar orbital to spin ratios have been calculated by Oda et af. lo) using an enlarged configuration space. In a subsequent paper Liu and Zamick ‘I) investigated the effect of different nuclear effective interactions on the energies and strengths of these Ml excitations. Following these predictions of strong low-lying Ml excitations in fp shell nuclei with open proton and neutron shells the Darmstadt group succeeded in detecting a Ml transition in 46Ti near 4.3 MeV and two transitions in 48Ti near 3.74 and 5.66 MeV fiefs. ‘2-‘4)] in high resolution electron scattering experiments. The observation of two Ml transitions in the axially asymmet~c rotator 48Ti [ref. 15)], as a splitting of the total orbital Ml strength at that time has been interpreted 12*13) due to the triaxially deformed nuclear shape 16,“). Today, detailed microscopic calculations of the strong Ml excitations in the even-even Ti isotopes (A = 44,46,48,50) are available 18). This description makes use of a deformed Woods-Saxon potential pius QRPA using a parameter free, self-consistent quadrupole force. The predicted RPA states have an overlap of 20-30% with the isovector scissors mode states. The ratio of orbital to spin contributions to the strength amounts to 0.4-0.7. These ratios can experimentally be determined by comparing (e, e’) data with the results of high resolution inelastic proton scattering 19*‘O). On the other hand, also low-lying, strong electric dipole excitations in 48Ti have been detected in electron scattering experiments 14).Within systematic (e, e’) studies the nature of these excitations observed in a wide mass range (48Ti, ‘64Dy, 232Th, and 238U) has been described in terms of a surface octupole vibration of deformed nuclei ‘I)_ Nuclear resonance fluorescence (NRF) with its high sensitivity offers the possibility to search for the fragmentation and fine structure of low-lying dipole strength of both electric and magnetic character, since in addition to their high selectivity to dipole transitions NRF experiments enable model independent parity assignments by using polarized photons. We started our NRF investigations of fp shell nuclei 22) with experiments on 48Ti since for this isotope, as discussed above, detailed calculations and data from experiments using other probes are available. 2. Experimental 2.1. NUCLEAR

RESONANCE

technique and set-ups

FLUORESCENCE

Nuclear resonance fluorescence (NRF) expe~ments represent an outstanding tool to investigate dipole excitations due to the well-known mechanism of the electro-

A. Degener ef al. / Dipoie excilaiions

31

magnetic interaction, its high selectivity to excite low-spin states (mainly dipole excitations) and the sensitivity achievable when using present day high-resolution y-spectroscopy 23). The scattering cross section integrated over one level is given by

Here iT is the reduced wavelength of the absorbed photon; W(e) represents the normalized angular distribution; Jo and J are the spins of the ground and excited states, respectively; and K,, r, fr are the ground-state decay width, the total width, and the decay width to the final state. In the case of elastic scattering (r, = r,) the scattering cross section is propo~ional to f,$‘K If the decay to other states can be observed or is known, then the ground-state width r, can be determined. r, is proportional to the reduced transition probability B(XL, E,) (h = E or M): r,=87r

m (L+ I)(E,/RC)2L+’ 2.&J+1 C E [NE& L[(2L+ 1)!!]2 L=l

ET)+ NM.& &)I.

(2)

For spin assignments it is sufficient, at least in the favourable case of even-even nuclei with pure dipole or quadrupole cascades (spin sequences O-I-O and O-2-0, respectiveiy), to measure the scattered photons at two different angles. Preferable are scattering angles of 6 = 90” and 127”. The intensity ratio W{90”)/ W(127”) amounts to 0.734 and 2.28 for dipole and quadrupole transitions, respectively. These values are slightly diminished for realistic geometries used in the experiments. Parity assignments are of crucial importance for the interpretation of the observed dipole excitations. IModel independent parity assignments can be achieved in photon scattering experiments by investigating polarization observables. The basics of polarization measurements in photon scattering have recently been summarized by Heil ef al. 24)Following the notations of the pioneering review by Fagg and Hanna *‘). There are, in principle, two methods available: (i) The measurement of the linear polarization of the scattered photons; (ii) The use of linearly polarized photons in the entrance channel. The linear polarization of scattered photons can be measured using Compton polarimeters. Recently, modern high-efficiency polarimeter set-ups 24)enabled parity assignments of low-lying dipole excitations in the energy range of the so-called Ml “scissors mode” near 3 MeV in heavy deformed nuclei 24,26).In the present experiments, however, we were interested in dipole excitations of excitation energies up to -10 MeV. Due to the decreasing polarization sensitivity of the Compton scattering process with increasing photon energies it is more efficient to use the second technique, i.e. polarized photons in the entrance channel. This method has been applied successfully in numerous photon scattering experiments 23) at the Giessen electron linear accelerator using partially linearly polarized off-axis bremsstrahlung. The azimuthal asymmetry in the angular distribution of the scattered polarized

32

A. Degener et al. / Dipole excitations

photons is given by (3)

Here NL,,, denote the numbers of photons scattered perpendicular and parallel to the polarization plane defined by the directions of the incoming photon beam and the electric field vector of the linearly polarized photons. P, is the degree of polarization of the photon beam. The analyzing power Z( 6) is maximal at a scattering angle of e = 90” for spin cascades O-1-0 and O-2-0, and amounts to +I for El and -1 for Ml and E2 transitions, respectively.

2.2. EXPERIMENTAL

SET-UPS

Most of the present experiments on 48Ti have been performed at the polarized bremsstrahlung facility installed at the 65 MeV Giessen electron linear accelerator 23). The high average currents (~300 FA) of this conventional linear accelerator (duty cycle 0.12%) enables the production of partially linearly polarized photons (P, r= 1530%) of reasonable fluxes by the selection of off-axis bremsstrahlung. The degree of polarization as a function of the photon energy can be measured simultaneously with the NRF experiments via the photodisintegration of the deuteron. The azimuthal asymmetry of the angular distribution of the scattered photons is measured by a set-up of 4 Ge detectors installed at azimuthal angles of d, = 0”, 90”, 180”, 270” and a scattering angle of 8 = 90” [ref. “)I. This fully symmetric arrangement together with the possibility to switch frequently the polarization in first order cancels all apparative asymmetries. In addition, energy spectra and angular distributions of the scattered photons have been measured using a set-up of four Ge detectors at scattering angles of 0 = 90” and 127”. The range of low excitation energies (E* ~4.1 MeV) has been investigated in photon scattering experiments performed at the bremsstrahlung facility installed at the Stuttgart Dynamitron Accelerator, which provides a very intense CW electron beam with a maximum current of s4 mA at energies up to 4.3 MeV. Three Ge y-spectrometers were used in the NRF experiments (without polarization sensitivity) to measure the energy spectra and angular distributions of the scattered photons. The bremsstrahlung facility and the detector arrangement are described in detail in ref. *‘).

2.3. CALIBRATIONS

2.3.1. Targets. The targets consisted of 5 g of TiO, powder, pressed into discs of appropriate diameters (2-3 cm). In the experiments at the Stuttgart facility (bremsstrahlung endpoint energy E0 = 4.1 MeV) the TiO, powder additionally was

33

A. Degener et al. / Dipole excitations

sandwiched below).

between

Aluminum

The target thickness

discs

used

for the photon

has been determined

by weighing.

flux calibration The isotopic

(see enrich-

ment of the 48Ti target material amounted to 97.8%. 2.3.2. Energy calibrations. The energy calibration of the Ge -y-spectrometers been

performed

using

a 56Co source

emitting

-y-quanta

of various

energies

has up to

-3.5 MeV. The calibration for higher energies could be obtained by a 13C(n, (Y)‘~O* source emitting a 6.129 MeV y-transition. Furthermore, the 6.917 MeV E2 transition in I60 excited

in the

NRF

experiments

(oxide

target)

delivered

an additional

high-energy calibration point. 2.3.3. Photon jlux calibration. Since absolute photon flux calibrations of an adequate precision represent a very difficult task all our experiments have been performed as relative measurements. In the experiments at the Giessen linear accelerator reported here (E. = 11 MeV) all transition strengths have been normalized to the well-known 6.917 MeV E2 since TiOz target material has transition in 160 [ref. ‘“)I excited simultaneously, been used. For the spectral shape of the bremsstrahlung spectrum a “Schiffspectrum” 29) h as b een assumed. This assumption has been verified experimentally for the geometry used in Giessen in previous measurements of the bremsstrahlung spectra via the deuteron photodisintegration 30). In the experiments at Stuttgart (E. = 4.1 MeV) the transition strengths have been normalized to well known transitions in 27A1 with transition energies up to -4 MeV [ref. ‘I)]. For the spectral shape of the low energy, thick target bremsstrahlung spectrum a linear shape near the endpoint energy E. has been assumed. This assumption has been checked by measurements of the bremsstrahlung spectra using a composite target consisting of different isotopes with known transitions 32) and by Monte Carlo simulations using the program GEANT3 33).

3. Results Fig.

1 shows

experiments

a typical

at the Giessen

pulse facility

height

spectrum

as measured

(E. = 11 MeV). Numerous

in the 48Ti(y, y’) peaks are evident

in

the excitation energy range 3.6-8 MeV. The transitions are labelled according to table 1 and the corresponding excitation energies are given for the strongest transitions. Single- and double-escape peaks are marked by one and two primes, respectively. In total, transitions from 31 levels in the excitation energy range 3.7-10 MeV could be detected, furthermore, 4 transitions corresponding to inelastic photon scattering (transitions to lower lying excited states) were observed. In fig. 2 the low-energy parts of the 48Ti(y, y’) spectra measured at Giessen (E. = 11 MeV, upper part) and at Stuttgart (E. = 4.1 MeV, lower part) are compared. The advantage of a considerably enhanced peak-to-continuum ratio when using a

34

A. Degener et al. / Dipole excitations

1500

1000

500

Energy [keV] Fig. 1. 48Ti( y, y’) spectrum measured at the Giessen bremsstrahlung facility (E,= the peaks correspond to the numbering in table 1. For the stronger peaks transition

lower bombarding energy for the demonstrated. The results of the angular W(90”)/ W(127”)) are summarized 48Ti are of dipole character, besides of 2716 keV corresponds to inelastic

investigation

of low-lying

11 MeV). Labels at energies are given.

transitions

is clearly

measurements (intensity ratios distribution in fig. 3. All observed transitions belonging to a weak E2 transition at 6236 keV (the transition photon scattering; decay of the 3699 keV level

to the first excited 2+ state at 983 keV). Parities could be determined for the stronger asymmetries E of these transitions are plotted

transitions. The measured azimuthal in fig. 4. The dashed lines show the

expected asymmetries for pure El and Ml transitions (negative and positive parities, respectively). The data points are nicely grouped in the upper or lower part of the figure thus enabling unambiguous parity assignments in most cases. The l+ state at 3738 keV ‘is fed from higher lying states in these experiments at bremsstrahlung endpoint energies of 12 and 14 MeV as could be shown by measurements using the lower bremsstrahlung endpoint energy of 4.1 MeV. This explains the somewhat reduced asymmetry observed for this transition. In table 1 the results are summarized and spin and partially parity assignments are given. Most of the observed dipole transitions have not been known so far. The distributions of the detected El and Ml strengths are shown in fig. 5. For the sake of clarity only the stronger transitions are depicted where parity assignments could be achieved.

35

A. Degener et al. / Dipole exciiations TABLE 1 Transitions

No.

1

2 3 4 5 6 7 8 9 10 11 12 13 14 1.5 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35

Energy

[keV]

9976 * 6 9024 f 5 8995 * 5 8932 * 5 8671*4 8591*4 8571*4 8254*4 8198*4 8009 * 4 7968 zt 4 7585*4 7483 * 4 7449 zt 3 7221* 2 7123i3 7109*5 7070*4 7040 * 4 6978 * 3 6604 * 3 6236 f 3 6138*4 6126*3 6086 f 4 5640 * 2 5620 f 4 5526*3 5340 * 3 4655 * 3 4310*2 37394 1% 3700* 1* 2755~t l* 2716* l*

Ti/T

in @Ti observed

[meV]

400 * 230 660 f 250 300* 140 200* 120 450* 180 610*220 300* 130 320* 130 170*90 340* 130 190* 110 880 * 380 16Ozt 100 320+ 110 1260*480 630 f 230 240+ 110 4001150 110*70 380* 140 520* 180 70*40 90*40 160*60 110*40 200 * 70 70*30 80+30 70120 58+7* 12*2*

in the present

W( 0 = 90”) W(O=127”) 0.79 * 0.25 0.71* 0.09 0.65*0.16 0.67 zt 0.24 0.76*0.11 0.615 0.08 0.80*0.17 0.65+0.11 1.22*0.31 0.66 * 0.09 0.74*0.17 0.66+0.12 0.94*0.16 0.86*0.10 0.72 * 0.03 0.74* 0.05 0.66*0.12 0.66 * 0.22 1.15*0.37 0.71 f 0.06 0.71 *to.04 1.73 * 0.47 0.75*0.16 0.73 * 0.10 0.93*0.14 0.69 * 0.06 1.11*0.22 0.8410.16 0.99*0.14 0.84 i 0.22 0.56+0.10 0.87 * 0.08* 0.29*0.11* 0.82+0.11* 1.00*0.12*

work

E [%]

46zt41 -13148 -24*35 12*54 6*51 11*41 38*47 0140 25*46 17*39 2*34 33*41 18*37 33*10 -14*4 43*15 30*50 -17zt 18 -9161 30*7 23+4 -30+42 21+52 4140 -26* 10 -8*27 -5zt48 26*33 -10157 -17*11 -9+22 17*35 8*31 1146

J”

Remarks

11 1(+)

a

1 1 1 1’-’

a

1 L2 1 1 1’-’ 1 11+ 11 1+ 1,2 112+ l(+) 1 1 1+ inel. 1 1’-’ inel. 1+ 1+ 1’-’ inel. inel.

31 “1 1 1 “1 “1 a 1 a 1 1 1 a

a a

1; a

C1

“1 a ) a

)

a

4 7 1 9 CI 7 a

a

“) 1

a

7 7 bs

1

hc

“) ?

The quoted errors contain both statistical and systematical errors. The systematical error due to the assumption of a “Schiff-spectrum” for the relative shape of the bremsstrahlung spectrum has been estimated to be +3% (see text). Data marked by an * have been taken at a low bombarding energy E, = 4.1 MeV. In these experimental runs a better absolute energy calibration could be achieved and, furthermore, the transition strengths could be determined since no feeding from higher lying states is to be expected. The linear interpolation used for the relative photon flux determination in these experiments causes a systematic error estimated to be *lo%, included in the quoted errors for the transition strengths. “) Transitions from levels unknown up to now. h, Transitions from levels known from previous ( y, y’) work 35,36). ‘) Transitions from levels known from quite recent (e, e’) work *“). d, Transitions from levels known from particle-induced reaction studies 34).

A. Degener ei al. / Dipole excitations

36

s 2500 24 a

,

2000

s1500 2 g u

1000 500 0

”

$

100

0

3000 Energy

3500

4000

[keV]

Fig. 2. Comparison of %( ‘y, yf) spectra measured at the Giessen linear accelerator (E,= 1I MeV) and the Stuttgart Dynamitron (i&=4,1 MeV). SE and DE labeled peaks correspond to single and double escape peaks, respectively.

4. Discussion 4.1. COMPARISON

WlTH PREVIOUS NRF WORK

Two NRF experiments on 4BTi had been performed in the past 35*J6).In these experiments only two low-lying states near 3.7 MeV and the transition at 6.605 MeV were observed. Already in 1975 Rasmussen investigated levels in ‘“-50Ti in the excitation energy range 1.5-4.3 MeV by photon scattering using electron bremsstrahlung, Ge(Li) detectors and a natural Ti target 35). Parities have been assigned to two transitions by linear polarization measurements using a Compton polarimeter of modest efficiency (Ge(Li) two-slab arrangement 37)). In the energy range of interest Rasmussen observed two dipole transitions at 3700 and 3739 keV. For both he assigned

37

A. Degener et al. / Dipole excitations

3.0

2.5

4000

2000

8000

6000

Energy

10000

[keV]

Fig. 3. Intensity ratios W( 0 = 90”)/ W( 0 = 127”) together with the expected values for pure dipole and quadrupole cascades, respectively (spin sequences 0-1-O and 0-2-O).

a positive parity (for the 3700 keV transition only tentatively). The reported energies and transition strengths of both transitions agree well with the results of the present epxeriments (see table 2). This gives confidence in the energy and photon flux calibrations of both experiments. However, the positive parity assignment for the 3700 keV level could not be confirmed in the present experiments. In the other previous NRF experiment on 48Ti Moreh et al. 36) used a quite different

technique,

i.e. monochromatic

photons

from (n, y) sources.

The random

overlap of the energies of the incident photons with the 6605 keV state in 48Ti enabled the photoexcitation of only this level. The decay properties of this state have been measured. The negative parity could be assigned by using a Compton polarimeter. The data of the present work. calibration at higher 6917 keV transition in

4.2.

reported by Moreh et al. are in a good agreement with those In addition this confirms the reliability of the photon flux excitation energies by normalization to the strength of the I60 as outlined above.

El TRANSITIONS

In fig. 5 (right-hand part) the strengths of the El transitions identified in the present experiments are shown. Obviously there is a strength concentration in the energy range 6-8 MeV. As already mentioned the strong 6605 keV transition has

A. Degener

38

et al. / Dipole excitations

1.0

!a

-0”

k?

0.5 ===z====

g Q)

E

E

2

!f

‘I =======

i

3 45;

II

=====

II

0.0

-0.5

” t -

8

-

A

===--___ ---__

__-_===I

=s====s=== 0

II-

‘; ‘I i’ ’ g.

(“=-js

;,

I

1

_ _--== _--

==,

I

3

d

8 8

;

;,

-1.0

=

3

8 I

+

I

I

I

I

I

I

I

4000

5000

6000

7000

8000

9000

10000

Energy [keV] Fig. 4. Azimuthal asymmetries polarized photons. The dashed transitions (lower

(

for the strongest dipole transitions measured using partially linearly lines show the expected asymmetries for pure El (upper part) and Ml part) and correspond to the degree of linear polarization.

48Ti Ml

‘------I

Energy Fig. 5. Ml and El strength distributions a tentative parity

48Ti El

[keV]

in 48Ti observed in the present experiments. assignment are shown as shaded bars.

Transitions

with

39

A. Degener et al. / Dipole excitations TABLE

Comparison Energy

Jw

lkevl

of the experimental

i r’m$ ro/(ro+rl)

2

data with the results

of previous

r.

ro/r

lmevl

BW)f [ph]

NRF experiments

B(El)T [10e3 e2 fm’]

3700* 1 370011

*‘+I 1’-’

10*1 12*2

0.48 * 0.3

20.4*2.3

1.2*0.1

0.44 f 0.03

0.42*0.03’)

28.514.5

1.6*0.3

3739*1 3739*1

1+ 1+

58*7

0.68 f 0.02

0.64 f 0.02 ‘)

101 f 10 91*11

6605”) 6604i3

1l-

0.86 * 0.02

0.75 b) 0.86 f 0.02

645*150 605*210

484* 520*

113 180

Ref. 35 ) this work 351 this work

0.50*0.05 0.45 f 0.06 6.4+ 1.5 6.0*2.1

361 this work

“) No errors are quoted by Moreh et al. 36). However, this transition energy is known very precisely from a ‘OV(n, y) work =) to be 6604.6 * 0.2 keV. ‘) No errcw quoted. ‘) In the present experiments for these low-energy levels only the ground-state decay (decay width r,) and the decay to the first excited 2+ state (decay width I‘, = r,+) could be detected. Therefore, an upper limit of 5% for a decay branching to higher lying excited states has been assumed (see ref. 27)).

been investigated previously by Moreh et al. 36). The other transitions could not be detected by Moreh et al. since these authors used monoenergetic photons of fixed energy from (n, y) capture reactions. For the low-energy transition at 3700 keV we tentatively assigned a negative parity in contrast to the work of Rasmussen 35). This assignment has been supported by recent (e, e’) experiments 2’). The excitation energy and strength observed in both the electron and photon scattering experiments are in a good agreement (see table 3). The electron scattering form factor could be explained in terms of an octupole vibration coupled to a deformed quadrupole spheroid 2’). A further experimental hint in favour of this explanation comes from the strong population of the first 2+ state at 983 keV in the decay of the 3700 keV I- level observed in the present NRF experiment [To/T = 0.42 f 0.03 corresponding to r2+/ r,+ = 1.27 f 0.17 and a ratio of the reduced transition probabilities of B( 1- + 2’)/ B( 1- + Of) = 3.21 f 0.431. This decay behaviour is quite different from those observed for the higher lying l- levels which predominantly decay to the ground state (see table 2). TABLE

3

Comparison of the experimental data of the 3700 keV El transition observed in recent (e, e’) experiments *‘) and in the present (y, y’) work Experiment (x Y’) (e, e’)

4.3. Ml

Energy [keV] 3700 3702

B(El)f

[10m3 e* fm’] 1.6kO.3 12

TRANSITIONS

In fig. 5 (left-hand part) the strengths of the Ml transitions identified in the present experiments are shown. The experimental data are compiled in table 4

40

A. Degener et al. 1 Dipole excirations TABLET

Comparison of Ml excitations observed in f y, y’) (this work), (e, e’) and fp, p’) experiments *O) ( y, y’) this work

(e, e’)

(P, P’)

energy

B(Ml)f

energy

[Mevl

[!&I

[Mevl

M-41

tMeV1

0.45 * 0.06 0.39*0.13 0.59 f 0.20 0.1010.04

3.741 4.263 5.64

0.50* 0.08 0.24*0.10 0.50 rt 0.08

3.74 4.26

125*45 95k30

6.38 6.79 6.97

7o;t7 7Oi=? 143 * 14

7.20 7.40 7.58 7.75 8.15 8.30

270 f 20

(8.86-9.00) 9.26

490 f 50 220*20

3.139 4.310 “) 5.640 6.138*

7.070 7.221

0.30*0.11 0.87 i 0.33

7.22

8.37 8.45 8.995*

O.llf0.05

B(Ml)t

0.80 f 0.06

0.10~0.04 0.10*0.03

energy

(do/da)

W/4

90* 10 114111 110*11 189+20 316*43

Transitions marked with an * have only tentatively assigned Positive parities. “) For the calculation of the B(Ml)f value experimental branching ratios 34) have been considered.

together with the results of recent (e, e’) and (p, p’) experiments ‘O). The good agreement for the low-energy transitions and the strong excitation at 7.22 MeV is clearly evident. In fig. 6 the present (y, y’) results are compared with the theoretical predictions of Nojarov et ~2.39) and Faessler et al. ‘*) and with electron scattering data *O).Both calculations predict remarkable Ml strengths near -4.5 and -6 MeV in fair agreement with the experimental data, even if the predicted strengths seem to be overestimated in these calculations. However, the strong transition at 7.22 MeV observed in the experiments is not reproduced by the calculations. According to the (e, e’) and (p, p’) experiments and the shell model calculations performed for their analysis *‘) the low-energy states around 4 MeV should correspond to isovector Ml excitations partially excited by the orbital electromagnetic interaction [the QRPA calcuhtions “) give ratios of orbital to spin contributions of 0.4 to 0.71. On the other hand, the transition at 7.22 MeV strongly excited in (y, -y’), (e, e’) and (p, p’) experiments is a nearly pure spin-flip excitation ‘O). The authors thank Prof. Dr. A. Richter and his collaborators Th. Guhr and H.J. Wijrtche for stimulating discussions. Thanks are due to W. Arnold and his team for their engaged help at the Giessen linear accelerator. The authors are gratefully indebted to Prof. Dr. K.-W. Hoffmann and his collaborators for the warm hospitality at the Jnstitut fiir Strahlenphysik at Stuttgart University and their kind support. The

41

A. Degener et al. f Dipole excirations

,

!

I

3

’ Theory 1 .o

[Nojarov

’

’

et al.]

0.5 0.0

Theory 1 .o

[Faessler

et al.]

I

B

0.5 I

0.0

I

I

1

I

I

I

Experiment [(e,e’)-data,

1.0

Willis et al.: I-

0.5

0.0

-

i

I

n I

Expeiiment

1 .o

[(y,y’)-data,

this

’ work]

0.5

0.0

-

4000

6000

8000

Energy [keV] Fig. 6. Comparison of theoretical predicted Ml strengths [Nojarov et al. 39) and Faessler et al. ‘*)I and experimental data [(e, e’), ref. ‘Of; ( y, y’) present work].

financial support by the Deutsche Forschungsgemeinschaft edged.

is gratefully acknowl-

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