MODEL - INDEPENDENT DETERMINATION OF M1 STATES BY NUCLEAR RESONANCE FLUORESCENCE

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MODEL - INDEPENDENT DETERMINATION OF M1 STATES BY NUCLEAR RESONANCE

FLUORESCENCE

U. Berg

To cite this version:

U. Berg. MODEL - INDEPENDENT DETERMINATION OF M1 STATES BY NUCLEAR RES- ONANCE FLUORESCENCE. Journal de Physique Colloques, 1984, 45 (C4), pp.C4-359-C4-373.

�10.1051/jphyscol:1984428�. �jpa-00224094�

JOURNAL DE PHYSIQUE

Colloque C 4 , suppi6rnent au n03, Tome 45, mars 1984 page C4-359

MODEL - INDEPENDENT DETERMINATION OF M 1 STATES BY NUCLEAR RESONANCE FLUORESCENCE

U.E.P. Berg

Justus-Liebig-Universitat, StrahZenzentrum, Institut fiir Kernphysik, 0-6300 Giessen, F.R. G .

Rgsumg - La t e c h n i q u e de f l u o r e s c e n c e p a r rgsonance n u c l g a i r e au moyen d ' u n f a i s c e a u de photons d e b r e m s s t r a h l u n g p o l a r i s 6 s lingaire- ment e s t p r g s e n t g e c o m e une n o u v e l l e mgthode d e d g t e r m i n a t i o n de l a n a t u r e 1' ( M I ) d ' u n c e r t a i n nombre d ' g t a t s l i e s du noyau. On dgcrira briGvement comment l e s s p i n s , p a r i t g s , l a r g e u r s d ' g m i s s i o n e t mglanges de t r a n s i t i o n s m u l t i p o l a i r e s peuvent 6 t r e o b t e n u s dans

l m g t u d $ d e l a d i f f u s i o n d e photons. Des mesures r g c e n t e s , 6 0 ~ i , 9 0 ~ r , 2 0 8 ~ b (y , y ) m o n t r e n t que d e t r G s f o r t e s e x c i t a t i o n s E l s o n t p&sentes

dans l e domaine a t t e n d u pour l a rgsonance MI; e l l e s s e r v i r o n t d ' i l l u s t , r a t i o n d l a mgthode. E n f i n l e s r g s u l t a t s d ' u n e e x p g r i e n c e 2 6 ~ g ( y , y ) s e r o n t p r g s e n t g s e t m o n t r e r o n t comment l e s f o r c e s d e tran- s i t i o n MI e n t r e & t a t s e x c i t g s du noyau c i b l e peuvent S t r e mesurges.

A b s t r a c t - The a p p l i c a b i l i t y o f n u c l e a r r e s o n a n c e f l u o r e s c e n c e u s i n g l i n e a r l y p o l a r i z e d b r e m s s t r a h l u n g a s photon s o u r c e i s d i s - c u s s e d a s a new method f o r t h e d e t e r m i n a t i o n o f bound M I s t a t e s . I t i s b r i e f l y mentioned how s p i n s , p a r i t i e s , decay w i d t h s and mixing r a t i o s of m u l t i p o l e t r a n s i t i o n s c a n be t a i n e d i n photon s c a t t e r i n g . Recent 6 0 ~ i (7, y) , 'OZI- ( B , ^{y )} and 28BPb ('if y ) measurements, which show t h a t s t r o n g El e x c i t a t i o n s a r e p r e s e n t i n t h e r a n g e o f the M I r e s o n a n c e , a r e d i s c u s s e d a s examples f o r t h e method. R e s u l t s from a 2 6 ~ g ( y , y ' ) experiment a r e p r e s e n t e d t o d e m o n s t r a t e how MI s t r e n g t h s between e x c i t e d s t a t e s can be measured.

1. I n t r o d u c t i o n

One of t h e t o p i c s of t h i s symposium i s t h e i n v e s t i g a t i o n of magnetic d i p o l e e x c i t a t i o n s and r e l a t e d Gamow-Teller t r a n s i t i o n s . There a r e s e v e r a l i m p o r t a n t c o n t r i b u t i o n s t o t h i s c o n f e r e n c e a b o u t d i f f e r e n t e x p e r i m e n t s t o l o c a t e M I s t r e n g t h i n an unambiguous way.clkle of t h e f a c e t s i n t h e mosaic of i n f o r m a t i o n o b t a i n e d a b o u t M I s t a t e s i n n u c l e i stems from n u c l e a r r e s o n a n c e f l u o r e s c e n c e (NRF). The e l e c t r o m a g n e t i c i n t e r a c t i o n i s w e l l u n d e r s t o o d a n d , t h e r e f o r e , photon s c a t t e r i n g i s a n i m p o r t a n t t o o l f o r s t u d y i n g n u c l e a r t r a n s i t i o n s o f low momentum

t r a n s f e r .

I n t e r e s t i n g c o n t r i b u t i o n s i n t h e f i e l d o f N R F come from e x p e r i m e n t s w i t h monoenergetic photons i n r a d i a t i v e n e u t r o n c a p t u r e r e a c t i o n s / 1 , 2 / . Another p o s s i b i l i t y t o o b t a i n monoenergetic h i g h energy y-rays a r e ( p , y ) r e a c t i o n s . NRF e x p e r i m e n t s u s i n g t h e s e photons a r e s u c c e s s - f u l l y c a r r i e d o u t a t Groningen / 3 , 4 / . The ( p l y ) photon i n t e n s i t i e s a r e s e v e r a l o r d e r s of magnitude s m a l l e r t h a n i n ( n , y ) r e a c t i o n s a t h i g h f l u x r e a c t o r s , b u t ( p l y ) r e a c t i o n s y i e l d photons which a r e l i n e a r l y p o l a r i z e d . P o l a r i z e d photons a r e n e c e s s a r y , i f p a r i t i e s of e x c i t e d s t a t e s s h a l l be d e t e r m i n e d .

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984428

C4-360 JOURNAL DE PHYSIQUE

The type of measurements I will focus on in this talk are NRF ex- periments in which the incoming photon beam has a continuous fre- quency spectrum. This has the advantage that all states with large ground state decay widths can be excited simultaneously and one is not limited by random overlap of incident y-ray energy and excitation energy of a particular state. Gn the other hand, bremsstrahlung pro- duced by electrons is one of the most intense photon sources (-109y/s).

Pioneering work in photon scattering of bremsstrahlung on low energy nuclear levels has been carried out by Metzger / 5 , 6 / . With the ad- vance of high current electron accelerators and with electron energies of more than 10 MeV, bound states in the region of the magnetic dipole resonance became accessible to NRF. Therefore, we nowadays have the possibility to study MI excitations with the high precision of y-ray spectroscopy.

2. Nuclear Resonance Fluorescence

The NRF method is described in a couple of review articles /5,7,8/

and only a short summary shall be given here for a better under- standing of results discussed later.

2.1. NRF USING UNPOLARIZED BREMSSTRAHLUNG

If a nucleus is irradiated by a continuous photon spectrum it will act like a monochromator, namely pick out photons with energies corres- ponding to the excitation energy of an excited state and then the nucleus will deexcite and emit monochromatic y-rays. These scattered photons are detected with the help of high resolution Ge(Li)detectors.

Fig. 1. Excitation process in a NRF experiment and subsequent decay of the excited state to a final state, which can be an ex- cited level or the ground state. The deexcitation y-ray of multipolarity L2 or mixed multipolarity L2 and L2' is ob- served in a NRF measurement.

As indicated in Fig. 1 ground state decay widths To of highly excited states, their spins, y-branching ratios ro/r, and mixing ratios 6 of multipolarities L contributing to a transition can be obtained in NRF experiments. If thin targets are used, selfabsorption is negli- gible, and the ground state decay width To of a state at excitation energy Ex can be directly calculated from the measured photon scattering intensity (cross section integrated.over a resonance;

dimension MeV . ^mb)

The threshold for particle emission, especially the neutron th esholdr 5

limits NRF to bound states, because usua1:y rn>>r0 and henc? r.o /f < < 1.

For elastic scattering the directly measured quantity is To /r, where

T is the total width of a level.

The information about the spins of the levels, which are involved in the scattering process, and the multipolarities of the transitions is contained in the angular distribution function kg/.

W(O) is plotted in Fig. 2 in polar coordinates for the case of pure dipole and quadrupole cascades in a spin J=O target nucleus. It is obvious from Fig. 2 that it is sufficient to measure photon scattering at only two angles (0=90° and 127') to determine the multipolarity of elastically scattered photons by a spin J=O target.

- detector s

Fig. 2. Angular distribution of dipole and quadrupole scattering on a J=O nucleus. W(0) is independent of the internal structure of a transition.

If inelastic photon scattering occurs on a spin J = 0 target, mostly the final state will be +=he first excited 2 ' level. In this case the angular correlation coefficient Av (2) in equ. (2)

for the deexcitation y-ray depends oq the ratio 62 of the matrix elements of the multipoles L2 and L2 = L2 + 1 contributing to the deexcitation process.

It is, therefore, possible to determine the matrix elements of

different multipoles involved and their relative sign in a NRF experi- ment. An example will be given at the end of this talk.

2.2. NRF USING LINEARLY POLARIZED BREMSSTRAHLUNG

Parities of excited states can be determined model-independently in NRF experiments with linearly polarized photons. The angular distribution W(O) for scattering of unpolarized photons has to be substituted by /9/

w (Or@) = W(O) + (f) cos 24 '

1 ( 1 + 6 ~ 2 ) (1+82 2, bLv (I) .Av (2) ) ' ( (~080) ( 5

C4-362 JOURNAL DE PHYSIQUE

where Lv ( I ) is the angular correlation coefficient for linearly polarized photons in the entrance channel. @ is the azimuthal angle between the polarization plane (parallel to the E-vector of the incoming photons) and the reaction plyne. The information about the parity is contained in the factor ( z ) ~ ~ , which is + I for excitations

of odd parity and -1 for excitations of even parity. The angular distribution for elastic scattering of linearly polarized photons on a bound dipole state in a ground state J=O+ nucleus is plotted in Fig. 3 for a scattering angle of 0=90°. If the polarization of the incoming photons is less than 100 % , the terms containing cos2@

have to be multiplied by the degree of photon polarization Py.

In a NRF experiment with linearly polarized bremsstrahlung we measure the azimuthal scattering cross section with the help of detectors, which are placed at 0=90° perpendicular and parallel to the polari-

zation plane.

Fig. 3. Azimuthal angular distribution for elastic electric and magnetic dipole scattering of photons on a spin J=O nucleus.

The asymmetry masured by the photon detectors

is t"ne product of bremsstrahlung polarization Py at the energy of the

excited state times the analyzing power

of the reaction. Since the absolute value of C(O=90°) equals 1 for a spin 0+1+0 cascade, the magnitude of the measured asymmetry is determined by the degree of bremsstrahlung polarization and equals

> 0 for 0++1-+of transitions (El)

E = [ ^{< o for} o ~ + I ~ + o + transitions (MI) and 0 +2 +0+ transitions (E2)

These asymmetries, which will be shown later for some nuclei, are the basis for parity determination.

2.2.1. LINEARLY POLARIZED BREMSSTRAHLUNG

We have developed a method at Giessen to produce a linearly polarized bremsstrahlung beam by selectingthe off-axis part of the brems-

strahlung cone by a narrow collimator. This part is linearly polarized

deflection magnets Al radiator collimator e-- beam

BrernsstraMung polarization:

F i g . 4 . P r o d u c t i o n of l i n e a r l y p o l a r i z e d b r e m s s t r a h l u n g a t t h e Giessen l i n a c .

and can be used f o r NRF e x p e r i m e n t s . The p r i n c i p l e o f t h e e x p e r i m e n t a l arrangement a t t h e p o l a r i z e d b r e m s s t r a h l u n g f a c i l i t y o f t h e Univer- s i t y of G i e s s e n 6 5 MeV e l e c t r o n l i n e a r a c c e l e r a t o r i s s k e t c h e d i n F i g . 4 . The a n g l e between t h e i n c i d e n t e l e c t r o n s and t h e brems- s t r a h l u n g r a d i a t o r can b e s l i g h t l y changed v e r t i c a l l y and h o r i z o n t a l l y by d e f l e c t i o n magnets s o t h a t o n l y a p o l a r i z e d o f f - a x i s p a r t of t h e b r e m s s t r a h l u n g can p a s s t h e subsequent c o l ~ m a t o r . Maximum p o l a r i z a - _{t i o n} i s o b t a i n e d a t an a n g l e of a b o u t moc 3 /E,-'.

The d i r e c t i o n of p o l a r i z a t i o n i s i l l u s t r a t e d i n t h e lower p a r t o f F i g . 4 . The e l e c t r i c f i e l d v e c t o r of t h e b r e m s s t r a h l u n g photons i s p r e f e r e n t i a l l y a l i g n e d t a n g e n t i a l t o a c i r c l e around t h e d i r e c t i o n of t h e inm~ning e l e c t r o n s The r e q u i r e m e n t of a v e r y t h i n b r e m s s t r a h l u n g r a d i a t o r ( 2 5 pm A 1 a t G i e s s e n ) makes a h i g h c u r r e n t a c c e l e r a t o r n e c e s s a r y t o o b t a i n enough b r e m s s t r a h l u n g i n t e n s i t y . The a v e r a g e e l e c t r o n c u r r e n t of t h e Giessen l i n a c i s a b o u t 300 uA.

/; ^ON.

F i g . 5. P h o t o d i s i n t e g r a t i o n of N,, d e u t e r i u m by l i n e a r l y - , n- Detector p o l a r i z e d p h o t o n s . Neutrons lNE213 -Scintillator) a r e d e t e c t e d p a r a l l e l and

p e r p e n d i c u l a r - t o t h e

p o l a r i z a t i o n p l a n e . The P S D spectrum shows an

enhancement of t h e n e u t r o n y i e l d p a r a l l e l t o t h e p o l a r i z a t i o n p l a n e .

- Pulse Shape

C4-364 JOURNAL DE PHYSIQUE

The degree of bremsstrahlung polarization can be measured with help of the photodisintegration of the deuteron. After absorption of El radiation protons and neutrons are preferentially emitted in the direction of the E-vector as depicted in Fig. 5.Bremsst ahlung

olarization is determined at Giessen with the help of 'H(7.n) and P H ( ~ t p ) polarimeters. Neutrons and y-rays in the neutron detector are distinguished by pulse shape discrimination (PSD). PSD spectra are shown in the lower part of Fig. 5. Clearly, the number of neutrons N,, detected parallel to the polarization plane is larger than the number of neutrons NI observed by the detector located perpendicular to the scattering plane, while the yield of y-rays remains constant.

Fig. 6.

> 2000 2~ (T,p) spectra measured 2 parallel and perpendicular

In

to the polarization plane. . ^m

The lower part depicts $ ^I000

the derived bremsstrahlung 2

polarization. 8

1 2~ (<PI

I Epx= 15MeV

I _I 25wm Al-radiator

I

~ I

I plane

t ' I t E 1 sc. plane

'

+ + * 0

+****+;:,

+::::.*.**

**._

- ' i l l 1 1 1 1

' 1 1 1 1

lllljll

- - - - - - I!,,+,---:

2 ~ ( T , p ) measurements have the advantage that the energy dependence of zhe+bremsstrahlung polarization can be determined more easily than in H(y,n) experiments. Figure 6 shows proton spectra detected with the help of semiconductor telescope detectors parallel and perpendicular to the bremsstrahlung polarization plane. The bremsstrahlung

polarization, which was derived from the measured proton yields, is plotted in the lower half of Fig. 6. The polarization is zero at the endpoint energy of the bremsstrahlung spectrum (15 MeV) and rises lineasly+to a degree of polarization of 33 % at 7 MeV photon energy.

This H(y,p) measurement was performed simultaneously during a

( T I y) experiment, which will be discussed later.

The polarization effect in the (y,y) spectra is demonstrated in Fig. 7. + This NRF measurement was g~rformed with four Ge(Li) detectors at @=OO, 90°, 180' and 270' and a Zr02 target. Plotted are escape peaks of the 6296, 6425 and 6762 keV transitions in 9 0 ~ r and the double escape peak of a transition to the 2 ' state at 6917 keV in 160. The upper spectrum shows the sum of counts of the Ge(Li) detectors placed parallel to the polarization plane and the lower spectrum those per- pendicular to the E-vector.

All peaks in Fig. 7 which stem from excitations in 'Ozr are enhanced in the lower spectrum being evidence that El transitions are excited.

The only peak which is stronger in the spectra recorded parallel to

the polarization plane stems from a weak E2 transition to a 2 ' state (ro=0.12 eV) at 6917 keV in 160. It should be noted that the energy resolution in the spectra is very high. The FWHM of the peaks in Fig.

7 is of the order of 4.8 keV.

3. E x p e r i m e n t a ~ s u l t s and Discussion

NRF experiments using bremsstrahlung have been carried out in the whole range of the periodic table /6-8,10/. Nuclei in the Op-shell were studied at Urbana and at Sendai /11,12/. For the odd sd-shell nuclei we now have a very complete systematics of ground state transition widths from NRF experiments performed by Shikazono and Kawarasaki and by Vodhanel /13,14/. The even sd-shell nuclei were investigated with unpolarized and polarized photons at Giessen /15/.

? l scattering plane

Scattering of linearly polarg6ed bremsstrahlung

on a Zr02 target. _700- Transitions with negative 2

5700 5800 5900

ENERGY (keV?

90Zr02 (7.y)

E y m = 6 & V 5 2

N x

. ,

a

c .

,

-

6 > >

I I

+ E II SC pl

- ^{k 2}

Highly excited bound dipole states in heavier nuclei were studied at Urbana /16/, Sendai /17,18/ and Giessen. Besides one NRF experiment on 208pb /19/ all other scattering experiments using polarized brems- strahlung were performed at Giessen. So far, isotopes in the sd-shell and the following heavier nuclei were investi ated at our polarized bremsstrahlung beam: 5 2 ~ r /20/, 5 4 ~ e , 5 6 ~ e , 5 8 ~ i /21/, 6 0 ~ i , 888, /22/ ,

9 0 ~ r and 2 0 8 ~ b /23/.

parity are enhanced in the lower spectrum while transitions with positive parity are enhanced in -

the upper spectrum re- corded with Ge (Li) detec- tors perpendicular and parallel to the polari- zation plane.

3.1. FOUR EXAMPLES

Instead of giving a review on all the experiments, I rather prefer presenting four examples of most recent measurements because of time and s ace limitations. In this connection I have chosen 6 0 ~ i , 9 0 ~ r and 298Pb and a new prospect of inelastic photon scattering, namely the extraction of B(M1) values between excited states from a NRF experiment.

The even Ni isotopes have been recently studied by inelastic

scattering of 200 MeV protons at Orsay /24,25/. Proton inelastic

C4-366 J O U R N A L DE PHYSIQUE

scattering at very forward an les is very sensitive to MI excitations and the ( p I p l ) spectra from 58~60r62r64Ni show sharp peaks in the range between 7 and 10 MeV, where the T<=To component of the MI re- sonance is expected to be located. On the other hand, El transitions can also be excited in (p,pl) by Coulomb excitation and these cross sections are also very forward peaked. In order to find out the parities of dipole excitations in a model independent way, 'ONi (t,y) experiments were performed in the course of a Ph.D. thesis by Naatz and first results shall be reported here.

v: .,

LD -4,

- T,= -

W 0

r

. N O

m m

Ey,,=17 MeV -

Energy (MeV)

Fig. 8. Asymmetries of dipole states measured in the 6 0 ~ i ( ~ , y ) reaction.

The asymmetries measured in a 6 0 ~ i (Y + ,y) experiment with linearly polarized bremsstrahlung of 17 MeV endpoint energyare represented in Fig. 8. If we apply the definitions in eqs. 7 and 8r0nly three states at 7950 + ^{2, 9258} + ^{5 and 9301} + 2 keV show definitely a negative asymmetry implying (together with the measurement of the spin) that I + states are excited. The values I'$/T for the 1' levels are 0.66 +

0.10, 0.43 + 0.20 and 0.95 + 0.28 eV, respectively. If a branching ratio of I',/T= 1 is assumed, we calculate for the respective B(Ml)+

values 0.34 + 0.08, 0.14 + 0.07 and 0.31 + 0.09 kO2.

The sum ZB(M1) f of these three states equals 0.79 + ^{0.24 b and is}

one order of magnitude smaller than the sum rule value 8.4 bO2 calculated for the T, component of the MI resonance in "Ni with help of the Zipparini sum rule / 2 6 / . Even if the unrealistic assumption is made that all states, for which we could not determine the parity, are MI excitations~only about one half of the aforementioned sum rule value would be reached.

Figure 9 shows the part of the 6 0 ~ i (y , y ) spectrum, in which the MI transitions occured (peaks number 13,14 and 22). It is obvious, if we take into account the result from the asymmetry measurement in Fig. 8 that the main dipole strength in 6 0 ~ i observed in NRF is due

o El excitations. A detailed comparison structures observed in

ON^ ^{(p,pl) with 61} and MI strengths from "Ni(y,y) is difficult at

present and has to await a careful analysis in the near future.

0

70 7.5 8.0 8.5 9 0

Energy ( MeV)

# fl

2 s '

6 0 ~ i ( Y , Y ) Eym,, = 13 MeV

11"

X

Fig. 9. Energy part of the 6 0 ~ i (y, y) spectra, where MI excitations were observed (peaks number 13, 14 and 22).

The MI resonance in 9 0 ~ r is still a challenging problem. Inelastic scattering of 200 MeV protons revealed a pronounced bump between 8 and 10 MeV excitation energy /27,28/. The observed MI strength could be readily reproduced in terms of a simple shell model /29/. More recent 90~r(p,p') measurements at LAMPF by Nanda et al. /30/ with 318 MeV protons were able to resolve fine structures in this 2 MeV wide bump. In addition, a similar enhancement of cross section around 9 MeV was observed in elastic scattering of tagged photons at Urbana /31,32/.

High resolution NRF experiments at Urbana and Giessen with unpolarized photons revealed that the structures observed in the Zr(y,y) experi- ment with tagged photons ( energy resolution of about 100 keV) con- sist of a large number of individual transitions.

The results from the tagged photon measurements and the NRF experi- ment at Urbana are depicted in Fig. 10. The dotted line represents the convolution of the Zr(y,y) cross section data from Cannell /32/.

The photon scattering strengths from the 9 0 ~ r (y, y' ) experiments using Ge(Li) detectors /35/ were divided by the square of the ex- citation energy for the comparison with the tagged photon data. A measurement of the angular distribution yielded that at least the strongest transitions above 5 MeV are dipole excitations /33, 34/.

A "Zr (y, y) spectrum is plotted in Fig. 1 1 in order to give an

impression how dense the distribution of peaks is. This spectrum re-

presents the sum of spectra obtained with the help of four Ge(Li)

detectors during the two runs at Giessen with linearly polarized

bremsstrahlung of 15 MeV endpoint energy. Four adjacent channels have

been added together for this survey plot.

JOURNAL DE PHYSIQUE

Fig. 10. Results from photon scattering experiments performed at Urbana ( Illinois ) /32,33/. The convolution of the Zr(y,y) data from scattering of tagged.photons dotted. These structures could be resolved in NRF experiments with brems- strahlung and Ge (Li) detectors (bars) .

First results of the 'Ozr (Try) measurements with polarized brems- strahlung are depicted as an asymmetry plot in Fig. 12. None of the strongest dipole excitations observed in NRF is due to a MI ex- citation. Spins and parities of the states at 6296, 6425, 6162, 6876, 7250, 7706, 8110, 8500, 8717, 9146 and 9332 keV are 1 . ^For

the states at 8131, 9194, 9678 and 10040 keV we give a tentative spin and parity assignment of (I-) .

The solid lines in Fig. 12 represent the avergge bremsstrahlung polarization obtained during the two runs on Zr. Determination of the bremsstrahlung polarization was explained by Fig. 7. The analysis of parities of weaker transitions is in progress. MI excitations re- ported by Meuer et al. /35/ are at the resent detection limit of NRF experiments, which raises slowly from lz/r= 0.2 eV at 6 MeV to 0.6 eV at 9 MeV and then ascents steeply to about 2 eV at 10 MeV for 9 0 ~ r . The parity of a state at 8496 was determined in Ref. /36/ to be positive. We observed a state at 850023 keV , which has almost the same ground state decay width, but the measured parity is negative

(four standard deviations away from the value for posith~e parity).

A negative parity is in agreement with the result from Zr(e,e1)/35/, where a state at 8.494 MeV was found, whose formation was cleady

compatible with an El excitation (but a Jn=2- assignment could not be

completely ruled out).

90 Zr (y,y)

Ey,= 15 MeV 8 = 90"

6 7 8 9 10

ENERGY ( MeV )

F i g . 1 1 . High e n e r g y spectrum from ' O Z ~ ( y , y) . T h i s spectrum r e - sembles t h e sum of s p e c t r a ob l n e d by f o u r 100 cm2 Ge ( L i ) d e t e c t o r s d u r i n g two r u n s on "Zr.

I n summary, shows a c o n c e n t r a t i o n of d i p o l e s t r e n g t h i n t h e energy r e g i o n around 9 MeV. Many s t a t e s a r e l e s s t h a n 30 kev a p a r t i n energy. So far, p a r i t i e s o f 11 s t r o n g s t a t e s were determined t o be T = - and a t e n t a t i v e s p i n and p a r i t y assignment o f ( I - ) f o r f o u r f u r t h e r s t a t e s can be g i v e n . Although ( p , p l ) e x p e r i m e n t s a t i n t e r m e d i a t e e n e r g i e s a r e v e r y s e n s i t i v e t o M I e x c i t a t i o n s , c a u t i o n seems t o be unavoidable t o i n t e r p r e t a l l t h e s t r e n g t h between 8 and 10 MeV a s M I s t r e n g t h , because many peaks observed i n ( p , p l ) /30/

l i n e up w i t h E l s t r e n g t h observed i n ( T I y ) .

F i g . 12. Asymmetries f o r s t r o n g

gdpole e x c i t a t i o n s i n oB Z r from NRF w i t h

. T I = - B p -

p o l a r i z e d photons. 0 6 - E n e r g i e s o f t h e e x c i t e d s t a t e s a r e l i s t e d i n

keV. The o n l y t r a n s i - 0 2 1

t i o n w i t h w s i t i v e m i t y stems f r o m a n E 2 ex- 3 ⁰

c i t a i o n i n 160 from

t h e 'oZr02 t a r q e t . The 2 - 0 2

s o l i d l i n e s r e p r e s e n t

t h e asymmetries e x p e ~ t e d -OL

on ground of t h e 2~ ( y t p ) - 0 6

measurement ( s e e F i g . 6).

- a 8

/

- : - ^I -

- 5 9

" ~ r (y;y) - Ey,,=15MeV - -

. T I = +

-..

' . . . . . .

. .

6 7 8 9 10

ENERGY I M e V )

JOURNAL DE PHYSIQUE

Search for MI strength in 2 0 8 ~ b has a long, exciting history /37/.

Two strong I + states are expected in the simple shell model having a pure v(ill/2, ii3j2) neutron configuration and a pure n(hg/2, h;;l2) proton configuration, respectively.

Our experiments with linearly polarized bremsstrahlung led to the discovery of a I + state at 5846.1 + 1.1 keV possessing a strength of T$/T = I .250.4 eV /38/. If To/T = 1 is assumed, we obtain B(M1) + = 1.650.5 NO. Fig. 13 shows the measured asymmetries in a run with

This width was calibrated relative to the strong El transitions.

In the meantime several experiments were able to detect this 1 ' state too /39-44/. The energy of the 1 ' level at 5846 keV is a little bit higher than the energy 5.5 MeV redicted by ergados /45/ for the isoscalar 1 ' state in 28QPb, but 2g9Bi ( d I 3 ~ e ) 288Ph /39,41/, 2 0 8 ~ b

( p I p l ) /44/ and 238~b(d,d1) experiments /43/ confirm the isoscalar character of this transition.

I

0.6 - 2 0 8 ~ b ( $ , ~ )

Fig. 13. 04- Eyme~10 MeV

Measured asymmetries for the 0 2 - -

+

. T I = -

2 0 8 ~ b (t,y) reaction. The

strength of the transitions !2 O: .,+

Furthermore, we were able to clarify the parity of the 4842 keV level and it was possible to determine the parities of the states at 6263, 6312 and 6363 keV. In this case, all levels are excited via El

transitions.

/16,38/ are plotted in the

2 ^-02-

lower part. The only MI ^-04 transitions observed is at -06

3.1.4. MI STRENGTH BETWEEN EXCITED STATES

- --- -

-

:

One of the exciting features of NRF is that also B(M1) values of transitions between excited states can be measured. In this case, we have the possibility to study how strongly the 1 ' states, observed in inelastic proton and electron scattering or elastic photon

scattering, couple to low lying excited states. Furthermore, E2/M1 interference terms with their relative sign of matrix elements can be determined from a precise measurement of the angular distribution of the inelastically scattered photons, since W ( O ) in equ. (3) depends on the mixing ratio 6.

5846 keV (hatched bar).

LO

0

6 7

It .

A p r e c i s e measurement of t h e a n g u l a r d i s t r i b u t i o n of i n e l a s t i c a l l y s c a t t e r e d photons i s e x p e r i m e n t a l l y d i f f i c u l t i n s o f a r t h a t good s t a t i s t i c s i n t h e s p e c t r a i s r e q u i r e d t o c a l c u l a t e mixing r a t i o s w i t h s u f f i c i e n t a c c u r a c y . A t a low d u t y c y c l e e l e c t r o n l i n e a r a c c e l e r a t o r a s t h e G i e s s e n l i n a c t h e r e p e t i t i o n r a t e o f t h e a c c e l e r a t o r h e a v i l y l i m i t s t h e p o s s i b l e c o u n t i n g r a t e .

2 F? 600

I- cn z g ^LOO

7 8 9 10

ENERGY ( MeV )

F i g . 14. High e n e r g y p a r t of a NRF spectrum from 2 6 ~ g r e c o r d e d w i t h a G e ( L i ) d e t e c t o r a t t h e 14 MeV s t a g e o f t h e Mainz r a c e t r a c k m i c r o t r o n .

T h e r e f o r e , a h i g h d u t y c y c l e e l e c t r o n a c c e l e r a t o r h a s a g r e a t advantage. The u s u a l l y lower e l e c t r o n c u r r e n t o f h i g h d u t y f a c t o r machines does n o t h u r t i n t h i s c a s e , because t h i c k b r e m s s t r a h l u n g t a r g e t s c a n be used f o r p r o d u c t i o n of u n p o l a r i z e d p h o t o n s .

We made a f i r s t a t t e m p t t o o b t a i n B(M1) v a l u e s between e x c i t e d s t a t e s i n a NRF e x p e r i m e n t w i t h b r e m s s t r a h l u n g a t t h e 14 MeV s t a g e o f t h e new r a c e t r a c k m i c r o t o n a t Mainz ( M A M I ) , which h a s a d u t y c y c l e o f 100%. Two G e ( L i ) d e t e c t o r s were p l a c e d a t 0=90° and 150' w i t h r e s p e c t t o t h e b r e m s s t r a h l u n g beam. A NRF spectrum from 2 6 ~ g i s shown i n F i g . 14. The t r a n s i t i o n o f i n t e r e s t i s l a b e l e d 31 and it p r o c e e d s from a 1' s t a t e a t 10148 keV /46/ t o t h e f i r s t e x c i t e d s t a t e i n 2 6 ~ g . Peak number 3 i s t h e c o r r e s p o n d i n g e l a s t i c l i n e . (Another p a i r o f t r a n - s i t i o n s , b u t w i t h n e g a t i v e p a r i t i e s , proceed t o a 1 s t a t e a t 8504 keV;

peaks number 9 and g l ) .

The t h e o r e t i c a l r a t i o o f c r o s s s e c t i o n s R = W ( 0 = 1 5 0 ° ) / ~ ( 0 = 9 0 0 ) f o r i n -

e l a s t i c photon s c a t t e r i n g i n v o l v i n g a s p i n c a s c a d e 0 +1*+2+ i s

p l o t t e d i n F i g . 15 a s a function o f t h e mixing r a t i o 6 ( s e e equ. 4 ) .

Two c u r v e s a r e o b t a i n e d depending on t h e s i g n of 6 . The e x p e r i m e n t a l l y

measured r a t i o of i n e l a s t i c photon s c a t t e r i n g i n t e n s i t i e s R=0.81+0.10

f o r t h e 1 s t a t e a t 10148 keV i s p l o t t e d o n t o t h e t h e o r e t i c a l r a t i o

c u r v e . We f i n d t h a t t h e r e l a t i v e s i g n between t h e E2 and t h e M I m a t r i x

e l e m e n t s i s negative and calculate a B ( M I ; 1 + (1 01 48 keV) +2+ (1 809 keV) )

v a l u e of 0.080 f ::::: 2 . T h i s s t r e n g t h t o t h e f i r s t e x c i t e d s t a t e

i s about 1 / 3 o f t h a t f o r t h e ground s t a t e .

JOURNAL DE PHYSIQUE

Fig. 15. Theoretical ratio R of photon scattering intensities at 0=150°

to 90' for inelastic photon scattering on J=l state, which decays to a 2+ level. The measured ratios for a y-ray cas- cade of even and one of odd parity are plotted with error bars onto the theoretical curve.

The error bars for the ratio of inelastic photon scattering cross sections (a second one is given in Fig. 15 for a case, where M2/E1 interference occurs) from this first test run are still relatively large. Further experiments are necessary to yield transition pro- babilities with smaller errors. The intention of this last example, however, was to discuss one of the new capabilities of NRF at modern accelerators in the near future.

Acknowledgments -

I wish to thank Dr. K. Ackermann, Dr. K. Bangert, C. Blasing, W. Naatz, A. Ruckelshausen, S. Schennach, R. Stock, H. Wickert and Dr. K.

Wienhard for a fruitful collaboration and for freely supplying me with their results and interpretations of experiments. I am indebted to Dr. W.C. Sallyey for permission to include results obtained by him and research workers at Urbana (~llinois) in this talk. I would also like to thank Professor B. Ziegler and Dr. A. Zieger for all the support we have received during the experiments at ~ainz.Finally, sup- of the Deutsche Forschungsgeneinschaft is gratefully achw1@&-

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