A high resolution study of stretched-spin states in 116Sn with the 115In(3He, d)116Sn and 115In(α, tγ)116Sn reactions

NUCLEAR PHYSICS A

Nuclear Physics A548 (1992) 271-307 North-Holland

A high resolution study of stretched-spin states in " 6Sn with the '111n(3 He, d) " 6 Sn and " 5 In(a, t y) " 6 Sn reactions J.M . Schippers, W.T.A. Borghols', M.A. Hofstee, R.F. Noor nan2, J.M. Schreuder3 and S.Y. van der Werf

Kernfysisch Versneller Instituut, Zernikelaan 25, 9747 AA Groningen, The Netherlands

N. Blasi

Dipartimento di Fisica dell' Università di Milano and INFN, Sezione di Milano, I-2013? Milano, Italy Received 13 February 1992 (Revised 13 April 1992) Abstract: The excitation-energy range 3.5-7.2 MeV in the nucleus "6Sn has been investigated via the proton stripping reactions 1151WHe, d)"6Sn, "SIn(a, t)" 6Sn and "51n(0, ty)"6Sn. Detailed information is obtained on the distribution of the spectroscopic strength for the different 1-transfers. By combining this information with a study of the gamma decay following the (a, t) reaction, important parts of the (nearly) stretched-spin configurations J' =10 - , 9 - and J' =7+ and several J' = 3+ and J' = 5+ states have been identified . The natural-parity configurations are strongly subject to spreading into a continuum, while unnatural-parity states exhibit much less fragmentation . A comparison is made with large basis model calculations. E

NUCLEAR REACTIONS " 51n( 3 He, d), E = 50 MeV ; "SIn(a, t), E =65 MeV; measured Q(0). " Sln (a,ty), E =65 MeV ; measured ty-coin . "6 Sn deduced levels, J, 7r, spectroscopic factors, stretched-spin states. Comparison with model calculations .

l. Introduction An important class of nuclear excitations is formed by stretched-spin states, one-particle-one-hole excitations of the highest spin possible (J =ip+ j,,) within a Ohw or 1 fiw particle-hole basis. The reason for the interest in stretched high-spin states is their simplicity, since these excitations are, within a restricted basis, rather pure shell-model states for which only few and sometimes just one IpIh configuration can couple to the same high total angular momentum . This makes these simple Correspondence to : Dr. J.M. Schippers, KVI, Zernikelaan 25, 9747 AA Groningen . ' Present address : Gasunie research, P.O. Box 19, 9700 MA Groningen, The Netherlands. 2 Present address: I .H.N. van Dorsser BV, Laan Corpus den Ho3rn 110, 9728 JR Groningen, The Netherlands . 3 Present address : AT&T Network Systems, P.O. Box 1168, 1200 BD Hilversum, The Netherlands. 0375-9474/92/$05 .00 © 1992 - Elsevier Science Publishers B.V. All rights reserved

J

Schilp

Ps et aL / "6Sn

roes to test e.g. various nucleon-nucleon forces on which the elastic scattering is based. cleus is very suitable for studying stretched-spin states : not only can studied y inelastic scattering experiments, but " 6 Sn is also the ature which may be reached by single nucleon stripping onto le- o e target nucleus ("'In) . e report on a study o the ('He, d), (a, t) and the (a, ty) reactions w or l w spin configurations of proton 1p l h tici ate stretched t redsi are: ° =1 (g9~®~, 11/~)', ' = 2)7r* d12 (g~®;' 97 ~), and J' = 7 + (g9i2, rot -transfer reactions e 2Jr + 1 dependence of the cross sections favors al states ig spin. Furthermore, because of their uniqueness, stretched-spin co gu ti ns of high sin are expected to be less subject to fragmentation than co figu ti lower sin. Consequently, most of the partial sum-rule strengths ') are ex este reside on relatively few states . e e is described ere are performed to locate the strength of stretched s states in "' n, an to study their -fragmentation. In previous work 2) it as been f that the proton stripping spectrum shows several types of structure : iscrete e s, a continuous distribution of strength which appears as a background a clusters unresolved peaks superimposed on this background . The 0116s "'In( in g. 1 extends up to 25 eV in excitation energy. It sect statt e ""background" is in fact part of a bump centered around E,, = 7 eV. s useful

115 I

(o,t) 116

EQ =75 3000 ~- O=100

Sn

eV

c

02000 U

g.s.

1000

0

-r 20

10

EX (MeV)

Fig. 1. Experimental spectrum of "6Sn observed via the (a, t) reaction on "51n at 6s MeV. The hatched peaks are due to contaminants in the target.

J.M. Schippers et al. / "6Sn

273

A pronounced second bump appears around 16 MeV. Similar structures have been observed by Galès et al. 3,4) in neutron and proton stripping experiments on eveneven nuclei and have been interpreted as lp-strength distributions associated with stripping into outer subshells across 1 hw and 2hw. Analogously we interpret these structures as due to 1 hw and 2 hw 1p I h excitations, with the hole in the g9/2 orbital. In the clusters that are superimposed on the 1 hw bump strong indications have been found 2 ) for the presence of stretched-spin state strength. We have repeated the " S In( 3 He, d) and (a, t) experiments with much higher resolution to study the region of 3 .5-7.2 MeV excitation energy (top of fig. 2). This interval exhibits the two types of strength fragmentation: resolved peaks and clusters 600

400

s

11 ln (Ot .t4) 116

Eo~ =65 MeV 0=0°

n

triton spectra singles

200

N 10

Co N

I

CD M

N C>

rCD,

T M

rn 0

coincident with POSITIVE PARITY BAND

Fig. 2. The singles-triton spectrum from the "Sin(cr,t)' 46Sn reaction at 0=0° (top) and the triton spectrum gated on indicated y-ray transitions in the positive parity band in 116Sn (middle). The bottom part shows the triton spectrum gated on the indicated transitions in the negative-parity band of 2qr neutron excitations .

2

J.M. Sehippers et aL / ""Sn

(line fragmentation) and the low-energy part of the 1 hw bump (quasi-continuum fragmentation) . e gamma decay of the structures was studied via the 6 " iln(a, t°y)" Sn reaction. This proves to be a strong tool in the identification of the observed states . We find that gamma decay shows a pronounced selectivity : the spectra gated on the gamma decay within the kno`in positive- or the negative-parity bands -), shown in fig. 2, demonstrate clearly the different characters ofstates excited in the (, t) reactions . Furthermore, several spin assignments of excited states can be made by gating on the sidefeeding of these bands. Among them, several states can be identified as stretched-spin states and the fragmentation of their underlying configurations can be established . In earlier studies of hole states the observation of the gamma decay has proven to be ofgreat help in understanding the admixture of collective components observed in the hole states. Such studies have been performed in the odd Pd and odd Sn isotopes'- ') and they evidence the coupling of the deep-hole states to collective states. e E, < 3.5 eV region of "6Sn has been studied by several groups) and the level scheme is well known '®). This information is, of course, essential for a proper interpretation of our gamma decay data. In the established level scheme of "6Sn two bands are known and their sates and gamma decay have been observed using the "4Cd(a, 2ny)"6Sn reaction at 28 MeV [refs. s. ")]. The spectra in fig. 2 are gated on either of these bands. The levels in the positive-parity band are of rather complex character: they form a quasi-rotational band built upon the 02 state which originates from a non-closed Z = 50 shell") and have been interpreted as of proton 2p2h nature. e negative-parity states) form the second band which has a two quasiparticle neutron (one broken pair) character, based on (97/2h,,/2),, (s,/2h  /2), (d3/ ,h  /?) and (d,,/ 2h, /5) neutron configurations . The state with the highest spin is the J" = 91 at E, = 3.522 MeV. Above E,t = 3.8 MeV, studies with the "SIn(3He, d) reaction have been performed before by several groups ''- '4), but due to the rather low bombarding energies (18 and 25 .6 eV) the small 1-values dominate the spectra and 1= 4 transfer was not observed . Almost no spin assignments were made in these studies . In the work of van der Werf et al. 2) a beam energy of 50 MeV has been used to emphasize also the higher 1-transfer (1= 4 and 1= 5) in the stripping reactions. They found the clusters at E., = 4.8, 5.8 and 6.3 MeV to be candidates for the proton 1 p 1 h J' =10and J' = 8 + stretched-spin states, but were unable to make definite spin assignments . The observed strength of these candidate states was found to be lower than expected from partial sum rules. Also in several inelastic scattering studies 15-17) it has been observed that the cross sections of high-spin states are much smaller than expected . The observations of too low cross sections of individual high-spin states may result from several mechanisms: (1) short-range correlations in the nucleon-nucleon force leading to a reduced occupancy of orbitals just below the Fermi surface, see e.g. refs. '8-26), (2) mixing

J. M. Schippers et al. / "'Sn

275

with other (usually collective) states with more complex configurations, like 2p2h states, see e.g. refs. 27-35) and (3) the addition to the wave function of lplh excitations of higher (e.g. 3hw) excitation energy (core polarization), see e.g. ref. 36). In sect. 2 of this paper the experimental setup of the (3He,d) and (a, ty) experiments is described, followed by their results in sect. 3. The analysis of the experimental data (sect. 4) is divided in a discussion of the observed strength distributions, the sidefeeding patterns and the spin assignments . In sect . 5 the results of the analysis are discussed and compared with sum rules and existing model calculations and in sect. 6 a summary of our observations and conclusions is given. 2. Experimental procedure The (3 He, d) and (a, t) reactions were studied by bombarding a self supporting "'In target of about 290 FCg/cm2 with momentum analyzed beams of 50.5 MeV 3He particles and 65 MeV a-particles from the AVF cyclotron at the KVI. The outgoing particles were detected with a multi-wire drift chamber 37) (MWDC) at the focal plane of the QMG/2 spectrometer, covering an excitation-energy range 3.5-7.2 MeV. Particle separation was performed by discriminating on the pulse height of a plastic scintillation detector mounted behind the MWDC. Deuteron spectra were taken at the spectrometer angles 1 .5°, 7°, 11°, 15°, 19° and 23°, using a solid angle of 5.56 msr, with AO = 6° in the horizontal plane. Since the angle resolution was about 0.6°, the horizontal opening angle could be split into two angle bins, having a width of 3° each, to get a finer angular distribution. In fig. 3 (bottom) the spectrum at Blab= 9.5° is shown. The energy resolution was 15 keV. A Si detector was used as a monitor for elastically scattered 3 He particles. Absolute cross-section calibration was done by measuring the elastic scattering cross section on the same target . The spectrum recorded in the angle bin at 0° had an effective opening angle ranging from -0.6° to +1 .5°, due to the fact that the region from -1 .5° to -0.6° was cut off by the beam dump in the dipole of the spectrometer. Triton spectra from the (a, t) reaction were recorded at a few forward angles with an energy resolution of 16 keV. The spectrum recorded at Blab = 0° is shown in fig. 3 (top). For both reactions the 1 ositions and intensities of the peaks in the spectra have been fitted using the peas-shape parameters obtained from fits of the four most prominent peaks at E,{ = 3 .887, 4.023, 4.076 and 4.285 MeV. The background was chosen to be a spline function through the positions of local minima, following the shape of the "flat continuum." The energy calibration has been done assuming a quadratic energy dependence and using excitation energies of the prominent peaks as determined in this work from the y-decay experiment . The proton stripping+ y-decay coincidence experiment was performed with an analyzed beam of 65 MeV a-particles from the KVI cyclotron. . The outgoing tritons were also detected with the MWDC mounted in the focal plane of the QMG/2

276

"'Sn J. M. Schippeers et nl. /

Ex

(MeV)

Fig. 3. Spectra of " 6Sn obtained with the proton stripping reactions. The (n, t) and the (3 He, d) spectra are displayed on the same energy scale.

magnetic spectrometer. e spectrometer was positioned at 0 = 0°.and its maximum opening angle of 0 sr was used. The "In target had a thickness of 120 Nag/cm 2. e triton-energy resolution, averaged over the whole experiment (1 .5 weeks of beam time) was 20 keV. The target was surrounded by four germanium detectors, three of which had a BG® anti-Compton shield . Seven 2" x 3" Nal detectors were used to measure the y-ray multiplicity. The 0" setup has the advantage that the beam is stopped rather far away from the target, on a Faraday cup inside the first dipole ofthe specrometer. Paraffin, cadmium and lead shielding was placed between the dipule and the y-ray detectors to further reduce the background (neutrons and fs) in the e detectors to an acceptable level. Also special care was taken to reduce beam halos. n event was defined by a coincidence between a triton in the spectrometer and a y-ray in one of the Ge detectors . The coincidence time window had a length of about five cyclotron beam bursts, corresponding to 700 ns. The time difference between the "triton event" and the "y-event" was measured in a Tl3C. Due to the rather low beam current of 20 nA, a prompt-to-random ratio of about 17 has been achieved (see fig. ). e signals of the Nal detectors were only read out for a coincidence between a triton in the WIC and a y-ray in a e-detector and did not play a role in the event definition . Their signals were also timed against the

J.M. Schippers et al. / ""Sn

200

400

time (ns)

277

600

Fig. 4. Spectrum of the time between the detection of a gamma ray and a triton.

plastic scintillator behind the MWDC and a prompt coincidence was required for the multiplicity determination . The triton-energy data and the -y-r,.-4y energies of the prompt coincidences were stored. in a 1024 x 4096 matrix. Separate triton spectra were updated for 0-, 1-, 2and 3-fold prompt coincidences with the NaI detectors, and from those the y-ray multiplicities were derived. A Monte Carlo simulation provided a calibration of the multiplicity measurement. After establishing the y-decay schemes of the strong peaks, the simulations were normalized to the multiplicities of the y-ray cascades coincident with these peaks. A calibrated 152 Eu source was used to determine energy calibrations and efficiencies of the Ge detectors and the y-ray data were corrected accordingly. 3. Experimental results 3.1. THE SINGLES MEASUREMENTS

"5In(3He, d)"6Sn data have been compared The differential cross sections of the with zero-range DWBA calculations 38). The optical model parameters and the parameters defining the geometry of the bound-state potential of the form factor are given in table 1 . In the DWBA calculations a finite-range correction of 0.77 has been used and non-locality corrections have been neglected . No lower cutoff was used. The calculated angular distributions for ? = 0, 2 and 4 transfer to the (3 He, d) reaction are sufficiently different to be distinguished experimentaïly . The 1= 5 transfer, however, cannot be distinguished from 1= 4 transfer. Also the angular distributions for d3/2 aid d5/2 transfer cannot be distinguished from each other.

2 78

J.

rs~n ~chip~ers et al. / ® °TABLE 1

ptica!

odes parameters for the

V'® [d1t~v] i

ß4'e [~7eV]

175 d ~) a `! t $) p

`') `') `)

55 .5

Vu, [94ieV]

6 .4 102 .9

175 V9ai

ßß~~ [Me~°] 17 .5

.R

2

(3

e, d) reaction at 50.5 MeV and the (a, t) reaction at 65 MeV on " S in

17 .5

ied

ibsofl et al. s= ).

A = 25 ~)

i-linterberger et ®1,

a

r®

r,

[fm]

[fm]

1 .14

1 .60

1 .05

1 .32

1 .254 1 .14

e)

1 .25

;).

`)

rn [fm]

a® [fmJ

ao

[fm]

aso

r~

[fm]

[fm]

0.723

0.86

0.913

0 .722

1 .254

0 .669

0 .669

1 .4

1 .60

0 .723

0 .86

1 .4

1 .05

0 .650

1 .4 C.i :3

~ .3

,

1 .2

Perey and Perey °°) .

Varied to reproduce the binding energy . e spin-orbit to

in the bound-state potential is

` fa V~ _d +A with dr(1+e~')-'(1-~), ~na c r

x=(r-r®A'r3)~a®-

o are with the (' e, ) reaction, the shapes of the angular distributions of i erent transfers i t e ( , t) reaction are not so sensitive to the 1-value. Therefore, no separate analysis has been made of the angular distributions obtained with the (a, t) reaction . owever, ue to angular-momentum mismatch in the (a, t) reaction, the co arison of the (a, t) and the (~ e, d) spectra in fig. 3 gives an extra indication n whic states ossess high !-values. For example, the clusters observed with the ( , t) reaction at E~ = 5.5 and 5.78 eV and the state at 6.295 MeV are almost not visible in the (' e, d) spectrum . This means that these clusters must have large !-transfer co ponents. Generally, states that are observed clearly in the ( a, t) reacticr but onlti® weakly in the ( ; he, ) reaction, are to be associated with ! = 5 transfer and are thus of negative parity. Based o this comparison we adopt in the calculation of spectroscopic strengths h ,, strength above and g,,~ strength below Ex = 5.5 MeV. For ! = 2 strength we adopt d;,, above 6 MeV and d S,, at lower excitation energies . Fi al-state wave functions of positive parity may be mixtures of diflferent particlehole con gurations and angular distributions may receive contributions from i erent !-transfers . ~egative-parity states can only be produced via h  ,, transfer, assu ing t at only the 1$~~cd valence orbits contribute at these energies. The amplitudes of the different cnrr'ponentç have been determined by a least-squares fit of angular distributions to the ( 3 1-Ie, d) data. Spectroscopic strengths have been de uce fro

wit ® = 4.42 for (; e, d). n the ab~~we fortraui~e ti = (2Jf T 1)/(2.1; t 1) z~ is the s ectrosco is strength . n g . the is of the angular distributions of the most prominent peaks are s own. e deduced strengths are given in table 2. Since the ! = 0 angular distribution

279

J.M. Schippers et al. / "'Sn

0

.O76 MeV E. =4

E.= 4.239 MeV

I=2 ~

E,t-4.285 MeV

E.= 4.365 MeV

7

Fig . 5 . Angular distributions from the " 5In(3 He,d)"*5Sn reaction of the resclAved Maces above E,, 3.3 WV-

"Mm et aL

280

/

'An

UBLE 2

reactions topic strengths'), concentrated in peaks, observed with the proton stripping reactions tree analysis of both transfer are from in this work. The J' assignments and tI , e -y-coincidence measurement

X

A)

338 3,65 3339 3.797 3SS7 3,95 4.00 4.023 4A76 4,24 4.285 4,33 4,362 4,39 4,765

%e

G(sj/2)

3" $+ 5+ (4 ,", SI 7'

(0.01) 0.06 0.03 103 0.01

7' (8-,8-,10-) (8 -,9 -)

5.5 533 534 5.78 (10- ) 1860 (10-) 6.295 (10-) total E,, > 3.8 MeV:

G(87/2)

dcr/dD (a, 0 (mb/sr) at 5*

0.12 0.04

0.07 114 038 138 1.90 0.28

('He, d)

J IT

0.13

G(d5/z) 0.009 0A12 0.07 0.07 0.42 0.08 1017 ;j .25 123 0.06 0.37 0.031 112 0.009

1 .59

0.28 118 0.06 0.85 0.025 0.012

1.57

230 138 0A8 3.61 0.27 0.43 0.09 018 IA8 7.8 0.43 016 1A7 017 0S8

') The spectroscopic strengths have an .incertainty of 20% due to systematic errors in the normalization and ambiguities of the optical model parameters.

has a strong maximum at 0", the weight of the cross sections at this angle is very large in the determination of the 1=0 strength . Due to the uncertainty in the calibration of the solid angle at 0", a systematic error of 20% is assumed for the I= 0 strength . Compared to this, the q tatistical error may usually be neglected. For the other I-transfers the statistical errors in the strengths (0.005 for I= 2 and 0.04 ford = 4) am approximately the same for all levels . The uncertainties mentioned in the foilowing are statistical errors, unless explicitly stated differently. S an alternative analysis of the strength distribution of the different subsht-Ils, angular distributions of 50 keV bins A!ere made in the reg ,!~rn 4.2 < E" < 7.1 MeV., without subtracting any background . These were fitted wi ,, .h DWBA calculations, using three /-values. The strength diet ibutions so obtained are shown in fig. 6. The summed strengths located in the peaks above the continuum and the summed in the bin-wise ariaiysis are iisted in ta le 3.

J.M. Schippers et aL / "6Sn 400

(3He,d) 116Sn E3He = 50 MeV 115(n

C

c ru

281

mei

~ Co

'Blab= 12.511

-

10 0

s m

0 u

r

0

'

IW'rr1J

~Y

y u

0.04 si/2

0.02 u

0 a

0.2

10

d3,12

Y

dsi:

c

.3

s

u

N

0 .0 i

hw2

0. 5 0

â N

o m

0

L

97n 0 ao

0.5

0

Ex (MeV) Fig. 6. The spectroscopic strength for I = 0, 2, 4 and I = 5 transfer, measured with the "S In(3He, d)"6Sn

reaction, in bins of 50 keV. The spectrum (taken at Blab = 12.5°) and the strengths are displayed on the same excitation-energy scale. The procedure ofbinning the excitation energy spectrum in 50 keV intervals makes no use of the high resolution. In order to maintain the high resolution and yet make a qualitative 1-transfer decomposition of the spectra a subtraction technique is used . This method is sensitive when, as in the case of the ('He, d) reaction, the shapes of the differential cross sections are sufficiently different . At 0 = 0° the distributions

J. M. Schippeis et al. / "6Sn

282

TABLE 3

Total observed strengths measured with the 115 1n( 3 He, d)" 6Sn reaction in ref. ') and in this work Orbital s,®2

ds/2+d3/2 97/2+hi1/2

Gel 0-4.3 MeV (peaks)

Gei 4 .3-7 .2 MeV (50 keV bins)

Gei (%) 2j+ 1 0-7.2 MeV

0.14 1.9 1 .8

0.77 4.7 7.8

46% 66% 48%

of I = 9 and I = 2 are the most prominent and the 1= 0 distribution has a pronounced maximum. e I = 2 angular distribution ' - s its maximum at 0 = 8 .5°. Thus the I = 0 transfer is made visible by subtracting the h, - .;ctrum taken at 0 = 8.5° from that taken at 0 = 0®. n the same way the I = 2 transfer is stressed by subtracting the spectra taken at 0 = 8.5® and 13.5° and the I = 4 and 5 transfers by subtracting those taken at 0 =16.5° and 8.5°. All source spectra were normalized such that the final spectra had almost no negative channel contents . The results of the subtractions are shown in fig. 7 and they will be used in the discussions on the spin assignments. Especially i cases where peaks within the clusters are analyzed, the resolution of these subtraction spectra helps in recognizing the 1-transfers invoived, rather than determining the strengths quantitatively . 3 .2. RESULTS OF THE " S In(a, ty)" 6Sn COINCIDENCE MEASUREMENTS

Gating on different excitation energy regions in " 6Sn several E,, projections have been made. The most important gates on the excitation energy of "6Sn are indicated in fig. 8 as the positions and widths of the multiplicity-measurement results. No background correction was made, since the background in the triton spectrum is of physical origin . In fig. 9 a prompt y-ray spectrum is shown with a gate on the total energy byte observed by the focal-plane detector . In this spectrum it is observed that the excited states decay predominantly via the positive- and negative-parity bands observed by clan Poelgeest et al. 5). In fig . 10 y-ray spectra gated on selected triton peaks are shown. These spectra show the strong selectivity of these measurements and illustrate that the very good energy resolution of the MWDC-equipped spectrograph is crucial in defining narrow gates on excitation energy (i.e. triton energy). The contents (efficiency corrected) of the peaks in the y-ray spectra are tabulated '°) . On request they are available from the authors . If the transitions are known or if they match a known level distance an identification is given . An uncertainty of 1 keV has been accepted in the matching of the gamma transitions. In this way decay schemes ofthe different parts ofthe excitation spectrum have been constructed.

J.M. Schippers et aL / "'Sn

600

115in

E3

He

400

»s sn (3He, d) =50.5 MeV

283

fli m M N O

OIab= 12.5°

200

0 (Ol ab= O ° ) - iolab= 8 .5 ° )

100

50

0 600

i ~n

v

(O1ab=83°)- lOI ab=13.5 °)

400 pl

200

W Z Z Q

û 150 w Z

ô

I

0

w^~

C

,8=4+5

100

9il i

50

0 7

6

5 E x (MeV)

4

Fig . 7. Differences of experimental "SIn( 3He, d)"6Sn spectra taken at angles that have been chosen such that one specific I-value is emphasized in each difference spectrum. The ratio of the source spectra were taken such that no negative channel contents were obtained in the difference spectra.

LM. Schippers et al. /

284

' "6Sn

I

--i

triton spectrum -ray multiplicity

Fig. 8. Coincident triton spectrum and the y-ray multiplicity as a functi Col of excitation energy for the 115ln(a, ty)"6Sn reaction. The horizontal error bars indicate the widths of the excitation-energy bins.

several spectra the first y-rays in the cascade are observed . These "primary y-rays" are easy to observe in spectra gated on strong peaks in the excitation-energy spectrum . In the spectra gated on clusters or parts of the continuum, these transitions show up as broad structures, having the same width as the gate or the cluster. By squeezing the E,, spectra some of these broad structures can be distinguished from the background . Fig. 1 i shows two such squeezed y- ray spectra, gated on two neighboring bins in the cluster at E x = 5 .78 MeV. The broad structures at E,, = 2274 "'Sn "'in (a,tï) prompt y-ray spectrum gate on 3.5 < Ex < 7 .5 MeV

,t~ Z;;

02

04

N 0 .6

0e

10

E . (MeV)

12

ta

wr. 16

1 .8

2 0

Fig. 9. The coaicident y-ray spectrum of the "S ln(a, ty)"6 Sn reaction gated on the excitation-energy region 3.5-7 .5 MeV in "6 Sn .

J.M. Schippers et al. / "'Sn

285

a

00 N II

x

s

a. v

0 II

x

v

O

0p u au G. ess

L. r

a E 0

a ô

J.

286

. Schippers et al. / ® 'Sn

30

1

y-ray spectrum

"s in (a, t1) i16Sn

gate on E .= 5 .740 MeV N 20

O M fV N

10

lV N~

gate on E,=5 .780 MeV

20

10

E, (MeV) Fig. 11 . Two y-ray spectra gated on different parts of the cluster of unresolved excited states at 5.8 MeV in "'Sn. The shift in y-ray energy from 2230 to 2274 keV corresponds to the different gate positions and reflects that these y-rays are primary transition from the states in the cluster to J' = 9; .

and 2230 keV are identified as primary y-rays emitted in the decay of the high- and the low-energy part of the cluster, respectively . All triton spectra gated on peaks in the y-ray spectrum were corrected for the background in the E,, spectrum, caused by the Compton effect. In fig. 2 a global division in the decay modes is shown : triton spectra gated on transitions in either the positive-parity or the negative-parity band. From the figure we see that the peaks assigned as positive-parity states mainly decay via the positive-parity band. An exception is the state at 4.285 MeV. The clusters at 4.8, 5.3, 5 .5, 5.8 MeV and the states at 5 .86 and 6.295 MeV decay via the negative-parity band, although small parts of them, like the state at 4.765 are weakly observed in the decay via the positive-parity band. The decay of the continuum is different from that of the clusters superimposed on it and transitions in both the negative- and the positive-parity band are observed in coincidence with the continuum .

J. M. Schlppers et al. /

"6Sn

287

Using the triton spectra gated on 0-, 1-, 2-, and 3-fold prompt coincidences with the Nal detectors, the -y-ray multiplicity coincident with the gates on the excitation energy in "6Sn, is determined . The multiplicity for each E,, gate is shown in fig. 8. The -y-ray multiplicity of the possible stretched-spin states is not significantly different from that of the continuum or neighboring peaks. A monotonous increase of the -y-ray multiplicity is observed with increasing excitation energy. 4. Analysis 4.1 . STRENGTH DISTRIBUTIONS

The excitation energy range investigated with the ('He, d) reaction covers about half of the 1 hcw bump that is shown in fig. 1 for the (a, t) reaction . About 45% of the cross section between 3 and 12 MeV is located in the 3 .5-7 .2 MeV range. In this range more than 70% of all the strength observed with the (3 He, d) reaction is located, in the "continuum" and in clusters of unresolved peaks (fig. 6 and table 3). The 1= 0 strength is not concentrated in distinct parts of the spectrum, except for the strong components in the peaks at 3.887, 4.023 and 4.076 MeV. The strength appears spread over the whole excitation energy range. From the difference spectra it is also clear that the s,/2 strength found by fitting the angular distribution in the peaks at 3 .739 and 3.797 MeV [see ref.')] is probably negligible . The fact that we locate about 46% of the s,/2 sum-rule strength in the energy region 0-7.2 MeV is consistent with the observation that this excitation-energy range contains about 45% of the total intensity of the 1 hw bump. The 1=2 strength appears to be present in the peaks between 3.5 and 4.5 MeV mainly, but also some 1= 2 strength is observed in the clusters at 4.8 and 5 .2 MeV, (see figs. 6 and 7). Since 66% of their combined sum rule is exhausted, configurations with ds/2 or d3/2 strength appear less fragmented into the continuum at higher excitation energy than configurations with s, /2 strength. The strength of configurations excited via 1=4 or 1=5 transfer is found in the clusters of states observed at 4.8, 5 .5 and 5 .8 MeV and in the continuum above 6 MeV. A small part of the strength is located in the four large peaks at E,, = 3.887, 4.023, 4.076 and 4.285 MeV, of which the peak at 4 .285 MeV has the strongest component. 48% of the combined g7/2 and h /2 sum rules is found below E,{ 7.2 MeV, suggesting that the missing part of the strength may be found in the high-energy part of the 1 hw bump. The clusters observed above 4.5 MeV contain both l = 4 or 5 and l = 2 components and must consist of many different and unresolved states. 4.2. SIDEFEEDING PATTERNS

Conclusions on the spins and parities of the excited states can be drawn when the primary y's from states populated in the (a, t) reaction are observed and when

288

J.

Schâppers et al. / "'Sn

the transitions can e place in the level scheme . It is not very probable that such a y-transition changes the spin with more than two units and therefore, the higher the spin of the state where the y-cascade enters the band, the higher the spin of the state excited in the (a, t) reaction . Since the matrix element of an electromagnetic transition is symmetric with respect to initial and final spin, it is possible that the sin of the excited state is lower than that of the state in the band to which it decays . I the primary y's are not detected, one can estimate a possible lower limit on the spin assignment by observing the sidefeeding to different states in the bands. Of course a definite sin assignment cannot be made without taking into account the -transfer established i the ( 3He, d) reaction . Fig. 12 shows the sidefeeding to several states in the negative-parity band. These spectra have been obtained by subtracting the triton spectra gated on intra-band gamma transitions that populate such a state, from the triton spectra gated on the known transitions that eexcite it. The source spectra were corrected for the -Y-ray detection efficiency. The most important result is that we see parts of some clusters 9 s state. Their excitation energies are 4.840, 5.780, that enter the band at the J . The big 5.860 an 6.295 eV cluster at 5.5 MeV does not decay via the J'r = 9-1 , but its high-energy part shows up in the sidefeeding of the J~ = 8 ; . The state at 4 .285 eV also does not decay via the J'r = 9 ; but it is present in the sidefeeding t the J - = 8 1 and 7, . n the sidefeeding to the J ~ = 7 ; we observe the state at 4.23 eV . e low-energy part of the 4.8 MeV cluster, i .e. the states at 4.765 and 4.799 eV are observed for the first time in the sidefeeding to the Jr = 6 ; state, n also the 5.5 eV cluster shows a feeding to the J' = 6 ; . The sidefeeding to several states in the positive-parity band is shown in fig. 13. The sidefeeding to the J ' = 6, is dominated by the decay of one of the four big peaks: the state at 4.023 MeV. Also small feeding from the 4.285 MeV state is observed . The same state also decays to the 6, and so does the state at 3.887 MeV. The sidefeeding to the 4, has not much statistics, but part of the cluster at 4.8 MeV is observed, together with the states at 3.943 and 4 .076 MeV. The peaks at 3.739, 3 .797, 3 .887, 4.023 and 4 .076 MeV are the main contributions to the sidefeeding of the , state, together with a small part of the state at 4.285 MeV and the cluster at 4.8 4.3. SPIN ASSIGNMENTS

From the ( 3 He, d) and the (d, t) reactions y), the state at 3.739 MeV has a preliminary spin assignment of J'r = 2+ or 3+. In this gate two almost equally intense "primary" -y-transitions of 1348 and 1373 keV are observed, which correspond to the transitions to the J, = 4; and the J' = 5 ; states, respectively . Based on the preliminary (2+, 3 + ) spin assignment, these are either E2 and E3 transitions or E2/ M1 and 2 transitions . However, in the decay (fig. 15) of the 4.023 MeV state (which will be assigned as J` = 5 + later) a 284 keV transition is found from this

J. M. Schippers et al. /

"6Sn

28 9

Fig. 12. The singles-triton spectrum from the "SIn(a, t)"6 Sn reaction at 0 = 0° (top) and triton spectra ,gated on the (side) feeding to different levels in the band of 2qp neutron excitations in 116 Sn.

J.,Nf. Selt®ppers et al. / ' ®6Sn

290

1

"'In ( ,t ) 196Sn tritron spectra

N

Ici

~0 Ln CD

N

.

a _t

Fig . 13 . The singles-triton spectrum from the 115ln(a, t)" 6Sn reaction at 0 = 0° (top) and tritoi, spectra gated on the sidefeeding to different levels in the positive-parity band in 116Sn .

J. M. Schippers et al. / "6Sn

29 1

E,, = 4.023 MeV state to the state at E,{ = 3 .739 MeV. Thus a spin assignment J' = 2+ for the 3.739 MeV state would imply a low-energy M3 transition which is very unlikely against a low-energy E2 transition in the case of the J' = 3+ assignment . We therefore propose for the state at 3.739 MeV a spin assignment J' = 3+. The states at 3.887 MeV, 4.023 and 4.076 MeV are populated in the (3He, d) reaction via I = 0, 2 and 4, indicating that their parities are positive. Furthermore, the presence of a significant I = 0 component restricts the possible spins to 4+ or 5+. In fig. 14 the y-decay scheme of the state at 3.887 MeV is shown. The intensities are represented by the thickness of the arrows . The state at 3379 keV may be the state observed but not identified by Wienke et al. 39) at 3381 keV. Assuming transitions via this state, it becomes possible to place the 509 keV (as a "primary" y) and the 989 keV y-lines in the decay scheme of the 3 .887 MeV state and also in those of the states at 4.023 and 4.076 MeV. The state at 3379 keV is then seen to decay only to the three 4+ levels in the positive-parity band. In the decay of the 3.887 MeV state the y-lines at 841 and 1754 keV can be placed as the decay to and from the known J' = 4+ state at 3046 keV, which is not a member of any of the two positive-parity bands. The strong decay and weak feeding of the J' = 3 ; state indicates a strong sidefeeding to this level. Although there are still a few unidentified y-lines, we cannot reconstruct this sidefeeding. It should also be noted that the J' = 5, level 3887 keV

o.

»6 Sn

Fig. 14. TIP y-decay pattern of the state at E,, = 3.887 MeV in

116

Sn (assigned as J' = 5+).

J. M. SchippemueomL /

292

^

l'5S m

at 2366 keV ham a lifetime longer than the time window we applied in the measurement, so that a large part of its decay was missed. e decay of the 3.887 MeV state proceeds mainly to the states with J' = 4' and to the state at 3379 M Also some weak transitions to the J "' =5 and J' =6 states where the decay to the 6- im weak, but significantly stronger than the are decay to the 5 ~ , An assignment of J' =4' would thus be very improbable, since it would be faced with a 1114 keV M2 transition to the J ' =6~ level, being stronger . than a 0521 keV El transition to the J' = 5~ level .lFherefmr m, the state at 3.887 MeV -

-

+

im assigned as T' =5 ^

4.023 MeV (fig . 15) shows that it decays strongly to the J` 6 2' at 3277 keV in the quasi-rotational band. Other transitions lead to 2' or 3') also excited in the the Jy` =5 ` state and the state at Ex = 3739 keV (J neutron pickup reactions A transition to proton stripping reactions and in the the state ak 3 .88 7 MeV is also observed, together with the decay pattern of this state. Although the decay pattern of the state at 4.023 MeV differs from that of the state at 3 .887 MeV , the possible spin assignment J/' =5+ of the 4.023 MeV state im in agreement 4' with the transitions observed, but a J ' is also a possible assignment . It should be noted however, that the state at 4.023 MeV im the only state excited in the proton transfer reactions which is observed in the decay of the state at 4.285 MeV. As will be shown later, the state at 4.2@5 MeV im assigned am J' =7 . Since an E2 The decay scheme of the state at

9).

+

4023 keV

o

66

SD

Fig. 15 . The )+deouy pattern oy tbe state au E,, = 4LU23 Me vio / / 6 8o (assigned aa J ' =5') .

J.M. Schippers et aL / "6Sn

293

transition to a 5+ is much more likely than a M3 transition to a 4 + state, certainly for this low y-ray energy, the state at 4.023 MeV can be assigned as J' = 5+. Strong y-lines at 888 and 911 keV, could not be placed in the level scheme. Their equal intensity suggests that they form two successive transitions leading to a known J' = 2 + state at 2225 keV, but no subsequent decay of this state is observed, and possible intermediate levels have also not been reported . The way the state at 4.076 MeV decays is very similar to that of the state at 3.887 MeV; even a "primary" y-transition to this latter state is observed, followed by y's from its most important decay mode. In fig. 16 we observe a strong transition to the J' = 4 ; state at 2392 keV. Also the decay via the 3379 MeV state is observed, and so is the decay via the J'r = 4+ at 3046 keV. A difference, however, is that the sidefeeding to the J'r = 3 - is not very strong and that we observe a transition to the J'T = 62 in the quasi-rotational band. Also in this case the previous spin assignment of J'r = 4+ , 5 + is in agreement with these observations. No primary -y-transition to the J'r = 6, state is seen, but we do observe its decay. It might be possible that this 1303 keV -y-ray is not resolved from the tail of the strong transition 2 ; --~ g.s. of 1293 keV. It this is the case, the same arguments applied for the spin assignment of the 3.887 MeV state will hold for the J' = 5 + spin assignment of the state at 4.076 MeV, but no definite assignment can be made. Combining the conclusions on the spin assignments of the three states at 3.887, 4.023 and 4.076 MeV, it is likely that all three states have spin J'r =-,".+ . The partial 4076 keV

»6S n Fig. 16. The y-decay pattern of the state at Ex = 4.076 MeV in

116

Sn (assigned as J~ = 4+ or 5+).

.I.

~~ Sc lirX'~ee°s e~ cal / ® °~Sn

s~+n~ rule for .l ~ = 5} is ~ = l.l, so that the total observed ds,, strength for these states, being 0.90, almost exhausts this sum rulz. This would imply that almost abl states with (g~~d;;~ ; 5~)~ have been found. The J~ = 5~ states are described by the wa`=e functions °y' with =+ °'-~- < 1 [and c~` = is, ®~)/ l.l, . . . etc.], which is true for all three states. Thus a Ja = 5~ assignment is possible for all three states. Furthermore, if one of these states `vere a 4~, apart from the observed decay to the collective bands, also strong transitions directly to known JT = 2+ levels should be observed. The nonobservation of these tranaitions, therefore, supports the J ~ _ ~+ assignments . e spectroscopic strength found for the g,; , orbital in the peak at 4.285 MeV is -= 0.8~ and this state also has a significant I = 2 component in its wa~e function, so that the highest possible spin is J °~ = 7+ , assuming dç,, for the 1= 2 cempone~~t . This state contains a large part of the J" _ 7 ' partial sum rule ' ), which is (2Js-a-1 ~/(2r,® + 1) _ ;1,, and the sum of the squared amplitudes in the ~~ave function yields ~' + ' = 0.81, with a and ß deduced from the corresponding ds,, and g,,z ~ strengths, so that this is a valid wave function. A J = 6t assignment would exhaust the partial sum rule for 94°/® and an assignment of the 4.285 MeV state as a J~ = 5+ woul~3 exceed the partial sum rule with 11% for this state alone. Therefore, from the ( 2fle, d~ reaction we can give this state a spin assignment of J~ =6+ or 7+. e special character of the peak at 4.2851l~IeV is visible also in its Y-decay (see . ~. ,Although transitions to the quasi-rotational band are observed (to the two 17 g _ J - ~~ states) the major y- decay is to the J~ = 8~ state at 3228 keV. Strong transitions to the J °' = 5 ; , 6; and 7 ; are also observed . The decay of the JT = 3 ~ suggests some sidefeeding to this JT = 3 ; level, but transitions to this level different from that ;~ corning from the J = 5 ; are not observed, although also in this gate some weak 1a-lines could nvt be placed in the level scheme . For a J ~ = 7+ assignment, the relatively large intensity of the M2 transition to the JT = 5 ; can be explained by the large energy of this transition. If the 4.285 MeV state were J~ = 6+, the transition to the J' = 5 ; would be E1 and its intensity would be expected to be much larger than that of the E 1 / M2 transition to the J~ = 8 i state, which is not the case. We c®nclude that the spin assig~iment for the state at 4.285 MeV can be established as J . _7 + . figs. l â and 9~ the v-decay patterns of the cluster at 4.811~eV are shown. Due to the good energy resolution in the triton spectrum we co~.~ld separate the EX = 4.84 ~ eV part showing strong 1= 4 or 5 transfer, from the state at 4.765 NIeV, showing predominantly I = 2 transfer (see figs 6 and 7). The major difference between the two states is that the E~ = 4.840 keV state decays via the negative-parity band,

J.M. Schippers et al. / "'Sn

295

4285 keV

F:g. 17. The y-decay pattern of the state at E,, = 4.285 MeV in " 6 Sn (assigned as J' = 7+).

already entering at the J 7T = 9 ; state, whereas the state at 4.765 MeV decays via the positive-parity band mainly. This strong selectivity within a single cluster is also visible in the spectra showing the sidefeeding to states in the negative-parity band (fig. 12) . The decay of the 4.765 MeV state enters the negative-parity band at the J' = 8 ; state and although the connecting y-lines are not observed directly, there is a strong sidefeeding to the J' = 6-, and 7 ; levels . The positive-parity band is entered at the J'r = 6' level . Since the (;He, d) results show strong d 5 / 2 transfer, the highest possible spin is J' = 7+ . The 1537 keV transition to the J'r = 8, level in the case of a 7+ assignment is much more probable to be E1 than M2 in the case of a 6+ assignment . Therefore, the 4.765 MeV state can be assigned as J' = 7+ . The feeding of the J' = 9 ; makes it very probable that the state 4.840 MeV contains part of the fragmented J' =10-, 8- or 8+ configurations . The observed decay to the J 'r = 7 ; is mainly from the higher and lower parts of the cluster (see also fig. 12). It should be noted that also the decay to the J'r - 8; is observed in this gate. In fig. 3 it can be seen, that the (a, t) reaction does not emphasize the 4.840 MeV peak as much as the clusters at higher excitation energy. Together with the observed strong high-1 transfer in fig. 6, this leads to the conclusion that also a J'r = 8+ component with G = 0.3 is present in the 4.840 MeV state.

J.

Schippers et al. / "'Sn

4765 keV

1555

i I

1 1 1 1

31~ -545

-3227.9 `3209.9 7_3105.6 _ 2908.8 2773.1

6 2365.9 355ns 2266.1

973 1293.5

1293.5

0. Fig. 18. The y-decay pattern of the low-energy part of the cluster at EX = 4.8 MeV: the state at EX = 4.765 MeV in 116Sn (assigned as J'=7 + ).

The "primary y-rays" from the states in the cluster at 5.5 MeV show a strong y-decay to the J'r = 8, and 71 levels, but not the 9, , see fig. 20. The y-ray transitions to the positive-parity band are not seen directly, but it is clear that the J' = 6' level at 3033 keV is populated in the decay process, although probably part of this feeding will originate from the decay of the continuum underlying the 5 .5 MeV cluster. The absence of the decay to the J' = 9, state indicates that the spin of the states involved roust be smaller than 10. Combining these results with the strong 1= (4 or) 5 transfer shown in fig. 6, we conclude that at least part of the cross section may be due to J' = 8- or 9- strength . A strong similarity is observed between the y-decay patterns of the high-excitation energy part of the cluster at 5.78 MeV and the peak at 6.295 MeV, as can be seen in figs. 21 and 22. Although the statistics was very poor, we have observed the primary y-transitions from these states to the J" =9- and 8- levels . In fig. 12 it is

J.M. Schippers et al.

/

" 6Sn

4840 keV

29 7 4880

1936

3522 .5 3492 .9 3227 .9

2529 .4

2908.8 2773 .1

4

2365.9 2266.1

2112.6 2:

355ns

?- 1293 .5 1293.5

116S

n

Fig. 19. The y-decay pattern of the high-energy part of the cluster at EX =4.8 MeV: the state at EX = 4.840 MeV in 116 Sn (assigned as J' = 8 ;, 8 - or 10 - ).

seen that there is a strong feeding from these clusters to the J 7r = 9- state. Considering also the enhanced h  /, strength at Ex = 5.8 MeV observed in the ('He, d) reaction and a comparison with the (a, t) spectrum (see figs. 7 and 3), we conclude that the states in the high-energy part of the cluster at 5 .78 MV and the states at 5.86 and 6.295 MeV are most probably J' =10 - states and carry part of the stretched (g9i2h i2) . .configuration. Both the peak at 6.295 MeV and the cluster at 5.78 MeV show decay via the J' = 8+ states, present in both the negative- and the positive-parity band . This reflects the contribution of the continuum under the peak and cluster gated on . This conclusion is based on the observation of the -y-decay gated on the continuum between 6.4 and 7.5 MeV excitation energy shown in fig. 23 . The continuous part of the spectrum does not show a clear preference for decay through either the positive-parity band or the negative-parity band. This reflects that it is a mixture of strongly fragmented states of many multipolarities .

amultiplicity 26result STRENGTH the They-decay reactionmeasurement ofhigh-spin a OF ismixing I pattern observed STRETCHED-SPIN performed of states of the a the the with cluster have -y-ray in particle-hole Aincreasing coincidence Schippers been of multiplicity CONFIGURATIONS unresolved =n 8-identified eîexcitation or MeV al strength, 9-)with /states (see 1"'Sniny-decay fig atenergy increasing spectra E8)== a5 This continuous measureMeV obtained 102 with can in be level via interpreted increase (assigned density protonIn the of as

.

J. I.

298

440-160

1

.9 8-Y-322T9 3209.9

i

2"3

"'S Fig,

.

J"

.,

.

In the as

.

5.

.5

"Sn

.

.

iscussion

5.1 . Stretched transfer

.,nts .

J. M. Schippers et al. / "'Sn

299

5.66-5 .81 MeV

(2250)

9 - 3522 .5 3492 .9 3227 .9 3209 .9

681 3033 .2 6

0. u

116S

n

Fig. 21 . The y-decay pattern of the high-energy part of the cluster at E,, --= 5.8 MeV: the states at E,, = 5.66-5.81 MeV in "'Sn (assigaed as J' = 10-).

proton stripping reactions the spins of the isolated states observed around 4 MeV excitation energy could be assigned and were all found to be of unnatural parity (J' = 3 +, 5+, 7+), suggesting that unnatural-parity states are less fragmented than natural-parity states. Above 4 .3 MeV the (3 He, d) and (a, t) transfer data exhibit clusters of unresolved peaks that have negative parity and which are candidates for high-spin states . An important part of the fragmented J ' =10- strength has been located with the coincidence measurement. Although the (3He, d) and the (a, t) experiments have given much information on the 1-transfer, most of the spin assignments were based on the high-resolution (a, ty) experiment. The y-decay also provides us with insight in the nature of the continuous background observed in the transfer reactions:

5chippers el al. / ®'ßsn

6295

eV

2770 3712 .9

6.

3033 .2

6,41

x -~ 2802 .1

2529 .4

416 .9

2112.6

4 2392 .2 1099

1293.5

0.

116S

n

Fig. 22. The y-decay pattern of the state at E,, = 6.295 MeV in "6Sn (assigned as J' =10 - ).

fragmented parts of the high-spin states give a large contribution to the cross section o the continuum. Although high-spin configurations of natural parity, such as (g9 129 2 ; 8+ )" and (gy',h  ,, ; 9- ), should in principle be observed with a large cross section, the only possible J" = 8 + candidate (at EX = 4.840 MeV) that has been observed, would only carry a strength of about 17% of the partial sum rule . The strength of the J" = 9- configuration might be located in the clusters, since these have been identified as having negative parity. Evidently, most of the strength of the J" = 8 + and probably also of the P = 6+ configuration is fragmented over many doorway states in this region and it may have an important contribution to the continuum. This might then also be true for the J" = 9 - state. The spectroscopic strength present i the clusters assigned as J" =10- can be obtained from the (a, t) differential cross sections by normalizing to the 99/2 spectroscopic factor of the

J.M. Schippers et al. / "'Sn

30 1

ground state 9 ) . Then, even neglecting the contribution of the state at EX = 4.840 MeV because of its uncertain spin assignment, the strengths located in the clusters at 5 .78, 5.860 and the state at 6.295 MeV and observed with the singles (a, t) reaction add up to: Y, G(J' =10- ) - 1 .1, which is about 50% of the partial sum rule for J, =10-. 5.2 . CORE POLARIZATION

There is strong evidence or the admixture cf neutron components in dominantly proton excitations and vice versa for proton components in neutron 2qp excitations: a pure proton excitation cannot decay in one step via a gamma transition to a pure neutron excitation. The observation of this decay indicates that neutron- and protonexcitation components must coexist in the initial and/or final wave function. These components may originate from core polarization . The case study 36) of the J' = 9; state at EX = 3 .522 MeV has shown that its wave function has a two-neutron quasiparticle configuration as leading component, but that core polarization gives additional proton components. These may be responsible for the gamma decay of the proton excitations to the band of neutron excitations. But also a neutron component in the wave function of proton lplh states excited in a proton transfer reaction, can cause a gamma-decay branching to neutron excitations. On the other hand, it is observed that proton excitations decay mainly via the negative-parity band . Even stranger, the purer the wave function of the excited state, the more it decays via the band of neutron excitations. For stretched-spin states the seniority of the leading configuration is 2 and since electromagnetic transitions proceed via the action of a one-particle operator, seniority cannot change by more than 2 units in one gamma-ray transition. This makes it impossible for these states to decay to e.g. a collective J ' = 6+ 3-phonon state, which has seniority 6. The preference for the negative-parity bared in the gamma decay of isolated parts of the stretched-spin states may thus be explained by the relatively simple character and low seniority of the excitations in the negative-parity band. The contribution of core polarization is probably very large in the gamma decay of the J' = 7+ at 4.285 MeV. With the ( 3 He, d) reaction we have found that its leading configuration is a jg9~2d5/2 ; 7+),, + F' 1g9l2g,/2 ; 7+) .. . However, its most important gamma decay proceeds via the band of 2qp-neutron excitations . Therefore, lplh excitations >2hw (core polarization) are expected to have an important contribution to this state and the states in the negative-parity band. The configuration a jg9/2 d5/2 ; 7+),, + ß 199/'297/2 ; 7+), should have a large cross section in (p, p') and (e, e') . This is not observed, as can be seen from fig. 1 in ref. 2 ), suggesting a large contribution of core polarization terms that interfere in a destructive way. This is in agreement with the general observation that interference of core polarization terms is constructive for natural-parity yrast states (e.g. the J' = 9, ), but the different amplitudes interfere destructively for yrast states with unnatural parity.

J, Al, Schippers et al. / `Sn

302 $3 . LEVEL

ENSITY AND SPREADING OF UNNATURAL-PARITY STATES

he observation of well-resolved unnatural-parity states and the non-observation natural-parity states around 4 MeV excitation energy, and the localization of pans the stretched J' =10- configuration at higher excitation energies may be related to a level density argument 4®). The fragmentation proceeds in stages with increasing complexity; 1p1h " 2p2h -> 3p3h -> . . . The initial configuration is (1p1h), and, therefore, the most important step is the fragmentation into 2p2h states . The partial level density for 2p2h states can be written as 41)

a

p(Ej

= [F, - ( EY(J) +,A)1 -1

where U is the excitation energy, offset by the energ; 6 Y of the yrast state of spin J and the pairing gap. Due to their collectivity, great states of natural parity have lower excitation energy than configurations of unnatural parity. Therefore, at the osition of unperturbed (1p1h), configurations the level density of natural-parity states is higher than that of unnatural-parity states, so that the possibilities for unnatural-parity states to mix with other configurations are decreased . Since no other J_ = 5' and 7' states are known, it may well be that the J' = 5' at 3 .887 MeV and the J" = 7' at 4185 MeV are themselves yrast states. e localization of parts of the stretched J' = 10- configuration, the J' = 7' state nd well resolved J" = 5' and V states, the absence of strong clusters or peaks with J` = 8' and probably also with J` = 9- strength, suggests that the strength of natural-parity configuration is spread over many other states in the spectrum. This is also supported by the y-decay observed in coincidence with the continuous background" (see fig. 23). Therefore, the strength that can be localized depends n the configuration and parity of the stretched-spin state. 5.4. COMPARISON WITH MODEL CALCULATIONS

Shell-model calculations of "'Sn and "'In within a basis adequate to describe the strength fragmentation at these excitation energies properly, are impossible due to computer limitations . The most extensive calculations available in this respect are the ones performed by Waroquier et al. 35), who include in the model space proton 1p1 h, neutron two-quasiparticle configurations and couplings of the two. ey predict a strong peak for the J' = 8+ state at E, = 4.1 MeV with strength G = 01. In 6g. 24 the results of the calculations are shown, along with a different spectrum enhancing the I = 4 and 5 strength . States with a (gW1297/2),, configuration in their wave function are located around 4 MeV excitation energy where the strong J_- = 8 + state should be clearly visible . The only state at this excitation energy, possessing the strong I = 4 component is the state at 4 .285 MeV, which has however been assigned J' = 7 +. The only possible 8' candidate was found at higher energy

J.M. Schippers et al. / 1 16Sn

480

30 3

2~95 583

9 3522.5 3492 .9 --4:-3227 .9 3209.9 `- 3105 .6 :2908.8 2773.1

973

116S

n

Fig. 23. The y-decay pattern of the continuum in the region Ex = 6.4-7.5 MeV. (E,, = 4.840 MeV) than predicted and it would carry only 30% of the calculated strength . The calculated positions of the unnatural-parity states are in good agreement with the observations (see also fig. 25 for the 1= 0 and 2 strength). According to the calculations of Waroquier the 10- strength is spread over many small fragments except for one peak having a spectroscopic strength of 1 .0, which is not too far from the total strength we observed . The same is true for the 7+ strength. In conclusion we may say that, although the spreading of the natural-parity states is not reproduced at all by the calculations, for the unnatural-parity states the results of Waroquier et al. are in rather good agreement with the data. The spreading for

"ssn J.M. âchippers et al. /

Fig. 24. Difference (top) of experimental " 5 1n( 3 He, d)" 6Sn spectra taken at Blab=18 .5° and 8.5°, enhancing of the 1=4 (g,/2) and 1=5 (h/2) strength. The bottom part shows strength distributions for 35) . proton stripping into these shells resulting from shell-model calculations of Waroquier et al.

these states is more than predicted but several fragments could be traced back that together exhaust an important part of the sum rule. In the calculations usually only one fragment possessing important strength is predicted. 6. Conclusions

e high resolution obtained with the ( ;He, d) and (a, ty) reactions on "'In, enabled us to study the gamma decay of states in " 6Sn at an excitation energy between 3.5 and 7 .5 MeV. Spin assignments of mostly unnatural-parity states have been made and important parts of the strengths of (nearly) stretched-spin states of unnatural parity in "6Sn have been measured by observing the feeding via gamma decay of known states of high spin in ''6Sn. At the rather high excitation energy investigated, the cross section is fragmented into isolated peaks, unresolved peaks grouped together in clusters and a continuum, starting at E,, = 4.3 MeV. We have found that the strength in the clusters and isolated peaks is mostly from unnatural parity states, whereas the cross sections of natural-parity states and low-spin states are fragmented into the continuum. This continuum, which has its maximum at about 7 MeV, may thus be interpreted as an addition of many unresolved lplh excitations, based on a proton g9i2 hole and a particle in the next major shell above the Z = 50 shell gap.

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E x (MeV) Fig . 25. Difference (top) of experimental "51 .( 3 "rle,d)" 6 Sn spectra enhancing the I=0 (s,/2) strength, together with the results of shell-model calculations 35). The bottom part shows the same comparison for I = 2 (d5/ 2 and d3/2) transfer.

Stretched-spin states, both of natural and unnatural parity, have rather simple and unique configurations . Stretched-spin states of natural parity are found to be subject to strong fragmentation since they have not been observed in peaks and their cross section adds up to the continuum of the 1 hw bump. Particle-hole states of unnatural parity appear to be less fragmented, since, above 3.7 MeV excitation energy (almost) all resolved states are found to be of unnatural parity . We have found these unnatural-parity states to be mostly of high spin . For unnatural-parity states our observations are shown to be in qualitative agreement with model calculations on the spreading mechanism of spectroscopic strength, although an extension of the configuration space, with couplings of ph excitations to quadrupole and octupole phonons, would be necessary to describe the spreading mechanism of high-lying particle-hole excitations better. An intriguing question regarding the identified parts of the J =10- stretched-spin configuration remains. The strength is fragmented into clusters that decay to the same final state. Each cluster looks like a broadened, single peak with a width about

J. M. Schippers eî al. / "'Sn

five times larger than the instrumental resolution . All these clusters are stable against isle decay, so that finite lifetime does not play a role. It is not clear why this configuration, although it seems a little fragmented, groups together into different clusters. is work was performed as part of the research program of the "Stichting voor amenteel Onderzoek der Materie" (FOM) with financial support from the erlandse Organisatie voor Wetenschappelijk Onderzoek" (NWO) . eferences 1) D.C.J.M. Hageman, M.N. Harakeh, R .H. Siemssen and S .Y. van der Werf, Nucl. Phys. A290 (1977) 1 2) S.Y. van der Werf, N. Blasi, M.N. Harakeh, G. Wenes, A.D. Bather, G.T. Emery, C.W. Glover, W.P. Jones, H. Nann, C. Olmer, P. den heijer, C.W. de Jager, H. de Vries, J. Ryckebusch and M . Waroquier, Phys. Lett . B166 (1986) 372 3) S. Galès, C.P. Massolo, S. Fortier, E. Gerlic, J. Guillot, E. Hourani, J.M. Maison, J.P. Shapira, . Zwieglinski, P. Martin and V. Comparai, Phys. Rev. Lett. 48 (1982) 1593 4) S Gals, Proc. Conf. on nuclear structure at high spin, exitation and momentum transfer, Indiana 1985, , A.I.P. 142 (1986) 272 ; Proc. Niels Bohr Symp. on nuclear structure, Copenhagen 1985 and references therein 5) A. van Poelgeest, J. Bron, W.H.A. Hesselink, K. Allaart, J.J.A. Zalmstra, M.J. Uitzinger and . Verheul, Nucl. Phys. A346 (1980) 70 6) H. Sakai, R.K. Bhowmik, S. Brandenburg, J.H . van Dijk, A.G. Drentje, M .N. Harakeh, Y. Iwasaki, R.H. Siemssen, S.Y. van der Werf and A. van der Woude, Phys. Lett. 8103 (1981) 309 7) H. Sakai, R. K. Bhowmik, S. Brandenburg, J. H. van Dijk, A.G. Drentje, M.N. Harakeh, Y. Iwasaki, R.H. Siemssen, S.Y. van der Werf and A. van der Woude, Nucl. Phys. A441 ( :985) 640 8) F. Azaiez, S. Fortier, S. Ga*s, E. Hourani, J.M. Maison, J. Kumpulainen and J.P. Schapira, Nucl . Ph1°s. A444 (1985) 373 9) J.M. Schippers, J.M. Schreude, S.Y. van der Werf, K. Allaart, N. Blasi, and M. Waroquier, Nucl. Phys. A510 (1990) 70 and references therein 10) J. Blachot and G. Marguier, Nuclear Data Sheets 59 (1990) 333 and references therein 11) J. Bron, W.H.A. Hesselink, A. van Poelgeest, J.J.A. Zalmstra, M.J. Uitzinger, H. Verheul, K. Heyde, M. Waroquier, H. Vincx and P. Van Isacker, Nucl. Phys. A318 (1979) 335 12) M. Conjeaud, S. Rarer and J. Picard, Phys. Lett. 23 (1966) 104 13) J.A. Biggerstaff, C. Bingham, P.D. Miller, J. Solomon and K.K. Seth, Phys. Lett. B25 (1967) 273 14) R. Shoup, J. D . Fox and G. Vourvopoulos, Nucl. Phys. A135 (1969) 689 15) R.J. Peterson, B.L. Clauses, J.J. Kraushaar, H. Nann, W.W. Jacobs, R .A. Lindgren and M.A. Plum. Phys. Rev. C33 (1986) 31 16) A.D. Bather, G .T. Emery, W.P. Jones, D.W. Miller, G.S. Adams, F. Petrovich, W.G. Love, Phys. Utt B97 (1980) 58 17) R.A. Lindgren, M. Leuschneq B.L. Clausen, R.J. Peterson, M .A. Plum and F. Petrovich, Proc. A.I.P. Conf. 142 (1986) 133 18) V.R. Pandharipande, C.N . Papanicolas and J. Wambach, Phys. Rev. Lett. 53 (1984) 1133 19) M. Jaminon, C. Mahaux and H. Ng6, Nucl. Phys. A440 (1985) 228 20) S. Frullani and J. Mougey, Adv . Nucl. Phys. 14 (1984) 1 21) H .P. Blok, Proc. 5th Mini Conf. on non-nucleonic degrees of freedom and intermediate energy electron-nucleus scattering, Nikhef-K, Amsterdam, 1987 22) G. van der Steenhoven, H.P. Blok, E. Jans, M. de Jong, L. Lapikas, E.N.M. Quint and P.K.A. de Witt Huberts, Nucl. Phys. A480 (1988) 547 23) C. Mahaux and R . Sartor, Nucl. Phys . A475 (1987) 247 24) C. Mahaux and R. Sartor, Nucl. Phys. A481 (1988) 381

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