A study of excited states of 21Ne

El 1.E.l: LE.4

Nuclear Physics Al89 (1972) 641-664;

@ North-Holland

Publishing

Co., Amsterdam

Not to be reproduced by photoprint or microfilm without written permission from the publisher

A STUDY OF EXCITED STATES OF “Ne C. ROLFS and H. P. TRAUTVETTER Physikalisches

Institut der Universitiit Freiburg i. Br., Germany 7 and University of Toronto, Toronto, Canada 77

E. KUHLMANN Physikalisches

Institat

and F. RIESS ttt

der Universitiit

Freiburg

i. Br., Germany

Received 22 March 1972 Abstract: The 2.79, 3.66,3.73 and 3.88 MeV states in ZINe have been studied with the ‘sO(a, ny)“‘Ne reaction at E, = 4.9 to 6.6 MeV. Both neutrons and y-rays were observed. The investigation involved measurements of y-ray angular distributions as well as ny and w angular correlations for procuring spin and multipole mixing information. The results of this investigation combined with previous work lead to respective Jn assignments of a-, +-, &’ and #-. The states above 4 MeV excitation energy have been studied at E, = 7.5 to 11.5 MeV with a Ge(Li)-NaI(T1) pair spectrometer. Two new states at E. = 5525.0& 1.5 and 5682f2 keV have been found. Evidence is also presented for a doublet comprising the states with excitation energies 5821+3 and 5823*3 keV. Excitation energies, y-ray decay schemes, mean-lives and limits on spin assignments to all the states are reported. Some of the results are compared with the available data for the mirror nucleus *INa and discussed in terms of the rotational model. E

NUCLEAR REACTIONS ‘sO(a, ny), E = 4.9-l 1.5 MeV; measured EY,~, ny-coin, c(E; EY, BY, &, t&J, DSA. a’Ne deduced levels, J, n, 6, t. Enriched target.

1. Introduction

Recent studies of the low-lying states of “Ne resulted le3) in the assignment of a K” = 3’ ground state rotational band consisting of all members up to the J” = y’ fifth band member. In addition, the q’ sixth member of this band has been tentatively identified “) with the state at E, = 6.45 MeV. The properties of these band members are well described “) by the Nilsson model ‘). This model also predicts excited rotational bands below 5 MeV excitation energy with K” = $+, 3- and 3’. Some of the low-lying states have been tentatively identified ‘*3, ‘) as the lowest members of these predicted bands. However, the lack of rigorous J” assignments for some of these states makes such identifications highly speculative. The present work describes an attempt to obtain unique spin assignments for the 2.79,3.66,3.73 and 3.88 MeV states. t Work supported by the Bundesministerium Forschungsgemeinschaft. tt Present address. ttt Now at Universitat Miinchen, Germany.

fiir wissenschaftliche

641

Forschung

and Deutsche

C. ROLFS et al.

642

TABLE I Summary of excitation energies (keV) of the excited states in zlNe Present work

Previous work ref. *)

350.010.9 1747.1+1.1 2790 +2 2795.8&1.8 2866.7&1.5 3663 *2 3735 12 3884.8f1.5 4433 12 4525.8h1.5 4685.8&1.5 4730 +2 5336.9& 1.2 5432.1fl.5 552&O&-1.5 5550 *2 5630.2&1.0 5682 12 5691 *3 5778 +3 5821 &3 5823 +3 5993.9+1.8 6033.8fl.7 6177 *3 6266.7kl.6 6448 12 6554.3 + 1.2 6606.6il.S 6642 &3 6747.451.8 7008 f3 1043 &-4 7109 +4 7154 *5 7226 15 7320 &5 7375 &5 7426 +5 7600 f5 7655 +5

350.2f0.8 1747.4kl.O 2790.2f1.5 2795.6fl.5 2866Sf1.5 3663 &2 3734.8hl.5 3885.5f1.5 4431 *2 4526.111.5 4687.4fl.5 4730.1 -cl.5

ref. ‘)

ref. 6,

ref. 11)

352110 1736&10 2789f10

349*7 1750f7 2800&7

28641-10 3664&10 3733*10 3883&10 4433*10 4525110 4681 f 10 4727+10 5334*10 5427 &-10

287017 366617 373717 3889k7 4435*7 4528&t 468519 472918 5338+8 5434&t

350.5+0.1 1745.4+0.2 2788.5f0.3 2797 fl 2865.5 f0.2 3662.0+0.4 3733.7ztO.2 3882.7-cO.3 4432.0fl.0 4523.710.5 4683.1+0.7 472X011.0 5332.6+1.0 5429.1 fl.O

554lflO 5624+ 10

555o+s 5632fQ

5626

5683*10 5770&10 5813&10

569419 5777&Q 5822&Q

5683.OkO.6 5772 t2

5986+10 6029&10 617OztlO 6259flO 6446&10 6543ilO 6597110 6633 f 10 6743&10 7002&10 7039&10 7104*10 7148flO 7225110 7326f10 7354&10 7413*10 7597rt10 7644110

5997+9 6036110 6176+10 6267*10 6450+12 65.55+10 6606110 6647112 6748&10

5991 +3 6030.410.5 6165 12 6263 22 6445.9rrtl.O 6548 &2 6600 15

&2

The investigation involved measurements of y-ray angular distributions and ny and yy angular correlations associated with the reaction ‘*O(g, ny)‘lNe (Q, = -712 keV). Higher members of these proposed excited bands could not be identified since most bound states (neutron binding energy = 6.76 MeV) above 5 MeV excitation energy

ZINe EXCITED

STATES

643

have not yet been thoroughly studied ex~rimentally. No y-ray decay schemes of the 16 observed bound states 6*‘) with E, 1 5 MeV (table 1) had been reported except for the 5.69, 5.99 and 6.45 MeV states 3*“). Some spin assignments for these states, based only on stripping data of the “Ne(d, p)“Ne reaction, have also been reported [refs. ‘* ’ “)I. Therefore an investigation of excitation energies, y-ray decay schemes, mean-lives and spin assignments of these states was carried out. A Ge(Li)-Na~~Tl) pair spectrometer was utilized in this investigation. The description of the experimental equipment (sect. 2) and procedure (sect. 3) precedes the method of analysis (sect. 4) and the results and conclusions (sect. 5). A comparison of the level schemes of the mirror nuclei “Ne and “Na as well as a discussion of some of the data in terms of the Nilsson model is presented in sect. 6. Since the completion of the present work, additional studies on excited states in 21Ne have been reported 11) which utilized similar techniques. Comparisons with these data are incorporated in the text and tables. 2. Experimental equipment The singly and doubly charged 4He beam of 60 to 200 nA was supplied by the 5.5 MV Van de Graaff accelerator of the Physikali~hes Institut Freiburg. The target consisted of a 0.5 mm thick sheet of tantalum which was anodized in 98.8 % I80 enriched water to produce a TazO, layer of 200 pg/cm’ thickness (energy loss = 50 keV for 9 MeV 4Hef+ ions). The y-rays were observed with 15 and 55 cm3 Ge(Li) detectors, as well as various NaI(T1) crystals of sizes 10.2 cm diam. x 10.2 cm and 12.5 cm diam. x 12.5 cm. In addition, a Ge(Li)-NaI(T1) pair spectrometer was used with the 55 cm3 Ge(Li) detector as the central detector flanked by four NaI(T1) crystals. A detailed account of the system and its performance is given elsewhere 3*12). Neutrons were detected with a 5.1 cm diam. x 7.6 cmNE213 scintillator. The neutron-y discrimination was effected with standard pulse-shape techniques ‘, with a neutron discrimination threshold set at 300 keV. 3. Experimental procedure 3.1. THE 2.79 MeV STATE It has been found 13) that in the region E, = 2.79MeV a doublet exists, with excitation energies of 2790 and 2796 keV. The upper member of this doublet is known ’ “) to decay 100 % to the ground state with a mean-life “) of s 25 fs. The J” = 3’ assignment to this state is based on stripping data 9*14) from the 2oNe(d, P)~‘N~ reaction. The lower member of this doublet decays r3) 85 % to the first excited state and 15 % to the ground state with a mean-life ’ ‘) of 845 10 ps. This 2.79 MeV state is strongly excited in the “Ne(p, d)“Ne reaction with an I = 1 stripping pattern “) r ORTEC, catalogue number 1000, p. 20.

400

600

100

t

15co

(b) GATE

(01 GATE

ON

ON

350

keV

LINE

NEUTRONS

2 3

2500

2790

2’

1

1397

-7

-1

2’Ne

’

2

96

I

2517

0

1747

Fig. 1. Gamma-ray spectra taken with the 55 cm3 Ge(Li) detector at 0 = 0” and Ea = 4.92MeV in coincidence with (a) neutrons (0, = 30”) and (b) the 350 keV y-ray. Random counts were negligible. The observed y-rays can be accounted for as shown in the inset level diagram.

E

z

: 0

8

0”

r 5

200

400

“Ne

EXCITED STATES

SlNIl03

33N3013Nl03

646

C. ROLFS et al.

implying J” = +- or 3-. A J = 3 spin assignment to this state has been suggested 1“) from an application of the (2J+ 1) rule to the 23Na(d, a)‘lNe cross sections. On the other hand, the ny angular correlations of the 2.79 + 0 and 2.79 + 0.35 MeV y-ray transitions, observed in the ‘sO(a, ny)21Ne reaction at Ea = 4.92 MeV, have been reported “) to be slightly anisotropic excluding therefore J(2.79) = +, in disagreement with the results of ref. 16). It should be pointed out, however, that the results of the ny angular correlations of ref. ‘) were obtained with a NaI(T1) detector. In addition, the possible presence of an energetically similar y-ray of Ey = 2.52 MeV originating from the transition 2.87 + 0.35 MeV was not taken into account. A repetition of these measurements with a high-resolution Ge(Li) detector was therefore desirable. In the present work, ny angular correlations were measured with the 55 cm3 Ge(Li) detector (distance to the target D = 7 cm) and for E, = 4.92 MeV as in ref. ‘). In one experiment the neutron detector was at 0” (D = 8 cm), in a second one at 30” (D = 10 cm). In the second experiment, a yy angular correlation for the 2.79 + 0.35 + 0 MeV cascade in geometry 20) VII was measured concurrently with the ny angular correlation. The pulses from the Ge(Li) detector were gated by those pulses corresponding to the 0.35 MeV y-ray from a fixed NaI(T1) detector (0 = 90”, I$ = 90”, D = 15 cm). The y-ray spectra obtained from a second 40 cm3 Ge(Li) detector at a fixed position, as well as the number of counts from the neutron detector and NaI(T1) gate crystal, served as monitors for the final normalization of the angular correlation data. Sample coincidence spectra are shown in fig. 1. Due to the negative Q-value of the reaction, only the population of states below E, = 3 MeV was allowed for a bombarding energy of E, = 4.92 MeV. At this beam energy the 2.79 MeV state was strongly excited while the 2.80 and 2.87 MeV states were much more weakly populated (fig.1). It was assumed previously ‘) that the 2.87 MeV state was not populated at this beam energy. 3.2. THE 3.66, 3.73 AND

3.88 MeV STATES

Previously reported yy angular correlations of the 3.66 + 0.35 + 0 and 3.88 + 0.35 + 0 MeV cascades as well as the 3.73 + 0 MeV ny angular correlation resulted “) in assignments of J(3.66) = 4, J(3.73) = Q or 3 and J(3.88) = j or 3. Since these results were also obtained with NaI(T1) detectors, a remeasurement of these correlations was performed with a Ge(Li) detector. The use of a Ge(Li) detector also permitted the measurement of the yy angular correlations for the 3.73 + 0.35 + 0 MeV cascade. The 3.66 MeV state is particularly interesting because of its 38 % branch to the 2.79 MeV state. This 3.66 MeV state was found to be strongly excited at E, = 6.10 MeV (fig. 2). At this beam energy only states below E, = 4 MeV can be populated. The ny angular correlations were performed with the 55 cm3 Ge(Li) detector at 0 = 30” to 90” (D = 7 cm) and the neutron detector at 8 = 0” (D = 8 cm). A sample coincidence spectrum is shown in fig. 2a. From these spectra, the ny angular correlations of the 3.66 + (2.79+2.80), 3.66 -+ 0.35 and 3.73 + 0 MeV y-ray transitions have been deduced.

zlNe EXCITED STATES

647

The 3.73 and 3.88 MeV states were found to be strongly populated at E, = 6.60 MeV. At this beam energy, angular distributions as well as yy angular correlations of the y-ray transitions from the 3.66,3.73 and 3.88 MeV states have been measured. In one experiment, the 15 cm3 Ge(Li) detector + was fixed at 6’= 90”, 4 = 0” (D = 6 cm) and operated in coincidence with either of two NaI(T1) detectors. One of these detectors was moved between t3 = 0” to 90” with $J fixed at 180” (D = 15 cm) while the other was moved between 4 = 90” to 180” for 0 fixed at 90” (D = 20 cm). In a second experiment, the Ge(Li) detector was moved between 0 = 0” to 90” for C$fixed at 0” (D = 6 cm) and operated in coincidence with one NaI(T1) detector lixed at 8 = go”, 4 = 180” (D = 21 cm). Since the cascades from all three 3 MeV states proceed via the 0.35 MeV state, the Ge(Li) spectra for all the yy correlation runs were obtained in coincidence with the 0.35 MeV y-rays observed with the NaI(T1) detectors. The coincidence spectra are similar to the one shown in fig. 2b. 3.3. THE STATES ABOVE

E. =

4 MeV

For the study of the states above 4 MeV excitation energy via the “O(U, ny)21Ne reaction, a Ge(Li)-NaI(T1) pair spectrometer was necessary since the y-ray spectra at E, 2 7.5 MeV are very complex. These experiments are similar to those described previously ‘). Briefly, direct ny as well as yy coincidence spectra were measured concurrently with the pair spectrometer (0, = 90”, D = 8 cm) for selected energies in the range Em = 7.5 to 11.5 MeV. For the ny coincidence spectra the pulses from the pair spectrometer were gated by the coincident pulses from the neutron detector (0, = 0”, D = 7 cm). The yy coincidence spectra were obtained with the pulses from the pair spectrometer in coincidence with the appropriately selected pulses from any of three additional NaI(T1) detectors. Gates were set on pulses corresponding to the y-ray transitions from the 0.35, 1.75,2.790+2.796 and 2.87 MeV low-lying states in each of these detectors and the logic output signals were combined in the coincidence arrangement. The three NaI(T1) detectors were located close to the target opposite to the pair spectrometer. A set of coincidence spectra obtained at E, = 10.5 MeV is shown in fig. 3. These sets of direct as well as coincidence pair spectrometer spectra obtained at several beam energies along with threshold arguments were used in the determination of the decay schemes. In order to account for y-rays from the inelastic a-particle scattering on the IsO target ++,a gate was also set on the pulses corresponding to the 1.98 + 0 MeV y-ray transition since most of the higher states in “0 cascade “) through the 1.98 MeV first excited state. This information was corroborated by the results of the ny coincidence spectra.

+ The experiments were performed at a time when only the 15 cm3 Ge(Li) detector was available. +t Observed contaminant y-rays are: (i) the 4.43 and 6.13 MeV y-rays from the inelastic a-particle scattering on 12C and I60 contaminations in the target; (ii) the 1.63 and 2.61 MeV y-rays from the first two excited states in ?ONe populated via “O(a, ny)‘ONe (Q. = 614 keV) due to x 1 % I70 contamination in the target. This latter identification was corroborated by the ny coincidence spectra and was verified with the use of a WO1 target 80 % enriched in 170.

CHANNEL

NUMBER

GATE

DIRECT

ON

SPECTRUM

NEUTRONS

(a)

PAIR

s-2’ !

5.Q’

7722’

92’

2’Ne

GATE GATE

E,>5Me”

GATE

FROM

STATES

AT

TRANSiTlONS

0

0.55

1.75

2.67

takenat 8 = 90” and E, = 10.5 MeV: (a) direct pair spectrum; (b), (c), (d) and (c)pair spectromFig. 3. Portions of the pair spectrometerspectra eter in coincidence with neutrons, the 0.35, 1.40 and 2.52 MeV gate y-rays, respectively. All y-rays are discussed in the text The spectra were taken over a period of 10 hours with a beam current of 60 nA.

20

40

60

20

40

I

5.28 10.63.035l

*‘Ne

EXCITED

STATES

649

Angular distributions were carried out with the pair spectrometer located at 6 = 0”, 45” and 90” (D = 8 cm) for E, = 8.0, 9.5 and 10.5 MeV. These data were used to obtain information on branching ratios and attenuated Doppler shifts.

4. Analysis procedure 4.1. EXCITATION

ENERGIES

AND

BRANCHING

RATIOS

The peak positions of the y-ray lines in the spectra obtained with the 55 cm3 Ge(Li) detector as well as with the pair spectrometer were determined by centroid as well as Gaussian line-shape analyses. The excitation energy of a given state was then obtained as the sum of the transition energies in the cascade to the ground state. In the event that more than one cascade to the ground state existed, the excitation energies obtained for the various routes were averaged. The energy calibration was obtained with the use of radioactive sources as well as the contaminant y-rays from the inelastic CIparticle scattering on “0 and ’ 60. The branching ratios of the states below E, = 4 MeV were obtained from the y-ray spectra of the 55 cm3 Ge(Li) detector. Its efficiency curve was taken from ref. 18). The branching ratios of the states above E,= 4 MeV were deduced from the pair spectrometer spectra. The determination of its efficiency curve has been described in ref. 3). 4.2. ANGULAR

DISTRIBUTION

AND

CORRELATION

MEASUREMENTS

In the analysis of the angular distribution and correlations, it was assumed that quadrupole radiation was the highest multipole present. The effects due to the finite size of the y-ray and neutron detectors were included in the analysis. In the ny angular correlations, only y-rays from lml = + substates were observed due to the geometry and reaction used. The finite solid angle, 52 = 0.3 sr, subtended by the neutron detector allowed ‘“) the lml = 3 substates to be populated, but by 10% at most for 1, 5 3. The phase convention of Rose and Brink ‘l) was used for the mixing ratios. The 0.1 % confidence limit has been taken as the basis for J” assignments. All details of the method of analysis have been described previously 1,“). 4.3. LIFETIME

MEASUREMENTS

The attenuated Doppler shifts of the y-ray transitions from the states above E, = 5 MeV were determined from the pair spectrometer spectra obtained in the angular distribution measurements. All details of the method of analysis of these data have been described recently “). 5. Results and conclusions The results on excitation energies, branching ratios, mean lives, angular distribution as well as angular correlation measurements are summarized in tables l-5, respectively.

C. ROLFS

650

et al.

TABLE 2 Summary of branching ratios Final state (MeV)

Initial state (MeV)

3.66 3.73 3.88 4.53 4.69 5.34 5.43 5.52 5.50 5.63 5.68 5.69 5.78 5.821 5.823 5.99 6.03 6.18 6.27 6.55 6.61 6.64 6.75 7.01 7.04 7.11 7.15 7.23 7.32 7.38 7.43 7.60 7.65

0 <2 80*3 35*5 28&5 3413 t2 <5 < 15 70115 10+8 < 20 43&3 85&10 <5 <5 100 <2 6&5 < 10 t9 5&4 < 15 78f3

0.35 54*4 12&3 65rt5 72f5 66&3 100 5814 < 15 30+15 90&8 < 20 <2 15&l 52&5 < 10 < 30 <3 36&7 20*10 <3 95+4 <5 22&3 20*10

1.75 <2 8rfr2 <3 t5 t.5 <5 26&4 100 < 20 < 10 100 < 30 <2 <2 12&S <2 58&3 43f7 < 15 76rir1.5 <3 100 <5 40*10 50f30

2.79

2.80

38&4 <2 <3 <5 t5 <2 t2 <2 < 20 < 10 < 15 t2 <2 <2 t2 t2 t2 <5 t2 <3 t2 t5 t5

<2 <3 <5 <5 <2 <2 <2 < 20 < 10 < 20 S7f3 <2 48+5 < 10 <2 <2 <5 <2 t4 t2 <5 <5

811

2.87 <2 <2 <3
3.66 -.._.

3.73

3.88

<2 <2 < 10 < 10 <3 <2 1SflO <3 <4 <2 <6 <4

t2 ts 12 <2 <4 <2 < 10 t5

t2 t2 t2 <2 <2 <3 t2 <2 <2 t4 <3 <5 t4

(100) “)

uw

(100) “)

“)

WV “1 lo&lo

90flO

(100) “) (100) “)

“) Assumed. 5.1 THE 2.79 MeV STATE

The ny angular correlations of the 2.79 -P 0 and 2.79 -+ 0.35 MeV y-ray transitions as observed at E, = 4.92 MeV are isotropic within statistical error and restrict J(2.79) to $, 5 or 4. The allowed J = 4 assignment was ruled out in previous work ‘) by the observation of anisotropies for these ny angular correlations at the same beam energy. This discrepancy may be explained if, in the previous results, spurious yy coincidences were present in the ny coincidence set-up + as well as a contribution to the ny angular t The discrimination between neutrons andy-rays was performed l) by placing 5 cm of lead in front of the NE213 detector in order to reduce the y-ray intensity.

21Ne EXCITED

STATES

651

TABLE 3 Summary of the lifetime measurements State

Mean-life (fs)

Fcxp “)

(MeV) 5.34 5.43 5.52 5.55 5.63 5.68 5.69 5.78 5.821 5.823 5.99 6.03 6.18 6.27 6.55 6.61 6.64 6.75

0.97f0.03 0.98 f0.04 0.75f0.15 0.90*0.03 0.95 L-0.03 0.96kO.03 1.0 *to.2 0.95kO.05 0.79*0.04 0.99kO.07 0.95*0&I 0.91&0.05 0.8910.03 0.89 10.03 0.95 10.03 0.93 i-o.03 0.92&0.09 0.96f0.03

preseht work

ref. rl)

< 35 < 35 100:~:: 40f13 < 40 < 40 < 80 < 40 80&18 < 35 < 35 35*30 35&18 35&18 < 35 25:;: < 95 t40

< 10 < 20

< 10 < 10 40*12

< 10 < 20 < 20 < 20 45f30 < 10

“) All shifts measured at Ea = 10.5 MeV, except for E. = 5.63 MeV at E, = 9.5 MeV. TABLE 4 Results of the angular distribution measurements Transition (MeV) 5.34 5.43 5.43 5.52 5.55 5 63 5.69 5.69 5.821 5.821 5.823 5.823 5.99 6.03 6.03 6.18 6.27 6.55 6.61 6.64 6.75 6.75

+ -+ + -+ -+ --f + + --f + -+ --f -+ -+ + --f -+ --f + + -+ -+

0.35 0.35 1.75 1.75 0 0.35 0 2.80 0.35 2.80 1.75 2.87 0 1.75 2.87 0 0.35 1.75 0.35 1.75 0 0.35

“) Corrected for finite solid angle.

02 “)

04 “)

(MS) 10.5 8.0 8.0 10.5 8.0 9.5 9.5 9.5 10.5 10.5 10.5 10.5 9.5 10.5 10.5 10.5 9.5 9.5 9.5 10.5 10.5 10.5

-0.41*0.02 -0.52f0.04 0.00*0.09 0.75+0.15 0.0210.09 -0.38&0.09 0.06iLO.15 0.05*0.11 -0.28&0.13 1.0 *o.z 0.2 *0.2 0.09+0.11 0.19hO.12 -0.23 ho.08 0.5110.12 0.84+0.12 0.39hO.10 -0.16*0.09 0.10~0.05 -0.05 hO.09 -0.45+0.09 0.4 +0.2

0.15 bO.03 O.llfO.05 -0.01 &O.ll 0.4 f0.2 0.05f0.12 0.01*0.11 -0.10f0.15 0.06&0.12 0.17f0.20 -0.09~0.11 0.4 &0.3 0.23 kO.13 0.02f0.14 0.00~0.10 0.40*0.14 -0.04~0.09 0.21 fO.10 -0.09~0.11 -0.08&0.06 -0.31 kO.12 0.16f0.13 0.1 kO.2

x x

6.10 6.10

6.10

6.60 6.60 6.60

6.60 6.60 6.60

3.66 -+ 0.35 3.66 -+ 2.790

3.13 -+ 0

3.73 -+ 0 3.73 -+ 0.35 3.73 --f 0.35 -+ 0 d)

3.88 -+ 0 3.88 -+ 0.35 3.88 + 0.35 -+ 0 d)

X

X X

ny

I II V

I I1 V

I II V

rv”)

coincident

Spectra

-0.10*0.03 0.22~0.04 0.16&0.04 -0.35&0.02 -0.43 10.01 0.43 AO.03 0.61 kO.06 -0.06f0.01 -0.26fO.01

-0.50~0.01

-0.63&0.04

-0.49~0.04 -0.36&0.02 0,37&0.03 --0.16&0.10 -0.57f0.08

0.03 10.02 -0.10~0.04 -0.12&0.04 -0.09kO.02 0.04kO.03 1 -0.02*0.02 0.03*0.04 0.05*0.06 0.06hO.02 0.00~0.02 I

0.08kO.07

0.17*0.08 0.05f0.02 o.o2i-0.02 I 0.2 f0.2 0.04~0.07

; 8

.$

+ # 8 _d:

:

?z 4

-

J3

# Q

at $f

4 +j P t

) s

J,

3.7 1.3 ‘) 4.5 ‘)

41.0 ‘)

3.0 3.0 3.0 3.0

3.0 3.0

5.4 5.4 5.4 5.4 4.6 5.4 5.4 4.6

1.0 0.6 0.6 0.5 26.5 1.9 0.5 51.6

3.5 ‘) 1.1 ‘)

3.2 3.1 f2.6

Mixing ratio ‘)

+0.49&0.08 -0.09~0.04 -0.50+0.10

+10 jls t0.55f0.10

1-1.3 kO.4 -tO.17+0.06

-0.1 &O.l or > 5 0.09f0.04 or 1.7fO.5 *) 2.2 11.5 “) -0.5 hO.28) or < -2 ‘)

-2.5

X20.,%

21.0 0.9

x2

*) Geometries in the yy angular correlations as defined in ref. 20). “) Corrected for finite solid angle. ‘) Mixing ratio of they-ray transition or of the primary in the case of a w cascade. The errors quoted correspond to one standard deviation of x2 if not stated otherwise. “) A mixing ratio of a(O.35 --f 0) = +0.06+0.03 was used *) in the analysis. ‘) Simultaneous x2 fit of the angular distribution andw angular correlation data. The mixing ratio for the 3.73 -+ 0 MeV transition has been limited to the values obtained from the ny angular correlations at E, = 6.10 MeV. ‘) Simultaneous x2 fit of the angular distribution and w angular correlation data. For the acceptable J(3.88) = 8 assignment, a mixing ratio of a(3.88 + 0) = 0.00&0.05 has been found. *) Solution for the mixing ratio at the 0.1 ‘A confidence limit.

x x

6.60

direct Y

3.66 -i 0.35 -+ 0 d,

WW

Transitions

TABLES

Results of the angular distribution, ny andyy angular correlation measurements ic;

*lNe

EXCITED

STATES

653

correlations from the highly anisotropic 2.87 + 0.35 MeV y-ray transition which, in the NaI(T1) spectra ‘), was unresolved from the 2.79 --, 0 and 2.79 + 0.35 MeV y-ray transitions. These difficulties were avoided in the present work with the use of a pulseshape discrimination technique (spurious yy coincidences less than 0.1 %) to detect the neutrons and a high-resolution Ge(Li) detector to observe the y-ray transitions. The yy angular correlation of the 2.79 + 0.35 + 0 MeV cascade in geometry VII was also observed in the present work to be isotropic within statistical error in disagreement with previous work ‘). Again, a small contribution of the unresolved 2.87 + 0.35 + 0 MeV cascade can very well explain the reported anisotropy ‘). A rigorous spin assignment for the 2.79 MeV state has been obtained from the analysis of the 3.66 ($-, subsect. 5.2) + 2.79 MeV ny angular correlation. However, the observed correlation contained an 8 % unresolved contribution from the 3.66 -+ 2.80 MeV branch (fig. 2a). The assumption that the M2 strength for the latter +- + 3’ y-ray transition is less than 50 W.U. yields a mixing ratio limited to 6 = O.O+ 0.1. This implies an a2 coefficient of a2 = -0.5 + 0.2 for the 3.66 -+ 2.80 MeV ny angular correlation. This correlation was subtracted from the combined correlations (a2 = -0.54 kO.04, a4 = 0.04&0.07) resulting in a2 = -0.57+0.08 and u4 = 0.04+0.07 for the pure 3.66 + 2.79 MeV ny angular correlation. The latter coefficients have then been used in the x2 analysis to determine the spin of the 2.79 MeV state. As shown in table 5, J(2.79) = 3,s and 3 assignments fit the data equally well. However for the 3 + 4 and 3 + 3 spin sequences, the mixing ratios (at the 0.1 % confidence limit) together with the mean life ~(3.66) = 68 rf: 16 fs and the 38 % branching ratio imply E2 transition strengths 2 110 W.U., thus providing grounds for the rejection of these spin sequences. In the case of the 3 --, t spin sequence, two values for 6 have been found, but only that for S = 0.09 & 0.04 is acceptable on the grounds of transition strength arguments. It is concluded therefore that J(2.79) = *. This result together with the observation of an 1 = 1 stripping pattern for this state in the 22Ne(p, d)“Ne reaction “) leads to J”(2.79) = +-.

5.2. THE

3.66, 3.73 AND

3.88 MeV

STATES

The spin of the 3.66 MeV state is restricted to J = 3 or 3 on the basis of its meanlife “) r = 68 + 16 fs and the 8% branch to the 2.796 (+‘)MeV state (e.g. for J(3.66) = 5, E2(M2) > 210(5450) W.U. at the 0.1 % confidence limit). The yy angular correlations of the 3.66 + 0.35 -+ 0 MeV cascade (table 5) as well as the anisotropic 3.66 + (2.79 + 2.80) MeV ny angular correlation (see subsect. 5.1) exclude J = 3. The 1 = 1 transfer to this state in the 22Ne(p, d)21Ne reaction ‘) as well as a recent y-ray polarization experiment 22) yield a negative parity assignment, hence J”(3.66) = 3-. The x2 analysis of the 3.73 --, 0 MeV ny angular correlation limits J(3.73) = 3 or 3 (table 5). Furthermore, the 3.73 + 0 and 3.73 + 0.35 MeV y-ray angular distributions as well as the 3.73 + 0.35 + 0 MeV yy angular correlations can only be fitted simultaneously by J(3.73) = 3 (table 5). The I = 2 transfer to this state in the “Ne(d, p)

C. ROLFS

654

et al.

“Ne reaction “) as well as the recent y-ray polarization experiment “) determine the parity of this state to be positive, hence J”(3.73) = 3’. The simultaneous x2 analysis of the 3.88 + 0 and 3.88 + 0.35 MeV y-ray angular distributions as well as the 3.88 + 0.35 --f 0 MeV yy angular correlations yields a unique J(3.88) = 3 assignment (table 5). The recent y-ray polarization experiment [ref. “)I determines the parity of this state to be negative, hence J”(3.88) = 5-. 5.3. THE

4.53 AND

4.69 MeV STATES

The observed y-ray decay schemes for the 4.53 and 4.69 MeV states (table 2) are in good agreement with previous work ‘*rl). Stripping data ‘*lo) result in J” = 3’ or 3’ assignments for both states. From an application of the (W+ 1) rule to the 23Na (d, a)“Ne cross sections, a J(4.53) = 3 assignment has been suggested 16). Recent py angular correlations applied to the ’ gF(3He, py)‘rNe reaction 23) strongly favour a J(4.69) = 3 assignment. 5.4. THE

5.34 MeV STATES

The 5.34 MeV state’is reported il) to decay 100 % to the 0.35 MeV state in excellent agreement with the present work (table 2). The non-zero a4 coefficient of the 5.34 + 0.35 MeV y-ray angular distribution (table 4) implies J 2 +jand stripping data ‘, 24) J” = 3” or _5+,hence J”(5.34) = 3’. 5.5. THE

5.43 MeV STATE

Bailey et al. ’ ‘) report two transitions from the 5.43 MeV state, namely 5,43 -+ 0.35 and 5.43 3 1.75 MeV. However no branching ratios are quoted. Both transitions have been verified in the present work. Furthermore a new 5.43 4 2.87 MeV branch has been found (fig. 3). The observed decay scheme (table 2) together with the mean-life (table 3) lead to the restriction J” = 3+, 8 or $+. It should be pointed out that the intensity of the 5.43 4 1.75 MeV branch relative to the other branches appears to increase by w 40 0/Ofor E, = 10.5 MeV compared with the intensities for E, = 8.0 MeV. This apparent change in branching ratio for the higher beam energy can be explained by the presence of an additional y-ray transition 6.55 4 2.87 MeV (fig. 3e and subsect. 5.14). The information presented in tables l-4 has therefore been deduced only from the data obtained at E, = 8.0 MeV, which is 800 keV below the threshold for populating the 6.55 MeV state. 5.6. THE

5.53 AND

5.55 MeV STATES

The 5.55 and 5.20 MeV y-rays (fig. 3) have been identified as transitions to the ground state and first excited state from the known state “) at 5550&-a keV on the basis of the following grounds: (i) both y-ray transitions were observed only for E, 2 8.0 MeV implying E, 5 5.85 MeV and (ii) the 5.55 MeV y-ray was observed only in coincidence with neutrons while the 5.20 MeV y-ray was in coincidence with neutrons as well as with the 0.35 MeV y-ray. The resulting excitation energy Ex = 5550 _+2 keV

21Ne EXCITED

STATES

655

is in excellent agreement with previous work (table 1). The y-ray decay scheme (table 2) and the mean-life (table 3) restrict J”(5.55) 5 3’. Stripping data yield ‘, 24) J” = 3’ or 3’. The 3.78 MeV y-ray (fig. 3) was observed for E, 2 8.0 MeV and was found to be in coincidence only with the cascade y-rays from the 1.75 MeV state. These results lead to the identification of a new state at E, = 5525.0& 1.5 keV. The non-zero a2 coefficient of the 3.78 MeV y-ray angular distribution (table 4) and transition strength arguments yield J”(5.53) 2 3’. 5.7. THE 5.63 MeV STATE

The 5.63 MeV state decays to the ground and first excited state (fig. 3 and table 2). The ground state decay has not been reported in previous work ‘l). The observed properties of this state limit J to +,3 or 3. Stripping data indicate lo) J” = 8’ or 3’. It should be noted that at E, 2 10 MeV each of the 5.28 and 5.63 MeV y-ray transitions have superimposed on them contributions from the transitions 5.26 (7.01 + 1.75)+5.29 (7.04 --, 1.75) and 5.63 (7.38 + 1.75) MeV, respectively. This superimposition is indicated by the broadening of these lines above E, 2 10 MeV and corroborated by the results of the coincidence pair spectra (fig. 3). The information presented has been therefore extracted from the data obtained at E. 4 9.5 MeV. 5.8. THE 5.68 AND

5.69 MeV STATES

The 3.93 MeV y-ray (fig. 3) is identified as a transition to the 1.75 MeV state from a state at E, = 5682 + 2 keV. This state has not been reported previously. The observed branch together with the mean life (table 3) limit J”(5.68) 2 3’ (for J” = 3-, M2 > 120 WA.). The observed y-ray decay scheme, mean-life and excitation energy for the 5.69 MeV state (tables l-3) are in good agreement with previous work *#“). Stripping data ‘) restrict J”(5.691) = &- or +-. The isotropic angular distributions for both y-ray transitions from this state (table 4) slightly favour J” = $-. 5.9. THE 5.78 MeV STATE

In addition to the reported ground state transition from the 5.78 MeV state ‘I), a new 5.78 --f 0.35 MeV branch has been observed in the present work (fig. 3 and table 2). Stripping data yield lo) J” = $- or 3-. 5.10. THE 5.821 AND

5.823 MeV DOUBLET

STATES

From previous work only one state has been reported “) at E, = 5822 + 9 keV with a J” = 3’ or 3’ assignment g*24). Four y-ray transitions of energies 5471,4077,3024 and 2955 keV have been observed in the present work for E, 2 8.5 MeV. The assumption that these y-rays are primary transitions to the states at 0.35, 1.75,2.80 and 2.87 MeV, respectively is supported by the coincidence pair spectra (fig. 3). These

656

C. ROLFS et al.

results lead to excitation energies of E, = 5821 f 2, 5824+ 5, 5820& 4 and 5822+ 2 keV with an average value of E, = 5822-t-2 keV. The y-ray decays to the 2.80 ($‘) and 2.87 (3’) MeV states together with the mean-life (see below) yield J(5.822) = 2. However, the large a2 coefficient (LQ = 1.0-L-0.2) of the 5.822 + 2.80 MeV y-ray angular distribution is compatible only with a .7(5.822) = 3 spin assignment. Furthermore, the attenuationfactorsFof thefoury-ray transitionsasextractedfromtheDoppler shift measurements overlap only in pairs, i.e. (a) P(4077) = l.OO+O.lO, F(2955) = 0.98f0.03 and (b) F(5471) = 0.81+0.03, F(3024) = 0.76+0.05. These results suggest therefore the existence of a doublet. One member decays to the 0.35 and 2.80 MeV states with a ratio of 52 : 48( f 5). The excitation energy and spin are determined to be E, = 5821+3 keV and J = 4. The other doublet member decays to the 1.75 and 2.87 MeV states with a ratio of 12 : 88( + 5). Its excitation energy and spin are 5823 f 3 keV and J” 2 3’. The mean lives are ~(5821) = 80+ 18 fs and ~(5823) S 35 fs. Bailey et al. ‘i) o b served a 5821+ 3 keV y-ray at E, = 9.0MeV and identified it as a 100 % ground state decay from a state at 5821 keV. This y-ray was also observed in the present work at E, 2 9.0MeV but was identified as a primary decay to the 0.35 MeV state since it was found to be in coincidence with neutrons and the 0.35 MeV y-ray (fig. 3). These results establish that the transition originates from the known state at E, = 6177 keV (subsect. 5.12). 5.11. THE 5.99 AND

6.03 MeV STATES

The reported 11) decay schemes for the 5.99 and 6.03 MeV states as well as their mean-lives are in good agreement with the results from the present work (tables 1-3) and restrict the spin assignments to J”(5.99) $ 3’ and J”(6.03) 2 3’. 5.12. THE 6.18 MeV STATE

In addition to the reported 6.18 --, 1.75 MeV y-ray decay 11) two new branches 6.18 --) 0 and 6.18 + 0.35 MeV have been observed in the present work (fig. 3). Furthermore, the existence of a 6.18 -+ 3.73 MeV y-ray branch is supported by the following observations: (i) the intensity of the 3.73 + 0 MeV y-ray transition in the various yy coincidence pair spectra (fig. 3) indicates the presence of a primary transition of Ey M 2.5 MeV to this state, (ii) the 2440 (2790 + 350) keV y-ray, which is completely unresolved from the expected 2442 (6177 + 3735) keV transition, shows an anisotropic angular distribution even though J(2.79) = 4 (subsect. 5.1) thus indicating an additional y-ray component in the peak; (iii) since the two proposed y-ray components in the 2.44 MeV peak originate from states with quite different mean lives [r(2.79) = 84 ps, r(6.18) = 35 fs], the two superimposed y-rays are expected to separate at 0 = 0” by 27 keV due to different Doppler shifts. This separation has been observed at forward angles. The observed decay scheme (table 2) together with the mean life lead to J”(6.18) = 3 +, 2 or 3’.

‘INe 5.13. THE

6.27

EXCITED

657

STATES

MeV STATE

addition to the reported 6.27 + 2.87 MeV decay i1), a new branch 6.27 -P 0.35 MeV has been observed (fig. 3 and table 2). These results together with the mean-life (table 3) yield P(6.27) = t+, 3 or 3. R ecent yy angular correlation measurements of the cascade y-rays from this state resulted “) in J(6.27) = 4 (3). In

5.14. THE

6.55 MeV STATE

The observed 6.55 + 1.75 MeV y-ray decay 11) has been verified in the present work (fig. 3). Furthermore a new 6.55 + 2.87 MeV transition has been found (fig. 3e). This decay scheme and the mean-life (table 3) restrict J 1 3. It should be pointed out that the intensity of the 3.68 (6.55 + 2.87) MeV branch had to be corrected for a contribution from the nearby peak of the 3.68 (5.43 --f 1.75) MeV y-ray transition. The latter contribution was estimated from the results described in subsect. 5.5. 5.15. THE

6.61 MeV STATE

The decay scheme for the 6.61 MeV state (table 2) is in substantial agreement with previous work ll). The present work adds no new information to the reported J” = 3’ or 3’ assignment from stripping data ‘). 5.16. THE

6.64 AND

6.75 MeV STATES

The 6.64 -+ 1.75 MeV y-ray branch (fig. 3) together with the mean-life (table 3) limit J(6.64) 2 4. The observed y-ray decay scheme for the 6.75 MeV state (fig. 3 and table 2) and its mean-life (table 3) restrict J(6.75) = 3, 3 or 3’. Stripping data indicate lo) J” = 3’ or 5’. 5.17.

GAMMA

DECAY

FROM

UNBOUND

STATES

The gamma decay from 10 unbound states? at E, = 7.01-7.65 MeV has been observed (table 2). The observed full Doppler shifts of the y-ray transitions imply total widths of r 2 60 meV for all these states. It is concluded furthermore that the y-ray width of these states must be of the order of magnitude of their neutron width due to the above observations. This result can be explained by the inhibitions of neutron emission due to high centrifugal barriers if these neutron unbound states have high spins. 5. Discussion Fig. 4 illustrates a summary of the available information

on the states below

E, = 5 MeV of the mirror nuclei “Ne and ‘lNa. From the y-ray decay schemes and

J” assignments, all “Ne states can be interpreted as analogues of states in 21Na except + The

neutron and a-particle

binding energies are 6.760 and 7.368 MeV, respectively.

658

Xl

I

I

I

II

I

I

I

II

!

!

I

j

I

“‘Ne EXCITED

659

STATES

I t/2-

E,(keV)

J*

7/2+ 9/2+

6747 6642 6607

3/2+.5/2+ Z3/2 3s5’2+

13/2*l9m

66% 6267 e0-P-F PI,, 6034

/ ~5/2+ I/z3/2I

4730 4686 4526 4433

13/2+ 9/2-

9/2(7/2l 3/ 2+,5/2,7/2+ 25, + s7/ H+

7/2+

7/2-

3/2 l/Z-_3/2’ 23/2’ 3/2+,5/2+ 3/2*5/2+ 23/i?+

1/27/2-

3/2-

3/23/2+(5/2+l

3/2+ 5/ 2+

5/2+ (3121 11/2*

3005 3735 3663

,112+

\

5/Z-

n;I

350

5/z+

0

3/2+

2'Ne

= 5/2+

s/2+ 1/2+

1/2K”

= 1/2-

t+41

7/2*

5/t+

3/2-

9/2* i/2+ 112-

1747

=‘1/2-

(+14)

5/25/2+ 3/2-

2867 2796 2790'

K*

CT = 1:2* (

f9)

7/2+

5/f*

312’ Kr;

3/2+

c+71

Fig. 5. Summary of excitation energies and spin-parity assignments to the bound states in 2’Ne. The neutron binding energy is at 6.76 MeV. The tentative identification of some of these bound states as members of rotational bands is indicated.

660

C. ROLFS et al.

for the y’, 4.43 MeV state in “Ne. The ex~rimentai information for the states above E = 5 MeV in 2fNa is rather incomplete, hence no comparison could be performid with their analogues in “Ne . The striking feature of large energy shifts between some of these analogues can be ascribed to the Thomas-Ehrman effect. This shift is, in general, effective on unbound or nearly unbound states with low angular momentum barriers and large reduced particle widths t 8’. Since the proton binding energy for ‘“Na is 2.43 MeV, the observed shifts can give valuable information on the intrinsic structure of these analogues (see below). A calculation of the shifts for the lowest six states has been made recently by De Meijer et al. * “)_ A tentative identi~~ation of these states as well as of states at higher excitation energy in “Ne as members of rotational bands is indicated in fig. 5. These identifications will be tested below by comparisons with the predictions of the Nilsson model neglecting band mixing. 5.1. POSITIVE-PARITY

ROTATIONAL

BANDS

The properties of ‘the K” = 3’ gr’ound state band have been discussed recently “). The 3+, 2.80 (21Ne) and ++, 2.43 (21Na) MeV states are strongly excited ‘* 9v14) in the one-particle transfer reactions “Ne(d, p)‘rNe and 20Ne(d, n)‘lNa, respectively. They have been interpreted 3**, 9*2’) as single-particle states where the odd nucleon has been promoted from Nilsson orbit no. 7 (d4, K” = 3’) to no. 9 (st, K” = 3’). The observed large level shift between these two analogue states (AE = 364 keV) and the stripping data “) require a reduced particle width (or spectroscopic factor) of t12 x 0.6. The Nilsson model predicts f o2 = 1.0, 0.64, 0.28 and 0.14 for deformations of q = 0, +2, i-4 and + 6, respectively, suggesting therefore q z +2 for this K” = 3’ rotational band. For this deformation, a large decoupling parameter of a = + I is predicted, implying a distorted I? = t’ rotational band with the 3” and 3* (3’ and Qf, . .) higher band members close in excitation energy. The 3+, 5’ pair is expected at E, zs 4.1 MeV for a moment of inertia of z 0.21 MeV, which represents a typical value for nuclei in this mass region. These two predicted levels are associated with the 4.69 (4.47) and 4.53 (4.29) MeV states in 21Ne (21Na) (figs. 4,5). These identi~~ations are based on the following considerations: (i) For the 3’ analogues, the reduced particle width calculated from the observed Thomas shift of AE = 219 keV is consistent with the value obtained from the observed ‘“) proton width of rP = 27 keV for the 3+, 4.47 MeV state in 2’ Na, both giving 8’ x 0.13, in good agreement with the predicted value of O2= 0.10 for ?J = f2. (ii) For the 3’ analogues, the Thomas shift of AE = 233 keV, the proton width 28) of rP = 17 keV for the 3’ 4.29 MeV state in 21Na, and the stripping data ‘) for the (3’) 4.53 MeV state in 21Ne all yield d2 x 0.15, again in fair agreement with * In the present work the reduced particle widths are defined as in ref. 27). treating unmixed model states, the particle reduced width is given by 8’ = 2(C~~)~/@.i+ 1) wherej refers to the spin of the level and C,, is the Nilsson expansion coefficient. The index CL it In

specifies the Nilsson orbit.

‘INe

EXCITED

661

STATES

the predicted value of 8’ = 0.06 for q = +2. (iii) The predicted y-ray decay schemes are in fair accord with their experimental equivalents (table 6). The 3+, $+ pair of band members are expected at E, x 7.0 MeV. The 7.01 and 7.04 MeV unbound states may be tentatively identified with these two band members on the basis of their excitation energies and observed y-ray decay schemes, which are not in disagreement with the model predictions (table 6). However more experimental data are necessary to test such a speculative identification. TABLE 6 Experimental and theoretical dynamic properties of the Kn = 4’ (orbit no. 9) and Kn = #+ (orbit no. 5) band members State

J”, K=

Mean life (fs)

(MeV)

2.796 4.69

exp

theor

< 27 < 36

1 1

4.53

P+r t+

< 38

1

7.01

(%+), 4’

< 10

0.6

7.04

(%+)P t+

< 10

0.1

3.73

it’, P’

< 38

3

6.18

(f+)* %’

35&18

2

Transition

Branching ratios ( %)

(MeV) exp 2.796 4.69 4.69 4.69 4.53 4.53 4.53 4.53 7.01 7.01 7.01 7.04 7.04 7.04 3.75 3.75 3.75 6.18 6.18 6.18 6.18

+ 0 -+ 0 -+ 0.35 + 2.796 +o -+ 0.35 + 1.75 --f 2.796 -+ 0.35 --f 1.75 -+ 2.87 --f 1.75 --f 2.87 --f 4.43 --f 0 -+ 0.35 -+ 1.75 +o -to.35 -+ 1.75 + 3.75

100*1 34*3 66+3 <5 28f5 72+5 <5 <5 20*10 40*10 40110 50+30 so+30 “) 80+3 12&3 8zt2 655 36+7 43*7 15&10

theor 100 46 54 1 13 67 20 0.1 25 52 23 32 57 11 75 24 1 1 69 21 9

“) Not determined.

The $+, 3.73 (3.54) MeV state in ‘lNe (‘lNa) has been proposed 3*9*27) as the head of a K” = 3’ rotational band based on orbit no. 5. For 22Ne, the 6.18 MeV state could be considered as the J” = 3’ second member of this band with the observed transition to the 3.73 MeV state as an in-band transition. Thus, the observed y-ray decay scheme is not in disagreement with the model predictions (table 6) if this state has J” = 3’. A rigorous spin-parity assignment is desirable. Further rotational bands of K” = 3’ and 4’ based on orbits no. 11 and 8 respectively are expected at E, z 5.5 MeV.

662

C. ROLFS ef al.

5.2. NEGATIVE-PARITY

ROTATIONAL

BANDS

The Nilsson model predicts the existence of two low-lying K” = 4- negative-parity rotational bands 5), one built on a p* hole in orbit no. 4 and the other built on a particle promoted from orbit no. 7 to orbit no. 14. The +-, 3- and $- states at 2.79 (2.80), 3.66 (3.68) and 3.88 (3.86) MeV excitation energy in 2”Ne (21Na) can be identified as the lowest members of the K” = 3- hole band based on orbit no. 4: (i) The 2’Ne states are strongly excited ‘) in the pick-up reaction 22Ne(p, d)?‘Ne with a large p+, hole strength but are weakly excited 9P14*2g) in the particle transfer reaction 2oNe(d, p)21Ne. (ii) The 3- 3.68 MeV and _IT-3.86 MeV states in 21Na have been observed 30) to have small reduced proton widths of 0.0015 and 0.0024, respectively. These small reduced proton widths imply no level shifts for these negative-parity states in excellent agreement with observation (fig. 4). TABLE7 Transition strengths (W.U.) of the negative-parity Transition (MeV)

JP

2.790 -+ 0 2.790 + 0.35 3.66 -+o

JP

states in 2’Ne El

< 8~10-~

M2

> 0.05 0.5~0.1

3.66 3.66 -+ + 0.35 1.75

f:

3.88 3.66 3.88 3.88 4.73 4.73

%f

--f -+ 02.796 -+ 0.35 --f 1.75 + 0.35 -+ 2.796

;”

5.69 -+ 0 5.69 -+ 2.796

o-) O-)

8-

< 8~10-~ (2.8f0.7) x 1O-4 (2&l) x 10-3 (1.8f0.f) x iO-“ (4.3*i1.3)x10-4 <9x10-5 > 4 x 10-d > 2x 10-a > 4x 10-5 > 4 x 10-d

< 2.8 l.lfl.1 < 67 < 50 0 05 ‘t$% ;.3+_;;: < 90

(iii) The decoupling parameter is predicted to be a = +0.6 for 9 = +4 in accord with the observed value of a = +0.7. (iv) The large E2 transition strength of 21::: (22:::) W.U. of the t- -+ _f” y-ray transition in 2”Ne (21Na) reflects a close intrinsic relationship between the two states as expected for members of the same rotational band’. (v) The observed El strengths of 10m3 to lo-’ W.U. for the y-ray transitions from these states (table 7) are in agreement with the inhibitions predicted by Benson and Flowers 31) for transitions from p+ hole states to pure particle states. (vi) The head of this p+ hole band is predicted at E, = 2.9 MeV from the formula of Zamick 37) and Wagner 38). Higher band members are expected at E, = 5.82 (3-J and 6.24 (p-) MeV. The states at E, (2”) = 5.823 (2 3’) and 6.27 (8) MeV may be t The 3.88 (&-) + 2.79 (&-) MeV in-band E2 transition in 21Ne has been observed recently “) +tQ W.U. implying an E2 strength of 61 _+s

“Ne

EXCITED STATES

663

identified as these band members on the basis of excitation energies and y-ray decay schemes (table 2). The +- 4.73 MeV and ($-) 5.69 MeV states in 21Ne can be identified as the first two members of the predicted K” = +- decoupled band based on orbit no. 14: (i) Both states are strongly excited in the one-particle transfer reaction “Ne(d, p)‘lNe with reduced particle widths of 0.6 [ref. ‘)I or 0.3 [ref. ““)I for the 4.73 MeV state and of 0.2 [ref. ‘)I for the 5.69 MeV state. The Nilsson model predicts these values to be 02(4.73) = 0.0, 0.10,0.23 and 0.30 and e2(5.69) = 0.00,0.02,0.04 and 0.07 for q = 0, + 2, +4 and + 6, respectively. These results suggest therefore a large deformation of q = +4 or +6. (ii) From the proton width 38) of Fp = 121 keV of the $- 4.18 MeV analogue state in “Na (fig. 4) a reduced particle width of t12(1= 1) = 0.29 is deduced in good agreement with the predicted value for q = +6. This reduced particle width implies a level shift of AE = 610 keV to be compared with the observed value of AE = 550 keV (fig. 4). (iii) The decoupling parameter is predicted to be a = -3.3 or -3.0 for q = +4 or +6 in accord with the observed value of a = -4.0. The large decoupling parameter implies a $-, $-, +-, y-, . . . spin sequence for the members of this K” = +- particle band. The expected excitation energies of the I- and yband members are indicated in fig. 5. Possible candidates for the I- band member are the states at E, = 5.43 or 5.52 MeV. The authors wish to express their thanks to G. Busch and the operating staff of the Van de Graaff generator for their co-operation. They are indebted to R. Kraemer and A. Mamis for their assistance in the course of the experiments. The co-operation of W. Trost in the initial part of the experiments was of great help. The discussions with Professor R. E. Azuma are gratefully acknowledged. The calculations were performed on the IBM 7090 computer of the Rechenzentrum der Universitat Freiburg and on the IBM 360 computer of the University of Toronto.

References 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14)

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