Investigation of 38Ar levels by 37Cl+p reactions

I2.C:I.E.4[

Nuclear Physics A220 (1974) 317--334; (~) North-HollandPublishin# Co., Amsterdam Not to be reproduced by photoprint or microfilm without written permission from the publisher

I N V E S T I G A T I O N O F 3SAr L E V E L S BY 37C1 + p R E A C T I O N S

([I). Lifetimes, spins, parities and theory C. ALDERLIESTEN and C. A. L. VAN LOON Fysisch Laboratorium, Rijksuniversiteit, Utrecht, The Netherlands Received 5 December 1973

Abstract: The mean lives of 16 bound states of 3SAr have been determined from DSA measurements at 14 selected 37C1(p, ~,)38Ar resonances in the range Ep = 0.9-1.5 MeV. Spins and parities of 20 (p,),) resonances have been determined from a precision comparison with corresponding 37Cl(p, ~o)34S resonances. Gamma-ray transition strengths combined with y-ray angular distribution measurements at two resonances determine (or limit) the spins of 15 bound states of 3aAr. The experimental data on aaAr are compared with the results from shell-model calculations. The negative-parity states, especially those with high spin, can be very well described in terms of a model with five active nucleons in the ld~. shell and one in either the lf~ or 2p~. shell. For the positive-parity states, the new data underline the importance of two-particle four-hole excitations. El

I

NUCLEAR REACTIONS 37C1(p,•), (p, ~t), E = 0.9-1.7 MeV; measured tr(O), DSA; 3aAr levels, resonances deduced Ti., j~r. Enriched target.

I. Introduction This paper describes a determination o f lifetimes, spins and parities o f 3SAr levels. The lifetimes are deduced f r o m D S A measurements in the 37Cl(p ' y)38Ar reaction (sect. 2). Spin and parity assignments for 3SAr levels are discussed in sect. 3. F o r twenty 37C1(p, y)3SAr resonances, J ~ values are obtained f r o m the correspondence [see paper 1 1)] with 37C1(p, a0)34S resonances. The J ~ values o f the (p, % ) resonances have been determined by Bognjakovi6 et aL 2); a few additional a-particle angular distributions have been measured. Spins and parities o f b o u n d states are based on p r i m a r y and secondary y-ray transition strengths and some ),-ray angular distributions. The data, c o m b i n e d with those in paper I, are c o m p a r e d with the results f r o m shell-model calculations in sect. 4. F o r negative-parity states the 3SAr wave functions o f Engelbertink and G l a u d e m a n s 3) are used to calculate spectroscopic factors a n d y-ray transition strengths. The positive-parity states are c o m p a r e d with calculations by G r a y et al. 4) and Skouras s) and with a new calculation in the relatively simple (ld~)6-n(lf~)" configuration space, with n -- 0 or 2. 317

318

C. ALDERLIESTEN A N D C. A. L. VAN LOON

2. Lifetimes of bound states 2.1. EXPERIMENTAL P R O C E D U R E

Fourteen selected 37Cl(p, ],)3SAt resonances in the Ep = 950-1500 keV region have been used for DSA measurements. The initial recoil velocity of the aSAr ion then is v(O)/c = 0.0012-0.0015. The experiments have been performed with a 45 or 60 cm 3 Ge(Li) detector. Singles ],-ray spectra (dispersion 0.5-2.5 keV/channel) were taken at 0 = 0 ° and 130° and stored in two 2048-channel groups of a pulse height analyser. The target-detector distance was D = 6 cm (45 cm 3) or 4.5 cm (60 cm3). The front face of the Ge(Li) detector was shielded with a 0.5 cm thick Pb sheet. The target material Ba37C12 was evaporated onto directly water-cooled tantalum backings (see paper I). The target thickness (20-90/tg/cm 2 in the beam direction) and the bombarding energy were chosen to guarantee stopping of the 3SAr ions in the target material itself. In view of the low melting point of the target material and the relatively large proton current (12/~A on a 0.5 cm 2 spot), the yield was checked regularly with a NaI(TI) monitor; if the target showed significant deterioration, it was replaced. In most experiments, more than one target had to be used. The total measuring time of 10-25 hours per resonance was subdivided into short runs ( ~ 15 min), at 0 = 0 ° and 130 ° alternately, to eliminate effects due to instrumental shifts. The target room was temperature controlled but no special gain stabilisation was applied. In several cases, ],-rays from radioactive sources (no shift) and/or primary ],-rays from the short-lived resonance state (full shift) served as checks on the stability. Some spectra from such a measurement are shown in fig. 1. 2.2. ANALYSIS

The Doppler shifts were derived from the first moments of the peaks in the y-ray spectra. The assigned errors include the purely statistical error and the uncertainty in the background subtraction. The observed shift is expressed as a fraction F of the full Doppler shift as calculated from the kinematics. Corrections for finite detector dimensions (2-3 ~ for isotropic y-rays) were taken into account. The correction for anisotropic ],-ray angular distributions never exceeds 1 ~ for the known anisotropies. The F ( % ) curve, which relates the measured F-values to the mean lifetimes % , was calculated with the formulae of Blaugrund 6), based on the slowing-down and collision theory of Lindhard et al. 7). The motion of 3SAr ions with v/c < 0.0015 in Ba 3 7C12 is determined mainly by the effect of the atomic collisions on both the magnitude and direction of the ion velocity. For lack of experimental data on this atomic part of the ion-target interaction, the unmodified Lindhard theory was used in the calculations. For the electronic part, several experimental corrections can be applied to the Lindhard formulae [see e.g. ref. a) for details]. For the present ion velocities only the corrections indicated by Ormrod et al. 9) are relevant.

2000-

2000

4000

0

X1/4

~xl/2

)'lk

~

,

L X1/2

F=41"-37°1°

~

X1/4

,

[

--441~--~1~-11.08keV 11,21keV I t i I

,.o,

.6B --+ 3.61

X1/5

~

' ~ ~ o

1%_ ll~

~

2,05

5.~--.3.6,

. 6.~o--.,,,,,,~.o,

"~to

" ~ i ~ I$

J,'l

l!l

F-98°15°1°

F=78-'28°1o

"Ik,

~',~

~ -~11~-- --~ Ic-O,O0*-O,05keV 112.gBkeV~ 3.81keV ~, III

,.6,

I e:o°188y

'~o

F'104'-16°1°

•

(I

,

'

,.,,-.~.,,

.

~,l

1',1

~1

III

F=65t6°I°

,~11 ~., "1~

l

I I __~p__ 14"2---~V~ I I 3.37keV I ~

~

6.50---3.6~

---~ I~-~4.21kev I

o

6.,o--,.,6

F=73'16°1°

¢p.

I

I

416 key

~'

.

6.~--~2.~7

r

J

.

/IX

I

,

I

900keY

'

,.

,

I

9 30keY

I

, t

I

i

[ I

19

I

;

:

9 32keV '

I I I I

F=99tg°l°

I

'

-

-

--" ~ 3Okay ' "

E T (MeV)

F= 93:14°1° F=104t3°l°

I

I

,

i

~1~ 18.29key

I

t

6.62 ---~ 2.17

1

I

6.~0

37CI2

SHIFTS

r --,6.50

Be

eV/CHANNEL

keV

DOPPLER

F=101'4°1° F=99t13°l"

I

,

i

I

L 19.79key

lI

6"82 4--~'1;

2.

t k

Ep=956

"CI(p,T)38Ar"

1 I

a

Fig. I. Doppler shifts measured at the Ep = 956 keY, a~Cl(p, 7)aSAr resonance with a 60 cm a Ge(Li) detector; proton current 14 #A, total measuring time 20 h. The aSAr ions were stopped in the target material (35 #g/cm ~, in beam direction). The s s y },-ray provides a check on instrumental shifts.

0

O

z

bO t,--

13_

w

IZ

0

< I

Z

LU Z

d

_Q

L~

+

9

< 6 350 -+900 -200

404- 11

1104- 30

124-

3104-160

2104- 70

224- 10

454- 16

>600

(Is)

Trtt a)

5.86 --> 3.81 5.97 --> 2.17 6.04 --* 3.81 3.81 4.48 4.48 4.48 6.05 ~ 5.35 6.214 --->0 0 3.94 6.34 --> 3.81 6.35 --->0 6.50 --> 3.81 3.81 6.57 --->0 6.60 --->4.48 4.48 4.48 6.62 -->4.57 6.67 --> 4.59 6.68 ---> 5.73 6.77 --->0 6.82 --->2.17 7.10 --->2.17 7.43 --->2.17

(MeV)

Exl -9- .Exf

Ell

956 1062 1104 1304 956 1104 1304 1501 1214 1312 929 929 1051 956 1214 1051 956 1095 1104 1054 1095 1062 1051 956 956 1214

(keV) 0.984-0.15 --0.08-t-0.10 0.374-0.12 0.444-0.19 0.4 4-0.4 0.624-0.29 0.8 4-0.4 0.474-0.08 0.874-0.09 0.91:1:0.17 1.074-0.27 0.94-4-0.14 0.934-0.04 0.824-0.12 1.034-0.18 !.00:[:0.06 1.044-0.16 0.774-0.08 0.964-0.18 0.58+0.16 0.784-0.03 0.304-0.40 0.994-0.04 0.934-0.14 1.034-0.06 0.924-0.10

F(Tm)

40 7 40 4 20 4 19

5 10

< 16±

50± 234> < < < <

19 4 10

40 8

< 6+__ 114-

1004104-

< 16 >2500 904- 40

(fs)

"i'ma)

F--AF (F+AF); the limits t h u s f o u n d are increased (decreased) by 25.%0to take into a c c o u n t the uncertainty

0.284-0.27 --0.024-0.08 0.654-0.06 0.534-0.07 0.734-0.16 0.824-0.07 0.744-0.22 0.784-0.18 0.71 4-0.27 0.234-0.16 0.28 4-0.04 0.164-0.07 0.244-0.10 0.884-0.09 0.854-0.19 0.8 4-0.4 0.604-0.20 0.41 4-0.07 0.704-0.15 0.58 4-0.06 0.674-0.02 0.974-0.04 0.144-0.12 0.3 4-0.4 0.7 4-0.8

F(-g.m)

") T h e upper (lower) limits correspond to in the F(zm) curve.

1062 1106 956 956 956 1304 1312 1054 1304 1304 1479 1214 1214 1054 1054 1214 1054 1346 1095 1095 1095 1051 929 929 929

4.71 -+ 3.94 3.94 4.88 --> 2.17 3.81 5.16 --->2.17 2.17 2.17 3.94 3.94 5.35 ~ 2.17 2.17 5.51 --~ 2.17 4.48 5.55 --> 3.94 4.57 4.57 5.59 -"> 3.38 3.38 5.66 --->3.81 4.48 4.59 5.73 ~ 0 5.82 --~ 3.81 4.88 5.08

El a

(keV)

Ell ---9..Elf (MeV)

Lifetime m e a s u r e m e n t s o f a SAr b o u n d states

TABLE 1

0 0 Z

.< > Z

0 .>

r~ -] r~ Z >

~v

t"

0

t.o

37C14-p (II)

321

2.3. RESULTS T h e F - v a l u e s a n d the d e d u c e d m e a n lifetimes Zm are given in t a b l e 1. T h e F - v a l u e s for one level m e a s u r e d at different resonances a r e t r a n s p o s e d t o one F(zm) curve a n d a v e r a g e d t o d e d u c e Zm. T h e lifetimes o f the 4.88 a n d 5.35 M e V levels a r e c o r r e c t e d for indirect feeding via the 6.50 a n d 6.05 M e V levels, respectively. T h e e r r o r in z m is the q u a d r a t i c s u m o f the statistical e r r o r a n d a 25 % e r r o r f r o m the u n c e r t a i n t y in the F(%a) curve, m a i n l y originating f r o m the s t o p p i n g theory. T h e a g r e e m e n t between F - v a l u e s o b t a i n e d f r o m different 7-rays deexciting the same level a n d f r o m m e a s u r e m e n t s at different Ep values ( a n d different targets) is quite satisfactory. TABLE 2 Mean lifetimes (in is) of aaAr levels E, (MeV) 4.71 4.88 5.16 5.35 5.51 5.55 5.59 5.66 5.73 5.82 5.86 5.97 6.04 6.05

(13,7) present exp. refs. 8. 10) a) >600 454- 16 224- 10 2104- 70 3104-160 12q- 9 1104- 30 404- 11 < 6 350 + 920000 < 16 >25OO 904- 40 1004- 40 --

(0%P7) ref. 1,)

E. (MeV)

2500+sooo 774- 20 b) 404- 19 1954- 60 2704- 80 < 45 --10oo

394-15 >300 644-19

934-

40

6.211 6.214 6.34 6.35 6.50 6.57 6.60 6.62 6.67 6.68 6.77 6.82 7.10 7.43

(P, 7) present exp. refs. a. 1o) ,) 130=I=70 c) 10+ 8 <19 64- 4 114-10 < 5 164-10 504-40 23 4- 7 >40 < 4 <20 < 4 < 19

254- 9

a) Thezm values are from ref. ,o); the errors which include a 30% uncertainty in the stopping theory are from ref. a). b) In addition, Zm(4.88) = 150-4-80 fs [ref. 12)]. c) This corrects the value zm(6.211) ----90-t-30 fs [refs. 8, ,o)] deduced from the shift xo) of the 6.211 --~ 4.48 MeV 7-ray for the contribution of the 6.60 --->4.88 MeV 7-ray transition observed in the present work (see paper I).

T h e p r e s e n t lifetimes are s u m m a r i z e d a n d c o m p a r e d with p r e v i o u s results in t a b l e 2. W h e r e c o m p a r i s o n is possible one finds a g r e e m e n t w i t h i n the e x p e r i m e n t a l e r r o r b o t h w i t h d a t a f r o m similar (p, ~) w o r k 1o) a n d f r o m n o n - c o i n c i d e n t 3 5 C l ( ~ , p ~ ) 3 S A r e x p e r i m e n t s t l ) . T h e l a t t e r were p e r f o r m e d with different t a r g e t m a t e r i a l (AgC1) a t h i g h e r velocities, o/c = 0.007. F o r the short-lived levels, the values f r o m the present w o r k are systematically l o w e r t h a n t h o s e f r o m ref. 1 i).

322

C. ALDERLIESTEN AND C. A. L. VAN LOON

3. Spin and parity determinations 3.1. PROCEDURE In this section, J ~ values of 3SAr levels are determined by combining arguments from: (i) The 7-ray widths deduced from lifetimes of bound states (sect. 2) or strengths of resonances (paper I). (ii) The a7Cl(p, 0to)34S angular distribution data from the present (subsect. 3.2) and other 2) work. Simultaneously measured aTCl(p, ~o)a4S and a7Cl(p,?)aSAr yield curves (paper I) then provide information about J ~ and the alignment of several a 7Cl(p ' ?)aSAr resonances. (iii) A few ?-ray angular distributions (subsect. 3.3). (iv) Literature data, especially lp values from the 37C1(T, d)3SAr reaction 13). The upper limits for acceptable strengths of E2, M2, and octupole radiation are set at 100, 3 and 100 W.u., respectively 14). In addition, M2 strengths of 1-3 W.u. are considered unlikely; spin assignments based on the latter criterion are tentative and will be given in brackets. In the present investigation the acceptance limits imply (i) that a 7-ray transition which is rejected as an M2 transition cannot have E3 character and (ii) that none of the transitions involved in the angular distribution measurements can have significant octupole admixture. 3.2. ALPHA-PARTICLE ANGULAR DISTRIBUTIONS Angular distributions were measured at seven 37C1(p, ~to)34S resonances in the Ep = 980-1580 keV region. A 0.7 #A proton beam bombarded a 10/~g/cm 2 Ba 37C12 target placed at the centre of the scattering chamber. The instrumental resolution (FWHM of a resonance) was 1.3 keV. Alpha particles were detected at seven angles. The angular distribution measurements were carried out in alternating peak-background runs [cf. ref. 15)]. The relative solid angles of the particle detectors were determined experimentally [cf. ref. 2)]. The analysis of ~-particle angular distributions on isolated 37C1(p, ~o)a4S resonances has been discussed by Bo~njakovi6 et al. 2). The measured and theoretical angular distributions were compared in a least-squares computer program, based on the channel spin formalism, written by De Meijer and Van Gasteren 15). In the analysis, orbital angular momentum mixing in the formation was restricted to the two lowest lp values and contributions from lp > 4 were excluded on the basis of Wigner limit considerations. A spin assumption was rejected if it implied a ~2 value exceeding the 0.1 ~ probability limit. The results are summarized in table 3. 3.3. GAMMA-RAY ANGULAR DISTRIBUTIONS The 37C1(p, ?)3SAr angular distributions were measured at a few selected resonances with the 60 cm 3 Ge(Li) detector placed at D = 4 cm. The analysis of the

37C1-~-p (II)

323

TABLE 3 Measured angular distribution coefficients a) and deduced spins of aTCl(p, go)a*S resonances E~ (keV)

A2

986 1323 1325 1346 1529 1554 1574

--0.924-0.04 --0.014-0.02 0.744-0.03 0.064-0.02 1.43 4-0.15 1.064-0.03 0.944-0.13

A6

A4

--0.064-0.03 0.584-0.17 0.09 4-0.04 0.834-0.20

-- 0.114-0.04 --0.264-0.18

,1n 10 +, 1-, 2 + a) 3- c) 0 +, 1-, 2 + a) 4 +, 5 b) 4+

a) Only coefficients exceeding one standard deviation are listed. b) No solution. c) The angular distribution analysis also allows J" = 1-, formed predominantly via lp = 3. This values has been rejected since it would imply a reduced width exceeding 20~. of the Wigner limit and an observable resonance in the (p, Po) reaction, in conflict with the experimental data (see paper I). a) Observation of an r ~ 0 7-transition excludes the possibility j~r = 0 +. TABLE4 Measured angular distribution coefficients from the 37Cl(p, 7)aSAr reaction Ep (keV)

Transition (Ex in MeV)

A2

1095

r --->6.67 6.67 --> 4.59 r --> 0 r --> 3.38 r --> 3.81 --> 5.35 --->5.82

+0.40 +0.07 +0.25 4-0.04 +0.1464-0.018 +0.26 4-0.07 -0.24 4-0.04 +0.40 4-0.08 -0.13 ±0.11

1262 1293 1479

A4. +0.024-0.08 +0.084-0.05 +0.054-0.03 -0.054-0.10 +0.064-0.05 +0.024-0.10 +0.104-0.16

four-angle a n g u l a r d i s t r i b u t i o n data, presented i n table 4, was performed i n a s t a n d a r d way 16). A d d i t i o n a l i n f o r m a t i o n o n J~s a n d the a l i g n m e n t f r o m previous w o r k 2, 17) was essential. The 0 . 1 % p r o b a b i l i t y criterion has b e e n applied. 3.4. RESULTS 3.4.1. R e s o n a n c e s t a t e s . The present spin a n d parity assignments to a 7Cl(p ' y)aSAr

resonance levels are p r i m a r i l y due to ~-particle a n g u l a r distributions at the corr e s p o n d i n g aTCl(p, ~o)a4S resonances; the correspondence o f (p, y) a n d (p, ~o) resonances is discussed i n p a p e r I. Some a d d i t i o n a l i n f o r m a t i o n for J ~ assignments has b e e n o b t a i n e d f r o m (p, y) a n g u l a r distributions or p r i m a r y y-decay. The resulting spins a n d parities are s u m m a r i z e d in table 5. 3.4.2. B o u n d s t a t e s . Spins a n d parities of b o u n d states deduced f r o m the present d a t a are s u m m a r i z e d i n table 6. A few assignments are discussed i n some detail below.

324

C. ALDERLIESTEN AND C. A. L. VAN LOON

TABLE 5 Spins and parities of 37Cl(p, ~)38Ar resonance states Ep (keV)

jn ~)

Ep (keV)

j r ~)

Ep (keV)

j n ,)

956 986 999 1104 1143 1156 1182

31- b) 1-, 2 + 3- c) 333-

1262 1293 1307 1323 1325 1333 1346

1- d) 1- a) 11-, 2 ÷ b) 3- b) 11 -, 2 + b)

1392 1411 1479 1574 1606 1633

4+ 14 ÷ e) 4 + b) 3-- c) 3--

a) As given in ref. 2) for the corresponding z7Cl(p, 0~o)34Sresonance, unless indicated otherwise. b) From the present (p, Co) experiment (table 3). c) A jTr = 1 - assignment, not excluded in ref. 2), would imply unacceptably strong M3 transitions. d) The second possibility, j~r = 2 +, can be excluded on the basis of the angular distributions of the r --->0 and r -+ 3.38 MeV 7-rays at the Ev = 1262 and 1293 keV resonances, respectively. c) A j~r = 5- possibility is ruled out by the strength of the r --->4.57 MeV transition and by the r --->3.81 MeV angular distribution.

The 4.57 M e V level. The o b s e r v a t i o n o f r ~ 4.57 M e V transitions f r o m J " = 1 (e.g. Ep = 1262 keV) a n d Y~ = 4 + resonances (Ep = 1479 k e Y ) implies J " ( 4 . 5 7 ) = 2 +, 3 - . T h e 37C1(T, d)38Ar r e a c t i o n a3) gives Ip = 0 a n d thus J " = (1, 2) +. Conclusion: J~(4.57) = 2 +. F r o m p r e v i o u s 37Cl(p ' 7)38Ar w o r k 1 o) J " ( 4 . 5 7 ) = ( 2 - ) was assigned tentatively. A J ~ = 2 + assignment was excluded o n the basis o f a n observed 2 ~ r ---, 4.57 M e V b r a n c h f r o m the Ep = 1732 keV, J " = 4 - , T = 2 resonance. Since the p r o t o n transfer d a t a 4, 13, 18) i m p l y rc = + , the 4.57 M e V level was suggested to b e a d o u b l e t 13). The present m e a s u r e m e n t o f the r ~ 4.57 M e V a n g u l a r d i s t r i b u t i o n conflicts with t h a t for a 4 - ~ 2 - transition. A l l the k n o w n facts can be e x p l a i n e d a s s u m i n g a 37C1(p, ~)38Ar m u l t i p l e t at Ep = 1732 keV, c o n t a i n i n g a s t r o n g J " = 4 - , T = 2, Ep = 1731.6 keV a n d a weak, low-spin, T = 1 c o m p o n e n t ; the latter m a y well c o r r e s p o n d with the Ep = 1732.0 keV, Y~ = 1 - , 2 + resonance [ref.2)] seen in the 37Cl(p ' ~o)34 S a n d 37Cl(p ' no) 3 TAr reactions [see ref. 19) a n d p a p e r I]. The p r e s e n t w o r k thus gives no i n d i c a t i o n for a 4.57 M e V multiplet. The5.35 M e V l e v e l . The r--* 5.35 M e V a n g u l a r d i s t r i b u t i o n m e a s u r e d at the Ep = 1479 keV, J ~ = 4 + resonance excludes J(5.35) = 2. T h e lifetime a n d b r a n c h i n g o f this level i m p l y IM(E2)I 2 = 28___9 W.u. o r IM(M2)I 2 = 1000___300 W.u. for the 5.35 ---, 3.94 M e V t r a n s i t i o n for J ~ = 4 + a n d 4 - , respectively, a n d thus rule o u t J~ = 4 - . Conclusion: J ~ = 3, 4 +, which agrees with the rejection b y James et aL al) o f the J~(5.35) = (1, 2) + assignment f r o m ref. 20). The 6.50 M e V l e v e l . I n the a7Cl('r, d)38Ar reaction, the 6.50 M e V level is m u c h m o r e strongly excited t h a n the u n r e s o l v e d 6.49 M e V level (see p a p e r I). T h e lp = 1 o r 1 + 3 value 13) f r o m (~, d ) w o r k thus implies J~(6.50) = ( 0 - 3 ) - . T h e strength o f the 6.50 ---, 4.48 M e V ( J ~ = 4 - ) t r a n s i t i o n excludes J " ( 6 . 5 0 ) = (0, 1 ) - a n d m a k e s J ~ = 2 - i m p r o b a b l e (IM(E2)I z > 27 W.u.). Conclusion: J=(6.50) = 3-(2-).

3, 4 ÷

6.05

3

1+ 3

1 + 3 or 1

1+3

0 or 0 + 2

0 or 0 + 2

37C1(.t.' d)3SAr lp

b)

3, 4 +

3 - , 4, 5 -

(2, 3 ) -

3-

1-

1 - , 2 + ~)

3, 4 +

2+

2+

Conclusion j~r

1, 2 + 1 - , 2, 3 ( 4 - ) , 5, 6 0 1, 2 + 1 - , 2, 3 -

6.57 6.62 6.67 6.77 6.82

1, 2 + ( 2 - ) , 3 , 4 + *)

1 or 1 + 3

1

3

(2)

1 or 1 + 3

1 or 1 + 3

1+3

1(-), 2 +

6.214

6.50

lp

jyr

6.35

37C1(1:, d ) a a A r b)

Present work a)

E~ (MeV)

F r o m strengths o f p r i m a r y a n d secondary y-ray transitions unless indicated otherwise. Ref. 13). F o r a discussion, see subsect. 3.4.2. T h e r ~ 5.82 M e V a n g u l a r distribution at the Ep = 1479 keV, j~r = 4 + resonance (see table 5) excludes J(5.82) = 2. See subsect. 4.1.

3 - , 4, 5 -

6.04

a) b) 0 a) ©)

> 2 d)

2, 3 -

1,2 +

5.73

5.86

1-, 2 +

5.59

5.82

2 c+)

3, 4 + ")

5.35

2 + , 3 - c)

4.57

5.16

Present w o r k a) j,r

Ex (MeV)

TABLE 6

Spin a n d parity a s s i g n m e n t s to aSAr b o u n d states

(!--3)-

I-

5-(4-)

(2 + )

1-

3-(2-)

1-

1t-), 2 +

jn

Conclusion

+ ~O

326

C. ALDERLIESTEN A N D C. A. L. VAN LOON

The 6.67 Me V level A s i m u l t a n e o u s l e a s t - s q u a r e s fit t o t h e a n g u l a r d i s t r i b u t i o n s o f t h e 7-rays in the r(Ep = 1095 keV, J " = 5 - ) ~ 6.67 ~ 4.59 M e V ( J ~ = 5 - ) c a s c a d e e x c l u d e s J ( 6 , 6 7 ) = 3. T h e l i f e t i m e e x c l u d e s J ( 6 . 6 7 M e V ) = 7. C o m b i n e d w i t h t h e a n g u l a r d i s t r i b u t i o n s , it also i m p l i e s a n i m p o s s i b l e a n d i m p r o b a b l e q u a d r u p o l e a d m i x t u r e in t h e 6.67 ~ 4.59 M e V t r a n s i t i o n f o r J ~ ( 6 . 6 7 ) = 4 + a n d 4 - , respectively. W i t h lp = 3 [ref. 13)] o n e t h u s c o n c l u d e s t o J ~ = 5 - ( 4 - ) .

4. Discussion T h e e x p e r i m e n t a l d a t a o n b o u n d states o f 3SAr a r e c o m p a r e d b e l o w w i t h results obtained from shell-model calculations. 4.1. NEGATIVE PARITY STATES T h e m o s t e x t e n s i v e c a l c u l a t i o n s o n n e g a t i v e p a r i t y states o f A = 38 n u c l e i h a v e b e e n p u b l i s h e d b y E n g e l b e r t i n k a n d G l a u d e m a n s 3). T h e c a l c u l a t e d e n e r g y levels are TABLE 7 Comparison of calculated and experimental proton stripping spectroscopic factors for ~r = -states of aSAr J"

Calculation a) Ex (MeV)

Sp (1 = 1)

Experiment b) Sp (l = 3)

Ex (MeV)

Sp (1 = 1)

Sp (l = 3) 0.33 0.48 0.008 c) 0.13

51 5z5a 5,~-

4.14 5.58 6.14 6.69

0.23 0.57 0.006 0.17

4.59 5.66 (6.04) 6.67

41 424a44-

4.32 5.88 6.17 6.78

0.02 0.34 0.018 0.41

4.48 6.211

31 3z 3a343s36-

4.32 5.08 6.28 6.83 7.18 7.48

0.0006 0.082 0.0008 0.099 0.21 0.28

0.013 0.24 0.22 0.34 0.06 4 × 10 -6

3.81 4.88 5.51

0.014 0.011 0.002

0.18 0.31 0.082

2t -

5.03 6.44 7.13 7.32

10 -6 0.0017 0.13 0.0010

6 × 10- 5 0.13 0.32 0.11

5.08 5.86

0.008

0.33 0.20

5.78 6.99

0.004 0.006

5.73

0.062

7.03

0.026

6.68

< 0.014 a)

2a2a2411 -

12

-

01 -

0.041 0.27 0.43

6.60

*) Excitation energies and wave functions from ref. a).

b) Ref. x3). ~) Estimated from the da/d.Q values at two angles reported in ref. 13). d) Estimated from the stripping pattern for the Ex = 6.67 MeV state given in ref. la).

(a)

(b)

(c)

(d)

NEGATV I EPARITY

-Z14 - 706 695 ~60 - 7 ~2------2 ''6;~5e'0

2"

6.83

3- "',

6.69

5-~_ ' , 6.679.68

628

6.19

',, 6.77

2"

6"486"49650 641 ',

:E

POSITIVE PARITY

7.13

6.44

3-

6

(e)

,,634635

', ,,'625~9.

(0-4)* O* {1-3)(1 -3)15-(4-) ~"34".p'-

6.6

3-(2__:1

/

1 (1-3)

6.17

1"

594

°

63

2:

', ,' ~'T1 ~'14

EY"7--'--~, ; ' / ~

e-,~----'Y}-,'~6.o 46.o5 5.9~ 6 - , ' / ' , ~ - 588 ~ 8 ~ 5.78 558

7.y 1-

",~----

3,4*3;4,5"`

__',

"-582586 ~5.73

5-.~.~55

.)7

.~'~1 5.35

....

3" 3.4" ",,

3"

::i:::

',

5.47 ',5.38

4 " ~

5.25

2"

4" )~

--/~ /

\ 0.~

5.16 508

I1-3)- ', ", 495 3', 4.92 ', 4.78 O" 4.74

2-~4.88 4.71

1+ 3* 4" 2"~"--..~473

4• " ~÷J

47

2*

43

2"

a857459 4-

414

5-/ ~ - ~ 8 1

394 538 2.17

2~3:----~ 3.77 o* 3.20 2+

~!7o Q25

O*

(ld312)5(lf 712)P(2P312)q

p,q=l

2" O* ~

0

O" EXPERIMENT

3.68 339

2÷/:~9.// O y

(3*

2.16

2*

2.1

2*

0

O"

0

0*

2"J 0

O"

(ld3/2)6"n(lfTi2) n n=O or2

~(Sd)"2

[(sd) "2

L(fp)=(sd) "4

L(1f 712)2(5d)"4

Fig. 2. Comparison o f experimental and theoretical level schemes of aaAr. The experimental data are from the present and several other investigations [see ref. 23)]. The calculated spectra (a), (c), (d) and (e) are taken from ref. 3), the present investigation, ref. 5) and ref. 4), respectively. The Ex values in spectrum (e) are not given as such in ref. 4) but are estimated from a level scheme in that paper. Speculative identifications are indicated by dashed lines.

328

C. A L D E R L I E S T E N A N D C. A. L. V A N L O O N TABLE 8 C a l c u l a t e d a n d e x p e r i m e n t a l ?,-decay d a t a of~z = --, T = 1 states o f 3 SAr

jl~. _~ jfTt a)

Exl ~ Exf (MeV) calc. a)

Branchings b, ¢) (%) exp.

calc.

rm c) (fs)

exp.

calc.

exp.

100

30

>

40

30

<

16

01-- - + 1 1 - -

7.03 -+ 5.78

6.68

--~ 5.73

100

2 2 - -->"31-+32-

6.44 ~ 4.32 5.08 ---> 5.03

5.86

--~ 3.81 -+4.88 --~ 5.08

86 11 3

87 13

32 - --->31--->.41--

5.08 --~ 4.32 4.32

4.88

--)-3.81 --~ 4.48

50 d'g) 4 a, g)

54

4-2 < 3

224 a'h)

454-16

3 a - ~ 3 1---> 4 1 --->-32-.---> 2 1 -

6.28 --->4.32 4.32 5.08 5.03

5.51

--->3.81 --> 4.48 "-~ 4.88 5.08

380'0 1 a,~) 28 a.8) 4 s)

11 37 23

4-4 13 ±2

245 a'h)

3104-160

4 1 - -+ 31-

4.32 --~ 4.32

4.48

-~ 3.81

100

4 2 - ~ 3 1--~ 4 1 ---->51-4-->33---> 5 2 -

5.88 --->-4.32 ---> 4.32 4-->4.14 ---> 6.28 ---> 5.58

6.211 --~ 3.81 --~ 4.48 4.59 ---> 5.51 --~ 5.66

25 47 8 18 2

4 3 - --->3 t --> 4 1--+514-->3252-

6.17 --~ 4.32 -+ 4.32 -+ 4.14 --> 5.08 -~ 5.58

6.0 i) ~ 3.81 4.48 --~ 4.59 4.88 5.66

54 23 1 20 1

4 4 - --->3 t -+ 4 1 --->51-32-4-->-3352 --,--> 53 ~ --~ 4 2 -

6.78 --~ 4.32 ---> 4.32 -+4.14 5.08 --~ 6.28 --~ 5.58 --~ 6.14 --~ 5.88

6.60

4-->3.81 ---> 4.48 ---> 4.59 ---> 4.88 --~ 5.51 -~ 5.66 --~ 6.04 6.211

14 a) 35 d) 22 d) 13 a) 10 a) 1 a) 1 4 a)

0.9-/-0.3 79.34-1.9 2.44-0.4 17.44-1.7 < 1 < 1

51- ~ 3141--

4.14 ~ 4.32 --~ 4.32

4.59

--~3.81 -->- 4.48

8 a) 92 a)

10.04-1.0 89.7 4-1.0

52- -->"3141--~ 5 1 -

5.58 --9- 4.32 4.32 4-->4.14

5.66

--~ 3.81 4.48 4.59

0 a) 3 a) 97 a)

2.44-0.3 8.64-0.7 89.04-0.9

5 3 - "-->31-->. 4 1 -.-->51-

6.14 --~ 4.32 ---> 4.32 4->4.14

6.04

-~ 3.81 4.48 4.59

38 19 43

58 -4-3 32 + 3 10.54-1.6

5 , - --,'.-4 1 -

6.69 --->4.32 --->4.14 -+ 5.58 5.91 5.88

6.67

--->4.48 4.59 5.66 --~ 5.9 i) --~ 6.211

2 79 17 1 1

7 93

---> 52-6 1--~ 4 2 -

a) a) a) d) d)

-4-2 4-2

100

394 d)

20004-800f)

< 5 92.9±0.9 7.1 4-0.9 < 2 < 3

73 a)

1304- 70

360

£:2 4-2 < 8

39 a)

164- 10

ll0xI0 aa) (185:k10)×103f) 41 a)

230

18

404- 11

904- 40

23:k

7

aTClq-p (II)

329

TABLE8 (continued) .r~,, -+ 7?, .)

"rm c)

Branchings b. ¢) (%)

(MeV)

calc.

exp.

(fs)

calc. a)

exp.

6a- ~ 5x7x-

5.91 ~ 4.14 -+ 5.88

6.4 l) _+ 4.59 --+ 5.9 l)

99 1

150

62- -+ 415t52-+ 53--+ 71-

6.19 -~ 4.32 ~ 4.14 --~ 5.58 --->6.14 --~ 5.88

6.4 I) ~ 4.48 --~ 4.59 -+ 5.66 ~ 6.0 l) --+ 5.9 t)

42 4 2 7 43

320

71- "-~ 5t-

5.88 ~ 4.14

6.41 -+ 4.59

100

100 • )

calc.

1000

exp.

1500±400

e)

a) The lowest j~r = 3-, 2- and 1- states, assumed at 3.81, 5.08 and 5.73 MeV, respectively, are omitted because their observed 7-decay proceeds (almost) entirely to n = + states. b) Calculated branchings of less than one percent are omitted if they are unobserved experimentally. c) From the present investigation unless indicated otherwise. d) Ref. a). e) Ref. 22). f) From other work [see ref. 23)]. ~) The calculated M1, E2 branching ratios are normalized to the sum of the experimental values to correct for E1 transitions. n) The partial lifetime, from the M1, E2 transitions only. l) Estimated excitation energy, chosen for comparison of different interpretations of the same observed state. given in fig. 2. T h e m o d e l assumes five active nucleons in the ld~, shell a n d one in either the lf~ o r the 2p~ shell; the m o d i f i e d surface-delta i n t e r a c t i o n 2x) ( M S D I ) was c h o s e n as residual interaction. S o m e s u p p o r t for the validity o f this r a t h e r simple m o d e l can be o b t a i n e d b y c o m p a r i n g the 37C1(-c, d)3SAr spectroscopic factors, c a l c u l a t e d with the wave functions f r o m ref. 3), a n d the e x p e r i m e n t a l d a t a f r o m ref. 13). I n the present calcul a t i o n o f the Sp values, listed in table 7, the a7Cl g r o u n d state was t a k e n as 100 ( l d ~ ) ~ . ~ , where the l a t t e r t w o subindices refer to J a n d T, respectively. O n e finds S p ( ] = 1) = A2, a n d Sp(l = 3) = A 2, where Ap a n d Af are, respectively, the a m plitu-xtes o f t h e ( l d t ) ~ , ~ 2 p ~ a n d ( l d ~ ) ~ , ~ l f ~ configurations in the final state. T h e excellent a g r e e m e n t b e t w e e n e x p e r i m e n t a n d calculations f o r J ~ = 4 - a n d 5 - states s u p p o r t s the c o n c l u s i o n a l r e a d y d r a w n a) f r o m p r e v i o u s 7-decay d a t a t h a t this m o d e l p r o v i d e s useful wave f u n c t i o n s for high-spin states. G a m m a - r a y s t r e n g t h s have been c a l c u l a t e d t for the transitions n o t y e t discussed in ref. 3). T h e y are p r e s e n t e d a n d c o m p a r e d with e x p e r i m e n t a l d a t a in t a b l e 8. Realistic g-factors were u s e d in the calculation o f M1 strengths. F o r the E2 strengths, e x p e c t a t i o n values o f the s q u a r e o f the r a d i a l c o o r d i n a t e were calculated f r o m h a r m o n i c oscillator wave ftinctions; effective charges were used o f ep = 2e, e, = e. We thank J. F. A. van Hienen for the use of his computer program. \ a

330

C. A L D E R L I E S T E N A N D C. A. L. V A N L O O N

Again the agreement is especially good for the high-spin states. The combined results from ref. 3) and the present work show that this simple model adequately describes the v-decay of the six negative-parity levels which are known to have J > 4. It should be stated, however, that the model produces five other states (J~ = 4 - , 5-, 6-, 6- and 7 - ) with E x < 6.2 MeV, which have not yet been located experimentally. For all known states with excitation energy below 7.0 MeV, other than those at 6.04 and 6.41 MeV, one can exclude J > 4, rc = - with high probability. For the 6.04 MeV level with J~ = 3-, 4, 5- and the 6.41 MeV level the spin of which is unknown but which exclusively decays to the 4.59 MeV, J~ = 5- level 22), the v-decay clearly suggests rc = - . If these states can be described in terms of the present model, the small experimental stripping strengths are incompatible with J ~ --- 3 - (see table 7). Branching ratios and lifetimes, calculated for the various interpretations of these levels (4~" and 5~" for the 6.04 MeV level and 43, 53, 6~-, 62 and 7~- for the 6.41 MeV level) are compared with the experimental data in table 8. The interpretation of the 6.04 and 6.41 MeV levels as the 53 and 7~ states, respectively, yields good agreement whereas the alternatives fit significantly worse. The 6.04 and 6.41 MeV levels were not observed in experiments 1o) on T = 2, (P, V) resonances and weakly, when at all, in the (z, d) reaction 13). This, and the fact that four more high-spin states are not observed either via T = 2, (p, V) resonances or in the (z, d) reaction, can also be understood on the basis of the model used. Such an excitation depends mainly on the amplitude of the (Id~)~, ~ lf~ component in the wave function of the final state. This component vanishes for the states with J " = 6- and 7- and is small for the missing J~ --- 4 - and 5- states (see table 7). Excitation of high-spin states via T = 1 resonances is improbable on general grounds. In the energy range studied, the proton width of high-spin resonances, with high lp values and low penetration factors, is on the average so small that these resonances escape detection. For the low-spin negative-parity levels the model is much less adequate. The calculations reproduce the Sp values and the v-decay of only one of the four known J ~ = 3 - levels. The calculated Sp values and decay properties of the 22 state agree with those of the 5.86 MeV, J~ = (2, 3)- state. The calculated properties of the 27 state, however, do not agree with those of the only lower-lying level (Ex = 5.08 MeV) which may have J " = 2-. The 1~- state, calculated at 5.78 MeV, may be identified with the 5.73 MeV, J~ = I level. The other obvious candidate, the J~ = (1-, 2+), ' 5.59 MeV level, mainly decays to states with large 2p-4h admixtures (see subsect. 4.2), which makes a J~ = 2 + assignment more likely. The lp = 1 + 3 assignme~lt from the (z, d) experiment 13) is not convincing in view of the very low cross s~ction. The 6.68 MeV level, which decays 100 ~ to the 5.73 MeV, J~ = 1- level and which is excited only at low-spin resonances, is the only acceptable candidate for identifications with the calculated 7.03 MeV, J~ = 0 - level. Its v-decay properties are well 3~

37C1+p (II)

331

r e p r o d u c e d in t h e c a l c u l a t i o n s if t h e 5.73 M e V l e v e l is i n d e e d (see a b o v e ) t h e l o w e s t d ~ = 1 - state o f t h e m o d e l . A s u m m a r y o f t h e s e c a l c u l a t i o n s , i d e n t i f i c a t i o n s a n d s u g g e s t i o n s is g i v e n in t a b l e s 7 a n d 8 a n d in fig. 2. 4.2. POSITIVE PARITY STATES A s h e l l - m o d e l d e s c r i p t i o n o f 3SAr w i t h t w o h o l e s in t h e sd shell p r o d u c e s 24) o n l y five n = + levels, o f a l m o s t p u r e (2s½) - 2 , ( l d ~ ) - 2 a n d ( 2 s ~ l l d ~ "x) c h a r a c t e r , b e l o w E~ = 7 M e V . TABLE 9 Comparison of calculated and experimental ),-ray strengths for ~ = + states of a SAr arl~r -+ jfTr

(Ext -+ E,r),xp') (MeV)

Strength (W.u.) experiment ~)

theory M1

E2

M1

Remarks

E2

4x + -+21 + -+ 22 + -+ 23 +

5.35 -+2.17 -+ 3.94 -+ 4.57

0.22 8.6 2.1

0.9 4-0.3 28 4-9 < 50

0 a) a)

42 + -+ 21 + -+ 22 + -+ 2a + -+ 4t +

6.05 -+ 2.17 -+ 3.94 --~ 4.57 ~ 5.35

0.40 0.50 3.6 0.14

5 22

< 0.12 4-2 4-9

~) a) a) 0

4.1 4-0.4

c)

2.1 +0.8 < 330

a) a) a)

< 0.03 < 9

a) e) a) a)

1.404-0.14

¢)

2a + -+0x +

2.17-+0

22 + -+ 21 +

3.94 -+ 2.17

-+01 +

-+0

-+ 02 +

-+ 3.38

23 + -+ 2x + -+ 22 + -+ Ot + -+ 02 +

4.58 -+ 2.17 -+ 3.94 -+ 0 -+ 3.38

02 + -+ 2x +

3.38 .-+ 2.17

0.56

0.5

4-0.2

4.90 0.012

0.01 0.27

0.0080 0.33 5.8

0.0074-0.003

0.48 3.0 0.10 1.3

0.0434-0.004 (0.053 4-0.013)

1.8

a) See discussion in subsect. 4.2. b) The branching ratios and the lifetimes of the 5.35 and 6.05 MeV levels are from the present experiments, the other lifetimes are from ref. 23). Mixing ratios are assumed to be zero. ~) Both states have small Sp (1 = 0) values. ~) One of the states has a small Sp (l = 0) value. • ) Both states have large Sp (l = 0) values.

T h e i m p o r t a n c e o f 2 p - 4 h e x c i t a t i o n s has b e e n d e m o n s t r a t e d e x p e r i m e n t a l l y 4, i s ) a n d d i s c u s s e d t h e o r e t i c a l l y . R e s u l t s o f c a l c u l a t i o n s o n t h e m i x i n g o f 2h w i t h 2 p - 4 h e x c i t a t i o n s a r e p r e s e n t e d in fig. 2. T h e l e v e l s c h e m e o f G r a y et aL 4) is b a s e d o n (lf~):2= o or 2 x (4h)s= o or 2 c o n f i g u r a t i o n s ; t h e t w o f - p a r t i c l e s a r e n e u t r o n s . I n t h e m o d e l o f S k o u r a s 5) t h e t w o n e u t r o n s a r e a d m i t t e d t o a n y s u b s h e l l o f t h e f p shell a n d c o u p l e d t o d e f o r m e d 4 h states w i t h J = 0, 2 o r 4. F o r t h e p r e s e n t d i s c u s s i o n , a t h i r d

332

C. ALDERLIESTEN AND C. A. L. VAN LOON

level scheme has been calculated in the relatively simple configuration space (ld~) 6-n (lf~) n, with n = 0 or 2, and is also presented in fig. 2. In this calculation, the two parameters 21) of the residual MSDI were estimated from known 3BAr states to be Ao = 0.63 MeV and At = 0.67 MeV and the I f ~ - l d , splitting to be AE = 1.76 MeV. Fig. 2 shows that this simple 2h, 2p-4h model with a closed 2s½ shell already produces many rc = + states below E x = 7 MeV. The wave functions of the present model have been used to calculate (see subsect. 4.1) M1 and E2 transition strengths. Since a closed 2s, shell is assumed, one may expect quantitative agreement with experiment only for transitions between states with small Sp(l = 0) values for pick-up 4, is). The results shown in table 9 bear out this expectation. It might be noted that the 6.05 ~ 5.35 MeV transition most probably has M1 character. The strength of this transition, which makes E1 character unlikely 14) and which is nicely reproduced only if the 6.05 and 5.35 MeV levels both have J~ = 4 +, is mainly due to the large [(ld~)04,o(lf~)2,114.1 and [(ld~)~, 2(lf~)], 1]4,1 components in the two states. There is also qualitative agreement for transitions to the 3.94 and 4.57 MeV 2 + states, both of which have strong 2s~- 1ld~ 1 components, if the 5.35 and 6.05 MeV states indeed have J~ --- 4 +. As expected, the agreement is poor for the transition between two states which are known 4, I s) to have appreciable 2s~ 1ld~ 1 admixtures. The calculations of Gray et al. 4) have already been compared with experiment. The present experimental data allow a few additional comments. Unambiguous J " = 2 + assignments have been made [see table 6 and ref. 23)] to the three lowest lp = 0 states. This confirms the previously suggested correspondence of calculated and measured levels. The interpretation of the fourth lp = 0 state at 5.55 MeV as the ( 2 s ~ l l d ~ l ) l , 1 state gives agreement with its 7-decay properties. In view of the large Sp(l -- 0) value of the 5.55 MeV state, one expects the strengths of the M1 transitions deexciting this level to be determined by the 2s~ 1ld~ 1 components in the 5.55 MeV and in the final states. The intensities of these components are proportional to the So(I = 0) values of these states. This conjecture is confirmed by a comparison of the experimental ratio of the strengths (assuming pure M1 character) of the 7-ray transitions to the levels at 2.17, 3.94, 4.57 and 5.16 MeV with the ratio of the corresponding Sp(l = 0) values 4). For the strengths one finds the ratio (0.028___0.004) : (0.39+0.04) : 1 : ( < 3) and for the Sp values (0.10_+0.04) : (0.33_+0.07) : 1 : (0.47__+0.09). For the absolute values, one calculates for the 5.55 ~ 4.57 MeV transition 0.9_+ 0.4 W.u. from the Sp values and experiment gives .n. .tz. +2.6 0.4 W.u. A J~ = 1 ÷ value for the 5.55 MeV level would also explain the fact that none of the many J~ = 3-, (p, ~) resonances decays via this level. The calculations of Gray et al. 4) and of Wildenthal et aL 24) yield one almost pure (2s~) -2 state at E~ = 5.7 and 6.8 MeV, respectively. The present ~-ray data rule out the candidature 13) of the 6.62 MeV, lp = (2) level and suggest that the 5.97 MeV level corresponds to this J~ = 0 + state. This would be in agreement with the low proton stripping 13) and pick-up 4, Is) strengths. The weakness o f the

37Clq-p (II)

333

5.97 ~ 2.17 MeV, E2 transition ( < 0.03 W.u.) and the correspondingly long lifetime are then due to the rather pure (ld~)- 2 character of the final state. The model of Skouras 5) produces one J~ = 2 + and two 4 + states in addition to the states given by Gray et al. 4). As to the 4 + states, identification with the 5.35 and 6.05 MeV states with J ~ = 3, 4 + has been discussed above. The additional J~ = 2 + state which has 96 % intensity 5) of (fp)2-4h configurations might be the 5.59 MeV level (see subsect. 4.1); its ~-decay and its very low cross section in the (z, d) reaction support this interpretation. Finally it might be noted that for J~(5.35) = 4 +, the excitation energies of the 3.38, 3.94 and 5.35 MeV levels, with J~ = 0 +, 2 + and 4 +, respectively, and the strength of the 5.35 ~ 3.94 MeV E2 transition, 28-t-9 W.u., are suggestive of a rotational band. Additional evidence, however, is required to substantiate this suggestion.

5. Summary and conclusions An extensive investigation of 37C1+p resonance states provided a starting point for further experiments on the bound states of 3SAr. The measurement of singles y-ray spectra and Doppler shift attenuations resulted in excitation energies, v-ray branchings and lifetimes for many bound states. From this information, some y-ray angular distributions and data from other investigations, several spin assignments for 3SAr bound states were deduced. The somewhat one-sided previous information on the nucleus aSAr, obtained from selective reactions such as (p, V) experiments on analogue resonances, is complemented by the present y-ray data. The experimental data from the present investigation and from the literature are compared with the results from shell-model calculations. For the negative-parity states, a (lf~2p~) t model works quite well for the high-spin states but it seems too restricted for the low-spin states. The experiment indicates many positive-parity states for which the intensity of 2p-4h configurations is at least significant. The results of previous and present shell-model calculations admitting (fp)2 excitations are encouraging. From the analogy of 2p-4h states in aSAr and 4p-2h states in 42Ca, collective features can be expected for the former; the present v-ray data already point that way. Further unambiguous J= assignments are, however, a prerequisite for further progress. We thank Prof. P. M. Endt and Dr. C. van der Leun for many helpful suggestions during the preparation of the manuscript. The kind criticism of Drs. H. A. Doubt, G.A.P. Engelbertink and P. W. M. Glaudemans is appreciated. It is a pleasure to thank J. F. A. van Hienen for many useful discussions and W. van Dalfsen for his assistance in the shell-model calculations.

334

C. ALDERLIESTEN A N D C. A. L. VAN LOON

T h i s w o r k was p e r f o r m e d as p a r t o f t h e r e s e a r c h p r o g r a m o f the " S t i c h t i n g v o o r Fundamenteel

Onderzoek der Materie"

(FOM)

with financial support from the

"Nederlandse Organisatie voor Zuiver Wetenschappelijk Onderzoek"

(ZWO).

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