Volume 29B, number 9 PHYSICS LETTERS 4 August 1969

POSITIVE PARITY STATES IN 4oK *

P. J. TWIN **, W. C. OLSEN and E. WONG Nuclear Research Centre,

The University of Alberta, Edmonton, Canada

Received 27 June 1969

Neutron and y-ray angular distributions, and y-ray polarisation correlations have been measured with the reaction 40Ar@,nr)40K. Positive paritv states have been established at 1.644 MeV (O+), 1.959 MeV (p), 2.261 MeV (3+) and 2.290 MeV (l+).

Recent interest in deformed states in nuclei around 40Ca [ 1,2] led us to search for such states in 40K, where they would be character- ised by their positive parity. The 39K(d, p) 40K reaction [3] was known to preferentially excite the single p-h (negative parity) states, the four lowest states at 0, 30, 800 and 893 keV are f: dzI states whilst those at 2047, 2070, 2103 and 2625 keV arise from the pr dzl configuration. We have studied the 4OAr(p, &ok reaction which strongly excites nearly all the low-lying levels in 40K including the four lowest levels not de- scribed by single p-h configurations. These are at 1644, 1959, 2261 and 2290 keV.

The lifetime of the 1644 keV state of 0.49 ps [4] together with its mode of decay to both the 800 keV (2’) and 30 keV (3’) states indicate a probable assignment of O+. Gn the other hand, a negative parity with JS 2 is favoured by a recent DWBA analysis of data from the 40Ar(r, t)4OK reaction [5]. We have measured the neutron an-

!” lar distributions from the five lowest levels in

OK at a bombarding energy of 5.5 MeV using the pulsed proton beam and neutron time-of -flight facility at the University of Alberta. The only distribution to exhibit any marked anisotropy was that for neutrons exciting the 1644 keV state. The symmetry about 90° of this distribution, shown in fig. 1, indicates the dominance of the compound nuclear mechanism. The theoretical distributions shown were calculated with a Hauser -Feshbach computer program [6] which uses the average potentials of Rosen [?I. Small changes in the potentials did not affect the shape

* Work supported in part by the Atomic Energy Con- trol Board of Canada.

** On leave of absence from University of Liverpool, England.

570

Q.o.M. idwee

Fig. 1. Neutron angular distribution for 1644 keV state in the reaction 40Ar@, n)4OK at a proton bom- barding energy of 5.5 MeV. The fitted curves are de- rived from a compound nuclear model calculation for spin assignments of 0, 1 and 5. The other possible spin values of 2, 3 and 4 give essentially isotropic dis-

tributions.

of the distributions which clearly indicate the 1644 keV state has spin 0. The negative parity assignment is ruled out as it requires an M3 en- hancement factor of 104 for the 1614 keV y-ray.

The 1959 keV state was studied at a proton energy of 4.44 MeV, 90 keV above its threshold excitation energy. We measured the angular dis- tributions of both the 1159 keV (- 800 keV 2- state) and 1929 keV (-30 keV 3’ state) y-rays with a 45 cm3 Ge(Li) detector; the y-y correla- tion between the 1159 keV and 770 keV y-rays with NaI crystals; and the polarisation direction correlation of the 1159 keV y-ray with a Comp- ton polarimeter with the 45 cm3 Ge(Li) counter as the scatterer and two NaI crystals as the ana- lysers. The data were fitted in terms of the 1959 keV level’s magnetic substate populations, which were limited to lie within values calcu- lated at threshold and 200 keV above threshold by the MANDY program of Sheldon and Van Pat- ter [8], which is based on the compound nuclear

Volume 29B. number 9 PHYSICS

model. The l a rge a 2 Legendre coefficients of 0,40 ± 0.01 and -0.22 ± 0.02 for the angular d i s - t r ibu t ions of the 1159 keV and 1929 keV T- rays , respec t ive ly , could only be fitted by a spin 2 a s s ignmen t to the level , which was conf i rmed by the T-T cor re la t ion . The pola r i sa t ion of the 1159 keV T-ray was measu red at angles of e = = 90o, 45 ° and 0 o re la t ive to the beam di rec t ion , the r e su l t s at 0 o being used to co r r ec t for slight d i f ferences in the efficiency of the ana lys ing c rys t a l s . The defini t ion of po la r i sa t ion used was

1 ~(e, ~ = 0) - N(e, 9 = 90) P=-EX~o,~ O)+ N(O,#, 901

where N(8, 9) is the number of counts in the photo-peak of the sum coincidence spec t ra at an- gles e, 4. The efficiency, R, of the po l a r ime t e r was calculated with the so l id-angle cor rec t ion technique suggested by Ferguson [9] and checked by the r e su l t s for the 2 + -. 0 + 1460 keV t r a n s i - t ion in 40Ar. The measu red po la r i sa t ion of

10000

i000

X 2 100

10

-co -2 -1 - . 5 - . 20 .2 .5 1 2 oo

8 ~159 KEy

Fig. 2. A plot of X 2 versus 5 for various spin and par- ity assignments to the 1959 keV state. The angular distributions of the 1159 keV and 1929 keV y - r a y and the polarisation-direction correlation of the 1159 keV

T-ray are fitted simultaneously.

L E T T E R S 4 August 1969

2.291 ~ ( I T ) I 2. 290

2. 261 ~ 2 i !0

2. 103 0

2.070 - - 4 0 ~ 50 I i

2.047 ~ 30 35 [

i. 959 15 i

1.6~1 60 I

O. 891 ~ 1 0 0 I i I

0. soo 1 lOO I t

o. 030 ~ lOO

4O K

r lO I

35 I

65 I

2O

4 6 5 ~ 1"

3*

1"

3"

2"

2~

0 ÷

5"

2"

3"

4"

-i PS/.z ci3/,z

-! f7/2 d3/2

Fig. 3. A summary of the spin and parity assignments together with the T-ray branchin~ ratios for the low~

lying states in OK.

-0.76 + 0.21 (the efficiency calculat ion con t r ib - uting most of the e r ro r ) enabled the definite as - s ignment of ~osi t ive par i ty to the 1.959 MeV state. The ×2 plot ve r sus 5, shown in fig. 2, for var ious spin and par i ty a s s ignment s indicates pure E1 charac te r for the 1159 keV T-ray. The 1929 keVT- ray was de te rmined to have an M2/E1 mixing ra t io of 0.10 ± 0.07.

F r o m s i m i l a r techniques the angular d i s t r i - but ions of the 2231 keV and 2261 keV r - r a y s i n - dicate that the 2261 keV state has a spin of 3 whilst the po la r i sa t ion cor re la t ion at e = 90 o, y i e ld ingp = -0.57 ± 0.28, shows that the state has posi t ive par i ty . The 2290 keV state observed in the 39K(d,p)40K reac t ion [3] has been shown to be a doublet of energ ies 2290 and 2291 keV. The 2290 keV state decays to the 1644 keV 0 + state (646 keV ~/-ray) and the 800 keV 2" s tate (1490 keV T-ray) whilst the 2291 keV state de- cays to the 4- ground state. The angular d i s t r i - bution of the 646 keV T-ray can only be fitted with a spin ass ignment of 1 and the compound nuc lea r calculat ion also gives a fit th ree t imes be t te r for 1+ than for 1-. The p re sence of the s t rong decay mode to the 0 + state also indicates the s tate has posi t ive par i ty . The angular d i s - t r ibut ion of the 2291 keV T-ray with an a 2 of 0.55 ± 0.03 can only be fitted for a spin of 4 to the 2291 keV state.

These resu l t s , s u m m a r i s e d in fig. 3, show that at leas t four posi t ive par i ty s ta tes , with poss ib le configurat ion f~d~ 2 and with sn ins 0 + 1 +, 2 + and 3 +, Iie at the ~ sa~me energy i n 40K as the s ingle p-h p~ d~ 1 s tates .

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Volume 29B, number 9 PHYSICS L E T T E R S

The au thors wish to thank Drs . W. J . McDon- ald, J . L. Honsaker and D. M. Sheppard, and Mr. T. P. G. Ca ro l a for the i r a s s i s t a n c e with va r i ous p a r t s of the exper iment .

References

1. W.J. Gerace and A. M. Green, Nucl. Phys. 93 (1967) 110 and A123 (1969) 241.

2. B.H. Flowers and L. D. Skouras, Nucl. Phys. Al l6 (1968) 529.

3. H.A. Enge, E.J . Irwin and D. H. Weaner, Phys. Rev. 115 (1959) 949.

4 August 1969

4. J .G. Main et al., Phys. Letters 26B (1968) 295. 5. J . J . Wesolowski, L.F. Hansen and M. L. Stelts,

Phys. Rev. 172 (1968) 1072. 6. N. E. Davison, Nuclear Research Centre Report,

University of Alberta, 1968 (unpublished). 7. L. Rosen, in Proc. Second Intern. Symp. on polari-

sation phenomena of nucleons (Birkhauser Verlag, Basel, 1966)V, 253.

8. E. Sheldon and D. M. Van Patter, Revs. Mod. Phys. 38 (1966) 143.

9. A.J. Ferguson, Angular correlation methods in gamma-ray spectroscopy (North-Holland Publishing Co., Amsterdam, 1965) p. 40.

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