On 1f2p nuclei: Structure of 53Cr with emphasis on high-spin states

Nuclear Physics A204 (1973) 561--573; (~) North-HollandPublishing Co., Amsterdam Not to be reproduced by photoprint or microfilm without written permission from the publisher

O N lf2p N U C L E I : S T R U C T U R E OF SaCr W I T H E M P H A S I S O N H I G H - S P I N STATES W. G U L L I - I O L M E R t a n d Z. P. S A W A

Research Institute for Physics, 104 05 Stockholm, Sweden

Received 23 October 1972 Abstract: The decsy properties of high-spin states of 5aCr have been investigated by means of the

5°Ti(,,, n)53Cr*(y) reaction at E~ = 10.2-14.2 MeV. From measurements of the yields, angular distributions and linear polarizations of ?'-ray transitions the following states have been assigned: 1006 keV ~-, 1290 keV ] - , 1536 keV ~[- (all confirmed), 2173 keV ~ - , 2233 keV ~-), 2827 keV ~ - ) , 3084 keV ~ - and 3593 keV ~.c-). Multipole mixing ratios are given for transitions. Coexistence of the spherical and the deformed states in 53Cr is suggested.

[ E

NUCLEAR REACTlONS S°Ti(~t,n)SaCr*(~,), E~= lO.2-14.2 MeV; measured tr(E, E~,, 0~,), ?'-linear polarization. SaCr deduced levels, j n iS. Enriched target.

I

1. Introduction

The valence nucleons of 53Cr are distributed in the lf2p major shell for low excitation energies. Accordingly it seems reasonable to assume that the corresponding four protons are in the lf~ orbit and the neutrons form a configuration of the closed lf~ shell plus one particle. However, similarly to the isotone 55Fe, one additional neutron may be promoted from the lf~_ shell to the higher-lying orbits 2p~, lf~ or 2p~, thus giving rise to hole states. These two degrees of freedom of motion are then supposed to be the main ones for 53Cr. The properties of states and transitions in 53Cr have been investigated rather extensively in the last few years 1). Recently Carola and Tamboer 2) have determined lifetimes, decay modes and spins of 53Cr states with E X < 3.3 MeV in the 5°Ti(~, n)53Cr*(~) reaction at E, = 4.9-6.0 MeV. In particular these authors have confirmed the tentative assignments jR = 7 - to the 1290 and 1536 keV states and determined accurate mixing ratios for several transitions. In an earlier investigation of the 51V(~, pn)53Cr*(~,) reaction at E, = 30 MeV using ),-), coincidences new energies states of 53Cr have been assessed at 2173, 2233, 2827, 3084, 3593 and 4697 keV [ref. 3)]. Considering the appreciable amount of angular m o m e n t u m transferred by the ~-beam these states are expected to have high spins ( J > ~). In the present work we have measured the angular distributions, polarizations and yields of transitions in 53Cr using the reaction 50Ti(ct, n)53Cr.(~,) at E, = 14.2 MeV and this enabled us to determine the spins of all these states but one. t On leave of absence from Chalmers University of Technology, GOteborg, Sweden. 561

562

W. G U L L H O L M E R A N D Z. P. S A W A

2. Experimental procedure We used a ~ 10 mg/cm 2 thick TiO2 target 80 % enriched in ~°Ti and glued with polystyrene to a 1 mg/cm 2 thick mylar foil. It was bombarded by the 14.2 MeV =beam from the 80 cm Stockholm cyclotron. Two 43 cm 3 coaxial Ge(Li) detectors with resolution of 2.6 keV F W H M at Er = 1332 keV were used in measurements of angular distributions (with one detector as the monitor). The beam spot on the target was limited to a diameter of 2.5 mm and the target-detector distance was 10 cm. The spectra were taken at 0 = 0 °, 24 °, 35 °, 45 °, 55 °, 66 ° and 90 ° from the beam direction. At the geometry used only the spectrum taken at 0 ° required the correction due to an absorption of the photons in the intervening thin Pb beam stopper. The yield of transitions was measured at an angle of 55 ° and also at two other beam energies, 12.2 and 10.2 MeV, which were obtained using Ta foils as beam energy degraders. The 12 cm 3 planar Ge(Li) detector, which was used to analyse the linear polarization of the y-rays [cf. ref. 4)], was positioned on a turntable at a distance of 16 cm beneath the target. The spectra were measured for two orientations of the planar detector: in and perpendicular to the reaction plane defined by the 0c-beam and the y-ray. The polarization efficiency calibration of this particular detector is given elsewhere s ) .

3. Experimental results The single y-ray spectrum obtained in the S°Ti+0c reaction at E~ --- 14.2 MeV is shown in fig. 1. The lines corresponding to transitions in the residual nucleus 53Cr were identified from the previous knowledge of transition energies in that nucleus 1, z) and from the y-y coincidence investigation in the 51V(0t, pn) 53Cr reaction at E~ = 30 MeV [ref. 3)]. The results of that experiment are also shown in fig. 2. The proper order in y-cascades was derived from the transition yield measurements, and the resulting level scheme of s 3Cr is presented in fig. 3. The measurements of y-ray angular distributions and the linear polarizations together with the investigations of yields (at various bombarding energies) of transitions enabled us to determine the spins for the corresponding states. These assignments were facilitated by the strong alignment o f the low-lying residual states formed in y-ray decay of the residual nucleus that is produced in an s-beam-induced reaction on a spinless target 6) at bombarding energies ~ 14 MeV. Assuming a Gaussian-like distribution with width a(Mi ) of magnetic substates Mi of an emitting state Ji, the values of Ji, the mixing ratios 6, and the widths a ( M i) were fitted to the measured angular y-ray distribution. Assuming, further Jf, the spin of the final state, the analysis yielded sets of Ji, tr(M~) and the multipole 2 mixing ratio 6 corresponding to a Qm~, (cf. fig. 4 and table 1). However, only the spins J~, Jr which satisfied the condition

I1~-Jfl ~ 2

Z

m

Z

~.. I

21)0

i

~

2832

t 2t.7.o

~ _ _ .

l.

1000

5302

.

~

593.8

L

696.4

__

i

..a..--..--..~-

882.8

•

2000

10063

a

~

~

1289.8

~

I

~

15 ,? 18,30 -i.-..~_

3000 CHANNEL NUMBER

I^.

0 x =90"

SINGLE 'y-SPECTRUH

Ea= 1/-,.2t4eV

S°Ti + ct

Fig. 1. The v-ray spectrum observed by a 43 cm 3 Ge(Li) detector at 0~ = 90 ° to the beam during bombardment of the enriched 5°Ti target with the 14.2 MeV or-beam. The spectrum lines corresponding to transitions in 53Cr from the 5°Ti(~, n)S3Cr*(7) reaction are indicated with energies in keV.

2

6

564

W. G U L L H O L M E R A N D Z. P. S A W A

GATE 1290 I~eV 53Cr"

10(

883

I 912 5C

z 10(2 o (J / 50I "

~:

GATE 883 keV 283 "

912 ,

.... i . "'":&"~ ~:;"~:~4"~"';~'~=~;~" ' : ; ~ ' "

o

1290 ~

1613

I .....

"

I '

•

'

883

1

283

1290

:|

1613

530

200

100

.-

• .

GATE 1006keV

59/, [ 696 I ' j766

•

0

GATE 530 keV

100

696 594

03

7:

1006

50 ,:,;...~.'..~..~.;.. :..

0 (J

,..

0 100 ,',n

z

50

•

247 .. ,283

53O 1594 " J :J| "/66

GATE 696 keY

1006

0 530

100 247

GATE 594 keV 696

5(2

0

500

1000

1500

2000

2500 .Ev.keV

Fig. 2. Th¢7- ~ comcidcnco spectra oloscrvcd in the 51V(¢c, pn)SZCr*(7) reaction z).

53Cr STRUCTURE

565

5°Ti(a,n) S3Cr*(y) '

~

' E~,,keV Ji---'Jf

,....,,.__------'1290

03 I.---

IZ

l.d

7

fl

JI----~GS

~1006 5 ~ - ~ G s

883 J3---"J1

696

53O

~

912 J6---*J3 594 Js~ J,766 JT~Js

Ex,keV

J7~

3593

36 ~

308,:

Js~ J=~ J3~

2827 2233

J2 - - - - - ~ - r - *

1536

2173

J1~

5/2"-~

3

/

2

1290

1006

-

~

53Cr

1'0

12

14

Ect MeV

Fig. 3. To the left, the yield of y-ray transitions in 5aCr observed at E= = 10.2, 12.2 and 14.2 MeV. To the right, the decay scheme of 5aCr.

were taken into consideration. Furthermore, a useful relationship between the Ji of the emitting states was obtained from the slopes of the transition yield curves when the energy of the o~-beam was varied. The corresponding yield changes reflect the redistribution of the available angular momentum among the residual states, and accordingly, indicate the values of the spins of, at least, those states which are involved in the same 7-ray cascade. We stress that under the conditions met in this kind of experiment any resonance effecting the alignment of low-lying states or the yield of the corresponding transition might be neglected. Other information on the multipolarity of a transition was supplied by measurement of the associated linear polarization 4, 7). The predicted polarization of the transition for the set of Ji, Jr and 6(E2/M1), that was obtained from the least-squares analysis of angular distribution, is compared in fig. 4 with the measured polarization that was derived from the observed counting rates of the polarimeter at the two positions. Summarizing, the following criteria have been applied in the considerations leading to the assignments of spins and parities of the states and of mixing ratios of the transitions: (i) The solutions corresponding to Q2i, of least-squares fits of angular distributions above 0.1 ~ confidence limit were rejected (Z2 test). (ii) The relationship of Ji and Jr ought to be in accordance with changes of the yield of associated transitions when the beam energy was varied. (iii) Consistency with available polarization data. The estimates of errors in mi~ing ratios ~5were obtained at Q2 corresponding to one standard deviation from anlill" 2

566

W. G U L L H O L M E R

a~

Q2

/ /#

)0 /

~

X~

x

Z. P. S A W A 1112

696 keY

~7/2

5/2"

/

100 ?/2

~/

101

AND

\

',

%/ /

/

10

\//~--

V 1

I

_/

X

10 I

\

-,

/

1

1512

##/

-90"

O"

*90"

arctg

i/~-.,./ //.," \ "~//,.,,'_.".,.\,~,,

O0

/ ,' V / 1, ; u

O*

arctg 6

d

\

-90"

/f

~: 9/21/2

\\ \.

/.#"/ ' /

+ 90"

//

~..,'~

l x. //~ooe keyx., \ \ /~,~. , ~3~ ,~ ~ !._ / ~~~~/. ~" 7~2" ~/

'~/:~', /1't912 I 7/2

"~ 312

112

$ I

tl

~[ ,/

,.,"

/"

,.~

x;

:

10

/2

Q 2 r ~

/

I

P

~ -.-,-

.I

'ooI../,."', ',,,oV//~ov\ • 17"

,oii , - 90 ° •I

,

I

'

i

3/2~3/2"

0

-'ti

-90"

I/

O"

arctg

+90"

0•

orctg

.90 °

?.J~s/2l,^ / I

\

V -90"

~11/2

V O"

orctg ~)

÷90°

Fig. 4. Results of the analysis of angular distributions and linear polarization for 5aCr. The least-squares Q2 fits v e r s u s arctg assuming the indicated Jl are shown for 1006, 1290, 883, 912, 530, 696, 594 and 766 keV transitions in 5aCr. The predicted linear polarizations P(90 °) are also shown together with the measured P(90 °) and the regions of arctg 6 allowed by the results from the Q2 fits. The correctly predicted curves intersect the cross hatched region, indicating the experimentally allowed regions of Pro,a,. (90 °) and arctg ~. Since P(M2, E l ) = --P(E2, M1) only Ppr=a.(90 °) for mixed E2, Ml are shown.

5aCr STRUCTURE

567

4. Reduction o f data

In this section comments will be given on the properties of states that were determined from y-spectroscopy studies in the present experiment, the results of which are summarized in table 1 and fig. 4. In addition we would like to point out that the positions of three high-spin states assigned here [2173 keV, 2827 keV, 3084 keV with spins 12 1-- ' 1.2 -1(--)' " 2 -1 5 - - ' respectively] agree well with the results of the proton inelastic scattering experiment 8) in which states in 53Cr were observed at 2.176 MeV, 2.828 MeV and 3.085 MeV excitation (with uncertainty 0.015 MeV). The 564 k e V state. This state of spin ½- [ref. 9)] was weakly excited in this experiment and was associated with an isotropic angular distribution of the corresponding 564 keV transition. The 1006 k e V state. The angular distribution and polarization of the 1006 keV transition are consistent with the assignment of spin .}- and mixing ratio 6(E2/M 1) = -0.36__+ 0.02 (the phase convention of Rose and Brink is used throughout this work). The value of the mixing parameter is in excellent agreement with that reported by Carola and T a m b o e r 2): 6 = - 0 . 3 4 + 0 . 0 4 . The 1290 keVstate. In accordance with the assignments by Carola and Tamboer this state is observed to decay throughout the E2 transition to the ground state and through the 283 keV pure M1 (cS(E2/M1) = 0+0.02) transition to the 1006 keV ~ excited state. These multipolarities were uniquely determined in this experiment from the measurement of the polarization of the 1290 keV transition. The 1536 k e V state. The assignments of the properties of the 1536, 530 and 247 keV transitions leading from this state as a pure E2 for the first, and mixed E2, M1 with mixing ratios +0.07+0.03 and 0+0.02 for the two latter, respectively, are also in fair agreement with Carola and Tamboer. In the present experiment these multipolarities were assessed from measurement of the polarization of the 530 keV transition. The 2173 k e V state. This state is established through its decay via the 883 keV transition to the 1290 keV ~- state. The measurements of the yield, the angular distribution and the polarization are consistent with an assignment of E2 to the transition and -~-- to this state. The 2233 k e V state. The decay of the 2233 keV state through the 696 keV transition to the 1536 keV ~ - state was observed in the 52Cr(d, py)53Cr reaction by Carola et al. 1o), who also proposed the spin ~ or ~2 for this state from the study of the corresponding P-7 angular correlation. According to our measurements of the yield and the angular distribution of the 696 keV transition the spin assignment for the 2233 keV state is 9, with good certainty. The polarization of the 696 keV transition was measured to be P = 0__+0.15 and did not allow a determination of the parity of the emitting state. However, the assignments ~2 - and fi(E2/M1) = +0.17+0.05 are consistent with the results obtained and are assumed accordingly in the forthcoming discussion.

+0.128i0.008

+0.2784-0.010

--0,2244-0.013

+0.3274-0.022

--0.298+0.013

+0,2794-0.002

+0.3234-0.008

0.462 d:0.009

64

100

7.2

5

30

t3

50

30

1006.3±0.2

1289.84-0.3

283.2±0.3

1537.3=[=0.8

530.24-0.2

247.04-0.3

882.8±0.3

696.4i0.3

1290->g.s.

1290--~1006

1536-->g.s.

1536-->1006

15364->1290

2173--~-1290

22334->1536

a4 d)

F0.009 i 0 . 0 1 1

--0.095 ~0.007

--0.001 :[:0.001

--0.0194-0.017

--0.0654-0.018

--0.0074-0.018

--0.0594-0.010

d 0.00210.008

isotropic

lO06->g.s.

a2 a)

564.1 4-0.2

9

E~, (keV) i,) Intensity ¢)

564->g.s.

E ~ E f a) (keY) (keY)

TABLE 1

1.40 2.20 1.81 1.25 1.67 2.0

2.01 42.1 1.00

½ --> ½--> ~ ~-- ~ ~-

-> ~,61.1 ~_t-) ...} Ya1.15 -~ ~ ½314.0

1.57

1.26 1.50 2.01

1.14 1.75 1.57

1.50

1.05 1.73 1.68

1.60 2.10

1.07

+0.024 -0.021

0

--0.25 --0.14

,1,0.49 -t-0.57

-I-0,014-0.02 .1.0.174-0.03

--0.27 --0.12

4-0.02

--0.10 -t-0.20 --0.37 --0.21

-t-0.41 -I-0.51

- 0.12 --0.05 +0.17 -} 0.28 --0.50 --0.35

predicted ~)

--1.18i0.07

0

+1.6 --0.3 +l.a -I-0.074-0.03

--0.034-0.03

4-0.02

0

- 4 . 7 4-0.8 -1-0.03±0.03 --0.364-0.02

a g) ~ ( E 2 / M 1 ) o r ~(M3/E2) h)

0.83

1.57 1.38 70.6

4.73 6.44 1.09

1.12

11.5 15.6 1.27

1.24 1.08 1.09 5.63

Q~i. f)

~-- --, ~--

~ -+ ~ ~- ~ t-+ ~ -

~ -+ ~ ~- -~- ~ ~- ~ t-

,~- ~'- ~ -

~- --~ ~ -~ ---> ~ ½- --~ t -

t - -~ t ,~ __> a -

~ -~" ~z-

½ -+ :~-

Ji --9"Jf ¢)

Summary o f the 7-ray data from the 5°Ti(oq n)sacr*(?') reaction

--0.15 +0.15

-t-0.36 +0.78

--0.45 --0.25

,1,0.22 +0.61

--0.69 --0.34

measured J)

Polarization

>

.N

> Z Z7

tr

O [-

tr-,

O

oo

911.8=}=0.3

766.0=/-0.3

3084 --->2 1 7 3

3593 ~ 2827

5

16

15

--0.501=/=0.010

+0.29210.025

--0.384-/-0.010

+0.015:/:0,014

--0.0675:0.022

--0.062~0.014

--> ~,1z

~ t - } -.~ ,~t-~ ~ - ->- ~

~

__> ~ t --> ~ J~'- ~ "~0.98 169

82.9

2.0 8.9 2.0

2.27 34.9

~

6.46

~ --> ~J!t~-~ --* ~{-) ~ -÷ ,~-

1.62 2.73

1.79

2.02 2.48 2.00

1.76 2.41

1.58

+0.144--0.05

--0.83+°~ ° -- 0.37_.o.o7 +o.oa 0 4-0.04

+0.114-0.04

--0.32 --0.14

--1.40 --0.67 +0.36 +0.60

--0.24 +0.14

--0.37 - 0.19

+1.0

--1.30 --0.33

0

--0.20 +0.20

J) Observed polarization at 0~, = 90 °. The proposed spins and parities of states and mixing ratios of transitions are indicated by boldface print. The following critieria were used in the considerations leading to these assignments: (i) the solutions which correspond to Q,,|~ 2 of least-squares fits of angular distributions above 0.1% confidence limit were rejected, (ii) the relationship between the assigned values of Ji and Jf was required to be reflected in the observed changes of yields of associated transitions when the energy of the or-beam was varied, (iii) consistency with available polarization data, (iv) the solutions for mixing ratios It~(M3/E2)I > 0 were considered less probable. The phase convention of Rose and Brink is adopted for the mixing ratios is).

f) Lowest value from least-squares fit, Q2, of W(07) , the theoretical expression for the angular distribution for (J~, Jr, 6), to Y(07), the measured angular distribution. ~) Width of the Gaussian distribution of magnetic substates of J~ at Qmin 2 • h) ~(E2/M1): Corresponding solution for mixing ratio of quadrupole and dipole radiation, here assumed E2, M1, respectively, g(M3/E2): Corresponding solution for mixing ratio of octupole and quadrupole radiation, here assumed M3, E2, respectively. The estimates of errors associated with mixing ratios 6 were obtained at Q~ corresponding to one standard deviation from 2 Qm|n • ') Predicted polarization at 07 : 90 ° of the transition associated with the corresponding set of J~, Jr and t~(E2/MI), using the measured a2 and a , coefficients in the Legendre polynomial expansion of the angular distribution of the transition. (For M2, E1 mixed radiation the reverse sign applies.) The mixing ratio t~(M3/E2) is assumed vanishing.

2) Transition from initial state at energy E~ to final state at energy El. b) Measured energy of the ),-ray. c) Intensity in arbitrary units (intensity of 1289 keV transition = 100). ~) Coefficients of the Legendre polynomial decomposition o f the measured angular distribution (corrected for the finite size of the detector). c) Assumed spins for initial and final states.

593.8:/0.3

2827 --~ 2233

C

-4

570

w. GULLHOLMER AND Z. P. SAWA

This state is established in this work by its decay to the 2233 keV 9 - state. The yield and the angular distribution of the corresponding 594 keV transition indicate spin 9 - for that state. Similarly as in the preceding case of the 696 keV transition the measured polarization of the 594 keV transition was almost zero but consistent with the _1~_- and 6(E2/M1) = +0.10_+0.04 assignments. No evidence of a cross-over transition to the 1536 keV z2- state was obtained in the )'-)' coincidence data because of the poor statistics achieved there. In the single )'spectrum from the 50yi(~ ' n)53Cr,()') reaction an eventual 1290.2_+ 0.6 keV ~ - - ~ ½ transition would almost coincide in energy on the other hand with the 1289.8_+0.3 keV 5 - ~ 3 - g.s. transition. The 3084 k e V state. The yield and angular distribution of the 912 keV transition, which feeds the 2172 keV 12-J-~-state, are consistent with the assignment -~-- for the 3084 keV state. The meagre polarization data indicate negative parity. The 3593 ke V state. This state is established by its decay to the 2827 keV -~-- state through the 766 keV transition of which the yield, angular distribution, and polarization indicate the assignment ~ - and 6(E2/M1) = +0.14_+0.05. Since there is no line of energy 1359.8 _+0.6 keV clearly present in the single )'-spectrum we conclude that a possible cross-over transition to the 2233 keV 9 - state would be associated with branching less than 15 %. The 4697 k e V state. The evidence for this state was found in the 51V(~, pn)53Cr * ()') investigation 3) in which a ),-ray with energy 1613 _+2 keV was observed in coincidence with two transitions: the 883 k e y ~ - - ---, ~2 and the 912 keV 1_~- _~ j~_(cf. fig. 2). A weak line of energy 1613_+ 1 keV is also present in this investigation in the single )'-spectrum. If these two )'-rays correspond to the same transition which then feeds the 3084 keV ~ - - state, we are led to the conclusion that the spin associated with the 4697 keV state is high but very plobably lower than -~-, 19 considering the highest angular momentum which might be transferred to the compound nucleus 54Cr by the 0~-beam of 14.2 MeV. The 2827 k e V state.

5. Discussion

5.1. ALIGNMENT OF STATES Before considering the properties of 53Cr, we would like to comment briefly on the alignment of residual states that was observed, e.g. in this investigation and which constituted the basis for spin and multipolarity assignment. Taking into account the rather narrow widths a(Mi) associated with the correct solutions obtained from the least-squares analysis (cf. table I) we are led to conclusion that the spread from the initial sole M = 0 (along the beam axis) in the entrance channel of the reaction apparently is small, indicating that both the neutron and the subsequent cascade of )'-rays carry small amounts of angular momentum. Of particular interest is thus the comparison of the results bearing implicitly on this matter concerning the 1006 keV {--, 1290 keV 5- and 1536 keV 5 - states which were

53Cr STRUCTURE

571

investigated both by Carola and Tamboer 2) and in this work. We recall that in the former work these residual states were produced and studied at ~-beam energies

jn

Ex.keV

17/2-.~ 19/2-~

15/2"----

("1 ¢D

4697

== 13/2 ~')

15/2--13/2---

~5/2-

~ ~o~,.

°

I

~

11/2H

(E2) II

I 6=*0M,=005

I

=m I

• I 6= 0.11t0.04

3593

2827

11/2".~ 9/T--

I

7/2-

~ 0.17=0.03

I1=

6._~t0.02 ~007o=0,03

7~--I~"LF~

5/2-

5/2"(D

1/2"

1/2--~

3/2-

3/2 - m

SH ~lf~'v(2p~'1f~'21~)1

~

~

1

~

~

129o I E L ,0o5

8=-0.36±Q02

564

53 24 Cr29

(VERVIER)

Fig. 5. Properties of states and transitions in 5aCr determined in the investigation of the 5°Ti(~, n)S3Cr*(7) reaction. To the right, the proposed hole-state band is shown. To the left are shown the positions of the low-lying high-spin states of the ~(lf~_) 4, v(2pk, lf~, 2p½) 1 shell-model configuration as calculated by Vervier 11).

572

W. GULLHOLMER AND Z. P. SAWA

1.5-2.5 MeV above the corresponding thresholds of formation of states.

Accordingly, these states were strongly aligned (since the outgoing neutrons having low energies contributed predominantly to s-waves) and furthermore the corresponding occupation numbers of substates M i were calculable using the compound-nucleus model of the reaction 2). From the inspection of the Legendre polynomial coefficients fitted to angular distributions of the transitions (cf. table 1) we infer that the corresponding states are nearly identically aligned in both experiments. From this we find support for the aforementioned mode of dissipating the angular momentum in the 5°Ti(~, n)53Cr*(~) reaction at E~ = 14.2 MeV. 5.2. PROPERTIES OF 53Cr As already mentioned, the properties determined here of the four states up to 1536 keV are in good agreement with the other work 2) in which the same reaction was employed. The assignment of an additional five states with high angular momentum is important since it enabled us to depict the main features of the structure of 53Cr. For instance, we identify the 1_~_- 2173 keV and the ~ - 3084 keV states as the shellmodel ones from the configuration nlf~, v(2p~, lf~, 2p~) 1, the energies of which were predicted by Vervier 1~) in 1966. Moreover, the high-spin state at 4697 keV is a strong candidate for one of the 1~-, 1 ; - states of this configuration. The calculated positions of the yrast levels of high-spin states are compared in fig. 5 with the results of this work. Evidently, the predicted energies are systematically shifted upwards with the differences in splittings which are fairly proportional to excitation of 53Cr. We conclude nevertheless that the ability of the simple shell model to account for the positions of the yrast levels is notable. Similar success was pointed out for the isotone 53Fe, where the detailed agreement of observed and calculated energies was still better 5). The 1536 keV-~-, 2233 keV ~2-, 2827 keV ~ - - and 3593 keV ~ - - states are associated with the other major degree of freedom of motion in 5aCt. The 1536 keV ½- state was strongly excited in the neutron pick-up reaction 54Cr(d, p)SaCr at Ep -- 17.5 MeV [ref. 12)], indicating that a rather pure ~- hole state was formed. The other aforementioned states are then proposed as the members of a band built upon the 1536 keV 5- hole state. Although their parities could not be explicitly established from the polarization measurements of corresponding transitions, none the less their decay properties are clearly in favour of that assignment. The origin of such a band can be explained by the rotational model when allowance is made for a prolate deformation (with/3 > 0.2) for 53Cr [cf. fig. 1 of ref. 13)]. Similarly as in the isotone 55Fe [ref. 5)], one neutron being excited from the lf.~ shell fills the Nilsson orbital K = ½- giving rise to a neutron K --- ~- hole band. We note that the energies within this band do not follow a clear rotational pattern.

5aCr STRUCTURE

573

H o w e v e r , the occurrence o f these K = ~ - b a n d s is strikingly similar to t h a t o f the positive-parity particle-hole b a n d s in 43Sc, 45Sc a n d 45Ti [ref. 14)].

6. Conclusions The simple shell m o d e l o f 53Cr, in which the four valence p r o t o n s are restricted to the lf~ shell a n d the t w e n t y - n i n t h valence n e u t r o n remains in one o f the 2p~, lf~ a n d 2p~ subshells, r e p r o d u c e s r e a s o n a b l y well the energy p o s i t i o n s o f the first ~ - - a n d 1_5- states which are p r o p o s e d in this work. F u r t h e r m o r e , we have presented the evidence for the existence o f the negative-parity h o l e - b a n d states with j r = ~ - ~ - , 12 1 - ~ 13 - in 53Cr. 2 A c c o r d i n g l y we conclude that in 53Cr b o t h the spherical a n d the d e f o r m e d states p a r t i c i p a t e in the low-lying excitations that are associated with high angular m o m e n t a . W e w o u l d like to t h a n k Prof. Nils R y d e a n d Prof. I n g m a r Bergstr6m for their kind interest in this work.

References 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) ! 1) 12) 13) 14) 15)

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