Unusually weak E1 transitions in 45Ti

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Unusually weak E1 transitions in 45Ti

A. Chevallier, Julien Chevallier, B. Haas, N. Schulz, M. Toulemonde

To cite this version:

239

Unusually

weak E1

transitions

in 45Ti

A.

Chevallier,

J.

Chevallier,

B.

_Haas,

N. Schulz and M. Toulemonde

Centre de Recherches Nucléaires et Université Louis-Pasteur, 67037 _Strasbourg, France (Reçu le 12 octobre 1979, accepte le 19 novembre 1979)

Résumé. 2014

Les décroissances des deux

_premiers

états de

_parité

_positive

du noyau 45Ti ont été réexaminées à l’aide

de spectres 03B3 obtenus en direct, en coincidence _03B3-03B3 et en coincidence _n-03B3. Les

_{probabilités}

de transition E1 observées

(de l’ordre de 10-7 unités

_Weisskopf)

_{sont jusqu’à} 30 fois inférieures à la transition E1 la _plus ralentie connue dans les autres noyaux de la région de masse 40 A 50.

Abstract. 2014 The

03B3 decay of the first two _{positive parity} states in 45Ti was reexamined _{using single} 03B3-ray, 03B3-03B3 coin-cidence and n-03B3 coincidence techniques. E1 transition _strengths down to 10-7 _Weisskopf units were deduced

from the _experimental data, which is 30 times lower than the weakest E1 transition known in the other nuclei of the mass _region 40 A 50.

J. _Physique 41 (1980) 239-241 MARS 1980, Classification Physics Abstracts 23.20 - 27.40 1. Introduction. -

Electromagnetic

transition

strengths

between bound states for

_light

nuclei have been reviewed

_by

Endt and Van der Leun

_[1].

The

values of the two weakest El transitions

_reported,

namely

the 2- -+ 2+ transitions in

22Ne and

44Sc,

have since been reevaluated

_{[2, 3]}

and found to be

equal

to 9.8 x 10-6 and 4.1 x 10-6

_Weisskopf

units

_(W.u.),

_{respectively. According}

to Ref.

_[1]

_]

and a _survey of all El transitions for 40 A 50

nuclei

_[4],

the weakest transition has a

_strength

of

(2.9

±

0.5)

x 10-6 W.u.

_(the

3- - 2+ transition in

44SC).

Theoretically

El transitions with even

_larger

hindrance factors can exist and the reason

_they

remain

_{largely unreported}

may be the

experimental

difficulties in

_observing

them. As a matter of

fact,

an El transition with a

_strength

of

has

_recently

been observed in the 2’Ne nucleus

_[5].

In order to find out if other El transitions with

unusually

weak

_strengths

_exist,

the

_y-decay

of the

first two

_{positive parity}

states in 45Ti was reexamined

(see

Fig.

1).

2.

_{Experimental procedure}

and results. - 2.1 THE

329 keV LEVEL. - The 329 keV level

was

populated

0.7 MeV above threshold

_through

the

_45Sc(p,

_ny)

reaction at a _{proton energy} of 3.6 MeV. The

bom-barding

energy was chosen in order to

_strongly

excite the state and at the same time to avoid _y-ray lines in the _spectra

_arising

from

_higher

excited states.

The _target consisted of a

deposit

of 600

f.lg/cm2

of scandium on a 0.1 mm thick

_{gold backing.}

Gamma

rays were detected in a 1.7 cm3

Ge(Li)

detector with a resolution of 1.0 keV at 300 keV and

_positioned

at an

_angle

of 550 to the beam axis.

’

Singles

y-ray spectra revealed a weak

line,

3.3 + 0.3 keV below the well known 329 -+ 37 keV

y-transition

(see

Fig.

2).

This line could

_correspond

to the 329 - 40 keV

_{y-transition,}

since the _energy difference between the two first excited states in 45Ti is _{3.5 +} 0.4 keV

_[6].

A

_branching

ratio of

0.55 + 0.25

_%

is deduced for this transition from the recorded

_spectra.

The

_origin

of this line was

_{unambiguously}

confirmed

by

y-y coincidences in the

42Ca(a,

ny)

reaction at

8 MeV

_bombarding

_{energy. The 40 keV y-ray} was

viewed

_by

the 1.7 cm3

Ge(Li)

detector,

whereas

more

_energetic

_{y-transitions}

were detected

_by

a

9

cm3

_Ge(Li)

detector

_having

a resolution of 1.4 keV at 300 keV. A time window of 30 ns was used in this

experiment

in order to

_accept

most of the

_delayed

40 keV y-rays since the mean life of the 40 keV level

is 16.6 ns.

2.2 THE 744 keV LEVEL. - The _y

_decay

of the

744 keV level was studied

_using

n-y coincidence

techniques.

Neutrons

_arising

from the

_45Sc(p,

_ny)

reaction at

_Ep

= 4.0 MeV _were detected _at 0° _to the

beam axis in a 5 cm

_thick,

12.5 cm diameter NE213

detector. _{The y-ray} flux in the neutron detector was

reduced

_by

a 4 cm thick lead shield. Neutrons were

240

Fig. l. -

Decay scheme for 45Ti levels below 1 MeV excitation

as _reported in Ref. _[6]. The J = _{5/2 assignment to the 40 keV level}

was made by Styczen et al. [7].

discriminated from the

_remaining

_y-rays

_by

time of

_flight,

with a

flight path

of 52 cm. Gamma _rays were detected in a 40

cm3

_Ge(Li)

detector

_positioned

at 550 to the beam axis. A lead absorber set in front of the

_Ge(Li)

counter attenuated the

_intensity

of the

low _{energy y-rays} and

_effectively

eliminated

_summing

effects. Two

_{parameter spectra}

were recorded with

the time

_spectrum

in a 256 channel resolution and the

Ge(Li) pulses

in a 1 024 channel resolution. Selected

parts of the coincident _{y-ray spectrum} obtained after random substraction are

_displayed

in

_figure

3.

Cor-rected for the relative detector

_efficiency,

the intensities of the three lines

_feeding

the lower

negative parity

states,

yield

the

_branching

ratios R for the

corres-ponding

three El transitions

_{(Table I).}

Fig. 2. - Portion of _a

singles y-ray spectrum taken in _{the 45Sc(p, ny)} reaction at _Ep = _{3.6 MeV.}

Fig. 3. - Selected

parts of the _spectrum of y-rays in coincidence

241

Table I. - El transitions

issuing

from

the 329 and 744 keV levels in 45Ti.

3. Conclusion. - Three weak El transitions have

been observed in the

_present

work. Whether the

reason similar weak transitions have not been

_reported

is due to

_experimental

difficulties or to a

special

aspect

of nuclear structure in 45Ti is _up to now an

unanswered

_question.

_Parity

_mixing

in the

_parent

level has been

_proposed

to

_explain

the _strong inhi-bition of the

_{1 /2-}

_3/2+

El transition in

2lNe

_[9].

Such an

explanation

would not hold for the

_present

transitions.

References

[1] ENDT, P. M. and VAN DER LEUN, C., Nucl. Phys. A 235 (1974) 27.

[2] MERDINGER, J. C., BOZEK, E., GEHRINGER, C. and STACHURA, Z.,

Phys. Lett. 59B (1975) 38.

[3] CHEVALLIER, A., CHEVALLIER, J., GROSS, J. L., HAAS, B.,

SCHULZ, N., STYCZEN, J. and TOULEMONDE, M., Z. _Phys. A 275 (1975) 51.

[4] CHEVALLIER, A., CHEVALLIER, J., HAAS, B., SCHULZ, N. and TOULEMONDE, M., Internal _{Report CRN/PN} 78-18,

Strasbourg.

[5] FREEMAN, R. M., HAAS, F., HEUSCH, B. and GALLMANN, A.,

Nucl. _Phys. A 261 (1976) 471.

[6] BEENE, J. R., Nuclear Data Sheets 22 ₍₁₉₇₇₎ 1.

[7] STYCZEN, J., CHEVALLIER, A., CHEVALLIER, J., HAAS, B., SCHULZ, N. and TOULEMONDE, M., Phys. Rev. C 15 ₍₁₉₇₇₎

1704.

[8] BLASI, P., MORANDO, M., MAURENZIG, P. R. and TACCETI, N.,

Nuovo Cimento Lett. 2A (1971) 63.