Recoil-distance lifetime measurement in 37Cl

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Recoil-distance lifetime measurement in 37Cl

J.C. Merdinger, N. Schulz, M. Toulemonde

To cite this version:

(Reçu

25 juin

1975,

1975,

accepté

septembre 1975)

Résumé. 2014

Des vies moyennes de (19,6 ± 3,0) ps et _(32,8 ± 2,0) ps pour les états 7/2- à 3 103 keV

et _9/2- à 4 011 keV dans 37Cl ont _{été mesurées par la méthode du parcours de recul dans la réaction}

27Al(12C, 2p)37Cl.

Parmi les transitions M2 connues dans les noyaux de la couche 2s-ld, celle issue du niveau à 3 103 keV dans 37Cl apparait comme étant la

_plus

intense. Une mesure de

dépla-cement _Doppler atténué a _permis d’évaluer une limite _supérieure de _{0,8 ps pour le temps} de peuple-ment dans la réaction de

_{fusion-évaporation}

_{27Al(12C, pn)37Ar.}

Abstract. 2014

Using the recoil-distance _technique in the

_{27Al(12C, 2p)37Cl}

_reaction, mean lives

of _(19.6 ± 3.0) ps for the 7/2-, 3 103-keV state and (32.8 ± 2.0) ps for the 9/2-, 4 011-keV state

in 37Cl have been measured. The _strength of the M2 _decay from the 3 103 keV level appears to be the _{strongest among known M2 transitions in 2s-ld nuclei. An upper limit of 0.8 ps for the}

_feeding

time in the

_{fusion-evaporation}

_{reaction 27Al(12C, pn)37Ar}

was estimated from an attenuated

Doppler-shift measurement.

1. Introduction. - A search for J1t =

7/2-

states

in odd mass s-d nuclei and a

_study

of their

electro-magnetic properties

are

_{currently underway}

in this

laboratory [1].

In the cases where

_spin,

lifetime,

branching

and

_mixing

ratios are well

_established,

it

appears that the

y-decays

of these states to the

J1t =

_{3/2+ ground}

states

_proceed

via mixed M2-E3 transitions and that the

partial

M2 lifetimes are

_always

retarded

_compared

to the

_{single-particle}

estimates

_[2].

In this _paper we

_report

on a recoil-distance method

(RDM)

measurement of the lifetime of the 3 103 keV

state in

3 7CI

for which

_only

a lower limit of 10 ps was known

_[3]

at the time the _present

_experiment

was

performed.

The

_{7/2- spin-parity assignment}

for this state, as well as the

branching

and

mixing

ratios for

its

_ground

state

_transition,

have been determined in

the _past

_[3],

so that the lifetime value

directly

leads

to the

_magnetic

_quadrupole

and electric

_octupole

ground-state

transition

_strengths.

A

_heavy-ion

induced

_{compound-nuclear}

reaction

was chosen for

_populating

the

37CI

states. Such reactions involve

_large

_angular

momenta transfer

resulting

in _strong

_population

of

_high-spin

states,

which are of interest here.

Furthermore,

such

reac-tions

_{yield large}

and forward directed velocities for the reaction

_{products. Consequently, only}

_singles

y-ray spectra are needed for the RDM lifetime

measu-rement. The bombardment of

2’Al

_by

31 MeV

12C

ions

_produced

the 37CI

states via the

_2p

exit channel. At that

_bombarding

energy other exit channels were

also _{open and lifetimes} for states in

_34S,

36CI

and

3’Ar were obtained as

_by-products.

2.

_{Experimental procedure}

and

_analysis.

- A 50 nA

beam of

12C

ions from the M. P. Tandem Van de

Graaff accelerator was used to bombard the alumi-nium _targets.

_Gamma-rays

were detected in a 84

cm3

Ge(Li)

detector. The detector _system had an intrinsic resolution of 2.8 keV for 1.33 _{MeV y-rays.}

A

_singles

spectrum recorded at 31 MeV

bombard-ing

energy, where the

yield

for the 3 103 keV line

was found to be

_maximum,

is shown in

_figure

1. A _target

_evaporated

onto a thick lead

_backing

was used and the

Ge(Li)

counter was

_placed

at an

_angle

of 55° to the beam direction. Most of the numerous lines _present in this _spectrum could be identified and

attributed to transitions in

_34S,

35CI,

36CI,

3’Cl

and 3’Ar.

2.1 RECOIL-DISTANCE LIFETIW MEASUREMENTS.

-The

_experimental

_set-up for the lifetime measurements

is identical to that described in ref.

_[4].

A 100

_flg/cm2

self

_supporting

stretched _target was used. A 7

mg/cm2

gold foil,

sufficient to stop the recoil

_ions,

was stretched

in the same manner as the _target and used for the

stopper. The beam was

stopped

in a thick tantalum

foil mounted behind the stopper foil. _{The y-rays}

were observed for various

_plunger

distances with the

Ge(Li)

detector

_placed

7 cm from the, _target and

at 0° to the beam direction.

The average axial

_velocity

of the

_recoiling

ions

2

FIG. 1. -

Gamma-ray spectrum recorded at 55° with respect to the beam axis for the 27 Al + 12C reaction at a _{bombarding energy}

of 31 MeV.

was determined from _{the energy} differences between

the

_{stopped Eo}

and shifted

_{E., peaks.}

The

correspond-ing

areas,

Io

and

Is,

were obtained with a

least-squares

fitting

program

using

Gaussian line

shapes

super-imposed

on a

_background

fitted to a

_polynomial

series. The

_experimental

ratios R =

Iol(Io

₊

I,)

of

the unshifted

_peak

area to the total area, were then

calculated at each

_plunger

distance. For cases where

the shifted

_peaks

were obscured

by

contaminants in

the _y-spectra, the intensities

_Io

were normalized to

the

_intensity

of the

_{stopped peak}

of the 1611 keV y-rays

deexciting

the

long-lived

37 Ar

1 611 keV

state, corrected for its own

_decay

with a lifetime

T = 6.38 _1: 0.15 ns

[5].

The lifetime values were obtained

by

comparing

the

_experimental

values of the ratio R to the values calculated for a level which is fed both

_directly

from

the reaction and

_by

_y-ray cascades from

_{higher lying}

levels. In this calculation the small direct

feeding-time

_(see

Section

_3.3)

was

_neglected.

The direct and cascade

_population

fractions were

taken from the _{y-ray spectrum} recorded at 550. Cor-rections due to _y-ray detection

_efficiency

and to the motion of the

_recoiling

nuclei were found to be

negli-gible

compared

to the statistical errors.

2.2 DOPPLER SHIFT ATTENUATION MEASUREMENT.

-By bombarding

the _target

_evaporated

on the thick

lead

_backing,

_y-ray

_singles

_spectra were measured at

the

_angles

of

00,

550 and 900. From there a value for

the attenuation factor F = 0.29 ₊ 0.04 _was deduced

for the 598 keV _y-ray transition

_decaying

the 7 071 keV

level

in 17 Ar.

The error on the attenuation factor arises

mainly

from the _presence of the 600 keV _y-ray line

produced by

neutron inelastic

_scattering

on

74 Ge.

3. Results. -

The values of the mean-lives obtained

in the _present work are listed in table I where

_they

are

_compared

to

_previous

measurements.

3.1 THE 5 689 keV LEVEL IN

34S.

-

Using

a mean recoil

_velocity

of 1.91

_%

the lifetime value for the

5 689 keV level was determined

_by

_analysing

three y-ray

transitions,

namely

the 5 689 - 4 622

_keV,

the

4 688 -+ 2 127 keV and the 3 304 -+ 0 keV transitions.

This could be done because the lifetime of the lower

states are much shorter than 1 ps

_[3].

Since the shifted

peak

for the 1 067 keV _y-ray transition was much

TABLE I

36Cl. strong

y-rays from the

decay

of

36CI

levels were observed

in the _spectra taken at

_55°,

_namely

the 1 730 keV and 789 keV _y-ray transitions from the 2 519 -+

789 - 0 keV cascade.

Whereas a

_single

value of 2.36 ± 0.16 ns is

_reported

for the lifetime of the 2 519 keV

_level,

the two values

reported

for the lifetime of the 789 keV level differ

by

a factor of ten

_[3].

In the _present

_x2

_analysis

of the

789 keV level

_decay

curve a best value of 45

_%

is

obtained for the

_population

fraction of this level

through

the 2 519 keV

level,

whereas a value of 59

%

is deduced from the

_photopeak

intensities in the 55°

spectrum taken at 31 MeV. This indicates that ano-ther 1 730 keV

line,

which could not be

_identified,

is

underlying

the _{2 519 - 789 keV y-ray} transition. This is corroborated

_by

the normalized 1 730 keV

stopped peak

versus _stopper distance curve which

does not

_display

a

simple exponential

decay.

So

_by

using

the value of 45

_%

for the cascade

_population

and the value of

_{r(2 519)}

= 2.36 ± 0.16 _{ns, a} value

of 32.3 ± 2.5 ps is obtained for the lifetime of the

789 keV level.

3. 3 THE 7 071 keV LEVEL IN

37 Ar. -

A lifetime

value of _il = 0.55 ₊ 0.12 ps has been determined

for this level in the

_34S(et,

_ny)

reaction

_[11].

In

the

present

27 AI(12c, pny)3’Ar

reaction no known

_higher

lying

levels could be identified and therefore the

measured attenuation factor in the DSA

measure-ment _may be due to the lifetime of the 7 071 keV

level and to the

_feeding

time _Tf,, i.e. the time interval between the reaction and the

_population

of this

state. A value of 0.34 ± 0.20 ps for the

_feeding

time

may be deduced from the attenuation factor

F using

the _{two-component}

_decay

chain formula

By using

this

formula,

one assumes that the

_feeding

of the 7 071 keV level _may be

_approximated

_by

an

exponential

decay.

Furthermore,

it should be remem-bered that

_large

uncertainties remain in lifetime determination

_by

the DSA method. Therefore

_only

an _upper limit of i f 0.8 ps may be

given

from the

present

analysis.

3.4 THE 6 473 keV LEVEL IN

37Ar. -

Lifetime

values for this level have been determined

_using

both

a recent RDM result in the

_34S(a,

_py)

reaction

_[10],

but

_supports

a less

_precise

value obtained

_by

direct

timing

using

a

pulsed

beam

[9].

FIG. 2. -

Intensity ratios as a function of _stopper distance for three transitions in 37 Cl. The _normalising factor _In is the area of the

stopped peak for the 1611-i 0 transition in _{37 Ar,} corrected for its _decay.

4. Discussion. - Transition

strengths

in 37CI

deduc-ed from the _present lifetime measurements are

reported

in table II.

4

TABLE II

level _appears to be the _{strongest among} the known M2 transitions in 2s-ld nuclei. In fact the transition

rate is close to the

_{single-particle}

estimate. Several shell model

_descriptions

for the first

_{7 j2-}

state have been used to

_predict

the transition rate and the deduced values _range from 0.3 to 5 W.u.

_[13].

The

great

majority

of M2 transitions in nuclei with A > 30 is

_severely

inhibited and this inhibition has been discussed

_by

Kurath and Lawson

_{[14]. They}

showed in

_particular

that M2 transitions in the

_3/27/2

region

will be inhibited in the frame of the

strong-coupling

model. In this model the intrinsic excited

state is formed

_{by raising}

a

_particle

from one of the

orbitals

_kd

_originating

from the

_d3j2

shell to an

orbi-tal

_{kf originating}

from the

_f7/2

shell. Kurath and

Lawson

_computed

the values of the hindrance factor for the

_{7 /2 - -+}

_3/2+

M2 transitions

_according

to

the different

_{(kd, kf)}

sets involved. The smallest factor

_{(- 2)}

was found for

_kd

=

3/2 and kf

=

7/2,

and these are indeed the Nilsson orbitals which would

be involved in the M2 transition

of 37CI

if this nucleus had an oblate deformation.

Large

and identical

_strengths

for the E3 transitions

arising

from the 3 103 and 4 Oil keV levels are

observ-ed. This may reflect the presence of a 3- core

excita-tion in the wave functions of the

_7/2-

and

_9/2-

states.

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