Nanosecond isomers in 186W

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Nanosecond isomers in 186W

H. Karwowski, S. Majewski, B. Pietrzyk, L. Wencel, J. Jastrzebski

To cite this version:

H. Karwowski, S. Majewski, B. Pietrzyk, L. Wencel, J. Jastrzebski. Nanosecond isomers in 186W.

Journal de Physique, 1975, 36 (6), pp.471-474. �10.1051/jphys:01975003606047100�. �jpa-00208275�

NANOSECOND ISOMERS IN 186W

H.

KARWOWSKI,

S. MAJEWSKI

(*),

^{B. PIETRZYK}

(*),

L. WENCEL

(*)

^{and J.}

JASTRZEBSKI

Institute of Nuclear

Research, 015Awierk

^near Warsaw, ^Poland

(Reçu

^{le 28 novembre}

1974,

^{révisé le 30}

janvier

1975,

accepté

^le

7 février 1975)

Résumé. ²⁰¹⁴ Les niveaux du 186W alimentés par la

désintégration

^{du 186Ta et par la} capture ^électro- nique ^{du 186Re ont} été étudiés. Les spectres simples ^et les coincidences retardées ont été mesurés. Les temps de vie des trois isomères dans 186W ont été déterminés :

et la valeur limite supérieure de la vie moyenne du niveau à 1 463 keV,

T1/2 ~

^{0,1 ns, a été} estimée. Les

énergies des états à deux

quasi-particules

^{ont été calculées et} leur attribution aux niveaux observés

expérimentalement

^est proposée.

Abstract. ²⁰¹⁴ The levels of 186W fed in the

03B2-decay

^{of 186Ta and EC}

decay

^{of 186Re were inves-} tigated. ^The

single

spectrum ^{and the} delayed coincidence spectra were measured. ^The half-lives ^of

three isomers in 186W were determined :

and the upper limit of the 1463 keV level half-life

T1/2 ~

^{0.1 ns was} estimated. Two-quasiparticle

excitations were calculated and their

assignment

^to

experimentally

^{observed states is} proposed.

Classification

Physics ^Abstracts

4.220 ^- 4.240 ^- 4.470

1. Introduction. ^- Levels of the heaviest stable tungsten

isotope ^186W

have been

investigated by

several authors since 1955. Monnand et al.

[1],

^Pathak

et al.

[2]

^and

Gujrathi

^{and Mark}

[3]

^measured energy levels of

186W

in the

fl-decay

^of

^186Ta.

^Coulomb

excitation methods were used

by

^{de Boer et al.}

[4],

McGowan and Stelson

[5]

and Milner et al.

[6].

^An

extensive

study

^{of 186W and}

lighter tungsten

^nuclei

were

performed by

Gunther et al.

[7].

The life-time of the first excited 122.3 keV level in

186W

has

already

been measured

by

^different

methods. The data obtained

by

electronic measu-

rements

[8, 9, 10]

^{have the mean value}

Experiments

^{based on} the Môssbauer effect

[11, 12]

gave

Tl,2

^{= 1.39 ± 0.12 ns and a} nuclear recoil

experiment [13]

gave

7B/2

^{= 1.30 ± 0.21 ns.}

An indication that the 952.4 keV level has a life- time of about 0.2 ns was

given by

Monnand et al.

[1].

The aim of the present

investigation

^{was to deter-}

mine the exact values of the lifetimes of the 122.3 keV and 952.4 keV levels.

Furthermore,

^{the half-life of}

the 1 661 keV level was measured and an upper limit for the lifetime of the 1 463 keV level was estimated.

Single

gamma-ray spectra ^{were also} measured but

only slight

modifications to the

decay

^scheme

proposed

in reference

[3]

^were introduced

[14].

These modi- fications are not

important

^{for the} present ^work.

Calculations of

two-quasiparticle

excitations in 186W and 184W were

performed

and the calculated confi-

gurations

^were

assigned

^{to some}

experimentally

observed states.

2.

Experimental procédure.

^- The excited states of

186W

can be reached in

the p decay

^{of 1116 Ta}

(T1/2

^{= 10.5}

^min)

^and

^by

^EC

from 186Re (Tl/2

^{= 90}

^h).

Natural tungsten

samples

^were irradiated with 14.5 MeV neutrons from a NE-246 type ^neutron

generator (typical

^{flux 5 x 109}

n/s cm2)

^{and the}

1116 Ta

activity

^{was obtained}

by

^the

(n, p)

^reaction.

For the measurements of the half-life of the first excited state

samples

of natural rhenium were acti- vated

during

^{1 h in} the flux of thermal neutrons of about

1014 n/s ^cm2

^{in the EWA} reactor at Swierk.

(*) From the Institute of Expérimental Physics, Warsaw Uni-

versity, Polaud.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:01975003606047100

472

The half-lives of the excited states were measured

using

^{the standard}

time-to-pulse-height

^converter

technique,

^NaI

(Tl) crystals,

^fast

plastic

scintillators connected with XP 1021

phototubes

^{and a true}

coaxial 20

cm’ Ge(Li)

^detector

[15]

^{were used as time}

signal

generators.

3. Results. ^- Two

plastic

scintillators were used to determine the half-life of the 952.4 keV level

(cf. Fig. 1).

^The

delayed

coincidence curve,

analyzed using

the convolution method

[16],

is shown in

figure

^2.

The value

T1/2 (952.4 keV)

⁼ 0.193 ± ^{0.015 ns was} obtained. The lifetimes of the 396.5 keV and 737.5 keV levels do not influence the measured value since

T1/2 ^{(737.5 keV)}

^{= 9 ps}

^{and, although} T,12 (396.5 keV)

^{= 40} ps, this level is

weakly populated

in

the 186Ta decay.

Fie. l. - The simplified decay ^{scheme of 186Ta} (ref. ^{[1, 3,} 14]). ^The intensity limit of the 952.4 keV transition is taken from refe-

rence [14].

It was

proved

^{in an} additional

experiment

^{that the}

observed isomer

in 186W corresponds

^to the 952.4 keV level and not to the 1 463 keV one

(cf. Fig. 1).

^{To avoid}

registration

of the 198.0 keV and 214.9 keV transitions the _energy windows in both

photomultipliers

^were

chosen between 300 and 500 keV. The value

T1/2

⁼ 0.200 ± 0.040 ^ns obtained in this measure-

ment is in

good agreement

^{with the}

previous

expe- riment.

An

electron-gamma

quantum coincidence _expe- riment was

performed

^to determine the half-life of the 1 661 keV level. The results of one of the measure- ments are shown in

figure

^{3. The value of the half-}

life obtained is

Tij2 (1. 661 keV)

^{= 4.92 + 0.10 ns.}

FIG. 2. ^- The half-life of the 952.4 keV level. The dashed line indicates the normalized prompt ^curve. The full line indicates the fitted time distribution assuming the transition intensities shown

in figure ^1.

FIG. 3. ^- The half-life of the 1661 keV level. The arrow indicates the beginning of the fitted area.

In these

experiments

^{the NE}

111-Ge(Li) assembly

allowed a

unique

determination of the isomeric level.

On the basis of the

analysis

^{of the}

delayed

^coinci-

dence curves the upper limit of the lifetime of 1463 keV level was estimated to be 0.1 ns.

The half-life of the 122.3 keV level was measured

by Xk-Xk

coincidence.

X-rays accompanying

^electron

capture ^and

X-rays produced

^{as a} result of the conver-

sion of the 122.3 keV transition were measured

by

two Pb loaded

plastic

scintillators PILOT B and the value

T,12 (122.3 keV) =

^1.036 ± ^{0.010 ns was} _obtained. This result is in

good

agreement ^{with the}

previous

electronic measurements and is

substantially

lower than the one obtained in the

experiments

^based

on the Môssbauer effect.

4. Discussion. ^- 4.1 GENERAL CONSIDERATIONS.

- The

simplified decay

scheme of 186Ta based on

references

[1, ^3, 14]

^{is shown in}

figure

^{1. The}

^{1116 Ta} ground

^state

configuration

^{can be}

1/2 -(510), 7/2+(404)

with W ⁼ 3 - or

3/2-(512), 7/2 + (404)

^{with Kn = 5 - .}

Both

configurations

^were

previously proposed [1, 3]

and are consistent with the

experimental

^data.

The low

log ft

^{value of}

the f3

transition

leading

^to

the 1661 keV level

(ah

^{or 1 u}

transition)

indicates that this level should be

mainly

^{of a}

two-quasiparticle

nature with one

particle

^{in the same orbital as in} 186Ta

ground

^state.

The 1 661 keV level

decays preferentially

^{to the}

1 463 keV state. The 198.0 keV transition has a retar- dation factor

FsP

^{= 2.1 x}

105, typical

^{for the one}

particle

^El transition in deformed nuclei.

Therefore, although

^{a two}

phonon interpretation

^was

proposed

for the 1 463 keV level

[3],

^a

two-quasiparticle

^confi-

guration

^can also be considered. In this case one should find for this level a

configuration

^{in which}

only

^one

particle

^{is in a} différent orbital in

comparison

^with

the 1 661 keV state and for which the El transition is of the OSl

= 0,1

type. ^This

configuration

^should

also

explain

^the

decay properties

of the 1 463 keV state,

taking

^{into account the nature of the 952.4 keV}

and 1 045.0 keV levels.

4.2 THE 952.4 keV LEVEL. - The Kn ⁼ 2- characteristics of the 952.4 keV and 1 045.0 keV levels are now well established

[1, 3, 7].

^{For the analo-}

gous level at 1 290 keV excitation _energy

in 182W

the main

two-quasiparticle configuration contributing

to this

octupole

excitation was found to be

[17]

pp,

§’(514), 5/2 + (402).

^Our numerical calculations for

186W

indicate that this

configuration

^{is at 0.98 MeV.}

Also the two neutron

configuration 2/7 - (503), 11/2 + (615)

has low _energy

(Ecale

^{= 1.1}

MeV)

^and may contribute to this excitation.

The

decay

characteristics of the 2-

octupole

^vibra-

tional level were

investigated previously

ⁱⁿ many nuclei in this

region.

^{It has} been shown

[18-23]

^{that a}

significant mixing

^{of E 1 + M 2 + E 3}

multipolarities

exists in the 2-

> 2g+

transition. A recent

compilation

of the transition

probabilities

^{from this state}

in 174 Yb, 176Hf, 180W ^{and 182W}

nuclei can be found in refe-

rences

[18, 19].

In the case

of 186W

the _energy difference of the 2-

octupole

level and the

2+

level is much

higher

^{than in}

the

previously

mentioned nuclei and the main

decay

of the 2- level

proceeds by

^{an El} transition to the

2+

state.

Assuming

pure

multipolarity

^the

single particle

retardation factor for this transition is

equal

^to

1.0 x 104.

Although

^{as in} other nuclei in this

region

^the

2- ->

2+

830.0 keV transition is

probably

^{also an}

El + M2 + E3

mixture,

^no

experimental

^{data are}

available to evaluate the

mixing

^ratios.

Assuming by analogy

^{with 18 2B W}

[19]

^{that the El} component takes about

50 %

^of this transition

intensity,

^one

obtains the El retardation factor of 4 ^x

10’,

^i.e.

of the same order as in

176Hf (5

^x

10’), ^180W (4

^x

10’)

^and

^i82W (7

^x

10’).

4. 3 THE 1463 keV ANn 1661 keV LEVELS. ^- Direct

experimental

^evidence

concerning

^the

spin-parity

of the levels discussed is not decisive.

However,

^based

on the

qualitative

considerations in section

4.1,

^the

nature of the 952.4 keV leBtel and our

two-quasipar-

ticle state calculations

(cf. Fig. 4)

^the

following

^confi-

gurations

for these levels can be

proposed :

FIG. 4. ^- Calculated two-quasiparticle excitations. Calculation

procedure ^is described in references [17, 27]. Deformation parame- ters are taken from reference [28].

The 1 463 keV level is connected with the

2-,

952.4 keV state

by

^{the 511 keV} transition. _{The expe-}

rimentally

determined

[1]

conversion coefficients of this transition

indicate,

^within the limit of errors

(Monnand,

^{E. and}

Moussa, A., private

^communi-

cation),

^{El or E2}

multipolarity

and therefore do not contradict the

proposed

^{values of}

spin

^and

parity

of the 1 463 keV level.

For both

previously

^mentioned

configurations

^of

the 186Ta ground

^{state the}

fl-transition

^{to the 1 661 keV}

level is classified as 1 _u, in agreement ^{with the} expe-

rimentally

determined

[1] log ft equal

^6.2.

With the

configuration proposed

for the 1 661 keV and 1 463 keV levels the 198.0 keV transition would

474

occur between

2 +(404)

^and

2 -(514)

^{orbitals. In the}

odd-A nuclei the retardation factor of the

analogous

E 1 transition

[24]

varies from 4 ^x 104 for

181Ta

to 3 x 106 for 177Lu. The

preferential decay

^{of the}

1 463 keV level to the 2- state is

explained

^{as E2}

transition between

2 + (411 )

^and

2 + (402)

^orbitals.

The 1n’ ⁼ 5 - state of nn

2 - (510), 2 + (615)

^confi-

guration

^was

recently

identified in

184w (cf.

^ref.

[25, 26])

^{at the} energy of 1 285 keV

(Ecale

^{= 1.3}

MeV).

The calculated _{energy of} this state in

186W

is

equal

to 1.1 MeV. The

feeding

^{of such a} level from the 1 661 keV and 1 463 keV states is not observed

[1, 3, 4].

This fact can be

easily explained

within the

proposed

configurations

^because transitions to this 5 - state would

require

^a

change

^{of two}

particles.

It should be mentioned that no other

configurations

for the considered levels were found which could

explain

all of the

experimental

data discussed above.

We would like to thank Mr A. Sulik for

running

the neutron generator ^for many

hours,

^{to Doctors}

J. Blocki and W. Kurcewicz who

put

^{to our}

disposal

their

two-quasiparticle

calculation code and to Mr W.

Szymczyk

^{for his}

help

^{in some}

experiments.

We are

greatly

^{indebted to Prof. J. Zakrzewski}

for his kind interest in the _progress of the present work.

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