The y activity from 11Li beta decay

HAL Id: jpa-00231821

https://hal.archives-ouvertes.fr/jpa-00231821

Submitted on 1 Jan 1980

HAL is a multi-disciplinary open access

archive for the deposit and dissemination of

sci-entific research documents, whether they are

pub-lished or not. The documents may come from

teaching and research institutions in France or

abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est

destinée au dépôt et à la diffusion de documents

scientifiques de niveau recherche, publiés ou non,

émanant des établissements d’enseignement et de

recherche français ou étrangers, des laboratoires

publics ou privés.

The y activity from 11Li beta decay

C. Détraz, D. Guillemaud, M. Langevin, F. Naulin, M. Epherre, R. Klapisch,

M. de Saint-Simon, C. Thibault, F. Touchard

To cite this version:

L-459

The

_y

_activity

from

11Li

beta

_decay

C.

Détraz,

D.

_Guillemaud,

M.

_Langevin,

F. Naulin

Institut de _{Physique Nucléaire, B.P. 1,} 91406 _Orsay, France

M.

_Epherre,

R.

_Klapisch,

M. de

_Saint-Simon,

C. Thibault and F. Touchard

Laboratoire _{René-Bernas,} Centre de _{Spectroscopie} Nucléaire et de _{Spectrométrie} de Masse, 91406 Orsay, France

(Reçu le _{24 juin} 1980, accepte le 18 aot2t 1980)

Résumé. 2014

Les

_énergies

et les intensités absolues des raies 03B3 consécutives à la

désintégration 03B2

de 11Li ont été mesurées. La

_{transition 03B2}

vers le niveau fondamental du 11Be n’est pas observée. Le pourcentage de

_{désintégration}

03B2 ne conduisant pas à l’émission de _{particules retardées n’est que de (5,2} ± 1,4)

%.

On observe de nouvelles voies

de neutrons retardés vers des états excités de 10Be et on en déduit la _probabilité totale d’émission de neutrons

retardés.

Abstract. 2014 The

energies

and absolute intensities of the 03B3-rays from the 03B2-decay of 11Li are measured. There is no

sizable 03B2

branch to the 11Be

_ground

state.

_Only

_{(5.2 ± 1.4)}

_%

of the

_{03B2-decay strength}

does not lead to

_03B2-delayed

particle

emission. New

_03B2-delayed

neutron branches to excited states of 10Be are observed and the total

_delayed

neutron emission

_probability

is deduced.

Tome 41 No 19 1 er OCTOBRE 1980

LE

JOURNAL

DE

_{PHYSIQUE - LETTRES}

J. Physique - LETTRES 41 (1980) L-459 - L-461 1 ler _OCTOBRE

1980,

Classification

Physics Abstracts 23.20 - 23.90

Since its first observation

_[1],

the 11 Li

_isotope

has been

_actively

studied. A 8.5 + 0.2 ms half-life

_[2]

and

a 40.94 ± 0.08 MeV mass excess

_[3]

have been

measur-ed. Its

_#-delayed

total neutron emission

_probability

[Pn

=

(60.8

±

7.2)

%]

has been observed

[2].

More

recently,

the neutron _{energy spectrum} has been

mea-sured

_[4]

which led to the first observation of

_p-delayed

multiple

neutron emission

_reported

with a

_probability

of

_P2n

=

(9

_±

3)

%

value

[4].

A sizable

_probability

of

_{#-delayed light charged particle}

emission is observed in a

_study

currently

in _progress in our _group

_[5].

In view of this wealth of

_information,

it is somewhat

paradoxical

that the

_y-activity

of 11 Ll

had never been observed

_previously.

This work was thus undertaken

to measure the _energy and absolute

_intensity

of the various _y-rays emitted from the

_#-decay

of 11 Li.

From the _{present measurements,} a new value for the

~-delay-ed neutron emission

_probability

is deduced which resolves the

_discrepancy

between the earlier

_Pn

measu-rement

_[2]

and a recent theoretical estimate

[6].

The

_experimental

method is the same as described

in our _report on

y-activities

from neutron-rich Na

isotopes

[7].

To summarize

it,

a

pulsed

beam of 24 GeV

protons from the CERN

_synchrotron

_(1),

induces

nuclear

_{fragmentation}

in a

_target

of a

_heavy

element,

in the _present case Ir. The

recoiling

nuclear

fragments

are thermalized in heated

_graphite,

out of whièh the alkali elements

_selectively

diffuse. The

selectivity

is enhanced

_by

a surface ionization mechanism. At

last,

alkali ions are extracted and

_analysed

_by

a mass

spectrometer. This insures a

complete selectivity

in Z

and A for the collected ions. The

_resulting

_y-activity

measured with a

_Ge(Li)

detector is observed in coinci-’

dence with the beta

_activity

detected

_by

a

_plastic

scintillator.

As an

_improvement

over our

previous

work

[7],

three

_pieces

of information are stored on

_tape

for

each

_~-y

coincidental event : the _{y energy}

_(E),

the time

(T)

elapsed

between the

_proton

beam burst and the detection of the

_~.y

_coincidence,

and the time between

the #

and _y

_signals.

Off line

_analysis

of the data allows the constitution of E _spectra

_according

to the T

para-meter in order to discriminate between activities of different half-lives.

Figure

1 shows a _{y energy spectrum} with no

res-triction on T. As a

result, y-rays

from the 8.5

ms llLi

coexist with the 2 125 keV _y-ray from its 14 s

11 Be

daughter.

One

_major

aim of this work was to measure the absolute

_intensity

_Iy

_{of the observed y-rays. The}

L-460 JOURNAL DE _{PHYSIQUE -} LETTRES

Fig. 1. -

y-ray spectrum observed in the _decay of 8.5 ms 11 Li.

Peaks are labelled in keV and their origine are indicated in table I.

(s) and (d) stand for single and double _escapes.

-lute

_efficiency

of the

_Ge(Li)

detector was determined

from calibrated sources of

56CO,

85Sr

and

203 Hg

with

an estimated

_uncertainty

of 12

_%.

The measurement

of the number of

_{decaying "Li}

ions collected was more

difficult. It was determined in two _{ways :}

_i)

before and after each data

_accumulating

run, direct measure-ments were made of the number of ions collected from

the mass _{spectrometer per} beam

burst,

i. e. for a

cer-tain number of incident _protons;

_ii)

the

_#-activity

was multiscaled to

_identify

the

_#-particle

due to the short-lived

11L1

from those due to

_background

or

long-lived

descendants. From the

_efficiency

of the

#

detector,

the

_{corresponding}

number of

collected 11 Li

ions was deduced.

Although

the results from these two methods were

found to _agree within 10

_%,

a more realistic estimate

of 20

_%

was retained for the

uncertainty

on the number of "Li ions collected.

Table I lists the

y-activities

observed and their

abso-lute- intensities.

_Three

conclusions can be

_readily

drawn from these results :

1)

The

_intensity

of the 2 125 keV _y-ray due to the

#-decay

of the

_{daughter "Be}

nucleus,

with a known

_[8]

Iy

=

(33

±

3) %,

is

fully

accounted for

by

the

measur-ed

_intensity

of the 320 keV y-ray

activity

from the

Table I. -

y activities observed

from

the

# decay of

11 Li.

Ey (keV) ~ ~ ~ (keV) 7y 0 (%) 219 6179 -~ 5 960 in 1 °Be 0.95 + 0.35 320 320 -~ 0 in 11 Be 5.2 + 1.4 (2125) (b) 2125 -~ 0 in 11B 1.8 ± 0.7 2 590 5 958, 5 960 3 368 in 1 °Be 3.5 + 1.0 2811 1 6179 -~ 3 368 in 1 °Be 1.65 ± 0.70 3 368 3 368 -~ 0 in 1 °Be 21 + 6

(") The uncertainties include estimated uncertainties of 20 %

for the number of collected 11 Li ions and _{12 %} for the Ge(Li) efficiency.

(b) This 14 s y activity from 11 Be decay is listed here for intensity comparison with the 320 keV _y-ray from the beta decay of 11 Li

(see text).

-decay

of the first excited state

of 11 Be

which is the

_only

bound excited state

_{against particle}

emission

_[8].

Therefore,

no

_{sizable ~}

branch to the

11 Be

_ground

state is observed within the

experimental

uncertainties,

as

_expected

for a

_{(1/2) - --+}

1 /2 + ~

transition.

2) Only (5.2

±

1.4) %

of the

_{~-decays strength}

of

11 Ll,

which feeds the 320 keV level

of 11 Be,

does not

give

rise to

_{p-delayed particle}

emission. All the

remain-der,

which

_populates

the other excited states

_{of 11 Be,}

must then lead to one or several channels of

_particle

emission,

lOBe

+ n, x +

6He

+ n,

9Be

+ 2 _n, 2oc + 3 n,

8Li+t.

As a

_result,

the total

_particle

emission

_probability

is thus deduced to be

-Our current

_study

of

_{p-delayed light}

charged

_particle

emission indicates a

_probability

of the order of 5

_%

for the emission of 2 rx or a +

6He

and a

_negligible

one for 8Li + t. This leads to a total

_delayed

neutron

intensity

per beta

disintegration of 11 Li,

varying

from 95 to 105

_%,

_depending

on whether the

emission of a or

6He-particles

is associated with In or 3n emission.

This value is in

_strong

_disagreement

with the

_only

earlier measurement

_[2]

which determined

_Pn

as the ratio of the measured numbers of detected neutrons to

~i-particles.

Whether a

_systematic

error was introduced

by

an incorrect

Pn(9Li)

normalizing

value,

by

an

inac-curate estimate of the

_efficiency

for

_high-energy

neu-trons, as

_{suggested by}

Barker and

_{Hickey [6],}

or

_by

an

improper

determination of the fraction

of P

count-ing

due

to 11 Li

as

_opposed

to the

_background

remains unclear. However it is felt that the new

value,

which is

in

_qualitative

_agreement

with a theoretical estimate

[6]

should be free of

_systematic

errors for the

_following

reasons :

_i)

the number of

_decaying

11 Li

has been measured

_{consistently by}

two

_independent

methods described

_above,

_ii)

the neutron

_branching

to the 2 +

state of

IDBe

of

₍₁₄

±

5)

% (see Fig.

2)

is in

_good

agreement

with the

_independent

measurement of Jonson and his coworkers

_[9]

who

_give

a value of 11

%,

with an estimated

uncertainty

of half of that value.

The observed _y-rays

from Li

_{decay give}

evidence for the

population by

p-delayed

neutron emission of

at least some of four states of

1 °Be

_lying

around 6 MeV excitation _energy.

_Only

an _upper limit can be set for

the

_feeding

of the 6 263 keV level but the _two ;~-rays

associated with the

_decay

of the-6179 keV level are

observed. The _y-rays from the doublet of levels at

5.96

MeV,

only

1.6 keV _apart, cannot be resolved but

a transition between this doublet and the 3 368 keV 2+

L-461 y ACTIVITY OF l1Li

Fig. 2. - A

partial p-decay scheme of 11 Li as established from the

observed _{y-activities.} Beta _decay intensities are _{given per} 100

disinte-grations of 11 Li.

1 oBe

level is defined as the difference between the

y intensities from and to this level.

Figure

2 summarizes the y transitions observed and

shows

_{the #}

and

_p-delayed

neutron intensities

measur-ed. It is clear that the

_major

_part

of the

_~-decays,

which

goes unobserved in the

present

experiment,

feeds the

lOBe

_ground

state

_through

neutron emission.

Even

if 11 Li

is an extreme case due to the low _energy thresholds of many

particle

emission

channels,

the

present

results are in

_agreement

with more

general

trends described elsewhere for Na

_isotopes

_[10].

More

specifically,

for odd-Z

elements,

Pin

tends towards

100

_%

for odd-A

_isotopes

as observed

here,

while for

even-A

_{isotopes, P2n}

tends towards

_Pln

as observed

for

32Na

[11].

In any case, the

high probability

of

#-delayed

neu-tron emission is

_clearly

the dominant _aspect of

#-decay

of _very neutron-rich

_isotopes.

Acknowledgments.

-The authors

_gratefully

acknow-ledge

valuable comments from P. G. Hansen and B.

_Jonson,

and a critical

_reading

of the

_manuscript

by

S. K. T. Mark.

References

[1] POSKANZER, A. M., COSPER, S. W., HYDE, E. K. and _CERNY,

J., Phys. Rev. Lett. 17 ₍₁₉₆₆₎ 1261.

[2] ROECKL, E., DITTNER, P. F., DÉTRAZ, C., KLAPISCH, R., THIBAULT, C. and _{RIGAUD, C., Phys.} Rev. C 10 ₍₁₉₇₄₎ 1181.

[3] THIBAULT, C., KLAPISCH, R., RIGAUD, C., POSKANZER, A. M., PRIEELS, R., LESSARD, L. and REISDORF, W., Phys. Rev. C 12 (1975) 644.

[4] AZUMA, R. E., CARRAZ, L. _{C., HANSEN,} P. G., JONSON, B., KRATZ, K. L., MATTSON, S., NYMAN, G., OHM, H.,

RAVN, H. L., SCHRÖDER, A. and _{ZIEGERT, W., Phys.} Rev.

Lett. 43 ₍₁₉₇₉₎ 1652.

[5] LANGEVIN, M. et al., Unpublished data.

[6] BARKER, F. C. and HICKEY, G. T., J. Phys. G 3 (1977) L-23.

[7] DÉTRAZ, C., GUILLEMAUD, D., HUBER, G., KLAPISCH, R.,

LANGEVIN, M., NAULIN, F., THIBAULT, C., CARRAZ, L. C. and TOUCHARD, F., Phys. Rev. C 19 (1979) 164.

[8] AJZENBERG-SELOVE, F., Nucl. _Phys. A 336 ₍₁₉₈₀₎ 1.

[9] JONSON, B. et _al., Private communication.

[10] DÉTRAZ, C., Fizika 10 (1978) 19.

[11] DÉTRAZ, C., EPHERRE, M., GUILLEMAUD, D., HANSEN, P. _G.,

JONSON, B., KLAPISCH, R., LANGEVIN, M., MATTSON, S., NAULIN, F., NYMAN, G., POSKANZER, A. M., RAVN, H. L.,

DE SAINT-SIMON, M., THIBAULT, C. and TOUCHARD, F.,