Excited states of 19N and 21O

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Excited states of 19N and 21O

F. Naulin, C. Détraz, M. Roy-Stéphan, M. Bernas, J. de Boer, D. Guillemaud,

M. Langevin, F. Pougheon, Pierre Roussel

To cite this version:

L-29

Excited

states of

19N

and

21O

F.

_Naulin,

C.

Détraz,

M.

_{Roy-Stéphan,}

M.

_Bernas,

J. de Boer

_(*),

D.

_Guillemaud,

M.

_Langevin,

F.

_Pougheon

and P. Roussel Institut de

_Physique

Nucléaire, B.P. n° _1, 91406

_Orsay,

France

(Reçu le 10 novembre 1981, accepte le 24 novembre 1981)

Résumé. 2014 Les réactions nucléaires

(18O, 19N)

et (18O, 21O)

sur une cible de 18O permettent

d’obser-ver des états excités à 1,12 et 1,59 MeV dans 19N et à 1,35 et 3,00 MeV dans 21O. Une valeur _plus

précise

de la masse de

19N, 15,856

± 0,050 MeV, est obtenue.

Abstract 2014

(18O, 19N)

and

_{(18O, 21O)}

nuclear reactions on a 18O target

provide

measurements of

excited state

_energies

at 1.12 and 1.59 MeV for 19N and at 1.35 and 3.00 MeV for 21O. The 19N mass is remeasured as 15.856 ± 0.050 MeV.

LE

JOURNAL DE

_{PHYSIQUE-LETTRES}

J.

_Physique

- LETTRES 43

(1982)

L-29 - L-32 15 JANVIER _1982, Classification Physics Abstracts 21.lOD - 25.70

For neutron-rich

_nitrogen

_{and oxygen}

_{isotopes, ground}

state masses are known up

to 19N

_[1,2]

and

220

_{[3]. Two-body}

nuclear reactions which measure the

_Q-value

and in this way determine an unknown mass excess have

vanishing

cross sections for even more exotic

isotopes.

It seems

nevertheless worthwhile to use them to

_study

excited states of nuclei with known mass excesses.

We have

_performed

such measurements for

14B and 18N

[4]

and we

_report

in this letter the first observation of excited states

of 19N and 210.

_Quantitative

results of this

_type,

far from the

_valley

of

_#-stability,

set

_stringent

constraints on the

predictions

of nuclear models

[5].

The

_{complex-rearrangement}

reaction

_180(180,

_19N)17F

and the three-neutron transfer

reaction ~0(~0, ~0)~0

were induced

by

a 180

beam from the

_Orsay

MP-Tandem on a

self-supporting Al203

target,

72

_Ilg.cm-2

thick and 90

_%

enriched

ion 180.

The nuclei emitted were

analysed by

a 1800

magnetic

_{spectrometer,}

within a 4° to 8°

_angular

_{range in the} horizontal

_plane

and a 4.8 msr. solid

_{angle. They}

were detected in the focal area of the

_magnet

_by

a

_system

_consisting

of two resistive-wire

_proportional

counters and an ionization chamber with a

_split

_anode,

pro-viding

two

_energy-loss

and one

_{residual-energy}

_{measurements,} with 2

_%

and 1.5

_%

_resolution,

respectively.

This

_system

allows off-line kinematical

correction,

through

ray

tracing,

and

_provides

redundant identification of the nuclei detected

[4, 6].

The energy

spectra

of

19N

and

21 0

are

presented

in

_figures

1 and 2.

L-30 JOURNAL DE _{PHYSIQUE -} LETTRES

Fig.

1. -

Energy

spectrum of the 19N nuclei emitted in the 180 + 18 0 reaction. The

_Q-value

calibration

is deduced from the observation of known transitions of nuclei of

_{neighbouring A}

and Z values. The _ground

state cross section is about 0.5

_{Jlb. sr-1.}

Fig.

2. -

Spectrum of the

_position

of Z 10 nuclei, produced in the 180 + 180 reaction,

_along

the focal

plane of the _{spectrometer.} The abscissa is the

_{magnetic rigidity (Bp}

in tesla _meter) of the observed 210

nucleus. The

_experimental

resolution is about 7 x 10-4 tesla _meter, but the

_experimental

_peaks, labelled

by

the excitation

_energies

of 210 and

_150,

are

_{Doppler-broadened}

for excited states of 210 (the

corres-ponding

width is about 0.2 MeV per MeV of

y-decay energy).

The six events of lower _Bp can be

_assigned

to

various combinations of excited states of 210 and 1 S 0. One count

_corresponds

to a cross section of

7 nb.sr-1.C.M.

The reasons

_why

we chose to detect the exotic

_nucleus,

for instance

210

rather than

~0,

from

the ’ 80 + 180

_two-body

reaction are

vividly

illustrated

_by

a

comparison

of

the 210

spec-trum (Fig. 2)

and

the 150

_energy

_spectrum

obtained in a

previous

measurement of

the 210

mass

[7].

Because it

_corresponds

to the less

_negative

_{Q-value, 210}

from

_{an 180 target}

appears at

_higher

kinematical _{energy than from}

the 160

and

27 Al

contaminants.

_{Quite generally [4],}

in a

_two-body

reaction the detected neutron-rich nucleus has an _energy

_spectrum

free of contaminants. There

are however two drawbacks.

First,

one can observe

as 210

nuclei in the detectors

_only

those

formed in a

particle-bound

_state, thus

limiting

the observable range of excited states.

_Second,

because of their

_{in-flight y-decay,}

the bound states _appear

_{Doppler-broadened}

in the _energy

spectrum, as seen in

_figures

1 and 2. The beam _energy had to be chosen _very

_carefully

for

the 210

the 180(8+)

elastically

Actually,

it was

_impossible

to observe the

_{ground 210}

state without

_saturating

and

_{possibly damaging}

the detectors. It was found that a 92 MeV incident

energy offered a broad range of observable excitation

_energies

for

210.

In that _case, the field of the

_magnetic

_spectrometer

was set at such a value which would

_put

the scattered beam

_slightly

off the focal

_plane

detectors. As a result no

possible

excited state

of 210

could be observed below 1.2 MeV.

The _energy

_spectrum

of

_figure

1

_provides

a remeasurement of the

19N

mass as

15.856 _± 0.050 MeV. This is in

_agreement

with but more accurate than

_previous

values of 15.810 _± 0.090 MeV

_[1]

and 15.960 + 0.150

_{MeV [2].}

Two transitions are identified to

19N

excited states at

_(1.12

±

0.04)

MeV and

_(1.59

±

0.04)

MeV.

_They

could not be observed in our

previous study [1]

of 19N

since these

_peaks

then fell outside the detector _range.

_According

to the

straightforward

shell-model

_prediction,

the

i 9N

_ground

state should have J1t =

1 /2 -.

The

same J ~ value is found

_[8]

for 17N

with one neutron

_pair

less in the

_sd-shell,

which should not

perturb

too much the

_proton

hole-state. One can

_speculate

that indeed

the 19N spectrum

_might

closely

parallel

the 17N

one. Then

_low-lying

excited states would have J~ =

3/2-

and

5/2-

with

a

_{weak-coupling}

[2 +

0

(1

py)’

1]

configuration

[8].

Since the 2 + state lies lower in

2°0

than in

180,

the lower excitation

energies

of the observed

19N

states, as

compared

to the

17 N

ones

(Fig.

3a),

would find a natural

_explanation.

Fig.

3. -

Comparison

of : a) the

_experimental

level schemes of 17 N and

19N;

b) the

_experimental

level scheme of 210 and the shell-model

_prediction

of reference _[9].

Two excited states

of 210

at 1.35 and 3.00 MeV are deduced from the energy _spectrum of

figure

2,

within the _{energy range}

available,

i.e. between 1.2 MeV due to the counter cut-off dis-cussed

_above,

and 3.72

_MeV,

the

_peutron

_{binding-energy}

of 210.

This result is

_compared

_(Fig.

_3b)

to a shell-model calculation

_[9]

of

the 210

_{energy spectrum.} If the

_{experimental peaks}

of

_figure

2

correspond

to

_{single 210}

levels,

the

_uncertainty

of the measured excitation _energy is ± 150 keV.

However,

the low

statistics,

the finite _energy resolution of the

_detecting

_system,

and the

Doppler-broadening

due to

_{in-flight y-decay}

of

excited 210

nuclei do not

_preclude

the occurrence of more

than one level

within

each

_{experimental peak.}

In

_fact,

since the shell-model calculations agree rather well with the

_spectra

of

_light

neutron-rich sd-shell

_nuclei,

as noted

_[10]

in the case

of 25Ne,

L-32 JOURNAL DE _{PHYSIQUE -} LETTRES

References

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Phys.

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Phys.

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HEB-BARD, D. F.,

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ROUSSEL, P. and ROY-STÉPHAN, M., J.

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