Mass excess and excited states of 14B and 18N from the (14C, 14B) and (18O, 18N) reactions

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Mass excess and excited states of 14B and 18N from the

(14C, 14B) and (18O, 18N) reactions

F. Naulin, C. Détraz, M. Bernas, D. Guillemaud, E. Kashy, M. Langevin, F.

Pougheon, Pierre Roussel, M. Roy-Stephan

To cite this version:

Mass

excess

and

excited

states

of 14B and 18N from the

(14C,

14B)

and

_(18O,

_18N)

reactions

F.

_Naulin,

C. Détraz, M. Bernas, D.

Guillemaud,

E.

Kashy

(*),

M.

_Langevin,

F.

_Pougheon,

P. Roussel and M.

_Roy-Stephan

Institut de Physique Nucléaire, B.P. n° 1, 9140b Orsay, France

(Reru lo 2/ doomhro /979, accepts l0 3 _{janvier 1980)}

Résumé. 2014 Les réactions

(14C, 14B)

et

_{(18O, 18N)}

sur le

_noyau

14C ont été utilisées _pour mesurer l’excès de masse

des

_noyaux

14B et 18N. Un état excité de 18N est observé à 575 ± 25 keV. Abstract. 2014

The

₍₁

4C, 1

4B)

and

_{(18O, 18N)}

reactions on a 14C target have been utilized to determine the mass excess of the 14B and 18N nuclei. An excited state of 18N is observed at 575 _{± 25} keV.

LE JOURNAL

DE

_{PHYSIQUE - LETTRES}

Classification Physics Ahstracts 2t.tOD-25.70

1. Introduction. - Neutron rich odd-odd

light

nuclei with T =

_2,

such

as

4B

and

18 N,

are much

less known than a number of

_{neighbouring isotopes}

further _away from the

_valley

of

_~-stability.

Although

this series of nuclei is

_only

a

single

charge-exchange

reaction away from stable

isotopes,

experi-mental information is scarce and sometimes

contra-dictory.

This is due to two reasons. The first one is

that the

_{charge-exchange}

reaction has to be of the

(n,

p)

or

(t,

3He)

kind with

light particles,

which

carries obvious

experimental

difficulties. The second

one lies with the somewhat

high

density

of

low-lying

excited states of the residual odd-odd

nucleus,

which makes the characterization of the

_ground

and indi-vidua~ excited states sometimes

_ambiguous.

A _progress in this situation

requires

the use of

_heavy

ion

beams,

as

_long

as

good particle

identification is

achieved and an

effort

is made towards the best

possible

energy resolution.

The

_qualities

of the

heavy-ion

beams which result from the recent

_equipment

of the

_Orsay

MP tandem with a Laddertron

charge system,

together

with

recent

_improvements

in the

_experimental

_apparatus

have allowed the collection of

_{low-background, good}

energy resolution data

bearing

on the mass excess

and excited states

of

4B and

18 N,

which are

presented

in this _paper.

(*) On leave from Michigan State _University, East Lansing, Michigan, U.S.A.

2. Available information on

14B

and

18 N.

- The

mass excess and the

_energies

of the first excited states

of

14B

have been measured once

_only,

_through

the

14C(7 Li , 7Be)I4B

reaction

[I].

Another

_study

of

14 B

through

a different nuclear reaction seems

worth-while. The situation for

18N

is more

_complicated.

An

_{early ~0(t,}

_3He)18N

_{experiment [2]}

observed a

single

broad

_peak

in the proton energy spectrum,

corresponding

to a 13.274 ₊ 0.030 MeV mass excess

for

1$N.

An even earlier measurement

_[3]

of the

_#

end-point

energy in the

decay

of

18 N

had determined

a 13.1 1 ± 0.4 MeV mass excess.

_Recently

the

_(d,

_2He)

reaction on

180

was used

_[4]

to further

_study

the energy spectrum.

Again

a broad

_peak

centred around the mass excess value of ref.

_[2]

was observed. However

the

_height

of the

_{background precluded}

_any firm conclusion

_although

there was indication that the

broad

_peak

_might

consist of at least 3

_peaks

seen as

shoulders.

3.

_Experimental

method. - Low statistics

are

una-voidable when one studies nuclei far from the

valley

of

_P-stability

since their formation

_requires

the use

of exotic nuclear reactions. To turn the few observed

events into a

_meaningful

_quantitative

measurement

supposes first an

_unambiguous

identification of the

ions detected and second an

_optimization

of the

peak-to-noise

ratio in their _energy measurement. This second

_requirement

is

_pursued

_by

_obtaining

the best

possible

energy resolution and

drastically

reducing

the

background.

Hence

three experimental

factors are of

L-80 JOURNAL DE PHYSIQUE - LETTRES

major importance : 1)

ion

identification,

2)

energy

resolution,

3)

height

and nature of the

_background.

Items 1 and 2 are dealt with in the next section.

Item 3 deserves a few comments.

In the

_experiments

under

consideration,

the mass

excess of an exotic nucleus is deduced from the

measu-rement of the

_Q-value

of a

_{A(a, b)B}

so-called

two-body

reaction,

where the mass excess of b or B is unknown. If the residual nucleus B is the exotic

neu-tron-rich

_species

under

_study,

the

_{(a, b)}

reaction is

found,

for all

_practical

cases, to have a

_Q-value

which

is more

_negative

than all the

A’(a, b)B’

reactions

which can take

_place

on the

_possible

A’

_isotopic

or

chemical contaminants in the A _target. Hence the

energy

spectra

of interest for

_particle

b sits on

_top

of the _energy

_spectra

due to these contaminants. On

the contrary, if the exotic nucleus under

_study

is the detected b

_particle,

the

_Q-values

of the

_{A’(a, b)B’}

reactions are more

_negative,

and the _energy

_spectrum

of b is free of any

background

due to contaminants.

This is the reason

_why

14B

and I8N

are

_investigated

in the

_present

_{study through}

the

_{(14C, 14B)}

and

(~0, ’~reactions.

The

_only

setback of this method lies with the

_study

of the excited states of the exotic nuclei.

If,

in the

A(a, b)B

reaction,

b is formed in a bound excited

state, the kinematics of the reaction

_corresponds

to

a

_Q

=

Qg.s. -

excitation

value,

and the energy of b is

accordingly

affected.

_However,

since the b nucleus

decays

in

_flight

_by

emission of a _y-ray, its detected

energy

peak

is

_{Doppler-broadened.}

For the

_light

nuclei studied

here,

this loss in energy resolution is

rather

_important.

Its order of

_magnitude

is a fifth of

the _{y-ray energy.}

Furthermore,

only

the bound _energy levels can be observed in this way. Those which

decay

by

particle

emission no

_longer

_give

rise to the

obser-vation of a b nucleus. For

instance,

while excited

states _up to

_nearly

3 MeV have been observed

_[1]

in

148,

this nucleus is bound

_{by only}

976 + 30

keV in its

ground

state. Hence

_only

the

_reported

0.74 ±0.04

MeV

excited state can be observed

_by

_detecting

14B

in the nuclear reaction.

4.

_Experimental

_apparatus.

- The

recently

deve-loped 14C

beam

_[5]

from the

_sputtering

ion-source of

the

_Orsay

MP-Tandem can reach an

intensity

on

target

of _up to 100 nA of

14C5+

ions. The

180

beam

is also

_produced

_by

_sputtering.

The

_energies

used are

87.4 MeV and 92.2

MeV,

respectively.

The beams are

incident on a 70

_Jlgjcm2

self

_supported

carbon

_target

with an 80

% isotopic

abundance of

14C.

As in

pre-vious studies of

19N

_[6],

17C

_[7]

and

210

_[8],

the ions emitted are

_analysed

within a

_{large (4.8}

_msr)

solid

angle

by

a 1800

_{double-focusing}

_magnetic

spectro-graph.

The focal

_plane

detectors measure several parameters

(AE,

E,

position)

to determine the nature and _energy of the ion. However several

_major

impro-vements over

_previous

_experiments

are achieved.

The

_{14C(I4C, 14B)14N}

and

_{~C(~0, ~N)~N}

reac-tions suffer from a very severe kinematical

_effect,

i.e.

_{angular dependence}

of _{the energy} of the emitted

ion,

which tends to worsen _{the energy} resolution.

Therefore,

the detectors must be set in a

_displaced

focal

_plane

distinct from the

_genuine

focal

_plane

corresponding

to K -

d/?/d0

= 0. But this is _not

B

p

usually

sufficient. For a wide

_angular

_aperture,

K cannot be considered as constant

(d2p/d02

is not

negligible)

and the

_displaced

focal

_planes

are different for the various reactions of interest.

_Therefore,

a _ray

reconstruction

_procedure

has to be used. For each event, two successive radial

_positions

_Xl

and

_X2

are

measured at the exit of the _magnet. Elastic

_scatterings

and reactions with known

_Q-values

observed at various

_angles

are used to establish

_general

calibration

functions

_{Bp(Xl X2)}

_{(magnetic rigidity)}

and

_{O(XI X2)}

(angle

of

_reaction).

These in turn determine the reaction

_Q-value

of the event. From the raw

parame-ters of each _event, a

_Q-value

_spectrum is built in this

way

(see,

e.g.

Fig. I).

Fig. I. -

Energy spectra of 3B, ’ 4B and 18N produced in the labelled reactions. The abscissa is _proportional to the reaction

Q-value (see text). In two cases, note the logarithmic scale. The energy range of the spectra corresponds to the width of the ioniza-tion chamber.

But

_again,

this is not

_usually

sufficient

since,

unless the beam has a

_negligible

_energy

_dispersion

and a

negligible geometrical

emittance,

the reaction

_angle

at the _target does

_depend

on the

_angle

of the incident ion

_trajectory

with _respect to the exit ion

_trajectory.

Hence,

two

correlations,

angle/position

and

_energy/

position, coupled

one to the other, must be achieved

on the _target in order to _{preserve the} existence of a

focus in the

_displaced

focal

_plane

of the _magnet

_[9].

These correlations are obtained

_{by i)}

_focusing

the

The detectors in the focal

_plane

of the _magnet

consist of two

_proportional

counters and an ionization

chamber. The two

_{position-sensitive}

counters are set

27 cm _apart one from the other in order to

_perform

the necessary ray reconstruction

_[10]

_but,

_contrary to

previous

experiments,

the two AE informations from

these counters are not used for ion identification.

These informations are obtained from the ionization chamber itself where the anode is sliced into three

parts

to measure _{the energy} lost

_by

the ion

_along

three successive _segments of its

_trajectory

in the chamber. The first two measurements

_provide

the

redundant AE information. To avoid distortion of the electric field established between the anode and the

cathode near the thin

_mylar

windows of the

chamber,

those are coated with thin horizontal

_strips

of

alumi-num which insure the

_regularity

and

_parallelism

of

isopotential

surfaces. At

last,

the vertical location of

the

_trajectory

can be determined

by measuring

the

time difference between the

_signals

of a

_proportional

counter and of the cathode of the ionization chamber.

The difference arises from the slowness of the drift of the

_charges

collected in the chamber. This is used to correct the observed

_systematic

dependence

of the

E

_signal

with the vertical location of the

_trajectory.

With these

_{improvements,}

the low _energy tail of

the E

_signal

is

_negligible

and

_typical

resolutions of

2

_%

for AE and I

_%

for E are

_routinely

achieved.

A detailed account of the

_performances

of this ioni-zation chamber will be

_published

elsewhere

_[11].

5. Results. -

Figure

1 shows the _{energy spectra} of

13 B, 14 B

and

18N.

The _energy resolution of

_nearly

200 keV

results,

under the

_present

conditions of a

strong kinematical

effect,

from the

_angular

uncer-tainty (0.25°)

of _{the ray}

_tracing.

_{An energy} calibration is

_provided

_by

the

_{(1 4C, 13 B)}

and

_(180,

_I7N)

reactions. The

_angular

distributions reconstructed from the ray

tracing procedure [10]

are shown in

_figure

2. Within the

_angular

_range

covered,

sharp

differences are

observed between the

_shapes

of these distributions,

some of which are flat while the cross section of the

18N

excited state decreases

_by

a factor of 4.

Only

one

_peak

_appears in the energy

_spectrum

of

14 B.

It

_corresponds

to a cross section of

daldo

(C.M.)

= 0.9

~b . sr-1

1

and a mass excess of

Fig. 2. - Four

partial angular distributions within the angular aperture of the _magnet. Each _{point corresponds} to the events

observed within a half degree bin, as obtained _by the ray tracing procedure (see text). Typical uncertainties are _given. There is an

overall uncertainty of 0.6~ over the absolute value of the angle.

23 590 _± 60 keV. While 18 events are

_assigned

to

the

_ground

state

transition,

no event is observed at the

_expected

location of the first excited state of

14B

at 0.74 MeV which was

_populated

in the

(7 Li , 7 Be)

reaction with a cross section

_only

a factor of 2 or 3

smaller than the

_ground

state

_[1].

The

_{suggested [1]}

_]

spins

and

_{configurations}

for the

14B

states

_provide

no

_{straightforward explanation}

for its non-observa-tion in the

_{(14C, 14B)}

reaction.

Two

_peaks

are observed in

the

8N

_energy

_spectrum

with centre of mass cross sections of 16.5 and

3.3

_{Jlb. sr- 1.}

Within the limitations due to the

_nearly

200 keV _energy

resolution,

they

show no evidence of

being

due to more than one state. The

_assigned

_ground

state

_peak

_corresponds

to a mass excess of

Table I. -

L-82 JOURNAL DE PHYSIQUE - LETTRES

13 217 ± 40 keV for

_{I 8N,}

while an excited state is found at 575 ± 25 keV. The results of

_{(p, p’)}

and

(d, d’)

scattering

on

180,

mentioned in a

_meeting

abstract

_[12]

and never

_published

_suggested

that two

T = 2 levels of

180

might

be located at 16.40 and

17.02 MeV

_(±

30

_keV).

A calculation of Coulomb

energy differences makes

possible

that the lower one

is the

_analog

of the 18 N

_ground

state.

_Furthermore,

the energy difference between these tentative T = 2

states

of

I 80

is close to the _{energy of}

the

I8N

excited state measured in the

_present

work.

Table I summarizes the results and the

_origin

of the uncertainties.

6. Conclusion. - The

present results confirm the

mass of

1~’B.

_They

contribute to

_finally

_clarify

the

situation for I8N since

_they

are in _agreement with the

early

result of Stokes and

Young

[2].

An excited state

is observed for the first time in 18N while the

_only

reported

bound excited state of

14B

is not observed.

To discard any doubt as to its

_reality,

another

mea-surement of the

_{(’4C, I4B)}

with better statistics is in order to take

_advantage

of _{the very} low

_background

attained in this

_experiment.

Further

_knowledge

of the T =

2,

A = 4 n + 2

nuclear states could result from the observation of

the

_analog

T = 2 states in

_e.g.

4C

and

~0.

No

unambiguous

result has been obtained _yet, but the

availability

of a

14C

beam makes, it

possible

to at last characterize these states in nuclear reactions.

References

[1] BALL, G. C., COSTA, G. J., DAVIES, W. G., FORSTER, J. S., HARDY, J. C. and MCDONALD, A. M., Phys. Rev. Lett. 31 (1973) 395.

[2] STOKES, R. H. and YOUNG, P. _{G., Phys.} Rev. 178 (1969) 1789.

[3] CHASE, L. F., GRENSCH, H. A., MCDONALD, R. E. and VAUGHN,

F. J., Phys. Rev. Lett. 13 (1964) 665.

[4] DE MEIJER, R. J., STAHEL, D. P., BICE, A. N., JAHN, R. and

CERNY, J., Lawrence Berkeley Laboratory, Nuclear Science Annual _Report 1977-1978 L.B.L. 8151 (1978) 9.

[5] DUMAIL, M., Nucl. Instrum. Methods 163 (1979) 61.

[6] DÉTRAZ, C., NAULIN, F., LANGEVIN, M., ROUSSEL, P.,

BER-NAS, M., POUGHEON, F. and VERNOTTE, J., Phys. Rev. C 5 (1977) 1738.

[7] NAULIN, F., DÉTRAZ, C., KASHY, E., BERNAS, M., LANGEVIN,

M., POUGHEON, F. and RoussEL, P., in Physics of Medium

Light Nuclei _{(Topical Conference)} ed. Editrice composi-tori, Bologna, 1978, p. 65.

[8] NAULIN, F., DÉTRAZ, C., BERNAS, M., KASHY, E., LANGEVIN,

M., POUGHEON, F. and ROUSSEL, P., Phys. Rev. C 17

(1978) 830.

[9] KASHY, E. and ROUSSEL, P., to be _published.

[10] ROUSSEL, P., BERNAS, M., DIAF, F., NAULIN, F., POUGHEON, F., ROTBARD, G. and ROY-STEPHAN, M., Nucl. Instrum.

Methods 153 ₍₁₉₇₈₎ 111.

[11] NAULIN, F., ROY-STEPHAN, M. and KASHY, E., to be published. [12] NERO, A. V., OHANIAN, R. S., SONEIRA, R. and