Investigation of Continuum States in the Unbound Neutron-Rich N=7 & 9 Isotones via Breakup and Knockout

Abstract

The structure of the unbound nuclei 9He, 10Li and 13Be has been explored using breakup and proton-knockout from intermediate energy 11Be and 14,15B beams. In the case of both N=7 isotones, virtual s-wave strength is observed near threshold together with a higher-lying resonance. A very narrow structure at threshold in the 12Be+n relative energy spectrum is demonstrated to arise from the decay of the 14Be*(2+), discounting earlier reports of a strong virtual s-wave state in 13Be. Comment: 5 pages, 4 figures, Contribution to NIIGATA2010 - "International Symposium on Forefronts of Reseach in Exotic Nuclear Structures", Tokamachi, Japan, 1-4 mars 2010 [Proceedings version will not include Fig. 2 owing to limit on number of pages].

Full text

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arXiv:1004.3233v1 [nucl-ex] 19 Apr 2010
April 20, 20104:26WSPC/INSTRUCTION FILEarXiv-Niigata-2010
International Journal of Modern Physics E
c ? World Scientific Publishing Company
INVESTIGATION OF CONTINUUM STATES IN THE UNBOUND
NEUTRON-RICH N=7 AND 9 ISOTONES VIA
BREAKUP AND KNOCKOUT∗
H. Al Falou†, A. Leprince, N.A. Orr‡
for the LPC–CHARISSA–DEMON Collaboration
LPC-ENSICAEN, IN2P3-CNRS et Universit´ e de Caen,
14050 Caen cedex, France
The structure of the unbound nuclei9He,10Li and13Be has been explored using
breakup and proton-knockout from intermediate energy11Be and14,15B beams. In the
case of both N=7 isotones, virtual s−wave strength is observed near threshold together
with a higher-lying resonance. A very narrow structure at threshold in the12Be+n
relative energy spectrum is demonstrated to arise from the decay of the14Be∗(2+),
discounting earlier reports of a strong virtual s−wave state in13Be.
The light nuclei have long provided a fertile testing ground for our understand-
ing of nuclear structure. From an experimental point of view, this is the only region
for which nuclei lying beyond the neutron dripline are presently accessible. The-
oretically, models incorporating explicitly the continuum are being developed. In
addition, the structure of unbound systems, such as10Li, is key to constructing
three-body descriptions of two-neutron halo, such as11Li, and related nuclei.
One of the approaches well adapted to the study of nuclei far from stability is
that of “knockout” or breakup of a high-energy radioactive beam. Here we report
on measurements using secondary beams (35 MeV/nucleon) of11Be and14,15B to
investigate the low-lying level structures of9He,10Li and13Be. The experiments
employed beams delivered by the LISE3 separator at GANIL. The beam velocity
charged fragments and neutrons emitted in the forward direction from the reactions
on a carbon target were identified and the momenta determined using a Si-Si-
CsI array coupled to a large-scale neutron array. These measurements allowed the
fragment+neutron (f-n) relative energy spectra to be reconstructed. In order to
interpret the spectra, simulations, which were validated using known resonances
(such as7Heg.s.), were developed to model the response function of the experimental
setup. Detailed accounts of the work presented here may be found elsewhere1,2,3.
∗Contribution to NIIGATA2010 - “International Symposium on Forefronts of Reseach in Exotic
Nuclear Structures”, Tokamachi, Japan, 1–4 mars 2010.
†Present Address: TRIUMF, 4004 Wesbrook Mall, Vancouver, BC V6T 2A3 Canada
‡Presenting author, e-mail: orr@lpccaen.in2p3.fr
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H. Al Falou, A. Leprince, N.A. Orr
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C(11Be,9Li+n)X
(a)
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C(14B,9Li+n)X
(b)
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C(11Be,8He+n)X
(c)
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C(14B,8He+n)X
(d)
Counts
Ef-n (MeV)
Fig. 1. Relative energy spectra for the9Li+n and8He+n systems for different reactions. The
dotted lines represent the uncorrelated background. The thin solid lines are the virtual s−states,
while the dashed lines are the resonances. The thicker solid line is the overall adjustment.
The results obtained for the8He+n and9Li+n systems are shown in Fig. 1.
As demonstrated in our work on the C(17C,15B+n) reaction3, the description of
the relative energy spectra require, in addition to discrete final states, a broad
and rather featureless continuum of uncorrelated events which may be generated
via event mixing4. Both spectra for9Li+n (Fig. 1(a) and (b)), as well as that for
8He+n derived from two-proton knockout from11Be (Fig. 1(c)), exhibit significant
strength just above threshold which can be most satisfactorily described by the
presence of a virtual s-wave scattering state. The results for the C(11Be,9Li+n)
and C(11Be,8He+n) reactions are in line with what may be expected on the basis
of simple considerations, whereby proton only removal from the projectile should
leave the neutron configuration undisturbed5. Given the dominant s−wave neutron
component in11Beg.s., proton knockout to10Li and9He should populate preferen-
tially s−wave final states. In the case of9Li+n, a scattering length around -17 fm

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INVESTIGATION OF CONTINUUM STATES ...
3
was deduced, whereas that for8He+n is close to 0 fm (as= -3 – 0 fm at the 3-
sigma level), signifying a very weak fragment-neutron interaction. The10Li result
is in line with other studies, including high-energy neutron knockout from11Li6,7,
whilst that for9He, despite being in conflict with similar work at slightly lower
energies5, has been confirmed by a very recent study at relativistic energies8.
The9Li+n spectrum from the breakup of14B clearly displays the presence of
a higher lying state some 0.5 MeV above threshold, which may be identified with
the expected p−wave resonance observed in other studies6,7. Interestingly, breakup
of15B exhibits an enhanced yield to this resonance relative to the s−state2, as
displayed in Fig. 2. Despite suffering from limited statistics, the8He+n relative
energy spectrum obtained from breakup of14B is consistent with the presence of
the very weakly interacting s−wave virtual state and a resonance around 1.2 MeV
above thresold9.
0
50
100
150
200
250
300
350
400
0246
Ef-n (MeV)
Counts
Fig. 2. Relative energy spectra for the9Li+n from breakup of a15B beam. The dotted lines
represent the uncorrelated background, whilst the thin solid lines are the virtual s−state and
resonance (see text). The thicker solid line is the overall adjustment.
Turning to the N=9 system13Be, we have investigated an earlier report of pro-
nounced s−wave virtual strength at threshold10, which is at varience with all other
studies4,7,11,12. The distinguishing features of the earlier work were: a collinear ge-
ometry for the fragment and neutron detection, which limited the range of accessible
relative energies to close to threshold, and the fragmentation of18O to produce the
12Be+n system. Given that in such reactions (where protons and neutrons are re-
moved) the outgoing channel of interest will tend to be produced via the neutron
decay of more neutron-rich unbound states, it is likely that the majority of the

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H. Al Falou, A. Leprince, N.A. Orr
0
5
10
15
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01234
Ef-n-n (MeV)
56
Counts
C(15B,12Be+n+n)
0
20
40
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100
120
0123456
Ef-n (MeV)
Counts
C(15B,12Be+n[+n])
Fig. 3. Left panel: Relative energy spectrum for the12Be+n+n system. Right panel:12Be+n
relative energy spectrum derived from12Be+n+n events, where events arising from the decay of
14Be∗(2+) are shown as the dashed histogramme close to threshold.
12Be+n yield from18O results from the decay of unbound states in14Be. Any nar-
row resonances in the14Be continuum could then leave an imprint on the12Be+n
relative energy spectrum as a narrow, resonance-like structure.
In this spirit, we have investigated the decay of14Be∗through its population
in single-proton removal from15B. The resulting12Be+n+n relative energy spec-
trum (Fig. 3) exhibits a well populated peak, some 300 keV above threshold, which
corresponds to the 2+
structed from these events is also displayed in Fig. 2. When the12Be+n events are
confined to those arising from the decay of14Be∗(2+), they are seen, unsurprisingly,
to constitute the very narrow structure at threshold (Fig. 3, right panel). That is
to say, the threshold strength in the12Be+n relative energy spectrum is an artefact
of the limited energy available in the decay of14Be∗(2+).
We can proceed further and reproduce conditions similar to that of the18O beam
experiment by restricting the neutron detection to very forward angles (θn <3◦)
and analysing the12Be+n events. The resultats are displayed in Fig. 4, including,
as in the18O study, the12Be–n relative velocity spectrum10. Unsurprisingly, the
restricted acceptances privilege almost exclusively the peak at threshold produced
by the decay of14Be∗(2+) and result in a relative velocity spectrum identical to that
seen in the18O beam experiment. Moreover, both the relative velocity and energy
spectra are well reproduced by the combination of an uncorrelated background and
an s−wave virtual state lineshape with as= -10 fm as in the original analysis of
the18O data10. In summary, we conclude from the present analyses, that there is
no evidence for a strong s−wave virtual state at threshold in13Be.
1state of14Be13. The12Be+n relative energy spectrum recon-
The authors would like to thank their colleagues in the LPC–CHARISSA–

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INVESTIGATION OF CONTINUUM STATES ...
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0
-30 -25 -20 -15 -10 -5
25
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0510 15 20 25 30
vn-vf (mm/ns)
C(15B,12Be+n)X
θn<3 ˚

10
Counts
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600
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6
C(15B,12Be+n)X
θn<3 ˚

Ef-n (MeV)
Fig. 4.12Be+n relative velocity (left panel – FWHM=10 mm/ns) and energy spectra (right panel)
from the breakup of15B for θn<3◦. The dotted line is the uncorrelated background and the thin
solid line a virtual s−wave state lineshape.
DEMON Collaboration and acknowledge the excellent support provided by the
technical staff of LPC and GANIL.
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