ELSEVIER
Nuclear
Physics
A7 19 (2003)
229~232~
www.elsevier.com/locate/npe
Resonance states of hydrogen
reactions with exotic beams
nuclei 4H and 5H obtained
in transfer
A.S. Fomichev”;
M.S. Golovkov”,
Yu.Ts. Oganessian”,
S.I. Sidorchuk”,
D.D. Bogdanov”,
A.M. Rodina, R.S. Slepnev”, S.V. Stepantsov”,
G.M. Ter-Akopian”,
R. Wolski”,
\:.A. Gorshkov”;
M.L. Chelnokov”,
M.G. Itkis”, E.M. Kozulin”,
A.A. Bogatchev”,
N.A. Kondratiev”;
I.V. Korzyukov”,
A.A. Korsheninnikovb,
E.Yu. Nikolskib and
I. Tanihatab
“Flerov
bRIKEN,
Laboratory
Hirosawa
of Nuclear
Reactions
2-l;F-14076,
Saitama
JINR,
Dubna,
141980 Russia
351-0198,Japan
Experimental
search for 4H and 5H has been performed
in the reactions 3H(t)p)5H,
2H(6He,3He)5H
and 2H(t,p)4H.
A resonance state situated at 1.810.1 MeV above the
t+n+n decay threshold is obtained in the missing mass energy spectrum of the 5H nucleus.
Also, there is an indication
that another resonance located at 2.710.1 MeV presents in
this spectrum.
Visible peak widths are governed mostly by the instrumental
resolution,
which came to about 0.4 MeV. We set an upper limit of 0.5 MeV on the true width of any
of the two states. The resonance state of ‘H with E,,,=3.3+0.2
MeV and I,,,=4.1&0.3
MeV was obtained in the 2H(t,p)4H reaction.
I. EXPERIMENTS
We studied the formation of superheavy hydrogen isotopes in three reactions:3H(t,p)5H,
were performed at the separator ACCULINA
2H(6He,3He)5H
and 2H(t,p)4H. Experiments
[l] in FLNR (JINR, Dubna).
The primary
triton beam delivered
from the cyclotron
U400M and the secondary one of 6He were used. The projectile
energies made 58 and
150 MeV for tritons and ‘He ions, respectively.
Liquid tritium
and deuterium
of the
thickness of 2.102r cm-’ were used as targets.
Two telescopes were intended
for the
detection of coincident
charged particles. Besides, 41 scintillation
modules of the neutron
carried out
spectrometer
DEMON
[2] were used for the neutron detection in experiments
with the triton beam. The estimated instrumental
resolution
obtained
by means of the
complete Monte-Carlo
simulation
of the experiments
was 400 keV for the 2,3H(t,p)4,5H
reactions and 600 keV for the 2H(6He,3He)5H
reaction.The
background
contribution
to
the spectra of 4,5H measured with the empty target was found to be negligibly
small for
all three reactions studied.
0375-94741031s
~ see front matter
doi:10.10161S0375-9474(03)00924-2
8 2003 Elsevier
Science
B.V.
All rights
reserved.
XI. Sidovchuk et al. /Nuclear Physics A719 (2003) 229c-232~
230~
2. RESULTS
2.1. 2H(t,p)4H
We identified the following processes accompanying the 4H formation: quasifree scattering of incoming tritons on a proton bound in deutron (QFS), proton-neutron and protontriton final state interactions (FSI,, and FSIPt).Detector arrangement provided the most
favorable conditions for the observation of the 4H resonance for protons and neutrons detected in coincidence. Besides in this case QFS and FS& do not contribute to the missing
mass spectrum shown in Fig. 1 due to the low laboratory energy of neutrons. The analy-
0
2
4
E,,
6
MeV
8
10
12
Figure 1. Energy spectrum of 4H for
p-n coincidence.
Figure 2. Energy spectrum of 4H for
p-t coincidence.
sis of data corresponding to proton-triton coincidence was done for the two ranges of 4H
decay angles 120”<6,,<180” and 90”<(pt,<270” taken in the polar rest frame of 4H. This
restriction leads to the removal of QFS and FSIpt from the final spectrum. The missing
mass energy spectrum of 4H obtained at the proton angle 19gm=500I-t 2” is shown in Fig.2.
The fit to the data gives the same resonance parameters for the two spectra shown in Fig.
1 and 2: E,,,=3.3&0.2 MeV and I’,,,=4.1?~0.3 MeV. The cross-section of the reaction
was estimated to be 0.8f0.4 mb/sr within the proton angular range 0Gm=32” f 52”.
2.2. 3H(t,p)5H
The measured inclusive proton spectrum does not exhibit any prominent structure
which could be ascribed to ‘H. The selection of p-t coincidence events eliminates from the
spectrum all events corresponding to the reactions on the target impurity nuclei. Nevertheless the processes competing to the 5H formation still dominate in the p-t coincidence
spectrum. The most severe contribution to the continuum observed in this spectrum
makes the quasifree scattering (QFS) of projectile tritons on the protons bound in the
target tritium nuclei. However, the two spectator neutrons confined in the target nucleus
acquire in this process low- energy in lab system. Therefore in the 5H spectrum obtained
with the selection of triple p-t-n events involving neutrons with lab energy more than 2.5
MeV we removed the QFS process. The corresponding missing mass energy spectrum
S.I. Sidorchuk et al. /Nuclear
Figure
3. Energy
spectrum
of ‘H measured
Physics A 719 (2003) 229c-232~
in the reaction
23ic
3H(t;p)5H.
obtained for ‘H is shown in Fig.3.
Despite of the low statistics, the narrow peak at 1.8 MeV is crearly seen in the spectrum.
Another state of ‘H could be assumed at an energy of about 2.7 MeV. The shape of the
measured spectrum was analyzed in a suggestion of different models. The instrumental
resolution and detection efficiency were taken into account in the complete Monte-Carlo
simulation
of the experiment.
The best fit to the data without
resonances in the 5H
system is shown as a thick solid curve. This curve is the sum of curves corresponding
to
the free 3-particle
phase space, FSI,, and FSIt, with the parameters
of the 4H resonance
obtained in our experiment.
The inclusion of the two 5H resonances to the fit improves
the reduced x2 value from 1.4 to 0.96.
The cross-sections of the 2n-transfer
reaction populating
the ground (E=1.8 MeV) and
excited (E=2.7 MeV) states make 18f20 pb/sr and 35f20 pb/sr, respectively.These
estimated cross-sections are based on the data averaged over the angular range covered by
our measurements
30”<6~~<50”.
2.3. 2H(“He,“He)5H
In this experiment
we detected the coincidence
of “He nuclei and tritons from the “H
decay. The resulting missing mass spectrum is shown in Fig.4. The position of the narrow
peak at energy 1.8 MeV is consistent with that obtained in the reaction 3H(t,p)5H.
We
note the absence of the second, 2.7 MeV peak in this spectrum.
One can assume that the
lack of this peak is due to the specific mechanism of the reaction 2H(6He,3He)sH,
which
is a pick up of a proton from 6He. It is reasonable to expect that neutrons in the ground
state of 5H occupy the same orbitals as in ‘He. Therefore
the l/2+ ground state of ‘H
should be preferably
populat,ed in the reaction studied.
232~
XI. Sidorchuk et al. /Nuclear Physics A719 (2003) 229c-232~
‘0
Figure
4. Energy
spectrum
, ,
, , ,
2
4
of 5H measured
, , , , / ,
6
,
10
Et+,+
MeV
in the reaction
8
2H(6He,3He)5H.
3. CONCLUSIONS
The obtained resonance parameters
or’ 4H E,,,=3.3&0.2
MeV and F,,,=4.lf0.3
MeV
are consistent with the data resulting
from an analysis assuming the charge-symmetry
reflection of 4Li R-matrix
[3]. The analysis of the angular distributions
did not reveal any
sign of the interference
between the 2- and I- states shown for 4H by this author.
The
measured cross-section of the reaction is in agreement with the result of our preliminary
DWBA analysis.
The striking features of the 5H l/2+ ground state are its low energy 1.8 MeV and the
extremely small width - the observed value is mainly due to the instrumental
resolution.
This result is in agreement with data obtained
in the work [4] but contradicts
the theoretical estimations
done in the frame of 3-body models [5,6]. Taking into account the
relatively
low cross-section of the 5H formation
one can conclude that the 5H resonance
might have a structure very different from the 3-body one. The peak at 2.7 MeV seems
to be a candidate
for the 5/2+ state predicted by theory [5].
The partial support of this work
02-16550) is acknowledged.
by Russian
Basic Research
Foundation
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