CORRELATION STUDIES OF THE 5H SPECTRUM
M.S. Golovkov1, L.V. Grigorenko1, A.S. Fomichev1, S.A. Krupko1, Yu.Ts. Oganessian1, A.M. Rodin1,
S.I. Sidorchuk1, R.S. Slepnev1, S.V. Stepantsov1, G.M. Ter-Akopian1, R. Wolski1,2,
A.A. Korsheninnikov3, E.Yu. Nikolskii3, P. Roussel-Chomaz 4, W. Mittig4, R. Palit5, C. Angulo6,
V. Bouchat7, V. Kinnard7, T. Materna7, F. Hanappe7, O. Dorvaux8, L. Stuttge 8, A.A. Yukhimchuk9,
V.V. Perevozchikov9, S.K. Grishechkin9, Yu.I. Vinogradov9, S.V. Zlatoustovskiy9.
1
Flerov Laboratory of Nuclear Reactions, JINR, Dubna, Russia
The Henryk Niewodniczanski Institute of Nuclear Physics, Krakow, Poland
3
RIKEN, Hirosawa, Japan; On leave from the Kurchatov Institute, Moscow, Russia
4
GANIL, Caen Cedex5, France
5
GSI, Darmstadt, Germany
6
Centre de Recherche du Cyclotron, UCL, Louvain-La-Neuve, Belgium
7
Universite Libre de Bruxelles, PNTPM, Bruxelles, Belgium
8
Institut de Recherches Subatomiques, IN2P3/Universite Louis Pasteur, Strasbourg, France
9
RNFC – All-Russian Research Institute of Experimenntal Physics, Sarov, Nizhni Novgorod region, 67190 Russia
2
Very asymmetric nuclear matter is a
phenomenon in subatomic physics remaining far
from a good understanding. The study of the
superheavy hydrogen isotope 5H could shed light
on this subject. Controversy in results obtained
to date on the 5H system [1-5] gave rise to an
intense discussion that has appeared even in the
popular literature. Essentially, the question was
whether the 5H ground state (g.s.) is located at
1.7-1.8MeV above the t+2n decay threshold
[2,3], or at about 3 MeV [4] or even higher [1,5].
Theory calculations are sensitive to model
assumptions and also give diverse predictions on
5
H. New experiments could resolve this
important question.
In the present work we studied the same
reaction 3H(t,p)5H as in Ref.[4] but in a different
kinematical region. The experimental setup is
shown in Fig.1. A 58 MeV triton beam was
produced at the Dubna U-400M cyclotron. The
ACCULINNA separator was used to reduce the
angular spread and energy dispersion of the
primary triton beam to 7 mrad and 0.3 MeV
(FWHM), respectively. Finally, the triton beam
with intensity of 3*107 particles/s was focused in
a 5 mm spot on a cryogenic tritium target [6].
The 4 mm thick target cell was filled with tritium
to a pressure of 860 mbar and cooled down to
25°K.
Slow protons emitted to the back from
the target hit a 300 µ thick Si detector with
double-sided strips. This annular detector,
having an active area with inner and outer
diameters of 32 and 85 mm, respectively, was
installed 100 mm upstream of the target.
Charged particles moving in the forward
direction were detected by a telescope consisting
of four annular Si detectors of the same radial
dimensions as the one used to detect protons.
Neutrons were detected by 48 scintillation
modules of the time-of-flight spectrometer
DEMON. Being installed at a distance of 2.5 m
from the target the modules covered an angular
range of θlab = 5° – 40°. The forward array
detected the 5H→t+2n decay products emitted
almost at any angle.
Fig. 1. Experimental set up.
Analysis was made for the detected triple
ptn coincidence events. Such events uniquely
identify the p+5H outgoing channel and make
complete kinematical reconstruction possible.
Results are shown in the CM system of the decay
products of 5H (t+2n). The direction of the
momentum transfer, kbeam - kp, occurring in the
reaction 3H(t,p)5H was chosen as Z axis (see Fig.
1).
The most striking result was the
observation of a sharp oscillating picture in the
triton angular distribution shown in Fig.2 by
points. The analysis showed that the bulk of the
data observed in the present experiment can be
explained on the assumption that the direct twoneutron transfer dominates in the 3H(t,p)5H
reaction leading to the population of the broad,
overlapping 3/2+ and 5/2+ states in 5H. This is
supported by the following arguments:
One could consider the 5H system as a
“proton hole” in 6He. So, definite similarity
between these systems can be expected.
Theoretical predictions give Jπ=1/2+ for the
ground state of 5H. Its low-lying excited states
are supposed to be a 3/2+ and 5/2+ doublet. One
should expect a weak population of the 5H g.s. in
the 3H(t,p)5H reaction. This is due to the
statistical factor and also is a consequence of the
“angular momentum mismatch” arising from the
fact that the light proton can not carry away as
much angular momentum as the heavier triton
projectile brings in. DWBA calculations confirm
this expectation.
with the doublet of the excited 3/2+ and 5/2+
states of 5H and excited doublet.
Fig. 3. The missing mass spectrum of 5H. Bottom
panel: data are shown by points, histogram is the
result of model calculations. Top panel: shown are the
efficiency corrected results of the fit. The dashed and
dotted curves show the contributions of the 1/2 + g.s.
and 3/2+, 5/2+ doublet, respectively.
Fig. 2 Distribution over the triton angle observed in
the CM system of 5H for different ranges of the E5H
energy. Points show the experimental data,
histograms present the results of model calculations.
We employed the following procedure for
data analysis. Correlations occurring at the 5H
decay are described as
W(E)=

†
J'M' ρ A J'M' A J'M'
JMJ'M'
where AJM are the decay amplitudes depending
on the 5H decay dynamics and ρ is the density
matrix describing the polarization of the 5H
states populated in the 3H(t,p)5H reaction. We
expanded the amplitudes AJM over a limited set
of hyperspherical harmonics. A similar approach
was employed in Ref. [7].
Histograms in Figs. 2 and 3 show the
results of calculations which take into account
the detection efficiency. The missing mass
spectrum of 5H (presented in Fig. 3) shows a
broad structure above 2.5 MeV. The strong
correlation patterns seen in Fig. 2c,d allowed us
to unambiguously identify this structure as a
mixture of the 3/2+ and 5/2+ states. Such a
correlation is a rare phenomenon for transfer
reactions involving nuclei with nonzero spin and
means that the 3/2+ and 5/2+ states are either
almost degenerate or the reaction mechanism
causes a specific interference of these states.
It should be noted that at E5H<2.5 MeV
(see Fig. 2a,b) excellent agreement with
experimental points could be achieved only at a
condition that the calculations assumed the
population of the 1/2+ 5H g.s. and its interference
The data analysis lead the authors to a
conclusion that the g.s. resonance of 5H is
located close to 1.8 MeV and has a width of
about 1.3 MeV. The observed g.s. resonance
position is in a good agreement with the
experimental observations of Refs. [2,3].
Furthermore, the present data show that the
doublet of the wide excited 3/2+–5/2+ states
populated in the 3H(t,p)5H reaction achieves its
maximum at E5H5 MeV. The correlation picture
observed in the present work at E5H<2.5 MeV
shows the interference of the 3/2+–5/2+ doublet
with the 1/2+ g.s. This is consistent with the
alternative explanation presented in Ref. [3] for
the small width of the 1.8 MeV g.s. peak of 5H
reported in this paper.
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