Experimental evidence for the existence of 7H and for a specific structure of 8He.
ABSTRACT Experimental search for the superheavy 7H isotope was performed in the reaction p(8He,pp)7H with the 8He beam at 61.3A MeV. The evidence for existence of the 7H state near the t+4n threshold was obtained. In the same experiment, the p(8He,t) reaction populating the ground and excited 2(+) state of 6He was investigated. The obtained results argue on a specific structure of the 8He ground state containing the 6He subsystem in the excited 2(+) state with a large weight.
- SourceAvailable from: Sergey Ivanovich Sidorchuk
Article: Superheavy hydrogen (5)H.[show abstract] [hide abstract]
ABSTRACT: Experimental search for (5)H using a secondary beam of (6)He has been performed. The transfer reaction (1)H((6)He,(2)He)(5)H was studied by detecting two protons emitted from the decay of (2)He. A peak consistent with a (5)H resonance at 1.7+/-0.3 MeV above the n+n+t threshold was observed, with a width of 1.9+/-0.4 MeV. The angular distribution of the (1)H((6)He,(2)He)(5)H reaction was measured as well as the energy correlation of the two protons.Physical Review Letters 09/2001; 87(9):092501. · 7.94 Impact Factor
- [show abstract] [hide abstract]
ABSTRACT: A simple five-body cluster orbital shell model approximation is proposed to describe the ground state wave function of the 8He neutron-halo nucleus. The spatial angular correlations, the geometry of the system, the alpha-particle, and the valence neutron momentum distribution are calculated analytically. With a single free parameter the model is able to reproduce the experimental geometry of 8He as well as the experimental transverse momentum distribution of the 6He nucleus from 8He fragmentation on a carbon target, at high energy.Physical Review C 08/1994; 50(1):R1-R4. · 3.72 Impact Factor
Experimental Evidence for the Existence of7H and for a Specific Structure of8He
A. A. Korsheninnikov,*E.Yu. Nikolskii,*E. A. Kuzmin,*A. Ozawa, K. Morimoto, F. Tokanai,
R. Kanungo, and I. Tanihata
RIKEN, Hirosawa 2-1,Wako, Saitama 351-0198, Japan
University of Surrey, Guildford, Surrey GU2 7XH, United Kingdom
M.S. Golovkov, A.S. Fomichev, A. M. Rodin, M. L. Chelnokov, and G. M. Ter-Akopian
JINR, 141980 Dubna, Moscow region, Russia
W. Mittig, P. Roussel-Chomaz, and H. Savajols
GANIL, BP 5027, F-14076 CAEN, CEDEX 5, France
DAPNIA-SPhN, CEA Saclay, France
A. A. Ogloblin
Kurchatov Institute, Kurchatov Square 1, 123182 Moscow, Russia
Chalmers University of Technology and Go ¨teborg University, S-41296 Go ¨teborg, Sweden
(Received 8 October 2002; published 27 February 2003)
Experimental search for the superheavy7H isotope was performed in the reaction p?8He;pp?7H with
the8He beam at 61:3A MeV. The evidence for existence of the7H state near the t ? 4n threshold was
obtained. In the same experiment, the p?8He;t? reaction populating the ground and excited 2?state of
6He was investigated. The obtained results argue on a specific structure of the8He ground state
containing the6He subsystem in the excited 2?state with a large weight.
DOI: 10.1103/PhysRevLett.90.082501PACS numbers: 27.20.+n, 25.60.Je
Progress in experimental technique including usage of
secondary beams of short-lived radioactive nuclei has
enabled studies of exotic nuclear systems in the vicinity
of and beyond the neutron drip line. Recently, experimen-
tal studies of
p?6He;2He?5H , t?t;p?5H , and d?6He;3He?5H .
These three measurements show a peak of
?1:8 MeV above the t ? 2n threshold. Note that two
other recent experiments [3,4] devoted to5H show a5H
peak at somewhat higher energy. Because of the known
systematics for helium isotopes, where8He having two
more neutrons than6He is more bound relative to the
separation of two neutrons than6He as well as7He is
less unbound than5He, the results for5H allow us to
speculate that7H may exist as a state in a vicinity of
the t ? 4n threshold.
Hardly7H exists as a bound state. During the years, in
numerous measurements performed using the ?E-E
method, the region of hydrogen and helium isotopes
was studied in detail and no hyperbole of the bound7H
in ?E-E plots was observed. However,7H may exist as an
unstable state. Being close to the threshold,7H could be
an especially interesting system. It should undergo the
unique five particle decay into t ? 4n and its width may
5H were performed in the reactions
be very narrow. An extreme fraction of neutrons in7H,
N=Z ? 6, is comparable with that in neutron stars.
Experimental search for unstable7H presents a diffi-
cult task, and this resonance has never been observed.
Recent calculation of7H within the seven-body hyper-
spherical functions method suggests that7H can be un-
bound with respect to the t ? 4n decay by 840 keV .
We made an additional attempt to estimate the energy
range for7H to be measured. In Ref. , the binding
energy of7H has been obtained by exponential extrapo-
lation of energies calculated up to K ? Kmin? 6. Such an
extrapolation can give only a rough evaluation of the
converged binding energy. In the present work, we study
proton separation energy for8He, Sp?8He?, rather than
total7H energy. Calculating proton separation energy in
6He also, we have revealed that, unlike the total binding
energy, Spis only slightly sensitive to the increase of
the model space. The reason for this lies in the well-
pronounced cluster structure of6;8He and5;7H for which
the binding energies of the cores4He and3H converge
In the present calculations, we used all variants of
the Volkov NN potential , trying to find a case
which reproduces the binding energies of
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2003 The American Physical Society082501-1
4;6;8He simultaneously. The best results have been ob-
tained with potential V6 modified as follows: Vmod
? ?2 ? ??V13, Vmod
? ?1 ? 2M?Vmod
Majorana parameter M ? 0:6. The results for Sp?6He?
and Sp?8He? obtained with this potential are shown in
Fig. 1 together with those obtained with potential V1 .
The Sp?6He? values are reasonably close to the experi-
mental value  also shown in Fig. 1. One can see that the
proton separation energy for8He is about 26 MeVfor both
potentials. Since the experimental8He binding energy is
31.4 MeV , we can expect the
energy to be ?5:4 MeV which is about 3 MeVabove the
t ? 4n threshold. Because of the remnant convergence
over K, which is seen in Fig. 1 for6He, the estimated
energy of7H can be regarded as an upper limit for the
Considering the experimental search for7H, we took
into account the following. It is reasonable to suppose that
neutrons in the ground state of
orbitals as in8He. Hence, by pickup of one proton from
8He, we should get a good chance to populate the ground
reaction p?8He;pp?7H for experimental search for7H.
The experiment was performed in RIKEN (Japan)
using a secondary beam of8He produced by the fragment
separator RIPS from fragmentation of a primary beam of
18O. The obtained8He beam had a very high intensity
of ?300000 particles per second and an energy of
61:3A MeV with an energy spread of 2:6A MeV.
As a proton target, we used a cryogenic target from
GANIL (France), which was filled with hydrogen gas at a
temperature of 35?K and pressure of 10 atm. The target
thickness was 6 ? 1021protons=cm2.
The experimental setup is shown in Fig. 2. Two plastic
scintillators were used for the identification of each par-
ticle of the secondary beam and for the measurement of
its energy by time of flight. The trajectories of individual
8He ions were measured by a pair of two-dimensional
multiwire proportional chambers (MWPCs). The two
? ?1 ? 2M?Vmod
with ? ? 1:15 and standard
7H seven-body binding
7H occupy the same
7H. Consequently, we have chosen the
protons originating from the reaction p?8He;pp?7H
were detected in coincidence by the RIKEN telescope
which is a stack of Si strip detectors. This telescope
allows one to detect several particles in coincidence, to
identify each particle, and to measure its energy and
angles. The beam passed through the central hole in the
annular strip detectors. Using this telescope, we detected
the two protons at small angles in the laboratory system.
Note that it differs from geometry used in studies of
quasifree pp scattering, where two protons are measured
with an angle of 90?between them, whereas the geome-
try used in the present experiment corresponds to our
expectation that the two protons from the reaction
p?8He;pp?7H can undergo final state interaction being
emitted as a virtual singlet state2He. This method was
used in the study of5H in the reaction p?6He;pp?5H .
When detecting the two protons by the RIKEN telescope,
we obtain kinematically complete information about
these two protons and, due to energy and momentum
conservation in the reaction p?8He;pp?7H, we can un-
ambiguously reconstruct a mass of the residual system
7H. Also, we detected tritons and neutrons from the
consisting of a dipole magnet and plastic scintillators.
This part of setup was the same as in our previous
experiment described in Ref. .
Experimental results obtained in the pp coincidences
are shown in Fig. 3. Spectra measured in coincidences
with triton and neutron emitted downstream have a very
similar character, but lower statistics. In Fig. 3, spectra
are shown as a function of the7H energy, E7H, relative to
the t ? 4n decay threshold. The solid histogram shows
the result obtained with the proton target. The cutoff at
?20 MeV reflects the detection limit due to the RIKEN
telescope. The dashed histogram presents a background
obtained with an empty-target and normalized according
to the8He beam integral. The shift of the background
with and without hydrogen gas in the target is smaller
than the size of the bin in the shown spectra. This back-
ground accounts for events at negative energies, where7H
would be bound. At positive energies, a definite effect
from the reaction on the proton target is observed: The
solid histogram exceeds the dashed histogram. At the
same time, a high contribution from the empty-target
background puts obstacles in a detailed analysis of the
7H spectrum. However, one very interesting feature can
be seen in the obtained results. This is a very rapid
increase of the7H spectrum near the t ? 4n threshold.
7H using a downstream detection system
∆K = Kmax - Kmin
tions of difference in maximum and minimum hypermoments.
Proton separation energies for6He and8He as func-
FIG. 2.Scheme of the experimental setup.
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It is more clearly seen in Fig. 4 obtained in the follow-
ing way. We fitted the empty-target background by a
polynomial of the fifth order (it is shown by the thin solid
curve in Fig. 3) with ?2per degree of freedom smaller
than unity and subtracted this polynomial from the solid
histogram in Fig. 3. Figure 4 shows the obtained result.
Error bars present statistical errors for the solid histo-
gram in Fig. 3. Figure 5 shows the spectrum obtained by
direct subtraction of the empty-target background with
error bars including statistical uncertainty of the back-
ground subtraction. Experimental resolution in these
spectra was 1.9 MeV (FWHM).
In Figs. 4 and 5, the very sharp increase of the spec-
trum starting from the t?4n threshold is obviously seen.
Such a behavior does not correspond to a nonresonant
continuum, which is illustrated by curves 1–3 in Fig. 4.
When calculating curve1, we described the motion in the
t?4n system by the five-body phase volume, dN=dE7H/
in Fig. 5, and the experimental resolution were taken into
account. Curve 2 was obtained, when the motion of t?4n
was taken as the three-body phase volume, dN=dE7H/
process, when two pairs of neutrons in the t?4n system
undergo final state interactions in singlet states, t?2n?
2n, and when the relative motion in the both pairs of
neutrons is described by delta functions for simplicity.
If we use a more realistic distribution for the relative
motion of twoneutrons in a singlet state, curve 2 becomes
less steep and more similar to curve 1. At last, curve 3
shows the most extreme case. It shows a calculation using
the two-body phase volume for t?4n, dN=dE7H/E1=2
This can be considered as a model for a process, when
each pair of neutrons is emitted in a singlet state and
when the relative motion of two neutrons in each pair is
described by delta function. Again, usage of more real-
istic distribution for the relative motion of twoneutronsin
singlet state makes curve 3 less steep and more similar to
curve 2. Also, curve 3 can be considered as a model for
the emission of a bound tetraneutron with a small binding
As seen in Fig. 4, the experimental spectrum increases
near the t ? 4n threshold even more sharply than the
most extreme curve 3. This provides a strong indication
on the possible existence of7H state near the t ? 4n
threshold. In this region
?10?2mbsr?1MeV?1. For further studies of7H, apart
d?8He;3He?7H, and t?8He;4He?7H seem to be promising.
In the same experiment, by detecting tritons by the
RIKEN telescope, we studied two-neutron transfer reac-
tions p?8He;t?6Heg:s:and p?8He;t?6He??2??. The produc-
tion of6Heg:s:was identified in coincidences t ?6He,
1:8 MeV in t ? ? coincidences. The measured cross
sections for population of the ground state and excited
2?state of6He are shown in Fig. 6.
7H. The detection efficiency, which is shown in the inset
7H. This can be considered as an extreme model for a
the cross section is
6He??2?? was observed as a peak at E??
30 20 10
to the empty-target background. Curves show nonresonant
continuums explained in the text.
Spectrum of7H after subtraction of the polynomial fit
20 100 -10 -20
target background. Inset in the lower part shows the detection
efficiency in arbitrary units.
Spectrum of7H after direct subtraction of the empty-
30 20100 -10-20 -30
solid histogram was obtained with the proton target. The
dashed histogram shows the empty-target background.
Spectrum of7H from the reaction p?8He;pp?7H. The
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VOLUME 90, NUMBER 8
If theground state of8He would contain subsystem6He
mainly in the state 0?, then the8He?p;t? reaction would
populate preferentially6Heg:s:?0??, whereas the cross sec-
tion for the population of6He??2?? wouldbe somewhat an
order of magnitude lower, because the latter would be
the second order process. It requires a two-neutron peak-
up from8He, as in p?8He;t?6Heg:s:, but in addition in
this case the6He?0?? subsystem should be excited to
the 2?state. As a result, the cross section of the
p?8He;t?6He??2?? reaction would be suppressed.
Experimental data in Fig. 6 is just the contrary: The
cross section of the p?8He;t?6He??2?? reaction is higher
than that of p?8He;t?6Heg:s:. It means that theground state
of8He contains subsystem6He in the state 2?with a large
Qualitatively it is consistent with a consideration of
8He in a simple model of Ref. , where a five-body (? ?
4n) wave function of8He was constructed with the four
antisymmetrized neutrons in the P3=2states relative to
the ? core. Under these assumptions, one can strictly
investigate the spin parity of the6He subsystem in8He.
The obtained weights for various J?in the6He subsystem
are the following: P?0?? ?1
P?3?? ? 0. That is, the excited 2?state of6He is pre-
dominant in the8He ground state wave function.
Now we can roughly estimate a ratio of spectro-
scopic factors for the reactions p?8He;t?6Heg:s: and
p?8He;t?6He??2??. Assuming that in the ?p;t? reaction a
dineutron cluster in a singlet state is transferred from8He
to proton, apart from the obtained weights of1
need to take into account squares of product of 9J symbol
and Talmi-Moshinski coefficient:
6He?0?? and6He?2??, respectively. At last, unlike the
ground state6He?0??, which contains valence neutrons
mainly in the P3=2states as it was found in microscopic
calculations (e.g., see ), in the excited state6He??2??
the P3=2configuration has a weight of 33% according to
microscopic calculations of Ref. . It gives for the
6He??2?? channel an additional factor of1
all mentioned coefficients, we obtain the ratio of the
spectroscopic factors close to 1.
We have performed calculations in the distorted-wave
Born approximation using several optical potentials taken
from literature. Values of the spectroscopic factors re-
quired for agreement with the experimental crosssections
depend on the choice of potentials, but the ratio of spec-
troscopic factors is rather stable. A typical result of the
calculations is shown by curves in Fig. 6. In this case, the
spectroscopic factors have been taken equal to each other.
The calculations are in satisfactory agreement with the
Further detailed theoretical analyses will require a8He
model going beyond the simple jj coupling, microscopic
calculations of the spectroscopic factors, and an inclusion
of two-step mechanism of the ?p;t? reaction.
Recently in Dubna (Russia), the ?p;t? reaction was in-
vestigated at lower energy of the8He beam, 26A MeV,
6, P?1?? ? 0, P?2?? ?5
and also higher cross section for the6He??2?? channel
than that for6Heg:s:was observed . Indications on the
6He?2?? ? 2n configuration in the ground state of8He
were previously obtained in Refs. [12,13].
In summary, we have performed the experimental
search for7H in the p?8He;pp?7H reaction. The evidence
for existence of the7H state near the t ? 4n decay thresh-
old was obtained. In the same experiment, the p?8He;t?
reaction populating the ground and excited 2?state of
6He was investigated. The obtained results argue on a
specific structure of the ground state of8He containing
subsystem6He in the state 2?with a large weight.
*On leave from the Kurchatov Institute, Kurchatov Square
1, 123182 Moscow, Russia.
 A. A. Korsheninnikov et al., Phys. Rev. Lett. 87, 092501
 G. M. Ter-Akopian et al., in Proceedings of the VII
School-Seminar on Heavy Ion Physics, 2002, Dubna,
Russia (World Scientific, Singapore, to be published).
 M.G. Gornov et al., LII Meeting on Nuclear Spec-
troscopy and Nuclear Structure, Books of Abstracts,
Russia (Moscow State University, Moscow, 2002), p. 70.
 L.V. Chulkov,VII School-Seminar on Heavy Ion Physics
 N.K. Timofeyuk, Phys. Rev. C 65, 064306 (2002).
 A. B. Volkov,Nucl. Phys. 74, 33 (1965).
 A. A. Korsheninnikov et al., Phys. Rev. Lett. 82, 3581
 M.V. Zhukov et al., Phys. Rev. C 50, R1 (1994).
 A. A. Korsheninnikov and T. Kobayashi, Nucl. Phys.
A567, 97 (1994).
 B.V. Danilin et al., Nucl. Phys. A632, 383 (1998).
 R. Wolski et al., in Proceedings of the International
Conference on Nuclear Physics at Border Lines, Lipari,
Italy, 2001 (World Scientific, Singapore, 2001), p. 368.
 R. Wolski et al., Nucl. Phys. A701, 29c (2002).
 K. Markenroth et al., Nucl. Phys. A679, 462 (2001).
Cross sections of the reactions8He?p;t?6Heg:s:and
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