Vol. 44 (2013) ACTA PHYSICA POLONICA B No 3

OBSERVATION OF GROUND-STATE TWO- DECAY∗

M. Thoennessena,b, Z. Kohleya, A. Spyroua,b, E. Lunderbergc P.A. DeYoungc, H. Attanayaked, T. Baumanna, D. Bazina B.A. Browna,b, G. Christiana,b, D. Divaratned, S.M. Grimesd A. Haagsmae, J.E. Fincke, N. Frankf , B. Lutherg, S. Mosbya,b T. Nagic, G.F. Peasleec, W.A. Petersh, A. Schillerd J.K. Smitha,b, J. Snydera,b, M. Strongmana,b, A. Volyai

aNSCL, Michigan State University, East Lansing, MI 48824, USA bDepartment of Physics and Astronomy, Michigan State University East Lansing, MI 48824, USA cDepartment of Physics, , Holland, MI 49423, USA dDepartment of Physics and Astronomy, Ohio University, Athens, OH 45701, USA eDepartment of Physics, Central Michigan Univ., Mt. Pleasant, MI 48859, USA f Department of Physics and Astronomy, Augustana College Rock Island, IL 61201, USA gDepartment of Physics, Concordia College, Moorhead, MN 56562, USA hDepartment of Physics and Astronomy, Rutgers University Piscataway, NJ 08854, USA iDepartment of Physics, Florida State University, Tallahasee, FL 32306, USA

(Received November 14, 2012)

Neutron decay spectroscopy has become a successful tool to explore nuclear properties of nuclei with the largest neutron-to-proton ratios. Res- onances in nuclei located beyond the neutron dripline are accessible by kinematic reconstruction of the decay products. The development of two- neutron detection capabilities of the Modular Neutron Array (MoNA) at NSCL has opened up the possibility to search for unbound nuclei which decay by the emission of two . Specifically, this exotic decay mode was observed in 16Be and 26O.

DOI:10.5506/APhysPolB.44.543 PACS numbers: 21.10.–k, 21.10.Dr, 25.60.–t, 29.30.Hs

∗ Presented at the Zakopane Conference on Nuclear Physics “Extremes of the Nuclear Landscape”, Zakopane, Poland, August 27–September 2, 2012.

(543) 544 M. Thoennessen et al.

1. Introduction Exploration of nuclear systems with the largest neutron excess requires the study of neutron-unbound states and nuclei. The first neutron-unbound nucleus was already discovered in 1937 by Williams, Shepperd, and Haxby who used the transfer reaction 7Li(d, α)5He to deduce the existence of 5He from the measured α-particle spectrum [1]. It took almost 30 years until the next unbound resonance was unambiguously identified with the deter- mination of the scattering length of the dineutron system in 1965 [2]. An overview of the discovery of light neutron-unbound nuclei can be found in Ref. [3]. Transfer reactions with stable beams were initially the best method to populate and study unbound states, although pion induced reactions were also an effective tool by utilizing the missing mass method. These reactions are limited to the lightest unbound nuclei, where the dripline is relatively close to the stable isotopes which have to be available as targets. In or- der to reach heavier unbound systems, new methods had to be developed. Figure1 gives an overview of the different methods which were first used to observe neutron unbound states from helium to fluorine. Nuclei where the unbound states were first observed with transfer reactions with stable or

17F 18F 19F 20F 21F 22F 23F 24F 25F 26F 27F 28F 29F 30F 31F

13O 14O 15O 16O 17O 18O 19O 20O 21O 22O 23O 24O 25O 26O 27O 28O

12N 13N 14N 15N 16N 17N 18N 19N 20N 21N 22N 23N 24N 25N

9C 10C 11C 12C 13C 14C 15C 16C 17C 18C 19C 20C 21C 22C

8B 9B 10B 11B 12B 13B 14B 15B 16B 17B 18B 19B 20B 21B

7Be 8Be 9Be 10Be 11Be 12Be 13Be 14Be 15Be 16Be

6Li 7Li 8Li 9Li 10Li 11Li 12Li 13Li Stable

3He 4He 5He 6He 7He 8He 9He 10He Transfer/Missing Mass

1H 2H 3H 4H 5H 6H 7H β-Delayed

1 n RI-Beams/Invariant Mass

Fig. 1. Section of the chart of nuclei. Stable nuclei are indicated by black squares and the solid thick/dark blue line denotes the neutron drip line. Note that the drip line is experimentally verified only up to oxygen and the status of heavier fluorine isotopes has not been determined in experiments. Nuclei for which neutron- unbound states have been measured are color coded, according to their detection technique: missing mass with transfer reactions (dark gray/red), β-delayed neutron decay (medium gray/blue) and invariant mass (light gray/yellow) measurements. The white squares beyond the drip line indicate isotopes which have been shown to be neutron unbound (adapted from Ref. [4]). Observation of Ground-state Two-neutron Decay 545 pion beams dominate the region close to the valley of stability and are shown in dark gray/red. Beta-delayed neutron emission was utilized to observe un- bound states in 18N, 21O, and 22O for the first time (medium gray/blue); this method was also used to study many of the nuclei closer to stability. Invariant mass measurements, shown in light gray/yellow, were necessary to populate even more neutron-rich nuclei. The diagonally shaded nuclei were explored with the missing mass as well as invariant mass method. The white squares beyond the dripline (solid thick line) show the isotopes which have been demonstrated to be unbound but no spectroscopy data has been mea- sured. Although not explicitly stated, 17Be and 23C are almost certain to be unbound because at least one heavier isotone for these isotopes has already been shown to be unbound. Similarly, 18Be is not expected to be bound because already 16Be is unbound. Thus, all bound isotopes up to oxygen have been observed. However, there are still several bound isotopes, where no unbound excited states have been observed as well as several unbound isotopes, where the resonance parameters are not yet known. The first nucleus demonstrated to be unbound with respect to two- neutron emission was 5H[5]. Subsequently, resonances in 10He [6] and 13Li [7] were reconstructed using invariant mass measurements with radioactive beams. In the present paper, we discuss results of recent two-neutron mea- surements of 16Be [8] and 26O[9].

2. 16Be 16Be was predicted to be bound with respect to one-neutron emission but unbound with respect to two-neutron emission, and is thus an ideal case to search for correlated two-neutron or dineutron-like decay. In the previous cases (5H and 10He), the intermediate unbound systems (4H and 9He, respectively) have broad resonances, which can extend below the single- neutron emission threshold and thus favor sequential decay. The situation is predicted to be different as shown in Fig.2. The energies levels of 14Be, 15Be and 16Be calculated with the shell model in the s–p–sd–pf model space using the WBP interaction [12] indicate that the only open decay path is the direct emission of two neutrons. This is supported by the non-observation of 14Be in the recent two-proton removal reaction from 17C which determined the lower limit of the 15Be ground state to be at the energy of the first excited state of 14Be [11]. The decay of 16Be was measured following the one-proton removal re- action from 17B at the Coupled Cyclotron Facility at NSCL/MSU. Neu- trons were measured with MoNA in coincidence with charged fragments. The three-body decay energy as well as the neutron–neutron-14Be corre- lations were measured [8]. The neutron interactions were simulated with 546 M. Thoennessen et al.

GEANT4 [14] using the physics class MENATE_R [15] in order to distin- guish true two-neutron events from a single neutron interacting twice in the detector array [16].

4.0

3.5 + 1/2 3.0 + + + 2 2.5 3/2, 5/2 2.0 2+ 1.5

Energy (MeV) Energy 1.0 0+ 0.5

+ 0.0 0 14Be+2n 15Be+n 16Be Fig. 2. Level and decay scheme for neutron-rich isotopes of beryllium. The solid Figure 5. Energy and decay level diagram for 16Be. The solid blue lines represent previous light gray/blue lines represent previous experimental observations of the first ex- experimental observations of the first excited state in 14Be [36] and a lower limit on the position cited state in 14Be [10] and a lower limit on the position of the 15Be ground of the 15Be ground state [37]. The 16Be ground state measurement from the current work is state [11]. The dashed black lines represent shell model calculations within the shown as the solid red line. The dashed black lines represent shell model calculations using the s–p–sd–pf model space using the WBP interaction [12]. The recently measured WBP interaction [35]. 16Be ground state [8] is shown as the solid dark gray/red line (adapted from Ref. [13]).

component estimated by the Monte Carlo simulation. The false 2n events consist of a significant portion of the spectrum, yet the 26O ground50 state resonance40 appears to consist of mostly true 20 (a) (b) (c) 2n events. Removal or reduction of the false 2n events can provide additional evidence for the 40 30 Data presence of the15 ground state resonance. Two methods were used Seq foruen this:tial (1) the simulated

nts 30 false 2n events were subtracted from the experimental 3-body spectrum, Phase-space Fig. 3(c), and (2) the

20 ou 10 Dineutron

causality cuts wereC applied to the data,20 Fig. 3(d). After subtracting the simulated false 2n events, the spectrum5 contains only the10 ground state resonance10 and a background level of counts at higher decay energies. Applying causality cuts of D12 > 25 cm and V12 > 7 cm/ns to both the simulated and0 experimental data shows0 a reduction in the0 false 2n events by a factor of 3 0 1 2 3 4 5 0.0 0.5 1.0 1.5 2.0 -1.0 -0.5 0.0 0.5 1.0 with the ground state resonance observed well above the false 2n component. The persistence of E (MeV) E (MeV) cos(q ) the low decay energy peakdecay after the removaln+ andn reduction of the false 2nnn events demonstrates 26 strong evidenceFig. 3. for (a) the Three-body observation decay of energy the fromO ground the reconstruction state resonance. of 14Be + n + n, (b) two-body relative energy of the two neutrons, (c) opening angle θn−n between 5. 16Be the two neutrons in the center-of-mass frame of 16Be. The experimental data are Shell modelshown calculations, in black triangles, as shown sequential in Fig. 5, emission have predicted in dashedthat lines,16 simultaneousBe is bound phase- with respect to one-neutronspace decay emission and unbound in dot-dashed with lines respect and dineutron to two-neutron decay in decay solid [35].lines (adapted This provides from a unique scenario inRef. which [8]). the 16Be should decay through the simultaneous emission of two neutrons. Experimental measurements (blue solid line) of the first excited state in 14Be [36] and a lower- limit on the 15Be ground state [37] both indicate that the shell model calculations appear to be reasonable, which suggests that the 16Be ground state energy should be around 1.0 MeV. Based on the scenario presented in Fig. 5, three modes of decay for the 16Be can be envisioned. The two-neutron emission could proceed via sequential decay through the tail of the broad 1/2+ s-state predicted by the shell model calculations. The two-neutrons could also be emitted simultaneously in an uncorrelated phase-space decay or as a correlated pair in a dineutron-like decay. Examination of the correlations in the 3-body decay can provide insight into the different decay modes. The 16Be ground state was observed through the coincidence measurement of the 14Be + n + n system. The 3-body decay energy spectrum is shown in Fig. 6(a) with causality cuts of D12 > 50 cm and V12 > 11 cm/ns applied. A clear peak is observed in the 3-body decay Observation of Ground-state Two-neutron Decay 547

Figure3 shows the three-body decay energy from the reconstruction of (a) 14Be + n + n, (b) the two-body relative energy of the two neutrons, (c) the opening angle θn−n between the two neutrons in the center-of-mass frame of 16Be. Simulations corresponding to three different decay modes were performed. Sequential emission (dashed lines) and simultaneous phase- space emission (dot-dashed lines) cannot reproduce the two-neutron relative energy nor the two-neutron opening angle. Only the dineutron decay sim- ulation can reproduce all three spectra. It should be mentioned that the dineutron decay as simulated in a two-body model is only an approximation and full correlated three-body model calculations are necessary in order to describe the decay of 16Be more realistically.

3. 26O Another example of a possible dineutron emitter is 26O. The predictions for the two-neutron separation energies show significant differences ranging from 5 MeV bound to 3 MeV unbound (see Fig.4). Several experimental searches for bound 26O were unsuccessful [46–49]. The definite proof that 26O is indeed unbound was established in a recent invariant mass measure- ment at NSCL/MSU. The setup was similar to the previously discussed 16Be experiment. 26O was produced with the one-proton removal reaction from a secondary 27F beam. The three-body decay energy spectrum shown in Fig.5 shows a clear peak near threshold. The extracted decay energy was +50 150−150 keV [9]. The observed width was dominated by the experimental resolution so that the expected very narrow width of the ground state (see discussion below) could not be determined. The statistics were not sufficient to extract any correlations and thus the details of the decay could not be determined in this measurement. How- ever, it is an even stronger candidate for a dineutron-like emission than 16Be, because the ground-state of the intermediate unbound system of 25O is unbound by 0.77(2) MeV [50] and thus is located about 600 keV above the 26O ground-state (see Fig.6). The figure also shows neutron-unbound states in the last two bound neutron-rich oxygen isotopes 23O and 24O. With the exception of the second excited state of 23O[51] and the negative parity state in 24O[52] all measurements were performed with MoNA at NSCL/MSU [9, 50, 53–55]. The energies of the first two excited states of 24O were recently confirmed by the recent RIKEN measurement [52]. The low decay energy of 26O very close to the two-neutron separation energy gives rise to speculations that 26O even represent a case for two- neutron radioactivity with a potentially fairly long lifetime. Grigorenko et al. [56] have calculated the decay widths and half-lives for the two-neutron emission of 26O for different angular momenta of the neutrons (see Fig.7). 548 M. Thoennessen et al.

5.0 AMD HFB+THO SM BHF HSFM SSM CSM MCSM CCM HF RCHB 2.5 HFB RMF

0.0 (MeV) 2n S

-2.5

-5.0

[23] [23] [23] [23] [39] [39] [39] [39] [39]

[16] [16] [26] [26] [31] [31] [32] [44]

A A A A A A

A. [18]

olya - PRL

uan - PRC [43] uan - PRC [43]

Y Y

Nakada - PRC [36]

. Mod. Phys. E [33] . Mod. Phys. E [33] . Mod. Phys. E [33]

2005: V

2012: 2012:

2008: Zhou - PRC [35] 2008: Zhou - PRC [35] 2002: Meng - PLB [27]

2009: Masui - EPJ [37] 2009: Masui - EPJ [37]

2012: Hagen - PRL

2005: Brown - PRC [30] 2005: Brown - PRC [30] 2006: Brown - PRC [34] 2006: Brown - PRC [34]

1987: Blumel - NP 1987: Blumel - NP

2010: Otsuka - PRL 2010: Otsuka - PRL 2010: Otsuka - PRL 2010: Otsuka - PRL 2010: Otsuka - PRL

2006: Nakada -NP

1997: Kruppa - PRL 1997: Kruppa - PRL 1997: Kruppa - PRL 1997: Kruppa - PRL

Thiamova - Czech J [29]

2002: Nakada - NP 2002: Nakada - NP 2006: Nakada - NP

2008: Nakada - PRC [36] 2008:

2003: Stoitsov - PRC [28]

imofeyuk - J. Phys. G. [22] imofeyuk - J. Phys. G. [22] 1: Shukla - J. Phys. G [40]

1996: Grummer - PLB [20] 1996: Grummer - PLB [20]

1999: Siiskonen - PRC [25] 1999: Siiskonen - PRC [25]

T T

1995: Ren - PRC Rapid [19]

1: Coraggio - J. Phys. G [41] 1: Coraggio - J. Phys. G [41]

201

2004:

1987: Sagawa - J. Phys. G. [17]

201 201

1997: Ormand - PRC Rapid [24] 1997: Ormand - PRC Rapid [24]

1993: Poppelier - Z. Phys.

1: Bhagwat - J. Mod. Phys. E [42] 1997: Prochniak - J. Phys. G. [21]

1999: 1999:

2009: Satula - J. Mod. Phys. E [38] 2009: Satula - J. Mod. Phys. E [38]

2006: Gridnev - J. Mod. Phys. E [33] 2006: Gridnev - J 2006: Gridnev - J 2006: Gridnev - J

201

26 Fig. 4. Predictions for the two-neutron separation energy S2n for O from various theoretical models.

Fig. 5. Decay energy spectrum for 26O (adapted from [9]). Observation of Ground-state Two-neutron Decay 549

3/2+ 4.0 1.3 + + ~0.6 (4 ) ~7.5 + 5/2 2.8(1) 0 ( ̶ ) ~7.3 0.045(4) ~3.2 1+ 5.33(12) 22 1.24(7) O 2+ 4.72(11) 1/2+ 0.63(4) 23O + 0.77(2) 3/2 0.77(2) 0+ 0+ ~0.15 24O 25O ~0.15 26O Fig. 6. Unbound levels in neutron-rich oxygen isotopes near and beyond the neutron dripline. All energies are in MeV and the decay energies are from [9, 50–55].

The valence neutrons in 26O are most likely in a [d2] configuration. A lower limit of 10−14 s can be extracted from Fig.7 for the d2 configuration using the upper limit of the decay energy from the present experiment (200 keV). The unsuccessful searches for 26O using fragment separators yield an upper limit of the half-life of about 200 ns. Thus, the possible range of half-lives 26 −7 −14 for O is still about 7 orders of magnitude with 2×10 s < T1/2 < 10 s.

-21 -1 10 2 6 [d ] ~E 10 -18 10 10-4 -15 10 10-7 -12 -10 (s)

10

10 2 / 1

(MeV) -9

-13 10 2 4 T G [p ] ~E 10 -6 10 10-16 -3 10 [s2] ~E2 10-19

10-3 10-2 10-1 100

Edecay (MeV) Fig. 7. Calculated widths and half-lives as a function of decay energy for two- neutron emission in different orbital configurations (adapted from Ref. [56]).

4. Conclusion and outlook Neutron decay spectroscopy with radioactive beams is an effective tool to explore nuclei at and beyond the neutron dripline. The two-neutron emission of 16Be corresponds to the first observation of a dineutron-like ground-state 550 M. Thoennessen et al. decay and the coincidence measurement of two-neutrons with 24O following the one-proton removal reaction from 27F determines unambiguously that 26O is unbound with respect to two-neutron emission. The MoNA Collaboration has recently commissioned an additional neu- tron detector array. LISA, the Large multi-Institutional Scintillator Array, was designed, built, and installed at the NSCL by a collaboration of un- dergraduate institutions [57]. The addition of LISA improves the efficiency, acceptance, and resolution for one- and especially two-neutron experiments. The data of the first experiment, where the decay of unbound excited states of 24O was measured with significantly improved resolution, are currently under analysis.

Fig. 8.Figure Typical 2: Typical layout layout of ofthe the MoNA-LISA MoNA-LISA in in at theat theNSCL. NSCL. The beam The enters beam the enters vault the vaultfrom from the the bottom bottom right. right (figure from [57]). Nuclear structure measurements with the MoNA-LISA Thisand material the is Sweeper based upon work supported by the NSF under Grants PHY- 02-44953, PHY-06-06007, PHY-07-57678, PHY-08-55456, PHY-09-69058, To create neutron-unbound states of oxygen, for example, as was previously done by the PHY-10-68217,MoNA Collaboration and PHY-11-02511,[7], a beam of 26F from and the NSCL the DOE A1900 spectrometer Awards DE-NA0000979 was incident and DE-FG02-92ER40750.on a 9Be target to create neutron-unbound excited states of 24O as well as the ground state of 25O. All charged fragments (including non-reacted beam) were deflected with a rigidity up to 4 T-m through the Sweeper [8] [9] [10] [11]. Neutrons are not deflected and a gap in the Sweeper magnetREFERENCES allows transmission of these neutrons; the MoNA and the LISA are therefore at zero degrees relative to the 26F beam. The MoNA and now [1] J.H.the LISA Williams, record both W.G. the Shepherd, neutron’s position R.O. and Haxby, time signatuPhys.re Rev. (with51 multiple-hit, 888 (1937) capa-. bility). The neutron energy is determined based on its time-of-flight (TOF). The TOF [2] R.P.spectra Haddock and the positionet al., ofPhys. the neutron Rev. Lett. yield14 the, neutron’s 318 (1965) momentum. vector [1]. The [3] M.LISA Thoennessen, was built and benchmarkdAt. Data in Nucl. 2010 andData installed Tables in98 the, N2 43 vault (2012) of the. NSCL in the spring of 2011. Like the MoNA, the LISA consists of 144 rectangular plastic scintillators, each 2 m x 10 cm x 10 cm. A typical configuration of the MoNA, the LISA and the Sweeper with necessary charged particle detectors is shown in Figure 2.

All isotopes are identified by a set of charged-particle detectors that determine each frag- ment’s energy loss, position, and total kinetic energy. Position information comes from

3 Observation of Ground-state Two-neutron Decay 551

[4] T. Baumann, A. Spyrou, M. Thoennessen, Rep. Prog. Phys. 75, 036301 (2012). [5] M.G. Gornov et al., JETP Lett. 45, 252 (1987). [6] A.A. Korsheninnikov et al., Phys. Lett. B326, 31 (1994). [7] Y. Aksyutina et al., Phys. Lett. B666, 430 (2008). [8] A. Spyrou et al., Phys. Rev. Lett. 108, 102501 (2012). [9] E. Lunderberg et al., Phys. Rev. Lett. 108, 142503 (2012). [10] T. Sugimoto et al., Phys. Lett. B654, 160 (2007). [11] A. Spyrou et al., Phys. Rev. C84, 044309 (2011). [12] E.K. Warburton, B.A. Brown, Phys. Rev. C46, 923 (1992). [13] Z. Kohley et al., arXiv:1208.2969 [nucl-ex]. [14] S. Agostinelli et al., Nucl. Instrum. Methods A506, 250 (2003). [15] P. Désesquelles et al., Nucl. Instrum. Methods A307, 366 (1991) adapted for GEANT4 by B.T. Roeder, 2008. [16] Z. Kohley et al., Nucl. Instrum. Methods A682, 59 (2012). [17] R. Blumel, K. Dietrich, Nucl. Phys. A471, 453 (1987). [18] H. Sagawa, H. Toki, J. Phys. G 13, 453 (1987). [19] N.A.F.M. Poppelier, A.A. Wolters, P.W.M. Glaudemans, Z. Phys. A346, 11 (1993). [20] Z. Ren et al., Phys. Rev. C52, R20 (1995). [21] F. Grummer et al., Phys. Lett. B387, 673 (1996). [22] L. Prochniak, S. Szpikowski, W. Berej, J. Phys. G 23, 705 (1997). [23] N.K. Timofeyuk, P. Descouvemont, I.J. Thompson, J. Phys. G 25, 933 (1999). [24] A.T. Kruppa et al., Phys. Rev. Lett. 79, 2217 (1997). [25] W.E. Ormand, Phys. Rev. C56, R1678 (1997). [26] T. Siiskonen, P.O. Lipas, J. Rikovska, Phys. Rev. C60, 034312 (1999). [27] H. Nakada, M. Sato, Nucl. Phys. A699, 511 (2002)[ Erratum ibid. 714, 696 (2003)]. [28] J. Meng, S.-G. Zhou, I. Tanihata, Phys. Lett. B532, 209 (2002). [29] M.V. Stoitsov et al., Phys. Rev. C68, 054312 (2003). [30] G. Thiamova, N. Itagaki, T. Otsuka, Czech. J. Phys. 54, 329 (2004). [31] B.A. Brown, W.A. Richter, Phys. Rev. C72, 057301 (2005). [32] H. Nakada, Nucl. Phys. A764, 117 (2006)[ Erratum ibid. 801, 169 (2008)]. [33] A. Volya, V. Zelevinsky, Phys. Rev. Lett. 94, 052501 (2005). [34] K.A. Gridnev et al., Int. J. Mod. Phys. E15, 673 (2006). [35] B.A. Brown, W.A. Richter, Phys. Rev. C74, 034315 (2006). [36] X.-R. Zhou et al., Phys. Rev. C78, 054306 (2008). [37] H. Nakada, Phys. Rev. C78, 054301 (2008). 552 M. Thoennessen et al.

[38] H. Masui, K. Kato, K. Ikeda, Eur. Phys. J. A42, 535 (2009). [39] W. Satula et al., Int. J. Mod. Phys. E18, 808 (2009). [40] T. Otsuka et al., Phys. Rev. Lett. 105, 032501 (2010). [41] A. Shukla, S. Aberg, S.K. Patra, J. Phys. G 38, 095103 (2011). [42] L. Coraggio et al., J. Phys: Conf. Ser. 312, 092021 (2011). [43] A. Bhagwat, Y.K. Gambhir, Int. J. Mod. Phys. E20, 1663 (2011). [44] C. Yuan et al., Phys. Rev. C85, 064324 (2012). [45] G. Hagen et al., Phys. Rev. Lett. 108, 242501 (2012). [46] D. Guillemaud-Mueller et al., Phys. Rev. C41, 937 (1990). [47] M. Fauerbach et al., Phys. Rev. C53, 647 (1996). [48] O. Tarasov et al., Phys. Lett. B409, 64 (1997). [49] A. Schiller et al., Phys. Rev. C72, 037601 (2005). [50] C.R. Hoffman et al., Phys. Rev. Lett. 100, 152501 (2008). [51] Z. Elekes et al., Phys. Rev. Lett. 98, 102502 (2007). [52] K. Tshoo et al., Phys. Rev. Lett. 109, 022501 (2012). [53] A. Schiller et al., Phys. Rev. Lett. 99, 112501 (2007). [54] C.R. Hoffman et al., Phys. Lett. B672, 17 (2009). [55] C.R. Hoffman et al., Phys. Rev. C83, 031303 (2011). [56] L.V. Grigorenko et al., Phys. Rev. C84, 021303 (2011). [57] S.L. Stephenson et al., Proc. of the 19th Int. Sem. on Interaction of Neutrons with Nuclei: Fundamental Interactions & Neutrons, Nuclear Structure, Ultracold Neutrons, Related Topics, JINR, Dubna 2011.