Table Of ContentFirst observation of 13Li ground state
Z. Kohley,1,∗ E. Lunderberg,2 P. A. DeYoung,2 A. Volya,3 T. Baumann,1 D. Bazin,1 G.
Christian,1,4,† N. L. Cooper,5 N. Frank,6 A. Gade,1,7 C. Hall,8 J. Hinnefeld,5 B. Luther,9 S.
Mosby,1,4,‡ W. A. Peters,1,4,§ J. K. Smith,1,4 J. Snyder,1,4 A. Spyrou,1,4 and M. Thoennessen1,4
1National Superconducting Cyclotron Laboratory, Michigan State University, East Lansing, Michigan 48824, USA
2Department of Physics, Hope College, Holland, Michigan 49422, USA
3Department of Physics, Florida State University, Tallahassee, Florida 32306, USA
4Department of Physics & Astronomy, Michigan State University, East Lansing, Michigan 48824, USA
5Department of Physics and Astronomy, Indiana University at South Bend, South Bend, Indiana 46634, USA
6Department of Physics and Astronomy, Augustana College, Rock Island, Illinois 61201, USA
7Department of Physics & Astronomy, Michigan State University, East Lansing, Michigan 48824, USA
8Department of Physics, Hope College, Holland, Michigan 49422-9000, USA
3 9Department of Physics, Concordia College, Moorhead, Minnesota 56562, USA
1
(Dated: January 21, 2013)
0
2 Thegroundstateofneutron-richunbound13Liwasobservedforthefirsttimeinaone-protonre-
movalreactionfrom14Beatabeamenergyof53.6MeV/u. The13Ligroundstatewasreconstructed
n
from 11Li and two neutrons giving a resonance energy of 120+60 keV. All events involving single
a −80
J and doubleneutron interactions in theModular Neutron Array(MoNA) were analyzed, simulated,
and fitted self-consistently. The three-body (11Li+n+n) correlations within Jacobi coordinates
8
showed strong dineutron characteristics. The decay energy spectrum of the intermediate 12Li sys-
1
tem (11Li+n) was described with an s-wave scattering length of greater than −4 fm, which is a
smaller absolute value than reported in a previous measurement.
]
x
e PACSnumbers: 21.10.Dr,25.60.-t,29.30.Hs
-
l
c
u The increasing availability of rare isotope beams has emission of an s-wave neutron with a scattering length
n made it possible to extend nuclear structure measure- of−13.7(1.6)fm. We repeatedthis protonremovalreac-
[ ments tonucleifar awayfromstability. Inlightneutron- tionata14Bebeamenergyof53.6MeV/uonaberyllium
1 rich nuclei these studies have been extended beyond the target. In addition to measuring a new low-lying reso-
v neutrondripline [1]. The existence of two-neutronhalos, nance in 13Li at 120+60 keV, which is presumably the
−80
0 first observed in 11Li [2, 3], sparked the interest in un- ground state, we extract a limit on the scattering length
1 bound two-neutron systems where measurements of the of a >−4 fm for 12Li.
s
4
decay products could yield information about neutron-
4 The experiment was performed at the Coupled Cy-
neutroncorrelationsinsidethenucleus. Recentexamples
. clotron Facility of the National Superconducting Cy-
1 include 10He [4, 5], 13Li [5, 6], 16Be [7], and 26O [8]. In
clotron Laboratory (NSCL) at Michigan State Univer-
130 1w6aBseotbhseerfivresdtbevecidaeunsceesfeoqrugenrotiuanlddesctaatyevdiaine15uBtreonwadseceany- saity4.113Amg1/2c0mM2 eBVe/upro18dOuctpiornimtaarrygebteatompbroodmubcaerdthede
: ergetically not possible [7]. 53.6 MeV/u 14Be secondary beam. The A1900 fragment
v Inthis article,wereportona newmeasurementofthe separator,with a 1050mg/cm2 Al achromaticwedgede-
Xi 13Lidecayproducts. Veryfewcalculationsexistthatpre- grader, was used to separate the 14Be secondary beam
dict the energy levels of 13Li. Over 25 years ago, a shell
r from other reaction products and the primary beam.
a model calculation predicted the ground state of 13Li to The rate of the 14Be secondary beam was about 500 pps
haveaspinandparityof 3− locatedabout3MeVabove at the reaction target and had a momentum spread of
2
the11Li+2nthreshold[9]. In2008Aksyutinaetal.[6]re- 2.5%. Contaminant nuclei in the secondary beam were
portedaresonanceatadecayenergyof1.47(31)MeVin excludedfromanalysisbasedonevent-by-eventmeasure-
aprotonremovalreactionfrom14Beat304MeV/uwitha ment of the time-of-flight from plastic scintillator tim-
liquidhydrogentarget. Inthesamereactiontheunbound ing detectors located at the A1900 focal plane and just
intermediate nucleus 12Li was observed to decay via the before the reaction target. The 14Be beam impinged
on a 477 mg/cm2 Be reaction target and produced 13Li
through one-proton removal. The two-neutron unbound
13Li decayed immediately into 11Li and two neutrons.
∗ [email protected] The 11Li fragments were deflected by the Sweeper su-
† Present address: TRIUMF, 4004 Wesbrook Mall, Vancouver,
perconducting dipole magnet[10] thatwas set to a mag-
BritishColumbiaV6T2A3,Canada
‡ Presentaddress: LosAlamosNationalLaboratory, LosAlamos, netic rigidity of 3.735 Tm. The detection setup was
NewMexico87545, USA identical to the experiment by Hall et al. [11], which
§ PresentAddress: OakRidgeAssociatedUniversities,OakRidge, measuredexcited states in 12Li following the two-proton
TN37830,USA removal from 14B. The deflected nuclei passed through
2
twox-ypositionsensitivecathode-readoutdriftchambers 250 11Li + 2n (a)
(CRDCs), separated by 1.816 m, a 5 mm thick plastic
scintillator, and were stopped in a 150 mm thick plastic 200
s
scintillator. Aninverseion-opticalmatrixforraytracing nt Exp.
150
was created with the program cosy infinity [12] using u Sum
o
the measured magnetic field map of the Sweeper to pro- C100 13Li
vide tracking of the 11Li trajectories back to the target 12Li
50
position.
The neutrons produced from the 13Li breakup were 0
11Li + 2n 1.0 (b)
detected using the Modular Neutron Array(MoNA) [13, 60
Causality Cuts
14]. Theindividualplasticscintillatorbarswerearranged 0.5
50
inninewalls,each16barshigh,resultinginanactivearea s
2 m wide by 1.6 m tall. The distance from the target to nt 40 00 1 2 3
u E (MeV)
the center of the first layer was 8.44 m. o 30 decay
C
Thethree-bodydecayenergyof13Licanbe calculated 20
withEDecay =M13Li−M11Li−2Mn,whereM13Li (M11Li) 10
is the mass of 13Li (11Li) and M is the neutron mass.
n 0
The invariant mass, M13Li, was calculated from the ex- 0 0.5 1 1.5 2 2.5 3 3.5
perimentallymeasuredfour-momentaofthe11Liandtwo EDecay(MeV)
neutrons. It should be noted that all experimental de-
cayenergyspectraarepresentedwithoutunfoldingofthe FIG. 1. (color online) Measured three-body decay energy
resolution or acceptance. spectrum of 11Li+2n without (a) and with (b)causality cuts
The reconstruction of the 13Li decay energy relies asdescribedinthetext. Theexperimentaldataisfitwithtwo
components: (1) the dineutron decay of 13Li→11Li+2n (red
on the correct identification of two neutrons interacting long-dashed line) and (2) the 12Li→11Li+n decay (blue dot-
in MoNA. It is critical to distinguish real two-neutron dashed line) The input 13Li resonance distribution is shown
eventsfromeventswhereasingleneutronre-scattersand in the insert of panel (b).
interacts twice in MoNA. It has been shownthat causal-
ity cuts appliedto the distance andthe velocitybetween
twosuccessiveinteractionscaneffectivelyenhancethese-
lection of real two-neutron events [7, 8, 15, 16]. Fig. 1 total inelastic reaction cross sections for neutrons above
shows the reconstructed three-body decay energy spec- 20 MeV, menate r uses cross sections for the differ-
trum of 13Li (11Li+2n) without (a) and with (b) the ent reaction channels, including (n,np), (n,p), (n,nγ),
causalitycuts applied. The cuts require that twosucces- and (n,α). It has recently been shown that the use of
sive interactions be separated by more than 50 cm and menate r is important for correctly simulating the re-
thatthetimebetweenthefirstandsecondinteractionbe sponseofplasticscintillatordetectors[17]. Distortionsof
shorterthanthetimeitwouldtakeaneutronatbeamve- theexperimentaldecayenergyspectraarepresentdueto
locity to travelbetween the respective interactionpoints the efficiency and acceptance of the detector setup [see
(see Ref. [17] and references therein for additional infor- the inserts of Figs. 1(b) and 2(a)]. The accuracy of the
mation on 2n cuts). The data exhibit a resonance-like Monte Carlo simulation was estimated through varying
structure below 500 keV, which was not observed in the input parameters, such as the momentum distribution
previous measurement of 13Li [6]. However, the earlier of the residual 11Li fragment, and examining how this
experiment had zero efficiency for the detection of two- changed the efficiency curves. The results showed that
neutroneventsbelow200keV[18],whichwasalsoshown largevariationsoftheinputparametersresultedinmini-
to have significant effects on the reconstructed shape of malchanges(less than 5%) to the shape ofthe efficiency
the decay energy spectrum of the excited 11Li [15]. curves.
In order to analyze the observed peak for the pres- In addition to the three-body decay energy (13Li), the
ence of a resonance, detailed Monte Carlo simulations simulations had to consistently describe other observ-
were performed [17]. The simulations included the in- ables, such as the decay of 12Li, which could be directly
coming beam characteristics, the reaction mechanism to populated from pn-stripping, and the overall multiplic-
populate specific states, and their subsequent decay as ity distribution of interactions in MoNA. Limiting the
wellasthedetectorresolutionsandefficiencies. Theone- data to multiplicity = 1 events should enhance the con-
proton and pn-removal reactions were simulated using tributionsofthedirect12Lipopulation. Fig.2showsthe
the Glauber reaction model. As mentioned earlier the overall two-body (11Li+n) decay energy spectrum (a),
detailed response of MoNA to the interactions of multi- the multiplicity = 1 gated 11Li+n decay energy spec-
ple neutrons is especially important and was simulated trum (b), as well as the multiplicity distribution (c) for
with Geant4 [19, 20] using the custom neutron interac- eventsin coincidencewith 11Li. The two-bodydecayen-
tion model menate r [21]. In contrast to the standard ergy was calculated as EDecay = M12Li −M11Li − Mn,
intranuclearcascademodelsinGeant4,whichusesonly where the invariant 12Li mass was calculated from the
3
11Li + n cy00..68 (a) 200 11Li + n 1080000 (b) (c)
400 Efficien00..42 150 Mult = 1 2c246000000 103
nts300 00 0.5E1Decay1(.5Me2V)2.5 3 0s-1c2at-t1e0rin-g8 le-n6gth-4 (fm-2) 102
u 100
o200 Exp.
C
10
Sum
100 13Li 50
12Li 1
0 0
0 0.5 1 1.5 2 2.5 3 3.5 0 0.5 1 1.5 2 2.5 3 3.5 0 2 4 6 8 10 12 14
E (MeV) E (MeV) Multiplicity
Decay Decay
FIG. 2. (color online) Measured decay energy spectrum of 11Li+n including all multiplicities (a) and with a multiplicity = 1
gate (b). Panel (c) displays the overall multiplicity distribution for all events in coincidence with a 11Li fragment. The insert
inpanel(a) showstheefficiency fordetectingoneneutronfrom a1ndecay(solid line)andtwoneutronsfrom a2ndecaywith
(dashed line) and without (long-dashed line) thecausality cuts. The2n decay was simulated with each neutron beingemitted
with50%ofthetotaldecayenergy. Theχ2 asafunctionofthe12Lis-wavescatteringlengthisshownintheinsertofpanel(b).
detected 11Li fragment and neutron. Y system are particularly sensitive to the decay mecha-
TheMonteCarlosimulationincludedtwocomponents: nisms,three-bodyphasespaceversusdineutron. IntheT
(1) the decay of 13Li from the one-proton removal reac- system Ex/ET represents the energy of the n-n system
tion and (2) the decay of 12Li from pn-stripping. The relative to the total three-body energy. In the Y sys-
blacksolidlinesinFigs.1and2representthesumofthe tem θk is the angle between the frag+n center-of-mass
13Li and 12Li contributions to the corresponding spec- and the additional neutron. The difference between the
tra. As shown in Fig. 1(a), a component of the decay three-body phase space anddineutrondecay simulations
of 12Li→11Li+n (blue dot-dashed line) is present in the canbe clearlyseen in the top-left andbottom-rightpan-
three-body decay energy spectrum from events in which elsofFig.3. Whilethephasespaceemission(reddashed
the single neutron interacted twice in MoNA and thus line) does not fit the experimental data, the dineutron
addedfalsetwoneutroneventstothe13Lispectrum. Ap- decay (green solid line) agrees well with the data show-
plying the causality cuts to Fig. 1(a) should greatly re- ing a strongly correlated n−n emission. This presents
ducethepresenceoffalsetwoneutroneventsinthethree- the second case (the first being 16Be [7]) in which a
body decay spectrum. The effectiveness of the causality ground state dineutron-like emission has been observed.
cuts can be seen from the near absence of the 12Li com- A strong dineutron component (∼ 50%) in the decay of
ponent in Fig. 1(b). Similarly, in the multiplicity = 1 13LiforEDecay >3MeVwasalsoreportedbyJohansson
two-bodydecay spectrum, Fig.2(b), a componentofthe et al. [5].
13Li→11Li+2n decay (red dashed line) is present from Since the three-body correlations indicate that the
events in which only one neutron is detected. dominatedecaychannelof13Liistheemissionofadineu-
The correlations of the three-body 11Li+n+n system tron(or twostronglycorrelatedneutrons), the two-body
can provide additional insight into the structure and de- andthree-bodydecayenergyspectraneedtobeanalyzed
cay of 13Li. A complete description of the three-body accordingly. Thus,thedecayof13Liwassimulatedasthe
correlations can be obtained from the relative energy emission of a dineutron, while the 12Li decay was simu-
(E /E ) andthe angle (θ )calculatedwithin the T and lated through the emission of a s-wave neutron with an
x T k
Y Jacobi systems. The relative energy is defined as the associated scattering length.
energy of the two-body system (frag+n or n+n), E , The dineutron decay was simulated as a two-steppro-
x
relative to the total three-body energy, E . A detailed cessfollowingtheformulismofRef.[28]wherethedineu-
descriptionand illustration ofthe JacobicToordinatesys- tronwasemittedwithenergyEy (intheT,“cluster”sys-
tems can be found in Refs. [22–25]. In Fig. 3 the rel- tem)fromthe13Lifollowedbythedecayofthedineutron
ative energy and angle for both the T and Y systems with energy Ex (13Li →11Li+2n→11Li+n+n). The
are shown from the experimental data with the causal- totalamplitude forthe two-stepdineutrondecayprocess
ity cuts applied and compared to a three-body phase is constructed from the one-body decay amplitudes as a
space and dineutron simulated emission. The three- second order process [29]:
body phase space distribution was calculated according
A (E )A (E )
1 y 2 x
tfiot Rtheefs.th[2re6e,-b27o]dywidthecaayBerneeitrgWy.igFnuerrthlienresdheatpaeilsusoefdthtoe A(Ey,Ex)= Ex−(cid:0)EV − 2iΓV(Ex)(cid:1), (1)
dineutrondecaycalculationareprovidedlaterinthetext. hereA (E )andA (E )representthe amplitudes ofthe
1 y 2 x
The relative energy in the T system and angle in the dineutron emission with kinetic energy E and the sub-
y
4
T Y 11Li + n
80 70
s 60
nt60 Exp.
u s 50 Sum
o40 nt 210 keV BW
C u 40 525 keV BW
20 Co 30 -3.8 fm s-wave
0
0 0.2 0.4 0.6 0.8 0 0.2 0.4 0.6 0.8 1 20
E/E E/E
x T x T 10
T Y
0
80 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8
E (MeV)
s Decay
nt60
u
o40 FIG. 4. (color online) Measured decay energy spectrum of
C Exp. 12Li populated in thetwo-proton removal reaction from 14B.
Phase-space
20 Three components are shown to fit the data: two Breit-
Dineutron
Wigner resonances (BW) and a −3.8 fm scattering length
0
-1 -0.5 cos0(q) 0.5 -1 -0.5 cos0(q) 0.5 1 s-wave, corresponding to the extracted as > −4 fm limit
k k from the current work. Experimental data was taken from
Ref. [11].
FIG.3. (coloronline)Therelativeenergy(Ex/ET)andangle
(cos(θk))asdefinedintheTandYsystemsareshownforthe
experimental data (black filled circles) with causality cuts,
three-body phase space decay simulation (red-dashed line), [Er and Γr(E) in Eq. (3)] and the 12Li s-wave scatter-
and dineutron decay simulation (green solid line). inglength. The simultaneousfitting ofallthe three-and
two-body decay spectra greatly increased the sensitiv-
ity and robustness of the fit in comparison to individ-
sequent dineutron breakup with intrinsic energy Ex, re- ual fits of each spectrum. The best overall fit (χ2 min-
spectively. EV and ΓV(Ex) represent the energy and imization) was achieved with a resonance state in 13Li
widthparametersofthepropagatordescribingthedineu- at E = 120+60 keV with Γ = 125+60 keV. A limit
1 −80 r −40
tron virtual state. These parameters along with A2(Ex) on the scattering length of as > −4 fm for the 12Li s-
are taken from the low-energy n− n scattering theory wave was determined from the χ2 fit which is shown in
in free space, where the scattering length as = −18.7 the insert of Fig. 2(b). A reduced chi-square (χ2/ν) of
fm[30],assumingthatthe dineutronisa1s0 n−n state. 1.18 was obtained from the fit with 181 degrees of free-
The Fermi Golden Rule gives the partial decay width dom. The mass excess of 13Li can be determined from
distribution for the sequential process as the 120+60 keV resonance and the mass excess of 11Li
−80
dΓ(E ) (40.72828(64)MeV [32]) to be 56.99(8) MeV.
T 2
=2πδ(E −E −E )|A(E ,E )| (2)
T y x y x The extracted scattering length limit of a > −4 fm
dE dE s
y x for 12Li from the present data seems to be inconsistent
whereET,again,representsthetotalenergyofthethree- with the previous result of −13.7(1.6) fm by Aksyutina
body system. The probability distribution then follows et al. [6]. While Aksyutina et al. used the calculation
the usual Breit-Wigner form presented in Ref. [6] to calculate the s-wavelineshape in
comparison to Ref. [31] used in the present work, it was
dP(E ,E ) 1 dΓ(E )
y x T
∝ , (3) verified that the difference in the calculated lineshapes
dE dE (E −E )2+Γ2(E )/4 dE dE
y x T r r T y x couldnotaccountforthedifferenceintheextractedscat-
where E and Γ (E) are the resonance energy and total teringlengths. However,onepossibleexplanationforthe
r r
width of the initial state in 13Li. The delta-function in differencewouldbe thatthe eventsfromtheunidentified
Eq. (2) enforces that E = E +E in Eq. (3). From low-energypeakin13Li,duetothezerotwo-neutroneffi-
T y x
thepresentedformalism,the13Lidecaywassimulatedas ciencybelow200keV,wereincludedinthe12Lispectrum
theemissionofadineutronwithkineticenergyE which of Ref. [6]. This would enhance the yield at low energy
y
proceeded to breakup with energy E . thus shifting the extracted scattering length of the fit to
x
The decay of 12Li, which was populated through a larger negative values.
pn−removal,wassimulatedusingas-wavelineshapecal- A previous experiment populating 12Li [11] in a two-
culated from the model of Ref. [31]. The potential pa- proton removal reaction from 14B included the -13.7 fm
rameters used in Ref. [31] to describe the 14Be and 14B scattering length s-wave from Aksyutina et al. [6] along
projectilesareusedinthepresentwork. Thiscalculation with two narrow resonances in the fit of the 12Li decay
wasalsousedtodescribethes-wavelineshapeofthe12Li energy spectrum. In order to have a consistent analysis,
decay populated from the 14B(-2p) reaction of Ref. [11]. the 12Li decay energy spectrum from Ref. [11] has to be
The four decay energy spectra of Figs. 1 and 2 were re-fit using an s-wave scattering length greater than −4
fitsimultaneouslyvaryingthe 13Liresonanceparameters fm,asextractedinthecurrentwork. Fig.4showsthere-
5
1500 1470 in 13Li [6]. The 12Li scattering length is not shown in
Fig. 5 since it corresponds to a very broad distribution
1250 inexcitationenergy[1]. Thusthestateat210keVhasto
V) 1000 be considered the ground state of 12Li. The mass excess
ke of 12Li is then 49.01(3)MeV using the most recent mass
y ( 750 measurements of 11Li [32].
g
ner 500 525 The 210 keV ground state of 12Li produces a scenario
E
in which 13Li is bound with respect to 1n emission and
250 210
120 unbound with respect to 2n emission. The decay of the
0 13Li ground state is, thus, a genuine three-body process
11Li + 2n 12Li + n 13Li and, while a purely sequential decay through the n−n
system is unlikely, the three-body correlations indicate
FIG. 5. Level scheme of 13Li and 12Li determined from the
the presence of the n−n virtual state (a = -18.7 fm).
currentwork. The1470keVresonancein13LiisfromRef.[6]. s
The broad s-statewith as>−4 fm in 12Li is not shown. The recent work by I. A. Egorova et al. [37] has demon-
stratedthecomplexitypresentinthethree-bodydynam-
ics of the two-proton decay of 6Be through the use of
sults of the fit (χ2/ν = 1.08)where the s-wavewas fixed a three-cluster model and similar calculations could pro-
atascatteringlengthof−3.8fm,whiletheresonancepa- videnewinsightsintothetwo-neutronemissionfrom13Li
rametersoftheothertwopreviouslyobservedresonances and 16Be [7].
were free fit parameters. The values of the lower (pink In summary, the population and decay of 13Li and
long-dashed lines) and higher (green short-dashed line)
12Li were measured in p− and pn−removal reactions
resonances had to be adjusted slightly from 250(20)keV from 14Be. A low-lying resonance of 120+60 keV was
to210(30)keVandfrom555(20)keVto525(25),respec- −80
observed in 13Li for the first time, corresponding to a
tively.
mass excess of 56.99(8) MeV. Within a consistent de-
One-neutronremovalexperimentsandshellmodelcal-
scription of all measured decay spectra the decay of
culations have indicated that the 14B ground state has
12Li could be described by an s-wave with a scattering
a dominant (64%−89%) s configuration [31, 33–35].
1/2 length of greater than −4 fm, which differs significantly
Thus, a strong population of the 12Li s-wave from 14B
from the −13.7(1.6) fm scattering length extracted in
would be expected along with a smaller population of
Ref. [6]. The a > −4 fm scattering length limit, from
s
d-wave resonances. While the fit shown in Fig. 4 has
thiswork,wasusedtore-fitthedecayenergyspectrumof
a 50% s-wave component, the statistical uncertainty of 12Li populated with a two-proton removal reaction from
the data allows for fits of similar quality (albeit having 14B [11]. The observed 210 keV resonance in 12Li repre-
slightly larger χ2) to be obtained with s-wave compo-
sents the ground state and corresponds to a mass excess
nents ranging from 45% to 90%, which is in agreement of49.01(3)MeV. This 12Ligroundstate produces condi-
with s and d components of the 14B ground state. In tionsfavorablefortruetwo-nucleonemissionfrom13Lito
populating12Lifrom14Be(-pn),inclusionofthetwonar- 11Li. The angular and energy correlations in the three-
row resonances in the fit of Figs. 1 and 2 did not have a body system of 13Li showed strong dineutron character
significant effect and slightly increased the χ2/ν to 1.2.
for the ground state decay.
Thus, the narrow resonances were not included in the
presented fit of Figs. 1 and 2. The results suggest that Theauthorsgratefullyacknowledgethe supportofthe
14Bemusthaveadominants configuration,whichisin NSCLoperationsstaffforprovidingahighqualitybeam.
1/2
agreement with the Coulomb dissociation measurements This material is based upon work supported by the De-
of Labiche et al. [36]. partment of Energy National Nuclear Security Adminis-
The new level schemes of 13Li and 12Li, based on the tration under Award Number de-na0000979 and DOE
presentdataandthedataofRefs.[6]and[11],areshown Award number DE-FG02-92ER40750. This work was
inFig. 5. Thepresentexperimenthasverylowdetection alsosupportedbytheNationalScienceFoundationunder
efficiencyfortwoneutroneventsabove1MeVandisthus Grant Nos. PHY06-06007, PHY09-69058, and PHY11-
insensitive to the previously observed level at 1.47 MeV 02511.
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