Underground nuclear astrophysics: why and how

0.65 MB·

by  A. Best

EPJmanuscriptNo.
(willbeinsertedbytheeditor)
Underground nuclear astrophysics: why and how
A.Best1,A.Caciolli2,3a,Zs.Fu¨lo¨p4,Gy.Gyu¨rky4,M.Laubenstein1,E.Napolitani2,5,V.Rigato5,V.Roca6,andT.Szu¨cs7
1 INFN,LaboratoriNazionalidelGranSasso,ViaG.Acitelli22,67100Assergi(AQ),Italy
2 DipartimentodiFisicaeAstronomia,Universita`diPadova,andINFN,SezionediPadova,ViaMarzolo8,Padova,Italy
3 INFN,SezionediPadova,viaMarzolo8,35131Padova,Italy
4 InstituteforNuclearResearch(MTAAtomki),Debrecen,Hungary
5 INFN,LaboratoriNazionalidiLegnaro,Legnaro,Italy
6 6 DipartimentodiFisica,Universita`diNapoli“FedericoII”,andINFN,SezionediNapoli,Napoli,Italy
1 7 Helmholtz-ZentrumDresden-Rossendorf(HZDR),Dresden,Germany
0
2
January5,2016

n
a
Abstract. The goal of nuclear astrophysics is to measure cross sections of nuclear physics reactions of interest in
J
astrophysics.Atstarstemperatures,thesecrosssectionsareverylowduetothesuppressionoftheCoulombbarrier.
2 Cosmicrayinducedbackgroundcanseriouslylimitthedeterminationofreactioncrosssectionsatenergiesrelevant

to astrophysical processes and experimental setups should be arranged in order to improve the signal-to-noise ratio.
]
x Placingexperimentsinundergroundsites,however,reducesthisbackgroundopeningthewaytowardsultralowcross
e section determination. LUNA (Laboratory for Underground Nuclear Astrophysics) was pioneer in this sense. Two
- acceleratorsweremountedattheINFNNationalLaboratoriesofGranSasso(LNGS)allowingtostudynuclearreactions
l
c close to stellar energies. A summary of the relevant technology used, including accelerators, target production and
u characterisation,andbackgroundtreatmentisgiven.
n
[ PACS. 25.60.Dz Interactionandreactioncrosssections–26. Nuclearastrophysics–29.20.Ba Electrostaticacceler-

ators–29.30.Kv X-andγ-rayspectroscopy
1
v
8
8 1 Introduction measurement in a laboratory at the Earth’s surface, where the
1 signal-to-backgroundratioistoosmallbecauseofcosmic-ray
0 Itiswellknownthatstarsgenerateenergyandsynthesisechem- interactions. Hence, cross-sections are measured at high ener-
0 icalelementsinthermonuclearreactions[1,2].Allreactionsin- gies and expressed as the astrophysical S factors from eq. 1.
1. ducedbychargedparticlesinastartakeplaceinthesocalled TheS(E)factoristhenusedtoextrapolatethedatatotherel-
0 Gamow peak. For the 3He(4He,γ)7Be reaction, for example, evantGamowpeak.AlthoughS(E)variesslowlywithenergy
6 which is important both for understanding the primordial 7Li forthedirectprocess,resonancesandresonancetailsmaycom-
1 abundance and for studying the solar neutrino problem, the plicatetheextrapolation,resultinginlargeuncertainties.There-
v: Gamow peak lies at about 22.4±6.2 keV at a temperature of fore,theprimarygoalofexperimentalnuclearastrophysicsre-
i T61 =15,whilefortheBigBangNucleosynthesis(BBN)sce- mainsthemeasurementofcross-sectionsatenergiesinsidethe
X
nariotheregionofinterestisabout160-380keV. Gamowpeak,oratleasttoapproachitascloselyaspossible.
r Thecross-sectionσ(E)ofacharged-particleinducedreac-
a
tiondropssteeplywithdecreasingenergyduetotheCoulomb
barrierintheentrancechannel:
Theimportanceoftheaccuratedeterminationofcrosssec-
S(E)
σ(E)= exp(−2πη) (1) tionsarisesfromthefactthatthisparameterentersdirectlyinto
E thereactionrater =< σ(ν)·ν >,whereσisthenuclearcross
section and ν is the relative velocity of particles. Precise and
whereS(E)istheastrophysicalS factorandηistheSommer-
feldparameterwith2πη=31.29Z Z (µ/E)1/2;Z andZ arethe accurate cross section data are therefore needed as inputs for
1 2 1 2
stellarreactionnetworks.Inparticular,duetothesteepdepen-
chargenumbersofprojectileandtargetnucleus,respectively,µ
dence of the cross section as a function of center-of mass en-
is the reduced mass (in amu), and E is the center-of-mass en-
ergy,anaccuratedeterminationoftheaverageenergyatwhich
ergy(inkeV).Inthequiescentburningofstars,thecrosssec-
a cross section is measured is of utmost importance. This im-
tion is very low at Gamow energies and this prevents a direct
plies that for this research field it is necessary to increase the
a e-mail:[email protected] accuracyonthemeasuredcrosssections,especiallyatverylow
1 T isthetemperatureexpressedinT/106K energies.
6
2 A.Bestetal.:Undergroundnuclearastrophysics:whyandhow
2 Experimental setups characterisedbyhigh-currentandsmallenergyspread,incom-
binationwithhigh-efficiencydetectionsystems.Thenextyears
TherealisationoftheLUNAoftheINFN(ItalianNationalIn- willseetheinstallationatLNGSofanewMVacceleratorfor
stitute for Nuclear Physics) under the Gran Sasso mountain wideningtheexplorationrangeofthelaboratory.
chainprovidedauniqueopportunitytocarryoutinmorefavourable
conditions the study of reactions of high astrophysical impor-
tance and to achieve results that in a surface laboratory (or
3 The accelerators of LUNA
in less shielded laboratories) could not have been obtained.
This goal could be reached due to the reduction of the cos-
Consideringtheverysmallcrosssectionsandtherapidvaria-
mic background through the rock overburden, which dimin-
ishes the muon flux to ∼ 10−8 cm−2s−1 [3]. Thanks to this tionoftheirvaluewiththeenergy(seeeq.1),themostimpor-
tantcharacteristicsofanacceleratorarethebeamintensityand
favourable position, LUNA was able to measure for the first
timethe3He(3He,2p)4Hecross-sectionwithinitssolarGamow thelongtermstabilityoftheterminalhighvoltage(HV).The
twoacceleratorsuseduptonowatLUNAfulfiltheserequire-
peak[4].Subsequently,awindowlessgastargetsetupanda4π-
ments.
BGOsummingdetectorhavebeenusedtostudytheradiative-
capturereaction2H(p,γ)3He,alsowithinitssolarGamowpeak The50kVLUNAacceleratoratLNGS,describedinmore
[5].Forsomereactions,theundergroundpositiondoesnotgrant detailselsewhere[13],consistedofaduoplasmatronionsource,
the possibility to measure directly at energies of the Gamow an extraction/acceleration system, and a double focusing 90◦
peak. Therefore direct data at higher energies are needed and analysingmagnet.Theenergyspreadoftheionsourcewasless
othermethodscanbeinvestigatedtoreducethesignaltonoise than20eV.Theionsource,evenatthelowestextractionener-
ratio,asforthecaseofRecoilMassSeparator(RMS)[6].This gies, provided stable proton beams with currents of few tens
apparatusallowstocountfewreactionproductsalsowhenin- ofµAforperiodsupto4weeks.TheHVpowersupplyhada
tense projectile beams have to be produced to overcome the typicalrippleof5×10−5 andalong-termstabilitybetterthan
smallcross-sectionsattheinterestingenergies(see[6]forde- 1×10−4.
tails). The 400 kV electrostatic accelerator (High Voltage Engi-
In some cases, when the reaction products are radioactive neering Europe, Netherlands) was installed in 2001. It is em-
with a half life of at least hours, direct measurements can be beddedinatankwhichisfilledwithagasmixtureN2-CO2 at
followedbyactivationanalysisontheirradiatedtargets[7],or 20 bar. The high voltage is generated by an Inline-Cockcroft-
bycountingofthereactionproductsbyacceleratormassspec- Waltonpowersupply(locatedinsidethetank)capabletohan-
trometry (AMS) techniques [8]. In both cases, the irradiation dle1mAat400kV.TheHVattheterminal(ionsource)isfil-
can be done either in underground or in overground laborato- teredbyastabilisationsystemconsistingofaRC-filterlocated
rieswithanhighintensitybeamasthatprovidedbyLUNA. at the output of the HV power supply and an active feedback
In the case of measurements at energies higher than the loop based on a chain of resistors which measures the HV at
Gamowpeak,thevalueoftheSfactorisobtainedbyextrapo- theterminal.
lationofthehigherenergydata. In one set of experiments [14] the absolute proton energy
An interesting case is the 14N(p,γ)15O reaction, where to Ep ranging between 130 keV and 400 keV was determined
constrain the S(0) extrapolation the low energy data must be fromtheenergyofthecaptureγ-raytransitionin12C(p,γ)13N.
combinedwiththehighenergyonesandalsowithexperimental Theresultshavebeencheckedusing(p,γ)resonanceson23Na,
measurements of the resonance parameters needed in the R- 25Mg and 26Mg. These resonances were also used to measure
matrixcalculations(see[9,10]fordetails). theenergyspread∆EBeamandthelong-termenergystabilityof
Low energy data taken in underground laboratory still re- theprotonbeam.
mainamajorconstraintintheextrapolation.Inthecaseofthe The radio-frequency ion source provides ion beams of 1
3He(4He,γ)7Be reaction, the high precision of the low energy mAhydrogen(H+)and500µAHe+ overacontinuousoperat-
dataprovidedbyLUNAwithtwodifferenttechniquesreduced ingtimeofabout40days.Theionsourceismounteddirectly
theuncertaintyon theS(0) valueof themore recentcompila- ontheacceleratortube.Theionsareextractedbyanelectrode,
tionfrom9%[11]to3%[12]. whichispartoftheacceleratortube:itsvoltageisthusincluded
ThisroughoutlineshowsthattheobjectivesofNuclearAs- in the overall HV at the terminal. With a 45 degrees magnet
trophysicscannotbeobtainedwithonetechniqueorwithone andaverticalsteererlocatedbeforethemagnet,theionbeam
kindofinstrument.Theastrophysicalscenariosaresowidethat isguidedandfocusedproperlytothetargetstation.Intheen-
the proper apparatus to study a specific reaction has to be se- ergyrangeof150−400keV,theprotonbeamcurrentontarget
lected.Inthenextsectionsexamplesofdifferentsetupsthatcan canreach500µA.Thisintensityismandatorytoimprovethe
beusedinundergroundnuclearastrophysicswillbediscussed. statistics in nuclear astrophysics experiments, but it could af-
The intensity and stability of the beam must be granted fectthetargetstabilityduetothedeteriorationofsolidtargets
since the cross section varies steeply with the energy. Targets (see section 6) or increase the beam heating effects (see sec-
have to be stable over time and any possible source of beam tion5.3).Theaccelerator,theexperimentalequipment,andthe
induced background has to be minimised. Specific detection datahandlingarecontrolledbyaPLCbasedcomputer,which
systemstofollowtheexperimentalrequirementsandmeasure allows for a safe operation over long periods of running time
thedifferentpropertiesofeachreactionareadopted. withouttheconstantpresenceofanoperatoronsite.
Themainroleinthiscontextisplayedbytheaccelerators. In2008asecondbeamlinewasinstalledtoallowtwoex-
LUNA used in its history: a 50 kV and a 400 kV machines, perimentalsetupstobemountedsimultaneously.
A.Bestetal.:Undergroundnuclearastrophysics:whyandhow 3
4 Nuclear Astrophysics Underground 104 40
LUNA has focused its work on quiescent hydrogen burning, 110023 214Bi 214Bi K 214Bi 214Bi 208Tl SUunrdfearcgeround
withrelevanttemperaturesinarangefrom20to100MK[15, 101
16]. The corresponding Gamow peak lies at energies from 10 V h100
tboechauunsdereodfsthoefdkeraVs.tiMcereadsuurcitniognaotfthtehseecernoesrsgsieecstiisonchdaulleentogitnhge counts / ke111000---321
Coulombbarrier.Theexperimentalstatisticscanbeimproved 10-4
byincreasingthebeamintensity,thetargetdensityandthede- 10-5
tection efficiency, but it can also be achieved by reducing the 10-6
twocomponentsofbackground:laboratoryandbeaminduced. 10-7
0.0 0.5 1.0 1.5 2.0 2.5 3.0 4 5 6 7 8 9 10 11 12 13 14 15
Energy / MeV
Fig.1.EnvironmentalbackgroundacquiredwithaHPGedetectoron
surfacelaboratory(black)andinsidetheLNGS(red).AtLNGSthe
4.1 LaboratoryBackground
HPGedetectorwasincludedinashieldingmadeofleadandcopper
asdiscussedin[18].
The laboratory background comes either from the signal pro-
duced by cosmic rays or from the one produced by radionu-
clidespresentintheexperimentalenvironment.Ingammaspec- trons from spontaneous fission and (α,n)-reactions, and resid-
tra each of these two components dominate a specific energy ual cosmogenic activation from the above-ground production
range. In particular, the γ-ray spectrum at energies up to 2.6 of the detector. Of course, also radon gas becomes important
MeV is dominated by the signal of radionuclides, especially andhastoberemovedcarefullyfromtheenvironmentaround
40Kandthedecaychainsof238Uand232Th. thedetector[19].Inaddition,aprerequisiteforobtainingultra-
Acarefulselectionprocedureofsetupmaterialsisrequired lowbackgroundis,ofcourse,thatthedetectorisradiopure.
tominimisethisbackgroundandthesetupisusuallyshielded
Ithastobenotedthatthebackgroundreductionfromcos-
with passive materials (i.e. copper and lead). Active shielding micradiationhasarelevanteffectalsofortheγ-energyregion
can also be applied, increasing the complexity of the acquisi-
below 3 MeV. This is evident if we compare shallow under-
tionelectronics.
groundlaboratorieswiththeresultsachievedattheLNGSwith
Atenergiesabove3MeV,thebackgroundintheγspectrum
afullyshieldedsetup[15].
isbasicallyduetocosmicradiation.Atsealevel,thisradiation
consists of 70% muons, 30% electrons, and <1% protons and
neutrons.Muonsarethemostpenetratingcomponentandthey
4.2 Neutronbackground
producebackgroundincountingfacilities.Theyloseenergyby
ionisationandcancontributetothedetectorbackgroundindif-
ferent ways: i) by losing energy while traversing the detector Besides the effect on γ spectroscopy, locating an experiment
itself; ii) by producing energetic electrons which induce sec- deepundergroundstronglyinfluencesalsobackgroundsincharged
ondaryelectronsandγradiation;iii)byproducinginteractions particle spectroscopy [20] and neutron detection. The latter is
in materials surrounding the detectors followed by X-rays, γ an important aspect considering the great interest in neutron
rays,andneutronemission.Inaddition,neutronsproducedby sourcesfors-processinnuclearastrophysics.
muon spallation induce radioactivity in the experimental en- The astrophysical s process is responsible for the produc-
vironment. Such background can be overcome by performing tionofabouthalfoftheelementsheavierthaniron[21].Itstwo
experimentsinadeepundergroundlaboratory,wherethemuon main neutron sources are the 13C(α,n)16O and 22Ne(α,n)25Mg
fluxisgreatlyreduced.ForexampleintheINFNNationalLab- reactions [22]. The standard approach to measure this kind of
oratoriesofGranSassothemuonfluxabove1TeVisreduced reactionsconsistsinusingamoderatingneutrondetector,e.g.,
by 6 orders of magnitude thanks to its depth of 3800 m.w.e. [23].Thethermalisationprocessimpliesthelossofinformation
(meters of water equivalent) [3,17]. This is clearly shown in ontheinitialenergyofthemeasuredneutrons.
figure1wheretwospectraacquiredwiththesamedetectorare Therefore, to be able to measure at the very low energies
compared.Thespectruminblackisacquiredonsurfacewhile required for the astrophysical scenarios of the s process, the
thespectruminredisacquiredinsidetheLNGSwiththedetec- neutronbackgroundneedstobesuppressedasmuchaspossi-
tor shielded with copper and lead (the details of the setup are ble. Forthe reaction 22Ne(α,n)25Mg [24]the energyregion of
discussedin[18]).Intheγspectrum,above4MeVthereduc- interest lies around 200 - 300 keV, below the lowest already
tionisduetotheundergroundposition,whilebelow3MeVthe measureddatapoint.Thenaturalneutronbackgroundremains
effectoftheshieldingisclearlyevident.Themainbackground the limiting factor towards lower energies. To reach the stel-
linesarelabeledintheblackspectrum. lar energy region another two or three orders of magnitude of
Theintegralbackgroundcountingratefrom40to2700keV backgroundreductionarenecessary.Thiscanbedonebygoing
for High Purity Germanium (HPGe) detectors, often used in deepunderground.
underground laboratories, asa function ofthe shielding depth The neutron flux on the surface of the earth is dominated
levels off at a certain depth, even though the muon part of by cosmic-ray induced neutrons. Generally, it is of the order
the cosmic rays is still decreasing exponentially with depth. of 10−3 cm−2s−1. By going deep underground one can reduce
This is a clear indication that other background sources be- thecosmicrayfluxby6ordersofmagnitude.Then,theradio-
come more important, i.e. intrinsic natural radioactivity, neu- genic component becomes the main neutron background, two
4 A.Bestetal.:Undergroundnuclearastrophysics:whyandhow
to four orders of magnitude higher than the cosmogenic neu- 0.0010000
beam on vacuum
tronflux.Asalreadydiscussedinsection4.1,theunderground beam on deuterium gas
neutron sources are mostly spontaneous fission of 238U in the
cavity walls and (α,n) reactions induced by α-particles from
0.0001000
thenaturalradioactivityoftheundergroundenvironment.
C
-2 ]m 10 counts/ µ 0.0000100
c
-1 s
-6 0 1 0.0000010
1
x [
on flu 10-1 0.000000 14500 5000 5500 6000 6500 7000
r
eut 10-2 energy [keV]
N Fig.3. Comparisonoftwospectraacquiredatthesameprotonbeam
2S0o9u0d amnwe 3L8N0G0S mwe 3S6U0R0F m 4w10e04S3U0R0F m Dwaevis4S3U0R0F m TwCeR 1K4U0R0F mwe 2W0I0P0P mwe energyusingastargetvacuum(inred)anddeuterium(inblue).The
peaksduetothep+dreactionareclearlyvisibleinthebluespectrum,
Site
but they are not present in the blue one where the peaks due to the
Fig.2.Thermalneutronfluxatvariousdeepundergroundlaboratories. 19F(p,αγ)16O reaction still remain. This suggests that the fluorine is
depositedonthesurfacesoftheexperimentalsetup.
Theexactvalueoftheneutronfluxisverysensitivetothe
local environment, like the composition of the shotcrete and
the surrounding rocks and on the radon concentration in the higherZareusuallynegligibleatastrophysicalenergies.Fluo-
vicinity of the detector. The thermal component of the neu- rineisoneofthemostcommoncontaminantsthatcanbefound
tron background comprises the majority of the total flux and and the 19F(p,αγ)16O reaction has a high cross section at en-
has therefore the most significant impact on the sensitivity of ergies below 400 keV (the one of interest for LUNA400kV
experiments,especiallywhenusingneutroncounterdetectors. accelerator). In particular, there is a resonance at E = 340
p
Various measurements of the thermal neutron background at keV with a large width that produces many structures in the
theLNGShavebeendone;noneofthemagreewitheachother, 5-8 MeV energy region of the γ-ray spectrum. In figure 3 a
possiblyduetoavariationinthebackgrounddependingonthe spectrum acquired with deuterium gas as target for the study
location in the laboratory or due to unknown systematic un- ofthe2H(p,γ)3Hereactionisshowninblue.Thepeaksdueto
certainties. The reported values are: (2.05±0.06)·10−6cm−2s−1 thereactionofinterestandthecorrespondingstructuresdueto
[25],(1.08±0.02)·10−6cm−2s−1[26],and(5.4±1.3)·10−7cm−2s−1 the beam induced background on 19F are visible in the spec-
[27].Arecentmeasurementshowninfigure2agreeswiththe trum. For comparison a spectrum acquired without gas in the
latter value [28]. Figure 2 shows a comparison of the thermal targetchamberisalsoshown(inred):onlythepeaksfromthe
neutronfluxesindifferentdeepundergroundlaboratoriesmea- 19F(p,αγ)16Oreactionarestillvisible.
sured with 3He counters. It can be seen that even in the same
laboratoriesthefluxcanvarybyuptoanorderofmagnitude, Boronandoxygenareothercommonsourcesofcontamina-
makingefficientairventilationandlocalshieldinganimportant tion.Especiallyboronisfrequentlypresentintheexperimental
componentintheplanningofnewfacilities.AtLNGSthether- surfaces and it is difficult to remove it. The (p,γ) reaction on
malfluxisbelow10−6cm−2s−1,thebackgroundsuppressionis boron-11mainlyemitsthreeγrays:around4.4MeV,12MeV
atalevelsofarnotreachedbysurfaceexperiments. and16MeV.Infigure4,theboronsignalisclearlyvisibleboth
in the spectrum with argon as target and in the one with neon
(enrichedin22Ne)fillingthescatteringchamber.Anotherpeak
isvisibleinbothspectra,duetothe18O(p,γ)19Freaction.Sub-
4.3 BeamInducedBackground
tractingthetwospectra,itispossibletoobtainaclearpeakat
theenergyofinterestofthe22Ne(p,γ)23Nareaction(labelledin
Environmental background is a critical issue for direct mea-
figure4).
surementsinnuclearastrophysicsbutinexperimentswithion
beams, impurities in the apparatus and in the targets can in- Whentheeffortofreducingcontaminantsthatproducebeam
ducereactions thatgiverise tobackground(mainly γ-rayand inducedbackgroundinreactionstudiesisnotsuccessful,care-
neutronbackground). ful study of the reactions involved in the beam induced back-
Such beam induced background is independent of the un- groundcanhelpinunderstandingtheirbehaviourandtheircon-
dergrounddepthandmustbedealtwithbyreducingtheinven- tribution to the acquired signal [29]. Many of these reactions
tory of materials hit by the ion beam and taking appropriate are already deeply studied in literature and this helps in man-
precautionstoeliminateworrisomecomponents. agingtheircontributionindataanalysiscomparingspectraac-
MaincontaminantsarelowZelementslikedeuterium,boron, quiredwithblanksamples[30]and/orusingMonteCarlosim-
carbon,oxygen,andfluorine.Contributionsfromelementswith ulations[31].
A.Bestetal.:Undergroundnuclearastrophysics:whyandhow 5
1e+06 Windowlessgastargetscanbeofextendedorjettype.Quasi
Ar-156keV
Ne-156keV point-like gas targets can be formed with a jet. In this case, a
40K supersonic gas stream is let to the target chamber through a
100000
smallnozzlewhereitishitbytheparticlebeam.Sincejettar-
208Tl gets were never used at LUNA, this section concentrates on
10000
extendedgastargets.Detailsaboutjettargetscanbefounde.g.
in[35].
Counts 1000 11B(p,γ)12C 18O(p,γ)19F 11B(p,γ)12C 11B(p,γ)12C
22Ne
100
5.1 Propertiesandcharacterisationofanextendedgas
target
10
Anextendedgastargetconsistsofafinitevolumeofgas,agas
1
0 5 10 15 20 filling system, pumping stages and some auxiliary equipment
Eγ [MeV] for, e.g. gas purification or beam intensity measurement. The
physical length of the gas volume is typically between a few
Fig.4. Comparisonoftwospectraacquiredatthesameprotonbeam
centimetres up to several tens of centimeters. The volume is
energyusingastargetargongas(inblack)andneongas(inred).The
peaksduetothereactionsonboronandoxygenareclearlyvisible,as confinedeitherbytwosmallapertures,oronlyoneapertureto-
discussedinthetext. wardstheaccelerator.Inthelattercasethebeamstopisplaced
insidethegasvolume.
Extended gas targets operate typically at pressures in the
5 Gas Target mbar range. The accelerator on the other hand requires high
vacuum, i.e. a pressure of several orders of magnitude lower
thaninthegastarget.Asthereisnowindowseparatingthegas
In nuclear astrophysics, reaction cross section measurements target from the rest of the beam line, the necessary pressure
veryoftenneedtheuseofagastargetsystem.Thisisespecially dropisrealizedbyaseriesofaperturesandpumpingstages.In
trueinthecaseofreactionsrelevantforhydrogenandhelium thefirstpumpingstagetheheavyloadoftargetgasispumped
burningprocesses,wherethecrosssectionmeasurementonthe away by high speed pumps, like a roots blower, resulting in
isotopesofgaseouselementslikehydrogen(deuteron),helium, a pressure drop of roughly 3 orders of magnitude. The subse-
nitrogen, oxygen, neon, etc. are necessary. In some cases, ex- quentpumpingstagesarenotpumpingsuchahighamountof
perimentscanalsobeperformedwithsolidstatetargetsusing gas and therefore turbomolecular pumps can be used to reach
asuitablecompound,likeTiNorTa2O5inthecaseofnitrogen therequiredvacuumoftheaccelerator.
or oxygen, respectively [32,33]. These targets, however, have Sincegasislostthroughtheapertures,thetargetgasmust
theirdisadvantagesconcerning,e.g.thelimitedtargetstability becontinuouslysuppliedinordertokeepaconstantpressurein
and the uncertain stoichiometry. Moreover, no chemical com- thetarget.Thisisachievedbygasinletvalvesregulatedbythe
poundcanbeformedfromthenoblegases,reducingthestabil- pressuresensorsofthetargetsystem.Inthecaseofexpensive
ityofsolidtargetsfortheseelements.Therefore,gastargetsys- gases, like isotopically enriched ones, it is necessary to recir-
temsareextensivelyusedinnuclearastrophysicsexperiments culate the gas lost from the target. In such a case, the outlets
andrepresentanimportantaspectofundergroundexperiments of the pumping stages are fedbackto the gas inlet system. In
aswell. ordertokeepthechemicalpurityofthegas,apurifiermaybe
Besides being unavoidable in some cases, gas targets also necessary. Such a purifier can ideally be used in the case of
havesomeclearadvantagesoversolidstatetargets.Thetarget a noble gas target, as the air contamination of the gas is effi-
thicknesscanbeeasilyregulatedbychangingthegaspressure ciently absorbed by the chemical getter of the purifier, while
and the number of target atoms can be determined rather pre- thenoblegasisletthrough.Figure5showstheblockdiagram
cisely using the pressure and temperature measurements. No ofatypicalextendedgastargetsystem.Suchasystemwasused
targetdegradationoccurs,whichisaclearadvantageinnuclear byLUNAinthecaseofseveralexperiments,e.g.3He(α,γ)7Be
astrophysicswheretypicallyhighbeamintensitiesareencoun- [37],2H(α,γ)6Li[38]and22Ne(p,γ)23Na[39].
teredinordertomeasurelowcrosssections. For a cross section measurement, one of the parameters
Gas targets can be categorised into two distinct groups: which must be known is the number of target atoms per unit
windowlessgastargetsandthinwindowgascells.Forthelat- area. In the case of an extended gas target this number is ob-
ter, the beam enters the gas target through a thin metal win- tainedfromthephysicallengthofthechamberandthepressure
dowwhichseparatesthetargetsetupfromtheaccelerator[34]. andtemperatureofthegas.Ifthecrosssectionismeasured,e.g.
In underground nuclear astrophysics experiments, low energy bythedetectionofthepromptγ-radiationemittedinthestud-
beamsaretypicalnotexceedingthefewhundredkeVrange.In ied capture reaction, then an effective target length is defined
thisenergyrange,eventhethinnestpossibleentrancewindow which takes into account those parts of the target from where
would cause too high energy loss and large energy straggling theγ-radiationcanreachthedetector.
increasing substantially the uncertainty of the measurements. Anypressurevariationalongthetargetchambermustalso
Thinwindowgascellsarethereforeusuallyavoidedinlowen- betakenintoaccount.Therefore,thepressureprofileofthetar-
ergynuclearastrophysics. get must be determined by measuring the pressure of the gas
6 A.Bestetal.:Undergroundnuclearastrophysics:whyandhow
I3 I2 I1 PT2
3rd STAGE 2nd STAGE 1st STAGE PT1
CALO
beam AP1
AP3 AP2
VT
P buffer
Pur 2
TP3 TP2L TP2M TP2R RUVAC 2000 MKS 248A
Buf 2
BUFFER
V buffer
V12 UP RUVAC 500 Buf 1
V2 V natNe
ACP28 Pur 1 PURIFIER V 22Ne 14N
V1 V ACP28
V12 DOWN V purifier
P purifier
ECODRY L
22Ne natNe 14N
Fig.5. Blockdiagramofatypicalextendedgastargetpumpingsystem[36,18].
at several positions, along the beam line. The pressure pro- Measurement of the elastic scattering of the projectile on
file outside the actual gas target, in the first pumping stage, the gas target nuclei can in principle give information about
mustalsobeknownasanon-negligiblepartofthetotaltarget theproductofthenumberoftargetatomsandprojectiles.This
thicknesscanbelocatedthere.Especiallyifahightemperature methodisoftenusedinthecaseofajetgastarget.Forextended
beam calorimeter is used (see below), the temperature profile targets, on the other hand, this method is not ideal as it gives
of the target gas must also be measured precisely in order to information only about a small part of the target length and
obtainanaccurategasdensityandhencetargetthickness[18]. thereforeintroduceshighuncertainties.
Of course the pressure measurement alone does not give any
information about the composition of the gas. Any gas impu- The usual solution is the application of beam calorimetry.
ritycanfalsifythedeterminednumberoftargetatomsandcan Here, instead of measuring the charge carried by the beam to
also cause unwanted background. Gas impurities (most typi- the beam stop, the deposited power is measured. If the beam
callynitrogenleakinginfromtheatmosphere)canbestudied energy is known, the beam intensity and therefore the num-
by sampling the gas occasionally and measuring its composi- berofprojectilescanbeobtained.Aprecisewayofmeasuring
tionwithaquadrupolemassspectrometer.Anotherpossibility thebeampoweristheapplicationofaconstantgradientbeam
isthemeasurementoftheelasticscatteringofthebeamparti- calorimeter.Insuchadevice,aconstanttemperaturedifference
cles in the gas. Details about such a method can be found in is sustained between the thermally coupled cold and hot side
[40]. ofacalorimeterbyapowerresistor.Ifthebeamisheatingthe
beam stop, the resistor is able to keep the temperature differ-
ence with a reduced power. The beam intensity can be calcu-
5.2 Beamcurrentmeasurement latedfromthedifferencebetweenthepowermeasuredwithand
withoutbeam[41].
Theotherimportantinputquantityofacrosssectionmeasure-
ment is the number of projectiles impinging on the target. In Inordertopreciselydeterminethebeamintensity,thebeam
the case of a solid state target setup, this quantity is usually calorimetermustbecalibrated.Thiscanbedoneusinganevac-
determinedthroughchargeintegration.Inthecaseofagastar- uatedgastargetandchargeintegration.Withoutgas,thebeam
getsetup,however,thebeamparticlespassingthroughthegas intensity can be measured with the conventional charge inte-
undergo charge exchange reactions with the target atoms and grationmethod.Ifthebeampowerismeasuredsimultaneously
secondary electrons are also produced. Therefore, the beam with the calorimeter, then the latter one can be calibrated. An
particles arrive at the beamstop with highly uncertain charge important condition for such a calibration is that the chamber
statetogetherwiththecreatedsecondaryelectrons.Thesimple aroundthebeamstopformsagoodFaradaycupforareliable
chargeintegrationmethodisthusnotapplicable. chargemeasurement.
A.Bestetal.:Undergroundnuclearastrophysics:whyandhow 7
5.3 Beamheatingeffect and the target. An electric triplet is also placed between the
analysismagnetstoimprovethebeamfocusing.
If an intense particle beam passes through a gas target, the Solid targets are susceptible to the deposition of contami-
power of the beam is in part deposited in the gas itself caus- nantsontheirsurface.Whileinthegastargetsetupnewgasis
ing a local heating effect. As a result of the local temperature constantlyintroducedintothescatteringchamber,solidtargets
rise,thegasdensityandhencetheeffectivetargetthicknessde- shouldbekeptfreefromcontaminantdepositionsontheirsur-
crease.Thiseffectcannotbeobservedwiththepressuresensors face. Contaminants decrease the beam energy and cause also
astheyarelocatedoutsidethebeampath. beam straggling. This is a critical problem in nuclear astro-
There are different ways to measure the beam heating ef- physics measurements where the cross section changes dras-
fect. The total energy loss of the beam in the gas target de- tically with the beam energy. Therefore the vacuum system
pendsonthegasdensityalongthebeampathandthereforeon shouldbeoptimisedinordertohavepressuresintherangeof
the beam heating effect. If the energy of the beam at a given 10−7mbar.Thisgoalcanbeachievedalsobyintroducingacold
pointalongthebeampathcanbemeasuredprecisely,thenthe trapclosetothetargetposition.IntheLUNAsolidtargetsetup,
reduced energy loss and thus the beam heating can be mea- a20cmlongcoppertubeisplacedat2mmfromthetarget.The
sured.Anuclearreactionexhibitinganarrowresonancecanbe tubeiscooledwithLN inordertocatchresidualcontaminant
2
used for such a measurement. The excitation function around particlesinthelastpartofthebeamline.Thankstothissystem,
the resonance is measured with different beam intensities and nobuild-upofanycontaminanthasbeenobservedduringlong
gas pressures, and the beam heating can be derived from the timeirradiationinLUNAexperiments[43,44].
positionoftheresonanceedge[42].
Anotherpossibilityisofferedbythedetectionofelastically
scattered beam particles on the gas as the yield of the scatter- 6.2 Targetproductionandanalysis
ing changes linearly with the gas density. The elastic scatter-
ing yield can be measured at different gas pressures and the There are several techniques used in solid target production.
measurementcanbeextrapolateddowntozeropressurewhere The most common ones are implantation, reactive sputtering,
no beam heating occurs. The measurement can be carried out oxidation,andevaporation.ReactivesputteringTiNtargetswere
at various beam energies and intensities and, if the flexibility usedatLUNAtostudythe14N(p,γ)15Oand15N(p,γ)16Oreac-
of the scattering setup allows, at different positions along the tions,thankstotheirgreatstabilityunderbeambombardment
beamline.SuchanextensivestudywascarriedoutatLUNAin andtheirveryprecisestoichiometry.ExperimentsatLUNAare
relationtothe3He(α,γ)7Beexperimentandabeamheatingef- characterisedbyhighbeamintensitiesinordertomaximisethe
fectofupto10%wasfoundwithbeamintensitiesoftypically statisticsandthereforethetargetsarecriticallystressed.Forthe
300µAandgaspressureofupto1.3mbar[40]. productionofTa O targets,adedicatedsetupwasinstalledat
2 5
the LNGS [45]. Those targets were shown to keep the same
characteristicsinstoichiometryandisotopicratioafter20Cof
6 Solid Target accumulated charge. As already outlined, in gas target setups
the gas is constantly refurbished in the scattering chamber by
ThesecondbeamlineoftheLUNA-400kVisdevotedtoreac- means of the circulating system. Solid targets should be con-
tion studies with solid targets. The advantages of a gas target stantly monitored in order to keep under control their charac-
setup have been explained in section 5. The solid target ap- teristics. Narrow resonances with well known strength can be
proachcanbechosenwhengastargethandlingiscomplicated used for this purpose. The scan of the integrated yield of the
ortooexpensive,andmostimportantlyincaseswheretheiso- resonancecontainsmanyparametersofthetargetcomposition
topetobestudiedisnotofagaseouselement,and/ordoesnot suchasthethickness,∆E,andthenumberofactiveatoms.The
formanyreasonablegascompound.Inaddition,asolidtarget yieldforanarrowresonancecanbewrittenas:
canbeconsideredapoint-likesourceinexperimentsinvolving
γemissionandthissimplifiestheexperimentalneedstostudy (cid:111)2ωγ
Y = (2)
angulardistributionsand/orcorrelations. 2 (cid:15)
r
Duetothereduceddimensionofthetargets,thescattering
chamberscanfiteasilytherequestofthedetectionsetupandit iftheenergylossinthetargetthicknessismuchhigherthanthe
isusuallyeasiertoputthedetectorsinclosegeometry(which resonancewidth(∆E >>Γtot)[2].Ineq.2,(cid:111)istheDeBroglie
is important in low statistics measurements). As already dis- wavelength, ωγ is the resonance strength and (cid:15)r the effective
cussed briefly in section 5, the disadvantages of solid targets stoppingpower.Thelatteristhestoppingpowerweightedfor
setupsareduetothedifficultiesincharacterizationandstabil- the reactive ions in the target. Repeating this scan in regular
ityofthetargetsamples.Thereforeacarefulstudyofthetargets time intervals (see Figure 6) allows to monitor targets con-
withseveralionbeamanalysistechniquesisneeded. sumption or contaminant deposition on the target surface (i.e.
observingashiftinthefrontedgeoftheresonancescan).
Contaminants are one of the most problematic issues of
6.1 TheLUNAsolidtargetbeamline solidtargets.Contaminantscanbeintroducedduringtargetprepa-
ration, be present in the backings (e. g. fluorine in tantalum
Thebeampassesthroughthe0◦ portofthefirstdipolemagnet backings)ortheycanbedepositedduringirradiationfromresid-
andthenitisbentby45◦inaseconddipolemagnet.Thebeam ualparticlesinthebeamlinevacuum.Impuritiesinthetargets
is collimated using two apertures between analysing magnet areremovedbycarefullypolishingthetargetbackingsandthe
8 A.Bestetal.:Undergroundnuclearastrophysics:whyandhow
3.5
Q = 0.4 C
3.0 Q = 17.4 C
2.5
u.] 2.0
a.
d [
el 1.5 FWHM = 26.6 keV
Yi
1.0
0.5
0.0
150 155 160 165 170 175 180 185
E [keV]
Fig. 6. Thick-target yield profile for the 151 keV resonance in
18O(p,γ)19FfortwovaluesofaccumulatedchargeonthesameTa O
2 5
target.Theerrorbarsarestatisticalonly.Thefittedvalueforthetarget
thicknessisreportedinthefigure.Thedashedlineisafittothethick-
targetyieldprofileobtainedafteratotalaccumulatedchargeontarget
Fig.7. RBSspectrameasuredwith2MeVαbeamonaTa O sam-
of17.4C:areductioninthicknessisvisible. ple.Thespectraareacquiredafterdifferentaccumulatedch2arg5eson
thesample.Thesignalfromtheoxygenisnotvisible,whilethelayer
oftantalumoxideisclearlyseparatedfromtheTabackingasshown
inthesimulation(lines).After12Caccumulatedcharge(blackspec-
target preparation setups and with proper target handling pro-
trum)thereductionofthicknessisclearlyvisiblewithoutanychange
cedures.Mostcommoncontaminantsarefluorine,carbon,oxy-
ofthestoichiometryTa:Owithrespecttotheoriginaltarget(red).
gen,anddeuterium.
Iftargetscannotbecheckedonlineduringtheirradiationor
ifitisnotpossibletoextrapolateallinformationfromasingle chemicalcompositions,contaminants)canbeachievedreduc-
technique, offline analysis has to be used. Typical techniques ingcostsandtimeneeds.
areRutherfordBackscattering(RBS),SecondaryIonisedMass SIMSwasusedfortheTa O targetsinordertodetermine
2 5
Spectrometry(SIMS),ElasticRecoilDetectionAnalysis(ERDA),the isotopic enrichment in 17O and 18O [45]. SIMS is a pow-
andNuclearReactionAnalysis(NRA).ThecontentsoftheTiN erful technique [48] to investigate also the abundance profile
targetswithnitrogenenrichedin15Nwereinvestigatedbyares- along the target thickness. Differently from RBS, it is not af-
onance scan of the 429.5 keV resonance of the 15N(p,αγ)12C fectedbytheproblemofhighZbackingsthatoverwhelmsthe
reactionattheHZDRTandetronaccelerator[46]andwiththe RBSspectrum.
HighZERDAtechniqueattheQ3Dmagnet[47]tounderstand InFigure8,anexampleofaSIMSstudyontheTa O ir-
2 5
theisotopicratioachievedduringthetargetpreparation.Inof- radiated at LUNA is shown. Two regions of the target were
flineanalysis,thesamplesareinvestigatedintheregionirradi- selected to study its properties before and after the usage at
atedbythebeamduringtheexperimentandinaregionoutside LUNA-400kV. During SIMS analysis, the sample surface is
the beamspot and comparison are also made with not irradi- continuouslysputteredawaybyafocusedandrasteredprimary
atedsamplestodescribeinthebestwaythesamplebehaviour ion beam. Secondary isotope ions that are emitted from the
during high intensity beam irradiation. In Figure 7 two spec- sample surface are selected with a high resolution mass spec-
traacquiredwithαbackscatteringtechniqueonTa O samples trometer andeventually collected.The resultis ayield profile
2 5
areshown.ThesemeasurementsweredoneattheAN2000ac- oftheselectedisotopesasafunctionoftheerosiontime.Depth
celeratoroftheINFNNationalLaboratoriesofLegnarousing profilesofisotopicabundancesof16O,17Oand18OinLUNA
anαbeamwithabeamsizeof1mm2 andαenergyof1.8and isotopicallyenrichedTa O /TalayersweremeasuredbySIMS
2 5
2.0MeV.Theelasticallyscatteredαparticlesweredetectedby at the Department of Physics and Astronomy of the Univer-
a silicon detector at 160◦ with respect to the beam axis. The sityofPadua(Italy),usingaCAMECAIMS-4fspectrometer.
sampleisTa O layeronTabackingwithoxygenenrichedin A 14.5 keV, 3 nA Cs+ primary beam is rastered over a crater
2 5
18Oat90%.Theblackspectrumisacquiredafteranirradiation area of 100 x 100 µm2. The secondary ions 16O−, 17O−, and
of 12 C at the LUNA-400kV and it is possible to derive the 18O−arecollectedduringsputteringonlyfromacentralregion
reduction of the Ta O layer thickness, while the stoichiom- of 30 µm diameter in order to avoid crater-edge effects. High
2 5
etry, which can be deduced from the height of the plateau at mass resolution (M/∆M = 5000) is used in order to eliminate
channels 770-790, remains stable also after long irradiation. mass interferences of 17O− and 18O− with the molecular ions
RBS and NRA are useful to get information on different ele- 16O1H− and 17O1H−, respectively. Measurements of a pure Si
ments and isotopes and their chemical composition in differ- samplewerecomparedtothenaturalSiisotopicabundanceas
ent layers. Therefore with a unique non destructive measure- a check for systematic errors. Fig. 8a shows the 16O−, 17O−
ment,manypiecesofinformation(i.e.thickness,isotopicand and 18O− yield profiles as a function of the erosion time col-
A.Bestetal.:Undergroundnuclearastrophysics:whyandhow 9
CombiningtheinformationfromalltheseIBAtechniquesit
ispossibletoreachaprecisionoffewpercentonthenumberof
targetatoms,whichisthegoalofnuclearastrophysicsstudies.
17O- (a)
105 7 Conclusions
16O-
LUNA started its activity almost 25 years ago with the goal
c/s] ofexploringthefascinatingdomainofnuclearastrophysicsat
eld [104 18O- verylowenergy.Duringitsactivity,LUNAhasproventhatdi-
Yi rect measurements in nuclear astrophysics benefit from going
in an underground environment, like the National Laborato-
ries of Gran Sasso. For the first time, the important reactions
103 Beam-spot Out-spot thatareresponsibleforhydrogenburningintheSunhavebeen
70 studied down to the relevant stellar energies. In recent years,
(b) manyothersimilarprojectshavebeenstarted:newaccelerators
60 17O
willbeinstalledinundergroundlaboratories,e.g.,LUNA-MV
50 in Europe [22], CASPAR in the US [49], and JUNA in China
%]
ance [ 40 16O [a5n0d].wTilhlebneefwocLuUseNdAo-nMhVelimumacbhuinreniwngillresatacrttioonpser[a2t2in].gin2018
nd 30
bu TaO /Ta
A 20 in2te5rface
Acknowledgments
10 18O
FinancialsupportbyINFN,FAI,DFG(BE4100-2/1)andOTKA
0
0 100 200 300 400 500 (K101328)isgratefullyacknowledged.
Sputter time [s]
References
1. C.E.Rolfs,andW.S.Rodney,CauldronsintheCosmos(TheUni-
versityofChicagoPress,1988).
Fig. 8. (Colour online) (a) Depth yield profiles of secondary 16O−,
2. C. Iliadis, Nuclear Physics of Stars (New York: Wiley-VCH,
17O− and18O− ionsasafunctionoftheerosiontime,asobtainedby
SecondaryIonMassSpectrometryanalysesofaTa O /Tasampleen- 2007).
2 5 3. D.Bemmereretal.Eur.Phys.J.A24,313(2005).
richedin17O.(b)16O,17Oand18Oisotopeabundancedepthprofiles
4. R.Bonettietal.,Phy.Rev.Lett,825205(1999).
calculatedfromtheyieldprofilesreportedinpanel(a).Thedepthof
the Ta O /Ta interfaces are also indicated with vertical lines. Mea- 5. C.Casellaetal.,Nucl.Phys.A706203(2002).
2 5 6. L.Gialanellaetal.,RevistaMexicanadeFisica43169(1997).
surementsweretakenintheregionofthetargetexposedtotheproton
7. D.A.Scottetal.,Phys.Rev.Lett.109,208001(2012).
beam at LUNA (beam-spot, dashed lines) and in peripheral regions
8. B.Limataetal.,Phys.Rev.C82,015801(2010).
notexposedtotheprotonbeam(out-spot,continuouslines).
9. M.Martaetal.,Phys.Rev.C78,022802(2008).
10. M.Martaetal.,Phys.Rev.C83,045804(2011).
11. E.G.Adelbergeretal.,Rev.Mod.Phys.70,1265(1998).
12. E.G.Adelbergeretal.,Rev.Mod.Phys.83,195(2011).
lectedbothinout-spotandbeam-spotregionsonarepresenta- 13. U.Greifeetal.,Nucl.Inst.MethA350327(1994).
tivetargetenrichedwith17O.Fig.8breportsthecorresponding 14. A.Formicolaetal.,Nucl.Inst.Meth.A507609(2003).
isotope ratio profiles extracted from the SIMS profiles of Fig. 15. C.Brogginietal.,Ann.Rev.Nucl.Part.Sci.60,53(2010).
8a.Dataclearlyshowabroadcentralthicknesswithflatabun- 16. H.Costantinietal.,Rep.Progr.Phys.72,086301(2009).
dances.Outsidetheplateau,yieldsmaybeaffectedbyseveral 17. T.Szu¨csetal.,Eur.Phys.J.A44,513(2010).
18. F.Cavannaetal.,Eur.Phys.J.A50179(2014).
phenomena,suchastransientsofthesputteringprocessdueto
19. A.Caciollietal.,Eur.Phys.J.A39,179(2009).
thesurfaceandpresenceofcontaminantsatthesurfaceandat
theTa O /Tainterface.Anadditional16Ocontaminationisal- 20. C.Brunoetal.,Eur.Phys.J.A5194(2015).
2 5 21. F.Ka¨ppeleretal.,Rep.Prog.Part.Phys.43419(1999).
waysobservedinbeam-spotmeasurement,mostprobablydue
22. M.Aliottaetal.,Heliumburningandneutronsourcesinthestars
to beam-induced O migration both from the surface and from
inthistopicalissue.
the Ta O /Ta interface. However, its effect on the total thick-
2 5 23. S.Falahatetal.,Nucl.Instr.Meth.A70053(2013).
ness remains usually negligible. In addition, beam-spot mea-
24. Jaegeretal.,Phys.Rev.Lett.87202501(2011).
surement shows a thickness reduction, indicated by the drop 25. A.Rindietal.,Nucl.Inst.Meth.A272871(1988).
in the yields occurring at earlier sputter times with respect to 26. P.Bellietal.,IlNuovoCimentoA101959(1989).
the out-spot sample, in agreement with results from RBS and 27. Z.Debickietal.,Nucl.Phys.B-ProceedingsSupplements196
NRA. 429(2009).
10 A.Bestetal.:Undergroundnuclearastrophysics:whyandhow
28. A. Best et al., NIM A to be published, doi:
10.1016/j.nima.2015.12.034.
29. M.Andersetal.,Eur.Phys.J.A49,28(2013).
30. D. Bemmerer et al., J. Phys. G: Nucl. Part. Phys. 36 045202
(2009).
31. F.Striederetal.,Phys.Lett.B70760(2012).
32. A.Formicolaetal.,Phys.Lett.B591,61(2004).
33. D.Scottetal.,Phys.Rev.Lett.109,202501(2012).
34. C.Bordeaunuetal.,Nucl.Instr.Meth.A693220(2012).
35. A.Kontosetal.,Nucl.Instr.Meth.A664272(2012).
36. H.Costantinietal.,Eur.Phys.J.A27,s01,177(2006).
37. D.Bemmereretal.,Phys.Rev.Lett.97122502(2006).
38. M.Andersetal.,Phys.Rev.Lett.113042501(2014).
39. F.Cavannaetal.,Phys.Rev.Lett.115252501(2015).
40. M.Martaetal.,Nucl.Insr.MethA569727(2006).
41. C.Casellaetal.,Nucl.Insr.Meth489160(2002).
42. J.Go¨rres,K.U.Kettner,H.Krawinkel,C.Rolfs,Nucl.Insr.Meth
177,295(1980).
43. A.DiLevaetal.,Phys.Rev.C89015803(2014).
44. A.Caciollietal.,A&A533A66(2011).
45. A.Caciollietal.,Eur.Phys.J.A48,1(2012).
46. M.Martaetal.,Phys.Rev.81055807(2010).
47. A.Bergmaier,G.Dollinger,andC.M.Frey,Nucl.Inst.andMeth.
B638136(1998).
48. F.G.R.A. Benninghoven, H.W. Werner, Secondary Ion Mass
Spectrometry: Basic Concepts, Instrumental Aspects, Applica-
tions,andTrends(Wiley,NewYork,1987).
49. F.Striederetal.,BulletinoftheAmericanPhysicalSociety-2015
FallMeetingoftheAPSDivisionofNuclearPhysics60(2015).
50. WUYu-Chengetal.,ChinesePhysicsC370860012013.