Preparing isomerically pure beams of short-lived nuclei at JYFLTRAP

0.26 MB·English

by  T. Eronen

Preparing isomericallypure beamsofshort-livednuclei atJYFLTRAP
∗
T.Eronena, ,V.-V.Elomaaa,U.Hagera,1,J.Hakalaa,A.Jokinena,A.Kankainena,S. Rahamana,
J.Rissanena,C.Webera andJ.A¨yst¨oa
aDepartment of Physics, P.O. Box 35 (YFL), FIN-40014 Universityof Jyv¨askyl¨a, Finland
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2Abstract

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Anewproceduretoprepareisomericallycleansamplesofionswithamassresolvingpowerofmorethan1×105 hasbeendeveloped
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Jat the JYFLTRAP tandem Penning trap system. The method utilises a dipolar rf-excitation of the ion motion with separated
oscillatory fields in the precision trap. During a subsequentretransfer to thepurification trap, thecontaminants are rejected and
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as a consequence, the remaining bunch is isomerically cleaned. This newly-developed method is suitable for very high-resolution
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cleaning and isat least a factor of fivefaster thanthemethodsused sofar in Penningtrap mass spectrometry.

]
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eKeywords: Penningtrap, isobaricseparation, isomericseparation
-PACS: 07.75.+hMass spectrometers, 21.10.DrBindingenergies andmasses
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c
u
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[1. Introduction The JYFLTRAP system consists of a radiofrequency

quadrupole (RFQ) cooler and buncher [4] and two Pen-
1
v A contamination-free ion beam is desirable for any ex- ningtraps[5]situatedinsidethesame7Tsuperconducting
4periment with radioactive ions. Mass spectrometry with solenoid.TheRFQisusedforcoolingandbunchingofthe
0Penning traps, where the cyclotron frequency in a strong ion beam from the IGISOL separator. Isobaric as well as
9magnetic field is determined [1], is no exception to this; isomericseparationandmassmeasurementsareperformed
2
.contaminants do not just deterioratethe lineshapes of the withthe Penningtraps.Thefirst,purificationtrapisused
1obtained spectra but also introduce systematic frequency for isobaric cleaning [5] and in most of the cases studied,
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shifts. it has been sufficient to provide contaminant-free samples
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0 Several purification steps are taken before the ions end forexperiments.Thesecond,precisiontrapisusedforiso-
:up in the actual measurement stage. At the IGISOL fa- meric cleaning and high-precision mass measurements. In
v
cility in Jyva¨skyla¨ [2], or at ISOL facilities in general, for this article, complete purification schemes for separating
i
Xinstance at ISOLDE,CERN [3], a coarsemass selectionis isobarsandclose-byisomerswiththeJYLFTRAPPenning
rperformed with magnetic separators soon after extracting trapsetuparepresented.
a
ionsfromtheionsource.Typicallyamassresolvingpower
R = M = ν from 200 to 5,000 is obtained,
∆MFWHM ∆νFWHM 2. Isobariccleaningwiththe purificationtrap
whichisadequateforaselectionofaparticularmassnum-
ber A, delivering a beam composed of isobars. These can
Thegas-filledpurificationtrapisusedtoseparateisobars
bepartiallyselected,forexample,bychemicalmeansorby
by employing the sideband-cooling technique [6]. At the
selective laser ionization. A mass separation in a Penning
traprequiresaresolvingpowerof104 to105.Isomericsep- JYFLTRAPexperiment,thefollowingisobaricpurification
schemeis used:
arationismoredemanding:evenamassresolvingpowerof
morethan106 mightbe needed. (i) AnionbunchfromtheRFQiscapturedinthepurifi-
cation trap. The injection side potential wall is low-
ered for a short period of time when ions are trans-
ferredinto the trapandonce the ions are inside, the
∗
Correspondingauthor
potentialwallisrestored.
Email address: [email protected] (T. Eronen).
1 Present address: TRIUMF, 4004 Wesbrook Mall, Vancouver, (ii) Theionsarekeptinthetrapfor20to200msinorder
BritishColumbia, V6T 2A3, Canada tolet the axialandthe cyclotronmotioncooldown.
Preprintsubmitted toElsevier 2February 2008
(iii) A dipolar magnetron excitation is applied typically
for 10 ms to establish a magnetron radius of more

than 1 mm for all ions. This assures that none of
the ionscanbe transferredthroughthe 2-mmorifice
1000
betweenthe traps. 54 +
(iv) A mass-selective quadrupolar excitation with a fre- Fe
quency of νc = 21πmq B is applied. This sideband ex- a.u. 100
cνicta=tioνn+a+ttνh−ewsuilml cfornevqeuretntchyeomfbaogtnhetrraodniamlmotoiotinonosf nts / 54mCo+ & 54Co+
theionsintoacyclotronmotion,whichismuchfaster ou
C 10
cooledunderthepresenceofabuffergas.Asaconse-
quence, the ions are mass-selectively driven back to
the axisofthe trap.
1
Theachievedmassresolvingpowermostlydependsonthe
50 100 350 400 450
buffer gas pressure and on both the duration and the am-
(Excitation frequency - 1991800) / Hz
plitude ofthe excitation.As longasthe trapis notloaded
withtoomanyions,R≤105canbeobtained.Anexample
Fig. 2. Purification scan for mass A = 54. The isomeric and the
of a quadrupolar rf-frequency scan around A = 26 in the groundstateof54Coarenotseparated.Theresolutioncouldnotbe
purificationtrapisshowninfig.1.Sucharesolvingpoweris improved, since the yield of stable 54Fe is overwhelming compared
even enough to separate the long-livedisomer 26mAl from withthe yieldof54Co.
itsgroundstate.Whenthepurificationtrapisloadedwith
3. Isomericcleaningwiththe precisiontrap
Sincetheprecisiontrapissituatedinultra-highvacuum
(p≤10−7 mbar)andboththecoolingandre-centeringef-
fect of the buffer gas are missing, the ion motions have to
be excited differently in order to produce clean ion sam-
26 +
Mg ples. Here, a dipolar excitation at the reduced cyclotron
100 frequencyν+ is typicallyused.
u. 26 + To remove contaminant ions prior to the measurement
a. 26mAl+ Al process,their radialamplitudes need to be increasedsuch
s / thattheywillnotaffectthe ionsofinterest.Thetimepro-
unt 10 file ofthe excitationresultsinafinite lineshapeinthe fre-
o
C quencydomain,givenbytheFourier-transformation,where
∆νFWHM of the resonance is determined by the time du-
ration T of the excitation, ∆νFWHM ∝ T1. In addition to
1
the normal dipolar cleaning, a Stored Waveform Inverse
50 100 150 200 700 750 800 850
FourierTransform(SWIFT) [7]canbeusedforaselective
(Excitation frequency - 4135000) / Hz broadbandcleaning.
Fig. 1. Purification scan for mass A=26. The excitation time that
was employed was sufficient to separate the isomer and the ground 3.1. Dipolar cleaningwith arectangularpulse
state of 26Al havingdifferent frequencies byapproximately 40 Hz.
When using a rectangular excitation pulse, the corre-
spondingFourier-transformationresultsinasincfunction,
more ions, the trap loses its capability to efficiently bring sin(x)/x,showingacharacteristicsidebandstructure.This
the ions of interest back towards the axis of the trap due isnotinconvenientifthe frequency difference ofthe ionof
to increased space charge. Increasing the quadrupole am- interest and the impurity is well known. Excitation times
plitude and the buffer gas pressure enhances the center- andfrequenciesneedtobeproperlysetthatcontaminants
ing process—unfortunately withexpense ofresolution.If are excited while the ion of interest is not. An excitation
the resolution is compromised (up to ∆νFWHM of 100 to with a rectangular pulse is shown in fig. 3 (a). One of the
200 Hz), the transmission of the trap can be maintained. frequencies for an ion of interest remaining unexcited is
An example ofa purificationscanwith reducedresolution markedwith(2).Despiteallcaretoavoidexcitationofthe
isshowninfig.2.Theisomerandthegroundstateof54Co ionofinterest,theremightstillbesomeexcitation,forex-
arenotresolved,sincethedifferenceoftheircyclotronfre- ampleduetotheresidualgasdampingeffect.Ontheother
quenciesν isonlyabout7Hzandthetraphasbeentuned hand,thecleaningprocesswithasquarewaveexcitationis
c
forbetter transmissionatthe costofhighresolution. more than factor of two faster than that with a Gaussian
2
envelope. f(t) (a) F(ω)
3.2. Dipolar cleaning witha Gaussian envelope
ByusingaGaussianenvelopeintheexcitationtimepro-
FT
file, the excitationpatterninthe frequency domainis also (b)
Gaussian.Thisway,theionofinterestjustneedstobesuf-
ficientlyseparatedfromcontaminantsinordertoavoidan (c)
unwantedincrease ofits motionalamplitudes. The resolu-
tion can be adjusted with the excitation time. This corre-
sponds to the excitation pattern (b) shown in fig. 3. The
contaminant, marked (0), is excited maximally while the 0 1 2 3
t ω
ionofinterest(3)remainsunaffected.
This method is routinely used in many trap setups like Fig. 3. Examples of excitation time profiles f(t) (left side) for (a)
ISOLTRAP[8]atCERN,CPT[9]atANLandLEBIT[10] arectangularexcitationpulse,(b)aGauss-modulatedenvelopeand
atMSU.Theadvantageinusingdipolarcleaningisthatit (c) an excitation with time-separated oscillatory fields. The corre-
canbeappliedeitherinlow-orhigh-resolutionmodes:Us- spondingFouriertransformationsF(ω)inthefrequencydomainare
shown on the right. The position (0) indicates the frequency of the
ingaveryshortdurationandahighamplitudewillremove
contaminantionspeciesand(1)to(3)indicateapossiblefrequency
ionswithinabandwidthofseveralkHz.Or,theamplitude
ofan ionof interest. For the discussionseetext.
can be set low and the duration long to obtain a narrow-
capturedin the purificationtrap,where the ions areaddi-
bandcleaning.
tionally recooled and recentered as described in section 2.
ISOLTRAPhasdemonstratedaselectionofnucleariso-
Inthisway,itispossibletoperformavery-highresolution
mers [11] in combination with a selective laser ionisation
and decay spectroscopy. Here, states of 70Cu were sepa- cleaninginatime-efficientmanner.Forinstance,the mass
doublet 115Sn and 115In has a cyclotron frequency differ-
ratedwithaPenningtrap.Inthecleaningprocess,amass
resolving power of about 2×105 was obtained with a 3-s ence of about 4.5 Hz. Using an excitation time pattern of
(10-80-10)ms (On-Off-On) the different isotopes are fully
excitationtime.Althoughtherewasonlyonecontaminant
separated. An example of a transmission curve after the
to be cleaned with the Penning trap, it should be noted
secondrecenteringinthepurificationtrapisshowninfig.4.
thatusually severalcontaminantsarecleanedinparallel.
80
3.3. Dipolar cleaning withseparated oscillatory fields u. 115Sn (a)
a. 60
The resolution can be further enhanced by using an nts / 40 115
excitation scheme with time-separated oscillatory fields u 20 In
o
C
[12,13,14].Ithasbeenshownthatthelinewidthisreduced 0
by almost 40 % [15]. The reduction is illustrated in fig. 3,
3.0 (b)
comparing the width of the main peak for (a) and (c). In u.
case (c) the ion of interest remains unexcited at the fre- / a.+ 2.0
quencyposition(1).Asinthecaseofarectangularexcita- ρ 1.0
tion profile,bothfrequencies have to be knownaccurately
0.0
beforehand and can be conveniently used for example in
-20 -10 0 10 20
measurementsofsuperallowedbeta-emittersorstableions.
detuning from ν of 115Sn+ ions / Hz
Thismethodislesssuitedforstudyingshort-livednuclides +
withunknownmassvalues.
Fig. 4. Number of detected ions at the microchannel-plate detec-
4. Isomeric cleaning with time-separated tor behind the trap setup as a function of the applied dipolar fre-
oscillatoryfieldsand additionalcooling quency in the precision trap (a). Here, an excitation time pattern
of (10-80-10) ms (On-Off-On) was used. Prior to their ejection the
ionshavebeenextractedbackwards,capturedandre-centeredinthe
In the methods introduced so far, the cleaning is per-
purification trap. The individual peaks are the transmitted ions of
formed in such a way that the contaminants are drivento 115Sn (grey) and 115In (white). The lower panel (b) shows the ex-
orbits with large radii such that they might hit the ring pectedincreaseofthecyclotronradiusρ+ for115Snasafunctionof
electrode.Toavoidexcessiveexcitation,theionsamplecan theapplieddipolarfrequency.Here,thehighesttransmissionoccurs
whenleast excited.
be extracted towards the purification trap: The contami-
nants will hit the electrode surface surrounding the 2-mm After a repeated centering and cooling in the purifica-
diaphragmbetweenthetraps,whiletheionsofinterestcan tiontraptheionbunchistransferredtothe precisiontrap
passthrough.Theremainingcleanedbunchofionsisthen for the actual cyclotron frequency determination. Figure
3
5 demonstrates the prospects of this cleaning procedure
applied to the A = 115 isobars of indium and tin. If no 250
cleaning is applied, 115In is barely visible in the cyclotron
s
roefs1o1n5aSnncetocu11r5vIen(ias).5Ttohe1.raTthioebneutmwebeenr cthane dbeeteqcutaendtiifioends µht / 200 54Co+ (0+ g.s.)
g
wthhaenn3s5e0lecµtsi,nwghoinclhywtheorseeaiffoencstweditbhyaatirmeseoonfaflnitgehxtcsihtaotritoenr e of fli 150 T1/2 = 193 ms
m
aintgthneuimrrbeesrpoefctdievteecctyecdloitornosnifsrienqduiecnacteydνbc.yTvheertcicoarlrebsaprosn.Idn- an ti 250
e
(b)115Snwasremovedbyadipolarexcitationatν+ inthe m 200
precisiontrap,asdescribedinsection3.3,resultinginclean 54mCo+ (isomer)
resonanceof115In.In(c),115Inwasremovedinanalogyto 150 T1/2 = 1.48 min
case(b).
-10 0 10 20
(Excitation frequency - 1991660) / Hz
115In+ 115Sn+
500
400 Fig. 6. Time-of-flight ion cyclotron resonances for ground and iso-
µs (a) meric state of 54Co measured in November 2006. The unwanted
me of flight / 345000000 (b) alised counts sctatilatlitaoetnsorhtyiamvfieeelbodefse.2n0F0colrematsnheewdacuyscsuilnosetgrdod.nipforleaqrueexncciytatdieotnerwmitinhasteiopnaraantedexocsi--
n ti 300 orm clotronresonances[15].Thetheoreticallineshapeforfitting
ea N purposes is extensively described in [14]. Figure 7 shows
m 500
anexamplewhereboththe cleaningandthetime-of-flight
400
resonance were produced with an excitation by separated
(c)
300 oscillatoryfields.
-2 0 2 4 6 8 10
(Excitation frequency - 935100) / Hz
54Co+
Fig.5.Time-of-flightioncyclotronresonancesandatwo-peakfitfor 200
anuncleanedionsample(a).The115Inresonanceisbarelyvisible.In
s
themiddlepanel(b),115Snhasbeencleanedaway,whichremarkably µ
enhances the 115In resonance. In the bottom panel (c), 115In has ht /
g
nboeermnarelimseodvendumfrbomertohfeiopnrsechisaivoinngtraapt.imTheeovfeflritgihcatllbesasrsthinadnic3a5t0eµthse. of fli 150
e 54mCo+
m
5. Application to superallowed QEC-value an ti 200
measurementsof50Mnand54Co me
The motivationto developa fast anduniversalcleaning 150
schemearisesfromarecentproposalforQECmeasurements -10 0 10 20
for50Mnand54Co.Bothnuclideshaverelativelyshorthalf- (Excitation frequency - 1991670) / Hz
lives T of 300 ms and 200 ms, respectively. In addition,
1/2
both have long-lived isomers with low excitation energies
Fig. 7. Time-of-flight ion cyclotron resonances for ground and iso-
ofabout200keV.Ascanbeseenfromfig.2,thestatesare meric state of 54Co using separated oscillatory fields for both the
not resolved with the conventional experimental sequence dipolarcleaningaswellasforthecyclotronresonancemeasurement.
inthe purificationtrap. Adipolarexcitationpatternof(10-55-10)msandaquadrupolarex-
citationpatternof(25-150-25)mswereused.Thedatawereobtained
Thefastestwayofseparatingthegroundandtheisomeric
inMay 2007.
state is to use dipolar cleaning with separated oscillatory
fields.Forthedipolarexcitation,atimeof80msandforthe
repeatedcoolingandcenteringoftheionsinthepurification Thelinewidthsofthecyclotronresonancesshowninfigs.
trap 100 ms are required. The resulting decay losses are 7 and 6 are 5.9 Hz and 3.7 Hz, respectively. In both cases
about 50 %, which are counterbalanced by a rather high atotaltimedurationof200mswasspentintheexcitation
productionrate.Examplesoftime-of-flightresonancesare process. The resonances obtained with time-separated os-
showninfig.6. cillatory fields are 35 % narrower. The uncertainty in the
Theexcitationwithseparatedoscillatoryfieldshasbeen determination of the line center is reduced by a factor of
demonstrated to be well suited also for time-of-flight cy- 2.5.
4
6. Conclusions [11]K. Blaum et al., Laser ionization and Penning trap mass
spectrometry — a fruitful combination for isomer separation
and highprecisionmassmeasurements, Hyp. Int. 162(2005) 1.
Anewhigh-resolutioncleaningschemeemployingexcita-
[12]N.F.Ramsey,Experimentswithseparatedoscillatoryfieldsand
tionwithtime-separatedoscillatoryfieldshasbeendemon- hydrogen masers,Rev. Mod.Phys. 62 (1990) 541.
strated. Unwanted isobaric or isomeric contaminants will [13]G. Bollen, H.-J. Kluge, T. Otto, G. Savard, H. Stolzenberg,
be completely removed after the cleaning excitation when RamseytechniqueappliedinaPenningtrapmassspectrometer,
the ions are retransferredfrom the precision to the purifi- Nucl. Instrum. Meth. Phys. Res.B 70 (1992) 490.
[14]M. Kretzschmar, The Ramsey method in high-precision mass
cationtrapthroughthe narrowchannel.Oncethe cleaned
spectrometry with Penning traps: Theoretical foundations, Int.
ion sample has been recooledandrecenteredin the purifi-
J. Mass.Spectrom. 264 (2007) 122.
cation trap, the sample is ready for extraction to the ex- [15]S.George,K.Blaum,F.Herfurth,A.Herlert,M.Kretzschmar,
periments.Recently,thisnewly-developedcleaningmethod S.Nagy,S.Schwarz,L.Schweikhard,C.Yazidjian,TheRamsey
has been succesfully used to prepare clean samples of the methodinhigh-precisionmassspectrometrywithPenningtraps:
ground states of 50Mn and 54Co, where a mass resolving Experimental results,Int. J. Mass.Spectrom. 264 (2007) 110.
powerofR=3×105 wasneeded.Thiscleaningmethodis
nowroutinelyusedatJYFLTRAP toprovideisomerically
cleanbeamsfor experiments.
Acknowledgements
This work has been supported by the EU’s 6th Frame-
work Programme “Integrating Infrastructure Initiative –
Transnational Access” Contract Number: 506065 (EU-
RONS,JointResearchActivitiesTRAPSPECandDLEP)
and by the Academy of Finland under the Finnish Centre
of Excellence Programmes 2000-2005 (ProjectNo. 44875,
Nuclear and Condensed Matter Physics Programme) and
2006-2011 (Nuclear and Accelerator Based Physics Pro-
grammeatJYFL).
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5