Time-separated oscillatory fields for high-precision mass measurements on short-lived Al and Ca nuclides

0.3 MB·English

by  S. George

epl draft
Time-separated oscillatory fields for high-precision mass measure-
ments on short-lived Al and Ca nuclides
S.George1,2 (a),G. Audi3, B.Blank4,K. Blaum1,2,5, M. Breitenfeldt6, U.Hager7 (b), F. Herfurth1,A.
Herlert8, A. Kellerbauer5, H.-J. Kluge1,9, M. Kretzschmar2, D. Lunney3, R. Savreux1, S. Schwarz10,
L. Schweikhard6 and C. Yazidjian1
8
0 1 GSI, Planckstraße 1, 64291 Darmstadt, Germany
0 2 Institut fu¨r Physik, Johannes Gutenberg-Universita¨t, 55099 Mainz, Germany
2 3 CSNSM-IN2P3-CNRS, 91405 Orsay-Campus, France

n 4 Centre d’Etudes Nucl´eaires de Bordeaux-Gradignan, 33175 Gradignan Cedex, France
a 5 Max-Planck-Institut fu¨r Kernphysik, 69117 Heidelberg, Germany
J
6 Institut fu¨r Physik, Ernst-Moritz-Arndt-Universita¨t, 17487 Greifswald, Germany

7 7 Department of Physics, University of Jyva¨skyla¨, P.O. Box 35 (YFL), 40014 Jyva¨skyla¨, Finland
1
8 CERN, Physics Department, 1211 Geneva 23, Switzerland

] 9 Physikalisches Institut, Ruprecht-Karls-Universita¨t, 69120 Heidelberg, Germany
x 10 NSCL, Michigan State University, East Lansing, MI 48824-1321, USA
e
-
l
c
u PACS 07.75.+h–Mass spectrometers
n
[ PACS 21.10.Dr–Binding energies and masses
PACS 27.30.+t–Properties of specific nucleibetween mass 20 and mass 38

1 PACS 27.40.+z–Properties of specific nucleibetween mass 39 and mass 58
v PACS 32.10.Bi–Atomic masses, mass spectra, abundances, and isotopes
3
9 Abstract. - High-precision Penning trap mass measurements on the stable nuclide 27Al as well
5 asontheshort-livedradionuclides26Aland38,39Cahavebeenperformedbyuseofradiofrequency
2
excitation with time-separated oscillatory fields, i.e. Ramsey’s method, as recently introduced
.
1 for the excitation of the ion motion in a Penning trap, was applied. A comparison with the
0 conventional method of a single continuous excitation demonstrates its advantage of up to ten
8 timesshortermeasurements. Thenewmassvaluesof26,27Alclarifyconflictingdatainthisspecific
0 mass region. In addition, the resulting mass values of the superallowed β-emitter 38Ca as well
v: as of the groundstate of the β-emitter 26Alm confirm previous measurements and corresponding
i theoretical corrections of theft-values.
X
r
a
Introduction. – Superallowed0+ →0+ β-decaysare isospinT =1 analogstates irrespectiveof theoreticalcor-
sensitive probes for testing fundamental concepts of weak rections. Including these modifications the corrected ft-
interaction. Hardy and Towner [1] addressed 20 such de- value can be written in the form
cays covering the nuclear chart from 10C to 74Rb and
showedaconsistentpictureallowingaconfirmationofthe Ft ≡ ft(1+δ′ )(1+δ −δ )
R NS C
conserved-vector-current (CVC) hypothesis. Three prop-
K
ertiesofthenucleartransitionsaremergedinthecompara- = =const., (1)
2G2(1+∆V)
tivehalf-lifeft: thetransitionenergyQ ,thehalf-lifeof V R
EC
the mother nuclei t , and the branching ratio R. Since
1/2 whereK isanumericalconstantandG isthevectorcou-
the Fermi-type β-decays are only effected by the vector V
plingconstant. δ denotestheisospin-symmetry-breaking
part of the hadronic weak interaction, the CVC hypoth- ′ C
correction, δ and δ are the transition-dependent
esis claims identical ft-values for all transitions between R NS
parts of the radiative correction, while ∆V is transition-
R
(a)E-mail: [email protected] independent [2]. Moreover, δC and δNS depend on the
(b)presentaddress: TRIUMF,Vancouver, B.C.,V6T,2A3 nuclearstructureofthespecificnuclei. In[1]theaveraged
p-1
S. George et al.
Ft-valueofthe12bestknown-caseswasdeterminedtobe MCP3 or
Channeltron
Ft=3072.7±0.8 s. (2)
precision
Penning
trap
MCP2
28Si
preparation
Penning
(p,α) p,γ) trap
(
stable alkali MCP1
26Al 27Al reference ion 2nd pulsed
source drift tube
(p,γ) (p,n) (p,γ) I(ScoOnLtiDnEuo buesa)m RFQ trap 1dsritf tp tuulbseed
60 keV
24Mg 25Mg 26Mg HV platform
(n,γ) (n,γ)
Fig. 2: Experimental setup of the mass spectrometer
Fig.1: Sectionofthenuclearchartintheregionofinterestfor ISOLTRAP. The RFQ trap, the preparation and precision
theAlmassmeasurementsindicatingthereactionsconnecting Penning traps as well as the reference ion source are shown.
thenuclides. Grey boxes denotestable isotopes. Monitoring the ion transfer as well as the time-of-flight mea-
surementsforthedeterminationofthecyclotronfrequencyare
RecentlySavardet al.[3]publishedanewQ valueof done with micro-channel-plate (MCP) detectors or a channel-
EC
the superallowed decay of 46V measured with the Cana- tron[20]. Intheinsetacyclotronfrequencyresonancefor26Al
dian Penning trap facility. Their Q-value differed by ions with an excitation time of 1.5s is given.
2.19 keV corresponding to 2.5 standard deviations from
a combination of two contradictory data [1,4,5]. The
27Al address a problem of conflicting data derived from
high Ft-value that resulted was conspicious with respect
various reaction Q-value measurements [16]: The masses
to the average Ft-value of the 12 best-known superal-
of the stable nuclides 24,26Mg [17] and 28Si [18,19] have
lowed transitions. Together with an updated data set of
all been measured with Penning trap facilities. They are
these 12 transition it is leading to a new average value
also related via reaction Q-values, namely 24,26Mg by a
of Ft = 3073.66(75)s. This triggered a careful remea-
pair of (n,γ) reactions through the stable isotopes 25Mg
surement of the Canadian Penning trap result as well as
and 26Mg, and 28Si by a pair of (p,γ) reactions through
a search for possible systematic differences between Pen-
the isotope 27Al. In addition, the masses of 25Mg and
ning trapmeasurements and Q-values fromreactionmea-
26Mg are also related by a sequence of a (p,γ) reaction
surements as performed by Hardy and coworkers[6]. The
and a (p,n) reaction through the isotope 26Al (see fig. 1).
result of Savard et al. has meanwhile been confirmed by
The two most precise values of the 25Mg(n,γ)26Mg reac-
thePenningtrapfacilityJYFLTRAP[7]. Obviously,there
tion agree neither with each other nor with the results
is a strong demand to remeasure the reaction Q-values of
from the combined 25Mg(p,γ)26Al and 26Mg(p,n)26Al re-
superallowed decays with high-precision Penning trap fa-
actions. Asimilarinconsistencyoccursforthenuclide27Al
cilities.
in relation to the magnesium isotopes and 28Si. Thus we
The Penning trap mass spectrometer ISOLTRAP has
performed direct mass measurements of 26,27Al to resolve
previously already addressed three superallowed emitters
this unsatisfactory situation.
of the 12 best-known cases: 22Mg [8], 34Ar [9], and
74Rb [10]. In the present paper, we contribute with mass Experimental Setup and measurement pro-
measurements of 26Al, itself a daughter in a superallowed cedure. – All measurements reported here have
decay, and 38Ca. For the mass measurements Ramsey’s been performed with the triple-trap mass spectrometer
methodofseparatedoscillatoryfields forexcitationofthe ISOLTRAP [21–23] (see fig. 2) at the online mass sepa-
ion motion in a Penning trap was applied. In [11–15] rator ISOLDE/CERN [24]. The experimental procedure
the adaption of Ramsey’s method to Penning trap mass is as follows: The 60-keV continuous ISOLDE ion beam
spectrometryhasbeenderivedandexperimentallydemon- is accumulated in a linear gas-filled RFQ ion beam cooler
strated. The precision of the frequency determination and buncher [25]. After a few milliseconds the accumu-
could be improved by more than a factor of three com- lated ions are transferred in a bunch to the first Penning
paredtotheconventionalexcitationmethodandscanpro- trap [26], which is used for mass-selective buffer-gas cool-
cedure. This allows one to reach a given precision up to ing [27] to remove isobaric contaminations. In the sec-
ten times faster than before. ond Penning trap the actual mass measurements are per-
Furthermore, the direct mass measurements of 26Al and formed. 39K+ ions from the stable alkali reference ion
p-2
High-precision mass measurements
source for the measurements of the calcium isotopes and
23Na+ ionsfromISOLDEforthealuminiumisotopeswere
used for calibration of the magnetic field strength.
s
The measurements are based on the determination of ht / 220
g
(a) of fli
9/2 e
m
n ti 200
τ a
e
1 M 27 +
nits -1 0 1500 trf/ ms 180 Al
u -2 -1 0 1 2
e / arb. τ1 τ0 τ1 (b) C - 3366727.9 Hz / Hz
d 9/2
plitu Fig. 4: Time-of-flight cyclotron resonance of 27Al+. A Ram-
m sey excitation scheme was chosen with two excitation pulses
A 0 t / ms
rf of 100ms duration interrupted by a 1.3s waiting period. The
solidlineisafitofthetheoretically expectedlineshapetothe
-9/2 data[11,13].
0 100 1400 1500
Fig.3: (a)Conventionalexcitationschemewithacontinuousrf
ment[13]. Insteadofexposingtheionscontinuouslytothe
pulseofτ=1.5s,(b)excitationwithtwo100-msRamseypulses
τ interruptedby a τ =1.3s waiting period. externalradiofrequencyfield[32],twoexcitationpulsesin-
1 0
terruptedbyawaitingperiodareusedtoexcitetheradial
ion motion (see fig. 3). The shape of the TOF-ICR curve
the ions cyclotron frequency ν = qB/(2πm), which is
c
(see fig. 4) differs significantly from the conventional one
proportional to the charge-to-mass ratio q/m as well as
(see inset of fig. 2). The smaller width of the central res-
to the magnetic field strength B [28]. The ion motion is
onance peak and the more pronounced sidebands allow a
excited by an azimuthal quadrupolar radiofrequency field
more precise or faster frequency measurements [12,13].
via the four-fold segmented ring electrode of the Penning
Penningtrapmassmeasurementsontheisotopes26,27Al
trap [29] before the ions are extracted and transported to
aswellason38Ca19F+and39Ca19F+havebeenperformed
a detector. The frequency of the excitation field is varied
with conventional excitation of the ion motion as well
around the expected value to record a time-of-flight ion
as with Ramsey’s method of separated oscillatory fields.
cyclotronresonance (TOF-ICR) [30] (see inset of fig. 2) a
The number of recorded ion events per resonance is ap-
well-established method in use at several facilities [31].
proximately 3000, which allows a comparison of system-
The aluminium isotopes were produced by bombarding
a silicon carbide target with 3·1013 1.4-GeV protons per atic uncertainties between the two methods as well as a
demonstration of the advantage of the new technique. In
pulse from the CERN proton synchrotron booster. A hot
fig. 5 the cyclotron-frequency ratios and their uncertain-
plasma sourceionized the atoms releasedfrom the heated
ties are shown (see also tab. 1). For simplicity the aver-
target. Isotopicseparationwasperformedwiththegeneral
age ratio of each ion species is subtracted. Measurements
purpose separator [24] with a resolving power of typically
withthe conventionalexcitation(filledsymbols)andwith
1000.
Ramsey-type excitation (empty symbols) can be directly
For the calcium isotopes a heated titanium foil target
compared. The total excitation time used for the alu-
was used in combination with a hot tungsten surface for
minium isotopes was 1.5s. During the Ramsey-type ex-
ionization. The high-resolution separator [24] served for
citation two 100 ms excitation pulses were interrupted by
mass separation with a resolving power of about 3000.
a 1.3s waiting period, so that the total excitation cycle
In order to suppress isobaric contaminations by 38K ions,
remained 1.5s.
CF was added in the ISOLDE ion source and the ions
4 Duetotheshorthalf-lifeof39Ca(T =859.6(1.4)ms)
were delivered to ISOLTRAP in form of the molecular 1/2
theexcitationtimewasreducedto1.2sandfor38Ca(T
sidebands 38Ca19F and 39Ca19F. The mass of the ion is 1/2
= 440(8)ms) to 900ms, respectively. As before the dura-
obtainedbymeasuringthefrequencyratiotoawell-known
tion of the Ramsey excitation pulses was 100ms and the
reference ion.
total cycle length has been kept constant.
The Ramsey technique. – In order to improve First, for all ion species, no significant difference of the
the precision of the frequency determination, separated extractedfrequencyratios(fig.5)andthusforthederived
oscillatory fields, as introduced by Ramsey to nuclear- mass excess values (fig. 6) is found between conventional
magnetic-resonance experiments, were adopted [11,12] and Ramsey excitation. All final mass excess values with
and used for the first time in an online mass measure- theirtotaluncertaintiesderivedfromtheISOLTRAPmea-
p-3
S. George et al.
Table 1: Ratios of the cyclotron frequencies of the isotopes Table 2: Mass excesses of themeasured nuclides.
investigated in this work.
Ion T Mass excess / keV
1/2
Ion Reference Frequency ratio,ννrioenf 26Al 717(24)ky -12210.18(22)
27Al stable -17196.92(23)
26Al 23Na 1.1303707761(104)
38Ca 440(8)ms -22058.11(60)
27Al 23Na 1.1736365541(108)
39Ca 859.6(1.4)ms -27282.59(61)
38Ca19F 39K 1.4622576087(162)
39Ca19F 39K 1.4877789303(165)
V 26 27 38 39
ke 0.3 Al Al 1.8 Ca 1.8 Ca
ce / 0.2 0.4 1.2 1.2
n
ere 0.1 0.2 0.6 0.6
s diff 0.0 0.0 0.0 0.0
s
e
exc -0.1 -0.2 -0.6 -0.6
ss -0.2 -1.2 -1.2
a
M -0.4
-0.3 -1.8 -1.8
Fig.6: Mass excessesof all fourradionucidesderivedfrom the
ISOLTRAP measurements (full dots). In addition the mass
excessesderivedfrommeasurementsusingtheconventionalex-
citation method (squares) as well as using theRamsey excita-
tion(triangles) arepresented. Theerrorbarsnexttothedata
pointsrepresent thestatistical uncertainties.
Fig. 5: Difference between the measured cyclotron frequency
ratiosRandtheiraveragevalueRave. Eachdatapointcontains
thesamenumberofrecordedions. Datapointswithfilledsym- The masses of 26Al and 27Al. – Five different
bolshavebeentakenwiththeconventionalexcitationmethod, measurements of four types of reactions are contributing
while empty symbols denote data points where the Ramsey to the mass value of 26Al as published in the Atomic-
method was used. In (a,b) the differences for the ratios R- Mass Evaluation AME2003 [34]. All of them are shown
Rave for 26,27Al+ to 23Na+ are shown. In (c,d) the same is in fig. 7(a). The highest impact on the final AME2003
plotted for the ratios of the molecular sidebands 38,39Ca19F+ mass have two independent measurements of the reaction
compared to 39K+. The grey band indicates the uncertainty 25Mg(p,γ)26Al [35,36]. In addition Q-values of the reac-
of theaverage ratio.
tions 26Mg(p,n)26Al [37], 26Mg(3He,t)26Al-14N()14O [38],
and 42Ca(3He,t)42Sc-26Mg()26Al [38] contribute to the
mass of 26Al. The value derived from the reaction
surements are presented in tab. 2. Second, the statistical 26Mg(p,n)26Aldisagreeswithformermeasurementsofthe
uncertainties of the 38,39Ca measurements, which are less samegroup[39,40],as wellas alsodisagreeswith the four
than 2 · 10−9 (shown separately in fig. 6), show clearly other measurements by more than three standard devia-
thatwithRamsey-typeexcitationonlythe systematicun- tions.
certainties are limiting the precision of the mass values. A new atomic-mass evaluation was performed with the
Thesystematicuncertaintiesaredominatedbyundetected ISOLTRAP data determined in this work which are the
magnetic field drifts during the measurements [33]. How- first derived from a Penning trap mass measurement.
ever, there is no difference in the statistical uncertainty The data from the 26Mg(p,n)26Al reaction is not taken
between the continuous 1.5s excitation and a Ramsey- into account anylonger for a new AME value. Instead
type excitation of equal length in the measurements of the ISOLTRAP value is now contributing with 8.3% to
the aluminium isotopes. Thus, the Ramsey-type excita- the new atomic-mass evaluation, whereas the reaction Q-
tion is favorable for shorter excitation cycles: It allows values of 26Mg(p,n)26Al, 26Mg(3He,t)26Al-14N()14O, and
for a 900ms excitation a frequency determination which 42Ca(3He,t)42Sc-26Mg()26Al are contributing with 78.4%,
is three times more precise than using the conventional 8.3%, and 5.0%, respectively (see fig. 7(b)).
excitation. This gain in precision in dependence of the Altogether five reaction Q-value measurements of the re-
overall excitation cycle has been observed for a constant actions26Mg(p,γ)27Aland27Al(p,γ)28Sihavecontributed
number of collected ions [12], comparable to the data of tothe massvalue of27Al. Togetherwithsomeotherreac-
the mass measurements presented here. tionsQ-valuestheyaredisplayedinfig.8(a). Thethreein-
p-4
High-precision mass measurements
dependentdata points ofthe reaction26Mg(p,γ)27Alcon-
tributed 16.0%and the two measurements of the reaction
27
27Al(p,γ)28Si contributed 84.0% to the mass value. The Al 2 AME 2003
(a) 16.0%
eV 1 84.0%
k
e / 0
c
26Ae / keVl 01..50 A(Ma)E 6270.20%3 26Mg(p,n)26Al 6.9% 4 2 C -a26(M3Hge(),t2)64A2Slc Mass differen ----4321 27 24 26Mg(p,)27Al 27Al(p,)28Si
c Al(p, ) Mg
n
e 0.0 -5
ass differ-0.5 25Mg(p,)26Al 21.7% 2 6 M -1g4(N3H()e14,Ot)26Al 4.2% AME 2003 + ISOLTMReAaPsurement #
M-1.0 2
V 1 (b) 12.9% 27Al(p,)28Si
e 19.9%
Measurement # e / k 0
c
ce / keV 01..50 A(MbE)7 82.04%03 + I2S6MOgL(TpR,nA)2P6Al 8.3%4 2 C -a26(M3Hge(),2t)64A2Slc8.3% Mass differen ----4321 27Al(p, )24Mg 26Mg(p,)27Al 67.2% ISOLTRAP
n
ss differe-00..50 25Mg(p,)26Al 2 6 M -1g4(N3H()e14,Ot)26Al5.0% ISOLTRAP -5 Measurement #
a
M-1.0 Fig. 8: Same as fig. 7 butfor 27Al.
Measurement #
38CaatISOLTRAP.Ourmassuncertaintyis0.59keV[12],
Fig. 7: (a) Mass difference of 26Al between various reaction whereasbothvaluesdifferonlyby0.43keV.Thereasonfor
experimentsand theaverage value of theatomic-mass evalua- ourlargeruncertaintydespiteusingthe favorableRamsey
tionfrom2003[34],heredenotedaszeroline. Theboxesmark excitation technique is the factor of five higher cyclotron
themeasurementswhichcontributetotheacceptedvalue. The frequency at MSU, since they measured doubly charged
impact of the measurements is reflected by the percentage of 38Ca2+ ions in a 9.4T magnetic field while we measured
thecontribution to the final value. (b) Same as graph (a) but the singly charged, heavier molecular sideband 38Ca19F+
including the recent ISOLTRAP measurement. Note that the
in a 5.9T magnetic field. The higher frequency leads to
zero line is shifted due to the new average value in the new
a smaller relative uncertainty, since the absolute uncer-
atomic-mass evaluation.
tainty of the frequency determination is identical. In the
mass measurement of ISOLTRAP is contributing 20.0% new atomic-mass evaluation the ISOLTRAP value con-
to the new average value (fig. 8(b)). Therefore the influ- tributes nevertheless 17.7%. The main contribution of
ence of the reaction measurements of 26Mg(p,γ)27Al and 82.3%comesfromtheLEBITmeasurement(seefig.9(a)).
27Al(p,γ)28Siisreducedto12.9%and67.2%,respectively. Intotalthe mass value is increasedby 0.08keV compared
All contributing data points agree within their uncertain- to the LEBIT value.
ties. Note that no Q-value measurement of the reaction Two Q-value measurements of the reaction 39K(p,γ)39Ca
27Al(p,α)24Mgcontributestothemassvalue,eventhough have so far been performed (see fig. 9(b)). Despite their
themostprecisedatapointhasanuncertaintyofonly0.21 relativesmalluncertaintiesof1.8 keV[43]and6keV[44],
keV [41]. However, this measurement has been rejected they differ by more than 12 keV. Until now the value of
since it deviates by more than four standard deviations Rao et al. [43] was used as the AME value. The new
from the accepted mean value and disagrees with earlier ISOLTRAP value disagrees with this value by more than
published values by the same group. fourstandarddeviations,butisinperfectagreementwith
the value of Kemper et al. [44]. Since their uncertainty is
The masses of 38Ca and 39Ca. – Until Bollen et
afactor often larger,the ISOLTRAP value determines in
al. performedamassmeasurementwiththe Penningtrap
the new atomic-mass evaluation the value by 100%.
mass spectrometer LEBIT [42], the mass of 38Ca was de-
terminedfromthe mostpreciseofthree Q-valuemeasure- Conclusion.– Wehavedemonstratedthatseparated
ments of the reaction 40Ca(p,t)38Ca. The uncertainty of oscillatory fields are an excellent tool for high-precision
the 38Ca mass value was 5 keV. The LEBIT measure- mass measurements. The final mass values are limited
ment reduced the uncertainty by more than an order of by our systematic uncertainties, since statistical uncer-
magntitude to 0.28keV and loweredthe value by 3.3keV. tainties of less than 2 · 10−9 have been achieved. This
We confirmed this new value by measuring the mass of shows the potential for future mass measurements on the
p-5
S. George et al.
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be dominated by the statistical uncertainty of the mea-
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This work was supported by the German BMBF un-
[32] K. Blaum et al.,J. Phys. B, Vol. 36 2003, p. 921.
der contracts 06GF151, 06GF186I and 06MZ215, by the
[33] A. Kellerbauer et al., Eur. Phys. J. D, Vol. 22 2003,
EU under contracts HPMT-CT-2000-00197 (Marie Curie p.53.
Fellowship) and RII3-CT-2004-506065 (TRAPSPEC), by [34] G. Audi and O. Bersillon and J. Blachot and A.H.
the European Union Sixth Framework through RII3- Wapstra,Nucl. Phys. A, Vol. 729 2003, p.3.
EURONS (contract no. 506065), and by the Helmholtz [35] F.J. Bergmeister et al., Z. Phys. A, Vol. 320 1985,
associationofnationalresearchcenters(HGF) undercon- p.693.
tract VH-NG-037. We also thank the ISOLDE Collab- [36] S.W.Kikstraetal.,Nucl.Phys.A,Vol.5291991,p.39.
[37] S.A. Brindhaban and P.H. Barker, Phys. Rev. C,
oration as well as the ISOLDE technical group for their
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