Mass measurements in the endpoint region of the rp-process at SHIPTRAP

Hyperfine Interact (2006) 173:133–142DOI 10.1007/s10751-007-9550-3

Mass measurements in the endpoint regionof the rp-process at SHIPTRAP

M. Block · D. Ackermann · K. Blaum · A. Chaudhuri ·Z. Di · S. Eliseev · R. Ferrer · D. Habs · F. Herfurth ·F. P. Heßberger · S. Hofmann · H.-J. Kluge · G. Maero ·A. Martín · G. Marx · M. Mazzocco · M. Mukherjee ·J. B. Neumayr · W. R. Plaß · W. Quint · S. Rahaman ·C. Rauth · D. Rodríguez · C. Scheidenberger ·L. Schweikhard · P. G. Thirolf · G. Vorobjev · C. Weber

Published online: 15 June 2007© Springer Science + Business Media B.V. 2007

M. Block (B) · D. Ackermann · S. Eliseev · F. Herfurth · F. P. Heßberger · S. Hofmann ·H.-J. Kluge · G. Maero · A. Martín · M. Mazzocco · M. Mukherjee · W. Quint ·S. Rahaman · C. Rauth · C. Scheidenberger · G. VorobjevGesellschaft für Schwerionenforschung mbH, Planckstr. 1, 64291 Darmstadt, Germanye-mail: [email protected]

A. Chaudhuri · G. Marx · L. SchweikhardInstitut für Physik, Ernst-Moritz-Arndt-Universität, 17489 Greifswald, Germany

K. Blaum · R. Ferrer · C. WeberInstitut für Physik, Johannes-Gutenberg-Universität, 55099 Mainz, Germany

D. Habs · J. B. Neumayr · P. G. ThirolfDepartment für Physik, Ludwig-Maximilians-Universität München,Am Coulombwall 1, 85748 Garching, Germany

Z. Di · W. R. Plaß · C. ScheidenbergerII. Physikalisches Institut, Justus-Liebig-Universität, Heinrich-Buff-Ring 16,35392 Gießen, Germany

D. RodríguezIN2P3, LPC-ENSICAEN, 6 Bd. Marechal Juin, 14050 Caen Cedex, France

Present Address:M. BlockNSCL, East Lansing, 48824 MI, USA

Present Address:M. MukherjeeUniversität Innsbruck, Technikerstr. 25, 6020 Innsbruck, Austria

Present Address:S. Rahaman · C. WeberUniversity of Jyväskylä, P.O. Box 35 (YFL), 40014 Jyväskylä, Finland

134 M. Block, et al.

Abstract The Penning-trap mass spectrometer SHIPTRAP was designed for preci-sion mass measurements of radionuclides produced in fusion–evaporation reactions.The latest measurement campaign covered heavy nuclei (A > 90) related to theastrophysical rapid proton capture process. The masses of 34 neutron-deficientradionuclides have been measured since February 2006 with relative uncertaintiesbetween 5 × 10−8 and 1 × 10−7. Furthermore, the use of an octupolar RF excitationfor the time-of-flight ion-cyclotron-resonance technique was investigated and anincrease of the resolving power by a factor of ten was observed in agreement withsimulations. This will allow to resolve isomeric states with excitation energies of afew 10 keV only.

Keywords Penning trap · Atomic mass · Binding energy · rp-process

PACS 07.75.+h · 21.10.Dr · 26.30.+k · 27.60.+j

1 Introduction

Mass measurements of radionuclides [1] support the understanding of the forcesinside a nucleus giving access to nucleon binding energies. The evolution of thenuclear shell structure can be studied by mass measurements along isotopic andisotonic chains. Separation energies, that can be derived from mass differences, allowdetermining the location of drip lines, i.e. the limits of stability. Nuclear massesprovide also important input for astrophysical network calculations relevant for theunderstanding of stellar nucleosynthesis [2], and contribute to tests of the standardmodel as for instance in the study of superallowed β-decays [3].

Penning trap mass spectrometers [4] are versatile tools for precision mass mea-surements [5, 6] providing an unmatched resolving power sufficient not only for iso-bar selection but even for the resolution of isomeric states. Hence, an unambiguousdetermination of the ground state mass is possible resulting in precise and reliablemass values.

The application of buffer gas-filled ion catcher systems facilitate the fast andefficient stopping of fusion–evaporation residues [7] as well as fragmentation prod-ucts [8] and enables their extraction at low energies. Radio-frequency quadrupole(RFQ) ion-beam coolers allow improving the beam emittance and provide bunchedbeams adapted for an efficient injection into Penning traps [9]. The combination ofthese techniques has opened the door to access exotic nuclides including refractoryelements and elements heavier than uranium that were not accessible for precisionexperiments at low energy before. At present, several Penning trap mass spectrom-eters installed at radioactive-beam facilities around the world apply a buffer-gasstopping scheme [10–12].

2 Experimental setup

SHIPTRAP [13] was designed for mass measurements of radionuclides produced infusion-evaporation reactions and separated in-flight by the velocity-filter SHIP [14].A schematic overview of the experimental set-up is shown in Fig. 1.

Mass measurements in the endpoint region of the rp-process 135

Gas Cell Buncher Transfer Penning Traps

DC cage RF funnel

ExtractionRFQ

Quadrupoledeflector

Superconductingmagnet

MCP-detector

Purification trap Measurement trap

Surfaceion source

Entrancewindow

SHIPion beam

Diaphragm

Fig. 1 Schematic overview of the SHIPTRAP setup

The reaction products from SHIP with energies of 200–500 A·keV are stopped ina buffer-gas stopping cell [7] in helium at pressures of around 50 mbar. The ions areextracted by a combination of RF and DC electric fields and eventually swept outthrough a nozzle in a supersonic gas jet. Subsequently the ions are pre-cooled in theextraction radiofrequency quadrupole (RFQ) operated as ion guide before they aretransferred to an RFQ ion-beam cooler and buncher, where they are cooled within afew milliseconds and extracted as a low-emittance bunched beam. This facilitates anefficient injection into the Penning trap system, consisting of two cylindrical traps ina superconducting 7 T magnet. The first trap is utilized for isobar selection, while ahigh-precision mass determination is performed in the second trap by application ofthe time-of-flight ion cyclotron resonance (TOF-ICR) technique [15, 16].

This paper summarizes mass measurements of nuclei related to the rapid protoncapture process above A = 90. Among the technical improvements envisaged, ex-perimental studies of the application of an octupolar RF excitation for the TOF-ICRtechnique are also addressed in this paper.

3 Mass measurements along the rp-process path

The astrophysical rapid proton-capture process (rp-process) is relevant for thestellar nucleosynthesis in environments, where explosive hydrogen burning at hightemperatures and high densities occurs, and powers type I X-ray bursts [17]. Thedetails of the process depend strongly on the nuclear densities and the tempera-ture in the stellar environment. To draw conclusions about the stellar conditionsfrom the astrophysical observations such as light curves of an X-ray burst or theelemental abundances in the solar system, precise nuclear physics data are requiredas ingredients for astrophysical network calculations. The predominant reactionsinvolved in the rp-process are proton capture (p-γ ) reactions, photodisintegration(γ -p reactions) and β-decay. Hence, the required nuclear physics data comprisemasses, half-lives and reaction rates.

The precise knowledge of nuclear masses is especially important since the elemen-tal abundances in an equilibrium state depend exponentially on the proton separa-tion energy [2]. Moreover, the reaction rates for proton capture (p,γ ) are determined


Ru

Rh

Pd

Ag

Gd

In

Sn

Sb

Te

Xe

I

Tc

113

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102

111

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113111

111

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109108

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105107104103102

106105 107

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107104103102

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1011009998

100999897 101

102 1039796

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95949392

9392

9392

93

91

9190

91908988

89

99

N=50

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95

97

N = Z

100

Fig. 2 Projected rp-process pathway. The grey shaded squares mark the nuclides, whose masses weremeasured at SHIPTRAP. The dark grey shaded squares indicate nuclides for which masses weredirectly measured for the first time

by the Q-value and influenced by the nuclear binding and shell-correction energies.The required precision for masses sufficient for a reliable determination of therp-process pathway is in the order of 10 keV, where the highest precision is neededfor waiting point nuclei [2].

The projected rp-process pathway proceeds along or close to the proton drip lineas indicated in Fig. 2, where precise experimental data is sparse. In contrast to earlierassumptions, the rp-process also contributes to the formation of heavy neutron-deficient elements up to tellurium [18]. The precision obtained by theoretical pre-dictions as for instance state-of-the-art shell model calculations or the extrapolationof mass values [19] is not better than 100 keV, and hence, still not sufficient for areliable determination of the rp-process pathway.

The nuclides with mass numbers A between 80 and 100 and equal numbers ofprotons and neutrons N = Z are subject of special interest. For lighter nuclei upto tin the drip line follows the N = Z line, while heavier nuclear systems (Z > 60)with fewer neutrons may be stable especially for even Z [20]. The neutron and protonshell closure at N = Z = 50 and in particular the doubly magic nucleus 100Sn providebenchmarks for shell model calculations. The nuclear shape changes rapidly in thisregion and gives rise to long-lived isomers with small excitation energies.

Many of the nuclei of interest can be produced in fusion-evaporation reactionswith relatively high rates. SHIPTRAP is well suited for these investigations exploit-ing the high primary beam intensities of the UNILAC accelerator and the highperformance of the SHIP velocity-filter.


-6 -4 -2 0 2 4 6

96

98

100

102

104

106

108

110

112

114

111Te+mea

n T

OF

/ μs

excitation frequency - 969998.6Hz / Hz

Fig. 3 Cyclotron resonance of 111Te+. The solid line is a fit of the theoretical lineshape[21] tothe data

In three runs in February and July 2006, several of the relevant nuclides wereproduced irradiating a 58Ni target (0.5 mg/cm2) with 40Ca, 50Cr, and 58Ni beams. Thebeam energies ranged from 3.6 to 5.0 MeV/u with average beam intensities between40 and 400 particle nA. As an example, a cyclotron resonance of 111Te+ is shownin Fig. 3. The masses of 34 radionuclides along the projected rp-process pathwaywere measured with uncertainties on the order of 10 keV. They are indicated by thegrey shaded squares in Fig. 2. Nuclides for which the masses were experimentallydetermined for the first time are marked by dark grey squares. Preliminary resultsof the mass measurements for some nuclides were presented in [22]. In Figs. 4 and 5some of the preliminary results are compared to the Atomic-mass Evaluation (AME)2003 values [19].

Except for 105In and some tellurium isotopes, the measured mass excesses agreewith the values of the AME 2003 quite well. However, the differences between thepresent values and the literature values show a certain trend for the indium isotopes.The absolute value of the mass excess difference seems to increase systematicallywith increasing neutron number.

As discussed by Schatz et al. [23], the rp-process proceeds along the tin isotopicchain until the proton separation energy of the antimony isotopes becomes largeenough to delay photo-disintegration and hence, causes the rp-process to proceedto heavier elements. This happens around 105Sn, but the current uncertainties in theproton separation energies of antimony isotopes prevented an accurate prediction,which of the various SnSbTe-cycles dominates. Now, by using the set of mass valuesmeasured at SHIPTRAP, more accurate proton separation energies can be deducedin order to clarify the situation.


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d

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n

106S

n

99Ag

101A

g

103A

gD

(S

T -

AM

E20

03)

/ keV

AME2003 SHIPTRAP

Fig. 4 Difference D between the measured mass excess (preliminary results) and the results fromthe AME 2003 [19] for nuclides produced in the reaction 50Cr+58Ni. The uncertainties of the AMEvalues are depicted by the solid lines

4 Technical developments and improvements

The envisaged technical improvements of the system concentrate presently onincreasing the efficiency and decreasing the uncertainty. At present relative uncer-tainties of 5 × 10−8 and an overall efficiency of up to about 0.5% [24] can be achieved.A further increase of the efficiency is required to access more exotic nuclides, inparticular in the region of nuclei heavier than uranium, where the production crosssections drop exponentially from about 2 μbarn for 254No (Z = 102) to about 1 pbarnfor Z = 112.

Modifications of the buffer-gas stopping cell as demonstrated for the MLL Ion-Catcher [25] allow an operation at a higher buffer gas pressure of about 100 mbarand are expected to result in an efficiency gain of up to a factor of three.

Another approach that results in a higher sensitivity of the system is the applica-tion of a different detection scheme. For longer-lived nuclides with low productionrates – a typical scenario for many transactinides – the non-destructive FourierTransform-Ion Cyclotron Resonance (FT-ICR) technique will be advantageouscompared to the TOF-ICR method as a mass value can be obtained from a singleion. In the FT-ICR method a trapped ion is detected by its image currents picked upby a resonant LC tank circuit connected to two segments of the ring electrode of thePenning trap. If a circuit with a high quality factor is used, single-ion sensitivity canbe obtained. A cryogenic trap system for the FT-ICR detection at SHIPTRAP [26]is presently being commissioned [27].

An improvement of the achievable uncertainty in a mass measurement is also ofimportance. On the one hand a high precision is required for certain mass valuesas for example in the case of the nuclei undergoing superallowed β-decay [3]. Onthe other hand an improvement is needed for heavy ions where the lower cyclotron


-400

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e

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e

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e

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111I

113X

e

107S

b

111S

b

109S

bD

(S

T -

AM

E20

03)

/ keV

AME2003 SHIPTRAP

Fig. 5 Difference D between the measured mass excess (preliminary results) and the results fromthe AME 2003 [19] for nuclides produced in the reaction 58Ni+58Ni. The uncertainties of the AMEvalues are depicted by the solid lines

frequency results in a reduction of the relative uncertainty. To measure the mass ofan ion with a mass of A = 250 to a precision of 10 keV corresponds to a relativeuncertainty of 4 × 10−8.

The relative statistical uncertainty in a Penning trap mass measurement usingthe time-of-flight ion cyclotron resonance method can be described by the empiricalrelation [28]

δmm

∝ 1

νc · TRF · √N

, (1)

where νc is the cyclotron frequency, TRF is the excitation time, and N is the numberof detected ions. For short-lived radionuclides the half-life of the nuclide underinvestigation imposes an ultimate limit for the excitation time and therefore, limitsthe achievable uncertainty. As the number of accumulated ions has to be traded forthe measurement time, given a generally low production rate for exotic nuclei, a gainin precision requires a higher resolving power. Different approaches can be followedto achieve this.

An increase of the precision is possible by increasing the cyclotron frequency usinghigher magnetic fields [10] or higher charge states [29–31]. Both solutions increasethe cost of the mass spectrometer and are difficult to incorporate in an existingapparatus. In addition, the use of highly-charged ions for mass measurements ofradionuclides with short half-lives or low production rate is challenging and has yetto be demonstrated.

The use of higher harmonics for the RF excitation, in particular the use ofan octupolar RF field at νoct = 2νc = (1/2π) (2q/m) B provides in contrast to aquadrupolar excitation a gain in resolving power by at least a factor of two for thesame excitation time. It should be noted that a factor of two gain in precision is


100

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Aoctu

=0.8V Aoctu

=1.3V

-30 -20 -10 0 10 20 30 -30 -20 -10 0 10 20 30100

110

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Aoctu

=1.4V Aoctu

=1.75V

old baseline

new baseline

ν - (Hz)RF

νoctu

TOF

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Fig. 6 Octupole resonances for different amplitudes of the RF excitation. The octupolar conversioncurves are shown as an inset and the chosen amplitude is marked by a dot

equivalent to a reduction of the measurement time by a factor of 4. The feasibility ofthis approach was demonstrated at LEBIT [32, 33].

To realize this excitation scheme, an eightfold segmented ring electrode is re-quired, which is available at SHIPTRAP. The response of the ion’s motion to theexcitation by an azimuthal octupolar RF field was investigated and compared tosimulations [34]. It was found that a small transversal emittance and a small energyspread of the ion bunch injected into the trap are prerequisites for a successfulapplication of octupolar excitation. The typical mean radius of the magnetron motionafter the dipolar excitation ranges from 1 to 2 mm. The ions occupy initially avolume with a radius of δr ≈ 0.3 mm in the SHIPTRAP measurement trap. It hasbeen experimentally demonstrated that a mass resolving power R = νoctu/δνoctu ≈800, 000 can be achieved for an octupolar excitation with an excitation time of onlyToctu = 100 ms for 133Cs+ ions. This exceeds the resolving power of the TOF-ICRmethod based on a quadrupolar excitation by a factor of ten. A set of octupolarresonances for different amplitudes of the RF excitation is shown in Fig. 6. Therespective octupolar conversion curve, i.e. the mean time of flight as function of theoctupolar RF excitation amplitude, is shown as an inset and the chosen amplitudeis marked. With increasing amplitude a very narrow resonance develops while thebaseline changes. For more details see [34]. For an excitation time of 900 ms (thestandard excitation time at SHIPTRAP), a resolving power of up to R ≈ 7 × 106


can be expected which corresponds to a mass uncertainty of 18 keV for 133Cs+.However, damping has to be small to avoid a reduction of the resolving power forlonger excitation times. Therefore, a reduction of the diameter of diffusion barrierseparating the two Penning traps is foreseen to obtain a lower pressure in themeasurement trap. Then the resolution of isomeric states with excitation energiesof only a few 10 keV should become possible even for moderate observation timesand hence, short half-lives of the nuclide under investigation.

The improved resolving power of the octupolar excitation results in a gain inprecision for the cyclotron frequency determination by at least a factor of two.An even higher gain is expected by reducing the range of the scan around thesharp resonance. However, the lineshape is not yet analytically known and thus, theoctupolar excitation scheme cannot be applied to high-precision mass measurementsyet, but it may be used to resolve isomeric states with low excitation energy.

5 Conclusion and outlook

The masses of 34 radionuclides along the rp-process pathway have been measuredwith the Penning trap mass spectrometer SHIPTRAP with uncertainties in the orderof 10 keV. The data supports astrophysical reaction network calculations to allowfor a more accurate determination of the rp-process pathway in the region aboveA = 90. In addition, an improvement of the resolving power of the TOF-ICR methodwas demonstrated using an octupolar excitation scheme. This will facilitate to resolveisomeric states with lower excitation energies and possibly help to achieve a highprecision for heavy ions. The implementation of the FT-ICR detection techniquewill enable mass measurements with only a single ion and allow to access very exoticnuclides with low production rates.

Acknowledgements This project was supported by the BMBF, the GSI F&E program, the ECunder contract number RII3-CT-2004-506065 (EURONS / JRA11 / TRAPSPEC), and by the HGFunder contract VH-NG-037.

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Mass measurements in the endpoint region of the rp-process at SHIPTRAP

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