High Precision Mass Measurements with the Penning Trap Mass Spectrometer ISOLTRAP at ISOLDE

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F. Ames1, G. Audi2, D. Beck3, K. Blaum#,3, G. Bollen4, J. Dilling3, O. Engels1, F. Herfurth1, A. Kellerbauer5, H.-J. Kluge3, D. Lunney2, R.B. Moore6, M. Oinonen5, C. Scheidenberger3, S. Schwarz4, G. Sikler3, E. Sauvan5, J. Szerypo7, C. Weber3 and the ISOLDE collaboration.

1Muenchen, 2Orsay, 3GSI, 4MSU, 5CERN, 6Montreal, 7Jyvaeskyla

Introduction

The ISOLTRAP experiment [1] is a tandem Penning trap mass spectrometer at the on-line isotope separator ISOLDE at CERN [2], which has been designed particularly for high-accuracy mass measurements of short-lived isotopes. It allows direct mass measurements by the determination of the angular cyclotron frequency

ωc = q / m * B   (Eq. 1)

of ions with a charge-to-mass ratio q/m confined in a magnetic field B. The value of the magnetic field B required for the mass determination can be determined from the cyclotron frequency ωc of an ion with a well-known mass.
Accurate experimental mass values serve as a stringent test of nuclear models, help to improve such models for predictions of the nuclear properties of very unstable nuclei that cannot be produced, and can reveal general nuclear structure.

The Penning Trap and the Ion Motion Inside

picture3.pngA Penning trap is a confining device made of a homogeneous magnetic field superimposed by an electrostatic quadrupole field. The magnetic field B is used for radial confinement. The electric field prevents the ions from escaping along the magnetic field lines and hence leads to axial confinement.
The motion in a Penning trap is a combination of three harmonic eigen-motions, an axial oscillation (ωz) and two circular motions commonly referred to as magnetron (ω-) and reduced cyclotron motion (ω+). The sum of the two circular eigen-frequencies equals the cyclotron frequency ωc= ω+ + ω- with ωc as given in Eq. 1. This frequency is used to determine the mass of the ion stored in the Penning trap.

The Isotope Separator ISOLDE

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The On-Line Isotope Mass Separator ISOLDE [2] is a facility dedicated to the production of a large variety of radioactive ion beams for a great number of different experiments, e.g. in the field of nuclear and atomic physics, solid-state physics, life sciences and material science. The facility, located at the Proton-Synchrotron Booster (PSB) of CERN, is operated by the ISOLDE Collaboration. The radioactive nuclides are produced by bombarding thick high-temperature targets with either 1 or 1.4 GeV protons via spallation, fission or fragmentation reactions.

Overview of the ISOLTRAP Experimental Setup

The essential parts of the ISOLTRAP spectrometer are:
(A) A linear radiofrequency quadrupole ion trap which has the task to stop, accumulate, cool and bunch the 60 keV ISOLDE beam to prepare it for efficient transfer into the cooler trap.
(B) A Penning trap to accumulate, cool, and mass separate (R <= 105) the ions delivered from the RFQ trap and to bunch them again for an efficient delivery to the second Penning trap.
(C) A precision Penning trap to determine the cyclotron frequency ωc (Eq. 1) of the ion under investigation.
Both (B) and (C) are located in a superconducting magnet with a 4.7 T and 5.9 T, respectively, magnetic field and a homogeneity of < 10-7 T in a volume of 1 cm3.
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Example Cyclotron Resonance

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The cyclotron frequency is determined using a resonant RF-excitation of the ion motion, followed by a time of flight (TOF) measurement. If the excitation frequency is in resonance, the TOF shortens. In the example cyclotron resonance spectrum of 36Ar the TOF of the ions from the trap to the ion detector is plotted as a function of the applied radio frequency. The solid line is a fit of the theoretical line shape to the data points.

Specifications of the Method and Experimental Results since 1995

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ISOLTRAP has now been operated at ISOLDE for many years. More than 200 masses have now been measured [3 and references therein]. A resolving power exceeding one million, which allows also to resolve isomers, and an accuracy in the mass determination of δm / m = 10-7 (in special cases 10-8) was achieved for practically all of these isotopes. The red points in the nuclear chart and the list give the isotopes investigated with ISOLTRAP from 1995 until the end of 2000.
Highlights are the measurements carried out in a long chain of mercury isotopes, 179-197Hg, which have given very precise information on binding energies in this region, and mass measurements on 33Ar and 74Rb. With half-lives of T1/2 = 173 ms and T1/2 = 64 ms these two nuclides are the shortest-lived ever studied in an ion trap.

References

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  1. G. Bollen et al., ISOLTRAP: a tandem Penning trap system for accurate on-line mass determination of short-lived isotopes, Nucl. Instrum. Methods A 368, 675 (1996).
  2. [2] E. Kugler, The ISOLDE facility, Hyperfine Interactions 129, 23 (2000).
  3. [3] D. Beck et al., Accurate masses of unstable rare-earth isotopes by ISOLTRAP, Eur. Phys. J. A 8, 307 (2000) and references therein.