SMILETRAP—A Penning trap facility for precision mass measurements using highly charged ions

doi:10.1016/S0168-9002(01)02178-7

Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment

Volume 487, Issue 3, 21 July 2002, Pages 618-651

https://doi.org/10.1016/S0168-9002(01)02178-7 Get rights and content

Abstract

The precision of mass measurements in a Penning trap increases linearly with the charge of the ion. Therefore we have attached a Penning trap, named SMILETRAP, to the electron beam ion source CRYSIS at MSL. CRYSIS is via an isotope separator connected to an ion source that can deliver singly charged ions of practically any element. In CRYSIS charge state breeding occurs by intense electron bombardment. We have shown that it is possible to produce, catch and measure the cyclotron frequencies of ions in the charge region 1+ to 52+. The relevant observable in mass measurements using a Penning trap is the ratio of the cyclotron frequencies of the ion of interest and ion used as a mass reference. High precision requires that the two frequencies are measured after one another in the shortest possible time. For reasons of convenience the precision trap operates at room temperature. So far it has been believed that warm traps working at 4 K are required for high mass precision with exactly one ion in the trap at a time. In this paper we demonstrate that mass precision of a few parts in 10¹⁰ also can be obtained in a warm trap at a pressure of about 5×10⁻¹² mbar by stabilizing the pressure in the He-dewar, the trap temperature and the frequency synthesizer. In order to reduce the influence of changes of the magnetic field to a level below 10⁻¹⁰, the scanning of the frequencies close to the resonances of both the ion of interest and the reference ion is done in a total time <2 min. Trapping of ions is a statistical procedure, allowing more than one ion to be trapped in each measurement cycle. However, after completing the measurements it is possible to reject all information except for events based on 1 and 2 trapped ions. The procedures of producing, transporting, catching, exciting and measuring the cyclotron resonance frequencies of highly charged ions and the mass reference ions with the time-of-flight method are described. In routine measurements with 1 s excitation time lasting for about 24 h, atomic masses can be determined at an uncertainty of about 1 pbb. In the case of q/A doublet measurements a mass uncertainty close to 0.1 ppb can be obtained as illustrated by a mass measurement of ⁴He²⁺. The mass measurements so far performed are either related to fundamental constants or to masses the accuracy of which is needed for some current questions in physics.

Section snippets

The development of mass spectrometry

From the very first mass measurements on an atomic scale by J. J. Thomson [1] in the beginning of the 20th century the precision has increased dramatically. In Thomson's cathode ray tube, mass spectra appeared as parabolas on a photographic film due to the large energy spread of the ions created in a high-voltage discharge tube used as ion source. With the vacuum technology available at that time it was impossible to avoid appreciable amounts of hydrogen in the ion source. Thus, in the mass

Merits of highly charged ions

The cyclotron frequency ν of a single ion with rest mass m and charge qe moving perpendicular to a magnetic field B is given by

$ν_{c} =qe B/(2π m).$ Though this situation never can be achieved in laboratory experiments this simple formula can be used to discuss the merits of HCI. Let us assume that it is possible to measure this cyclotron frequency somehow (see Section 5.3). In any frequency determination, involving a resonance phenomena, the figure of merit can be defined as m/Δm =ν/Δν, i.e. the mass

Mass determinations using highly charged ions

Mass measurements in a Penning trap are based on a comparison of the cyclotron frequencies of the ion of interest and that of a mass reference ion. If the reference ion is denoted by subscript 1 and the ion whose mass is to be measured by subscript 2 the following relation is obtained:

$ν_{1} /ν_{2} =(q_{1} /q_{2})(m_{2} /m_{1}).$ It is here assumed that the change in the magnetic field can be neglected during the time the cyclotron frequencies of the two ion species are measured. This condition is only fulfilled if

Equations of ion motion in an ideal penning trap

Eq. (1) represents a condition that cannot be realized in laboratory experiments. The magnetic field confines the ions only in a plane perpendicular to the magnetic field. But injected ions have a velocity component also along the magnetic field. Therefore, trapping of ions is done by introducing two retardation electrodes separated by a distance 2z, which limits the ion motion in the direction of the magnetic field. These two electrodes are both required to have surfaces of hyperboloidal shape

Experimental set up and sequence of measurements

The SMILETRAP—experiments involve mainly the following parts;

an electron beam ion source CRYSIS connected to an isotope separator for production of HCI,
an ion transport system,
a charge selection magnet,
a retardation trap where also H₂⁺ ions can be produced as a mass reference,
beam elements between the retardation trap and the precision trap,
a hyperboloidal Penning trap where the precision mass measurements are done by exciting the ions and detecting their time-of-flight (TOF).

In the following

The real trap versus the ideal trap

In the discussion of Penning trap properties we have so far identified a number of measures which have to be taken for reaching highest possible precision. The order given is related to the magnitude of the effects in terms of changes in the cyclotron frequency.

a.
A very stable trap temperature close to the trap is required. Changes cause magnetic field disturbances due to the susceptibility of the construction material, in particular, that one of the precision trap and the stainless steel vacuum

Accuracy limitations

There are several sources of error to be considered due to limitations in our experimental equipment and procedures which can cause frequency shifts and thus give rise to systematic uncertainties

1.
Instabilities in the magnetic field
2.
Instabilities in the trap voltage
3.
Ion interaction with the mirror charge in the ring of the precision trap
4.
Ion–ion interaction
5.
Ion-rest gas interaction
6.
Relativistic mass increase
7.
Uncertainty in the total binding energy of the HCI
8.
Uncertainty in the mass of the reference ions
9.

Present accuracy of stable isotopes and summary of results

The recent technical improvements of SMILETRAP allow mass uncertainties of about 1 ppb in routine measurements of light ions lasting for about 20 h. In doublet mass measurements with q/A=0.5 uncertainties of a few times 10⁻¹⁰ can be achieved. There are several hundred stable isotopes many of them having a mass uncertainty ⪢1 ppb (Fig. 22). It would be rather meaningless to systematically improve hundreds of mass values only in order to make the mass tables more attractive. We have, therefore,

Future and outlook

There are a few obvious measures to be taken which would improve the performance of SMILETRAP.

a.
Increase of the excitation time
b.
Implementation of the Ramsey fringe technique
c.
Cooling of the ions in the pretrap
d.
Combining SMILETRAP with an electron beam ion trap (EBIT) at a 250 kV platform.

Conclusions

It should be emphasized that the SMILETRAP facility is the only one that has measured masses of ions with q⪢8+. At the accelerator facility GSI in Darmstadt there is a recently approved project named HITRAP that aims at producing very HCI of radioactive as well as stable isotopes, as for example ²³⁸U ions with the three highest charge states. These ions are produced at very high energies and slowed down, first in the storage ring ESR and then further retarded in some decelerator device. Our

Acknowledgements

We gratefully acknowledge the support from the Knut and Alice Wallenberg Foundation, The Carl Trygger Foundation, The Bank of Sweden Tercentenary Foundation, The Swedish Natural Research Council, The European Community Science and TRM-Programs and The Manne Siegbahn Laboratory.

In particular, we appreciate the confidence that the Wallenberg Foundation has shown us from the start of the SMILETRAP project, which in the beginning looked rather hypothetical with several seemingly week features.

The

References (59)

F. Di Filippo
Phys. Rev. Lett.
(1994)
F. di Filippo et al.
Phys. Scripta, T
(1995)
T. Schwarz, Thesis, (Hochpräzise Massenbestimmung hochgeladener Ionen mit SMILETRAP) Precision mass measurements using...
J.J. Thomson
Philos. Mag.
(1907)
F.W. Aston, Bakerian Lecture, Philos. Mag....
J. Mattausch et al.
Z. Physik.
(1934)
A.J. Dempster
Proc. Am. Phys. Soc.
(1955)
A.O. Nier
Phys. Rev.
(1951)
H.E. Duckworth
Rev. Sci. Instrum.
(1950)
H.E. Duckworth
Phys. Rev.
(1950)
R.C. Barber
Rev. Sci. Instrum.
(1971)
J.K. Hykawy
Phys. Rev. Lett.
(1991)
L.G. Smith
Phys. Rev. C
(1971)
G. Audi et al.
Nucl. Phys., A
(1995)
D. Lunney
Hyperfine Interactions
(1996)
D. Lunney, et al., Proceedings of the Conference Trapped Charged Particles and Fundamental Physics, Asilomar, 1998, p....
M. De Saint Simon
Phys. Scripta, T.
(1995)

D.L. Farnham et al.

Phys. Rev. Lett.

(1995)

R.S. van Dyck et al.

Phys. Scripta, T

(1995)

R.S. van Dyck Jr., Proceedings of Trapped Charged Particles and Fundamental, Physics, Asilomar, 1998, p....

G. Gabrielse

Phys. Rev. Lett.

(1986)

G. Gabrielse

Phys. Rev. Lett.

(1989)

G. Gabrielse

Phys. Rev. Lett.

(1990)

D. Philips et al.

Phys. Scripta, T

(1995)

G. Gabrielse

Phys. Rev. Lett.

(1995)

G. Gabrielse, Presented at the Symposium Trapped Particles and Fundamental, Physics, Asilomar,...

M.P. Bradley

Phys. Rev. Lett.

(1999)

R.S. Van Dyck Jr., APAC Conference Cargese, Corsica, Sept 2000, Hyperfine Interactions 132 (2001)...

G. Bollen

Phys. Scripta T

(1995)

G. Bollen

Nucl. Instr. and Meth. A

(1995)

R. Jertz, Thesis, Direkte Bestimmung der Masse von 28Si als Beitrag zur Neudefinition des Kilograms (direct...

R. Jertz

Phys. Scripta

(1993)

Johannes Schönfelder, Thesis, (Hochpräzise Massenbestimmungen von 28Si und des Protons durch...

C. Carlberg

Phys. Scripta T

(1995)

R.L. Kelly, J. Phys. Chem. Ref. Data 16 (Suppl. 1)...

J. Scofield, Ionization Energies, LLNL Internal report, Livermore, 94550 California,...

J. Scofield, Private...

G. Rodrigues, et al., Phys. Rev. A. 63 (2001)...

L.S. Brown et al.

Rev. Mod. Phys.

(1986)

S. Brunner, et al., Europhys. J., in...

E. Bebee et al.

Phys. Scripta

(1993)

Cited by (104)

Nuclear Structure and Decay Data for A=76 Isobars
2024, Nuclear Data Sheets
The experimental nuclear spectroscopic data for known nuclides of mass number 76 (Fe, Co, Ni, Cu, Zn, Ga, Ge, As, Se, Br, Kr, Rb, Sr, Y) have been evaluated and presented together with Adopted properties for levels and γ rays. With the exception of structure data for ⁷⁶Ga nucleus, significant new data have been incorporated for all the other nuclides of A=76 since the previous 1995 update in ENSDF database and NDS publication 1995Si03. No data are yet available for excited states in ⁷⁶Fe, ⁷⁶Cu and ⁷⁶Y. Decay scheme characteristics for the decay of ⁷⁶Co, ⁷⁶Ni, and ⁷⁶Y are unknown while those for decays of ⁷⁶Cu and ⁷⁶Sr seem incomplete. For ⁷⁶Ni, very little structure data are available, and for ⁷⁶Ga and ⁷⁶As, only low-spin ( $J < 4$ or so) information is available. This work supersedes the data presented in the previous (1995Si03) NDS evaluation of A=76.
Vacuum requirements for Penning trap mass spectrometry with highly charged ions
2020, Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms
Experimental investigations employing highly charged ions (HCI) typically require very strict vacuum quality. In Penning trap mass spectrometry (PTMS), HCI can significantly improve the precision and resolution of mass measurements, but they are also subject to charge-exchange reactions with neutral gas particles, which may hamper the quality of the measurement. In this study, we employed a simple method to determine that the maximum pressure levels required for an undisturbed PTMS measurement of HCI is on the order of $10^{- 11}$ mbar. We also evaluate the residual gas pressure in the Penning trap mass spectrometer at TITAN facility by measuring the rate of charge-exchange reactions with HCI, revealing that the trap’s vacuum must be improved in about two orders of magnitude to reach the desired level.
Nuclear Data Sheets for A=40
2017, Nuclear Data Sheets
The experimental nuclear structure data and decay data are evaluated for the known nuclides of mass 40 (Mg, Al, Si, P, S, Cl, Ar, K, Ca, Sc, Ti). Detailed evaluated nuclear structure information is presented with the best values recommended for level energies, half-lives, γ-ray energies and intensities, decay properties (energies, intensities and placement of radiations), and other spectroscopic data. The ⁴⁰Ca and ⁴⁰K nuclides remain as the most extensively studied from many different reactions and decays; no excited states are known in ⁴⁰Mg, ⁴⁰Al, ⁴⁰P and ⁴⁰Ti. This work supersedes the earlier full evaluation of A=40 by J. Cameron and B. Singh (2004Ca38).
Development of a high-precision Penning trap magnetometer for the LEBIT facility
2015, International Journal of Mass Spectrometry
Citation Excerpt :
The mass of an atom or nucleus is one of its most fundamental properties and is important for various studies in physics. Many Penning Trap Mass Spectrometry (PTMS) facilities throughout the world [1–13] have been used to perform high-precision mass measurements on stable, long-lived, and short-lived isotopes in recent years to investigate nuclear shell structure, halo nuclei, nuclear astrophysics, determinations of beta decay Q-values, and tests of fundamental interactions and of the Isobaric Multiplet Mass Equation (IMME). PTMS facilities have achieved mass measurement fractional precisions as small as 7 parts in 10−12 for stable isotopes [14] and less than 10−8 for unstable isotopes [15,16].
A high-precision magnetometer based on a miniature Penning trap was developed to support Penning trap mass spectrometry of rare isotopes. Magnetic field monitoring with a relative precision of 2 parts in 10⁸ with a temporal resolution of 15 min was demonstrated. The use of this magnetometer at the Low-Energy Beam and Ion Trap (LEBIT) facility at Michigan State University will virtually eliminate the need to perform reference measurements during on-line mass measurements of rare isotopes and allow for the best use of precious on-line measurement time.
Classical calculation of relativistic frequency-shifts in an ideal Penning trap
2014, International Journal of Mass Spectrometry
The ideal Penning trap consists of a uniform magnetic field and an electrostatic quadrupole potential. In the classical low-energy limit, the three characteristic eigenfrequencies of a charged particle trapped in this configuration do not depend on the amplitudes of the three eigenmotions. No matter how accurate the experimental realization of the ideal Penning trap, its harmonicity is ultimately compromised by special relativity. Using a classical formalism of first-order perturbation theory, we calculate the relativistic frequency-shifts associated with the motional degrees of freedom for a spinless particle stored in an ideal Penning trap, and we compare the results with the simple but surprisingly accurate model of relativistic mass-increase.
The most precise atomic mass measurements in Penning traps
2013, International Journal of Mass Spectrometry
Citation Excerpt :
In parallel with image-charge detection, time-of-flight (TOF) detection [7,8] of cyclotron excitation has been developed by nuclear physicists whose usual aim is to measure masses of short-lived nuclei at ∼keV/c2 absolute precision, see the accompanying article by H.-J. Kluge. The SMILETRAP group at the University of Stockholm has also used TOF detection with highly charged ions to produce some of the most precise measurements of stable isotopes, especially of light ions [9]. The rest of this article is organized as follows.

View all citing articles on Scopus

View full text

SMILETRAP—A Penning trap facility for precision mass measurements using highly charged ions

Abstract

Section snippets

The development of mass spectrometry

Merits of highly charged ions

Mass determinations using highly charged ions

Equations of ion motion in an ideal penning trap

Experimental set up and sequence of measurements

The real trap versus the ideal trap

Accuracy limitations

Present accuracy of stable isotopes and summary of results

Future and outlook

Conclusions

Acknowledgements

Phys. Rev. Lett.

Phys. Scripta, T

Philos. Mag.

Z. Physik.

Proc. Am. Phys. Soc.

Phys. Rev.

Rev. Sci. Instrum.

Phys. Rev.

Rev. Sci. Instrum.

Phys. Rev. Lett.

Phys. Rev. C

Nucl. Phys., A

Hyperfine Interactions

Phys. Scripta, T.

Phys. Rev. Lett.

Phys. Scripta, T

Phys. Rev. Lett.

Phys. Rev. Lett.

Phys. Rev. Lett.

Phys. Scripta, T

Phys. Rev. Lett.

Phys. Rev. Lett.

Phys. Scripta T

Nucl. Instr. and Meth. A

Phys. Scripta

Phys. Scripta T

Rev. Mod. Phys.

Phys. Scripta