Precision Penning Trap Mass Spectrometry and Single Ion Spectroscopy (New Program!)
Some of the most precise measurements of atomic masses, at <0.1 ppb relative precision, have been obtained with the single-ion, “Ion Cyclotron Resonance” Penning trap mass spectrometer developed by David E. Pritchard and co-workers at MIT [1,2]. In their final work the MIT ICR group succeeded in pushing atomic mass comparisons below the 10 ppt level by simultaneously measuring the cyclotron frequencies of two ions in the same Penning trap [3], leading to a precision direct test of Einstein’s “E=mc2” [4]. In the process they discovered a new perturbation of the cyclotron frequency of molecular ions due to polarizability, which can be used to accurately measure dipole moments [5]. In May 2003, following a pre-arranged agreement, the MIT lab was closed and the apparatus was moved by the FSU atomic physics group to Tallahassee. Here it was set up again to form a new precision ICR laboratory.
Following apparatus set-up and shake-down our initial measurements were of the mass of 32S and of the most abundant isotopes of the heavier rare gases 84,86Kr and 129,132Xe, all at sub 10-10 relative precision [6]. We then developed a technique for mass comparison with two ions simultaneously trapped in a Penning trap, but with the ions alternately cooled to the center of the trap - where the cyclotron frequency is measured - or else parked in a large radius cyclotron orbit. This technique was used to measure the mass of 31P [7], to re-investigate polarizability induced cyclotron frequency shifts in CO+, to investigate such shifts in 31PH+, and for preliminary work at using polarizability shifts to detect laser-induced transitions. These precision mass measurements have immediate value in providing improved reference masses for many other atomic masses and hence strengthening the global Atomic Mass Evaluation [8]. We have also recently measured the mass of 136Xe. This is needed for determining the Q-value for searches for the neutrino-less double-beta decay of 136Xe to 136Ba. In parallel we are re-developing the ultra-high precision simultaneous cyclotron frequency measurement technique. We intend to apply this to more high-precision mass comparisons relevant to fundamental constants in general, and in particular to a high precision measurement of the mass difference between tritium and helium-3. This will determine the beta-decay Q-value and hence the end-point of the tritium beta-decay electron spectrum, an important parameter in the determination of limits to the mass of the electron neutrino.
References:
[1] M.P. Bradley, J.V. Porto, S. Rainville, J.K. Thompson and D.E. Pritchard, "Penning trap measurements of the masses of Cs-133, Rb-87,Rb- 85, and Na-23 with uncertainties <= 0.2 ppb", Physical Review Letters 83, 4510-4513 (1999).
[2] F. DiFilippo, V. Natarajan, K. Boyce and D.E. Pritchard, "Accurate masses for fundamental metrology", Phys. Rev. Lett. 73, 1481 (1994).
[3] S. Rainville, J.K. Thompson, and D.E. Pritchard, "An ion balance for ultra-high precision atomic mass measurements”, Science 303, 334 (2004).
[4] S. Rainville, J.K. Thompson, E.G. Myers, J.M. Brown, M.S. Dewey, E.G. Kessler, R.D. Deslattes, H.G. Boerner, M. Jentschel, P. Mutti, and D.E. Pritchard, "A direct test of E=mc2”, Nature, 438, 1096 (2005).
[5] J.K. Thompson, S. Rainville, and D.E. Pritchard, "Cyclotron frequency shifts arising from polarization forces”, Nature, 430, 58 (2004).
[6] W. Shi, M. Redshaw, and E.G. Myers, “Atomic masses of 32,33S, 84,86Kr, and 129, 132Xe with uncertainties <0.1ppb”, Phys. Rev. A72, 022510 (2005).
[7] M. Redshaw, J. McDaniel, W. Shi, and E.G. Myers, “Mass ratio of two ions in a Penning trap by alternating between the trap center and a large cyclotron orbit”, Int. J. Mass Spectrometry, in press.
[8] G. Audi, A.H. Wastra, and C. Thibault, “ The Atomic Mass Evaluation”, Nucl. Phys. A729 (2003) 337.
(Last update March 2006)