We report our recent results of the measurement of the binding energy of the ground state of the calcium negative ion using electric field dissociation and accelerator mass spectrometry. The electron affinity of Ca was measured to be 17.5 -] meV for a p-wave electron emission, which is in good agreement with recent photodetachment experiments performed by Walter and Peterson [Phys. Rev. Lett. 68, 2281 (1992)]. PACS number(s): 35.10.Hn We report in this Rapid Communication our recent results of the measurement of the binding energy of the ground state of the calcium negative ion using electric field dissociation and accelerator mass spectrometry (AMS). The value of the electron affinity of Ca has recently become controversial, having been measured using laser photodetachment methods by two different groups leading to two irreconcilable results. Pegg et al. [1] have observed Eb =43+'7 meV while Walter and Peterson [2] obtained Eg =18.4+ 2.5 meV. Also a measurement of the black-body-radiation-induced decay of Ca adds information concerning the possible metastable states of the calcium negative ion [3] and suggests that the lower value of the electron affinity might be more likely.While these measurements were in progress [2,3], we were studying the negative ion of calcium. Combining the fact that weakly bound negative ions are destroyed in high electric fields with the sensitivity of AMS, we were able to track the destruction of Ca over 4 orders of magnitudes of intensity. Much to our surprise, Ca proved to be destroyed at a much lower electric field than expected from the earlier photodetachment measurement (i.e. , 43+'7 meV) and actually below the designed operating voltage range of the apparatus [4]. The value of the electron affinity of calcium deduced from our measurements is 17.5-+2 meV which is consistent with the later photodetachment measurement [2]. Even though the existence of stable or metastable calcium negative ions has been established for more than 20 years [5], the actual stability of the negative ions of atoms with closed subshells such as Ca, Sr, Ba, and Ra have been questioned for a long time. Theoretical calculations using multiconfiguration Hartree-Fock theory have shown that the extra electron of the ground state should fill an np orbital instead of the expected (n -1)d orbital [6]. The calcium electron a%nity has also been used to calibrate the theoretical predictions regarding the binding energy of negative ions of this series which has been extended to other atoms with closed subshells like Yb ([Xe]4f ' 6s ) [7]. This makes the accurate determination of the calcium binding energy even more important. According to the calculations mentioned above, the ground state of Ca is dominated by the 4s 4p P configuration. The spin-orbit splitting between the lower state P]y2 and the excited state P3y2 has been calculated by Vosko, Chevary, and Mayer [7] and by Dzuba et al. [8]. Their calculations indicated a fine-structure splitting of 5.4 and 6.9 meV, respectively. The theory of electric dissocia...

As one in a series of articles on Canadian contributions to mass spectrometry, this review begins with an outline of the history of accelerator mass spectrometry (AMS), noting roles played by researchers at three Canadian AMS laboratories. After a description of the unique features of AMS, three examples, (14)C, (10)Be, and (129)I are given to illustrate the methods. The capabilities of mass spectrometry have been extended by the addition of atomic isobar selection, molecular isobar attenuation, further ion acceleration, followed by ion detection and ion identification at essentially zero dark current or ion flux. This has been accomplished by exploiting the techniques and accelerators of atomic and nuclear physics. In 1939, the first principles of AMS were established using a cyclotron. In 1977 the selection of isobars in the ion source was established when it was shown that the (14)N(-) ion was very unstable, or extremely difficult to create, making a tandem electrostatic accelerator highly suitable for assisting the mass spectrometric measurement of the rare long-lived radioactive isotope (14)C in the environment. This observation, together with the large attenuation of the molecular isobars (13)CH(-) and (12)CH 2(-) during tandem acceleration and the observed very low background contamination from the ion source, was found to facilitate the mass spectrometry of (14)C to at least a level of (14)C/C ~ 6 × 10(-16), the equivalent of a radiocarbon age of 60,000 years. Tandem Accelerator Mass Spectrometry, or AMS, has now made possible the accurate radiocarbon dating of milligram-sized carbon samples by ion counting as well as dating and tracing with many other long-lived radioactive isotopes such as (10)Be, (26)Al, (36)Cl, and (129)I. The difficulty of obtaining large anion currents with low electron affinities and the difficulties of isobar separation, especially for the heavier mass ions, has prompted the use of molecular anions and the search for alternative methods of isobar separation. These techniques are discussed in the latter part of the review.

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