Selective photoionization of palladium isotopes using a two-step excitation scheme

Abstract

Selective ionization using a narrow-linewidth laser to excite only specific isotopes, such as is used in the selective ionization of 235 U [3], is not applicable for palladium as isotope frequency shifts are smaller than the Dopplerbroadened absorption linewidths [4]. Instead, we base our work on a technique first proposed by Balling and Wright [5] wherein optical selection rules forbid excitation of evenmass number isotopes, but allow excitation of odd-mass number isotopes ( 105 Pd and 107 Pd) having non-zero nuclear spin. This is achieved using either two circularly polarized [6] (or orthogonal linearly polarized [7]) excitation lasers interacting with a stream of vaporized palladium, followed by a third ionizing laser. Ions are then separated from the vapor by an electric field, and non-radioactive isotopes remaining in the stream can be recycled, and odd-mass number isotopes (including radioactive 107 Pd) can be processed by means such as nuclear transmutation [8].This optical transition selection rule lends itself well to industrial scale-up, having notable advantages when compared to competing methods. Specifically, commercial high power excitation lasers can be used, without the need for narrow spectral linewidth nor long-term frequency stability, as is required for Atomic Vapor Laser Isotope Separation (AVLIS) [9]. Secondly, yield production is not limited, as may be the case when scaling up resonant ionization mass spectrometry RIMS [10] processes. This later limitation we have directly observed when a moderately high number of ions (of order 10 11 ) is produced in the interaction region and ion-ion Coulomb repulsive forces result in significant degradation of mass resolution (by either accelerating electric or magnetic fields), to the point where isotope resolution by a narrow slit is impossible.In previous work, we demonstrated the overall yield can be improved by tuning the third laser to autoionizing Rydberg states [11], where we used the J c K coupling scheme Abstract We present a novel two-step even-odd mass isotope selective excitation and ionization scheme, potentially applicable in resource recycling and management of palladium occurring in high-level nuclear waste. In contrast to the conventional three-step selective ionization process, the two-step scheme utilizes transition selection rules to an autoionizing Rydberg state, rather than to an intermediate state, resulting in an increase in efficiency of over an order of magnitude while retaining excellent selectivity of >99.7%. The reduction in the number of excitation lasers required allows several technical simplifications and reduces costs should the process be developed for large-scale resource recycling operations.

Selective Resonance Photoionization of Odd Mass Zirconium Isotopes Towards Efficient Separation of Radioactive Waste

“…However, alternatively suitable autoionizing Rydberg states, which potentially enhance the ionization efficiency and isotopic selectivity, have not been detected by searching to 58 000 cm −1 . These observations may indicate that zirconium has a more complex level structure than closed shell atoms, such as palladium 15 . In the J = 2–1–1–0 excitation scheme, however, we have identified several third intermediate excited states, which show both significant isotopic selectivity and high ionization efficiency for the odd mass Zr isotope.…”

“…In this paper, we present basic research on the selective photoionization of the odd mass Zr isotopes with multiple and stepwise laser excitations. Accompanied by the foregoing study of the laser isotope separation of palladium isotopes 15 , we revisited the three-step resonant photoionization of Zr vapour and examined the isotopic selectivity for 91 Zr from other naturally occurring even mass isotopes. The representative excitation scheme employed in this investigation is illustrated in Fig.…”

Laser isotope separation scheme of lithium by three-color photoionization

“…The photoionization of Li atoms can be carried out through different energy pathways whose common feature is selective excitation of one or more intermediate atomic states followed by the subsequent ionization of excited atoms either by an additional laser radiation, electric field or through collisions with other particles [5,6]. The simplest one is the two-step photoionization in the absence of external fields or collisions; however, the selectivity remains limited if auto-ionization levels are absent and such is the case with Li [11][12][13]. Higher selectivity can be obtained through multi-step resonant excitation, in which number of intermediate steps involved is decided by the ionization potential (I.P) of element and the photons energy of laser radiation used.…”

“…5 Watts) is required at 578.2 nm radiation. The high laser power favors to higher photoionization efficiency, however it may subsequently deteriorate the selectivity [11,27,31] of process.…”

“…Karlov et al [10] demonstrated an applicability of two-step RIS process for isotope separation of rare earths such as neodymium (Nd), samarium (Sm), europium (Eu), gadolinium (Gd), dysprosium (Dy) and erbium (Er). In recent, Locke et al [11] carried out selective photoionization of palladium (Pd) isotopes using two-step excitation method to recycle the Pd occurring in nuclear waste. Il'in et al [12] studied the Li isotopes photoionization in two frequency laser field using TOFMS and ionizing radiation of 266 nm.…”