Separation of Isotopes in Space and Time by Gas-Surface Atomic Diffraction

The separation of isotopes in space and time by gas-surface atomic diffraction is presented as a new means for isotopic enrichment. A supersonic beam of natural abundance neon is scattered from a periodic surface of methyl-terminated silicon, with the ${}^{20}\mathrm{Ne}$ and ${}^{22}\mathrm{Ne}$ isotopes scattering into unique diffraction channels. Under the experimental conditions presented in this Letter, a single pass yields an enrichment factor $3.50\pm 0.30$ for the less abundant isotope, ${}^{22}\mathrm{Ne}$, with extension to multiple passes easily envisioned. The velocity distribution of the incident beam is demonstrated to be the determining factor in the degree of separation between the isotopes’ diffraction peaks. In cases where there is incomplete angular separation, the difference in arrival times of the two isotopes at a given scattered angle can be exploited to achieve complete temporal separation of the isotopes. This study explores the novel application of supersonic molecular beam studies as a viable candidate for separation of isotopes without the need for ionization or laser excitation.

Figure 1

(a) Illustration of a monoenergetic beam of ${}^{20}\mathrm{Ne}$ and ${}^{22}\mathrm{Ne}$ diffracting from ${\mathrm{CH}}_3\text{−}\mathrm{Si}(111)$ into spatially well-separated final scattering angles; (b) schematic of the ultrahigh vacuum surface scattering instrument employed in this experiment; key components are a high-Mach number, triply differentially pumped molecular beam source with variable temperature nozzle, a UHV scattering chamber with a full suite of diagnostics (not shown), and a high sensitivity triply differentially pumped detector with a high degree of collimation that can rotate to detect a range of final scattering angles while maintaining constant incident kinematic conditions.

Figure 2

(a) Demonstration of angular separation for supersonic molecular beam diffraction from an ideal grating for the conditions reported herein; (b) lack of angular separation for an effusive source under identical incident conditions; $T_B=55\mathrm{K}$, ${\theta }_i=25.2°$.

Figure 3

Nearly complete angular separation of (11) diffraction peaks for ${}^{20}\mathrm{Ne}$ (black) and ${}^{22}\mathrm{Ne}$ (red) diffracted from ${\mathrm{CH}}_3\text{−}\mathrm{Si}(111)$. Further purification of the major ${}^{20}\mathrm{Ne}$ component or separation and enrichment of the minor ${}^{22}\mathrm{Ne}$ component can be realized through experimental considerations as discussed in the text.

Figure 4

$(\overline{1}\overline{1})$ diffraction peak for helium (black) and ${}^{20}\mathrm{Ne}$ (red) scattering from ${\mathrm{CH}}_3\text{−}\mathrm{Si}(111)$. Note that the width of these peaks is a consequence of the convolution of the instrument function, surface quality, and their incident velocity distributions. The He diffraction peak has a significantly narrower angular distribution than the Ne peak, due to the narrower velocity distribution of the He beam used ($\mathrm{\Delta }v/v=0.8\%$) as compared to that for the Ne beam ($\mathrm{\Delta }v/v=6.4\%$). Potential further improvement in angular separation, as compared to the data shown in Fig. 3, can be realized by additional narrowing of the beam’s incident velocity distribution. Inset: wide angular range diffraction scan for $\mathrm{He}/{\mathrm{CH}}_3\text{−}\mathrm{Si}(111)$, demonstrating the high quality of the substrate used in these experiments.

Figure 5

Time-of-flight spectra for ${}^{20}\mathrm{Ne}$ (black) and ${}^{22}\mathrm{Ne}$ (red) at the midpoint between the maxima of the ${}^{20}\mathrm{Ne}$ and ${}^{22}\mathrm{Ne}$ (11) diffraction peaks as shown in Fig. 3, providing a route for temporal separation of the isotopes.