Geant4 Simulations for the Radon Electric Dipole Moment Search at

(a) Formation and
breakup of an alkalimetal/noble-gas
van
der Waals molecule
(b) Binary collision
between an alkalimetal
atom and a
noble-gas atom
Figure 2.5: Polarization transfer process.
negligible. For heavier noble gases, such as Xe and Rn, the contributions of van der
Waals molecules dominate over the contribution of binary collisions at low pressures.
The pressure of the buffer gas participates in the creation and destruction of the
formed alkai-metal noble-gas van der Waals molecule (Figure 2.5). At high pressures
(multiatmosphere pressures) the collisions from N 2 greatly suppress the lifetime of
the formed van der Waals molecule. Thus binary collisions dominate the transfer of
angular momentum.
The RnEDM experiment will use high pressures in the measurement cell, where
the N 2 gas also plays a role in the optical pumping process. The N 2 gas will nonradiatively
de-excite (“quenche”) the excited rubidium atoms before they can reradiate
a photon. This avoids radiation trapping, where a photon is emitted by one
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