Nested Neutron Spectrometers™ (NNS) can be used to measure neutron fluence rate spectra under diverse circumstances with a working principle similar to Bonner sphere systems. Conventionally, the NNS consists of an active-readout He-3 detector core and concentric moderator shells. In environments where the neutron fluence rate exceeds ∽104 neutrons/s, these spectrometers may be operated in a current-mode to avoid the effects of pulse pile-up and deadtime. A current-to-pulse conversion factor is used to convert current-mode measurements to pulse-mode. However, the conversion factor can only be directly calibrated under low-flux conditions due to the pulse pile-up in high-flux situations. In order to have confidence in the use of the conversion factor in high neutron fluence rate environments such as in high-energy radiotherapy, its use must be experimentally validated. To perform this validation, we developed a passive-readout NNS with gold activation foils. Our work included the generation of system response functions using the Monte Carlo toolkit, GEANT4, and an experimental workflow. The passive NNS and the active NNS were then used to measure the secondary neutron fluence rate spectra produced by a Varian TrueBeam™ STx linac under identical experimental conditions. We found that the spectrum obtained using the active NNS agreed well with that obtained using the passive NNS within uncertainties. This serves as validation of the use of the current-mode of the active NNS in the high neutron fluence rate conditions encountered in radiotherapy.