We here review the procedure by which the injected power in bremsstrahlung-producing electrons is determined from the hard X-ray spectra that they produce. In particular, we note that low-energy photons are produced in large part by electrons with energies that do not greatly exceed the thermal energy of the target with which they interact, so that the commonly used assumption of "cold-target" energy loss is not applicable over the entire energy range. We show that this significantly reduces the inferred energy content of the injected electron distribution and even makes the oft-dwelt-upon concept of a "lower cutoff energy" in the injected electron spectrum moot. Convenient formulae are provided for the total power in the injected electrons.
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