The Department of Energy (DOE) has ruled that all sealed radioactive sources, even those considered exempt under Nuclear Regulatory Commission regulations, are subject to radioactive material controls. However, sources based on the primordial isotope potassium-40 (40K) are not subject to these restrictions. Potassium-40’s beta spectrum and 1460.8 keV gamma ray can be used to induce K-shell fluorescence x rays in high-Z metals between 60 and 80 keV. A gamma ray calibration source is thus proposed that uses potassium chloride salt and a high-Z metal to create a two-point calibration for a sodium iodide field gamma spectroscopy instrument. The calibration source was designed in collaboration with Sandia National Laboratory using the Monte Carlo N-Particle eXtended (MCNPX) transport code. The x ray production was maximized while attempting to preserve the detector system’s sensitivity to external sources by minimizing the count rate and shielding effect of the calibration source. Since the source is intended to be semi-permanently fixed to the detector, the weight of the calibration source was also a design factor. Two methods of x-ray production were explored. First, a thin high-Z layer (HZL) was interposed between the detector and the potassium chloride-urethane source matrix. Second, bismuth metal powder was homogeneously mixed with a urethane binding agent to form a potassium chloride-bismuth matrix (KBM). The two methods were directly compared using a series of simulations, including their x ray peak strengths, pulse-height spectral characteristics, and response to a simulated background environment. The bismuth-based source was selected as the development model because it is cheap, nontoxic, and outperforms the high-Z layer method in simulation. The overall performance for the bismuth-based source was significantly improved by splitting the calibration source longitudinally into two halves and placing them on either side of the detector. The performance was improved further by removing the binding agent and simulating a homogeneous mixture of potassium chloride and bismuth powder in a 0.1 cm plastic casing. The split plastic-encased potassium chloride-bismuth matrix would serve as a light, cheap, field calibration source that is not subject to DOE restrictions.