Three-Dimensional Alanine EPR Dosimetry for Spatial and Angular Dose Mapping in a 137Cs Cabinet Irradiator

Experiments were conducted to characterize spatial absorbed-dose variations in a Hopewell Designs BX3-BR 137Cs cabinet irradiator using alanine powder analyzed by electron paramagnetic resonance (EPR) spectroscopy. The alanine/EPR system was operated as a secondary-standard dosimetry system and calibrated for mass-normalized powder response, yielding linear dose-response behavior with coefficients of determination R2 > 0.99 and an estimated measurement uncertainty of approximately 2%. The experimental framework combined empty-field longitudinal profiling, multilevel spatial mapping under loaded conditions, and stationary angular mapping to evaluate dose heterogeneity under realistic sample-loading geometry. A Cartesian coordinate system was defined with the X-axis along the length of the irradiation chamber, the Y-axis horizontally across the beam, and the Z-axis vertically between shelf levels, and the "working zone" was taken as the manufacturer-specified longitudinal region used for routine sample irradiations. Empty-field measurements along the X-axis showed a systematic decrease in absorbed dose from about 9.9 Gy to 3.1 Gy across the manufacturer-defined working zone (480-855 mm), corresponding to a dose-uniformity ratio of approximately 3.2. When a multilevel glass rack filled with alanine powder dosimeters was introduced, the resulting three-dimensional dose distributions preserved this intrinsic longitudinal gradient but also exhibited shelf- and position-dependent perturbations. Within the working zone, mass-normalized alanine responses showed peak-to-valley variations of roughly 150-240% relative to the shelf-mean dose across the three occupied shelf levels, quantifying the local maxima and minima that arise under loaded conditions. Stationary angular mapping at a radius of 75 mm further revealed clear azimuthal nonuniformity, indicating that angular position can be as important as longitudinal placement for samples mounted on circular carriers. These findings demonstrate that alanine powder EPR dosimetry provides a practical beam-profiling tool for operational characterization of 137Cs cabinet irradiators. In particular, the combined longitudinal, spatial, and angular measurements support the definition of a dosimetrically validated working region and the design of geometry-specific placement constraints or correction factors for biomedical, preclinical animal, detector-testing, and radiation-effects applications that rely on this class of irradiator.