In this article, a sol–gel route was employed to prepare nanophosphor materials including Ca9−x CrxAl (PO4)7 series (where 0.05 ≤ x ≤ 0.5 (mol%)). All subseries were irradiated to a γ- test dose of 5 Gy and the thermoluminescence (TL) response of Cr-activated (0.1 mol%) Ca9Al (PO4)7 sample has the best TL response and was chosen for investigations. The phase, particle size, surface morphology, and elemental mapping of the chosen sample Cr0.1 in comparison with other ones (Cr0.05 and Cr0.5) were scrutinized using x-ray diffraction (XRD), transmission electron microscope (TEM), SEM-EDX mapping, and XPS techniques. XRD profiles showed that all diffraction peaks assigned to the trigonal R 3 c space group and the average of particle sizes obtained by the modified Scherer method and TEM data are closely related. The presence of Cr3+ ions in the Ca9Al (PO4)7 lattice was assured by XPS spectra. Cr0.1 chosen sample was studied after irradiation to different γ-doses and displayed a prominent glow curve at 205 °C ± 3.97 °C. It showed a linear dose–response curve from 200 mGy up to 10 Gy with fading of 24% upon 24 h and almost no significant signal loss for further interval times. The average Zeff value was ∼11 at both 662 and 1250 keV energies making this sample a bone-equivalent material (11.3–11.8). Values of activation energies (eV) calculated by different methods were comparable. Results the dosimetric properties of that the Cr0.1 sample can be surely considered as an additional recommended detector as other available ones in marketing.