Gamma irradiated MTB has been explored as a chemical gas sensor. For this purpose, MTB is used as the cladding in the sensing area of a multimode optical fiber in a home made fiber optic gas sensor. Solid state sintering and melting-quenching methods were followed for the synthesis of polycrystalline and glassy magnesium tetraborate (MTB). The structural and morphological studies were carried out using powder XRD and FESEM analysis. XRD peaks matched well with the Joint committee on powder diffraction standards(JCPDS) data. From EDAX analysis, the elemental composition of the sample showed the presence of cerium in MTB. Samples were irradiated in a 60Co gamma chamber and the dose of 20 Gy was given to the samples. UV–vis spectra of the as-prepared and gamma irradiated MTBs showed a marked variation in the absorbance. The sensor was used for the detection of ammonia gas at various concentrations. The gamma irradiated MTB showed a highly improved response for gas detection compared to as prepared MTB. In this paper, the experimental results and analysis are presented for ammonia detection. © 2018 Elsevier B.V.