Bremsstrahlung from ₉₀Sr-₉₀Y in Thin Absorbers

The Internal bremsstrahlung from ⁹⁰Sr−⁹⁰Y and the external bremsstrahlung yields in various target materials were measured, using a novel experimental approach. The bremsstrahlung targets employed consisted of materials from atomic numbers 13 to 73 and ranged in thickness from 4 mg/cm² past the range of the ⁹⁰Y beta particles. The experiment performed by means of pulse-height scintillator spectrometric techniques enabled the observation and semiempirical calculation of how the bremsstrahlung spectrum builds up and is simultaneously attenuated, as well as the corresponding changes in the beta spectrum transmitted by the absorbers. This approach allows determination of generated bremsstrahlung spectra without having to make large attenuation corrections to thick target data. A brief description of internal bremsstrahlung and the theoretical aspects of external bremsstrahlung generation is given. Although there are several thick-target theories, only the Evans approximation to thick-target external bremsstrahlung is considered in detail. The Evans theory provides a good fit to the high energy portion of bremsstrahlung spectra and is therefore in prevalent use. Besides, it provides a clear example as to how yield constants or values are determined.The bremsstrahlung yield constants were found to differ, depending upon whether photon number or energy is considered. Further, a difference was found between the yield constants for ⁹⁰Sr−⁹⁰Y in secular equilibrium and ⁹⁰Y alone, indicating that an energy dependence also exists. The yield constants determined are 0.34 × 10⁻³ and 0.24 × 10⁻³ MeV⁻¹ for ⁹⁰Sr−⁹⁰Y, and 0.50 × 10⁻³ and 0.41 × 10⁻³ MeV⁻¹ for ⁹⁰Y, for bremsstrahlung energy and photons, respectively. This result indicates that the yield constant is energy dependent and also differs depending upon whether bremsstrahlung photon number or energy is being considered.