Inner bremsstrahlung spectrum ofSr90andY

Abstract

The shape of the inner bremsstrahlung spectrum accompanying the P decay of 9 Y was previously reported by Narayana et al. as being very anomalous. We have remeasured this spectrum using a source of Sr in secular equilibrium with OY. The instrumentally distorted spectrum was predicted using the formulas of Ford and Martin in conjunction with a detailed Monte Carlo calculation of the detector response. The spectrometer was a low-background, well collimated 7.62 cm by 7.62 cm NaI(T1) detector, shielded by lead. The geometry was chosen to minimize distortions due to external bremsstrahlung from stopping the P particles. The results are in good agreement with the theoretically calculated spectrum, with detour transitions, when the theoretical spectrum is convoluted with the calculated detector response function. RADIOACTIVITY 9eSr, 9 Y: P decay: measured inner bremsstrahlung.Inner bremsstrahlung (IB) is a generic term for continuous photon spectra associated with radiative P decay. An exhaustive review of the theory of inner bremsstrahlung and related processes was published in 1976 by Erber, %hite, and Latal. ' A general review of experimental and theoretical results appears in an earlier article by Persson. In general, most of the measurements of IB spectra from allowed P decay agree with the theoretical predictions; however, a significant number of experimental IB spectra accompanying forbidden P decay have been reported which strongly deviate from theoretical predictions at the higher energy ends of the spectra. The theoretical treatment of Ford and Martin3 includes detailed calculations of the Coulomb effects as well as those of virtual excitations of the nucleus called detour transitions. These calculations have been very successful in explaining much of the data associated with forbidden transitions. Some of the interesting decays having IB spectra with reported strong deviations from theory are '9'W (Refs. 4 and 5), "9Er (Ref. 6), 'aY (Ref. 7) 'Y (Ref. 8), ' 'Pr (Ref. 9) "Cl (Ref. 10), '37Cs (Ref. 11), and '~'Ce (Ref. 12). Here we reportthe results of a new measurement of the IB spectrum from a source of 9 Y in secular equilibrium with the parent 9 Sr. Thc reason for using the 9 Sr source was to avoid the large corrections duc to thc 64 h half-life of 9 Y. Also the IB spectrum from 9cSr is weak and has an endpoint energy of 540 keV while that of 9 Y has an endpoint energy of 2250 keV. The correction for short half-lives is straightforward; however, the long counting times required to observe a statistically significant number of photons above 1 MeV, makes the use of the long lived source more desirable.The decay of interest is the unique first forbidden p decay from the 2 ground state of s Y to the 0+ ground state of Zr. This branch has 99.8% of the total P decay strength. The IB spectrum from this decay was originally observed by Hakeem' to have only a small departure from that calculated by Ford and Martin, but more recently a spectrum in violent disagreement with the theoretical spectrum of Ref. 3 was repor...