No abstract
Neutron-deficient radionuclides of praseodymium were produced by proton bombardment of enriched isotopes of cerium and neodymium. Two unreported activities were found, 22-min Pr 135 and 70-min Pr 136 ; the gamma and beta spectra of each were determined. Further study of 2.0-hr Pr 138 and 4.5-hr Pr 139 was also made, and gamma spectra of each are shown. The half-life of Pr 137 is shown to be <5 min or >1 year. Confirmation of assignment of 3.4-min Pr 140 is made. PRASEODYMIUM-135A N enriched stable isotope of Ce 136 was bombarded with 22.4-Mev protons in the Oak Ridge National Laboratory 86-inch cyclotron. Ion exchange methods were used to separate the products. 1 Following separation, the praseodymium fraction was found to contain a 22-min activity, a 70-min activity, 4.5-hr Pr 139 , and 19.3-hr Pr 142 . A second bombardment was made with the energy of the incident proton beam adjusted to 9.5 Mev by use of aluminum absorbers. Following separation the praseodymium fraction was found to contain a 70-min activity and 19.3-hr Pr 142 ; thus the 22-min activity was produced by either a (p,2n) or (p,3n) reaction. The 22-min activity was not observed as a result of bombarding enriched stable Ce 138 or natural cerium with either 22.4 or 9.5-Mev protons, thus eliminating its assignment to a mass higher than 136. In another bombardment of enriched Ce 136 with 22.4-Mev protons the praseodymium fraction from an initial separation was permitted to decay; then, after a second separation, 22-hr Ce 135 was found to be present in the cerium fraction. Thus, the 22-min activity is the parent of Ce 136 and is assigned to Pr 135 . A gamma-ray spectrum as measured with a Nal(Tl) scintillation spectrometer is shown in Fig. 1. The spectrum shows the presence of positron emission with gammas of 0.080, 0.22, and 0.30-Mev energy. Because of problems of counting efficiency and internal conversion it was not practical to distinguish between the x-rays due to electron capture and those due to internal conversion. However, it is reasonably certain that the activity decays by both electron capture and positron emission. The 0.033-Mev peak is the x-ray and the 0.51-Mev the annihilation peak. More energetic radiation was not observed. The gamma-ray spectrum was determined once every 20 minutes for ten half-lives and the contribution of the longer lived components of the praseodymium fraction subtracted in order to obtain a spectrum for the 22-min activity.The maximum energy of the positron emitted was determined by use of a scintillation spectrometer consisting of a cylindrical anthracene crystal 1J in. in diameter and 1 in. in thickness mounted on top of a DuMont type 6292 photomultiplier. Pulses from the photomultiplier tube were fed into a preamplifier. These were further amplified by a linear amplifier and fed into a single-channel pulse height analyzer and a scaler. The beta spectrum was determined several times over a period of ten half-lives. Decay of each experimental point was plotted and the decay curve analyzed for the 22-min component....
Proton bombardment of pure Tm 2 0 3 , followed by ion-exchange separations, has yielded an activity with an 18.5-minute half-life. Determination of the excitation function and identification of the daughter Tm 167 assigns the activity to Yb 167 . A gamma-ray spectrum shows it to decay by electron capture with a 0.118 Mev and possibly other gamma rays.
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