Radioactive phosphorus has been obtained by an n, p reaction on sulfur which had been electromagnetically enriched in S 33 . The beta spectrum of this phosphorus showed an increase in the low energy beta group approximately proportional to the isotopic enrichment. This confirms the previous identification of this activity with P 33 . The maximum beta energy was determined from several Kurie plots and found to be 0.249zfc0.002 Mev. The half-life was found to be 24.4±0.2 days. The spectrum appears to have an allowed shape as is expected from theoretical considerations.
An intermediate image beta-ray spectrometer has been constructed utilizing th.e focusing prope~ties of a U-shaped magnetic field distribution. The construction and performance of the m~t~me~t are dIscussed. A transmission of ten percent has been attained with a resolutlOn of 5.5 percent. A scmtillah~n spe~tr~meter is used in conjunction with the intermediate image spectrometer for the purpose of makmg comcidence measurements. 10 Pohm, Lewis, Talboy, and Jensen, Phys. Rev. 95, 1523 (1954).
The existence of a ring-shaped constriction in the electron beam of a thin lens beta-ray spectrometer is demonstrated experimentally by a photographic film method. Making use of a photographic mapping of the beam, an annular baffle is designed. Optimum relations between annular width, solid angle, and counter window diameter are investigated. A comparison of this type of baffle system with the original baffle system shows that at 2.5 percent resolution the spectrometer transmission may be increased by a factor of two by the use of an annular aperture in the region of beam constriction.
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