Investigation of the composition of a U-233 source for a new generation of optical atomic clocks based on a Th-229 nuclear transition, using alpha spectrometry

For the development at Division 4 of a new generation of optical atomic clocks with even lower uncertainties, a source of Th‑229 atoms is needed. One way to produce Th‑229 atoms is to obtain them as recoil ions from the alpha decay of U‑233. An initial analysis of a source produced at the Institute of Nuclear Chemistry of Johannes Gutenberg University Mainz was carried out using defined solid angle alpha spectrometry. This work included an investigation of whether, in addition to U‑233, further radioactive (a‑emitting) isotopes were present in the source. These isotopes, given their a‑decay and the associated recoil of their daughter isotopes, act as a source of further nuclides, which in turn makes them a source of increased background when Th‑229 is separated for the remaining experiments.

At the university in Mainz, U‑233 was deposited electrolytically on a titanium‑coated silicon wafer. This silicon wafer, 30 mm in diameter and with a 6 mm hole in the center, was characterized at PTB using defined solid angle alpha spectrometry and gamma spectrometry. Furthermore, an autoradiograph of the source was produced for this purpose (Figure 1).

Fig. 1: Autoradiograph of the U‑233 source with 30 mm diameter and central hole on a Ti‑coated Si wafer. The activity scale ranges from green (background) to yellow to red (highest activity level). Autoradiography is required for assessing the geometric correction factors for the defined solid angle alpha spectrometry of this specific source.

The identified nuclides and their activity were determined from the pulse height distribution of the measured spectrum by minimizing the deviation of the individual lines while taking account of the nuclear decay data. Besides the U‑233 lines we also find the lines of the first progeny, Th‑229. U‑232 and its progenies, including Th‑228, Ra‑224 and further to Po‑212, were identified as impurities.

Other impurities detected to date include various plutonium isotopes such as Pu‑238, Pu‑240 and Pu‑242. The measurement of this source by means of gamma spectrometry confirmed the presence of these isotopes.

Contacts:

A. Honig, Department 6.1, Working Group 6.13

S. Röttger, Department 6.1, Working Group 6.13

G.-A. Zitzer, Department 4.4, Working Group 4.44