Rare-Earth Hydroxide MicroPrecipitated
Alpha-Counting Sources ¾
an Attractive Alternative to
Traditional Fluoride Precipitation

Penet Melton

General Engineering Laboratories, Federal Division
pem@gel.com · (843) 556-8171

Michael K. Schultz

PerkinElmer Instruments
michael.schultz@perkinelmer.com · (865) 483-2446

Guebuem Kim and William C. Burnett

Florida State University, Department of Oceanography
burnett@ocean.fsu.edu · (850) 644-6703

Rare-earth hydroxide microprecipitation is an attractive alternative to the more common rare-earth fluoride method for preparing counting sources for alpha-spectrometry. Not only do the rare-earth microprecipitated sources eliminate the need for the use of hydrofluoric acid (removing a potential safety hazard), but also, it is possible that sources may be prepared without the need for redox control.

We investigated the quality and recovery of rare-earth-hydroxide microprecipitated alpha-counting sources using isotopic Am, Cm, Cf, Np, Pu, Th, and U. We used neodymium as our carrier element. Solutions of dilute hydrochloric acid were spiked with known quantities of radioisotopes of the actinides. In initial testing, 50 mg of Nd was added to each solution (10 ml 0.1 M HCl). The pH of the solutions were increased to >8 using concentrated ammonium hydroxide (NH₄OH, about 25 drops). Thymol blue indicator was added to monitor the pH of the solutions. Sources were prepared by filtration, using 47-mm diameter, 0.1 mm Gelmen-Metricell type filters. Sources were counted in vacuum-controlled alpha chambers, housed in an Octete PCÔ integrated alpha-spectroscopy system (PerkinElmer Instruments, Oak Ridge, TN), equipped with 450 mm² UltraÔ ion-implanted-silicon detectors (PerkinElmer Instruments). The system also includes a tandem recoil prevention system consisting of reversed-bias alpha-sample planchet holders (PerkinElmer Instruments) and integrated vacuum control system (Granville-Phillips Industries). Sources were counted at a distance of about 5 mm. Spectra were analyzed using PC-based AlphaVision^® integrated alpha-spectrometry software (PerkinElmer Instruments).

The results of these initial tests are very encouraging. Recoveries are generally greater than 85%, with the exception of Np (which may have been present in the highly soluble (V) redox state). Specifically, recoveries were: Pu-239/240 – 99.0%; U-238 – 96.0%; Am-241 – 86.0%; Cm-244 - 108.0%; Cf-252 – 88.0%; Th-232 – 103.0%; Np-237 – 82.0%; and Pa-231 – 92.0%. In terms of resolution, the average FWHM values for all actinides fell below 70 keV, which is acceptable for many applications.

An additional experiment was performed to determine the amount of rare earth that could be present without degrading spectral resolution. Direct precipitations were performed using Am-243, Am-241, and Cm-244. Increasing amounts of Nd³⁺ were added to the test samples (50, 100, 150, 200, 250, and 300 mg). Our data indicate that a limit of 100 mg rare-earth carrier may be the cutoff for the best resolution. This may limit the method for soils with high rare earth concentrations ¾ further testing is required.