Technical noteComparison of Cf-252 thin-film sources prepared by evaporation or self-transfer☆
Introduction
Californium-252 (Cf-252, Z=98) is a potent neutron generator due to its propensity to decay by spontaneous fission. Approximately 3.09% of Cf-252 atoms fission into fragments upon decay, releasing on average 3.77 neutrons per fission event. The balance of atoms decay by alpha emission to form Curium-248 (Cm-248, Z=96). Californium-252 has a 2.645 year half-life and emits 2.3×106 neutrons per microgram per second due to spontaneous fission (Haire, 2011). As a neutron generator, Cf-252 has found wide use in sealed-source form for prompt-gamma neutron activation analysis, nuclear reactor start-up, neutron radiography, and other uses (Osborne-Lee and Alexander, 1995).
Sealed sources containing Cf-252 are very useful tools, but they are less useful for scientific study of Cf-252 because the encapsulating material blocks detection of the alpha particles and fission decay fragments. An exposed Cf-252 source is needed for such studies.
A thin-film exposed source is better yet. A thin-film source is defined as a source that is sufficiently thin that the source material does not significantly interfere with its own particle emissions outside the plane of the source. Penetrating gamma and neutron emissions are generally unaffected by the thickness of a thin-film source, but the track lengths of alpha particles and fission fragments are very short in solids, and absolute particle counts and measurements of particle energies are adversely affected by source self-shielding. For those types of particle studies, a thin-film radioactive source must be sufficiently thin that self-shielding has little effect on the particle emission measurements.
This work was undertaken to determine whether thin-film Cf-252 sources prepared by simple evaporation are comparable to thin-film sources prepared by self-transfer in regard to alpha particle measurements. Evaporative sources are prepared by dripping a solution containing Cf-252 onto a solid surface and heating the surface until the droplet dries to form a solid residue. Self-transfer sources are prepared in a vacuum and rely on the spontaneous transfer or self-sputtering of Cf-252 from an exposed mother source to a receiver plate placed close to it (Pauker and Steiger-Shafrir, 1971). The self-transfer method is known to produce thin films, but the evaporation method is easier to perform and requires less source material. Data are presented to show the similarities and differences between sources prepared by these methods. The self-transfer sources exhibit less alpha energy straggling than the evaporative sources, but they also emit more gamma rays, which are attributed to the fission products that transferred along with the Cf-252. Work is ongoing to determine whether and when the self-transfer sources reach radioactive equilibrium, a state in which the rate of radioactive decay of the source is driven by the decay rate of Cf-252.
Section snippets
Instrumentation
Alpha spectroscopy measurements were made using a Canberra alpha spectrometer (Model 7401) with a Canberra passivated implanted planar silicon detector. Gamma spectroscopy measurements were made using a Canberra standard-electrode coaxial germanium detector (Model GC6020). The micrometer used to measure source areas was precise to within ±0.0025 cm. A Canberra Vacuum Chamber (Model 7400A) was used to make the self-transfer sources.
Evaporative source preparation and analysis
A 1 mL liquid sample containing 2.89×104 Bq Cf in 0.1 M HNO3 was
Results and discussion
The alpha spectra for the evaporative sources are shown in Fig. 1, and the alpha spectra for the self-transfer sources are shown in Fig. 2. Fig. 3 shows an overlay of the alpha spectra for Sources #1 (evaporation) and #5 (self-transfer). The data were normalized by dividing the number of counts per channel by the total number of counts for each sample, and the energy range of interest was narrowed to show just the californium peaks.
The gamma spectra were measured for all of the sources. Fig. 4
Conclusions
The differences between sources prepared by the two methods—evaporation and self-transfer—are discernable. The sources prepared by self-transfer exhibit less alpha energy straggling and alpha energy loss than sources prepared by evaporation. The sources prepared by self-transfer showed additional peaks in the gamma spectra, indicating the transfer of shorter-lived fission fragments to the receiver plate. The activities of the sources prepared by self-transfer were considerably weaker, however,
Acknowledgments
The U.S. National Nuclear Security Administration (NNSA) Office of Nuclear Materials Integration (NA-73) sponsored this work. This manuscript has been authored by UT-Battelle, LLC, under Contract no. DE-AC05-00OR22725 with the U.S. Department of Energy. The authors thank Dr. Rose Boll (Oak Ridge National Laboratory) for providing the Cf-252 solution and mother source for this work.
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Notice: This manuscript has been authored by UT-Battelle, LLC, under Contract no. DE-AC05-00OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. The Department of Energy will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan).
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