Laser Induced Fluorescent Tools in the Production of Californium-252 at Oak Ridge

[NNadir]

I'm so old that I actually remember working as a kid with a mass spectrometer that had a Californium (²⁵²Cf) radioactive ionization source, before the days of electrospray ionization.

(These kinds of radioactive ionization sources are still used on planetary space missions, since they don't need to draw a lot of power and can operate for many years.)

It's nice to see that Oak Ridge is still making ²⁵²Cf in the high flux reactor using ²⁴⁹Bk targets. In the neutron flux, some of the targets will undergo fission rather than capture, and thus the isolation of ²⁵²Cf will involve separation from fission products.

The following paper gives a nice insight to the process, and I found it fun to read:

Leveraging Calibration Transfer Techniques for Remote Monitoring of Samarium and Europium in LiCl Using Laser-Induced Florescence Spectroscopy for Radioisotope Production Applications Hunter B. Andrews, Jisue Moon, and Luke R. Sadergaski Industrial & Engineering Chemistry Research 2024 63 (25), 11082-11089.

The introductory excerpt:

Radioisotopes are critical resources for a plethora of applications, including radiopharmaceuticals, industrial radiography, powering deep-space exploration, and neutron sources. (1) The isotope ²⁵²Cf is used as a neutron source for nuclear reactor start-up, neutron activation analysis, and oil field exploration. (2) This ²⁵²Cf is produced at Oak Ridge National Laboratory by irradiating ²⁴⁹Bk with neutrons in the High Flux Isotope Reactor, after which the targets are processed to separate the ²⁵²Cf, as well as other isotopes, and recover the remaining 249Bk for further use. This processing occurs in radiological hot cells and glove boxes to protect operators from the radiation hazards associated with the irradiated material. Consequently, these chemical processes operate on limited analytical samples which require significant dilutions and time associated with removing material from the hot cell for analysis.

Other than californium neutron signals, obtaining radiometric signatures in the hot cell environment during radiochemical separations has been challenging. Optical spectroscopy is useful for process control in many nuclear processing applications. (3−5) Light can be transmitted in and out of glove box and hot cell facilities to obtain analytical data in situ with fiber-optic cables. Thus, developing optical monitoring approaches for lanthanides (e.g., Sm(III) and Eu(III)) and transplutonium species (e.g., Cm(III)) in both liquid samples throughout the various chemical processes (e.g., column separations) will provide benefits for making real-time adjustments, preventing faulty batches, and optimizing system performance and product quality. (6,7)

Atomic emission techniques, such as laser-induced fluorescence spectroscopy (LIFS), are useful for ultratrace analysis, complexation and speciation studies, and probing f-electron behavior. (8−12) LIFS requires a minimal sample quantity (∼10 ng), little to no sample preparation, atmospheric pressure, and no sample contact. Time-resolved (TR)-LIFS has been historically used with success for the detection of many lanthanides (e.g., Ce³⁺, Pr³⁺, Eu³⁺, Tb³⁺, Gd³⁺, Dy³⁺, Sm³⁺, and Tm³⁺,) and actinides (e.g., UO₂²⁺,, Am³⁺, Cm³⁺,, Cf³⁺, Bk³⁺, and Es³⁺). (13) However, for real-time monitoring, TR-LIFS is not realistic because measurements can last up to approximately 15 min. (12) Recently, direct LIFS analysis using a charge-coupled device (CCD) and multivariate chemometrics has improved the timeliness of the approach resulting in measurement times on the scale of seconds. It has been coupled with design of experiments and automated modeling approaches to successfully quantify Sm(III) and Eu(III), as well as LiCl relevant to Cf processing systems. (14) This previous proof-of-principle study was performed on a controlled laboratory benchtop experiment, but the deployment of these techniques for process monitoring applications will require using various spectrometers with varying resolution and sensitivity, continued deployment over extended periods of time, and measurements in different locations. This variety creates the need to evaluate calibration transfer methods to handle differences in spectra acquired with various instruments at different points in time and location...

It is interesting to see that the fission products in this case consist of the heavy lanthanides, transgadolinium lanthanides whereas fission in power reactors produces mainly the lighter lanthanides, lanthanum up to (small amounts) of gadolinium. Fission is not symmetric, and the yield vs. mass graphs show two maxima, the maxima on the right in the graphics shifting to right whereas the fission maxima on the left generally stays at a maximum around mass number 90, an isotope of strontium (that decays to zirconium) for asymmetric fissions. I'm not used to thinking about the heavy lanthanides when I think about fission products.

Very esoteric, I'm sure, but cool, a look in "how they do it."