Sample Preparation for Alpha Spectroscopy

---

More than any other factor, proper preparation of the sample is vital to achieving quality results in alpha spectroscopy. Sample preparation converts the sample into a thin layered, chemically isolated form that can be placed into a spectrometer and analyzed with a minimum of interferences and self absorption.

Conversion of the raw sample into a form suitable for alpha spectroscopy is often an extensive process requiring many steps. Hundreds of different methodologies have been published for specific applications (e.g. U/Pu/Am in soil, U/Th in marine sediments, etc.) A scheme for screening analysis for actinides in urine using extraction chromatographic resins is shown in Figure 1.

In this brief we will provide an overview of the techniques that are most commonly used. Canberra offers extensive training courses on sample preparation methodologies which cover these topics in detail.

There are three principal steps to the preparation of an alpha spectroscopy sample:

• Preliminary Treatment
• Chemical Separation
• Sample Mounting

Preliminary Treatments

These procedures are performed to homogenize and preserve the sample and to generally prepare the sample for subsequent chemical processing. Different procedures are used for solid samples (e.g., soils), liquid samples and filter/wipe type fibrous samples. If tracer nuclides are to be added to the samples, this is typically done early in the preliminary treatment phase.

For solid samples a typical pretreatment may involve drying, grinding the sample, sieving through a 100 mesh screen, combusting to remove organic matter, and finally dissolving in a strong acid solution - typically a combination of nitric, perchloric and hydrofluoric acids. When samples contain silicate material, hydrofluoric acid is necessary. Repeated evaporation of the acid dissolved sample will remove the silicates as silicon tetrafluoride.

Either microwave or conventional oven digestion bombs may be used to speed up the digestion process. The "bomb" approach is also desirable because it eliminates much of the acid fuming in the laboratory.

High temperature fusion techniques are also employed in dissolution of soil samples. This technique uses pyrosulfates, borates, and carbonates.

Of particular interest is the pyrosulfate technique which removes all silicates and assures complete dissolution.

An alternative method to total dissolution involves leaching the sample with a strong acidic solution such as 6 M hydrochloric acid. This is an effective approach when the radionuclide of interest is deposited on the surfaces of soil grains rather than incorporated within the crystal lattice.

Liquid samples are usually acidified with nitric or hydrochloric acid soon after collection.

This ensures that trace elements and nuclides are kept in solution and inhibits biological growth. There are exceptions such as iodine which should be preserved in basic solutions. Filtration of liquid samples will depend on the analytical requirements, i.e., is "soluble" or "total" concentration desired.

After addition of radioactive tracers and stable carrier elements, the liquid sample is then concentrated by various co-precipitation techniques. This initial "scavenging" of the radionuclide fraction serves as a preliminary purification step, as many common elements are left behind in the process.

Filter and "wipe" samples are typically processed by acid treatment. In the case of paper type filters they are usually combusted before being treated with strong acids as in a soil sample. Glass fiber filters can be dissolved with hydrofluoric acid and membrane filters processed with nitric acid. In some cases, membrane filters are simply ashed at high temperatures.

It is sometimes possible to directly count air filters thereby eliminating the chemical processing.

Chemical Separation

Techniques used for separation and purification include co-precipitation, liquid-liquid extraction, ion exchange and extraction chromatography. In some cases, two or more of these techniques are combined.

Co-precipitation is often used to preconcentrate radionuclides and remove them from the bulk of the matrix being analyzed. For example, precipitation of calcium oxalate will remove most actinides from urine leaving most organics and inorganics behind. The mechanism for co-precipitation in most compounds is inclusion or trapping. Many compounds such as iron hydroxide are amorphous in nature and usually contain many molecules of hydration.

When traces of an element such as a radionuclide are being assayed in a complex mixture, a preliminary separation is often made using a co-precipitation procedure. For example, iron and aluminum hydroxide can be used for the isolation of uranium from acid aqueous solutions by the addition of carbonate free ammonium hydroxide.

Uranium may also be co-precipitated with Al, Ti, Zr, or La as a fluoride or phosphate. Other effective precipitates are available to scavenge other nuclides.

In liquid-liquid extraction (often called solvent extraction) organic acids, ketones, ethers, esters, alcohol and organic derivatives of phosphoric acid are used for the extraction of uranium and other actinides. Methyl isobutyl ketone has been used extensively in the nuclear industry for the extraction of Pu and U from spent nuclear fuels.

In some cases solvent extraction can concentrate the nuclide of interest 10-fold or more, and can be reasonably selective for the nuclide or nuclides of interest. Extraction, however, presupposes that the nuclide is in true ionic solution in an aqueous medium. The nuclides cannot be extracted this way from suspended or colloidal material, or if the aqueous solution contains organic complexing materials.

Depending on the distribution coefficient, K_D, of an ion for a liquid extraction system, it may be necessary to perform several extractions on the sample to obtain high recovery of the nuclide of interest.

One of the most effective and popular approaches for chemical separation and isolation involves ion exchange techniques. Although this may be performed in a "batch" mode, it is more typically and conveniently run in a column. With the sample in an aqueous acidic medium, ions of interest (often actinide complexes) replace "counter-ions" on the exchange resin, while other ions pass through and are discarded. The desired element is later eluted from the resin by adjusting the type of acid concentration. With careful selection of parameters, ion exchange procedures can perform extremely well. In addition, since they can run largely unattended, they are amenable to large-scale processing of samples.

Figure 1
Flowchart for a proposed scheme for screening analysis actinides in urine using extraction chromatograhic resin (modified from Horwitz, et al. (1990). Analytica Chimica Acta, 238: 263-271).

Although operationally quite similar to ion exchange procedures, extraction chromatography is technically more similar to liquid-liquid extraction. By impregnating inert microporous beads with extractive reagents, an extraction chromatographic "resin" may then be loaded onto a column for more convenient operation. The primary advantage of the new generation of chromatographic resins is their high degree of selectivity for certain elements or groups of elements relative to other species. Further advances in this field should greatly reduce the time requirements for chemical separation of samples.

Sample Mounting

After separation, the next step is to mount the sample to produce a suitable source. Producing suitable sources from the separated sample material is one of the most important phases of sample preparation.

In order to obtain the best possible resolution with an alpha spectrometer, it is necessary to produce a thin, flat, uniform deposit for a counting source. Ideally, the source would have a monatomic layer of the alpha emitter with no foreign matter above this layer to attenuate the alpha radiation. The source must be capable of being handled, it must be chemically stable, and all traces of solvent and acid must be removed to prevent damage to counting chambers and detectors.

There are three principle methods of source mounting:

• Evaporation from an organic solvent
• Electrodeposition
• Fluoride precipitation/filtration as a thin source

Direct evaporation of an aqueous solution can be used to form acceptable alpha sources, but these tend to be less uniform than those produced by other methods. Spreading agents can be added to the solution, although this results in organic deposits that must be burned off before the sample can be counted. This often causes poor adherence of the nuclide to the backing as well as poor resolution.

Direct evaporation of organic solutions can provide nearly solid-free deposits of alpha emitters. Thenoyltrifluoroacetone (TTA) in benzene has been used to complex U and Th and TTA in Toluene is often used for Pu. The typical approach is to separate the sample with solvent extraction or ion exchange, evaporate to dryness, then treat with perchloric and nitric acids to oxidize residual organic matter. The purified ion in aqueous solution is then extracted into TTA, deposited onto a stainless steel disc, and evaporated.

Up until the last few years, eletro-deposition was the most common method used by radiochemists for the preparation of standard alpha sources. It is applicable to many elements and deposits can be made on discs of stainless steel, copper, nickel, and other materials. A cell is used in which the metal disc is the cathode with only one side exposed to the electrolyte. The anode is always platinum.

Most procedures for deposition of actinides use aqueous solutions in either slightly alkaline or acidic form. Uranium can be deposited as a hydrous oxide or as UF₄, depending on the electrolyte used. Similar methodology is used for plutonium. A general electrodeposition method for actinide elements (thorium through curium) plates these nuclides as hydrous oxides from a buffered, slightly acidic solution prior to oxidation.

Electrodeposition procedures can produce excellent sources for alpha spectroscopy. A good electrodeposition approach will result in a thin, uniform source which will produce superior resolution.  In addition, an electrodeposited source is durable and may be kept in the laboratory indefinitely.

Unfortunately, source preparation by electrodeposition is time consuming, often requiring one to two hours per sample (See Figure 2). Laboratories with high volume sample requirements, therefore, should consider other approaches for their routine work.

Figure 2
The results of an experiment designed to evaluate the rate of electrodeposition of Pu from 1 m H₂SO₄ (pH=3.2). Such experiments are used to optimize conditions and evaluate plating times (Burnett, B. (1992). Advanced Alpha Spectrometry course manual. Meriden, Connecticut: Canberra Industries, Inc.)

One of the most successful methods for producing sources without electrodeposition involves co-precipitation of an actinide element with small amounts (usually 50-100 micrograms) of a rare-earth element (cerium, neodymium, etc.) carrier. The carrier elements may be precipitated as either a fluoride (by addition of hydrofluoric acid) or a hydroxide (by addition of ammonium hydroxide). The small amount of precipitate is then filtered onto a 0.1 µm membrane filter which is dried, mounted onto a support backing, and used for alpha spectroscopy.

The method is fast, inexpensive, and produces resolution nearly as good as electrodeposition with comparable recoveries.

Conclusions

Alpha spectroscopy is a very valuable technique for the assay of alpha-emitting radioisotopes. Its relatively high efficiency and extremely low background results in high sensitivity. This makes the technique particularly useful for environmental samples.

Sample preparation for alpha spectroscopy is somewhat of an "art" as well as a science. Fortunately, new developments in this area are making this task less burdensome all the time. Since the quality of the final source will have a significant effect on the overall analysis result, it is worth investing the time and energy necessary to evaluate the best sample preparation approach to the problem at hand. This can be accomplished by experience and/or hands-on training such as that offered through the Training and Technical Services Department at Canberra Industries. Courses are now available in both Basic Alpha Spectrometry (SP-503-5) and Advanced Alpha Spectrometry (SP-506-4). Both of these courses contain extensive material involving fundamental and the latest developments in sample preparation techniques.

All in all, alpha spectroscopy is one of the most sensitive techniques available for the routine analysis of alpha-emitting nuclides. Results can be optimized and problems minimized by careful attention to sample preparation.

Canberra wishes to thank:
Dr. Bill Burnett
Department of Oceanography
Florida State University

Dr. Larry Burchfield
Oak Ridge Analytical Services

For their assistance in the preparation of this technical brief.

---

Top of Page