Tables for
Volume I
X-ray absorption spectroscopy and related techniques
Edited by C. T. Chantler, F. Boscherini and B. Bunker

International Tables for Crystallography (2022). Vol. I. Early view chapter

Radioactive samples

Jeff Terrya*

aDepartment of Physics, Illinois Institute of Technology, Chicago, Illinois, USA
Correspondence e-mail:

Synchrotron-radiation facilities are not populated with a large number of people with expertise in handling radioactive materials. This often leads to safety concerns that can delay or prevent experiments on radioactive materials from being conducted. Many of these concerns can be addressed in advance with well standardized research methods and procedures. This chapter discusses some of the methods and procedures that one can use to study radioactive samples at synchrotrons. It covers containment techniques and potential problems that can occur during measurements on radioactive samples.

Keywords: radioactive samples; containment techniques; synchrotrons.

1. Introduction

The study of radioactive materials at synchrotron-radiation facilities can often be challenging. There is added scrutiny to any experiment compared with those on nonradioactive samples. The majority of radioactive samples currently have to be measured within at least one level of containment, and often with as many as three levels, to minimize the risk of contamination of both the experimenters and the beamline equipment. These requirements, which are usually beyond those for more conventional materials, require the experimenter to significantly detail the experiment in advance. In addition, one usually has only one opportunity to perform the experiment correctly. Any abnormal conditions are often difficult to adjust at the synchrotron. For example, an adjustment that could trivially be performed on a nonradioactive sample or even in a laboratory that normally handles radioactive samples can be impossible. Remounting samples that have shifted during transportation can take a day and require the transfer of the sample to special laboratories near the synchrotron-radiation facility. These differences from working with nonradioactive materials require that the experimenter spends more time planning the experiments in advance. It is important to understand the conditions that will allow a successful measurement. In addition, one must evaluate the things that can lead to a failed measurement. It is also imperative to be ready to deal with a potential contamination scenario.

2. Advanced planning

The following information should be assembled before contacting the synchrotron about an experiment on radioactive samples. One should start by noting the technique or combination of techniques that will need to be used. For example, simple X-ray absorption fine-structure (XAFS) spectra can be measured with the sample in fluorescence or transmission geometry (Koningsberger & Prins, 1988[link]). The sample geometry can be adjusted to highlight surface sensitivity over bulk measurement by using a grazing-incidence geometry (Heald, 1992[link]; Den Auwer et al., 2003[link]). Will both XAFS and X-ray diffraction (XRD) be measured (van Beek et al., 2011[link])? XAFS and in situ electrochemistry is another common pairing of techniques (Soderholm et al., 1999[link]). Resonant photoemission is another technique that has been applied to the study of actinides, which requires simultaneous measurement of XAFS and photoemission (Terry et al., 2002[link]; Tobin et al., 2003[link]). There are certainly other techniques that could potentially pair well with XAFS in order to understand the physicochemical properties of radioactive materials. Every combination of techniques has different potential requirements and tradeoffs. One must make certain that the measurements can be made in a safe manner that still allows the required data to be collected.

Once the experimenter has decided on the technique or combination of techniques that will be utilized to study the radioactive sample, the next step is to determine the conditions under which the samples will be measured. The required conditions during the measurement play a large role in determining the size and activity of the sample. Common variables that can be controlled are temperature, pressure, pH, tensile properties and atmospheric composition. These parameters are often adjusted to systematically observe changes in both materials physics and chemistry. Temperature can be controlled using heating (Smith et al., 2019[link]) and cooling (Olive et al., 2016[link]). Pressure can be adjusted from ultrahigh-vacuum (UHV) conditions (Terry et al., 2002[link]) to kilobars (Borchert et al., 2004[link]). Actinide speciation is highly dependent upon pH in aqueous systems (Brendebach et al., 2009[link]). Speciation and surface reactivity is also strongly controlled by electrochemical potential (Yuan et al., 2015[link]). For the structural materials used in nuclear reactors, tensile properties (Liu et al., 2016[link]) depend upon temperature and radiation dose. Atmospheric composition may also be tightly controlled to either limit or probe reactivity (Caisso et al., 2015[link]; Rothe et al., 2019[link]). There are many additional parameters that one may want to control or measure during a synchroton run. Sample containment must be designed to allow the control and/or measurement of these parameters.

There are always tradeoffs to make as one designs the containment systems that allow these measurements to be conducted. These will be discussed in detail in Section 3[link]. In order to minimize potential exposure (dose) of personnel and equipment, it is best to reduce the sample activity by reducing the total amount (weight) of each radioactive isotope in the sample. This is sometimes limited by the amount of material needed for either synchrotron measurement or the measurement of the combined technique. For example, tensile testing often uses standard-size samples in a dog-bone shape. In the case of a dual tensile measurement, the sample dimensions are fixed by this measurement (Liu et al., 2016[link]). In other cases, the sample size may be limited by laser focus or the necessary geometry for levitation (Skinner et al., 2014[link]; Weber et al., 2016[link]).

The containment system also helps to minimize personnel exposure. The use of time, distance and shielding are the three main ways of controlling dose to the experimenter. Sample containment can add additional shielding, especially from α and β particles. The materials utilized are not as effective at shielding γ radiation. They also effectively increase the distance between the experimenter and the sample. These two effects act to reduce the potential dose. Poor design of the containment system can increase the amount of time that the experimenter has to spend handling and aligning the sample, which can increase the dose. The preliminary design of the containment system should address the potential for increased handling time and should be adjusted to minimize this effect.

Once an experimental plan and a rough design of the containment exist, the synchrotron staff should be contacted. One should note that the submission of a user proposal does not necessarily inform the radiation staff that there is a potential radioactive experiment to be conducted. Always inform the radiation-safety staff of any proposed experiment that will use radioactive samples. Do not assume that they are aware. This gives everyone time to address any potential complications and ensures that there are no last-minute surprises that can lead to the disapproval of an experiment.

Unfortunately, there are no uniform procedures between synchrotrons for conducting experiments with radioactive samples (see Appendix A[link]). The rules for handling radioactive materials in synchrotron-radiation facilities may vary by country, within facilities within the same country and even within different beamlines at the same facility (Denecke, 2006[link], 2016[link]). For example, BM20 at ESRF (Matz et al., 1999[link]) has a radioactive material-handling glove box installed in the beamline, and there are almost no restrictions on the condition of the sample when contained inside the box. It is beyond the scope of this work to provide a comprehensive description of all such facilities worldwide, but three methods are generally observed at synchrotrons for handling radioactive samples. Firstly, a synchrotron may not allow radioactive samples. Secondly, there are some synchrotrons that post detailed activity or dose limits. These facilities then review the safety documents for samples that are below these limits to see whether the experiment can be performed safely. Table 1[link] lists the allowed activities at representative synchrotrons following this process. Finally, there are other synchrotrons that do not publish activity limits but do allow radioactive materials to be measured. These facilities review all aspects of the safety documents but also include a review of the sample activity. This process can be beneficial when the experiment requires unusual sample handling or larger sample quantities, but can add review time to a simple experiment. Table 2[link] lists some of the synchrotrons that fully review all aspects of measurements on radioactive samples. The community would benefit from having standard experimental envelopes published at all synchrotron-radiation facilities.

Table 1
Synchrotrons with detailed activity or dose limits

Light source Activity limit
Advanced Photon Source 5 mR h−1 at 30 cm
Diamond Light Source 0.5 µSv h−1 at 30 cm
European Synchrotron Varies by isotope
National Synchrotron Light Source 0.1 rem h−1 at 30 cm
SOLEIL 18.5 GBq (0.5 Ci) per sample

Table 2
Synchrotrons relying on individual assessment

Light sourceRepresentative leading determination
Advanced Light Source Chemical Safety Specialist
Helmholtz-Zentrum Berlin Radiation Protection Department
Canadian Light Source Health, Safety, and Environment Coordinator

The last thing to consider in the advanced planning stage is what will you do if something goes wrong. Any number of things can go wrong when you are working with radioactive samples. These can range from samples moving within the containment cell, changing the mounting angle, to rupturing the containment. The plan that you put together for conducting the measurement should include contingencies for the things that could go wrong. Can you ship the sample to a neighbouring facility where you can remount the sample? Can you adjust the angle of the entire sample-containment cell to adjust for the motion? These are questions that you should address in your experimental plan. In your mounting strategy, have you mitigated any contamination that could occur if containment is lost? A simple sheet of aluminium foil placed to catch any loose material can save hours of decontamination. The surface cleaner fantastik from S. C. Johnson has been used by US National Laboratories for the decontamination of surfaces for decades ( ). My group always keeps this product on hand just in case of a surface-contamination incident. Prepare in advance for something to go wrong; do not be surprised. Do not panic when it happens, just calmly address the issue.

3. Sample containment

Having a well designed sample-containment cell is key to successfully carrying out experiments on radioactive samples. A poorly designed containment cell can prevent your experiment from being conducted at the synchrotron-radiation facility. It is imperative that the sample-containment cell serves two purposes. Firstly, the cell must successfully contain and protect the sample from accidental release. Secondly, the cell should allow you to make the measurements that you need to make. Sometimes, it can take years (Terry, 2018[link]) to successfully design, build and get approval to use a containment cell that will accomplish both of these needs. The procedures for qualifying and accepting a containment cell are often quite different among the various synchrotron-radiation facilities. One should always contact the facility that you intend to utilize once you have a preliminary design. It is beyond the scope of this article to cover the differences between facilities.

With experiments on actinides and radiopharmaceuticals, the cost of producing materials is not so great that a failed experiment due to a poorly designed containment cell is catastrophic. The loss is mainly in the time of the experimenter. This is not the case when the samples are irradiated nuclear fuels and structural materials. Here, the irradiations can take multiple years and cost millions of US dollars. A failed experiment on such irradiated materials should never occur. One must be willing to invest in the necessary containment infrastructure to allow the full measurement to be completed successfully. The synchrotron-radiation community could do better to learn and understand the magnitude and importance of these irradiation measurements.

The proper design of the sample-containment cells requires one to understand the physical requirements of the measurement, the physicochemical interactions of the samples with the containment and the transportation and disposal requirements of the containment. Issues can occur in any of these three areas and any of these issues can result in disapproval of the experiment or the halting of an experiment that is in progress.

3.1. Experimental considerations

The measurement geometry, excitation energy, detection requirements and facility prescriptions will all be used to determine the parameters of the sample-containment cell. Requirements in the past have ranged from no containment to triple-containment cells with multiple materials used in containment. Some things to consider when designing containment is that the window materials can have impurities and X-ray scattering properties that can negatively affect the proposed measurements. One should always try to run an experiment on a nonradioactive simulant sample as a test to identify potential issues without having to address them on a radioactive sample.

3.1.1. No containment

The need for no containment can arise for multiple reasons. The lower the excitation energy of the edge that one is trying to study, the harder it is to use containment. If the measurement is monitored using electrons, either photoelectrons or Auger electrons, containment can be nearly impossible as the electrons cannot pass through most containment. Finally, there is a movement to allow low-activity samples to be measured without containment at the Advanced Photon Source. At this time, the activity limits that can be studied without containment have not been set, but procedures are currently in development. While there is a demonstrated need to measure radioactive samples with no containment, this has been the most difficult situation to have approved at synchrotron-radiation facilities.

There is a risk to utilizing no containment. As described in the Guide of Good Practices for Occupational Radiological Protection in Plutonium Facilities (Swinth et al., 2005[link]): `the one characteristic that many believe is unique to plutonium is its ability to migrate with no apparent motive force. Whether from alpha recoil or some other mechanism, plutonium contamination, if not contained or removed, will spread relatively rapidly throughout an area'. The ability of radioactive contamination to spread is neither unique to plutonium nor to powdered samples. Radioactive powders exhibit this ability to self-spread contamination to a higher degree than bulk solid samples. However, even bulk solids can spread contamination beyond the sample holder, likely due to sputtering or spattering. Experimenters must always be aware of this possibility when working with uncontained samples. Samples with higher activity and those containing fine powders both exhibit this migration behaviour and it is wise to take precaution when studying them uncontained. Possible techniques for mitigation of migrating or spalled particles can include placing protective coverings over items that would be difficulty to decontaminate. UHV aluminium foil has been strategically placed in vacuum systems to capture spalled radioactive particles (Terry et al., 2002[link]). The availability of high-flux, high-energy laboratory-based X-ray sources (easyXAFS; ) may lead to more work being performed uncontained at laboratory sites rather than at synchrotrons (Bès et al., 2018[link]).

Fig. 1[link] shows a C K-edge map using scanning transmission X-ray microscopy (STXM) collected from halophiles from the Waste Isolation Pilot Plant (WIPP) grown in an aqueous medium including plutonium (Strietelmeier et al., 1999[link]). The low energy of the C K edge made it nearly impossible to encapsulate the sample while measuring the low-energy edge at 284 eV. The solution was to grow the halophiles on a gold mesh grid. The halophiles grew in the voids and it was possible to image individual microbes. The XAFS spectrum was collected at each point by varying the energy of each map across the edge. The risk of contamination was mitigated by lining the area near the sample with aluminium foil, which was removed after the experiment had been completed. After the experiment, the experimental vacuum chamber was surveyed to make sure that it was below the DOE free-release limits for the public. These limits are now set forth in DOE Order 458.1 Radiation Protection of the Public and the Environment (US Department of Energy, 2011[link]). This experiment would not have been possible given the technology at the time if containment has been required by the facility. Note one of the experimental difficulties of running uncontained was that the α particles emitted by the plutonium in the sample had a direct line of sight to the detector. They show up as bright spots throughout the image map.

[Figure 1]

Figure 1

Scanning transmission X-ray microscopy allows one to map out the X-ray absorption spectra of a material at high spatial resolution. This C K-edge map is of WIPP 1A bacteria grown in a plutonium-containing medium on a gold mesh grid. The individual bacteria can be seen in the voids of the grid. The bright spots are from α particles hitting the detector (Strietelmeier et al., 1999[link])

Another example of a situation where no containment is appropriate occurs when the X-ray absorption spectrum must be measured using either photoelectrons, as in resonant photoemission (Terry et al., 2002[link]; Tobin et al., 2003[link]), or with Auger electrons, as in partial electron-yield mode (Gao et al., 2009[link]). Resonant photoemission from α-Pu and δ-Pu bulk samples has been measured by Terry and coworkers (Terry et al., 2002[link]; Tobin et al., 2003[link]). The Pu O4,5 absorption edge was collected while monitoring the valence-band photoemission from the metallic plutonium samples. The absorption spectra followed a Fano profile and were used to try to understand the degree of localization of the valence f electrons in the two materials. Measurement of the photoelectrons required ultrahigh-vacuum (UHV) conditions and a clean sample surface. Fig. 2[link] (top) shows the plutonium sputter cleaning system that was designed and built to clean the plutonium surface before introduction into the analysis chamber. The system was very effective at removing the thin oxide layer that formed on the samples during transport (Fig. 2[link], bottom). As in the case of the STXM measurement on the WIPP halophiles described above, an aluminium boat was placed inside the photoemission UHV chamber to catch the majority of the material lost from the samples. The measured contamination from the plutonium samples was significantly less than that found near the ion gauges used to measure the pressure in the system, which used thoriated tungsten filaments. After the measurement, the vacuum chamber was decontaminated to meet the DOE free-release criteria (US Department of Energy, 2011[link]).

[Figure 2]

Figure 2

Top: the sample-handling integral transfer system for plutonium intense light experiments was used to sputter clean the metallic plutonium samples before introduction into the analysis chamber on the beamline. Bottom left: the plutonium disk in the centre is slightly darkened due to the presence of a thin oxide layer. Bottom right: after sputtering, the plutonium disk has a shiny metallic appearance (Terry et al., 2002[link]).

Presently, there is a wide variation in the activity limits allowed at synchrotron-radiation facilities, even among facilities in the same country. In many respects this is perplexing, as ionizing  radiation has the same effects worldwide. This leads to (needless) difficulties for those utilizing multiple facilities to study radioactive samples. Given the amount of contamination that one can normally find at a synchrotron-radiation facility due to the thoriated tungsten filaments used in much of the instrumentation, it would be reasonable to set an activity limit below which samples are treated as if they are not radioactive. In the United States, free-release levels are set by DOE Order 458.1 Radiation Protection of the Public and the Environment. This document sets conditions for the release of materials to the public (US Department of Energy, 2011[link]) and could be used to set some limits below which a sample could be treated as being nonradioactive. Pacific Northwest National Laboratory (PNNL) has been leading an effort to define these limits (Pacific Northwest National Laboratory, 2018[link]) at user facilities within the United States. At PNNL, they needed to reduce the activity of samples to place into an atom-probe microscope at a nonradiological user facility. They have developed `a free-release workflow document to enable easier, and safe, movement of tiny samples outside of radiological controls if certain thresholds are met'. Experimentally, these activity thresholds are reached by using using a focused ion beam (FIB) instrument to reduce the size and hence the activity of the sample. This methodology could be expanded to synchrotron-radiation facilities, where it would greatly simplify the study of low-activity samples. Facilities in other countries could adopt these limits or set their own. Standardization would be beneficial to all studying radioactive samples.

3.1.2. Simple containment

The majority of XAFS measurements on radioactive samples obtained by my research group utilize triple-containment sample cells. These cells provide easy mounting of powders (Fig. 3[link]) or liquids (Fig. 4[link]) for XAFS measurements at room temperature. The powder cell relies on the glue present on the Kapton tape for sealing. The liquid cell is available with either an O-ring seal or an indium wire seal. Both cells have air gaps between the windows of the outer cell and the next inner cell. The windows are the weakest part of the containment cells. The largest risk of a breach of containment is through the windows. The gap reduces the risk of a puncture through all three levels of containment.

[Figure 3]

Figure 3

Top: the triple-containment cell for powder samples consists of three containment sections. The outer two containment layers are made of aluminium and the inner containment layer is Kel-F. Bottom: the sample powder is placed within the inner Kel-F layer and sealed with Kapton tape. Each layer is nested within the next layer, with an air gap between each layer to act as puncture protection for the window.

[Figure 4]

Figure 4

Top: the liquid-containment cell has two layers of containment. The body of the cell is normally Kel-F because of its high radiation tolerance and high chemical stability. Bottom: the cell is then placed inside a second aluminium container and sealed with Kapton tape.

The powder cell is designed so that each layer nests inside the previous layer. In the case of powders, it is recommended that a thin piece of Kapton foil be placed over the sample so that the glue from the Kapton tape does not come into contact with the sample. This prevents any beam-induced chemical reactions with the glue from occurring. This adds some time and hence radiation exposure to the individual mounting the samples, but is usually worth the added effort to prevent the risk of radiation-induced chemistry.

The liquid cell is machined out of a solid block of Kel-F (polychlorotrifluoroethylene), although it can also be made out of Kapton if there is a chemical incompatibility (see Section 3.2[link]). The cell is filled from the side, where a stopper made out of nitrile is typically used to seal the liquid into the containment cell. The stopper is held in place by a plate bolted into the cell body. After the cell has been filled, it is placed inside a third level of containment machined out of aluminium. Kapton tape is used to seal the final containment layer.

These two sample-containment cells work very well for room-temperature measurements. They are relatively easy to align in the beam. The large windows allow a large collection area for fluorescence photons. They work well in both transmission and fluorescence geometry.

3.1.3. Surface sensitivity

The presence of the containment cells can make it very difficult to perform measurements in a grazing-incidence geometry. To utilize grazing incidence, the windows of the containment cell have to be impractically large. The windows become easier to puncture at this size. In addition, transportation becomes difficult due to the large size of the containment cells. To work around these issues, we developed triply encapsulated total electron-yield (TEY) cells (Fig. 5[link]). These cells allow measurement in both surface-sensitive electron-yield mode and bulk-sensitive fluorescence mode.

[Figure 5]

Figure 5

The total electron-yield (TEY) containment cell has two layers of containment. It is then placed inside a large third layer of containment with a large Kapton window. Left: the TEY cell for radioactive samples was based on this design of a total yield detector that includes A, fluorescence detector; B, polycarbonate window; C, copper retaining ring (100 mm diameter); D, gold TEY collection grid; E, BNC connector to power/amplifier; F, O-ring (95 mm diameter); G, copper clips; H, specimen; I, electrical contact point; J, TEY/cell base; K, retaining screws; L, aluminium shielding plate. Reproduced from Song et al. (1995[link]). Middle: the front of the TEY containment cell has a large Kapton window to allow fluorescence photons to escape. Right: the rear of the TEY containment cell has a large hole where a Phi sample puck is bolted in, compressing an indium wire seal. The samples are mounted onto the puck as in Fig. 2[link] (bottom).

The TEY cell was based on a nonradioactive TEY detector developed by Song et al. (1995[link]). The internal components of the TEY cell were similar to those in Fig. 5[link] (left), except that the windows were Kapton instead of polycarbonate. The detector gas used for amplification of the electron-yield signal was helium. Small particulate filters were placed inline on both the gas inlet and outlet to capture any material lost from the sample. A gold grid was used to detect the total electron-yield signal. Samples were mounted on a Phi sample puck, as shown in Fig. 2[link] (bottom). The metal sample pucks were bolted onto the TEY containment cell (Fig. 5[link], right), where they pressed against an indium wire which compressed to seal the plutonium within the containment. Fig. 6[link] shows both fluorescence-yield and electron-yield measurements from plutonium metal with varying gallium content (Terry, 2000[link]). The sample without gallium showed a higher level of oxidation than the gallium-stabilized sample. This is clear from the strength of the white line in the absorption edge. The white line is significantly larger in the surface-sensitive electron-yield spectra, indicating that the oxide was a surface species.

[Figure 6]

Figure 6

Fluorescence and total electron-yield data from 0.0% and 1.6% gallium-doped plutonium metal. The electron-yield measurements show the characteristic white line of the oxide on the surface of the metal. The fluorescence measurements are indicative of the bulk structure of the metal. Reprinted from Terry et al. (2000[link]), with the permission of AIP Publishing.

3.1.4. High temperature

High-temperature measurements on radioactive samples are in my experience the most difficult to attempt. Multiple components in the heater cell can exceed the melting points of the structural materials used in the containment cell. Two methods are often utilized to prevent accidentally melting a window. The first is to use ceramic windows with higher melting points; boron nitride and alumina are two that are often used. However, these thicker windows can greatly reduce the signal and hence can significantly increase the counting time necessary to obtain the needed signal-to-noise ratio. More commonly, then, one designs a system that isolates materials with a lower melting point so that they remain a significant distance away from the high-temperature areas (see Fig. 7[link] for an example; Smith et al., 2019[link]). This system was used to measure the XAFS signal from molten thorium salts that have potential for use in reactor fuels.

[Figure 7]

Figure 7

Smith and coworkers have designed a furnace for performing XAFS measurements on molten thorium salts. 1, Boron nitride cell; 2, Pt/Rh heating coil; 3, alumina tube; 4, Pt/Rh thermocouple; 5, molybdenum heating shields; 6, water cooling; 7, stainless-steel casing; 8, Kapton windows (75 µm); 9, copper feedthrough; 10, thermocouple feedthrough (Smith et al., 2019[link]).

Another method of heating radioactive samples has recently been utilized by Skinner et al. (2014[link]). They designed a heating cell for the study of molten UO2. This clever design used a laser to heat a small sample of UO2 that was levitated on a beam of argon gas. This setup allowed them to study the changes in atomic structure of UO2 as the temperature was increased above the melting point to 3000°C. Levitating the material allowed high temperatures to be reached without the potential of introducing contamination from any other material with which the material being studied could potentially come into contact. Two potential dangers to utilizing a heating element like this is that if the molten sample loses levitation then it could easily melt one of the lower temperature materials in the cell. The second is that the laser density has to be well controlled to make sure that it does not damage the containment cell and result in a breach of containment.

Relatively few samples containing elements heavier than uranium have been measured at elevated temperature at synchrotron-radiation facilities. To prevent unwanted chemical reactions from occurring at elevated temperatures, the atmosphere within the heating system has to be tightly controlled. The INE (Institute for Nuclear Waste Disposal) beamline of the ANKA synchrotron facility (Rothe et al., 2006[link]; Kalrsuhe Institute of Technology, Germany) has the capability to perform heating under carefully controlled atmospheric conditions (Rothe et al., 2019[link]). This facility has been utilized to study uranium (Prieur et al., 2018[link]) and americium (Caisso et al., 2015[link]) compounds at temperatures from 275 to 1400 K. The system can be used with either a static or a flowing atmosphere and has been used under controlled conditions utilizing both oxidizing and reducing atmospheres. Experimental apparatus such as this will allow a greatly expanded understanding of the chemical reactivity of radioactive species under simulated reactor conditions.

3.1.5. Low temperature

Los Alamos National Laboratory and Lawrence Berkeley National Laboratory (Pugmire et al., 2018[link]; Parsons-Moss, Tüysüz et al., 2014[link]; Parsons-Moss, Wang et al., 2014[link]) have collaboratively designed a cryogenic sample holder (Fig. 8[link]). This cryogenic containment cell consists of nested primary, secondary and tertiary layers. The primary containment layer can accomodate a sample that is approximately 10 × 2.5 × 3.5 mm in size. The final assembled tertiary containment cell is approximately 30.8 × 28.8 × 13.1 mm in size. For each of the containment layers (Fig. 8[link], top left), the holder consists of a body piece with a Kapton window on one side attached using a cryogenically compatible epoxy, which is vacuum leak checked prior to use. Sealing a sample or nested layer inside is accomplished by bolting down the lid, which compresses an indium wire seal against a piece of Kapton placed underneath the lid (Fig. 8[link], top right). Finally, the triple-containment cell is placed on the beamline in a once-through, liquid-helium cryostat (Janis), which offers relatively quick cooling times but requires fine-tuning of the helium flow so as not to expend the dewar too quickly.

[Figure 8]

Figure 8

Top left: from top to bottom, tertiary, secondary and primary sample-containment bodies and lids. Kapton windows are epoxied onto the rear surface of the bodies. Kapton sheets on the front surface are sealed using an indium wire seal which is crushed when the lids are bolted on. Top right: the assembled triple-containment cryogenic cell. Bottom: the triple-containment cryocell is placed into a once-through, liquid-helium cryostat from Janis. Reprinted from Olive (2018[link]).

There are multiple reasons to measure samples at cryogenic temperatures. One reason is to minimize X-ray beam-induced chemical changes (also called radiation damage). Radiation-induced chemistry is often minimized at low temperature. A second reason is to minimize the effect of sample vibration. Finally, some phenomena only occur at low temperature, one example being the observation of radiation-damage effects in plutonium metal (Booth et al., 2013[link]). At room temperature, defects in the plutonium crystal structure self-anneal out. Booth et al. (2013[link]) have demonstrated that these defects induced by α-particle collisions can be observed at cryogenic temperatures. The more parameters that you can control, the more likely you are to find an unexpected observation.

Low-temperature measurements on radioactive samples are much more common than high-temperature measurements. Experiments with a cryostat for radioactive samples are used, especially for XAFS measurements, in order to modify the thermal component of the Debye–Waller factor. While triple-containment sample holders are commonly used at facilities in the United States, other sample holders based on double containment have been developed, for example, at the ROBL beamline at ESRF (Reich et al., 2000[link]) . This facility has carried out XAFS measurements on samples ranging from environmental plutonium (Dumas et al., 2019[link]) to multivalent mixed actinide oxides (Epifano et al., 2019[link]).

3.1.6. Chemistry

Soderholm et al. (1999[link]) have shown that electrochemical control can be exerted over redox-active systems. They have used X-ray absorption near-edge structure (XANES) measurements to quantitatively determine the concentrations of the species that are present. To make these measurements, they designed an XAFS electrochemical containment cell. The cell is shown in Fig. 9[link]. By adjusting the electrochemical potential, they were able to demonstrate that they could probe two neptunium redox couples: neptunium(VI)/neptunium(V) and neptunium(IV)/neptunium(III). This technique is valuable as there are cases where XANES spectroelectrochemistry is the only available technique for probing a redox couple. Examples of this occur when multiple redox-active species are present or when other interferences occur in the optical spectrum.

[Figure 9]

Figure 9

Soderholm and coworkers have designed an electrochemical containment cell for electrochemical control of redox-active systems. The cell consisted of a working electrode (W) and an auxiliary electrode (A). The working electrode was a 52-mesh platinum gauze. The auxiliary electrode was platinum wire. The cell utilized a Ag/AgCl (3 M NaCl) BAS RE-5B reference electrode. N2 was introduced into the cell to promote mixing. The window material could be changed to maximize chemical compatibility. This cell was placed into another cell which became the triple containment for the experiment. Reprinted with permission from Soderholm et al. (1999[link]). Copyright 1999 American Chemical Society.

3.1.7. Manipulation

Historically, due to the containment requirements of radioactive samples, it has been very difficult to manipulate samples during an experiment. Two containment units with gloves where the experimenter can manipulate a sample have been used at the Materials Research Collaborative Access Team (MRCAT) beamline at the Advanced Photon Source (APS). The first system was used to study the Wigner effect in irradiated graphite (Lexa & Kropf, 2006[link]). More recently, a large glove-box system had to be designed to transfer multiple samples into a tensile testing rig in the X-ray beam (Liu et al., 2016[link]). The Illinois Institute of Technology, in collaboration with the University of Illinois, Urbana-Champaign, designed and built the in situ tensile system shown in Fig. 10[link] (top). This system can measure transmission XAFS and transmission XRD. The glove box serves as the containment for the samples while they are measured on the beamline. The samples must be loaded into the containment box in a hot cell facility located at Argonne National Laboratory but not within the synchrotron. The containment box is then loaded into a Type A shipping container (Fig. 10[link], bottom) for transport from the hot cells across the Argonne campus to the MRCAT beamline at the APS. Special Materials handled the transport between the two Argonne facilities. Argonne riggers removed the sample-containment glove box and aligned it onto the X-ray beamline. Once installed at the beamline, the experimenters were able to transfer samples with the glove box from internal storage to the tensile stage and back. The number of samples that can be stored depends on the total activity of the sample. The ability to manipulate samples while on the beamline opens up a number of new avenues of research on radioactive samples.

[Figure 10]

Figure 10

Top: the X-ray glove box with an in situ tensile stage with annealing capability is shown in position on the beamline. It is capable of both transmission XAFS and XRD measurements. Bottom: the glove box cannot be loaded at the Advanced Photon Source. It has to be shipped in a Type A shipping container to a hot cell facility for loading. Here, the riggers are securing the containment glove box for transfer within the Type A shipping container.

3.1.8. High-intensity beams

The use of high-intensity beams to heat or probe the radioactive sample comes with a risk of breaching the containment. This risk usually comes about when one performs an experiment with a focused X-ray beam, pink beam or white beam. Extreme caution should always be taken when performing experiments using these high-intensity X-ray beams. As mentioned above in Section 3.1.4[link], the use of high-power lasers also presents an added risk of a containment breach. One should always test the window materials that are used to introduce these beams to make sure that they can survive the duration of the experiment under the experimental conditions of temperature, pressure, pH etc. under which the measurements on the radioactive sample will take place. These test measurements can often be simultaneously conducted during other experiments just by placing a window material in the beam path. The accidental breach of containment due to window failure can often be avoided with advance testing.

3.2. Interactions

One of the easiest ways for an experiment to fail is to have a chemical incompatibility between the sample and the containment cell. It is of critical importance to choose materials for the containment cell that will not react with the sample under study. This is an area that not everyone considers in advance. My research group recently had a chemical incompatibility incident at a neutron scattering facility. During the study of a high-temperature alloy, the facility had us remount the alloy in a niobium can inside a heating cell. It was agreed that niobium would be the best material for the containment can based upon the alloy constituents. As the temperature was ramped, the niobium alloyed with the material being studied, and the new alloy melted and destroyed the heater. Fortunately, the outer containment cell contained the melt product. We chose to dispose of the mess rather than try to determine which alloy was actually formed, but this goes to illustrate that there are unknowns when you are working with complex materials. The experiment would have benefited from a test with a niobium can outside the neutron source.

Depending upon the type of samples that you are studying, it is necessary to explore the phase diagrams, chemical reactivity and other possible reasons for incompatibility such as radiolysis. One of the best sources of information on chemical incompatibility is the MasterFlex Chemical Compatibility Database ( ). It allows you to search for materials and chemicals that are compatible over a 48 h exposure. Two phase-diagram databases that can be used to predict any potential alloying are the ASM Alloy Phase Diagram Database ( ) and the ACerS–NIST database ( ). The ASM database contains information on binary and ternary alloy phase diagrams. The ACerS–NIST database contains temperature-dependent data on material interactions in ceramic and inorganic systems.

3.2.1. Structural materials

Common structural materials used in containment cells include aluminium, stainless steel, Inconel, Hastelloy, Kapton and Kel-F. Take the case where one wants to explore the reactivity of nuclear fuels in molten sodium. The containment cell has to have chemical compatibility with sodium at elevated temperature. The BalSeal Chemical Compatibility Chart ( ) shows that both grade 316 stainless steel and common aluminium alloys are incompatible with sodium at elevated temperature. The chart suggests that the use of Hastelloy C-276 is questionable. Finally, it suggests that Inconel X750 is acceptable for use with sodium at elevated temperature. In the case of Kapton and Kel-F, the MasterFlex Chemical Compatibility Chart ( ) indicates that Kapton is not suitable for use with strong bases such as potassium hydroxide and sodium hydroxide. Kel-F is the better choice in this case according to the MasterFlex Chemical Compatibility Database. Once you have identified the material that you think will have the best chemical compatibility, it is wise to actually test the material with nonradioactive materials to ensure that the actual use case is viable.

3.2.2. Sealing materials

Containment cells are often sealed with indium, nitrile O-rings, Viton O-rings, silicone sealant and epoxy. Again the MasterFlex Chemical Compatibility Chart can assist in choosing the correct material to use. For example, nitrile O-rings are incompatible with 100% hydrogen peroxide solutions, while Viton O-rings have excellent chemical compatibility. Indium wire seals are very effective at low temperature, but indium's low melting point of 157°C makes it ineffective in high-temperature applications. Different epoxies fail at different temperatures. MasterBond epoxy, EP29LPSP, can be used as a sealant at temperatures as low as 4 K. In addition, it can can withstand significant cryogenic shock. For example, it can survive a temperature ramp from room temperature to liquid-helium temperatures in a 5 min time period ( ). One should always verify that the sealing material can survive the conditions of the measurement by testing with nonradioactive materials as well.

3.3. Transportation

Shipping of radioactive samples is highly regulated and varies worldwide. It is very difficult to make generalized statements about the shipping and handling of radioactive samples. It is always recommended that you directly contact the radioactive materials specialist at the facility that you will be using. In the United States, my research group follows two rules regarding the transportation of radioactive material. These rules may not be applicable at all facilities. The first is never carry a radioactive sample to the synchrotron on your person. The second is do not ship a radioactive sample to a synchrotron without the express permission of the radiation group at the facility. The overwhelming majority of samples that my group has studied at synchrotron-radiation facilities have not required specialized nuclear transportation. At the activity levels that US synchrotrons usually approve, the materials can be shipped through a common shipping carrier such as FedEx. Only in rare occasions such as unique containment or a large amount of activity in multiple samples is it necessary to switch to specialized shipping. Only two experiments that my group has performed have required advanced transportation (US Nuclear Regulatory Commission; ) using either Type A (see Section 3.1.7[link]) or Type B (Terry et al., 2002[link]) shipping containers. One can never go wrong by contacting the synchrotron-radiation facility that you will use for assistance in determining the proper shipping procedures that they require. At a minimum, one should plan on having acquired the necessary shipping approvals at least several weeks prior to the scheduled experimental time. In some cases, several months have been required to obtain shipping approval depending on the sample isotope and activity (Terry et al., 2002[link]).

4. Absorption measurements

The actual measurement of the XAFS signal from a radioactive sample is not significantly different from that from a non­radioactive sample. The one thing that should be done before introducing X-rays onto a radioactive sample is to verify the beam parameters prior to exposing the containment cell to the X-ray beam. Verify that the beam type (monochromatic/pink/white beam) is the correct one for the experiment. Ensure that you have the correct beam energy when using monochromatic excitation or the proper energy cutoff in the case of a pink beam. Always check to make sure that the flux and beam size at the sample position are as expected. One does not want to risk breaching the containment because a white beam was accidentally put on the sample cell instead of monochromatic radiation.

The other concern that is unique to radioactive samples is that they are constantly transforming into other elements. These other elements can have absorption edges in the range of interest. This can limit the region over which usable data can be collected. It can be very difficult to see some of these edges at low concentrations, but they are still there and they can affect the ability to analyze XAFS data. Fig. 11[link](a) shows a U L3 edge from fissioned uranium oxycarbide (UCO) fuel. While the XAFS spectrum looks reasonable over the full range, if you zoom in, as seen in the inset, it becomes apparent that what one might think is an XAFS oscillation is really the Np L3 absorption edge. Problems arise if one tries to analyze the U L3 XAFS data beyond this point. This is a very common problem when studying radioactive materials. These tiny absorption edges from contaminants often limit the range over which one can analyze data. They are not always apparent at first glance, but you do need to look for them carefully.

[Figure 11]

Figure 11

(a) The U L3 edge from fissioned uranium oxycarbide (UCO) fuel is shown. Inset: an enlarged region around the Np L3 edge indicates that the tiny wiggle is actually an absorption edge. (b) The Hf L3 edge from a metallic Hf alloy shows an inverted tungsten absorption edge because the tungsten emission photons were windowed out of the measurement with a multi-channel analyzer.

One method that can be utilized to address this (if allowed by the energy range of the beamline) is to make the XAFS measurement at another edge of the element of interest. Sometimes by so doing one can extend the range over which the fine structure can be measured (Epifano et al., 2019[link]; Lebreton et al., 2015[link]; Prieur et al., 2013[link]). For (U,Am)O2−δ samples, which always contain a small amount of neptunium due to the decay of americium, collecting XAFS data at the U L2 edge rather than the L3 edge allows one to measure over a longer range of fine structure.

A second method that can be used to conduct measurements on samples with overlapping edges requires the use of an energy-resolving detector (Kropf et al., 2003[link]) to measure the fluorescence from the elements of interest. A window can be set to only include the fluorescence emission from the element to be studied. However, care must be taken when using this technique. Photons are still absorbed by the other elements in the sample and these are not being normalized out by the incident beam monitor. This technique often works well for measuring the XAFS spectrum of a low-concentration component riding on the smooth signal at high k of an element with a larger concentration. However, the opposite does not work as well. It is problematic to measure the weak high-k XAFS signal of an element with an overlapping absorption edge. Using a high energy-resolution, bent Laue detector (Kropf et al., 2003[link]), one can even observe an inverse edge due to the removal of all of the emitted fluorescence photons from the overlapping element. Fig. 11[link](b) shows an inverted edge in a fluorescence XAFS measurement of the Hf L3 edge due to photons absorbed in the sample by W atoms, but the emission photons from tungsten were not counted due to the use of a narrow emission energy window on a multi-channel analyzer. This leads to the inverted absorption edge. The photons were absorbed and cannot excite the XAFS of the element of interest, but there is no simple way to account for this in the normalization. Caution must be used when using fluorescence detection to try to separate overlapping absorption edges. My research group has had so much difficulty with this that we no longer utilize this method. We truncate the spectrum and limit analysis to the region below the absorption edge of the contaminating material.

5. Conclusion

Radioactive materials are important in the areas of the environment, medicine, national security, energy generation and fundamental physics. Unfortunately, given their importance in so many critical areas, it has been challenging to study these materials at synchrotron-radiation facilities. There are a number of problems that need to be solved that would benefit from exploration using synchrotron-radiation techniques. This chapter has presented a number of methods that have been utilized to study radioactive materials at synchrotron-radiation sources. If you have an experiment that would benefit from synchrotron-radiation techniques such as X-ray absorption or X-ray diffraction, you should not be afraid to attempt the measurement at a synchrotron source. Radioactive materials can be studied safely at these facilities. In addition, many of these techniques are now becoming available at laboratories that do specialize in handling radioactive samples. There are very few experiments that cannot be performed on radioactive samples given enough planning.



It is very important to have uniform procedures so that every experimenter is handling each step of the experiment in the same way. These procedures need not be words only. Pictures and images can be very helpful, especially when it comes to sample mounting. Each synchrotron will have its own safety-procedure requirements that will have to be followed when conducting the experiment. We have developed uniform procedures such as that given in Fig. 12[link] that are provided to all users conducting experiments on radioactive samples.

[Figure 12]

Figure 12

The mounting procedure for the triple-containment powder cells described in Section 3.1.2[link] is shown here. This allows everyone who utilizes these cells to mount and seal them in the same way.

It is recommended that every procedure that is repeated be detailed. These processes should be reviewed and updated with any improvements or issues that have been identified. These documents should be included with the safety documentation submitted to the synchrotron-radiation safety department. The more uniformity that you can install in the experimentation with radioactive samples, the easier it is to obtain experimental approval at the light source.


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