Compliance With DOE 5480.11: The Urine Bioassay Program at Y-12
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On January 1, 1989, DOE Order 5480.11 went into effect. Part of this order requires an internal dose calculation for uranium workers assessing urine samples through alpha spectroscopy.
In particular, it requires that the American National Standards Institute (ANSI) guideline N13.30 be followed. This "Performance Criteria for Radiobioassay" specifies criteria for accuracy, precision and detection capabilities for the measurement of specific radionuclides. Alpha spectroscopy is currently the only accepted method for making routine isotopic measurements at the detection levels mentioned in ANSI N13.30.
At Oak Ridge's Y-12 plant, a program has been put into place to comply with the DOE order. At the heart of the program is a Canberra VAX-based Genie 9900 system that automates calculations, calibrates equipment and ensures quality control.
The Y-12 Urine Bioassay program can be divided into several stages: sampling and sample tracking, sample preparation and mounting, alpha spectroscopy and quality assurance.
Dr. Larry Burchfield, Development Group at the Oak Ridge
Y-12 Plant.
Sampling and Sample Tracking
Worker Sampling
Workers at the Martin Marietta Y-12 plant who routinely handle radioactive materials or who are likely to be exposed to radiation are monitored on a monthly basis. Those who may be exposed only occasionally are monitored through a "work audit". Here, the Health Physics Department determines the number of employees to be monitored based on the work group's size, and from this number infers exposure levels for the entire group. Employees who do not work with radiation are also tested as a control group.
The actual sampling process involves the use of a sealed, two-liter take home kit. The voiding is done at home over a 24-hour period, or over two consecutive nights (simulated 24-hour period). The sample is then re-sealed, and a supplied bar-code label is attached to the container.
Sample Tracking
The samples are delivered to the counting lab each morning. The lab currently counts 60 samples per day to monitor 1350 employees per month. That number will increase to 160 per day — which the Genie 9900 system will easily be able to handle.
If the sample is received with a broken tamper seal, it is rejected. The samples are weighed, the bar code label is scanned into a VAX based computer, and the sample is then preserved and stored under refrigeration.
To protect the employee's right to privacy, each sample is assigned an internal tracking number (different than the original bar code) that is not associated with the employee's name. This tracking number uses the date and a sample number from 1 to 60. The results are then linked back to the original bar code label that indicates the employee identification.
Sample Preparation and Sample Mounting
Why Are These Steps So Critical?
Alpha spectroscopists will tell you that sample preparation and sample mounting are the most important steps in alpha spectroscopy. Why? Because improper separation and mounting techniques will result in poor resolution (smearing) or extreme tailing in the alpha spectrum. This makes identification of individual isotopes difficult if not impossible.
Many isotopes emit alphas at the same or similar energies and can not be discerned unless they are chemically separated from one another (i.e. separation of thorium from uranium). Therefore, proper identification of individual isotopes requires a chemical separation scheme. During this process, however, it is inevitable that some of the sample will be lost. Thus, you must have a way to calculate chemical yield - the percentage of analyzed species remaining at the end of the separation process.
To determine the chemical yield, a known activity of tracer is spiked into the sample prior to the separation chemistry. The tracer is an isotope of the element that you're looking for - i.e. U-232 for uranium samples. Since it is an isotope of the same element it will behave chemically in the same manner, so that you can assume the percent of tracer lost is equal to the percent of sample lost.
The tracer must be brought into chemical equilibrium (the same chemical form) with the isotopes being measured. This is important because different chemical forms of the same element behave differently. Since the analysis depends on an accurate accounting of how much of the species being analyzed is lost, both the tracer and sample must be in the same chemical form. Therefore, steps must be taken to bring about a complete exchange of the tracer with the sample.
Assuming a chemical yield, rather than introducing a tracer is a meaningless exercise since every sample behaves differently - no adequate prediction can be made. Chemical yields can typically vary from 25% to 95%, and can sometimes even be below 25%. With this type of variation (which occurs even with careful separation techniques) it is impossible to assume an average yield and have credible results.
Once the sample has been separated, the next step is to properly mount it. The objective is to mount the sample with as thin a layer as possible, to minimize self absorption. Alpha particles are easily absorbed by any material, even the material used in mounting. In addition to self absorption, a shift in energy due to partial attenuation can be a problem with samples that are poorly mounted.
Sample mounting should be done either by electroplating, or by precipitation with micro-gram quantities of some rare earth fluoride (neodymium is used at the Y-12 plant). Simple evaporation is not acceptable because it will leave a salt layer too thick for proper measurement and will fall prey to the problems mentioned above.
Techniques Used in the Y-12 Program
The basic sample preparation process for the urine bioassay program at the Oak Ridge Y-12 plant is 1) sample preservation (with acid), 2) introduction of the tracer and establishment of equilibrium between the sample and the tracer, 3) chemical separation and 4) mounting of the sample.
The specific process is indicated in the following flow diagram. Typical chemical yields are 85%. Any sample below a 25% yield is rejected.
Figure 1 - Flow Diagram of Y-12 Chemical Process
Alpha Spectroscopy
Each alpha-emitting radioisotope emits alpha particles at a distinct, characteristic energy. Because of this, we can use a simple distribution graph — counts versus energy — to determine the presence and amount of particular isotopes in an unknown sample.
Alpha particles are quite slow and large, hence they can easily be stopped. A single sheet of paper or even air itself can stop them. Therefore, alpha spectroscopy samples are counted in a vacuum. With the removal of the air surrounding the sample, the alpha particles have a much greater chance of reaching the detector to be counted. The detector used in alpha counting is a semiconductor composed of silicon.
One type of silicon detector is particularly useful for alpha spectroscopy — the Passivated Implanted Planar Silicon (PIPS) detector. This detector is characterized by a thin entrance window (for better resolution) and low leakage current (low noise contribution). Unlike the older technology Silicon Surface Barrier detectors it replaces, the PIPS can be cleaned with a small amount of acetone on a cotton ball.
The Appropriate Equipment
An alpha spec counting system with multiple inputs generally consists of the following:
Per input:
- vacuum chamber with a silicon detector and reverse sample bias
- preamplifier/amplifier
- analog to digital converter (ADC) or Multiplexer/Mixer-Router input
- power supply to power the detector
Per system:
- NIM bins or racks to house vacuum chambers
- vacuum pump and tubing
- mixer/routers or multiplexers
- multichannel analyzer/computer system
- analysis software
Figure 2 - A Typical Alpha Spectroscopy System
The Counting Equipment at Y-12
The current system at Y-12 consists of 128 NIM-based spectrometers multiplexed to a single Canberra Genie 9900 VAX-based multichannel analyzer system. The power of a VAX-based system is essential in this application since in addition to the 128 data inputs, there are thousands of quality data values to track.
Controlling the entire system is the Alpha Management Software (AMS), originally written specifically for Y-12 and now available directly through Canberra. The software was put into place to avoid all hand calculations, thus saving valuable time. It cuts down on the margin of human error and allows for extensive quality control tracking that would otherwise be too overwhelming to even contemplate.
The software controls every aspect of the alpha counting process - from starting the counters to archiving the results on tape. Functions include sample information entry, data collection, data analysis and archiving results. Additionally, AMS performs calibrations, collects backgrounds and counts standard samples. Reports and plots can be printed, and a bar code can be used to track samples.
Initially, energy regions for each expected isotope are entered so that the operator does not have to manually enter them for each count. Region of Interest (ROI) analysis, where the counts are determined for an established region, is a preferred method over the peak search method for alpha spectroscopy analysis. With ill-defined peaks and a FWHM (Full Width at Half Maximum) that varies widely, a computerized peak search is very difficult and often inaccurate.
AMS computes a concentration for each identified isotope based on the known activity of the tracer (U-232) and the percent chemical yield for the sample. Each spectrum must be reviewed by the operator to ensure integrity of the ROI setup. A report can be printed after review, or the data can be transferred to a Health Physics database, as in the Y-12 system.
Quality Assurance
QA plays a major role at Martin Marietta, where the NQA-1 standard is followed. At present, there are a total of 2732 quality control charts that are automatically generated by the AMS software. The parameters tracked include backgrounds, controls, daily checks, weekly checks, reagent blanks and chemical yields.
The operator can print out a variety of reports and plots simply by choosing the parameter, detector and date range.
Special thanks to Dr. Larry Burchfield, Development Group at the Oak Ridge Y-12 Plant for his technical assistance in the writing of this application note.
Figure 3 - Operating Screens
Figure 4 - A Quality Control Chart
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