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Detailed introduction
ISOTOPIC provides practical, integrated solutions for a variety of gamma ray measurement problems encountered in radioactive waste analysis and characterization applications. It analyzes high-resolution, high-purity germanium (HPGe) spectra and determines results for large sample volumes.
ISOTOPIC can be used "out of the box" as part of an easy-to-operate mobile system (such as the ORTEC ISO-CART-85) or integrated into automated systems, for example, for high-throughput measurements of large vessels in decommissioning projects.
Applicability
Applicable to the following geometries:
- Box, barrel, tube or surface (collimated detector)
- Closed geometric small containers (e.g. bottles with end caps)
- Large area determination of soil and surface (non-collimated detector: M-1 method)
ISOTOPIC offers a number of standard geometry "templates" from which specific measurement configurations can be developed. These templates include cylinders (from top and sides; including lined cylinders (pipes)), boxes, point sources (far field), closed geometric small containers with end caps, and infinite planes. The bottle counting option on the ISO-CART-85 is an example of a "with end cap" closed geometry. The Infinite Plane (Soil) mode provides non-collimated measurements of contamination, shedding or large-area leaks washed into or on an Infinite Plane, i.e. measurements of soil above the ground.
method
In container mode, for counting packages, pipes and surfaces, the detector is calibrated with a single point source measurement, even when using a collimator. This primary calibration meets any detector's certification standards and can be extrapolated or modeled to match the sample's physical conditions, container geometry, materials, and matrix composition. The model is based on a "point-kernel" approach, which decomposes the entire measurement problem into multiple source/matrix pixels and calculates and sums their contributions to the composite spectrum. This method is similar to the Monte Carlo method, utilizing user-supplied detector parameters (crystal diameter, crystal length, dead layer and end cap thickness) as part of the measurement configuration. No special individual measurements are required to calibrate the detector other than a single point source calibration.
ISOTOPIC includes an improved "closed geometry" algorithm where the distance between detector and container is less than 15 cm.
For non-collimated large area soil counting, the "1 meter" method developed by the US DOE EML2 and later extension 3 can be used. It works in many situations:
- Conduct decontamination assessments of previously used sites
- Assessment of nuclides deposited during emergencies
- Routine environmental monitoring near nuclear facilities
The EML method simplifies complex measurement problems into the product of three easily determinable factors. The gamma ray peak area is related to a specific nuclide activity by the product of three factors. Factors for a range of detector types and soil conditions have been identified and listed within the program. Efficiency calibration is determined using the efficiency specified by ANSI/IEEE 325-1996 at 1.33 MeV and the crystal length and diameter.
To improve accuracy at low energies, the user can use the same calibration as in vessel mode instead of the EML method.
No special (and expensive) Monte Carlo calibration of the detector is required. Determine the attenuation correction by selecting the soil type and nuclide distribution type: recent (surface) sediments, older (washed-in) sediments, or natural (uniform) sediments. Energy and peak shape calibration are performed using multiline sources and can be automated. If you use ISOTOPIC, there are no unexpected detector calibration costs.
Multiple measurements of a single container
When measuring any large waste container, it is common to take multiple measurements from different directions to ensure results are obtained. If only one hardware system is available, measurements can be completed sequentially, or if multiple sets of hardware are available, measurements can be completed simultaneously. ISOTOPIC can automatically combine the results based on a user-defined weighted average. When using multiple detectors at the same time, the real-time spectrum of each detector can be displayed simultaneously on the screen to ensure data accuracy.
Standard and custom reports
ISOTOPIC offers flexible reporting within a standard product. All changeable parameters can be included in the standard output report. Analysis results are stored in an MS Access compatible database for easy printing or export for further processing into summary reports. Customized reports can be generated using Crystal Reports.
Hardware compatibility
Like all ORTEC CONNECTIONS application software products, ISOTOPIC is compatible with all ORTEC MCA hardware. In particular, it is ideal for use with the IDM-200-V, a complete rugged portable HPGe spectrometer system that does not require the use of liquid nitrogen.
Support system integrators
System integrators are often required to develop automated systems in which the details of hardware control and analysis are largely hidden from human operators beneath a software layer designed to provide a simplified user interface and/or allow for unattended operation. The standard user documentation set contains extensive documentation that uses example material to illustrate how to control the analysis engine from the command line. Analysis parameters and analysis results will be saved to an ACCESS compatible database. It provides all required file structure information, including ISOTOPIC database file structure. The spectrum or "SPC" file structure is provided in a separate accompanying manual.
ORTEC hardware control is implemented through the so-called UMCBI, which provides a common API for all supported spectrum hardware. A Programmer's Toolkit is available as an option to provide the system integrator with instructions on how to easily control the MCA hardware from a program developed by him/herself. Typically, a basic ISOTOPIC program is used to set up the system hardware and calibrate, and then the integrator's application will control the system during the regular operating cycle. With these tools and the level of documentation provided, system integrators can easily develop complex measurement systems.
Use ISOTOPIC
ISOTOPIC has two modes: Administrator and Operator. Operators only need to make choices from a minimal subset of system options defined by the administrator. Administrator mode is used to define the actions an operator is allowed to perform. A wizard will guide the administrator in setting up the operator program. Wizards display parameters on logically grouped screens and emphasize the feasibility of the method.
Administrator/operator partitioning enables even semi-skilled operators to collect good data in the field while reducing wasted duplication time (lower cost per measurement project). Of course, skilled users can choose to run both modes.
Administrators can calibrate the system, create libraries, define sample geometries, matrices, collimators, and other features for later use by operators. Administrators can also define the functions that operators are allowed to access.
The operator home screen is determined by the permissions granted by the administrator and is much simpler than the administrator screen. In daily use, for container analysis, the operator only needs to start acquisition, select a configuration (the standard container configuration), and enter "log data" such as container ID, type, weight and key measurements (such as detector-to-vessel distance).
Standard container configurations and collimator configurations are defined and specified by the administrator. Container configurations include default dimensions, materials, and substrate details. The operator can specify and recall any number of these configurations when needed.
Analysis tools
Interactive results graph
Once the analysis is complete, the operator can optimize the results by adjusting vessel/matrix physical parameters (such as matrix density or vessel wall thickness) by using the nuclide map.
The graph shows the percentage difference between the corrected measured activity and the calculated reference peak activity for each nuclide. The administrator can select the reference peak. Operators can optimize the analysis, adjusting the vessel, matrix and uranium weight fraction to optimize results. If the points from a multimodal nuclide are normally distributed around the "zero line", this means a good result. When analyzing uranium, if U-235 enrichment is known, this value can be entered to more accurately calculate U-238 and U-234 values in samples containing weak uranium activity. This method allows homogeneous and inhomogeneous samples to be analyzed with greater precision. For parcels with uneven material distribution, the user will have parameter combinations that will flatten some nuclide activity maps. This plot, along with the spectrum, can form part of the output report.
Field of view calculator
The detector field of view is an important parameter in measurements. Software algorithms "correct" or adjust based on what is "seen" in the collimator's field of view to analyze the contents of the entire container. In general, the field of view is chosen so that it is filled by the bin, being further away from this will reduce the signal-to-noise ratio in the spectrum, and being closer to this will cause the measurements to be more susceptible to local inhomogeneities (the effects can be further reduced by taking multiple measurements in different directions). A convenient field of view calculator allows the operator to evaluate which part of the container is actually within the collimated detector's field of view.
report
Once fine-tuning is complete, the operator can select reports for each nuclide showing activity and weight. These results are then printed and archived. Report files can be written as a database summary or a full report, which will show all input and correction information. Use the report generator option to generate custom reports. Component tables of error estimates can be used to help reduce the overall uncertainty, for example, by extending the counting time or repositioning the detector. Users will also be warned if any corrections appear to be too large. Calculate the minimum detectable activity (MDA) for each nuclide. Multiple measurements of activity, grams of U or Pu, or MDA may be reported as a weighted average. Weighting is user-definable.
accuracy of results
The basic assumption for a single measurement is that the entire object contains the same matrix and specific activity as the volume of the part being measured. Inaccuracies due to this incorrect assumption can be reduced by taking multiple measurements from different points on the object's surface and comparing the similarities. These comparisons can be used to develop measurement strategies for individual objects that reduce such systematic errors. If required, ISOTOPIC can provide a weighted average report including the relevant minimum detectable activity.
Overall, the main influencing factors on the accuracy of the results are: statistical and counting times, calibration uncertainty, the number of repeated measurements made on a single object (random uncertainty), inhomogeneities in the matrix density and nuclide distribution and the number of measurements made on a single object from different directions (systematic errors).
An accuracy range of 10 to 50% should be considered representative, with smaller ranges for well-defined geometries in homogeneous and lightweight matrices.
Analysis Library Manager
ISOTOPIC includes a comprehensive library editor for building custom analysis libraries. The editor allows the operator to cut and paste nuclides and peaks from the main library, add identification markers (single escape peaks, X-ray or other) and analysis (keyline or exclusion from activity calculations) to individual peaks, and save the library under any name. It also integrates the complete Nuclide Navigator library tool. ISOTOPIC will use Nuclide Navigator, read the Nuclide Navigator library in Microsoft Access database format (no conversion required), and save the library in database format for use by Nuclide Navigator.
quality assurance
ISOTOPIC's quality assurance meets the requirements of ANSI N13.30. For each detector, the following is monitored:
- Detector total background
- Total (decay-corrected) activity of all calibration nuclides
- Average FWHM ratio (spectrum to calibration standard)
- Average FW1/10M ratio (spectrum to calibration standard)
- Average peak offset from library value
- actual peak center energy
Calculation details
A review of isotope patterning methods for containers
The activity of the isotope in the container is given by:
Among them
Aisotope = Reported isotope activity (Bq/μCi）。
PAmeas = The measured peak area count rate (c/s) of the isotope reference gamma ray. This quantity can be determined directly from the spectrum and acquisition live time. ORTEC's ZDT dead time correction algorithm is very useful if short half-life isotopes are present or if there are rapid changes in sample activity in flowing samples.
CFitem = Container, matrix and sample self-attenuation correction factors. ISOTOPIC calculates these data based on the physical data provided in the configuration.
CFcol=Collimator correction factor. Some of the gamma rays will penetrate the collimator surrounding the germanium detector. The collimator correction factor depends largely on the diameter of the collimator, the depth of collimation and the wall thickness of the collimator as well as the angle of the radiation and its energy.
The collimator correction factor can be determined by first calculating the portion of the activity that is not obscured by the collimator and then calculating the length of the remaining activity that penetrates the collimator. This is determined for each voxel of the item being measured.
If there is no collimator, set to 1.
BRray = Gamma ray branching ratio. This information is contained in nuclide libraries.
det = detector efficiency (cps/Bq, μCi) measured using a NIST traceable point source. A typical calibration distance is 30 cm, at which distance the detector and source can be viewed as point objects. At close range, detector length and diameter dimensions cannot be ignored. By providing these dimensions during the calibration process, simple "point detector" assumptions can be corrected automatically. Correction of closed geometries is described in the ISOTOPIC Administrator's Manual 1.
When it is necessary to report the gram quantity of an isotopeMassisotope, these are given by:
Among them
N = number of atoms of the reporting isotope.
λisotope = reports the decay constant of the isotope (sec–1).
At = atomic number of the isotope being measured (g/Av).
Av = Avogadro's constant.
Average the results of multiple measurements
After multiple measurements are combined, a weighted average can be calculated according to the following equation:
Aaverage = ∑ Ai wi/∑wi
Among them
Ai = Result for each activity (gram or MDA).
wi = user-defined weighting factor.
Soil
Specific activity A (Bq/m2 or Bq/g) is related to the net peak count rate Nf:
Among them
Nf /N0 = angular correction factor of the detector at that energy for a given source distribution in the soil.
N0 /Φ = peak count rate per unit of uncollision flux for a parallel gamma-ray beam of peak energy incident perpendicular to the detector plane (cpm/γ s–1)。
Φ/A = total uncollided flux reaching the detector at peak energy per unit inventory or nuclide concentration in the soil (γ cm–2 s–1) or (γ g–1 s–1)。
Methods for estimating calibration factors use information about the detector and the distribution of the radionuclides being measured:
- Detector efficiency (expressed as %)
- Detector direction (up or down)
- Detector aspect ratio (calculated as crystal length/crystal diameter)
- Sedimentary profile parameter α/ρ value
For all natural emitters, α/ρ is assumed to be 0 (uniform distribution). For settlement on undisturbed soil, α/ρ is assumed to be infinite (for surface distribution only).
The Beck method is applied in ISOTOPIC by calculating the value of each calibration parameter. Each gamma ray will be calculated for all identified nuclides.
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1Hagenauer, R.C., “Nondestructive testing and quantitative analysis of poorly characterized radioisotopes,” Proceedings of the Fourth Conference on Nondestructive Testing and Nondestructive Waste Characterization, Salt Lake City, 1995.
2 H. L. Beck et al., "In situ Ge(Li) and NaI(Tl) gamma-ray spectroscopic measurements," U.S. Department of Energy, Environmental Measurement Laboratory, HASL-258, September (1972).
3 I. K. Helfer and K. M. Miller, "Calibration factors for Ge detectors for in-situ spectrometry", Health Physics, Vol. 55, No. 1, pp. 15-29 (1988).
42012 NPL Nuclear Industry Proficiency Test Exercise. NPL Report IR 30 2013 (UK National Physical Laboratory). The ORTEC system is numbered 9.
5 For soil characterization in M-1 mode, it is recommended to use HPGe with crystal length/diameter in the range of 0.5 to 1.3. 80% of HPGe detectors meet this standard. ORTEC PROFILE M series detectors are ideally suited for this and ISOTOPIC vessel measurements. Specifications-
