MGA++ γ-ray isotope ratio actinide analysis

• Three suites of advanced analytics programs:
  • MGA analyzes Pu with single plane or one plane and coaxial Ge detector
  • MGAHI analyzes Pu with a single coaxial Ge detector
  • U235 uses only a planar Ge detector to analyze U
• The LLNL algorithm is used to analyze actinides.
• No calibration standards are required to correct for matrix or container effects.
• Quick operation with display of spectra, results and peak fit residuals.
• Parameter sets can be easily modified to obtain results.
• Flexible reporting: Instant results and archived copies maintained to Access database.
• Comprehensive data collection and analysis.
• Easy-to-use Windows graphical user interface.
• Complies with ORTEC CONNECTIONS standards.
• Operates with all ORTEC and many non-ORTEC MCAs.
• Developer toolkit options facilitate custom system development.

MGA++ consists of three software programs (MGA, U235 and MGAHI) for analyzing actinide spectra obtained by germanium detectors. MGA++ is the result of years of ongoing research and development at Lawrence Livermore National Laboratory. 1

The original MGA code was developed to determine the plutonium isotope abundance from gamma-ray data acquired by the germanium detector. MGA-B32 includes 1) an upgraded version of the original MGA code, which relies on the 100 keV region; 2) U235, a uranium isotope analysis code using gamma rays below 300 keV; and 3) MGAHI, a plutonium isotope analysis code using the 200 keV-1 MeV energy region. These codes analyze gamma ray data collected using HPGe detectors.

These procedures require no special calibration sources or calculations and only use information obtained from the sample spectrum to determine isotope ratios.

As a full member of the CONNECTIONS software family, MGA++ can display the data being acquired, then quickly analyze and report the results - all from one easy-to-use program.

user interface
For each operating mode, separate "viewer" programs (MGAView, MGAHIView and U235View) and analysis modules (MGA.EXE, MGAHI.EXE and U235.EXE) ensure the integrity of the analysis method. The Viewer program provides user interface and hardware control functions. In MGA++, it is possible to suppress the display of all spectra.

MGAView, MGAHIView and U235View provide the same user-friendly interface. The current status of the detector is shown on the right. A special count rate meter mode displays the instantaneous count rate for a selected area. Spectra can be viewed during acquisition.

Start/save/report functions enable one-click collection and analysis via keyboard or mouse, simplifying the wearer's operator's tasks! Restart/Save/Report will continue the current count when additional counting is deemed necessary.

Reanalyzing spectra from disk or MCB is easy - just click and select. All spectra collected by the MGA++ program are stored in the ORTEC Standard Spectrum (SPC) format, which can be read by many programs. This file includes a record of the analysis parameters and hardware description—that is, it includes all the data needed to verify the results.

In addition to the main ORTEC SPC file formats, various input file format spectra stored on disk can be analyzed by MGA++.

Analysis results can be displayed using the zoom function to visually confirm the analysis.

Peak fit results can be checked along with the fit residuals to ensure analysis quality. To aid understanding, individual peaks on the display can be turned on or off.

Flexible analysis
Analysis parameters can be specified in clear, easy-to-understand dialog boxes. Analysis parameters can be saved to disk for use on similar samples and recalled as needed.

At the end of each analysis, the results are automatically stored in an Access format database and printed or displayed for the operator. Three database tables (Analysis, Acquisition, and Isotope) can be viewed with MGAView (using the logging function) or Access. This efficient method of data storage makes it easy to create summary reports, exception reports, and other useful output.

CONNECTIONSIntegration
MGA++ programs running under Windows 2000/XP are integrated into the ORTEC CONNECTIONS environment. All ORTEC multi-channel buffering hardware is supported, including networked as well as standalone DigiDART and DSPEC MCA.

The CONNECTIONS structure facilitates the development of custom applications, such as automatic measurement systems. (ORTEC can provide a developer toolkit to assist with this development process.)

The common feature of all CONNECTIONS products is that it supports multi-tasking and multi-threaded concurrent analysis processes under Windows 2000/XP, supports both local and remote connection Ethernet link MCA, and security password locking of the detector.

Prerequisites
MGA++ will run on any system under Windows 2000/XP that supports ORTEC multi-channel buffering hardware. While MGA++ can directly control compatible MCA hardware, MAESTRO is a prerequisite for instrument setup.

MGA analysis mode

• Weight percent results of 238,239,240,241Pu, 241,243Am, 237,239Np and 235,238U were produced.
• OK 242Pu.
• Can operate with a single planar HPGe detector (0-300 keV) or a planar and coaxial HPGe detector (0-1000 keV).
• No calibration standards are required to correct for matrix or container effects.
• Automatic energy and peak shape recalibration.

MGA mode has two data analysis configurations: single detector mode and dual detector mode. In single detector mode, the MGA is designed for planar HPGe detectors in the energy range 0-300 keV to obtain Pu isotope information. In dual detector mode for high attenuation samples, measurements can be assisted by using a coaxial HPGe detector capable of providing higher energy data up to ~1000 keV. This coaxial detector measures gamma ray energies of ~1 MeV. Using this additional information, sample uniformity and isotope content can be improved. However, in dual detector mode, information from the planar detector is required and coaxial information alone is not sufficient. This requirement becomes even more apparent when the original MGA code is applied to the gamma-ray spectrum of tightly shielded samples. This problem can be solved to some extent by the introduction of ORTEC SGD GEM detectors, which in some cases can perform the functions of planar and coaxial detectors by using two independent spectra (low energy and high energy).

There are about 10 energy regions in the plutonium gamma-ray spectrum that can be used to calculate isotope abundances. Isotope abundance in the 94-104 keV region and peaks at 129 and 148 keV are evident. Although these areas are difficult to analyze, accuracy of 1% or better can be achieved.

Processed sample age
MGA can handle freshly separated and aged samples - this is important because recently processed plutonium samples often have very low 237U and 241Am contents. 237U activity increases within approximately 2 months of treatment, at which time it reaches equilibrium with the alpha decay rate of its 241Pu parent. Any analytical method that assumes an established decay equilibrium between these two isotopes cannot be used on freshly processed samples.

Presence of other radioactive materials
Other radioactive materials may be present in the plutonium sample as decay products, as contaminants from previous processes, or as mixed products. Uranium is a common radioactive substance that is mixed with plutonium to form mixed oxide (MOX) fuel. MGA can accurately determine the relative abundance of 235U in a sample. Other radioactive materials sometimes encountered are 237Np-233Pa, 243Am-239, and some low-level fission products (such as 95Zr-95Nb and 137Cs). If a dual detector system is used, 237Np can be detected down to about 50 ppm by analyzing the 312 keV peak of 233Pa (a daughter of 237Np). The MGA automatically identifies interferences from the 243Am and 239Np isotopes, the X-ray fluorescence of thorium in the sample, and the presence of 137Cs (if a dual detector system is used).

MGAHI mode
MGAHI uses physical parameters to account for gamma ray attenuation and emission and does not require efficiency calibration of the detector. No gamma ray information between 50 keV and 200 keV is required. MGAHI is useful when the source is heavily shielded and in a high-background, space-constrained environment. If shielded too much, the 100 keV energy region may be attenuated. In a high-background environment, the detector may see gamma rays from other nearby sources. Additionally, spectrometry systems may have very high dead times. Lead shielding can be used in this case to reduce dead time, but this may also cut low-energy gamma rays. MGAHI adopts the original MGA approach: 1) uses physical parameters to account for gamma ray attenuation and emission, and 2) does not require detector efficiency calibration. However, unlike the "original MGA", it no longer requires plane information. The MGA analysis relies heavily on the 100 keV energy region detector efficiency, absorber thickness and Pu thickness using²³⁹PuCalculated from spectral data of known decaying gamma ray peaks.

MGAHI mode performance
MGAHI Pu wt% results for two Pu(PIDIE) standards. Collect 3 hours of data using a 75% coaxial detector. Use two absorbers (5mm stainless steel (ss) and 2mm Pb). The results of destructive analysis (DA) are also presented in the table.

U235 analysis mode

• Relative ratios of 235U, 234U and 238U.
• Automatically checks for the presence of Pu using the 129 keV peak.
• Operated using a single planar HPGe detector from 0-300 keV.
• No calibration standards are required to correct for matrix or container effects.
• Corrected for internal sample absorption and absorber placed between sample and detector.
• Peak shape calibration is determined by user-defined spectral peaks (loadable from file) or default values.

U235 uses a single-plane HPGe detector with an operating energy range of 0-300 keV. It determines the relative isotope ratios of uranium. In a similar manner to MGA and MGAHI, no basic calibration is required for efficiency or absorbers in the matrix or sample container.

Typical U235 mode performance
U235 gives the weight percentage results of 234, 235, and 238U.

The U235 analysis mode gives the relative ratios of 235U, 234U and 238U; when operating in this mode, it can warn the user of the possible presence of Pu.

Precision depends on the statistics of the sample.

General considerations

Sample size and matrix
In practice, the lower limit of sample size is approximately 100 mg - determined by counting time and required statistical precision. In principle there is no upper limit to the sample size and in practice only the surface of large samples is measured because the low energy rays of plutonium and uranium have short mean free paths. Sample thicknesses exceeding the "saturation thickness" will not increase the count rate because the rays from the back of the sample have been absorbed.

Uranium or plutonium emit fluorescence via its own alpha or gamma to a much lower degree than solid materials. Entering detailed information about the solution will help analyze the X-rays.

shield
Shields or collimators can be used to reduce or eliminate spectral counts from nearby materials. Cadmium, copper, lead or tantalum foils are used to reduce the presence of low energy gamma rays, especially those at 59 keV. This reduces the dead time during which plutonium samples cannot be counted. These low-energy gamma rays are almost absent in uranium samples. No preferential filtering of low energy rays is required unless they are present.

Germanium Detector
For measurements up to 300 keV using MGA or U235, a planar HPGe detector with a nominal resolution of 550 eV at 122 keV is required. Although this requirement can be relaxed in some cases, 122-keV resolution should not exceed 700 eV.

A thicker planar detector will have a higher peak-to-contrast (p/C) ratio and therefore provide superior performance at low energies, assuming it has the area and 122-keV resolution to meet other measurement requirements.

To use higher energy regions in the analysis of plutonium-only samples, a coaxial detector with a resolution less than 2.0 keV at 1332 keV can be used in two detector MGA mode or MGAHI mode. For MGAHI, a detector with a resolution of 1.1 keV or higher at 208 keV is required. 239-keV gamma rays from 203Pu must be visible. Up to 5 mm of Pb absorber can be placed in front of the detector.

ORTEC's Safety Guaranteed (SGD) series of coaxial and planar detectors are optimized for this purpose and are highly recommended for use with the MGA++.

ORTEC signs MGA++ licensing and development agreement with LLNL
ORTEC and Lawrence Livermore National Laboratory have entered into a licensing and development agreement for software specialized in nuclear safeguards applications. The license covers existing versions of the MGA++ suite of codes that can be used to analyze HPGe detector gamma-ray spectra from a variety of nuclear safeguards samples to determine relative plutonium isotope abundance and/or uranium enrichment. The development agreement (CRADA) involves further improvements to the code and improvements to "seamless" integration within the ORTEC CONNECTIONS Windows 2000/XP spectrometry environment for ease of use and reliability.

¹ It is the subject of a Cooperative Research and Development Agreement (CRADA TSV-1368-96) and a license (License No. TL-1375-96) between ORTEC and the University of California, and ORTEC is trying to integrate these programs into the ORTEC software environment to increase usability.

² K. Debertin and R.G.Helmer, "Gamma-ray and X-ray spectroscopy measurements using semiconductor detectors", p. 185, Elsevier Science Publishers B.V., The Netherlands, 1988.