ML12191A013

Draft Regulatory Guide (SG 042-2), Guidelines for Germanium Spectroscopy Systems for Measurement of Special Nuclear Material

ML12191A013
Person / Time
Issue date:	07/31/1982
From:	Frattali S Office of Nuclear Regulatory Research
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References
SG 042-2 RG-5.009, Rev. 2
Download: ML12191A013 (16)

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0 U.S. NUCLEAR REGULATORY COMMISSION July 1982 OFFICE OF NUCLEAR REGULATORY RESEARCH Dvso 0*)T*(f, Division 5 5

• *Y.'*;Task SG 042-2

- DRAFT REGULATORY GUIDE AND VALUE/IMPACT STATEMENT

Contact:

S. D. Frattali (301)443-5976 PROPOSED REVISION 2 TO REGULATORY GUIDE 5.9 GUIDELINES FOR GERMANIUM SPECTROSCOPY SYSTEMS FOR MEASUREMENT OF SPECIAL NUCLEAR MATERIAL A. INTRODUCTION Section 70.51, "Material Balance, Inventory, and R cor Requirements,"

of 10 CFR Part 70, "Domestic Licensing of Special uclear Maerial," requires, in part, that licensees authorized to possess at any ne tme more than one effective kilogram of special nuclear material e and maintain a system ilabnr)is of control and accountability so that the tt oterror of any inventory difference, ascertained as a result of ja reasu r.d material balance, meets established minimum standards. The selpctionand proper application of an adequate measurement method for each o ;the material forms in the fuel cycle is essential for the maintenafftie of>'th'se standards.

Many. types of nondestngctive assay (NDA) measurements on special nuclear material (SNM) can involve, uX even require, a high-resolution gamma ray spectroscopy system This guide is intended both to provide some general guidelines accep ab 0to the NRC staff for the selection of such systems and to point out u fUresources for more detailed information on their assembly, optimization-, anduse in material protection measurements.

B. DISCUSSION

1. BACKGROUND Gamma ray spectroscopy systems are used for NDA of various special nuclear material forms encountered in the nuclear fuel cycle, both for quantitative This regulatory guide and the associated value/impact statement are being issued in draft form to involve the public in the early stages of the development of a regulatory positi(,n in this area. lhey have nut received complete staff review and do not. represent an official NRC staff pesition.

Public comments are being solicited on both drafts, the guide (including any implementation schedule) and the value/impact statement. Comments on the value/impact statement should be accompanied by supportincl data. Comments on both drafts should he sent to the Secretary of the Commission, U.S. Nucl Reg, latory Commission, Washington, D.C. 20555, Attention: Docketing and Service Branch, byStp 17 Requests for single copies of draft guides (which may lhereproduced) or for placement nn an automatic dlstrIlbut.fon list. for sinigle copies of future draft guides in specif it divisions should he made inl writ. inl to the (I.S. Nuclear Reol'lat.nry Commission, WastinIctol), I). C. 20555, Attenut.ion: D irector, Division of leclu i(cal Informatlin,i andu lDocuument. Conutrol.

determination of the SNM content and for the determination of radionuclide abundances.

Applications of high-resolution gamma ray spectroscopy have multiplied greatly in recent years. The samples encountered range from fresh fuel rods and reprocessing solutions to boxes and cans of uncharacterized waste material.

Measurement conditions also vary widely from controlled laboratory environments to the unpredictable plant environment that can be hostile to the measurement equipment and can often contribute serious background interferences to the spec-tral data. As a result, there is no single gamma ray assay system that can be effective in all cases. The system chosen for a particular NDA task must there-fore be determined from careful consideration of all factors that may affect the measurement and of the requirements for the precision and accuracy of the assay.

The scope of this guide is limited to the consideration of high-resolution gamma ray spectroscopy with lithium-drifted germanium, Ge(Li), or intrinsic germanium (IG) detectors. No discussion of thallium-activated sodium iodide, NaI(TI), gamma ray systems is presented. In addition, no discussion of spe-cific NDA applications of gamma ray spectroscopy is provided. The measurement procedures (including calibration), analysis methods, inherent limitations, and overall precision and accuracy attainable are specific to each application and are therefore the subject of separate application guides. Guidelines for measurement control, calibration, and error analysis of NDA measurements are dealt with in detail in Regulatory Guide 5.53, "Qualification, Calibration, and Error Estimation Methods for Nondestructive Assay," which endorses ANSI 15.20-1975, "Guide to Calibrating Nondestructive Assay Systems."*

ANSI 15.20-1975 was reaffirmed in 1981. A proposed revision to Regulatory Guide 5.53 has been issued for comment as Task SG 049-4.

All of the major commercial vendors of Ge(Li) and IG detectors and the associated electronics maintain up-to-date documentation on the equipment specifications currently available, as well as a variety of useful and infor-mative notes on applications. This literature is available from the manufac-turers upon request and should be utilized by the potential user as a continuing source of the most current information on the highest quality systems available.

Finally, the potential user should not fail to consult with those individuals currently active in the field of nondestructive assay of special nuclear material and seek their advice in the particular assay problem being considered.

• Copies of this standard may be obtained from the American National Standards Institute, Inc., 1430 Broadway, New York, New York 10018.

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2. BIBLIOGRAPHIC INFORMATION An annotated bibliography is included in this regulatory guide to provide more detailed information on spectroscopy systems and their use.

Elementary introductions to the concepts associated with the application of high-resolution gamma ray spectroscopy to problems of nuclear material assay are available in Augustson and Reilly and in Kull. These works discuss the physical processes of gamma ray detection and important instrumentation characteristics. More advanced discussion of gamma ray detectors and asso-ciated electronics may be found in Knoll and in Adams and Dams. A thorough treatise on the associated electronics is'available in Nicholson. In addition, extensive discussion of a variety of NDA techniques and the implementation of some of these techniques with high-resolution gamma ray spectroscopy may be found in Sher and Untermeyer, in Rogers, and in Reilly and Parker. Detailed descriptions of detector efficiency and energy calibration procedures are available in section D of Knoll and also in Hajnal and Klusek; in Hansen, McGeorge, and Fink; in Hansen, et al.; and in Roney and Seale.

Relevant technical information beyond the introductory level, including nomenclature and definitions, is contained in two useful standards of the Institute of Electrical and Electronics Engineers, IEEE Std 301-1969, "Test Procedure for Amplifiers and Preamplifiers for Semiconductor Radiation Detectors,"* and IEEE Std 325-1971, "Test Procedures for Germanium Gamma-Ray Detectors."* These describe detailed techniques for defining and obtaining meaningful performance data for Ge(Li) and IG detectors and amplifiers.

3. FUNCTIONAL DESCRIPTION A block diagram of a typical high-resolution gamma ray spectroscopy system is shown in Figure 1. In such a system, the solid state Ge(Li) or IG detector converts some or all of the incident gamma ray energy into a propor-tional amount of electric charge, which can be analyzed by the subsequent electronics. The detector output is converted into an analog voltage signal by the preamplifier, which is an integral part of the detector package. The preamplifier signal is further amplified and shaped and is then converted into

• Copies may be obtained from the Institute of Electrical and Electronics Engineers, Inc., 345 East 47th Street, New York, New York 10017.

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Spectroscopy I Analog-to-Digital DeterAmplifier Conversion I I. I 1I kI I

---

--- CountI Scaler Data storage, display, and data reduction and analysis I I components I I I Figure 1. A block diagram of a typical setup of a high-resolution gamma ray spectroscopy system. The dashed boxes indicate which sets of modules are usually packaged as one component in commercially available systems. Liquid nitrogen cooling of the detector is required for proper operation of the system, but the field-effect transistor (FET) in the preamplifier input stage may or may not be cooled, depending upon the type of detector used and the energy resolution desired. A scaler is shown connected to the main amplifier, a common method of monitoring the total system count rate. For long-term data acquisition, spectrum stabilization is recommended, and the method is indi-cated here by a stabilizer module in communication with the analog-to-digital converter (ADC).

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digital information that can be stored, displayed, and otherwise processed by the data reduction and analytical components of the system.

4. TYPES OF SYSTEMS High-resolution gamma ray spectroscopy systems are distinguished primarily by the type of detector used. For assay applications involving the measurement of low-energy gamma radiation (i.e., energies below approximately 200 keV), a thin planar IG or Ge(Li) crystal is most appropriate. A coaxial detector crystal with a larger volume is much better suited for higher energy gamma ray measurements (i.e, for energies above approximately 120 keV). The distinction between these two types of detectors is not sharp. For instance, there may be some applications above 120 keV where a planar detector would be useful to render the system less sensitive to interferences from ambient high-energy gamma radiation.

It should be noted that Ge(Li) detectors have no real advantage over IG detectors with comparable performance specifications. In addition, Ge(Li) detectors require constant liquid nitrogen (LN) cooling, even when not in operation. IG detectors must, of course, also be operated at LN temperature, but they can be stored at room temperature. This added convenience and the greater ruggedness of the IG detectors makes them especially attractive for in-plant NDA applications.

5. EQUIPMENT ACCEPTANCE PRACTICES Equipment descriptions and instructional material covering operation, maintenance, and servicing of all electronic components should be supplied by the manufacturer for all individual modules or complete systems. Such descrip-tions should include complete and accurate schematic diagrams for possible in-house equipment servicing. Carefully specified operational tests of system performance should be made at the vendor's facility, and the original data should be supplied to the user upon delivery of the equipment. Extensive performance testing of all systems by the user is generally not necessary.*

• Although the quality control and preshipment testing procedures of the commercial vendors of detectors and associated electronics have improved and are quite dependable, some user verification of the specifications claimed by the manufacturer is strongly recommended.

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However, qualitative verification of selected equipment performance specifications and detector resolution is recommended.

It is necessary to have calibration sources on hand to verify the operational capabilities of the system. The following radioactive sources (with appropriate activities) will provide sufficient counting rates to verify the energy resolution specifications of the manufacturer and to carry out any other performance tests desired by the user:

66Co 10-30 pCi, Gamma ray energies: 1173, 1332 keV 57Co 1-30 pCi, Gamma ray energies: 14, 122, 136 keV C. REGULATORY POSITION Ge(Li) or IG gamma ray spectroscopy data acquisition systems meeting the general guidelines outlined briefly below are considered more than adequate for use in SNM assay requiring resolution better than that obtainable with Nal detectors. The potential user should select the detector and associated elec-tronics that meet the needs of the particular assay task required, with careful consideration of all factors that could affect the quality of the assay.

1. DETECTOR PERFORMANCE Excellent performance, routinely available in coaxial Ge detectors, may be represented by energy resolutions (FWHM)* of approximately 1.7 keV at 1332 keV ( 6 °Co) and approximately 0.7 keV at 122 keV ( 5 7 Co) for detectors with efficiencies up to 20 percent.t The full width at 0.1 maximum (FWTM) for such The full width of the gamma ray photopeak at half of its maximum height (FWHM) and the full width at tenth maximum (FWTM) are defined in Section 3 of IEEE Std 301-1969. Measurement procedures for these quantities may be found in Section 4 of the same standard.

tThe full-energy peak efficiency (in percent) is defined relative to the full-energy peak efficiency of a 3-in. x 3-in. NaI(TI) scintillation detector for 1332-keV gamma rays ( 6 0 Co) at a source-to-detector distance of 25 cm.

The detailed procedures for determining the efficiency in accordance with this definition are presented in Section 5.2 of IEEE Std 301-1969.

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detectors is typically up to 1.9 times the FWHM. For these higher efficiency detectors, "peak-to-Compton ratios" (defined in Section 3.4 of IEEE Std 301-1969) are usually quoted in the range of 25 to 40. These ratios are strong functions of resolution, efficiency, and exact detector crystal geometry, and no typical values can be given without knowledge of all of these parameters.

Coaxial detectors with this kind of resolution will usually have cooled field-effect transistor (FET) preamplifiers and a count-rate capability of approximately 50,000 MeV/sec.* Room temperature preamplifiers have somewhat worse resolution but have rate capabilities on the order of 150,000 MeV/sec.

The resolution of planar detectors is a stronger function of the crystal size and shape than that of coaxial detectors, so representative resolutions cannot be given over a range of sizes. As an example from the middle of the range of sizes usually offered, an excellent 2 cm3 (i.e., 2 cm2 front face area x 1 cm thick) would have a resolution of approximately 0.5 keV at 122 keV (5 7 Co) and 0.21 keV at 5.9 keV (Mn X-ray from 5 5 Fe decay). Planar detectors will always have LN-cooled FET preamplifiers, in order to achieve the excellent resolution of these systems. The preamplifier feedback loop may be either pulsed optical or resistive,t and the system will have fairly modest rate capabilities in the range of 5000 MeV/sec.*

2. ELECTRONICS PERFORMANCE For ease of use, maintenance, and replacement of the components in a high-resolution gamma ray spectroscopy system, the electronic components should be standard nuclear instrument modules (NIM) (Ref. 1), with the possible exception of the pulse-height analysis (i.e., multichannel analyzer) components. Pulse signals should be transmitted from module to module in shielded coaxial cable Counting rate capabilities, expressed in MeV/sec, denote the maximum charge-to-voltage conversion rate of which the preamplifier is capable. For 6 °Co, a 50,O00-MeV/sec rate capability corresponds to a pulse counting rate limita-tion of approximately 80,000 counts/sec. For 5 7 Co, a 5000-MeV/sec rate capability also corresponds to a pulse rate limitation of approximately 80,000 counts/sec. Of course, nuclear material assays should be performed at count rates well below these limiting values in order to minimize rate-related losses from pulse pileup and dead time.

tFeedback methods for charge-sensitive preamplifiers are discussed thoroughly in Chapter 5 of Reference 2.

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to minimize the effects of possible electronic noise from nearby machinery at the measurement site. The cables should have a characteristic impedance that matches the terminations used in the NIM modules (generally 93 ohms).

The system power supplies (detector high voltage, preamplifier, and NIM bin) should be capable of operating the system within the operating specifica-tions when supplied with 115 volts (+/-10 percent) at 50 to 65 hertz (at constant room temperature). The output voltage of the detector bias supply is determined by the detector requirements; 5 kilovolts is sufficient for most applications.

The main amplifier, commonly referred to as the spectroscopy amplifier, should have variable gain and pulse-shaping controls for maximum setup flexi-bility. Most high-quality amplifiers are equipped with baseline restoration and pole-zero cancellation circuits (Ref. 2), which greatly improve the resolu-tion that can be achieved on a routine ba sis. Baseline restoration is essential for assay situations in which count rates in excess of several kilohertz are anticipated. Pulse pileup suppression is also a useful feature and may be found in some spectroscopy amplifiers, in many multichannel analyzers, and even in separate NIM modules designed for that purpose.

Electronic components should be obta ined with state-of-the-art linearity and temperature sensitivity. Maintenance of long-term gain stability may require the use of a spectrum stabilizer. Centroid variations of a stabiliza-tion peak of less than one channel in a 4096-channel spectrum are achievable with commercially available stabilizer modules. Stabilization peaks can be provided either by a pulser or by a radioactive source. Generally, a radio-active source is preferred because it contributes less distortion to the gamma ray spectrum and has a stable (although decaying) emission rate. Furthermore, stabilization peaks from natural sources may be obtained from existing peaks in the assay spectrum itself, which simplifies the assay setup. Dead-time and pileup corrections may also be performed using a pulser or a separate radio-active source fixed to the detector. The latter method is preferred for the reasons stated above.

3. SYSTEM SELECTION AND USE The detailed requirements and constraints of a particular measurement situation will cause wide variation in the optimum choice of systems, even within a fairly well-defined application. For example, a requirement for high 8

throughput may dictate higher efficiency detectors and highly automated data acquisition electronics. Anticipated interferences from uranium, thorium, or fission.products may make the best possible system resolution the most impor-tant consideration. Severe operating environments may make the use of digital stabilization highly desirable. Constraints of space and location could dictate an unusually small LN dewar with automatic filling capacity. The list of such considerations in a given situation can be long, and each situation should be considered carefully and individually in order to achieve a system that can acquire the required measurement data.

Beyond the choice of data acquisition systems, many other factors influence the successful use of gamma ray spectroscopy in quantitative assay measurements. Some of these are:

a. Gamma Ray Signatures: The energies and intensities of the relevant gamma rays place fundamental restrictions on the sensitivity, precision, and accuracy of any assay. The range of gamma ray energies of interest also determines the type of gamma ray detector appropriate for optimum efficiency.

b. Full-Energy Peak Area Determination: The procedure for extracting this fundamental information from the spectral data will be determined by the complexity of the gamma ray spectra as well as the intensity and complexity of the gamma ray background at energies near the peaks of interest.

c. Gamma Ray Attenuation by the Samples and Surrounding Materials:

Corrections for this effect are essential for accurate assays. The importance of this correction will increase as the gamma ray energies of interest decrease and the absorptive power of the sample and surrounding materials increases.

All of this emphasizes that by far the most important factor in choosing an appropriate data acquisition system, in implementing proper assay procedures, and in supervising the assay operations is a highly competent person, prefera-bly experienced in gamma ray spectroscopy and its application to assay measurements of special nuclear materials. Such a person, with the assistance of the existing literature and of others in the gamma ray field, will be able to consider a particular application in detail and choose an appropriate detector and electronics to create a data acquisition system that is well suited to the required assay task.

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REFERENCES

1. USAEC Technical Information Department, "Standard Nuclear Measurement Modules," Revision 3, TID-20893, 1969.

2. P. W. Nicholson, Nuclear Electronics, John Wiley and Sons, New York, 1974.

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BIBLIOGRAPHY Adams, F., and R. Dams, Applied Gamma-Ray Spectroscopy, Pergamon Press, New York, 1970.

This work provides a comprehensive coverage of background material pertinent to the gamma ray spectroscopist. Considerable information is provided on both NaI and Ge detectors.

Augustson, R. H., and T. D. Reilly, "Fundamentals of Passive Nondestructive Assay of Fissionable Materials," Los Alamos Report LA-5651-M, 1974.

This manual contains helpful introductory descriptions of NDA applications of gamma ray spectroscopy, as well as some discussion of gamma ray detection systems.

Hajnal, F., and C. Klusek, "Semi-Empirical Efficiency Equations for Ge(Li)

Detectors," Nuclear Instruments and Methods, Vol. 122, p. 559, 1974.

Hansen, J., J. McGeorge, and R. Fink, "Efficiency Calibration of Semiconductor Detectors in the X-Ray Region," Nuclear Instruments and Methods, Vol. 112,

p. 239, 1973.

Hansen, J., J. McGeorge, D. Nix, W. Schmidt-Ott, I. Unus, and R. Fink, "Accurate Efficiency Calibration and Properties of Semiconductor Detectors for Low-Energy Photons," Nuclear Instruments and Methods, Vol. 106, p. 365, 1973.

Knoll, G. F., Radiation Detection and Measurement, John Wiley and Sons, New York, 1979.

This book provides extensive discussion of all types of radiation detection systems, including high-resolution gamma ray spectroscopy systems. In particular, Section D deals exclusively with solid state detectors, and Section F is devoted to detector electronics and pulse processing.

Kull, L. A., "An Introduction to Ge(Li) and NaI Gamma-Ray Detectors for Safeguards Applications," ANL-AECA-103, 1973.

P. W. Nicholson, Nuclear Electronics, John Wiley and Sons, New York, 1974.

This is an extensive treatise on electronics systems associated with high-resolution detectors. Detailed descriptions are given of detector preamplifiers, pulse shaping, rate-related losses, pulse-height analysis, and spectral resolution.

Reilly, T. D., and J. L. Parker, "Guide to Gamma-Ray Assay for Nuclear Material Accountability," Los Alamos Report LA-5794-M, 1975.

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This report briefly covers the principles involved in using gamma ray spectroscopy in the quantitative assay of SNM and attempts to describe both capabilities and limitations of gamma ray assay techniques. The report also includes a description of procedures for determining plutonium isotopic ratios.

Rogers, D. R., "Handbook of Nuclear Safeguards Measurement Methods," Nuclear Regulatory Commission, NUREG/CR-2078, to be published.

Chapter 5, "Passive Nondestructive Assay Methods," contains descriptions of many applications of high-resolution gamma ray spectroscopy, as well as many references to original papers and reports.

Roney, W., and W. Seale, "Gamma-Ray Intensity Standards for Calibrating Ge(Li)

Detectors for the Energy Range 200-1700 keV," Nuclear Instruments and Methods, Vol. 171, p. 389, 1980.

Sher, R., and S. Untermeyer, The Detection of Fissionable Materials by Nondestructive Means, American Nuclear Society Monograph, 1980.

This relatively short book summarizes the principles of most nondestructive assay methods and briefly describes many typical applications, including those of high-resolution gamma ray spectroscopy. Chapters 3 and 5 are of particular interest since they deal, respectively, with nuclear detection methods and passive NDA techniques. The book also contains many references to original papers and reports.

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DRAFT VALUE/IMPACT STATEMENT

1. PROPOSED ACTION 1.1 Description Licensees authorized to possess at any one time more than one effective kilogram of special nuclear material (SNM) are required in § 70.51 of 10 CFR Part 70 to establish and maintain a system of control and accountability so that the limit of error of any inventory difference ascertained as a result of a measured material balance meets established minimum standards. The selec-tion and proper application of an adequate measurement method for each of the material forms in the fuel cycle is essential for the maintenance of these standards.

Many types of nondestructive assay (NDA) measurements on SNM can involve, or even require, a high-resolution gamma ray spectroscopy system. The proposed action is to provide some general guidelines in the selection of such systems and to point out useful resources for more detailed information on their assembly, optimization, and use in material protection measurements.

1.2 Need for Proposed Action Regulatory Guide 5.9, which provides guidance in this area, has not been updated since 1974 and does not contain a list of pertinent information currently available in the literature.

1.3 Value/Impact of Proposed Action 1.3.1 NRC Operations The experience and improvements in detector technology that have occurred since the guide was issued will be made available for the regulatory process.

Using these updated techniques should nave no adverse impact.

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1.3.2 Other Government Agencies Not applicable.

1.3.3 Industry Since industry is already applying the more recent detector technology discussed in the guide., updating these techniques should have no adverse impact.

1.3.4 Public No adverse impact on the public can be foreseen.

1.4 Decision on Proposed Action The guide should be revised to reflect improvements in techniques, to bring the guide into'conformity with current practice, and to provide a list of pertinent information currently available.

2. TECHNICAL APPROACH Not applicable.

3. PROCEDURAL APPROACH 3.1 Procedural Alternatives Potential RES procedures that may be used for the proposed action include the following:

Regulation Revision of a regulatory guide

• ANSI standard, endorsed by a regulatory guide

• Branch position NUREG-series report 3.2 Discussion of Procedural Alternatives Since a useful and usable regulatory guide exists and modifications are minimal, the simplest procedure is to revise the guide.

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3.3 Decision on Procedural Approach A revised regulatory guide should be prepared.

4. STATUTORY CONSIDERATIONS 4.1 NRC Authority The proposed action falls under the authority of the Atomic Energy Act through the Commission's regulations in § 70.51 of 10 CFR Part 70.

4.2 Need for NEPA Assessment The proposed action is not a major action that may significantly affect the quality of the human environment and does not require an environmental impact statement.

5. RELATIONSHIP TO OTHER EXISTING OR PROPOSED REGULATIONS OR POLICIES The proposed action is one of a series of actions to provide updated guidance on nondestructive assay techniques.

6.

SUMMARY

AND CONCLUSIONS A revised guide should be prepared to bring Regulatory Guide 5.9 up to date.

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UNITED STATES FIRST CLASS MAIL NUCLEAR REGULATORY COMMISSION POSTAGE & FEES PAID USNRC WASHINGTON, D.C. 20555 WASH D C PE RMITNo GE?

OFFICIAL BUSINESS PENALTY FOR PRIVATE USE, $300 11,940c002001 I QPSAS51S US NRL REG.ION I 631 PAR~K AVE~NUE REGION I KING L*- PRUSSIA PA. 1940o