Regulatory Guide 5.9

(Task SG 042-2), Revision 2, Guidelines for Germanium Spectroscopy Systems for Measurement of Special Nuclear Material

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Issue date:	12/31/1983
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Revision 2*

December 1983 U.S. NUCLEAR REGULATORY COMMISSION

REGULATORY GUIDE

OFFICE OF NUCLEAR REGULATORY RESEARCH

REGULATORY GUIDE 5.9 (Task SG 042-2)

GUIDELINES FOR GERMANIUM SPECTROSCOPY SYSTEMS

FOR MEASUREMENT OF SPECIAL NUCLEAR MATERIAL

to boxes and cans of uncharacterized waste materia

A. INTRODUCTION

urement conditions also vary widely from controlled laboratory environments to the unpredictable plant environ Section 70.51, "Material Balance, Inventory, and Records ment that can be hostile to the measurement equipment Requirements," of 10 CFR Part 70, "Domestic Licensing and can often contribute serious background interferences of Special Nuclear Material," requires, in part, that licensees to the spectral data. As a result, there is no single gamma authorized to possess at any one time more than one ray assay system that can be effective in all cases. The effective kilogram of special nuclear material establish and system chosen for a particular NDA task must therefore be maintain a system of control and accountability so that determined from careful consideration of all factors that the standard error (estimator) of any inventory difference, may affect the measurement and of the requirements for ascertained as a result of a measured material balance, the precision and accuracy of the assay.

meets established minimum standards. The selection and proper application of an adequate measurement method for The scope of this guide is limited to the consideration of each of the material forms in the fuel cycle is essential for high-resolution gamma ray spectroscopy with lithium-drifted the maintenance of these standards. germanium, Ge(Li), or high-purity germanium, HPGe (also referred to as intrinsic germanium, IG), detectors. No Many types of nondestructive assay (NDA) measurements discussion of thallium-activated sodium iodide, NaI(Tl), or on special nuclear material (SNM) can involve, or even lithium-drifted silicon, Si(Li), gamma ray systems is

> require, a high-resolution gamma ray spectroscopy system. presented. In addition, no discussion of specific NDA

This guide is intended both to provide some general guide applications of gamma ray spectroscopy is provided. The lines acceptable to the NRC staff for the selection of such measurement procedures (including calibration), analysis systems and to point out useful resources for more detailed methods, Inherent limitations, and overall precision and information on their assembly, optimization, and use in accuracy attainable are specific to each application and are material protection measurements. therefore the subject of separate application guides. Guide lines for measurement control, calibration, and error Any guidance in this document related to information analysis of NDA measurements are dealt with in detail in collection activities has been cleared under OMB Clearance Regulatory Guide 5.53, "Qualification, Calibration, and No. 3150-0009. Error Estimation Methods for Nondestructive Assay,"

which endorses ANSI N15.20-1975, "Guide to Calibrating

1

B. DISCUSSION

Nondestructive Assay Systems." ANSI N15.20-1975 was reaffirmed in 1980.

1. BACKGROUND

ýX of the major commercial vendors of Ge(Li) and Gamma ray spectroscopy systems are used for NDA of HPGe detectors and the associated electronics maintain various special nuclear material forms encountered in the up-to-date documentation on the specifications of currently nuclear fuel cycle, both for quantitative determination available equipment, as well as a variety of useful and infor of the SNM content and for the determination of radio mative notes on applications. This literature is available nuclide abundances. The substantial number of channes in this revision has made it Impractical to Indicate the changes with lines in the margin.

the American Applications of high-resolution gamma ray spectroscopy Covpies of this standard may be obtained fromNew

1 Standards Institute, Inc., 1430 Broadway, York, New have multiplied greatly in recent years. The samples encoun National York 10018.

tered range from fresh fuel rods and reprocessing solutions Comments should be sentCommissionto the Secretary of the Commission.

USNRC REGULATORY GUIDES U.S. Nuclear Regulatory Washington, D.C. 20555.

make available to the Attention: Docketing and Service Brancn.

Regulatory Guides are Issued to describe andstaff of implementing public methods acceptable to the NRC to delineate tech- The guides are Issued In the following ten broad divisions:

specific parts of the Commission's regulations, niques used by the staff in evaluating specific problems or postu 1. Power Reactors 6. Products Regulatory iated accidents or to provide guidance to applicants. with 2. Research and Test Reactors 7. Transportation Guides are nof substitutes for regulations, and compliance from those set 3. Fuels and Materials Facilities S. Occupational Health them Is not required. Methods and solutions different 9. Antitrust and Financial Review

4. Environmental and Siting out in the guides will be acceptable If they provide of a permit the a basis for or 5. Materials and Plant Protection 10. General findings requisite to the issuance or continuance license by the Commission. Copies of issued guides may be purchased at the current Government Printing Office price. A subscription service for future guides in spe This guide was Issued after consideration of comments received from Government Printing Office.

in these cific divisions is available through the the public. Comments and suggestions for Improvements will be revised, as Information on the subscription service and current GPO prices may guides are encouraged at all times, and guides be obtained by writing the U.S. Nuclear RegulatorySales Commission, appropriate, to accommodate comments and to reflect new informa- Washington, D.C. 20555, Attention: Publications Manager.

tion or experience.

from the manufacturers upon request, and the potential an integral part of the detector package. The preamplifier customer may use this literature as a source of the most signal is further amplified and shaped and is then converted current information on the highest quality systems available.

into digital information that can be stored, displayed, and otherwise processed by the data reduction and analytical Finally, the potential user ought to consult with those components of the system.

individuals currently active in the field of nondestructive assay of special nuclear material and seek their advice in the

4. TYPES OF SYSTEMS

particular assay problem being considered.

High-resolution gamma ray spectroscopy systems are

2. BIBLIOGRAPHIC INFORMATION distinguished primarily by the type (p-type or n-type) and the configuration (planar or coaxial) of detector used. For An annotated bibliography is included in this regulatory assay applications involving the measurement of low-energy guide to provide more detailed information on spectros gamma radiation (i.e., energies below approximately copy systems and their use. 200 keV), a thin planar HPGe or Ge(Li) crystal is most appropriate. A coaxial detector crystal with a larger volume Elementary introductions to the concepts associated is much better suited for higher energy gamma ray measure with the application of high-resolution gamma ray spectros ments (i.e., for energies above approximately 120 keV).

copy to problems of nuclear material assay are available in The distinction between these two types of detectors is not Augustson and Reilly and in Kull. These works discuss sharp. For instance, there maý be some applications above the physical processes of gamma ray detection and impor 120 keV in which a planar detector would be useful to tant instrumentation characteristics. More advanced dis render the system less sensitive to interferences from cussion of gamma ray detectors and associated electronics ambient high-energy gamma radiation.

may be found in Knoll and in Adams and Dams. A thorough treatise on the associated electronics is available in Nicholson.

It should be noted that Ge(Li) detectors have no real In addition, extensive discussion of a variety of NDA tech advantage over HPGe detectors with comparable perform niques and the implementation of some of these techniques ance specifications. In addition, Ge(Li) detectors require with high-resolution gamma ray spectroscopy may be constant liquid nitrogen (LN) cooling, even when not in found in Sher and Untermeyer, in Rogers, and in Reilly and operation. HPGe detectors are, of course, also operated at Parker. Detailed descriptions of detector efficiency and LN temperature, but they can be stored at room tempera energy calibration procedures are available in section D of ture. This is an advantage to potential users who may have Knoll and also in Hajnal and Klusek; in Hansen, McGeorge, extended plant shutdowns. It also prevents complete loss and Fink; in Hansen et aL; and in Roney and Seale. of a detector due to operator procedure error, which can happen with a Ge(Li) detector when Ll4 cooling is not Relevant technical information beyond the introductory level, including nomenclature and definitions, is contained continuously maintained. This added convenience and the greater ruggedness of the HPGe detectors make them K

in three useful standards of the Institute of Electrical and especially attractive for in-plant NDA applications.

Electronics Engineers, ANSI/IEEE Std 301-1976, "Test Procedures for Amplifiers and Preamplifiers for Semi 5. EQUIPMENT ACCEPTANCE PRACTICES

conductor Radiation Detectors for Ionizing Radiation," 2 ANSI/IEEE Std 325-1971, "Test Procedures for Germanium Equipment descriptions and instructional material Gamma-Ray Detectors" 2 (reaffirmed in 1977), and ANSI/

covering operation, maintenance, and: servicing of all IEEE Std 645-1977, "Test Procedures for Hifh-Purity electronic components are supplied by the manufacturer Germanium Detectors for Ionizing Radiation,"" which for all individual modules or complete systems. Such supplements ANSI/IEEE Std 325-1971. These describe descriptions should include complete and accurate sche detailed techniques for defining and obtaining meaningful matic diagrams for possible in-house equipment servicing.

performance data for Ge(Li) and HPGe detectors and Complete operational tests of system performance are to be amplifiers. made at the vendor's facility, and the original data are

/

supplied to the user upon delivery of the equipment.

3. FUNCTIONAL DESCRIPTION Extensive performance testing of all systems by the user is generally not necessary. 3 However, qualitative verification A block diagram of a typical high-resolution gamma ray of selected equipment performance specifications and spectroscopy system is shown in Figure 1. In such a system, detector resolution is recommended.

the solid state Ge(Li) or HPGe detector converts some or all of the incident gamma ray energy into a proportional It is necessary to have calibration sources on hand to amount of electric charge, which can be analyzed by the verify the operational capabilities of the system. The subsequent electronics. The detector output is converted following radioactive sources (with appropriate activities)

into an analog voltage signal by the preamplifier, which is

3 Although the quality control and presh.pment testing may be obtained from the Institute of Electrical and dures of the commercial vendors of detectors and associatedproce.elec.

Electronics Engineers, Inc., 34S East 47th Street, New York, New onuic, h~ave improved and are quite dependable, some user verifica.

York 10017. tion of the specifications claimed by the manufacturer Is strongly recommended.

5.9-2

I I

I \ I

I I

I Uquld I

Nitrogen High Dewa Voltage I (Cooling) Supply Spectrum f .Stabilization I

Spectroscopy I Analog-to-Digital I Detector Preamplifier Amplifier Conversion II

I

I. I , I

II

Count I

Rate Scaler Data storage, display, and data reduction and analysis 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 acquisi tion, spectrum stabilization is recommended, and the method is indicated here by a stabilizer module in communication with the analog-to-digital converter (ADC).

5.9-3

will provide sufficient counting rates to verify the energy will always have LN-cooled FET preamplifiers in order to resolution specifications of the manufacturer and to carry achieve the excellent resolution of these systems. The out any other performance tests desired by the user: preamplifier feedback loop may be either pulsed optical or resistive, 7 and the system will have fairly modest rate

60Co 10-30 pCi, Gamma ray energies: 1173,1332 keV

57 capabilities in the range of 5000 MeV/sec. 6 It is important CO 1-10 j0i, Gamma ray energies: 14, 122, 136 keV

to decouple the detector from noisy mechanical environ ments to avoid microphonic pickup.

C. REGULATORY POSITION

2. ELECTRONICS PERFORMANCE

Ge(Li) or HPGe gamma ray spectroscopy data acquisi tion systems meeting the general guidelines outlined briefly For ease of use, maintenance, and replacement of the below are considered more than adequate for use in SNM components in a high-resolution gamma ray spectroscopy assay requiring resolution better than that obtainable with system, the electronic components should be standard Na! detectors. The potential user should select the detector nuclear instrument modules (NIM) (Ref. 1), with the and associated electronics that meet the needs of the partic possible exception of the pulse-height analysis (Le., multi ular assay task required, with careful consideration of all channel analyzer) components. Pulse signals should be factors that could affect the quality of the assay. transmitted from module to module in shielded coaxial cable to minimize the effects of possible electronic noise

1. DETECTOR PERFORMANCE from nearby machinery at the measurement site. The cables should have a characteristic impedance that matches the Excellent performance, routinely available in coaxial terminations used in the NIM modules (generally 93 ohms).

germanium detectors, may be represented by energy resolutions (FWHM) 4 of approximately 1.7 keV at 1332 keV The system power supplies (detector high voltage,

( 60 Co) and approximately 0.7 keV at 122 keV ( 5 Co) for preamplifier, and NIM bin) should be capable of operating detectors with efficiencies up to 20 percent. 5 The full width the system within the operating specifications when supplied at 0.1 maximum (FWTM) for such detectors is typically up with 115 volts (+10 percent) at 50 to 65 hertz (at constant to 1.9 times the FWHM. For these higher efficiency detec room temperature). The power supplied for the detection tors, "peak-to-Compton ratios" are usually quoted in the system should be stabilized against voltage shifts in order to range of 25 to 40. These ratios are strong functions of maintain resolution. The output voltage of the detector bias resolution, efficiency, and exact detector crystal geometry, supply is determined by the detector requirements; 5 kilo and no typical values can be given without knowledge of all volts is sufficient for most applications.

of these parameters. Coaxial detectors with this kind of resolution will usually have cooled field-effect transistor The main amplifier, commonly referred to as the spectros (FET) preamplifiers and an energy-rate capability of copy amplifier, should have variable gain and pulse-shaping approximately 50,000 MeV/sec. 6 Room temperature pre controls for maximum setup flexibility. Most high-quality amplifiers have somewhat worse resolution but have rate amplifiers are equipped with baseline restoration and capabilities on the order of 150,000 MeV/sec. pole-zero cancellation circuits (Ref. 2), which greatly improve the resolution that can be achieved on a routine The resolution of planar detectors is a stronger function basis. Baseline restoration is essential for assay situations in of the crystal size and shape than that of coaxial detectors, which count rates in excess of several kilohertz are antici so representative resolutions cannot be given over a range of pated. Pulse pileup suppression is also a useful feature, if sizes. As an example from the middle of the range of sizes available; it may be found in some spectroscopy amplifiers usually offered, an excellent 2 cm 3 planar detector (le., and even in separate NIM modules designed for that purpose.

2 cm 2 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 Electronic components should be obtained with state-of at 5.9 keV (Mn X-ray from SaFe decay). Planar detectors the-art linearity and temperature sensitivit

y. Maintenance

4 The full width of the gamma ray photopeak at half of its of long-term gain stability may require the use of a spec maximum height (FWHM) is defined in ANSI/IEEE Std 301-1976. trum stabilizer. Centroid variations of a stabilization peak SThe full-energy peak efficiency (in percent) is defined relative of less than one channel in a 4096-channel spectrum are to the full-energy peak efficiency of a 3-in. J.n. NaI(TI) scintilla achievable with commercially available stabilizer modules.

tion detector for 1332-keV gamma rays v Co) at a source-to. Stabilization peaks can be provided either by a pulser or by detector distance of 25 cm. The detailed procedures for determining the efficiency in accordance with this definition are presented in a radioactive source. Generally, a radioactive source is Section 5.2 of ANSI/IEEE Std 301-1976.

preferred because it contributes less distortion to the

6 Counting rate capabilities, expressed In MSV/sec, denote the gamma ray spectrum and has a stable (although decaying)

maximum charge-to-voltay. conversion rate of which the pre emission rate. Furthermore, stabilization peaks from amplifier Is capable. For " Co, a SO,000-MeV/sec rate capability corresponds to a pulse iunting rate limitation of approximately natural sources may be obtained from existing peaks

80,000 counts/sec. For Co a 000-MeV/sec rate capability also in the assay spectrum itself, which simplifies the assay 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 7 Feedback methods for charge-sensithve preamplifiers are dis rate-related losses from pulse pileup and dead time. cussed thoroughly in Chapter 5 of Reference 2.

K\

5.9-4

on the sensitivity, precision, and accuracy of any assay. The setup. Dead-time and pileup corrections may also be range of gamma ray energies of interest also determines the performed using a pulser or a separate radioactive source type of gamma ray detector appropriate for optimum fixed to the detector. The latter method is preferred for the efficiency.

reasons stated above.

b. Full-Energy Peak Area Determination: The proce

3. SYSTEM SELECTION AND USE dure for extracting this fundamental information from the spectral data will be determined by the complexity of the The detailed requirements and constraints of a particular gamma ray spectra as well as the intensity and complexity measurement situation will cause wide variation in the of the gamma ray background at energies near the peaks of optimum choice of systems, even within a fairly well-defined interest.

application. For example, a requirement for high through put may dictate higher efficiency detectors and highly c. Gamma Ray Attenuation by the Samples and Sur automated data acquisition electronics. Anticipated inter rounding Materials: Corrections for this effect are essential ferences from uranium, thorium, or fission products may for accurate assays. The importance of this correction will make the best possible system resolution the most impor increase as the gamma ray energies of interest decrease and tant consideration. Severe operating environments may the absorptive power of the sample and surrounding mate make the use of digital stabilization highly desirable. Con rials increases.

straints 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 All of this emphasizes that by far the most important each situation should be considered carefully and indi factor in choosing an appropriate data acquisition system, vidually in order to achieve a system that can acquire the in Implementing proper assay procedures, and in supervising required measurement data. the assay operations is a highly competent person, prefera bly experienced in gamma ray spectroscopy and its appli Beyond the choice of data acquisition systems, many cation to assay measurements of special nuclear materials.

other factors influence the successful use of gamma ray Such a person, with the assistance of the existing literature spectroscopy in quantitative assay measurements. Some of and of others in the gamma ray field, will be able to consid these are: er a particular application in detail and choose an appro priate detector and electronics to create a data acquisition a. Gamma Ray Signatures: The energies and intensities system that is well suited to the required assay task.

of the relevant gamma rays place fundamental restrictions

5.9-5

REFERENCES

1. L Costrell, "Standard Nuclear Instrument Modules,"

2. P. W. Nicholson, Nuclear Electronics, John Wiley and U.S. Atomic Energy Commission, TID.20893, Revi Sons, New York, 1974.

sion 3, 1969.

BIBLIOGRAPHY

Adams, F., and R. Dams, Applied Gamma-Ray Spectros This is an extensive treatise on electronics systems copy, Pergamon Press, New York, 1970.

associated with high-resolution detectors. Detailed descriptions are given of detector preamplifiers, pulse This work provides a comprehensive coverage of back shaping, rate-related losses, pulse-height analysis, and ground material pertinent to the gamma ray spectros spectral resolution.

copist. Considerable information is provided on both Nal and Ge detectors. Reilly, T. D., and J. L Parker, "Guide to Gamma-Ray Assay for Nuclear Material Accountability," Los Alamos Augustson, R. H., and T. D. Reilly, "Fundamentals of Scientific Laboratory, LA-5794-M, 1975.

Passive Nondestructive Assay of Fissionable Material," Los Alamos Scientific Laboratory, LA-565 I-M, 1974.

This report briefly covers the principles involved in using gamma ray spectroscopy in the quantitative assay of This manual contains helpful introductory descriptions SNM and attempts to describe both capabilities and of NDA applications of gamma ray spectroscopy, as well limitations of gamma ray assay techniques. The report as some discussion of gamma ray detection systems.

also includes a description of procedures for determining Hajnal, F., and C. Klusek, "Semi-Empirical Efficiency plutonium isotopic ratios.

Equations for Ge(Li) Detectors," Nuclear Instruments and Methods, Vol. 122, p. 559, 1974.

Rogers, D. R., "Handbook of Nuclear Safeguards Measure ment Methods," Nuclear Regulatory Commission, NUREG/

Hansen, J., J. McGeorge, and R. Fink, "Efficiency Calibra CR-2078, 1983.

tion of Semiconductor Detectors in the X-Ray Region,"

Nuclear Instruments and Methods, Vol. 112, p. 239, 1973.

Chapter 5, "Passive Nondestructive Assay Methods,"

Hansen, J., et al., "Accurate Efficiency Calibration and contains descriptions of many applications of high K

resolution gamma ray spectroscopy, as well as many Properties of Semiconductor Detectors for Low-Energy references to original papers and reports.

Photons," Nuclear Instruments and Methods, Vol. 106, p. 365, 1973.

Roney, W., and W. Seale, "Gamma-Ray Intensity Standards Knoll, G. F., Radiation Detection and Measurement, for Calibrating Ge(Li) Detectors for the Energy Range 200

John Wiley and Sons, New York, 1979.

1700 keV," Nuclear Instruments and Methods, Vol. 171, p. 389, 1980.

This book provides extensive discussion of all types of radiation detection systems, including high-resolution Sher, R., and S. Untermeyer, The Detection of Fissionable gamma ray spectroscopy systems. In particular, Sec Materials by Nondestructive Means, American Nuclear tion D deals exclusively with solid state detectors, and Society Monograph, 1980.

Section F is devoted to detector electronics and pulse processing. This relatively short book summarizes the principles of most nondestructive assay methods and briefly describes Kuil, L A., "An Introduction to Ge(Li) and NaI Gamma many typical applications, including those of high Ray Detectors for Safeguards Applications," Argonne resolution gamma ray spectroscopy. Chapters 3 and 5 National Laboratory, ANL-AECA-103, 1974.

are of particular interest since they deal, respectively, with nuclear detection methods and passive NDA

P. W. Nicholson, Nuclear Electronics,John Wiley and Sons, techniques. The book also contains many references to New York, 1974.

original papers and reports.

5.9-6

VALUE/IMPACT STATEMENT

1.3.4 Public

1. PROPOSED ACTION

No adverse impact on the public can be foreseen.

1.1 Description

1.4 Decision on Proposed Action

-" Licensees authorized to possess at any one time more than one effective kilogram of special nuclear material The guide should be revised to reflect improvements in (SNM) are required in § 70.51 of 10 CFR Part 70 to techniques, to bring the guide into conformity with current establish and maintain a system of control and account practice, and to provide a list of pertinent information ability so that the standard error of any inventory difference currently available.

ascertained as a result of a measured material balance meets established minimum standards. The selection and proper

2. TECHNICAL APPROACH

application of an adequate measurement method for each of the material forms in the fuel cycle are essential for the Not applicable.

maintenance of these standards.

Many types of nondestructive assay (NDA) measurements

3. PROCEDURAL APPROACH

on SNM can involve, or even require, a high-resolution gamma ray spectroscopy system. The proposed action is to Of the alternative procedures considered, revision of the provide some general guidelines in the selection of such existing regulatory guide was selected as the most advan systems and to point out useful resources for more detailed tageous and cost effective.

information on their assembly, optimization, and use in material protection measurements.

4. STATUTORY CONSIDERATIONS

1.2 Need for Proposed Action 4.1 NRC Authority Regulatory Guide 5.9, which provides guidance in this Authority for the proposed action is derived from the area, has not been updated since 1974 and does not contain Atomic Energy Act of 1954, as amended, and the Energy a list of pertinent information currently available in the Reorganization Act of 1974, as amended, and implemented literature.

through the Commission's regulations.

1.3 Value/Impact of Proposed Action 4.2 Need for NEPA Assessment

1.3.1 NRC Operations The proposed action is not a major action that may significantly affect the quality of the human environment The experience and improvements in detector technology and does not require an environmental impact statement.

that have occurred since the guide was issued will be made available for the regulatory proces

s. Using these updated

5. RELATIONSHIP TO OTHER EXISTING OR

techniques should have no adverse impact. PROPOSED REGULATIONS OR POLICIES

1.3.2 Other Government Agencies The proposed action is one of a series of revisions of existing regulatory guides on nondestructive assay tech Not applicable. niques.

1.3.3 Industry 6. SUMMARY AND CONCLUSIONS

Since industry is already applying the more recent Regulatory Guide 5.9 should be revised to bring it up to detector technology discussed in the guide, updating these date.

techniques should have no adverse impact.

5.9-7

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