NIST Technical Note 1758

Justification of Shielding 

Requirements for Plutonium 

Sources

L. Pibida

M. Unterweger 

http://dx.doi.org/10.6028/NIST.TN.1758
NIST Technical Note 1758 

Justification of Shielding 

Requirements for Plutonium 

Sources

L. Pibida

M. Unterweger

Radiation and Biomolecular Physics Division

Physical Measurement Laboratory

http://dx.doi.org/10.6028/NIST.TN.1758

September 2012

U.S. Department of Commerce 

Rebecca Blank, Acting Secretary

National Institute of Standards and Technology 

Patrick D. Gallagher, Under Secretary of Commerce for Standards and Technology and Director 
Certain commercial entities, equipment, or materials may be identified in this

document in order to describe an experimental procedure or concept adequately.

Such identification is not intended to imply recommendation or endorsement by the

National Institute of Standards and Technology, nor is it intended to imply that the

entities, materials, or equipment are necessarily the best available for the purpose. 

National Institute of Standards and Technology Technical Note 1758

Natl. Inst. Stand. Technol. Tech. Note 1758, 10 pages (September 2012) 

http://dx.doi.org/10.6028/NIST.TN.1758

CODEN: NTNOEF
Justification of Shielding Requirements for Plutonium Sources

L. Pibida and M. Unterweger

National Institute of Standards and Technology, 100 Bureau Dr, MS 8462, Gaithersburg, MD 20899

Abstract

This report provides a justification for shielding plutonium sources when testing radiation detection 

instruments for standard performance requirements and test methods. The use of copper as a shielding 

material is proposed to provide comparable test results when testing radiation detection instruments in 

different locations using different plutonium sources with varying 241Am content.

Introduction

In 2008 the National Institute of Standards and Technology (NIST) participated in the Domestic Nuclear 

Detection Office (DNDO) test of advanced spectroscopic portal (ASP) monitors to characterize the 

performance of these systems. Several plutonium sources were used for testing the alarm and radionuclide 

identification response of the ASPs. During these tests it was found that the americium-241 (

241Am)

content in the various plutonium sources had a significant impact in the identification of plutonium-239 

(

239Pu). This report summarizes how this problem was addressed. 

Differences in the 241Am content are due to the isotopic composition (

241Pu content) and age of the Pu 

source. It became clear that it was necessary to limit the emission of the 59.54 keV gamma-ray line in 

order to have comparable instrument response results when testing with different sources in different test 

locations. At that point in time the DNDO and NIST scientists decided that for weapons grade plutonium 

(WGPu) sources the net count rate of the 59.54 keV gamma-ray line from 241Am shall be no more than 10 

times greater than that of the net count rate of the 414 keV line for 239Pu (e.g., if the count rate for 239Pu is 

100 cps then the count rate for the 59.54 keV line for 241Am shall not exceed 1,000 counts per second 

(cps)). This amount allows the detection of 241Am without distorting the high energy spectrum that could 

potentially prevent the identification of 239Pu. Based on NIST and DNDO calculations it was determined 

that a medium-z material, such as copper, was the best option to effectively reduce the 241Am contribution 

with minimum distortion of the energy spectrum in the energy range between 300 keV and 500 keV.

This approach was later used for other DNDO test campaigns including those named Eland and PaxBag. 

When Pu sources were used in test campaigns, it was clear that the identification response (radionuclide 

identification answer) provided by the radionuclide identifier detectors (RIIDs), ASP, mobile systems

with spectroscopic capabilities, spectrometric personal radiation detectors, stations with spectroscopic 

capabilities, and spectrometric pedestrian portal monitors, strongly depended on the amount of 241Am

present in the source. 

As performance standards were being developed or revised, the importance of using this technique 

became clearer. Testing of radiation detection instruments against the standards can occur at different 
testing facilities and having access to the same Pu source is almost impossible due to availability and 

shipping restrictions.

During the development of the RIIDs technical capability standard (TCS) document several 

measurements using different Pu sources were made at Los Alamos National Laboratory (LANL) by 

NIST staff. The measurements were performed with bare and shielded Pu sources. Results showed that 

the 241Am content varied over approximately three orders of magnitude indicating that it was also 

important to specify the shielding material. During the TCS working group meetings the use of cadmium 

(Cd) or copper (Cu) for shielding the Pu sources was discussed.

The ASTM C 993 – 97 standard [1] also discusses the 241Am content in Pu sources. This standard defines 

the following:

standard plutonium test source—a metallic sphere or cube of low-burnup plutonium containing at least 

93 % 239Pu, less than 6.5 % 240Pu, and less than 0.5 % impurities.

Discussion—A cadmium filter can reduce the impact of 241Am, a plutonium decay product that will slowly

build up in time and emit increasing amounts of 60-keV radiation. Begin use of a 0.04-cm thick cadmium 

filter when three or more years have elapsed since separation of plutonium decay products. If ten or more 

years have elapsed since separation, use a cadmium filter 0.08 cm thick. The protective encapsulation 

should be in as many layers as local rules require. A nonradioactive encapsulation material, such as,

aluminum (0.32 cm-thick) or thin (0.16 cm-thick) stainless steel or nickel, should be used to reduce 

unnecessary radiation absorption.

The guidance provided in the American Standard Test Method (ASTM) standard is a rule of thumb 

approach (i.e., providing shielding thicknesses based on the age of the Pu sources) but it does not provide 

the necessary accuracy for the comparisons of test results when testing against a performance standard. 

For the purpose of the ASTM standards this rule of thumb is enough as it does not compare the 

performance requirements between different instruments, especially spectrometric type instruments. 

Spectrometric type instruments may not identify the presence of 239Pu when a large amount of 241Am is

present in the spectrum. A more accurate determination of the emission of the 59.54 keV gamma-ray line 

from 241Am is obtained by measuring the energy spectrum of the Pu source using a high purity 

germanium (HPGe) detector and limiting the net count rate of the 59.54 keV gamma-ray line from 241Am 

to be no more than 10 times greater than that of the net count rate of the 414 keV line for 239Pu, as 

adopted by DNDO for testing of radiation detection instruments.

The use of Cd to shield the Pu sources was further discussed in the TCS working group meetings. 

Machining of Cd is hazardous if inhaled due to the toxicity of the dust particles. Therefore, the use of 

copper was suggested. LANL proposed the use of a specific copper alloy to minimize differences when 

testing at different facilities. The use of copper with more than 99.9 % Cu content listed in the ASTM 

B152 standard [2] was proposed for the TCS. 

Supporting data

During the development of the RIIDs TCS several measurements using different Pu sources were made in 

LANL. Figure 1 and Figure 2 show the energy spectra of different WGPu sources acquired using a 

NaI(Tl) and HPGe detector respectively. The differences in the structure in the 60 keV energy region due 
to the presence of 241Am is clearly seen. Figure 3 shows the energy spectra of a bare and shielded WGPu 

source acquired at Oak Ridge National Laboratory (ORNL). The shielding used for this measurement was 

5 mm steel. 

Calculations were performed to look at the difference in the attenuation coefficients and factors when 

using different materials for shielding the Pu sources in order to reduce the 241Am contribution while 

minimizing the attenuation of the lines in the 300 to 500 keV energy region. Table 1 summarized the 

calculations for the different proposed materials. The Photon Cross Sections Data (XCOM) Database [16] 

was used to obtain the numbers listed in Tables 1 and 3. This database does not provide uncertainties for 

the calculated attenuation coefficients. Therefore, it is not possible to estimate the uncertainties for the 

values listed in Tables 1 and 3. 

0 300 600 900 1200 1500 1800

100

101

102

103

104

105

Counts

Energy (keV)

Figure 1: Energy spectra of different bare Pu sources from LANL using a NaI detector. The black line represents 

the background.
100 200 300 400 500

100

101

102

103

104

Counts

Energy (keV)

Figure 2: Energy spectra of different bare Pu sources from LANL using a HPGe detector

Figure 3: Energy spectra of bare and shielded Pu source from ORNL using a HPGe detector. The black line 

represents the bare Pu spectrum and the red line represents the spectrum of the shielded Pu source using a

5 mm steel plate.

 

 

 Energy (keV) 

 100 200 300 400 500 600 

 Counts / keV 

101

102

103

104

105

106
Table 1: Summary of attenuation calculations

Shielding 

Material

Density 

(g/cm3)

Attenuation coefficient 

(cm2/g)

Attenuation Factor for 

1 cm material

Attenuation Factor for 

5 mm material

Aluminum 2.699 60 keV – 0.2778

400 keV – 0.09276

60 keV – 0.472

400 keV – 0.779

60 keV – 0.687

400 keV – 0.882

Iron 7.874 60 keV – 1.205

400 keV – 0.09400

60 keV – 7.57 × 10-5

400 keV – 0.477

60 keV – 8.70 × 10-3

400 keV – 0.691

Copper 8.960 60 keV – 1.593

400 keV – 0.09413

60 keV – 6.33 × 10-7

400 keV – 0.430

60 keV – 7.95 × 10-4

400 keV – 0.656

Cadmium 8.650 60 keV – 5.975

400 keV – 0.1129

60 keV – 3.58 × 10-23

400 keV – 0.377

60 keV – 5.98 × 10-12

400 keV – 0.614

There are many different copper and copper-alloy compositions that are grouped into the categories: 

copper, high copper alloy, brasses, bronzes, copper nickels, copper–nickel–zinc (nickel silver), leaded

copper, and special alloys. The following table lists the principal alloying element for four of the more 

common types, along with the name for each type.

Table 2: Classification of copper and its alloys and associated Unified Numbering System (UNS) number

Family Principal alloying element UNS number

Copper alloys, brass Zinc (Zn) C1xxxx–C4xxxx,C66400–C69800

Phosphor bronze Tin (Sn) C5xxxx

Aluminum bronzes Aluminum (Al) C60600–C64200

Silicon bronzes Silicon (Si) C64700–C66100

Copper nickel, nickel silvers Nickel (Ni) C7xxxx

The composition of these different alloys meets different standards requirements as shown in Table 3.

Due to the difference in elemental composition, the x-ray and gamma-ray attenuation through different 

materials can vary from alloy to alloy. It is therefore recommended that in order to gain consistency in 

testing, the Pu source be shielded using copper with a purity of 99.90% or better (i.e., C101 or UNS No. 

C11000) to reduce the low-energy emissions from the 59.54 keV 241Am line. The copper thickness will 

depend on the 241Am content of the source, in order to ensure that the net count rate of the 59.54 keV 

gamma-ray line from 241Am is not more than 10 times greater than the net count rate of the 414 keV line 

for 239Pu, while the 59.54 keV gamma-ray line from 241Am is still visible in the energy spectrum. 

 

Table 3: Nominal composition of copper alloys

Alloy Name Nominal composition Attenuation 

Coefficient (cm2/g)

Calculated Density 

(g/cm3)

Copper (ASTM B152, B124) More than 99.9 % Cu 60 keV – 1.593

400 keV – 0.09413

8.960

Gilding metal (ASTM B36) 95.0 % Cu, 5.0 % Zn 60 keV – 1.601

400 keV – 0.09419

8.847

Cartridge brass (ASTM B19, 70.0 % Cu, 30.0 % Zn 60 keV – 1.644 8.321
B36, B135) 400 keV – 0.09451

Phosphor bronze (ASTM B103, 

B139)

89.75% Cu, 10.0 % Sn, 

0.25 % P

60 keV – 2.448

400 keV – 0.09458

8.697

Yellow or High brass (ASTM 

B36, B135)

65.0% Cu, 35.0 % Zn 60 keV – 1.652

400 keV – 0.09782

8.223

Naval brass (ASTM B21) 60.0 % Cu, 39.25 % Zn, 

0.75 % Sn

60 keV – 1.728

400 keV – 0.09492

8.129

Muntz metal (ASTM B111) 60.0 % Cu, 40.0 % Zn 60 keV – 1.661

400 keV – 0.09464

8.127

Aluminium bronze (ASTM B169, 

B124, B150)

92.0 % Cu, 8.0 % Al 60 keV – 1.546

400 keV – 0.09408

8.780

Beryllium copper (ASTM B194, 

B196)

97.75 % Cu, 2.0 % Be, 

0.25 % Co or Ni

60 keV – 1.588

400 keV – 0.09411

8.320

Cupronickel 70.0 % Cu, 30.0 % Ni 60 keV – 1.569

400 keV – 0.09513

8.943

Ounce metal Copper Alloy 

C83600 (also known as "Red 

brass" or "composition metal") 

(ASTM B62)

85.0 % Cu, 5.0 % Zn, 5.0 % 

Pb, 5.0 % Sn

60 keV – 2.484

400 keV – 0.1154

9.198

Summary 

The use of any copper with more than 99.9 % Cu content listed in the ASTM B152 standard [2] is an 

adequate choice for reducing the amount of the 241Am gamma-ray emission from Pu sources. Commonly 

used copper types for this type of application are the UNS No. C11000 or the C10100. The C11000 

(equivalent to the C101 British designation) copper has a purity of 99.90 %. The C10100 (equivalent to 

the C110 British designation) copper is the oxygen free type and has the highest purity of 99.99 %. 

The thickness of the copper shielding shall be based on the 241Am content of the source. The copper 

thickness needs to be chosen to ensure that the net count rate of the 59.54 keV gamma-ray line from 

241Am to be no more than 10 times greater than that of the net count rate of the 414 keV line for 239Pu

while the 59.54 keV gamma-ray line from 241Am is still visible in the energy spectrum.

References 

1. ASTM C 993 – 97, “Standard Guide for In-Plant Performance Evaluation of Automatic 

Pedestrian SNM Monitors.”

2. ASTM B152/B152M-09, “Standard Specification for Copper Sheet, Strip, Plate and Rolled Bar.”

3. ASTM B124/B124M-12, “Standard Specification for Copper and Copper Alloy Forging Rod, 

Bar, and Shapes.”

4. ASTM B36/B36M-08a, “Standard Specification for Brass Plate, Sheet, Strip, and Rolled Bar.”

5. ASTM B19-10, “Standard Specification for Cartridge Brass Sheet, Strip, Plate, Bar, and Disks.”

6. ASTM B135-10, “Standard Specification for Seamless Brass Tube.”
7. ASTM B103/B103M-10, “Standard Specification for Phosphor Bronze Plate, Sheet, Strip, and 

Rolled Bar”

8. ASTM B139/B139M-07, “Standard Specification for Phosphor Bronze Rod, Bar, and Shapes.”

9. ASTM B21/B21M-12, “Standard Specification for Naval Brass Rod, Bar, and Shapes.”

10. ASTM B111/B111M-11, “Standard Specification for Copper and Copper-Alloy Seamless 

Condenser Tubes and Ferrule Stock.”

11. ASTM B169/B169M-10, “Standard Specification for Aluminum Bronze Sheet, Strip, and Rolled 

Bar.”

12. ASTM B150/B150M-08, “Standard Specification for Aluminum Bronze Rod, Bar, and Shapes.”

13. ASTM B194-08, “Standard Specification for Copper-Beryllium Alloy Plate, Sheet, Strip, and 

Rolled Bar.”

14. ASTM B196/B196M-07, “Standard Specification for Copper-Beryllium Alloy Rod and Bar.”

15. ASTM B62-09, “Standard Specification for Composition Bronze or Ounce Metal Castings.”

16. Xcom website, http://physics.nist.gov/PhysRefData/Xcom/html/xcom1.html