GAMMA-RAY EMITTING TEST SOURCES FOR PORTAL

MONITORS USED FOR HOMELAND SECURITY

L. Lucas, L. Pibida, M. Unterweger and L. Karam

NIST, 100 Bureau Dr. Stop 8462, Gaithersburg, MD 20899-8462

Received September 8 2004, amended October 5 2004, accepted October 26 2004

The National Institute of Standards and Technology developed a new set of test sources for testing of portal monitors against

the ANSI standard N42.35-2004. The design and source specifications are provided in this work.

INTRODUCTION

With the development of the new ANSI N42.35

standard, ‘Evaluation and Performance of Radiation Detection Portal Monitors for Use in Homeland Security’(1), as well as the related Test and

Evaluation Protocol for Homeland Security(2), the

National Institute of Standards and Technology

(NIST) has developed a set of test sources for the

testing of portal monitors. The standard(1) specifies

that the test sources should be point sources (relative

to the size of the portal monitors), they should be

single radionuclides and encapsulated in a 0.25 mm

thick stainless steel disc. The radionuclides required

for portal monitor testing are: 57Co, 60Co, 133Ba, 137Cs, 228Th and 241Am. These sources are useable

not only for testing and evaluating instrumentation,

but can also be used as calibration test sources and

for instrument user training.

SOURCE SPECIFICATIONS

The sources specified in the ANSI N42.35 standard

were chosen according to the half-life and gammaray energies of the radionuclide. Since radioactive

sources decay over time, the use of a radionuclide

with a long half-life, T1/2, is required if a source is to

be useable for an extended period of time. For the

types of detectors to be used in homeland security

applications, gamma-ray energies from 50 keV to

3 MeV are desired. In addition, the source design

requires specific encapsulation and geometry considerations. The durability of the source and resistance

to shocks, extreme temperatures and rough handling

must be considered; durability is a crucial aspect of

the source design adopted by NIST.

The ANSI standard for testing portal monitors

specifies both the construction of the source and its

nominal activity at the time of testing. The actual

activity in each source at the time of the testing may

be between 0.8 and 1.2 times (i.e. 20%) the nominal

activity. The specified activities are given in Table 1.

The radioactive material, with the activity listed

in Table 1, is welded between two 0.25 mm thick

316 stainless steel discs. Stainless steel was chosen

for its chemical resistance and mechanical strength,

so that a source could tolerate the force of a

vehicle driven over it without loss of radioactivity.

Note that the measured gamma-ray emission rate

is reduced due to the attenuation in the stainless

steel disc.

The source itself consists of the radioactive material, originally as a calibrated solution dissolved in

acid, weighed out and dried on a piece of glass-fibre

filter paper, 1.27 cm in diameter and 1 mm thick. The

filter paper is placed between two 316 stainless steel

discs, each 0.25 mm thick and 38.1 mm in diameter

(see Figure 1 for a diagram of the discs). Before

welding the stainless steel discs together, the

distribution of the radioactive material in the source

is verified using photo-stimulable luminescence



Corresponding author: leticia.pibida@nist.gov

Table 1. Radionuclides and activities for the portal monitor

test sources.

Radionuclide Activity at the time of testing (MBq)

Minimum Nominal Maximum

57Co

(T1/2 1⁄4 270 d)

2.75 3.44 4.13

60Co

(T1/2 1⁄4 5.2714 y)

0.119 0.148 0.177

133Ba

(T1/2 1⁄4 10.51 y)

0.681 0.851 1.021

137Cs

(T1/2 1⁄4 30.07 y)

0.474 0.592 0.710

228Th

(T1/2 1⁄4 1.9116 y)

0.208 0.259 0.310

241Am

(T1/2 1⁄4 432.2 y)

13.7 17.1 20.5

Radiation Protection Dosimetry (2005) Vol. 113, No. 1, pp. 108–111 doi:10.1093/rpd/nch422

Advance Access publication 7 December 2004 TECHNICAL NOTE

Published by Oxford University Press 2004.
imaging techniques [Fuji system], and the activity is

measured using high purity germanium (HPGe)

detectors. The measured activity is compared to the

activity calculated from the mass of the original

NIST calibrated radioactive solution. Then the

discs are welded around the edge using an automated

tungsten-inert-gas (TIG) welding system. Each

welded source is subsequently measured again

using the HPGe detectors to verify that the welding

process did not alter the activity. The stainless steel

disc is then mounted in a 6061-T6 aluminium holder.

The holder consists of two plates, each 5.08 cm wide,

8.89 cm long and 0.3175 cm thick with a 3.175 cm

hole for mounting the source, and three smaller

holes for mounting the holder to the experimental

set up for testing the portal monitor. See Figure 2 for

a diagram of the holder. Part of the holder area is

used for labeling the source, including source number, activity and reference date. The first set of

sources produced by NIST is pictured in Figure 3.

The technical drawings for the sources are given in

Appendix 1.

FUTURE CONSIDERATIONS

For the commercialisation and widespread distribution of this type of test source, a variety of additional

issues must be addressed. The National Institute of

Standards and Technology has developed the prototype, and the first set of test sources, for the testing

and verification of portal monitors with minimal

concern as to the cost and availability of the radioactive materials. If mass production is desired in

future, the cost of the radioactive material to be

used, as well as its availability in large quantities,

will play a major role in the practicality of specific

radionuclides to be used. In addition, the role that

manufacturers will play in the production and distribution of these sources must be considered. We

are aware of five companies that currently produce

or distribute radioactive sources in the USA: AEA

38.1 mm

12.7 mm

0.254 mm

1 mm

Glass-fiber filter paper 316 SS disk

Figure 1. Test source before welding.

Figure 3. Picture of the first set of test sources.

Marking 

Area

Source 

Disk

Mounting

Holes

5.08 cm

8.89 cm

Figure 2. Aluminium holder.

TEST SOURCES FOR TESTING OF PORTAL MONITORS

109
Technologies, Analytics, Isotope Products Laboratories, North American Scientific and Spectrum

Techniques, Inc. Several of these companies are participants in a Measurement Assurance Programme

(MAP) with NIST to assure traceability to the US

national standards in the production of sources for

nuclear power plant and medical applications. A

similar and more extended programme could be

implemented for homeland security applications.

These and other issues will need to be resolved

before there can be an effective programme to distribute these types of sources more widely.

CONCLUSIONS

The first set of test check sources have been produced and sent to the Oak Ridge National Lab

(ORNL), the Pacific Northwest National Lab

(PNNL), the Los Alamos National Lab (LANL),

the Lawrence Livermore National Lab (LLNL) and

the Environmental Measurement Lab (EML), where

they are being used for the testing and evaluation of

commercially available portal monitors. There is still

the need to discuss with source manufacturers the

requirements and logistics for larger scale production, if needed.

REFERENCES

1. ANSI N42.35-2004, American National Standard

Evaluation and Performance of Radiation Detection

Portal Monitors for Use in Homeland Security (2004).

2. T&E Protocol N42.35, April 2004, Testing and

Evaluation Protocol of Radiation Detection Portal

Monitors for Use in Homeland Security. DHS T&E

N42.35 (2004).

APPENDIX 1. TECHNICAL DRAWINGS

FOR THE SOURCES

Figures A.1–A.4 and Table A.1 show in detail the

technical drawings for the sources.

Figure A.1. DHS test source disc before welding.

Glass-fiber filter paper Welded at the edge

Figure A.2. DHS test source disc after welding.

L. LUCAS ET AL.

110
Figure A.3. Top plate for radioactive DHS test source holder.

Figure A.4. Bottom plate for radioactive DHS test source

holder.

Table A.1. List of drawing dimensions.

Dimension Values in millimeters

B Drill through 3.6576 mm diameter

and counter-sink for #6–32 flat head

machine screws (4 places)

C Drill through 5.1562 mm diameter

(2 places)

D Drill through 6.7564 mm diameter

(1 place)

E Drill through and tapped for #6-32

machine screw (4 places)

F 12.70

G 31.75

H 38.10

J 50.80

K 44.45

L 3.175

M 0.508

N 88.90

P 1.00

Q 0.254

R 5.08

S 27.94

Note: Material test source disc 1⁄4 316 stainless steel.

Material test source holder 1⁄4 6061-T6 Aluminium bar 3.175

 50.8  88.9 mm.

Tolerance 1⁄4 0.254 mm (unless otherwise indicated).

Tolerance for the counter-bore depth dimension ‘‘M’’ of

0.508 mm is 0.127 mm.

Tolerance for stainless steel disc thickness is 0.0127 mm

(0.0005 inches).

TEST SOURCES FOR TESTING OF PORTAL MONITORS

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