NBSIR 81-2407 (R)

Intruder Tagging and Identification

Using Luminescent Particles

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Center for Analytical Chemistry

Washington, DC 20234

Prepared by

Law Enforcement Standards Laboratory

National Engineering Laboratory

National Bureau of Standards

Washington, DC 20234

September 1982

Final Report

}

Prepared for —^'^ense Nuclear Agency

-yC - fN: OPNS

2Q(J tohington, DC 20305

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1982

C.2 j

OFFICIAL USE ONLY
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'• '^ NBSIR 81-2407 (R) ^

.. INTRUDER TAGGING AND IDENTIFICATION ' USING LUMINESCENT PARTICLES > T

J. R. DeVoe

R. A. Velapoldi

J. K. Langland

D. K. Hancock

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Center for Analytical Chemistry

Washington, DC 20234

Prepared by

Law Enforcement Standards Laboratory

National Engineering Laboratory

National Bureau of Standards

Washington, DC 20234

September 1 982

Final Report

This work was sponsored by the Defense Nuclear Agency under Subtask

Code B99QAXRF, Technology Development, and Work Unit Code 00027,

Taggant/Tracer,

Prepared for Defense Nuclear Agency

Attn: OPNS

Washington, DC 20305

OFFICIAL USE ONLY

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

ACKNOWLEDGMENTS

-PVia was prepared by the Law Enforcement Standards Laboratory of the

security Systems B?ickbS?A , and Dale Kauffman of the

lu!^ *S^ SI ImIISSS"- Development ??oup for =er«in P«ta of the

00027, TaggantAracer, William J. Witter, DHA Subtask Manager.

ii
]
FOREWORD

The Defense Nuclear Agency (DNA) Is engaged in a continuing effort to enhance

the security of nuclear weapons storage. In this effort, it is receiving technical

support from the National Bureau of Standards' Law Enforcement Standards Laboratory

(LESL) , whose overall program involves the application of science and technology to

the problems of crime prevention, law enforcement and criminal justice.

LESL is assisting DNA's physical security program with support in chemical

sciences, and the barrier and ballistic materials areas. Among the tasks being performed by LESL for DNA are the preparation and

publication of several series of technical reports on the results of its researches.

This document is one such report.

Technical comments and suggestions are invited from all interested parties.

They may be addressed to the authors, or the Law Enforcement Standards Laboratory,

National Bureau of Standards, Washington, DC 20234.

Lawrence K. Eliason, Chief

Law Enforcement Standards Laboratory

iii
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INTRUDER TAGGING AND IDENTIFICATION

USING LUMINESCENT PARTICLES

Contents

Page

Foreword iii

List of Tables vi

List of Figures vii

I. Introduction 1 II . Spectrof luorimetry 3 A. History, Description, and Application 3 B. Description of the Microspectrof luorimeter 6 III. Preparation and Emission Characteristics of Various Luminophor-Doped

Glasses 6 A. Glass Preparation 6 B. Emission Characteristics 7 C. Testing of Final Glass Composition 8 D. Instrumental Factors Affecting Peak Ratios 25

E. Sphere Preparation and Characterization 33

F. Reproducibility of Measurements 37

IV. Extent of Unique Types of Particles 38

V. Types of Particle Deployment 39

A. Apparatus and Experimental Procedure 39

B. Distribution of Particles 39

VI. Detection of Spherical Particles 46

A. Limit of Detection 46

B. Particle Dilution Experiments 4 7 C. Detection of Particles in a Background 50

D. Reflecting Glass Particles 53

VII . Particle Sampling Techniques 53

A. Vacuum System 53

B. Adhesive Tape 58

C. Practical Tests 58

VIII. Description of Field Unit 61

IX . Future Development of the System 64

References 65

V
LIST OF TABLES

Page

1. Optical characteristics of the Ploemopak filter cubes used for the

selection of excitation radiation . 8 2. Compositions of preliminary glass melts 11

3. Composition of Tb203 ~ EU2O3 doped glasses 25

4. Effect of changing parameters of the measurement system 38

5. Frequency distribution of particles 48

e. Effect of background particles on Ivuninescent particle detectability .... 80

vi
LIST OF FIGURES

Paqe

1 .

2 .

3.

4 .

5.

6 .

7 .

8 .

9.

10 .

11 .

12 .

13.

14.

15.

16 .

17.

Simplified Jablonski energy level diagram showing the processes of photon

absorption (la )internal conversion (Ic )

,

and luminescence (II) . . . . Schematic for a spectrof luorimeter ; M can be monochromators or selective

filters, Xy = exciting wavelength, Xm = emission wavelength

Diagram of microspectrof luorimeter used in this work

Luminescence spectra of glasses containing U 3O 8 (curve A) and EU 2O 3 (curve B) ; (2.5 mole %) ; cube A; photomultiplier voltage; curve A = 900 V, curve B = 890 V

Luminescence spectra of glasses containing Tb 203 (curve A) and Sm 203

(curve B) at 4.0 and 1.1 mole percent, respectively; cube A;

photomultiplier voltage; curve A = 800 v, curve B = 930 V

Luminescence spectrum of glass, melt K-1031 containing EU 2O 3 , peaks

labeled 1 and Tb 203 , peaks labeled 2; 1.5 and 1.0 mole percent,

respectively; cube A; photomultiplier voltage; curve A = 970 V,

curve B = 1300 V; monochromator slits =0.5,0.5mm

Luminescence spectra of glass, melt K-1032, containing Sm 203 , peaks

labeled 1 and Tb 203 , peaks labeled 2 ; 1.0 mole percent each:

photomultiplier voltage and cube used; curve A = 1020 V, cube A;

curve B = 960 V, cube D; monochromator slits =2,2mm ....

Luminescence spectra of glass, melt K-1045, containing EU 2O 3 , peaks

labeled 1 and CuO; 1.5 and 0.1 mole percent, respectively;

photomultiplier voltage and cube used; curve A = 840 V, cube A;

curve B = 1200 V, cube A; curve C = 830 V, cube D; monochromator slit

widths = 2,2 mm

Luminescence spectra of glass, melt K-1064 , containing EU2O3 peaks

labeled 1 and UoOg peaks labeled 2; 1.5 and 0.3 mole percent,

respectively photomultiplier voltage and cube usedr curve A = 780 V,

cube A: curve B = 730 V, cube D; curve C = 607 V, cube G; monochromator

slit widths = 2,2 mm

Luminescence spectra of glass, melt K-1077, containing EU2O3 , peaks

labeled 1, and Sm203 ; 1.5 and 1.0 mole percent, respectively;

photomultiplier voltage; curve A = 767 V, monochromator slit widths =

2,2 mm Luminescence spectra of glass, melt K-1078 , containing Eu203 f peaks

labeled 1 and Dy203 peaks labeled 2; 1.5 and 2.0 mole percent,

respectively; photomultiplier voltage and cube used; curve A = 1100 V,

cube A; curve B = 1500 V, cube A scale expanded XlO; curve C = 1130 V,

cube D; monochromator slit widths =2,2mm ..

Luminescence spectra of glass, melt K-1079, containing EU2O3 , peaks

labeled 1; Sm 203 , and CuO; 1.0, 1.0, and 0.1 mole percent,

respectively; photomultiplier voltage and cube used; curve A = 890 V,

cube A; curve B = 890 V, cube D; monochromator slit widths = 2,2 mm . . Luminescence spectrum of glass, melt K-1087, containing Sm203 , and Dy203 , peaks labeled 2 ; 1.0 and 2.0 mole percent, respectively;

photomultiplier voltage = 1460 V; monochromator slit widths =2,2 mm

Lvuminescence spectra of glass, melt K-1088 , containing U 3O8 , peaks

labeled 2 and Sm 203 , peaks labeled 1 ; 0.6 and 1.0 mole percent,

respectively; photomultiplier voltage and monochromator slit widths;

curve A = 1300 V, 2,2 mm; curve B = 1700 V, 0.5, 0.5 mm

Luminescence spectrum of glass, melt K-1124 , containing Tb203 , peaks

labeled 2 and Sm 203 peaks labeled 1 ; 2.0 weight percent;

photomultiplier voltage = 1350 V; monochromator slit widths =

0 . 5

,

0 . 5 mm

Luminescence spectrum of glass, melt K-1125 , containing Tb203 , peaks

labeled 1, and EU 2O 3 peaks labeled 2; 2.0 and 3.0 weight percent,

respectively; photomultiplier voltage = 1250 Vr monochromator slit

widths = 0.5, 0.5 mm

Luminescence spectrum of glass, melt K-1126 , containing Dy203 , peaks

labeled 1, and EU 2O 3

, peaks labeled 2; 2.0 and 1.5 weight percent,

respectively; photomultiplier voltage = 1490 V: monochromator slit

widths = 0.5, 0.5 mm

4

57

9

10

12

13

14

15

16

17

18

19

20

21

22

23

vii
18 .

19.

20 .

21 .

22 .

23.

24 .

25.

26 .

27.

28 . 29.

30 .

31a.

31b.

31c.

32.

33.

34 .

35.

36.

37.

38 .

39a .

39b.

40.

41

42.

43 .

Luminescence spectra of glass, melt K-1128 , containing U 3 O8 , peaks

labeled 1, and EU 2 O 3 , peaks labeled 2; 0.6 and 3.0 weight percent,

respectively; photomultiplier voltage and monochromator slits;

curve A = 1000 V, 2,2 mm; curve B = 1380 V,2,2mm

Luminescence spectra of glasses containing Tb^'*’ solid line, Eu^"^ dashed

line, cube D; monochromator slit widths = 2,2 mm

Partial Jablonski energy level diagram of europium and terbium showing

the transitions observed in the luminescence spectra of figure 19 . . Typical luminescence spectrum of a glass doped with Tb203 , peaks labeled

1, and EU 2 O 3 , peaks labeled 2; 3.0 and 1.0 weight percent,

respectively; photomultiplier voltage = 1000 V; monochromator slit

widths = 2 , 2 mm Luminescence spectra of glasses containing Tb3+ and of Eu^"*" using cube D

(table 1) . The numbers refer to the melt designations listed in table

3. Each spectrum was recorded at different gain settings. Relative

peak intensities in each spectrum are illustrated

Typical excitation spectra for Tb^"*" (dashed line) and Eu^+ (solid line)

in a glass matrix. Approximate excitation wavelength bandwidths are shown by the boxed in areas using filter cubes A and D (see appropriate

cross dashing and table l)

Luminescence spectra of melt K-1186 using cubes A and D (table 1) to

select excitation radiation and show difference in peak ratios for

the same melt

Luminescence spectra of all glass melts of composition listed in table 3 using cube A (table 1) . Each spectrum was recorded at different gain

settings. Relative peak intensities in each spectrum are illustrated.

Luminescence spectra of glass melt K-1202 as a function of monochromator

slit widths using filter cube D (see table 1); A = 2mm, B= 1mm,

C = 0.5 mm, D = 0.2 mm

Plot of relative luminescence intensity versus the third power of

particle radius

Plot of europium-terbium ratio versus the particle radius

Plot of europium-terbium ratio versus the weight percent ratio of

europium to terbium

Photograph of particle disperser; D = circular deflector disc; P = pipe

fitting in which particles reside; A = nitrogen pressure line . . . . Particle dispersal system; plastic covered chamber showing positions of

microscope slides

Particle dispersal system; close-up showing microscope slide placed on inverted 50 mL beaker

Particle dispersal system; top of chamber showing mounted disperser with

chamber in the background .....

Diagrcim showing slide positions (dimensions in millimeters)

Test results of particle dispersal. Numbers are average of five runs;

units = particles/cm^/g (dimensions in millimeters)

Curve 1 (dots)- Ratio of number density (16.7 ym diam) luminescent to

blank particles; calculated vs. experimental. Curve 2 (squares) - Experimental number density of luminescent particles vs. experimental

ratio of number density luminescent to blank particles

Photograph of luminescent particles in house dust

Photograph of pollen grain and particles in house dust

Luminescence spectrum of a pollen grain with K-1200 tagged particles

for comparison

Photograph showing light -ref lecting properties of the glass sphere

retroref lectors (diameters range from 40-85 ym)

Photograph of vacuum filter particle collection system; vacuiim pump and

filter housing

Photograph of vacuum filter particle collection system; filter housing . Spectra of luminescent particle (code K-1200) collected on; A) filter

paper, and B) adhesive tape. Dashed lines indicate background . . . . Photograph of the NBS microspectrof luorimeter in a van at the Naval

Weapons Supply Center , Crane , IN

Floor plan of ammunition bunker showing location of plastic strip and

positions where sampling was performed along with the results in

particles per square centimeter (dimensions in meters)

Photograph of microspectrof luorimeter (with pertinent controls labeled)

24

26

27

28

29

30

31

32

34

35

36

37

40

41

42

43

44

45

49

51

51

52

54

55

56

57

59

60

62

viii
INTRUDER TAGGING AND IDENTIFICATION

USING LUMINESCENT PARTICLES

J. R. DeVoe, R. A. Velapoldi, J. K. Lang land, D. K. Hancock*

National Bureau of Standards

Washington, DC 20234

This is the final report on the developnent of a unique particle

tagging system that can allow one to follow the egress of a person who

violates a secured area. The intruder comes into contact with glass beads

(20 Mm diam) which are of two types: one type acts as light reflectors

allowing immediate visual detection; the second type contains rare earths

whose emission spectra and quantity can be measured in a microspectrofluorimeter . Each site is tagged with beads containing a unique

concentration ratio of two rare earths, thereby enabling hundreds of unique

codes. The efficiency for detecting these particles has been measured and

particle densities as low as 5 particles/cra^ can be detected. Particle

concentration techniques were also developed. Practical tests demonstrated

the portability of the equipment and that this system is effective in such

environments as normal buildings and houses as well as in an ammunition

storage bunker.

Key words: fluorescence; intruder tagging and identification;

microspectrofluorimeter ; particles; tagging material.

I. INTRODUCTION

Conventional security systems rely upon a combination of intrusion alarms and

response forces to protect facilities and government property. Present day

intrusion detection systems sense the presence or action of an intruder and signal

an alarm for appropriate action by the response force. The need for improved site

security is often recognized, and several overall intrusion detection systems have

been developed. While these systems may have application for improving the security

of a site, it is desirable to augment such systems with components that react to the

penetration attempts of an Intruder, not simply signal an alarm.

The Defense Nuclear Agency has sponsored the development of a security system

concept called Forced Entry Deterrent Systems (FEDS) . The objective of this

development is to deny access to a secured site by impairing the senses of an

intruder sufficiently to cause him to discontinue his penetration attempt

(psychological deterrence) , while at the same time affording a tactical advantage to

the response force personnel.

Assuming that an intruder has activated a FEDS component, it would be desirable

to tag or label the individual so that the person could be positively identified as having attempted to Intrude into the site and aid in apprehension should the

intruder escape from the scene before response forces arrive. In addition, a tagging material that is dispensed at the perimeter of a site could assist in

tracking the intruder towards the target.

An intruder enters a secured area and then leaves with or without the object or information that was to be stolen. The question to be answered is whether or not a tag can be used to uniquely identify the intruder even if apprehension occurred much

later and quite remote from the secured area. This report describes one solution to

the problem, which is the use of luminescent particles that can be made so as to

cause them to be uniquely identifiable from all other types of particles.

The luminescent particles must be physically placed on or within the intruder.

There are two obvious ways that this can be accomplished. The surface areas of

interest (e.g., inventory, tables, walls, parts of the floor) can be coated with a film of these particles. Alternatively, a sensor can trigger the ejection of

particles in a cloud that fills the area. The intruders then get the particles on

* Center for Analytical Chemistry, National Measurement Laboratory.

1
their person and/or in the nostrils and lungs, and of course leave a trail. The

particles can be made so that immediate detection of even a few particles with a simple lamp is possible. To identify conclusively that the particles came from the

violated secured space, the particles roust be analyzed with a microspectrofluorimeter

There are only a few techniques available for identification involving

measurement of elanental concentration ratios of rare earths within single particles

[1]^. Electron or ion probes are expensive, but can be used in this application.

Very sensitive analytical techniques such as activation analysis could be used, but

the availability of facilities is limited. Other sensitive methods which dissolve

the sample could be used, but the cost of analyses at the expected resultant 100

pg/mL of solution is high due to the technical difficulties in analyzing at such low

concentrations

Producing and measuring luminescence of particles involve the use of a special

microscope in which particles are irradiated with ultraviolet (UV) light. This UV

energy excites the luminescent substance causing it to emit radiation of a different

wavelength than the incident radiation. Measurement of the emission or luminescence

spectrum verifies the identity of the substance and in this work the ratio of

specified peaks in the emission spectrum of two luminescent substances provides the

unique identification of the particle. Peak intensities in the emission spectra are proportional to the concentration of the luminescent species. Spectra can be

obtained frcxn rare earth glass spheres as small as 10 um in diameter.

There has been a limited amount of work done on this idea in the past.

Luminescent particles have been incorporated into explosives for the purpose of

tracing their origins. The Aerospace Corp. [2] contracted with Westinghouse Corp.

and 3 M Corp. to produce appropriate particles. Westinghouse used ceramic spheres

with a Ivuninescent coating, while 3 M used irregular-shaoed particles made of

layered luminescent substances. The layered system provides for a large number of

possibly unique codes, but the particle sizes are quite large, often greater than

100 um. This may limit dispersal except when explosives are used. Many of the

luminophors that were used in these systems were organic compounds whose

luminescence can be expected to fade when exposed to sunlight.

The glass spheres selected in this project incorporate the rare earth

luminophors as a solid solution and are more stable than the organic species. In

addition, it is much easier to detect via microscopic examination completely

spherical particles (as contrasted to the irregular shape of the layered particles)

from the various odd shapes of particles normally found in common house dust, for

example. Although no studies were made during this project, it is suspected that

there will be minimal dangers from particles inhaled into the lungs or nasal

passages. This assumption is based upon the fact that the surface composition of

the glass sphere, unlike the coated ceramic spheres, is mostly Si02 , which is like

road dust or sand, and it has been established to be relatively safe.

A benefit from this study has been the generation of a luminescent particle as a reference material . There is a need in several areas of science for an accurate

calibration of instruments that roeasiire luminescence intensity produced by small

particles. For example in the field of immunocytology , the use of a luminescent tag

of blood cells is commonplace, but there is a need for calibrating the apparatus

which measures the intensity of luminescence from a single cell. The glass spheres

developed in this work are ideally suited for this. They are very stable,

spherical, and can be measured reproducibly

The five phases identified in this project were carried out in sequence with the

understanding that a satisfactory conclusion would have to be derived from each

phase before beginning the next one. The phases are:

1. Identify and characterize candidate particles, fabricate some of the

promising ones, and test their properties of luminescence (sec. III).

• 1Numbers in brackets refer to references on page 65.

2
2. Explore the extent of unique types of tagged particles that can be used (sec.

IV) .

3. Investigate various methods for particle deployment (sec. V).

4. Evaluate the sensitivity for detecting particles (sec. VI).

5. Evaluate the effectiveness of the system by simulated field tests (sec. VII)

The subsequent sections of this report briefly describe the field of

microspectrof luorimetry , the work done in each of the phases, and finally, the

potential and future development of the system.^

II. SPECTROFLUORIMETRY

A. History, Description, and Application

The photoluminescence of molecules was first noted by Monaides in the early

1500 's [3], and in 1852, Stokes [4] correctly described this phenomenon and labeled it

fluorescence. Luminescence is a general term that includes phosphorescence and

fluorescence and will be used throughout this report. Luminescence techniques are

extremely sensitive and selective, and are used in both the macro or micro sense in

diverse disciplines including: environmental pollution, forensic science,

biochemistry, geochemistry, clinical chemistry, molecular biology, and cytology.

It is the application in these latter areas that has led to the development of

microscopic techniques and associated instrumentation to measxire luminescence.

Before addressing this topic, a brief description of the luminescence process and

its important variables is in order so that the bases for luminescent particle

selection and measurement are established.

Photoluminescence is the emission of radiation (light) by atrans, ions, or molecules excited by an external source (in this case, excitation is through photon

absorption). A simplified energy diagram, presented in figure 1, illustrates the

basic processes of radiation absorption and emission.

The ground state (Sq)

,

the excited states S^, S2 and the vibrationalrotational levels (for molecules) designated by 0, 1, 2 n are shown schematically

in figure 1. Radiation is absorbed in the ground state Sq. The molecule, atom, or ion is raised to some excited state, e.g.. Si or S2

. In general, the molecule, atom,

or ion loses energy through interaction with adjacent atoms, ions, or molecules and

thus reverts to the lower energy levels. Two examples of this process, called

internal conversion, are represented by I^, in figure 1. Internal conversion is

usually quite rapid with respect to photoemissxon from excited states higher than S^,

and consequently little photoemission is observed from these states. However, the

time for 1^, from to Sq is of the same order of magnitude as photoemission II

because Si and Sq are separated by a fairly large AE, which in turn results in a low

probability for 1^,. Thus, Ic and II from Si to Sq occur within the same time frame

and are competing processes. Note: For simplicity, we have neglected other

processes competing with photosmission such as intersystem crossing and quenching.

Additional information can be obtained from C. A. Parker's classic work as well as other treatises [5]

Each ion, atom, or molecule has different energy levels which

result in different excitation (absorbance) and luminescence spectra.

The definition of luminescence emission is:

IL ^L ^a ( 1 )

2Mention of trade names throughout this report is made to improve clarity of

presentation and does not constitute endorsement by any agency of the

U.S. Government.

3
where 1^ rate of light emitted in photons/s,

< quantxjm efficiency of luminescence/ and

> rate of light absorbed in photons/s.

The light absorbed can be shown to be

-kc'i

(1-e ) ( 2 )

where Iq is the intensity of the excitation radiation,

k is the cross-sectional area for absorption,

c ' is the concentration of the luminophor in roolecules/cc , and

i is the length of the absorption path.

Substitution of eq (2) into eq (1) followed by power series expansion and units

conversion yields

It = In (2.3 tct) r, _ 2.3 ECi ^ (2.3 tcl)^

1_^ 2 6 * Ql (3)

Tfjhere e is the molar absorptivity and c is the concentration in moles/L,

UJ

>-O

cc

lU

z

LU

S2

S1

So

Figure 1. Simplified Jablonski energy level diagram showing the processes of photon

absorption (1^^) , internal conversion (1^) , and luminescence (I^) .

4
A dilute solution approximation can be made with little error [5(a)] yielding a simplified eq (4)

= Iq (2.3 ecDQj^ (4)

which shows that the radiation emitted by a solution is proportional to iQf Qlf sind

the absorbance which equals cc^. Increasing any of these parameters increases the

intensity of the emitted radiation. For the same species in the same matrix, £ and

Ql are usually constant* Assuming Iq and A also to be constant, the equation

defining luminescence or the radiation emitted by a solid or solution simplifies to:

II “ C (5)

which specifies that the intensity of the emitted radiation is proportional to the

concentration of the luminescent species.

As a result of having different energy levels, each luminescent material has an excitation spectrum (absorbance spectrum, la as a function of wavelength, 1) and an emission spectrum (Ij, as a function of X) that is indicative of the energy levels

within the molecule or ion in that particular matrix. These spectra may be measured

by using a spectrofluorimeter . A simplified schematic of a spectrofluorimeter is

given in figure 2.

Figure 2. Schematic for a spectrofluorimeter ,* M can be monochromators or selective

filters, Xjj = exciting wavelength, emission wavelength.

5
Radiation from an excitation source (xenon arc, mercury arc, laser, etc.)

is selected by (bandpass filters or monochromator) and is absorbed by the sample.

Radiation is emitted by the sample (luminescence) and the spectral distribution

of this radiation is analyzed by M2 (bandpass filters or monochromator) and detected

by a photomultiplier tube. For the purpose of this discussion, and M2 are considered to be monochromators such that wavelengths for and Xm with fairly

narrow bandwidths (half -height bandwidths, hhbw) of 1-16 nm can be easily measured.

Filter hhbw's are fairly broad (10-200 nm) except for special narrow-bandpass

filters. An emission spectrum is obtained by scanning M2 while holding Mj constant

at some wavelength Xx at which the species absorbs radiation. Similarly, an

excitation spectrum is obtained by scanning M]^ while holding M 2 constant at some wavelength Xj^ at which the species emits.

Since different species have different excitation and CTiission spectra, and

since the intensity of the emission (or excitation) spectrum is proportional to

species concentration, the luminescence spectrum and the relative intensity cf that

spectrum can be used to identify a specific luminophor and thus a specific particle.

If two luminescent ions with different emission spectra are dissolved in a glass

that is fabricated into a sphere, it is probable that the sphere can be

unambiguously identified using spectrof luorimetric techniques. In this case the

ratio of the major emission peak heights for each luminophor would be deteimiined by

measuring the combined emission spectrum and comparing it to known spectra for

standard luminophor-doped glass spheres. To accomplish this, a microspectrofluorimeter was designed and constructed from commercially available parts.

B. Description of the Microspectrofluorimeter

A microscope (Orthoplan, Ernst Leitz Wetzlar GMBH, W. Germany) equipped with a Ploemopak vertical illuminator [6] for epi -illumination (incident excitation) and a microscope photometer (MPV, E. Leitz) was used as the basic microspectrof luorimeter

(fig. 3) . The excitation radiation, produced by a xenon (150 W) or mercury (100 or

200 W) arc Icunp, is passed through a BG 38 red suppression filter and through a Ploem illuminator cube system consisting of bandpass (a) and barrier filters (c) and

a dichroic mirror (b) . The bandpass filter transmits only the UV-blue exciting

radiation. The dichroic mirror reflects the UV-blue exciting radiation and

transmits the longer wavelength radiation emitted by the sample. The barrier filter

is used to screen out the scattered excitation radiation while transmitting the

longer emission wavelengths. The optical characteristics of the filters and

dichroic mirrors in the cubes used in this work are summarized in table 1. The

selected excitati-on radiation then passes through the microscope objective (25 and

50 magnification with numerical apertures of 0,50 and 0.80, respectively) and

irradiates the glass sphere. The luminophor eibsorbs the exciting radiation and

emits characteristic radiation at longer wavelengths ( > 400 nm) . The emitted

radiation is collected by the same microscope objective, passes through the dichroic

mirror, barrier filter, a mea.suring aperture, focusing optics, a scanning

monochromator (0.1 m grating monochromator GM 100, Schoeffel Instruments, Westwood,

NJ),and finally impinges on a photomultiplier tube (S-20 response, EMI 9659QB

extended red, EMI Electronics, Middlesex, England). The signal is amplified and

directed to an x-y recorder which plots a cuirve of intensity vs. emission

wavelength. Alternatively, a voltage to frequency converter (V-F) and a multichannel analyzer (MCA) operated in a multiscaling mode can be used to record

the emission spectrum. To increase signal reproducibility, the ratio of the

luminescence intensity to the intensity of the excitation radiation can also be

directed to the recorder or V-F converter - MCA combination. Note: Detailed

operating instructions for this instrument are given in a svibsequent section.

III. PREPARATION AND EMISSION CHARACTERISTICS OF

VARIOUS LUMINOPHOR-DOPED GLASSES

A, Glass Preparation

The glasses and the glass spheres or beads used in this study were prepared by

the Inorganic Glass Group at the NBS. The appropriate weights of the individual

metal oxides were melted and stirred at 1000-1100“C until a homogeneous mixture was

obtained. The melt was poured into a form and allowed to cool slowly to produce a block of glass of approximately 350 g. About 100 g of the block was crushed using a stainless steel mortar and pestle type of apparatus until jagged particles 200 Pm

and below were produced

6
Two base glasses were used that have low Intrinsic luminescence properties.

Their compositions are in weight percent: Base 1 • Si02 - 68.17, Na20 - 14.07, BaO

- 11.50, and ZnO - 6.26; Base 2 « SiOo - 39.00, BaO - 44.00, and ZnO - 17.00.

(Note: Base glass #1 %rats used previously [7]). Luminophors such as the lanthanides,

uranium, copper, etc., which emit in the green to red spectral regions, were

substituted in specific amounts for part of the BaO component.

B. Emission Characteristics

The emission spectra for several of the luminophor glasses are given in figures

4 and 5 . The combinations shown as well as others could be used to provide hundreds

of different, uniquely identifiable spheres by varying the luminophors and the ratio

of the luminophor concentrations

x-y RECORDER

LIGHT

SOURCE

Figure 3. Diagram of microspectrof luorimeter used in this work.

7
Table 1. Optical characteristics of the Ploemopak filter cubes

used for the selection of excitation radiation

Cube

Excitation range

50% transmittance Component

Approximate wavelength

in nm for designated

transmission (%)

2 50

A UV 340-380 nm Bandpass filter 330 340

390 380

Dichroic mirror 380 400

Barrier filter 415 430

D UV + violet 355-425 nm Bandpass filter 340 355

435 425

Dichroic mirror 420 455

Barrier filter 445 460

G UV + violet + blue Bandpass filter — 350

350-460 nm Dichroic mirror — 460

Barrier filter 510

515

Preliminary ccmipositions of the glasses containing tvo luminophors are

summarized in table 2, and the emission spectra from these specific melts are given

in figures 6 through 18. From these spectra, it is obvious that melts K-1031, K1032, K-1064 , K-1124 , K-1125 , and X-1128 containing the Tb-Eu, Tb-Sra, and U-Eu

lumincphor combinations could be easily developed to produce uniquely identifiable

glasses. Dysprosium containing melts K-1078 , K-1087 , and K~1126 may be suitable for

use but further work has to be done. On the other hand, melts containing copoer (K~

10^5, K-1079) would need additional development to produce a glass with discernible

copper luminescence. The major problem is that luminescence from copper-doped

glasses is produced by copper in the +1 oxidation state [8]

Procedures for

producing and stabilizing Cu'*’^ glasses would have to be developed. The melts

containing and Sm'*'3 (K-1077, K-1079) have emission peaks which overlap

extensively and although these two luminophors could be used together , sophisticated

instrumentation with extremely high spectral resolution would be needed to obtain

usable spectra

C . Testing of Final Glass Composition

Based on the above data and other considerations such as ease^of material

handling and glass preparation, the glass containing a combination of Tb^"^ and Eu^t

(melt K-1125) was chosen for extensive testing. A series of glasses with varying

Tb3+ and Eu3+ concentrations was prepared and their compositions are summarized in

table 3

8
100 r

Figure 4. Luminescence spectra of glasses containing 0303 (curve A) and EU2 O3 (curve B) ; (2.5 mole %) ; cube A; photomultiplier voltage; curve A • 900 V, curve B 890 V.

9
100 r

Pigur

Wavelength (nm)

e 5. Lximinescence spectra of glasses containing Tb203 (curve A) and Stn203

(curve B) at 4.0 and 1,1 mole percent, respectively; cube A; photomultiplier voltage; curve A » 800 V, curve B » 930 V.

10
Table 2.

Compositions

of

preliminary

glass

melts

03 CM 00 <a< VO 0 0

CM rH ov in 0 VO »H • • • • • 1 1 • 1 1—

in rH in CO 1 CO 1 1 0 1 00 1 VO rH rH rH

Ui

CO ov in CO 0 0

CM r- rH ov in in 0

rH • « • • 1 • 1 • 1 1 iH in rH in CO 1 rH 1 CM 1 1 r~ 1 VO rH rH rH

ui

in VO CM in 0 0

CM 0 00 CO 0 0

iH 1 1 1 'a* rH m CO CM CO 1 1 1 1 VO 1 VO rH rH rH

Ui

'9' TT rH rH 0 0

CM iH ov in 0 0

»H • • • • • 1 • 1 1 1 iH in rH in CO CM CM 1 1 1 in 1 VO rH rH rH

03 r~ 00 rH CO 0 c 00 00 rH r- VO

(U 0 • • • 1 1 1 • 1 • o rH VO CO 1 1 1 CO 1 0 'a' 1 n rH rH

a> Ui 04 p 0 r' VO VO r~ X CO 0 ov in VO

O' 0 • • • 1 1 1 • • 1 iH in OV <0 1 1 1 CO 1 CO 0) 1 cn CO rH rH G o^ 0 OV CO CO OV 0 0 M* 0 r r 0

•H 0 • • • 1 1 • • 1 1 • p iH VO CM CO 1 1 CO CO 1 1 0 CM •H 1 cn rH rH

COQ

Ui £ 00 0 'a* CO ov 0 r' r r» xr VO 0 0 • • • 1 1 • 1 • 1 1 00 CO 1 1 m 1 1 rH V 1 CO ro rH rH u

(U r" 0 VO 00 ov

C" VO ro in VO c 0 • • • 1 1 • • 1 1 1 rH in rH CO 1 1 in CO 1 1 1 0 p 1 fn 'a' rH rH

fH

0)

•sr in CO ov

VO rH (O <3V VO 03 0 • • • 1 1 • 1 1 • 1 rH VO CO CO 1 1 in 1 1 0 1 ov 1 CO •a* rH

Ui

in VO 1^ rH o>

>vr CO C' 0 0 0 • • • 1 1 1 1 1 • iH VO CO 1 1 in 1 1 1 0 00 1 (O M" rH «

CM CM VO 0 rH CM

ro OV in C3V ov 0 • • • 1 • 1 • 1 1 1 rH in CM CO 1 CO 1 CO 1 1 1 1 fO rH 1 «

rH \D CO ro 0 ov

cn a\ CM in CO oa* 0 • • • 1 • • 1 1 1 1 rH <<r CM m 1 m in 1 1 1 1 VO 1 1 m rH 1 G -« j 0 £

•p •H P c to P

<u to 4J c O-H 0 0 CO CO m P £ <U a CM 0 0 0 0 0 00 3 u a e 0 0 0 CM CM CM CM CM 0 0 tJV'-' to 0 •H nj G nJ 3 £ &

CO 3 •H

u cn CQ Z E-*

M COD

U IP

11
100 r

Wavelength (nm)

Figure 6 . Luminescence spectrum of glass, melt K-1031 containing EU2O3 , peaks

labeled 1 emd Tb203 , peaks labeled 2; 1.5 and 1.0 mole percent,

respectively; cube A; photomultiplier voltage; curve A = 970 V, curve B *» 1300 V; monochromator slits » 0.5, 0.5 mm.

12
100 r

Figure 7. Luminescence spectra of glass, melt K-1032, containing Sm203 , peaks

labeled 1 and Tb203 , peadcs labeled 2; 1.0 mole percent each;

photomultiplier voltage and cube used; curve A « 1020 V, cube A;

curve B » 960 V, cube D; monochromator slits = 2,2 ram.

13
100 r

Figure 8 . Luminescence spectra of glass, malt K-1045, containing EU2O3

, peaks

labeled 1 and CuO; 1.5 and 0.1 mole percent, respectively; photomultiplier

voltage and cube used; curve A « 840 V, cube A; curve B «« 1200 V, cube A;

curve C » 830 V, cube D; monochromator slit widths » 2,2 mm

14
Wavelength (nm)

Figure 9. Luminescence spectra of glass r melt K-1064, containing EU2O3 peaks

labeled 1 and 1)309 peaks labeled 2; 1.5 and 0.3 mole percent,

respectively; photomultiplier voltage and cube used; curve A 780 V,

cube A; curve B 730 V, cube D; curve C « 607 V, cube G; monochromator

slit widths « 2,2 mm.

15
100 r

Figure 10. Luminescence spectra of glass, melt K-1077, containing BU2O3 , peaks

labeled 1, and Sm203 ; 1.5 and 1.0 mole percent, respectively;

photomultiplier voltage; curve A « 767 V, monochromator slit widths

2,2 mm.

16
Figure 11. Lxunlnescence spectra of glass, melt K-1078, containing EU2O3 , peaks

labeled 1 and 07303 peaks labeled 2; 1.5 and 2.0 mole percent,

respectively; photomultiplier voltage and cube used; curve A = 1100 V,

cube A; curve B •= 1500 V, cube A scale expanded XlO; curve C = 1130 V,

cube D; monochromator slit widths = 2,2 mm.

17
100 r

Wavelength (nm)

Figure 12. Luminescence spectra of glass, melt K-1079, containing BU2 O3

, peaks

labeled 1; Sm203 » and CuO; 1.0, 1.0, and 0.1 mole percent, respectively;

photomultiplier voltage and cube used; curve A = 890 V, cube A; curve B = 890 V, cube D; monochromator slit widths » 2,2 mm.

18
100 r

400 450 500 550 600 650 700 750

Wavelength (nm)

Figure 13. Luminescence spectrum of glass, melt K-1087, containing Sm203 , and

peaks labeled 2; 1.0 and 2.0 mole percent, respectively;

photomultiplier voltage = 1460 V; monochromator slit widths =2,2 mm.

19
100

Figure 14. Luminescence spectra of glass, melt K-1088, containing U 3O9 , peaks

labeled 2 and Sm203 , peaks labeled 1 ; 0.6 and 1.0 mole percent,

respectively; photomultiplier voltage and monochromator slit widths; curve A = 1300 V, 2,2 mm; curve B = 1700 V, 0.5, 0.5 mm.

20
100 r

0 ^ \ \ \ \ ^ L_

400 450 500 550 600 650 700 750

Wavelength (nm)

Figure 15. Luminescence spectrum of glass, melt K-1124, containing Tb203 , peaks

labeled 2 and Sm203 peaks labeled 1; 2.0 weight percent; photomultiplier

voltage = 1350 V; monochromator slit widths = 0.5, 0.5 mm.

21
100 r

2

I

Wavelength (nm)

Figure 16. Luminescence spectrum of glass , melt K-1125, containing Tb203 , peaks

labeled 1, and EU2O3 peaks labeled 2; 2.0 and 3.0 weight percent,

respectively; photomultiplier voltage « 1250 V; monochromator slit

widths “ 0.5, 0.5 mm.

22
100 r

Wavelength (nm)

Figure 17. Luminescence spectrum of glass, melt K-1126, containing Dy203 , peaks

labeled 1, and EU2 O3 , peaks labeled 2; 2.0 and 1.5 weight percent,

respectively; photomultiplier voltage « 1490 V; monochromator slit

widths 0.5, 0.5 mm.

23
100 r

2

Wavelength (nm)

Figure 18. Luminescence spectra of glass, melt K-1128, containing U3O3, peaks

labeled 1, and EU2O3, peaks labeled 2; 0.6 and 3.0 weight percent,

respectively; photomultiplier voltage and monochromator slits; curve A « 1000 V, 2,2 mm; curve B »> 1380 V, 2,2 mm.

24
Table 3. Composition of Tb202-Eu20^ doped glasses

Melt

Composition, weight percent

TbaOj EUjOj SiO^ Na20 BaO ZnO Tb/Eu

K-1192 3.00 — 66.13 13.65 11.25 5.97

K-1198 5.00 0.50 64.43 13.29 10.96 5.82 10.0

K-1200 5.00 1.00 64.09 13.22 10.90 5.79 5.0

K-1202 4.00 1.00 64.76 13.37 11.02 5.85 4.0

K-1186 3.00 1.00 65.44 13.51 11.14 5.91 3.0

K-1201 3.00 2.00 64.76 13.37 11.02 5.85 1.5

K-1125 2.00 3.00 64.76 13.37 11.02 5.85 0.67

K-1194 — 2.00 66.31 13.79 11.37 6.03 —

The emission spectra for melts with Tb^"*" (K-1192) or (K-1194) are given in

figure 19. The assigned electronic transitions and approximate maxima for the peaks

are svunmarized in figure 20 [9] . The major transitions that are immediately

identifiable to Tb3+ are ^D4-^'^Fi^2,3,4,5,and 6

5

Do'»’'Fi 2,3/4,5faQd 6*

Additional transitions for each luminophor have been seen, (e.g., for Eu3+

^FS 6' ^^^1-^^Fo, 1,2 and for Tb3+, ,Z, A, S 6^' however, under present

instrument operating conditions, these are not^ visible. The transitions shown in

figure 20 indicate that several less intense peaks in the Tb^"*" spectrum overlap

intense peaks in the Eu3+ spectrum and, as expected, cannot be identified in a melt

containing both Ivminophors. The major peaks are immediately discernable in figure

21 at 542 nm (Tb2+) and 610 nm (Eu34>) , These two peaks are those on which the ratio

measurements will be made. Emission spectra for the eight melts listed in table 3 are given in figure 22 to show the relative peak max^a; although some contributions

to the 610 nm Eu^+ peak intensity are made by the Tb^"*" peaks at ''<58 9 and '^>623 nm, no corrections for these contributions will be made since a) the ratio is a relative

measurement, and most importantly, b) the peak ratios are dependent on instrumental

conditions and must be made re lative ^ the ratio for known standards .

D. Instrumental Factors Affecting Peak Ratios

3+ 3+ 1. Excitation Bands. Typical excitation spectra for Eu and Tb in glasses

are given in frgure 23. The excitation bands selected by Cubes, A and D also are superimposed as rectangular areas on the two spectra at the approximate wavelengths

(also see table 1). As can be seen, if Cube A is used to select exciting radiation,

very little or no radiation is passed to excite Eu^"*" through the major absorbance

peak at 394 nm. Use of Cube D, on the other hand, allows radiation of this

wavelength to impinge on the sample. The use of Cube D would be expected to yield a larger Eu/Tb peak ratio measurement than the ratio obtained by use of Cube A. This

is verified by the spectra given in figure 24. Thus, use of different cubes to

select exciting radiation has significant impact on the measured peak ratios. In

addition, if monochromatic radiation at 394 nm rather than a broad band of exciting

radiation were used to excite a glass containing both Eu^"*" and Tb3+ ions, only a Eu3+

emission spectrum would be observed since Tb^+ has very little if any absorbance at

this wavelength. To show the relative effects of using Cubes A and D, the spectra

for all melts using Cube A are illustrated in figure 25, while those obtained using

Cube D are given in figure 22.

25
100 r

Wavelength (nm)

Figure 19. Luminescence spectra of glasses containing solid line, Eu^"*" dashed

line, cube D; monochromator slit widths *= 2,2 mm.

26
28

26

5D3

IO

eoO

24

22

20

QC

OQ

Z3

Z

111 I

14

12

10

8

6

4

2

0

5Di

5Do

Figure 20. Partial Jablonski energy level diagram of europium and terbium showing

the transitions observed in the luminescence spectra of figure 19.

27
100 r

Figure 21. Typical luminescence spectrum of a glass doped with Tb203 , peaks labeled

1, and EU2O3 , peaks labeled 2; 3.0 and 1.0 weight percent, respectively;

photomultiplier voltage » 1000 V; monochromator slit widths 2,2 mm.

28
PLOEMPAK CUBE D, NO FILTER IN LAMP HOUSING

100 W HG LAMP

K-1200

K-1202

J\i VL M.

K-1186 K-1201 K-1125 K-1194

U J VL ^

Figure 22. Luminescence spectra of glasses containing and of using cube D

(table 1). The numbers refer to the melt designations listed in table 3.

Each spectrum was recorded at different gain settings. Relative peak

intensities in each spectrum are illustrated.

29
RELATIVE

INTENSITY

300 400 500

WAVELENGTH, nm

Figure 23. Typical excitation spectra for (dashed line) and (solid line)

in a glass matrix. Approximate excitation wavelength bandwidths are

shown by the boxed in areas using filter cubes A and D (see appropriate

cross dashing and table 1)

30
CUBE A CUBE D

WAVELENGTH

K-1186

Figure 24. Luminescence spectra of melt K-1186 using cubes A and D (table 1) to

select excitation radiation and show difference in peak ratios for the

same melt.

31
PLOEMPAK CUBE “A”

MELTS:

K-1192 K-1198 K-1200 K-1202

K-1186 K-1201 K-1125 K-1194

J M.

^

Figure 25. Luminescence spectra of all glass melts of composition listed in table 3

using cube A (table 1) . Each spectrum was recorded at different gain

settings. Relative peak intensities in each spectrum are illustrated.

32
2. Age of Excitation Source . Aging of the excitation source generally reduces

the intensities of the iKorter wavelengths; thus, the Eu/Tb peak ratio would

increase because the Tb^"^ excitation would decrease more than the Eu^'*' excitation.

3. Monochromator Slit Widths . In general, as the monochromator slit widths are decreased, resolution increases and the luminescence intensity passing through the

monochromator decreases. This phenomenon is illustrated in figure 26 which shows

the emission spectra of a single melt using four different monochromator slits.

Specifically, as the slit width decreases from 2 mm to 0.2 mm, the resolution

increases from 16 nm to 1.6 nm (hhbw) and the Tb^"*" peaks at ~475 and ~545 nm are split into double peaks. Similarly, shoulders appear at ~580 and ~615 nm on the Eu^"*" peaks which were not apparent previously. As a result of this splitting, the Eu/Tb

peak ratio changes from "“1. 30 for 2 - mm slits to ~0.7 for 0.2-mm slits. The

luminescence intensity decreases to such an extent that the signal-measuring

equipment must be operated at close to maximum gain thereby reducing the signal to

noise ratio.

Thus for comparative purposes, it is recommended that the monochromator

resolution be as large (~16 nm hhbw) and sensitivities be as low as practicable to

obtain spectra that have broad peaks and low noise to facilitate the peak ratio

measurements

4. Detection System Responsivities . The optical transmission or sensitivity

characteristics vary among optical devices Ce.g., microscopes, monochromators,

etc.). Photomultiplier responses produced by different manufacturers vary even for

similar equipment produced by the saune meinufacturer due to production differences

and component aging. Thus, different peak ratios will be obtained as a function of

equipment or time.

In summary, it is imperative that a series of standards (spheres of known

composition) is available to be used for the unambiguous identification of an

un)cnown sphere. The emission spectrum of the unknown must be measured on the same instrument, with the same instrumental parameters, and close in time to that of the

standard so that calculations and comparisons of the Eu/Tb peak ratios will not be

instrument or time dependent.

To facilitate the following discussions of sphere parameters, specific

measurements were made using 2 -ram monochromator slits (giving a hhbw resolution of

~16 nm) with a series of melts, or a particular melt being used at the same time on

the same instrument. Thus the ratio measurements and discussions relate only to the

effect of that single pareimeter on the stability of the spheres, luminescence

intensity as a function of Ivuninophor concentrations, sphere size, or instrument

parauneters

E. Sphere Preparation and Characterization

1. Sphere Preparation . Initially the spheres were prepared by placing the

crushed glass particles in a "hopper” attached to an oxy-acetylene flame gun. The

particles were passed through the flame at temperatures of 2000-3000 *C and were

collected in a water-filled container. Determination of the Eu/Tb ratios for

various sized spheres (10 - 100 um) showed that the ratios were dependent on sphere

dicimeter . It is believed that the high formation temperatures caused selective

luminophor loss at smaller sphere sizes resulting in individual luminophor

concentration differences and thus different luminophor intensity ratios.

An alternative procedure was developed to produce spheres 110] . This process

prepares spheres by passing an air suspension of crushed glass particles through an oven with an equilibration temperature at which the glass just becomes fluid (900-

1400 ®C, depending on glass composition) . The particles are formed into a spherical

shape by surface tension. The particles are collected on filters and passed through

appropriate sieves to yield glass spheres of specific size ranges.

2. Luminophor Homogeneity . Since the total sphere volume and thus all

luminophors are excited by the source, a mass proportional emission flux is

obtained. General eq (4) for the emission flux may be modified to yield

1^ = 1^1(2.303 el m/v)l (6)

33
100 r

400 450 500 550 600 650 700 750

Wavelength (nm)

Figure 26. Luminescence spectra of glass melt K-1202 as a function of

monochromator slit widths using filter cube D (see table 1) ; A = 2 mm,

B = 1 mm, C = 0.5 mm, D = 0.2 mm.

34
where concentration, c is replaced with ra/v (mass/volume) . At a constant luminophor

concentration, the mass of the luminophor should be proportional to the cube of the

sphere diameter

m « 4/3irr^ (7)

and since II * mass from eq (6)

,

then II has to be proportional to r^. This

relationship is verified for a melt in figure 27 which shows a straight-line

relationship between II for the Tb and Eu peaks versus the sphere radius cubed. As

shown in figure 28 , the Eu/Tb ratio is independent of the sphere size with this

method of bead preparation. The deviations from the straight line for beads with

radii <5 ym are due to high measurement imprecisions resulting from excessive

electronic noise and errors in radius measurement. It is recommended that beads

with radii in the 10-20 ym range be used for optimal ratio measurements.

(RADIUS)3,

Figure 27. Plot of relative luminescence intensity versus

the third power of particle radius.

35
qi/ng OliVU MVBd N0ISSII/\I3

36
If the Eu/Tb peak ratio is plotted versus the Iwt % Eu/wt % Tb] , a linear

relationship is observed (fig. 29) except for roelt K-1125 (i.e., Eu/Tb concentration

ratio » 1.5). We have not been able to establish a reason for the deviation

observed for melt K-1125.

F. Reproducibility of Measurements

The identification of origin of any particular glass sphere doped with a pair of

rare earths will depend upon a comparison of the concentration ratio of rare earths

in the unknown glass sphere with that of a standard or known sample of the original

tagged material. Consequently, it is important to know how reproducibly the ratio

of rare earth concentrations in the glass can be measured.

Determinations were made on the effects of particle size, concentration ratio,

operator, and measurement sequence on the variance of the measurement. An analysis

showed no significant differences of variance from the parameter changes: sphere

sizes with diameters 20 and 40 vm; two different operators; two different rare earth

ratios (Codes K-1202 and 1198) ; and a comparison of within-day and between-day

measurements (table 4) . From these data it may be assumed that the method is fairly

robust, although more detailed studies should be made. Pooling of all measurements

shows a measurement imprecision of about 1 percent relative standard deviation of a single determination based on a series of 16 measurements.

.o

"aUJO

<

QC

<

HI

Q.

o

(O

<0

UJ

Figure 29. Plot of europium-terbium ratio versus the weight percent

ratio of europium to terbium

37
Table 4. Effect of changing parameters of the measurement system

Peak ratios^

Variable/value Average Standard deviation

Operator 1 1.221 .018

Operator 2 1.219 .011

Particle size 41 ym 1.218 .016

Particle size 19 um 1.222 .013

Measurement (within day) 1.216 .018

Timing (between days) 1.224 .010

Code K-1202 1.222 .016

Code K-1198 1.217 .013

^Average calculated from eight replications; standard deviation

is for a single determination. Numbers for code K-1198 were

normalized to that of K-1202

IV. EXTENT OF UNIQUE TYPES OF PARTICLES

It is important to determine how many secured spaces can be identified uniquely.

Therefore, it is necessary to determine the reproducibility that can be obtained in

the measurement of the quantity of luminophors from the spectral data. As stated

before, it is best to measure the ratio of the peak intensities of the two

luminophors. In addition to the tests made in the previous section, measurements

were made to determine what effect various methods of quantifying the spectral data

had on reproducibility. The reproducibility of the ratios are measured by using

three methods of processing the data: 1) graphical inspection, 2) digital peak

height measurement with interpolation, and 3) a least squares fit of a Gaussian

function to the data in the region of the peak. All showed a relative standard

deviation of about 1 percent. It makes little difference upon the method used for

processing the data at the level of reproducibility inherent in these data. It is

always preferable if possible to process the data by computer in order to reduce

blunders

Based upon the analysis of variance and measurements of reproducibility in the

previous section, one can expect to be eible to distinguish a difference in peak

intensity of about 3 percent. Therefore, each peak can be divided into 'V'30 parts.

This would imply (30)* or 900 unique ratios less the redundancy generated by even multiples. A simple computer prograun was written to determine the number of unique

ratios by eliminating redundancy. For a two-component system at 30 units of

concentration for each component, 550 unique codes can be generated. At 20 units,

250 codes can be generated. For a three -component system at 10 units for each

component, 840 unique codes can be generated.

38
V. TYPES OF PARTICLE DEPLOYMENT

In order to investigate the efficiency of detection of these particles by

microscopic examination, it was necessary to develop a system that could put a knovm

concentration of particles randomly on a glass slide. This involved a study of

possible particle deployment methods.

Two media, liquid or gas, can be used for distributing the particles within the

secured area . The liquid medium would be very useful for large areas and could be

distributed as a mist or heavy spray. The liquid can contain an adhesive agent to

aid in retaining the particles on a surface. Quite often, however, the particles

will be electrostatically attracted to charged surfaces (e.g., nonconductors) and

will adhere without an adhesive.

We elected to investigate the use of gaseous dispersion. This procedure has the

attractive feature that the particles can be stored indefinitely as a powder in the

dispersing mechanism with little likelihood of degradation that might result from

dispersing the particles in a liquid. The dispersal technique selected for the

conduction of the sensitivity experiments is described in the next section.

A. Apparatus and Experimental Procedure

A chamber approximately 1.5 m (5 ft) on each side was fabricated of aluminum

angles. It was assembled so that a plastic sheet could be mounted totally within

the frame such that no frame member was exposed to the inside of the chamber. In

addition, sponge rubber strips were mounted on the bottom of the frame to fit flush

with the floor which was also covered with a plastic sheet to make the floor of the

chamber. The top was designed to fit flush with the frame by applying sponge rubber

strips to the contacting surfaces. The center of the top contained a housing for

the disperser system and a small fan. The fan was to be used to generate positive

pressure within the chamber in the event that the plastic did not hang smoothly

against the frame. However, with the design used, no difficulty of this type was observed and the fan was not used. The chamber was designed so that the plastic

sheeting could be changed within a half hour

The system for dispersal is shown in figure 30. The circular disc (D) acts as a deflector of the gas {N2 ) stream and the particles are blown outward in all

directions. The flowing gas stream produces a doughnut -shaped vortex that helps to

mix the particles quite evenly over a space of about 2.25 m^ (25 ft^) . The dispersal

system is made of 1.3-cm (1/2-in) pipe with a 90* elbow fitting (P) that is loaded

with a known weight of particles. A quick release valve is used to release the gas.

The required amount of particulate matter was weighed directly into a section of the

dispersal device, which was then mounted in place with a standard pipe connection.

The dispersal device was aligned by eye to be parallel with the floor. Microscope

slides appropriately labeled were then placed on the floor of the chamber (fig. 31)

at the positions indicated in figure 32. The nitrogen pressure was adjusted to

5.50x10^ Pa (80 psig) and the gas was turned on with a quick-acting lever valve and

allowed to run for 5 s. The particles were allowed to settle for 15 rain.

(Preliminary tests indicated that at least 95 percent of all particles had settled

by that time.)

After the particles settled, the top of the cheunber was carefully removed so as not to jostle either the top or sides of the chamber. The chamber was then

carefully tilted to expose the floor and allow easy removal of the glass slides.

The slides were placed in a covered metal tray and transported to the laboratory for

microscopic examination. (Plastic containers are unsatisfactory because static

charges on the surface can draw particles from the glass slides.)

The chamber was constructed so that the plastic could be easily removed without

producing contamination. Repeated tests for conteimination of the top and sides of

the chamber showed the presence of very few spheres. The plastic sheet on the floor

of the chamber was replaced periodically. Evidently the chamber was large enough to

prevent electrostatic attraction of the plume of particles that was produced.

B. Distribution of Particles , The distribution of spheres measured as an average of five separate runs normalized to unit weight of dispersed spheres is illustrated in figure 33. (For

39
40

Figure

30.

Photograph

of

particle

disperser;

D

=

circular

deflector

disc;

P

=

pipe

fitting

in

which

particles

reside;

A

=

nitrogen

pressure

line.
Figure 31a. Particle dispersal system; plastic covered chamber

showing positions of microscope slides.

41
42

Figure

31b.

Particle

dispersal

system;

close-up

showing

microscope

slide

placed

on

inverted

50

mL

beaker
43

Figure

31c,

Particle

dispersal

system?

hop

of

chamber

showing

mounted

disperser

with

chamber

in

the

background.
44

Figure

32.

Diagram

showing

slide

positions

(dimensions

in

millimeters)
o

z

z

ccL CM

^

g

CO

45

Figure

3’J

.

Test

results

of

particle

dispersal.

Numbers

are

average

of

five

runs;

units

*:

particles/cm'^/g

{dimensions

in

millimeters)

.
specifications on the glass spheres, see the next section.) The pooled relative

standard deviation of the averages between runs using data from each of the nine

positions is about 13 percent. The pooled relative standard deviation of the

averages within a run and between positions is about 10 percent. The grand average

over the replicates and positions (45 measurements) is 896 particles cm"^ g~^ with a pooled relative standard deviation of 12 percent. These results are acceptable for

such a dispersal system.

VI. DETECTION OF SPHERICAL PARTICLES

It is important to determine an approximate lower level of concentration of

spherical particles that can be detected by microscopic excunination. The simplest

and best procedure for measuring the particles per unit area is to allow the

particles to fall upon a surface as uniformly as possible and to measure the number

of particles per unit area on a microscope slide. This was accomplished by using

the system described in the previous section.

There are a number of factors that will determine the detectability of particles

on the microscope slide. The size of the particles is very important, since the

maximum resolving power of a microscope is about 1 ym. The type, size, shape, and

number of the particles in the microscope field of view can adversely affect the

selection of the desired particles. A decided advantage in their detection is the

fact that the particles of interest are spherical and that they show a color (e.g.,

the luminescent radiation) when irradiated with ultraviolet light. However, it is

the spherical shape that is most important, because it will be demonstrated that

most house dust contains brilliantly luminescent particles when excited by

ultraviolet light, but their shapes are irregular.

A. Limit of Detection

One can predict a limit of detection based upon the following assumptions:

1. The particles are dispersed randomly on the microscope slide and the law of

probability for this system follows a binomial distribution.

2. Random fields of view (FOV) are inspected.

3. Eye fatigue limits the continuous inspection of a single microscope slide to

10 min.

4. Only a binary decision is considered; namely, the FOV under inspection has or

has not a single particle of interest.

5. A selected particle will always be positively verified as a particle of

interest after spectroscopic examination. Also, the observer is 100 percent

efficient in detecting the presence of a particle in a field of view. The

most practical size of particle to work with ranges between 10 - 20 ym in

diameter. We have arbitrarily selected to work with particles that are one division wide within the optical graticule (16.7 ym) . Therefore, calculations

are based upon that size.

One needs to determine how many FOV can be inspected in 10 min. In practice, if

the magnification was too high, eye strain increased significantly. It was found

that with an objective setting of 25X and a lOX eyepiece, one could inspect 150

FOV in 10 min. At this magnification there are 950 FOV/cm^. When there are a large

number of FOV, and the probability of finding a particle is small, the Poisson

distribution can be used. One can therefore calculate the minimum detectable

particle density from the Poisson probability distribution function

P = (np)^e-^P

(r) (8)

46
where

n = niamber of FOV measured

p = probability of finding a particle in a single FOV

r = the number of FOV's that are found to have a particle

= probability of finding that r FOV contain a particle

Eg (8) reduces to

P

( 0 ) = e -np

( 9 )

since r must be 0 at the detection limit, and p

= where N is the total

950

number of

particles per square centimeter. Substituting into eq (9) allows calculation of P(0)

which is the probability of observing no particles in 150 FOV's when the total

number of particles (in all 950 FOV's) is N.

If N = 4, P(Q. = .53; N = 7, P(q) - .33; N = 14, P/q) = .11? N = 19, P(n) = 0.05. These results demonstrate that the particle density N particles/cm''^ can be

quite low and still the probability of detecting a particle in the 150 FOV is

reasonable, e.g., at N = 4, I-P^qj = .47.

B. Particle Dilution Experiments

It was thought desirable to verify experimentally these numbers for the

detection limit. It was important to be able to distribute the particles as randomly as possible over the surface of the microscope slide. The test chamber

proved to serve very well for this purpose

One could experimentally measure the limit of detection by simply reducing the

amount of dispersed particles, but this was deemed to be inadvisable because as the

total mass of particles changed appreciably, it was suspected that the dispersal

properties might vary. Therefore, we decided to dilute the luminescent glass

particles with plain glass (blank) particles so that the total mass of particles

dispersed is constant. The only complication might be in the relative particle size

distribution. If the distribution were sufficiently different, the aerodynamics of

the gas dispersal system might affect the blank particles in a manner unlike that of

the luminescent particles. It was therefore necessary to measure the distribution

of particle size. The size distribution was measured by using the microscope with a calibrated graticule while measuring the size of a particle in a randomly selected

number of FOV. The data in table 5 represent those particles w’hich fall within

the size indicated + 2 ym which is approximately two standard deviations for a single measurement. A total of 795 blank particles and 145 luminescent particles

were measured. Table 5 shows the distribution relative to the nvimber density in

particles per square centimeter for four different sizes. The second column shows

the mass fraction which is calculated by multiplying the size distribution cell

number fraction by the cube of the particle diameter (assuming that the density of

all particles is the same), and dividing by the sum of the cell products. Since it

is relatively easy to detect 16.7 ym diam particles with the microscope, all

subsequent measurements will relate to this particle size. There is probably a significant number density below 4 um, but it is assvimed that they are insufficient

to affect the mass fraction. It can be seen from the table that the mass fraction

for the blank (nonluminescent) particles is 0.38 and for the luminescent particles

it is 0.61. For the dilution work it is necessary to convert from mass fraction,

which is the mode used for diluting the luminescent particles with blank particles,

to a number density because this is the measurement unit obtained from the

microscope. From the distribution data of dispersed blank particles in figure 33, one obtains a grand average of 896, 16.7 ym diameter particles per square centimeter

per gram with a relative standard deviation of the average of 12 percent based upon

45 measurements. Therefore, correcting for the mass fractions one should observe

(89 .6) xO .61/0 .38 = 144 particles with 16.7 ym diameter per square centimenter per

100 mg of luminescent particles dispersed. Results of distribution experiments

identical to that described in the previous section gave a grand average of 149

luminescent particles per square centimeter per 100 mg with a relative standard

deviation of the average of 18 percent based upon nine measurements. Thus, it

47
appears that dilution by mass and verification by number density is a workable

process

The dilution experiments were made by weighing out the proper amount of blank

glass particles and the luminescent-compound, doped-glass particles and by

thoroughly mixing them. An aliquot was then weighed into the dispersal sample

chamber

Careful inspection of the particle distribution on the microscope slides showed

no evidence of nonrandom particle distribution across the slide. Although a large

amount of replication was not done , no evidence for nonrandom variation between runs was observed.

Glass slides were positioned as described above at positions 3, 4, 5, 6, 11, 14,

15, 16, and 17 (see fig. 32) . Each dilution was dispersed as described. The slides

were then examined using a microscope, random FOV were selected, and luminescent

particles with a diameter of one division (16.7 um diam) on the reticule were

counted. The total area scanned was recorded and the number density calculated.

Effort was made to keep the area measured close to 150 FOV.

The blank glass particles that were used are called glass reinforcement filter

spheres and are distributed by Potters Industries, Inc., Carlstadt, NJ 07072,

catalogue number 3000. These were passed through U.S. Screen Number, minus 325.

Both types of particles (luminescent and blank) were observed to be very uniform

spheres

In figure 34 the results of the dilution experiments as a plot of the ratio of

luminescent/blank particle number density calculated from mass dilution vs. the

experimentally measured density ratio are shown in curve 1 . Although the scatter in

the data is significant, the experimental data agree with that calculated from the mass dilution within experimental error. At the lower ratios, the ability to

distinguish between 10-15 particles/cm^ and no particles is impaired as indicated in

curve 2 . It should be noted that at the calculated dilution to put 3 to 4

particles/cm^ on the microscope slide, five of the nine slides did show 1 particle

each. This agrees with calculations of the limit of detection.

Table 5. Frequency distribution of particles

Particle

diameter

size , (ym)

Number fraction of

particles

Mass fraction of

particles^

Blank Luminescence Blank Luminescence

16 .7 .07 .21 .38 .61

12.5 .13 .22 .33 .26

8 .4 .34 .31 .25 .12

4.2 .46 .26 .04 .01

^Calculated from number fraction, Fjj , and particle diameter, P,

for each (i)th cell of distribution.

F , = mass fraction of first cell,

ml

ml

48
CALCULATED

NUMBER

DENSITV

RATIO 80

50

40

30

20

10

0

. 06.3 .6 .9 1.2 1.5 1.8 2.1 2.4 2.7 3.0 3.3

EXPERIMENTAL NUMBER DENSITY RATIO

Figure 34. Curve 1 (dots) - Ratio of number density (16.7 urn diam) luminescent to

blank particles; calculated vs, experimental. Curve 2 (squares) - Experimental number density of luminescent particles vs. experimental

ratio of number density luminescent to blank particles.

49

NUMBER

DENSITY

LUMINESCENT

PARTICLES

(part/cni*)
C. Detection of Particles in a Background

A series of experiments was devised to determine if detectability was affected

by the presence of other particles. The first experiment involved the sequential

dispersal of luminescent particles and 1.6 g of nonlximine scent particles onto the

slides without slide movement. Measurements were made on each of five microscope

slides positioned at 3, 4, 5, 8, and 11 (see fig. 32). These data appear as rows 1

and 2 in table 6, respectively. Within the precision of these measurements, no significant difference can be observed.

A series of microscope slides were coated with a thin film of dust by pouring a thick layer of dust on the slide, then removing the bulk by tilting the slide and

lightly tapping its edge. These slides were then placed in the positions indicated

above for experiments 1 and 2, and luminescent spheres were dispersed over the dust

layer. This result is shown in row 3(a) of table 6. Row 3(b) shows the same experiment except that a dilution of luminescent beads with blank beads was used.

Appropriate normalization was made for the mass dilution ratios that were used , and

the corrected result to zero dilution is shown in parenthesis.

The final experiment was identical to 3 (a) except that a different dust was

used. One can conclude from these experiments that blank glass particles do not

affect detectability, but dust has the effect of reducing the efficiency of

detection to about 60 percent. This cannot be considered to be a large decrease in

efficiency and probably this lack of effect is due to the fact that the Ixjminescent

particles are spherical and have a very characteristic color while many of the dust

particles are brightly luminescent but irregular in shape. A photograph of a luminescent particle in house dust is shown in figure 35. Here it is seen to be

quite easy to pick out the circular shape from the background of odd shapes.

However, in figure 36 a perfectly circular shape is found. It is even close to the

correct color, but when the luminescence spectrum is measured (fig. 37) , it is clear

that it is not one of the tagged particles, but rather a sphere of pollen.

Table 6. Effect of background particles on luminescent particle detectability

Experiment

identif ication

Particle niimber

density

Percent

p/cm^g error^ error

1. Luminescent particles (K-1200) 864 75 9

2. Blank particle overlay 783 78 10

3. SRM dust

(a) K-1200 542 38 7

(b) K-1200 dil 311 (520)*^ (100)^ 19

4. House dust 444 72 16

^Errors are the standard deviation of the average of five

measurements

^Corrected to undiluted value.

50
Figure 35. Photograph of luminescent particles in house dust

Figure 36. Photograph of pollen grain and particles in house dust.

51
CUBE A

WAVELENGTH, nm

Figure 37. Luminescence spectrum of a pollen grain with K-1200

tagged particles for comparison.

52
D. Reflecting Glass Particles

A limitation of the particle tagging system is the long turnaround time from

Scimpling a given area to the microscopic verification that there are coded particles

in the sample. The best possibility would be to mount the apparatus in a mobile

truck.

This particle tagging system can be enhanced by using retroref lector beads [11]

Retroref lector beads, available from 3 M Corporation are coated with aluminum

over one-half of their surface. These particles act as miniature reflectors such

that light incident on the particles will be reflected close to the angle of entry.

Some studies of their optical properties show that the intensity of reflected light

is not very great, and therefore, for light intensity used in normal automobile

driving, some 60-100 m is the maximum distance that the reflected light can be seen

[12] . The light is returned in the form of a ring about the entry axis [13] . A

microscopic view of these retroreflecting microspheres that shows these rings is

given in figure 38.

Mixing these particles with the coded ones or half coating the encoded particles

would allow their detection by using a high intensity lamp that throws a beam of

light about 10 cm wide. When the eye is placed close to a low intensity lamp such as

a flashlight, a single 40 ym diam particle can be easily seen at a distance of 10 m

in daylight away from the direct sun. Use of a small halogen lamp which provides

a narrow beam of high-intensity light would allow very sensitive detection of the

particles. Of course, it is possible that coated particles came from a legitimate

source, such as a broken road sign or from the ubiquitous Scotch-lite tape. However,

this can be considered improbable, and if one concludes that the presence of

retroref lecting particles strongly suggests the presence of the tagged intruder,

only those suspect samples need be taken to the micro spectrnfluorimeter for positive

identification

VII. PARTICLE SAMPLING TECHNIQUES

Two methods were investigated. One uses a vacuum cleaner type of device and the

other uses adhesive tape. There are other possibilities such as a simple dust cloth

or paper tissue that is swept over the surface to be sampled.

A. Vacuum System

The use of normal vacu\im cleaners with a collecting bag was found to be

unsatisfactory because of the porosity and thickness of the bag. It was not

possible to observe particles at a density of 300 particles/cm^ (16.7 um diam) when

removed by vacuum from a microscope slide,

A rather simple filter mechanism was devised from a commonly used filter system

for chemical solutions. A glass frit was mounted in the base of an aluminum tube to

which a vacuum pump was attached (fig. 39a) . A filter mat was placed on the frit

and an aluminum tube was screw-clamped on top, thereby holding the filter mat onto

the surface of the glass frit. A tube and vacuum nozzle was attached to the top of

the clamping tubes. The system is shown in figure 39b. It is very important that

the flow of air through the filter be as laminar as possible* otherwise, the

distribution of particles across the surface of the filter would be uneven. After

trying several commercial filter holders without success, we resorted to the fritted

glass support which gave a true laminar flow of air through the filter. A number cf

different filter materials were tried. A suitable one was determined to be Gelman

Metricell Black 6N, 0.45 ym, 25 mm diam. Part No. 60511. The use of these filters

produced a minimum of luminescent background, and the particles were well supported

on the surface of the filter paper. The surface of the filter paper must be amply

illximinated for microscopic examination. Luminescence spectra were easily obtained

from particles on this filter paper [see fig. 40(A)]

The obvious major advantage of the vacuum system is that it increases the

particle density and therefore improves detectability. Tests were made on the

vacuuming of a glass slide. Transfer of particles from the slide was 100 percent,

however, collection and detection on the raillipore filter was only 50 percent

efficient. Since the area of the millipore was smaller than the area of the glass

slide, a net overall increase in detection using the vacuum cleaner as a

53
Figure 38 Photograph showing light-reflecting properties of the glass

sphere retroref lectors (diameters range from 40-35 urn)

54
55

Figure

39a.

Photograph

o£

vacuum

filter

particle

colleotioa

system:

vacuum

pump

and

filter

housing.
56

Figure

39b.

Photograph

of

vacuum

filter

particle

collection

system

r

filter

housing.
RELATIVE

INTENSITY

450 550 650

WAVELENGTH, nm

Figure 40. Spectra of luminescent particle (code K-1200) collected

on: A) filter paper, and B) adhesive tape. Dashed

lines indicate background.

57
concentrator results in a 1.5 times increase in nizmber density. This factor can be

easily increased by a factor of 10 by simply vacuuming more area, if the particles

are evenly dispersed

B. Adhesive Tape

Another method studied was the use of cellophane tape containing the usual

adhesive material. One simply sticks the tape to the surface to be sampled, presses

it firmly on the surface, and peels the tape from the surface with the particles

embedded in the adhesive. The best procedure found was to use double-sided adhesive

tape, to make good optical contact between the no-particle side of the tape and a glass slide, and to look at the particles with transmitted light. Optical contact

is made by mechanically pressing the tape to the glass microscope slide. The

cellophane effectively attenuates the ultraviolet excitation so the particles must

be on the surface away from the slide. A series of experiments were performed by

transferring particles from microscope slide to tape. A detection efficiency of

about 50 percent was observed in transferring from particle densities of between 50-

150 particles/cm^ . Of course, no gain in particle density can result from using this

sampling technique. Again the spectrum from an individual particle on the tape is

easily obtained [see fig. 40(B)] .

C. Practical Tests

Using the two methods of particle collection, a general survey was made of our working laboratory where many of the particle experiments were performed. In

addition, a pair of old shoes, a white shirt, and a black sweater were coated with

particles in our test chamber to a level of about 350 particles/cm^ . Vacuuming of

the clothing revealed hundreds of particles on the filter medium. Even though the

clothing was worn for several days, many particles could still be picked up.

VJashing the clothing removed the particles. A few particles remained in the crack

between the shoe leather and the sole even after several months of wearing the

shoes. Sampling the experimental laboratory revealed particles on the floor of the

balance room and around the balance where the mass dilutions were made. A few were

found on the walls of the test chamber. Based upon these results, it appears that

the particles adhere tenaciously to clothing, probably by electrostatic attraction.

Another test that simulated field conditions more closely was conducted at the

Naval Weapons Supply Center, Crane, IN. The entire microspectrofluorimeter was

disassembled, packed into a station wagon, and transported from NBS, Gaithersburg,

riD to Crane, IN. The equipment was reassembled in a van in 3 h and verification of

particles was made (see fig. 41)

The test involved the dispersal of the particles in concentrated liquid ammonia

that was part of an ammonium chloride cloud -producing device which was being tested

in an ammunition bunker. Based upon our test results we calculated that an even

dispersal of 1 g of the coded particles throughout the ammunition bunker could be

easily detected by our sampling techniques. Plastic strips were laid on the floor

of the bunker in the manner shown in figure 42 to facilitate the sampling of the

dispersed particles. About 4.2 g of the K-1200 code of particles containing europium

and terbium in approximately equimolar quantities were placed in the ammonium

chloride smoke generator. After firing the smoke generator and after the smoke had

cleared (about 1 h) , samples were collected from the plastic surfaces. Even though

the plastic had become wet from the deliquescent ammonium chloride and moisture

condensation on the cold floor , the tape sampling technique gave the results shown

in figure 42. The numbers on the figure indicate the sampling locations and the

number of particles per square centimeter that were collected at that point. The

moisture totally prevented collection of the particles by the vacuuming technique.

Because of the moisture on the plastic surface, we estimate that the collection

efficiency was no more than 25 percent.

There were no problems in being able to distinguish the coded particles from the

dirt and debris collected along with the particles. Separate laboratory tests

showed that a coating of ammonium chloride on the particles did not affect the

luminescence spectrum. However, an excessive white coating probably reduced the

microscope particle detection efficiency although no quantitative measurements were

made of the effect.

58
Figure 41. Photograph of the NBS microspectrof luorimeter in a van at the Naval Weapons Supply Center, Crane, IN.

59
FLOOR PLAN AND

PARTICLE DATA

TEST WEAPONS

STORAGE BUNKER

NAD CRANE

SMOKE

Figure 42. Floor plan of cinununition bunker showing location of plastic strip and

positions where scimpling was performed along with the results in

particles per square centimeter (dimensions in meters)

60
The field test demonstrated that the microspectrofluorimeter is sufficiently

rugged to be used in the field, and that the particles can be uneguivocally

identified under field conditions in the presence of substantial amounts of dirt,

moisture, HCl and NH4 CI.

VIII. DESCRIPTION OF FIELD UNIT

The particle tagging system is at a sufficient stage of development to consider

the assemblage of a field unit which could be placed in a van. A large suitcase can

be used to house the necessary vacuum cleaner type of apparatus. The sane suitcase

can house a high intensity lamp although this is not included in the NBS kit.

DETAILED PROCEDURE FOR PARTICLE SAMPLING AND VERIFICATION

Particle Sampling

1. If light-reflecting particles have been incorporated into the tag, the highintensity light should be used to illuminate the area under surveillance. It is

important to keep the eye as close to the source of light as possible since the

spherical beads act as retroref lectors . For example, a miner 's-cap type of

arrangement would be ideal.

2. If a likely area has been found, the particles can be collected using the

vacuum system (fia. 39a) . The pump uses 110 V ac ; the filter clamps between the

upper and lower housing (fig. 39b). The hose containing the copper wand is attached

to the filter. Keeping the hose as short as possible is important because static

charges can build up on the hose material and a substantial number of the collected

particles will adhere to the inner surface of the hose.

If it is impractical to collect by vacuuming, cellulosive adhesive tape can be

used. Adhesive tape coated on both sides provides the best optical contact with the

microscope slide. The tape can be pressed onto the glass with the particle side up,

by using a curved metal implement. Sufficient contact can be made so that few air

bubbles are noticeable. Even though the particles are pressed into the adhesive by

this technique they remain easily identifiable under microscopic examination.

Particle Verification

1. The microspectrof luorimeter is set up as follows:

a. For a schematic of the apparatus refer to figure 3, and for a labeled

photograph refer to figure 43.

b. Place a sample of collected dirt or dust suspected of containing the

tagged particle on a microscope slide. If the particles are on a filter

medium, mount the filter on the slide; or if adhesive transparent tape is

used, mount this on the slide.

c. Turn the power switch on the excitation lamp power supply to "ON"

position, wait 30 s, then depress the ignite button for 1/2 s. If the

lamp does not light release the button and repeat after 30 s. Do not

hold the button in for a long period of time, i.e., 5 s or longer. The

"Lamp On" light will signal lamp ignition. Note: It is advisable to

operate the light source for 1 h before use in order to stabilize its

output intensity.

d. The slide is scanned in as random a fashion as possible so that no FOV

overlap. To minimize fatigue scanning should be no longer than 10 min.

This time should allow the analyst to inspect about 150 FOV using 312X

magnification (25X objective, lOX eyepiece, 1.25X lens).

e. When a particle is located, its identity must be verified by measuring

its luminescence spectrum, using the following procedure.

61
PhotoHiul tipi ier

Monochromator

X-V Focusing Optics

Measuring Aperture

Binoculars

Ploem illuminator

Mcasurinq Telescope

Figure 43. Photograph of microspectrof luor imeter (with pertinent

controls labeled)

62
2. Turn on power supply to photomultiplier (PM) tube 1000-1300 V, the current

amplifier (setting at 10^)

,

and the x-y recorder. Allow the lamp and

electronic systems to idle for 1 h before taking measurements.

3. Place bead(s) to be examined on a glass slide and secure slide on microscope

stage

4. Viewing through the binocular eyepiece, focus the microscope using first the

coarse, then the fine adjustments. Be careful not to drive the objective

into the sample slide.

5. When a bead is found using the x and y stage adjustments, center the bead in

the concentric circles seen through the binocular eyepiece.

6. Select the appropriate filter block by rotating the correct cube into

position. Note: For each rare earth composition, there may be a different

set of excitation and emission filters that will give the optimum peak

intensities. These have to be tested experimentally for each standard

particle

7 . Push in the lever that operates the beam splitter and observe the bead

through the monocular eyepiece.

8 . Refocus the bead and center it in the cross hairs seen through the monocular

eyepiece

9 . With most of the light directed up the tube and with the side-mounted light

on, adjust the field iris diaphragm so that the illuminated circular area is

slightly larger than the image of the bead. This shows the image whose light

will be transmitted to the monochromator. Note: The adjustment of the field

size with the iris diaphragm is made by adding a small source of light to

produce an image that superimposes on the microscope's monocular eyepiece.

10. Pull the beam splitter lever out to allow light to pass into PM tube. With

the monochromator adjusted to a peak maximum, maximize the signal by

adjusting the focusing lens. Note: The axis of the PMT focusing lens should

be colinear with the optic axis of the microscope. This is best done by

lateral movement of the lens with the two adjusting screws provided. Once

this adjustment is made it should be unnecessary to readjust unless the

optics have been disassembled.

11. Scan the emission spectrum with the scanning monochromator from 0,430-750 nm while recording the output on the x-y recorder.

12. Readjust the mechanical stage so that a bead-free area of the slide is

exposed and record a background spectrum. Note: A background "spectrum"

should have very low intensity. If not, look for stray light entering the

system. It might be possible for a strongly emitting particle to cause a distortion of the peak heights in the spectrum. The measurement should be

repeated for both the standard and unknown at a different diaphragm setting

(e.g., to change the amount of background light entering the optics). If the

ratio of peak intensities changes, it may be necessary to move the particle

to another location on the slide.

13. Repeating the above procedure, record an emission spectrum for a reference

bead and again take a background spectrum.

14. Measure the peak -height maximum each for Eu and Tb , from the background level

to the height of the peak and calculate a peak height ratio for the reference

and unknown particles. If the spectral shapes of the reference and unknovm

look identical and the peak height ratios agree within 2 percent relative, a positive identification is indicated. Replicate measurements of both

standard and unknown will statistically confirm the identity.

63
IX. FUTURE DEVELOPMENT OF THE SYSTEM

It is obvious that the process for identifying the presence of a tag is somewhat

lengthy. However, the use of the retroreflecting beads should provide an answer to

that problem because the particles can be rather easily seen at some distance that

facilitates the scanning of a large area in a short time. This is of particular

advantage when one wishes to follow a ’hot" trail involving the immediate pathway

that an intruder might have taken after setting off the alarm and releasing the tag

material. The trail of the intruder would be illuminated by the retroref lecting

spheres. Tests should be made v;ith a high intensity lamp in bright sunlight to see

if the retroref lecting spheres can still be easily seen. The obvious disadvantage

is that the tag is not covert in the sense that anyone with the appropriate

illuminator can locate the retroreflecting particles. Of course, the particle

verification still requires the use of the microspectrof luorimeter which is not

commonly available. The retroref lector particles can probably be admixed with the

luminescent particles. There seems to be no advantage to having the luminescent

particles coated with a reflecting substance. A low-cost light source, preferably

battery-operated, to be used to detect the retroreflector particles needs to be

developed.

Fluorescent chemical systems need to be developed to provide sufficient unique

tagging codes for use at a large number of facilities, however, preliminary studies

show that a sufficient number of codes can be generated.

More extensive studies should be made on the method of tagging the intruder

For example, thought should be given to the use of the particles in a paint that

dries into a powdery film which causes tagging by contact with the painted surface.

If a container housing the object to be protected were covered with the encoded

luminescent particles, the intruder could not avoid contact. Inconvenience may

outweigh the advantage of such a passive tagging method. For example, careful

decontamination would have to be performed each time the protected item were moved.

Finally, through development of the technique, it should be possible to predict

the proper areas for sampling. For example, during the limited practical testing

done in this work, it was found that the crack between the shoe leather and sole is an excellent place to find a tag even though the tagging occurred months before the

detection of the tag

64
REFERENCES

[ 1 ]

[ 2 ]

[3]

[4]

[5]

[ 6 ]

[7]

[ 8 ]

[9]

[ 10 ]

[ 11 ]

[ 12 ]

[13]

DeVoe, James R. Intruder detection by chemical analysis. Memorandum report

submitted to the Defense Nuclear Agency. Washington, DC: U.S. Dept, of

Defense; 1977 June.

DeVoe, J. R.; Sweger , D. M. ; Velapoldi , R. A. Intruder detection using trace

constituent analysis. Progress report to Defense Nuclear Agency.

Washington, DC: U.S. Dept, of Defense; 1978 April.

Harvey, E. N. A history of luminescence. Philadelphia, PA: The Amer. Phil.

Soc.; 1957.

Stokes, G. G. Phil. Trans. Roy. Soc. London. 463 (1852); Ibid.

A^3 , 385 (1853) .

(a) Parker, C. A. Photoluminescence of solutions. New York, NY: Elsevier

Publishing Co.; 1968.

(b) Binks, J. B. Photophysics of aromatic molecules. New York, NY r Wiley

Interscience; 1969.

Ploem, J. S. Z. Wiss. Mikrosk. ^ 129 (1967).

Velapoldi, R. A.; Travis, J. C.; Cassatt, W. A.; Yap, W. T. J. Microscopv.

103 293 (1975) . Seidle, A. R. ; Velapoldi, R. A.; Erickson, N. Appl . of Surface Science. 3^ 229 (1979); J. Inorg . Nucl. Letters. ^ 33 (1979).

Reisfeld, R. ; Velapoldi, R. A.; Boehm, L. J. Phys. Chem. 7^ 1293 (1972);

Reisfeld, R. ; Ilonigbaum, A.; Michaels, G.; Hard, L.r Ish~Shalom, M. Israel

J. Chem. 7 613 (1969)

Small, J. K.; Heinrich, K. F. J.; Fiori, C. E. ; Myklebust, R. L. ; Newbury, D.

E.- Dilmore, M. F. Scan. Elect. Micros. 1 445 (1978).

This idea was first suggested to us by Dr. H. Webster, U.S. Naval Weapons

Support Center , Crane , IN

Stoudt

,

M. D. and Vedam , K

Applied Optics 17 No. 12 1855 (1978)

Stoudt

,

M. D. and Vedam , KApplied Optics 11^ No. 12 1859 (1978)

65
NBS-114A (REV. 2-8C) U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

1. PUBLICATION OR

REPORT NO.

2. Performing Organ. Reoort No.

SHEET (See instructions) NBSIR 81-2407 (R)

4. TITLE AND SUBTITLE

Publication Date September 1982

Intruder Tagging and Identification Using Luminescent Particles

5. AUTHOR(S)

J. R. DeVoe, R. A. Velapoldi, J. K. Lanqland, D . K. Hancock

6. PERFORMING ORGANIZATION (If joint or other than MBS. see instructions) 7. Contract/Grant No. NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE 8. Type of Report & Period Covered

WASHINGTON, D.C. 20234 Final

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City, State, ZIP)

Defense Nuclear Agency

Attn: OPNS

Washington, DC 20305

10. SUPPLEMENTARY NOTES

Document describes a computer program; SF-185, '^IPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here)

This is the final report on the development of a unique particle

tagging system that can allow one to follow the egress of a person

who violates a secured area. The intruder comes into contact with

glass beads (20 ym diam) which are of two types; one type acts as

light reflectors allowing immediate visual detection: the second type

contains rare earths whose emission spectra and quantity can be measured

in a microspectrof luorimeter , Each site is tagg^ with beads containing

a unique concentration ratio of two rare earths, thereby enabling

hundreds of unique codes. The efficiency for detecting these particles

has been measur^ and particle densities as low as 5 particles/cm^ can

be detected. Particle concentration techniques were also developed.

Practical tests demonstrated the portability of the equipment and that

this system is effective in such environments as normal buildings and

houses as well as in an ammunition storage bunker.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolon s) fluorescence; intruder tagging and identification;

microspectrof luorimeter ; particles; tagging material.

13. AVAILABILITY

1 1 Uni imited IX 1 For Official Distribution. Do Not Release to NTIS

1 1 Order From Suoerintendent of Documents, U.S. Government Printing Office, Washington, D.C.

20402.

Order From National Technical Information Service (NTIS), Springfield, VA. 22161

14. NO. OF

PRINTED PAGES

15. Price

USCOMM-DC 6043-P80