NBSIR 74-502 ((^)

The Determination of lo(line-129 at

Natural Levels Using Neutron

Activation and isotopic Separation

Harry L. Rook, James E. Suddueth, Donald A. Becker

Analytical Chemistry Division

Institute for Materials Research

National Bureau of Standards

Washington, D. C. 20234

January 1974

Final Report

Prepared for Defense Advanced Research Projects Agency

Arlington, Virginia 22209

ARPA Order 1702

NBSIR 74-502

THE DETERMINATION OF IODINE-129 AT

NATURAL LEVELS USING NEUTRON

ACTIVATION AND ISOTOPIC SEPARATION

Harry L. Rook, James E. Suddueth, Donald A. Becker

Analytical Chemistry Division

Institute for Materials Research

National Bureau of Standards

Washington, D. C, 20234

January 1974

Final Report

Prepared for Defense Advanced Research Projects Agency

Arlington, Virginia 22209

ARPA Order 1702

U. S. DEPARTMENT OF COMMERCE. Frederick B. Dent, Secretary

NATIONAL BUREAU OF STANDARDS, Richard W. Roberts, Director

THE DETERMINATION OF ^"-""l AT

NATURAL LEVELS USING NEUTRON ACTIVATION AND

ISOTOPIC SEPARATION

by

Harry L. Rook, James E. Suddueth, and Donald A. Becker

Analytical Chemistry Division

National Bureau of Standards

Washington, D.C. 20234

ABSTRACT

Iodine-129 levels have been published for biological

matrices that naturally accumulate iodine such as animal

thyroid and kelp. However, published procedures have

insufficient sensitivity to determine ^^^l in other

environmental and biological species. A unique procedure

has been developed for the determination of ^^^i which

couples neutron activation with mass separation. The

procedure results in a significant improvement in sensitivity,

thus allowing analyses to be performed on a variety of

matrices which heretofore had not been investigated. Method

development and analytical procedures are presented and

analytical results at the lO'^^-lO"^"* gram ^^^I level

are given.

Key Words: Iodine-129; isotopic separation; neutron

activation
TABLE OF CONTENTS

PAGE

1. Introduction 1

1.1 Background 1

1.2 Purpose 2 2. Experimental . 3

2.1 Mass Separator 3

2.2 Preseparat ion 6

2.3 Irradiation 7 2.4 Post- Irradiation Separation 7

2.5 Counting 10

2.6 Standards 11

3. Results and Discussion 11

3.1 Quantitation 11

3.2 Analytical 13

3.3 Interferences and Errors 15

4. Conclusions and Proposed Research 18

Bibliography 20

ii
LIST OF TABLES

Table No. Page

1. Mass Separator Yield as a Function

of Arc Current 21

2. Separation Yields as a Function of Mass . . 22

3. System Blank for Normal Laboratory

Operation 22

12 9

4. I Analytical Results 23

iii
LIST OF FIGURES

FIGURE NO. PAGE

1. Backloading Assembly 24

2. Modified Backloader and Furnace 25

3. Beam Collector Plate 26

4. Sample Combustion System 27

5. Modified Backloader Boat 28

6. Collector Foil with 6 Scintillator 29

iv
THE DETERMINATION OF ""''''l AT

NATURAL LEVELS USING NEUTRON ACTIVATION AND

ISOTOPIC SEPARATION

Harry L. Rook, James E. Suddueth, and Donald A. Becker

Analytical Chemistry Division

^National Bureau of Standards

Washington, D.C. 20234

1. INTRODUCTION

1 . 1 Background

Iodine-129 is a nuclide of considerable interest. It is

produced in trace quantities by natural processes and has

7 a half life of 1.59x10 years, thus, a steady state

12 9 concentration of I had been established in the

biosphere prior to the advent of the nuclear age. Since

that time, its concentration is believed to be steadily

increasing. For health and safety reasons, as well as

nuclear monitoring, a reliable method for determining

129

I concentrations in biological and ecological matrices

is of great interest.

1 29 Due to the long half life of I, low level counting

techniques are not normally sensitive enough to enable its

direct determination at natural levels. Purkayastha and

Martin first reported a procedure to enhance the sensitivity

of the determination by neutron irradiation (1) . Samples

were irradiated in a high flux reactor and the 12.5 hour

-I -7 0 half life ^ 1 product was counted. By combining this

procedure with radiochemical separations for iodine,

12 9

numerous authors have reported I sensitivities in the

10"^-10"^-^ gram range (2-7).

1
However, -^-"^I is subject to several possible interferences.

These include direct production by the '-^^Cs (n ,a) -^^^I

and indirect production by the l^^I(3n, y)^^^l and the

235 129 U(n,£) I reactions. The above procedures eliminated

direct production interference by pre - irradiat ion chemical

separations and from numerous other trace activities by postirradiation chemical separations. However, the following

three secondary reactions could not be eliminated and all

13 0 authors used only the high energy photon of I for

discrimination from other iodine isotopes. These reactions

12 6

are: a) I produced via an (n,2n) reaction on natural

127 235

I; b) iodine fission products of U, exclusive of

13 0 128 shielded I; and c) neutron capture products of Te and

13 0

Te. As an added complication, bromine-82 offered a significant problem in all of the separation procedures

due to its similar chemistry to that of iodine. As a result, all previously published procedures do not have the

sen

(7)

17 9 sensitivity to determine ^ I in most environmental matrices

1 . 2 Purpose

This work was undertaken to demonstrate the feasibility

of using a mass separator to lower the detection limit

129 for the determination of I. It can be calculated that

129 if I is separated free of other radioactivity, less than

- 14 12 9 10 grams of I can be detected using low level 3-Y

coincidence sum counting techniques. Mass separation is an

extremely important tool as it offers the only means of

130 separating I from other iodine isotopes in the

irradiation products. This fact, plus the virtual elimination of other radioactive contaminants accounts for the

improved sensitivity. The use of an electromagnetic mass

separator to effect simple, interference- free separations

in activation analysis has been demonstrated (8)

2
In this project, the total analytical procedure for the

12 9 quantitative analysis o£ I was reviewed and improved

wherever possible:

a revised preseparat ion of total iodine

was developed based on sample combustion and

quantitative isolation of iodine.

a substantial reduction of system blank

was demonstrated by operation in a class 100

clean room;

a design change was made on the mass separator

which resulted in substantially improved yields;

the mass separator operation was optimized

and data obtained on yields, intermass contamination

and memory;

an improved procedure for 3-Y coincidence

counting was devised where the was

directly implanted into thin 3 scintillators;

finally, real samples were run to give a realistic

estimate of the detection limit, blank level and

its variations, and levels of ^^^J in selected

environmental matrices.

2 . EXPERIMENTAL

2 . 1 Mass Separator

All work was performed using a Harwell designed electromagnetic mass separator manufactured by Lintott Engineering,

Ltd.* (9). The separator uses a standard Harwell ion source

containing a hot cathode arc chamber with a 4 cm extraction

slit. The extraction potential is variable from 0 to 40 kV,

with the beam focused using conventional ion beam optics.

The mass separation takes place in a 60°, 0.4 m radius,

*See Disclaimer

3
homogeneous - f ield analyzing magnet. The magnet is fitted

with wide pole-tips with rotatable inserts so that its

effective deflecting radius is adjustable with respect to

the center of curvature. The combination of these adjustments permits considerable variation in the geometry of

the analyzing Qiagnet and allows the focal length, dispersion,

and divergence of the ion beam to be controlled over wide

limits.

A Harwell designed backloading assembly was incorporated

with one ion source so that solid samples could be introduced

into a res is tance -wire heated oven and vaporized directly

into the arc chamber (Figure 1) . The over has a dynamic

range of 20 to 1200 °C. The backloading system was

designed with a minimum dead volume thereby minimizing

pumpdown and outgassing time; a sample can be introduced

into the separator and be ready for operational startup

within 15 minutes.

The backloading assembly, as designed, had considerably

lower efficiencies ('^O.S-S^) and higher memory (up to ^41)

than the conventional closed oven. The loss in efficiency

and increase in memory was most obvious with metals such

as tin, lead and cadmium. Close examination of radioactive

tracers condensed inside the ion source after a run indicated

that a large portion of the vaporized sample was escaping

from the rear of the oven and condensing onto cooler metal

parts. This material was then vaporized and introduced

into the beam during subsequent runs.

The backloading assembly has been modified so that the

graphite sample container makes a seal with a conical boron

nitride insert placed in the front of the oven. The opening

4
leading to the arc chamber has been increased from 0.228

to 0.635 cm diameter (Figure 2). The net effect of these

modifications has been to introduce virtually all of the

vaporized sample directly into the arc chamber. This modification has doubled separation yields to ^10% and simultane-

- 2 - 3 ously reduced the memory to the 10 -10 I level. Yields

are defined here as the percent of collected isotope compared

to the quantity of input isotopic material. Improvement

of the backloader is important, since without it the ion

source must be removed from the machine for reloading of

samples requiring lengthy pump down and degassing procedures.

A second, and even more dramatic improvement in yield was

achieved by a modification of the ion source power supply.

A circuit modification was made to allow stable operation

at higher arc currents. This, in turn, caused a higher

ionization efficiency in the arc chamber. Before the modification, the maximum stable arc current obtainable was 2.0

amps. Now, arc currents of up to 5 amps can be routinely

used. It was noted from collector current readings that

the beam intensity increased proportionally with increasing

arc current up to about 4.5 amps. Subsequent tracer runs

have verified this with optimum yields at 4.2 amps of arc

current. Results of yield improvements subsequent to the

instrument modifications are given in Table 1.

The collection plate is a two-stage, movable, water cooled

graphite plate with a vertically centered slit 1 mm wide

and 4 cm long (Figure 3) . Behind the slit is an insulated

back collector plate. The bottom stage of the collector

is covered with commercial grade aluminum foil with a hole

cut around the center slit. With this type of a collector,

an oscilloscope image of the beam, over a limited mass

range, can be obtained by modulating a 5 kV, 3600 Hz signal

5
on to the accelerating voltage. This sweeps the ion beam

back and forth across the slit. The signal obtained from

the back plate is a beam image which allows peak identification and precise beam focus.

I'or beam collection, the modulating voltage was removed

and a high relative abundance stable isotope of the

clement of interest allowed to pass through the slit and

the current collected on the back plate monitored; all other

isotopic species in the beam impinge on the aluminum

collector foil. The current of the foil was also monitored.

A change in the current ratio of the front to back plates

gives an indication of defocus and/or instrument drift,

should it occur. A permanent magnet was mounted directly

behind the collector plate to reduce electron losses from

the collector via ion bombardment which would cause

variable and erroneous current readings.

2 . 2 Preseparat ion

13 3 130

In order to avoid interference from the Cs(n,a) I 23 5 1 3X and the U(n,f) I reactions, a pre - irradiation separation

and concentration of the iodine from the sample matrix

is essential. Previous work in this laboratory has demonstrated that iodine can be quantitatively separated from

biological matrices by combustion and collection on activated charcoal. The volatile materials trapped on the

charcoal were then vacuum distilled into a quartz tube,

sealed, and irradiated. In this work, tracer studies

confirmed that iodine was distilled from a sample at

>800 °C, and could be quantitatively trpped on activated

charcoal. However, difficulty was encountered in quantitatively transferring the idoine from the charcoal to quartz

tubing by vacuum distillation. A pressure increase in the

quartz tube by desorption of other volatile materials, mostly

6
organics, made the pumping time exceedingly long and rarely

were the transfers quantitative. To eliminate this problem,

it was found that the sample could be irradiated directly

on the charcoal if the charcoal was first desorbed under

vacuum at 850 °C. Blank levels for a 250 mg charcoal trap

- 14 12 9 corresponded to less than 5x10 g of I.

The preseparat ion procedure used is as follows:

A 2 gram sample was weighed into a combution boat

and placed into the quartz combustion tube (Figure 4)

A quartz tube, with a 250 mg activated charcoal trap was

fitted into a 19/38 standard taper Teflon plug and the

assembly mated with the end of the combustion tube. Oxygen

as '^ISO cc/min was passed over the sample and the sample

ignited. After combustion stopped, the ash was heated

to 800 °C for 10 minutes with a resistance furnace. The

quartz tube containing the charcoal trap was removed,

one end seated, evacuated, and the other end sealed. The

charcoal trap, containing the volatile combustion products,

was then vacuum sealed in the quartz ready for irradiation.

2 . 3 Irradiation

All neutron irradiations were carried out using the pneumatic

transfer facilities RT-3 and RT-4 of the NBS Research Reactor.

The thermal neutron fluxes obtained in these facilities are

5x10^"^ and 1 . 3xl0^"'"^n*cm s , respectively. RT-4, with a copper -cadmium ratio of greater than 500, was used for most

of the irradiations as the production of 126j f^om the (n,2n)

reaction was greatly suppressed.

2 . 4 Post- Irradiation Separation

12 5 Following irradiation, the sample, plus added I used

for yield calculations, was prepared in a known chemical

state by distillation and reaction with silvered quartz wool.

7
One requirement of radioisotope dilution as used for quantitation of mass separator results is that all isotopes used

must be in the same chemical form. This is required because

different chemical species of a given isotope will have

different ionization efficiencies, resulting in different

separation yields.

The following procedure was used for mass separation:

The outside of the irradiated quartz sample tube was

washed with dilute HCl, cooled to liquid nitrogen temperatures,

broken, and inserted into a quartz combustion tube similar

to the one used for preseparat ion . 100 yl of a standard

1 2 5

I solution and 5 mg of non-active potassium iodide were

added. Silvered quartz wool was packed into a threaded,

open-end, graphite backloader sample holder designed

specifically for this work (Figure 5) . The sample holder

was inserted into a 19/38 standard taper joint on the quartz

combustion tube. Oxygen was passed over the sample and

the graphite ignited. The combustion tube was heated to

800 °C for 10 minutes, cooled, and the graphite sample

holder removed. The threaded-end plug was inserted into the

back of the sample holder and the assembly attached to the

backloader delivery arm. The boat containing the sample

was then inserted directly into the ion source oven and the

system was evacuated to a pressure of <5xl0 ^ torr. Argon

support gas was introduced to give a total arc chamber

- 4

pressure of 2x10 torr, the filament current increased and

an arc struck in the arc chamber. The system was allowed to

equilibrate for 5 minutes to eliminate sample outgassing

effects as the arc chamber temperature increased to the

operating temperature. After volatilization of the sample

had begun, the argon support gas was reduced to a pressure

of '^8x10 torr, and a beam of iodine extracted to give a beam current of '^lO uA.

8
The beam focus and intensity were optimized while monitoring

the beam using the modulated high voltage, the top collector ' plate, and oscilloscope. Following optimization of the

instrument parameters, the 5 kV modulated voltage was removed,

the bottom collector plate raised into position, and the

iodine-127 isotope positioned so as to completely pass

through the central slit and fall onto the back collector

plate. Visual observation is possible by turning off the

room lights and observing the blue fluorescent lines through

the glass collector housing.

The oven temperature was raised slowly to '^^550 °C by conduction of heat from the arc chamber. During this time, the

total beam current increased to "^600 yA and then as steady

state temperature was achieved the beam current slowly

dropped off. During this time, instrument parameters were

periodically adjusted to maintain a maximum current ratio

between the back and front collector plates.

The oven temperature was then raised slowly to maintain

a stable iodine beam of 'V'300 yA current. The separator

was operated at this current with continuous monitoring

of the current ratio between the front and back plate until

all of the iodine was processed. An occasional increase

in oven temperature was used to maintain the beam current.

The time required for processing 15-20 mg of input sample

under the conditions described was about 1 hour.

After a complete run, the collector plate was isolated

from the rest of the system, brought to atmospheric pressure,

and the aluminum collector foil removed. The mass region

corresponding to mass 125, 130, and 132 were cut out and

counted for yield and analysis respectively.

9
2 . 5 Count ing

Counting of the mass separated samples was performed using

one of two systems. The first, used for photopeak identification and contamination checks, was an 84 cm Ge(Li)

detector coupled to a 2048 channel computer pulse height

analyzer. All samples were counted for a minimum of 100

minutes on this system to check for any gamma photopeaks

130

not associated with the decay of I. Selected samples

12 9 containing low levels of I were counted for 20 hours

to obtain a good estimate of contamination.

The second counting system, used for high efficiency, and

low background counting, was a 3-Y coincidence system.

The system consisted of a Pilot-B 3 scintillation disks

optically coupled to a 2.54 cm diameter photomultiplier

(PM) tube inserted into the well of a 12.5cm x 10cm well

type Nal(Tl) gamma detector. The outputs of each detector

were amplified and fed through the coincidence gate of

a 400 channel pulse height analyzer. The 3-Y coincidence

gamma spectrum was stored in the analyzer and summed from

0.1 to 3 MeV for quantitation.

The main advantages of this system are background reduction

and high efficiency counting. The normal background of

the system is '^-0.1 c/m when shielded in a 5 cm thick mercury

shield. The efficiency of the system is >70% if the sample

is deposited in a thin layer, relative to 3 absorption,

directly onto the 3" scintillation wafer. Thin deposition

of the sample was accomplished by using the scintillator

material as the collector material of the mass separator

in the mass 130 region (Figure 6) . A 5 mm wide slit was

cut in the normal aluminum foil target assembly, backed

with the scintillator plastic and the beam in the mass

130 region was allowed to implant directly into the

10
scintillator. The scintillator material was removed after

a run, heat sealed between two clean scintillators, and

optically coupled to the PM tube. The collection efficiencies of this procedure, at mass 125 implanted into aluminum

compared to mass 130 implanted into Pilot-B, were checked

and found to be equivalent to those totally collected on

aluminum foil. This method of direct implantation of the

isotope of interest into the 3 scintillator has increased

the counting efficiency by more than a factor of 2 over

previous methods of counting.

2 . 6 Standards

12 9 One deficiency of past work on the determination of I has been the availability of adequate standards. NBS has

recently issued as a Standard Reference Material (SRM 4949) , 12 9 12 9 a solution of I certified for absolute I activity.

129 From this, solutions of absolute I concentrations were

made by dilution and used for comparator standards. All

quantitative results were related directly to SRM 4949.

3. RESULTS AND DISCUSSION

3 . 1 Quant itat ion

It was found in previous work with the mass separator that

the most reliable method to quantify the percent recovery

of the separated isotopes was to use radioisotope dilution

(8). This technique involves the addition of a known quantity

of a second isotope of the element of interest and assumes

that the two isotopes behave in a similar fashion in the

ins trument

The assumption of similar quantitative behavior must be

verified

11
13 0 For the work with iodine, I was the isotope used for

analysis, as it was produced from the (n,Y} reaction on "'"^^I.

Iodine-125 was chosen for yield quantitation because it

has a long half-life, is not produced by neutron irradiation

of stable iodine, and most importantly, it is two mass

12 7 units removed from stable I. Past work with cadmium

indicated significant losses in the collection of separated

isotopes if they were adjacent to, or overlapping the mass

region of large abundance stable isotopes (8) .

This problem was investigated in detail with iodine isotopes

and the results are given in Table 2. Once again, the

12 7 two isotopes adjacent to stable I show reduced yields.

Usually, the low mass isotope has been most severely affected

by sputtering losses due to slight asymmetry in the beam

128 focus I however, in this work I was separated in lowest

yield. It was also noted that a measureable beam current

could be detected in the mass-128 region. Since there

128

are insufficient I ions in the beam to be measured by

current collection, it is believed that the hydride ion,

H-I^, with mass 128, is formed in sufficient quantity to

12 8 interfere with the collection of I. It was also noted

that the intensity of the peak at mass-128 decreased in

amplitude, relative to mass-127 during a given run. This

is consistent with the proposed hydride ion interference

in that trace water, from which ions can be found in

the ion source, is quickly evaporated out of the samples

at operating temperature. Although hydride ion formation

is only of passing interest to this analysis, it is important

to note for any work where the isotope of interest is a stable mass plus one.

Iodine-126 also gave low collection efficiencies, relative

to ^^^i and '"^'^I. In this case, however, the loss in

12
yield was similar to that seen with cadmium isotopes which

were adjacent to a stable isotopic mass and can be attributed

12 7 completely to sputtering losses from overlapping I.

125 130

In experiments where known quantities of I and I tracers were added, yields were found to be equivalent

for these isotopes. Since it was essential that this be

the case, numerous runs, under various instrumental

operation conditions, were carried out and in all cases

yields were found equivalent within the errors due to

radioisotopic counting statistics (Table 2)

3 . 2 Analytical

With all of the individual components of the analytical

scheme worked out and tested with tracers, analyses were

conducted for ^ I content of two natural samples. One

sample was a freeze-dried hog thyroid, obtained from the

U. S. Department of Agriculture, which represented samples

naturally enriched in total iodine. The second was a freeze-dried filleted albacore tuna fish sample, which is

currently being developed at NBS as a trace element

standard and on which numerous homogeneity studies have

been conducted.

Samples were weighed into a quartz tube 10 mm in I.D.,

15 cm long. The entire tube was inserted into the quartz

combustion tube and the pre- irradiation separation carried

out as described in section 2.2.

Following preseparat ion , an iron foil flux monitor was

attached to the sealed quartz tube containing the

distillates. The unit was sealed in a polyethylene "rabbit"

and irradiated in the NBSR pneumatic transfer facility

RT-4 for 12 hours at a thermal neutron flux of

1 . 3xl0"'"^n -cm ^s . 13
After irradiation, the sample was allowed to decay for

12 8

6 hours to reduce I and other short half-lived activity.

The flux monitor was removed and possible exterior contami.-..

nation of the quartz vial removed by acid washing. The

sample was then frozen in liquid nitrogen to condense any

gaseous material, the quartz vial broken open into a combus12 5 tion boat, and a 100 ml aliquot of the I standard tracer

solution along with 10 mg of potassium iodine carrier added.

The post irradiation separation into mass components was

carried out as described in section 2.4.

Mass regions 125, 130, and 132 were isolated and each was

counted using the Ge(Li) gamma detector to determine if

12 5 any interfering isotopes were present. The I was counted

on a low energy photon system (LEPS) using the iodine Xray for quantitation of yields. If no contamination was

130 present, the I was counted for from 100 to 1,000 minutes

using the 3-y coincidence system. To insure against fission

product interference, the mass-132 region was also counted

132

on every analysis. If I were found , its only source

could be from fission production during neutron irradiation.

Thus, using the mass separator, an additional advantage

13 2 was realized. The absence of I in the mass-132 region

verified the effectiveness of the sample preseparat ion

procedure

initial analyses were carried in a specially cleaned fume

hood in a normal working laboratory. One run each was

carried out on the hog thyroid and tuna fish. The analyses

were preceded and followed by blank runs where the entire

procedure was carried out with empty cleaned equipment

(boats, combustion tubes, charcoal traps, etc.). The results

are shown in Table 3. As can be seen, the blank, with

its associated variability, was the major limitation on

14
the analytical system. One major problem was that I standard solutions had been prepared in an adjacent laboratory

area and thus low level contamination of the entire laboratory

area was possible.

In light of the severe blank problem, the entire sample

preparation and preseparat ion aparatus was cleaned and

moved into a class 100 clean room. All sample manipulations

were carried out in the clean room. Only after the preseparated samples were sealed in quartz were they removed from

the clean room facility. Analysis on the hog thyroid and

tuna standard were repeated and a significant reduction

in blank observed. The results are given in Table 4.

As can be seen, blank levels were reduced by almost three

orders of magnitude by adopting clean room operation.

These blank levels were derived from variations in background

counting statistics and thus could be lowered by subjecting

the samples to higher neutron doses. For these samples,

however, that was not necessary. The samples contained

significant activity above background levels. These analyses,

although limited in number, have conclusively demonstrated

g the ability of determining as little as 10 total atoms

12 9 of I in real biological matrices. This sensitivity

is extremely important if samples of biological origin,

other than iodine concentrators such as kelp and thyroid,

are to be analyzed.

3 . 3 Interferences and Errors

A major source of possible error, when 3-Y coincidence

sum counting is used, is the contamination of the mass130 region with other isotopic species. This may occur

either by molecular ion formation or by alteration of ionic

species within the magnetic sector by ionic reflection

off walls and by gaseous collisions. Molecular ion

15
formation which was discussed in length in a previous

publication (8) was not found to be a contaminant here.

Contamination of a given mass region by isotopes more than

ten mass units removed from the mass of interest has been

measured and was found to be less than 10 ^. Normally a

separation specificity that great would be more than

adequate to reduce contaminants to a negligible level.

o However, in this work, the levels desired are in the 10 - 10^ atom range. Thus, if the sample contains 10^ or greater

activity from other volatile isotopes, general contamination

of the collector foil may occur. This type of contamination

is believed to be from low angle scattering of the beam

off the walls of the drift section of the instrument. Mass

species greatly different from that being collected will

fall in regions on the side wall of the drift section of

the instrument and during a run, some of these species

will be reintroduced into the beam plasma by reflection.

These ions will have a broadened energy profile, not

monoenerget ic like the accelerated ions. If this occurs

within the magnetic region of the drift tube, the scattered

ions will be deflected in a continuous broad band across

all surfaces, including the collector foil. This type of

contamination was observed for the first time in the lowest

level samples of tuna fish. Bromine-82 was observed in a 40 hour Ge(Li) count taken to give a good estimate of

possible contamination. The contamination occurred because

the activity level from '^10 ppm bromine in the tuna sample

gave greater than 10 times the activity derived from ^ I.

The activity level found from ^^Br would account for an

estimated 10% positive bias in the " I analysis if

3-Y coincidence sum counting were used.

16
The magnitude of the contamination can be reduced by using

a smaller region of the collector foil for analysis. Ninety130 five percent of the I mass is deposited in a line 1 mm

in width. Currently, 8 mm of collector foil are removed

and counted to insure complete inclusion of the desired

13 0 isotope. By exactly locating the I mass line to the

nearest 0.1 mm and reeucing the width of collector foil

8 2 counted to ^1 mm, an 8-fold reduction in the Br activity

could be realized.

o 2 Interference from ° Br or any other isotope has not been

detected in the hog thyroid samples or in any of the blank

runs. Investigations of this type of contamination is

continuing and a more fundamental solution is being sought,

such as an improved baffling system for the instrument

flight tube.

The second most important error in these analyses is that of

the blank. The use of the class 100 clean room was found

to be essential to the success of this project. More work

should be devoted to establish the actual blank level and

its main contributing factors. With detailed knowledge of

blank sources, more control can be applied to insure against

sporatic high level contamination of the sample before it is

proces s ed

Contamination of the sample from fission products or iodine126 was not a problem using mass separation. Iodine-130

is shielded from fission products formation by -^-^^Te. The

direct fission yield is so small as not to be of concern,

even in this work. Other iodine isotopes and possible

interferences from secondary reactions are removed by the

mass separation step and are thus no problem.

17
The I did not offer interference as it was 4 mass units

130 removed from I used for analysis and overlap amounted to

<10 ^ times the activity in the primary mass line. In no

126

case was I found as an interference in the mass-130

reg ion

The final problem which must be considered as an inter12 7 ference is triple neutron capture of stable I . Although

the probability of triple neutron capture is very small, it

must be considered, especially when very high flux reactors

are used. Edwards et al have calculated and experimentally

measured this interference (7). Based on their data, under

the irradiation conditions used here, the formation of

130 127

I from I (3n,Y) was not detectable.

Other errors were derived from those normally associated

with neutron activation and will not be repeated here.

4. CONCLUSIONS AND PROPOSED RESEARCH

The results of this work have demonstrated the

12 9 +8 +9 feasibility of measuring I at the 10 -10 atom level

using an electromagnetic mass separator. This is a substantial improvement over previous work and allows the

12 9 measurement of I in biological samples other than those

which naturally concentrate iodine.

129 With the analytical procedure for the determination of

completed, it would be extremely valuable to devote the

129

necessary time and effort to obtain a reliable I value

in one of the NBS biological SRM ' s . A particularly good

choice would be the new tuna fish standard since the

12 9

I level is at the lower measurable range of

12 9 conventional procedures for determining I and other

18
members of the scientific community could use this standard

as a control for their best possible work. The effort

129 involved in getting a reliable value of I in such a standard material would also yield a better understanding

of the repeatability and total system error in the

described procedure.

5. ACKNOWLEDGMENT

This research was supported by the Defense Advanced

Research Projects Agency of the Department of Defense and

was monitored by Hq USAF (AFTAC/TRE) , Patrick AFB, FL 32925

under Project Authorization VT/3416.

19
BIBLIOGRAPHY

1. B. C. Purkayastha and G. R. Martin, Can. J. Chem 54 , 293, (1956)

2. R. R. Edwards, Saienoe 157 , 851 (1962).

5. M. H. Studier, C. Postmis Jr., J. Mech, R, R. Walters

and E. N. Sloth, J. Inorganio Nucl. Chem. 5_4, 755

(1962) .

4. R. C. Koch and B. Keisch, L Analyse par Radioaotivation

et ses Applications Aux Soienaes Biologiques ^ Presses

Universitaires de Fance, 108, Blvd, Saint-Germain,

Paris, 6E, 47-68 (1964). (English). Pittsburgh, Pa,

5. G. Buzzelli, Anal. Chem., 56_, 1975 (1964).

6. B. Keisch, R. C. Koch, and A. S. Levine, Proceedings

1965 International Conference. Modern Trends in

r-.y: Activatlon Analysis, College Station, Texas, 284-290,

• April 19-22, 1965.

7. R. R. Edwards and P. Rey , NYO-5624-5, 50p., 1967, Department o£ Chemistry, Carnegie-Mellon Univ.,

Pittsburg, Pa.

8. H. L. Rook, P. D. LaFleur, and J. E. Suddueth, Bual.

Instrum. Methd. , Accepted for publication,

September 1975

9. J. H. Freeman, Nuol. Instrum. Method , 58, 97 (1965).

Disclaimer:

Certain commercial equipment, instruments, or materials are

identified in this paper in order to adequately specify

the experimental procedure. In no case does such identification imply recommendation or endorsement by the

National Bureau of Standards, nor does it imply that the

material or equipment identified is necessarily the best

available for the purpose.

20
Table 1 Mass Separator Yield as a Function of Arc Current

Isotope Arc Current Percent Yield

^^^I^ , 2.0 8.3

-"^^I^ 2.1 8.8

-•^^I^ 3.2 19.5

-"^^I^ 3 . 2 20.6

'•^^I^ 4.0 23.6

^^^I^ 4.0 22.0

"^^I^ 4.3 34.0

^^^I^ 4.3 31.8

Run made with original backloader

Run made with modified ion source power supply

21
Table 2 Separation Yields as a Function o£ Mass

Run 125

Iodine Isotopic Mass

126 128 130

1

2

3

4

: 5'

6

7

8

9

10

11

12

19

21

14

33

23

17

17 . 7

17.4

6.3

7.2

5.2

3.6

3.4

5.4

1.4

1.1

22 . 0

14.0

31 . 2 22 . 0

17.5

18.1

18.0

Table 3

System Blank for Normal Laboratory Operation

Sample

Blank

Tuna

Hog Thyroid

Blank

Wt Sample

= 1.0

0.973 g

2.131 g

= 1.0

Concentration

g

^ ^ ^ I / g s amp 1

2.39x10

1. 73x10

4.19x10

2.51x10

-10

-11

-11

11

22
Table 4

129

I Analytical Results

Sample

Blank

Hog Thyroid

Tuna

Tuna

Blank

Sample Wt . (g)

1 . 0

1 . 91

1 .22

1. 53

1.0

Concentrat ion

g^ ^ ^I/g sample

8 . 5x10 -14

9. 0x10

9. 3x10

12

13

7 . 9x10 -13

4. 5x10 14

23



27
28


NBS.114A (REV. 7-73)

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO.

NBSIR 74-502

2. Gov't Accession

No.

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

ine Determination or ioaine-129 at Natural

Levels Using Neutron Activation and Isotopic

Separation

5. Publication Date July 1974

6. Performing Organization Code

7. AUTHOR(S)

Harry L. Rook, James E. Suddueth and Donald A. Beckei

8. Performing Organ. Report No. NBSIR 74-502

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No. 310-5484

11. Contract/Grant No. AFTAC Auth. No.

VT/3416

12. Sponsoring Organization Name ancd Complete Address (Street, City, State, ZIP)

Defense Advanced Research Projects Agency

Office of the Secretary of Defense

1400 Wilson Blvd., Arlington, VA 22209

13. Type of Report & Period Covered

Final Report

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.)

Iodine-129 levels have been published for biological matrices that naturally accumulate iodine such as animal thyroid and kelp. However, published procedures have insufficient sensitivity to determine I m other environmental and biological species. A unique procedure has been developed for the determination of

I which couples neutron activation with mass separation. The procedure results in a significant improvement in sensitivity,

^u^^u^-^^^^^i analyses to be performed on a variety of matrices which heretofore had not been investigated. Method development and analytical procedures are presented and analytical resutls

at the 10-13-10-^'* gram ^^^l level are given.

KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons)

Improved limits of detection; iodine-129: isotopic separation; neutron activation

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