**URANIUM AGE DETERMINATION BY**

**MEASURING THE ^230^Th / ^234^U RATIO**

S.P. LaMont and G. Hall

Savannah River Technology Center, Nonproliferation Technologies Section

Bldg. 735-A, Aiken, SC 20808, USA.

stephen.lamont@srs.gov; gregory.hall@srs.gov

**ABSTRACT**

A radiochemical isotope dilution mass spectrometry method has been
developed to determine the age of uranium materials. The amount of
^230^Th activity, the first progeny of ^234^U, that had grown into a
small uranium metal sample was used to determine the elapsed time since
the material was last radiochemically purified. To preserve the sample,
only a small amount of oxidized uranium was removed from the surface of
the sample and dissolved. Aliquots of the dissolved sample were spiked
with ^233^U tracer and radiochemically purified by anion-exchange
chromatography. The ^234^U isotopic concentration was then determined by
thermal ionization mass spectrometry (TIMS). Additional aliquots of the
sample were spiked with ^229^Th tracer, and the thorium was purified
using two sequential anion-exchange chromatography separations. The
isotopic concentrations of ^230^Th and ^232^Th were determined by TIMS.
The lack of any ^232^Th confirmed the assumption that all thorium was
removed from the uranium sample at the time of purification. The ^230^Th
and ^234^U mass concentrations were converted to activities and the
^230^Th/^234^U ratio for the sample was calculated. The experimental
^230^Th/^234^U ratio showed the uranium in this sample was
radiochemically purified in about 1945.

*Key Words*: Uranium Series Radiochronology

## Introduction

Numerous radiochronology methods have been developed that rely on
determining the disequilibrium in uranium series radionuclides (Figure
1). Most of these are used to study geologic processes, and take
advantage of the long half-life of ^230^Th, which, if removed from
uranium by a chemical process takes approximately 5 x 10^5^ years to
regain secular equilibrium with its parent, ^234^U. Examples include
dating of carnotite minerals^1^, Fe-Mn deposits^2^, and travertine
carbonate formations^3^.

![](media/image1.wmf){width="5.513888888888889in" height="4.4125in"}

Figure 1: The first seven radionuclides in the uranium series with mode
of decay and half-life information. After uranium is purified, ^230^Th
activity slowly increases and can be used to determine when the uranium
was purified.

The ingrowth of ^230^Th can also be used to determine the time elapsed
since a source of uranium was last radiochemically purified. However,
uranium extraction from ores has only been occurring for just over 100
years, and the amount ^230^Th that has grown into purified uranium is
very small compared to geologic samples. To effectively determine the
^230^Th that has grown into a sample of uranium, either the Th has to be
separated from a very large sample of uranium, or extremely sensitive
analytical methods have to be used. In both cases, extensive
radiochemistry is required to purify the ^230^Th prior to analysis to
remove interfering species. The use of the ^230^Th / ^234^U ratio has
been used to determine the age of highly enriched uranium using isotope
dilution thermal ionization mass spectrometry (ID-TIMS), isotope
dilution alpha spectrometry (ID-AS), and inductively coupled plasma mass
spectrometry (ICP-MS).^4^ However, the authors concluded that
approximately 10 mg of HEU and 1 g of natural U would be required to
determine the age using ID-TIMS or ID-AS, and that ICP-MS could only
determine the age of enriched uranium containing at least 0.01 % 234U by
weight.^4^

The Nonproliferation Technology Section, at the Savannah River
Technology Center received a request to determine the age of a small
piece of uranium metal. Because the material was at least a year old,
the ^230^Th ingrowth, while very small, was the only isotope in the U
series that could be measured to determine its age. Radiochemical
isotope dilution mass spectrometry methods were used by NTS to determine
the uranium isotopic composition of the sample and measure ^230^Th
activity in the sample to estimate the year the uranium was
manufactured.

## Experimental

### **Sample Preparation**

The sample provided was a small uranium metal rod approximately 0.8 cm
diameter and 2 cm long. The sample had a small layer of oxidation had
formed on the surface in the form of a very finely divided powder. Some
of this oxidation had been transferred to the inside of the bag the
uranium was stored in, and served as a convenient method by which to
obtain a small sample of the uranium, while maintaining the integrity of
the bulk sample.

To sample the uranium, the rod was transferred to a clean bag, and an
approximately one inch square of the original bag containing most of the
oxidized material was leached in 25 mL of 8 M HNO~3~ to dissolve the
uranium dust. After 2 hours the bag was removed from the solution and
rinsed with 8 M HNO~3~ adding the rinse to the leach solution. The leach
solution was transferred to a 100 mL Class A volumetric flask and
brought up to volume with 8 M HNO~3~ (Solution 1). Two successive 1:100
dilutions of Solution 1 were made, labeled Solution 2 and Solution 3, to
produce a uranium concentration in the appropriate range for analysis by
mass spectrometry.

### **Uranium Purification**

Three 1.00 mL aliquots of Solution 3 were pipetted into 5 mL conical
Teflon vials. One of these was submitted directly for analysis by mass
spectrometry. One of the remaining two samples was spiked with ^233^U as
a radiochemical yield tracer, and remaining solutions were purified by
anion exchange. A blank sample and QC sample of natural uranium (both
spiked with ^233^U tracer) were run through the anion exchange procedure
with the samples. All of the samples were taken to dryness on a hotplate
and redissolved in 3.00 mL of 9 M HCl.

Anion exchange columns containing 0.50 mL of 100 - 200 mesh AG1x4 Cl^-^
form resin were used for the purification of the uranium samples. The
columns were prepared by passing 5 mL of 0.1 M HCl over each column,
followed by 5 mL of 9 M HCl. The samples were then passed through the
columns, and the vials rinsed twice with 1 mL of 9 M HCl, adding the
rinse to the columns. The columns were then rinsed with an additional
2.5 mL of 9M HCl followed by 1.25 mL of 8 M HNO~3~ to remove any iron
present. Finally, the uranium was eluted into 5 mL conical Teflon vials
with an additional 4 mL of 8 M HNO~3~ and submitted for isotopic uranium
determination by mass spectrometry.

### **Thorium Purification**

Three 20.0 mL aliquots of Solution 1 were pipetted into 50 mL Pyrex
beakers. Two aliquots were spiked with ^229^Th as a radiochemical yield
tracer. A reagent blank was also prepared by spiking 20.0 mL of 8 M
HNO~3~ (the matrix of Solution 1) with ^229^Th tracer. An aliquot of
^229^Th tracer was also submitted directly for isotopic analysis by mass
spectrometry to determine any ^232^Th and ^230^Th isotopic impurities.
The two spiked aliquots of Solution 1, the unspiked aliquot of Solution
1, and the spiked reagent blank were all taken to dryness on a hotplate
and redissolved in 25 mL of 8 M HNO~3~.

The thorium was purified using a 10 mL column anion exchange column
followed by a 0.5 mL anion exchange column in order to get sufficient
separation of the thorium from uranium. The first columns were prepared
by adding 10 mL 100 -- 200 mesh AG1x8 Cl^-^ form resin to each of the
four ion-exchange columns and rinsing them with 50 mL 0.1 M HCl to
remove any actinide impurities. After converting the columns to the
NO~3~^-^ form by adding 50 mL 8 M HNO~3~, the samples were added to the
columns. The beakers were rinsed 3 times with 10 mL 8 M HNO~3~ and added
to the columns. The columns were then rinsed with an additional 45 mL 8
M HNO~3~. Clean beakers were placed under the columns and the thorium
was eluted with 60 mL of 9 M HCl. These solutions were taken to dryness
on a hotplate, the residue was redissolved in 2 mL of 8 M HNO~3~, and
each sample was transferred to a 5 mL conical Teflon vial. The beakers
were rinsed 2 times with 1 mL 8 M HNO~3~, and the rinse solutions were
added to their respective vials.

The second columns were 0.5 mL AG1x4 100 -- 200 mesh anion exchange
columns rinsed with 5 mL 0.1 M HCl, followed by conversion to NO~3~^-^
with 5 mL 8 M HNO~3~. The samples were then passed over the columns, and
the vials rinsed 3 times with 1 mL of 8 M HNO~3~, adding the rinse
solutions to the columns. The columns were rinsed with an additional 2
mL of 8 M HNO~3~, and the thorium was eluted into clean 5 mL conical
Teflon vials with 5 mL of 9 M HCl. The samples were taken to dryness
under a heatlamp, wet ashed with 1 mL 8 M HNO~3~, and submitted for
analysis by mass spectrometry.

### **Mass Spectrometry**

The instrument used for this work is a Knolls Atomic Power Laboratory
(KAPL) design three stage thermal ionization mass spectrometer. Samples
loaded on rhenium filaments were vaporized and ionized, sorted by two
90° magnetic sectors and an electrostatic sector, and detected by single
ion counting. The instrument has a dynamic range of about 10^9^, but the
detector limit is about 10^7^. The normal uranium sample size is
approximately 1 ng.

Each of the uranium samples was reconstituted with 10 μl of 9 M HCl
before the addition of two ion exchange resin beads (DOWEX AG 1-X2
conditioned in nitric acid). The thorium samples were reconstituted with
8 M HNO~3~ but were otherwise treated the same. The sample vials were
stirred/gyrated overnight, and the beads were mounted to degassed "vee"
type rhenium filaments using collodion thinned with methanol as
adhesive.

The beads were decomposed by heating in vacuum to about 1000°C while
keeping the pressure below 3 x 10^-7^ torr, and allowed to cool for a
minimum of 30 minutes before analysis. This process takes from 12 to 16
minutes and reduces the resin beads to small carbon rich chips
containing the sample. Uranium filaments were reheated to about 1500°C
for tuning and calibration using a 12 minute process. The thorium
filaments were reheated to about 1550°C in a 16 minute process. The
timed reheat sequences are to allow the chemistry/kinetics that occurs
on the filaments to develop similarly for the entire set. The instrument
was tuned and calibrated using normal uranium methods for all of the
filaments due to the residual uranium in the thorium samples. The normal
uranium calibration uses 2 points (masses 238 and 235) to account for
the various offsets in the digital analog converter (DAC) and source
high voltage supply. Mass spectra were made on each filament, and the
calibration checked prior to raising the temperature to data collection
levels.

Normal temperatures for data collection with uranium filaments range
from 1550 to 1650°C. Higher temperatures will ionize more efficiently,
but the sample uranium evaporates faster reducing the time available for
data collection. Thorium filaments operate hotter than uranium
filaments, at about 1700°C. After adjusting a thorium filament to a
temperature for acceptable mass 232 signals, any residual uranium ion
current lasted only a few minutes.

While collecting data, the controller steps the accelerating high
voltage causing the instrument to switch from mass to mass, cyclically
from high to low to high, and spends the specified gate times counting
pulses from the detector. A single cycle through the masses took about a
minute. The filaments were counted for 30 -- 60 minutes, and were
typically not run to exhaustion. The resulting count data was scanned
for outliers, and reduced using normal uranium methods.

## Results

The isotopic composition of the uranium in the unspiked sample, the
spiked sample, and the QC sample are summarized in Table 1. Both the
spiked sample and the QC sample were corrected for isotopic impurities
in the ^233^U tracer. The sample has a uranium isotopic composition
similar to natural uranium, and the lack of detectable ^236^U indicates
that it was never irradiated in a nuclear reactor. It should be noted
that all uncertainties are reported as 2 sigma and were propagated in
quadrature. Table 2 shows the concentration of ^238^U determined in the
spiked sample (Solution 3) and the QC sample. The concentration of
uranium in Solution 3 was also used to calculate the concentration of
uranium in the original sample, Solution 1.

  ----------------------- ---------- ------------ ----------- -------------
  **Sample Description**  **238 Atom **236 Atom   **235 Atom  **234 Atom
                          %**        %**          %**         %**

  Unspiked Sample         99.26 ±    None         0.732 ±     0.0062 ±
                          0.59       Detected     0.012       0.0003

  Spiked Sample           99.27 ±    None         0.724 ±     0.0056 ±
                          0.59       Detected     0.012       0.0019

  QC Sample Measured      99.27 ±    None         0.724 ±     0.0056 ±
                          0.35       Detected     0.007       0.0005

  Natural Uranium and QC  99.275     0            0.720       0.0055
  Certified Isotopic                                          
  Composition                                                 
  ----------------------- ---------- ------------ ----------- -------------

Table 1: Uranium isotopic composition in samples. The bottom row shows
the certified values for the QC sample, which are in good agreement with
the experimentally determined values.

  -------------------- ---------------------- ---------------------------
  **Sample             **^238^U pg / mL**     **^238^U pCi / mL**
  Description**                               

  Solution 1           (5.50 ± 0.17) x 10^6^  1.85 ± 0.06

  Solution 3           550.2 ± 9.8            (1.85 ± 0.03) x 10^-4^

  QC Measured          3261 ± 326             (11.0 ± 1.1) x 10^-4^

  QC Certified Value   2990 ± 290             (10.0 ± 1.0) x 10^-4^
  -------------------- ---------------------- ---------------------------

Table 2: Uranium concentration in sample solutions and QC standard. The
bottom row shows the certified values for the QC material, which are in
good agreement with the measured value.

Thorium isotopic impurities were determined in the ^229^Th tracer so
corrections could be applied when calculating the thorium isotopic
concentrations in the samples and reagent blank. The ^232^Th/^229^Th
ratio in the tracer was 19.987 and the ^230^Th/^229^Th ratio was 0.0017.
Thorium isotopic concentrations determined in Solution 1 and the spiked
reagent blank are shown in Table 3. Measurable ^232^Th was found in the
reagent blank, and was statistically equal to the ^232^Th found in
Solution 1. This indicates that when the uranium was purified, no
significant thorium was present, a requirement for calculating the age
of the material based on ^230^Th growth.

  --------------- ------------- ------------ -------------- --------------
  **Sample        **^230^Th pg  **^232^Th pg **^230^Th pCi  **^232^Th pCi
  Description**   / mL**        / mL**       / mL**         / mL**

  Solution 1      0.0460 ±      37.8 ± 4.4   (9.5 ± 1.2) x  (4.16 ± 0.48)
                  0.0058                     10^-4^         x 10^-7^

  Solution 1      0.0469 ±      40.9 ± 4.6   (9.7 ± 1.2) x  (4.50 ± 0.51)
  (replicate)     0.0058                     10^-4^         x 10^-7^

  Reagent Blank   \< 0.0005     36.5 ± 4.8   \< 1 x 10^-5^  (4.02 ± 0.53)
                                                            x 10^-7^
  --------------- ------------- ------------ -------------- --------------

Table 3: Thorium isotopic concentrations in sample solution and reagent
blank

The age of the uranium was calculated from the ingrowth ^230^Th from the
radioactive decay of ^234^U, with the assumption that ^234^U and ^238^U
were in secular equilibrium and that no ^230^Th was present at the time
the uranium metal was produced. The equation for calculating the ^230^Th
at a given time after the uranium was radiochemically purified is given
in Figure 2, and is valid if no ^230^Th was present immediately after
purification, and the time elapsed since purification is less that 100
years.

Figure 2: Equation for ^230^Th activity in uranium at a given time after
radiochemical purification of the uranium. This equation is valid if no
^230^Th is present immediately after separation, and the elapsed time
since purification is less than 100 years.

A plot of the change in ^230^Th / ^234^U ratio as a function of time was
generated using the equation shown in Figure 3 and includes the
experimentally determined ^230^Th / ^234^U ratio showing the date
![](media/image2.wmf){width="5.998611111111111in"
height="4.727083333333334in"}of purification for the uranium. The
^230^Th / ^234^U activity ratios calculated using the data from the two
^230^Th measurements and the ^238^U measurement were (5.13 ± 0.67) x
10^-4^ and (5.23 ± 0.67) x 10^-4^, which correspond to production dates
of 1946 ± 7 years and 1945 ± 7 years, respectively.

Figure 3: Change in ^230^Th / ^234^U ratio as a function of when the
uranium was purified. The experimentally determined ^230^Th / ^234^U
ratio indicates the uranium was purified between 1938 and 1953.

## Conclusions

Isotope dilution thermal ionization mass spectrometry has sufficient
sensitivity to determine the age of 100 μg size samples of uranium. The
use of ^229^Th tracer allows for the use of extensive radiochemistry to
separate the Th from the U with no impact on the accuracy of the
measurement. With additional replicate measurements of the ^230^Th /
^234^U isotopic ratio, the precision in the experimental age of the
uranium could be improved.

##  

## References

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^2^ F. Chabaux, A.S. Cohen, R.K. O'Nions, J.R. Hein. Geochim. Cosmochim.
Acta. 59, 3 (1995) 633.

^3^ R. Mallick, N. Frank. Geochim. Cosmochim. Acta. 66, 24 (2002) 4261.

^4^ M. WALLENIUS, A. MORGENSTERN, C. APOSTOLIDIS, K. MAYER. Anal.
Bioanal. Chem. 374 (2002) 379.