Note: Descriptions are shown in the official language in which they were submitted.
2131942
DISCLOSURE of the INVENTION
The process and apparatus of the present invention, described below, provides
a
relatively small, inexpensive, accurate, and precise device to detect the
concentra-
tions of the radionuclides'4C and/or 26A1 and/or SSFe and/or'Z9I relative to
their
stable isotopes over a dynamic range of 1:10' to 1:10'2to's, using samples as
small
as lOp,g. Separation from interfering molecular and atomic mass isobars
employs
a completely difFerent approach than that used by traditional AMS,
untraditional
AMS, ICP-MS, or other methods. The process may also be used to increase the
efficacy of the assay of ultra-trace concentrations of nuclides when atomic
mass-
isomers need not be distinguished, compared with standard ICP-MS and other
methods.
BACKGROUND of the INVENTION
There exist two main categories of techniques to assay the quantity of a
radionul-
cide in a sample: Radioactive Decay Counting, whereby the nuclear radiations
emitted by a sample are detected and analyzed, and Direct Atom Counting, where
the individual atoms are separated analyzed by Mass Spectroscopy. The former
is
generally used when activities are relatively large, the latter for small
activities,
where halflives are long and / or abundances are small and / or the number of
samples to be processed is great, and individual size of the sample is small.
For ultra-trace assay of certain radionulcides, Accelerator Mass Spectroscopy
(AMS)
is used to isolate the radionulcides under study from the overwhelmingly
greater
quantities of competing molecular and atomic mass isobars, which is usually im-
possible even v~rith conventional conventional Mass Spectroscopy (MS, ICP-MS,
etc.). The principles behind AMS originated in 1977 (1,2~ and are summarized
a.s
follows:
~ chemical processing to make graphite samples (or other chemical forms) of
a few mg in mass
~ using a negative ion source to produce '9C- ions because the stable element
mass isobar'4N can not be negatively ionized
~ low-resolution magnetic analysis of the ions
~ accelerating from 1 to tens of MeV and stripping the ions to charge q>2+ to
eliminate molecular-ion mass isobars in post analyzed beam
~ using a high resolution magnetic spectrograph and a combination of medium
resolution electrostatic, or Wien (E/M velocity) filter
~ detection of final ions using special detectors to measure the energy loss,
range, velocity, and total energy to identify the final individual particles.
Useful Papers and summaries from several AMS workshops and symposiums may
be found in the literature (3-9,12. Some relevant patents are: US 4,973,841
3
2131942
11/1990 Purser~US 4,037,100 07/1977 PurseryUS 4,489,237 12/1984 Litherland
et al.
Other novel alternatives to AMS may also be found in the literature, such as
use of a small, low energy cyclotron to phase separate 14C from 14N (18~;
using a
laser source to generate q=+3 ions, analyzing them using time-of-flight,
charge-
exchanging to q=-1 using thin foils or gas-jets and reanalyzing with TOF or
mass
spectrometer (19~; using a tuned laser to selectively detach unwanted negative
ions
(eg 36S- from 36C1- (20~; etc. However, to date these alternatives still
suffer from
poor efficiency and /or discrimination are not yet viable.
Examples of conventional applications of ultra-trace radionuclide assay
include
(but by no means limited to) radiocarbon dating for archeology, geology, etc.,
where the ratio of C-14, v~~hich has a half-life of 5700 years, to C-12 ranges
from
10-12 for modern samples to 10-IS for samples 62,000 years old (RDC requires
samples up to several kg in size, bulk chemical processing and counting times
up
to days; AMS needs only O.lmg to lOmg samples, with scaled down processing,
and actual measurement lasting minutes to hours); monitoring of nuclear waste,
geological prospecting, etc.; "tagging" of chemical compounds using
radionuclides
as labels for studies in biomedicine, pharmacology, environment, etc. (with
con-
siderably higher doses and associated risks with RDC than AMS).
However, facilities needed to perform AMS are either very large and very
expensive
high energy (up to hundreds of MeV) laboratories that share beam time with
AMS,
or smaller (1-3MV), expensive (3-5M$) dedicated laboratories (eg ISOTRACE).
Furthermore, analyzing 100 samples per day stretches the limit of their
capacity,
especially for carbon samples older than 20Iia. There is an obvious need for
an
apparatus that is able to assay certain radionuclides in samples with
sufficient ac-
curacy, precision, sensitivity, efficiency and cost-effectiveness; hence the
impetus
behind the present invenvtion.
4
2131942
DETAILS of the INVENTION
The key details of the apparatus and methodology behind the of the present in-
vention are described as follows:
~ an Inductively Coupled Plasma source (ICP) is used as a source of ions and
as the primary molecular dissociator at the part-per-trillion level;
~ ions are accelerated only to relatively modest energies (few tens of KV)
using
simple high voltage DC potentials
~ high resolution energy and momentum analyzers are used to filter out and
monitor ion species of extraneous mass
~ a thin foil or gas/jet canal is used as an electron adder to convert some of
the ions to a negative charge state as well as act as a secondary molecular
dissociator
~ high resolution energy (E/q) and momentum (p/q) analyzers are used to
analyze the negative ions
~ a simple ion detector is used to count the final individual ions and measure
their energy (E)
A block diagram of the essential elements of the invention is shown in Figure-
1.
For simplicity, the discussion below concentrates on ions relevant to C-14,
with
the understanding that similar techniques may be employed for the other ions
of
interest.
1. A sample (of order lmg) is introduced to the system after initial prepara-
tion. Depending upon the final desired sensitivity of measurment, the sample
may be in the form of a. gas (C02 purified from the raw sample), a liquid
(Hydrocarbon, etc.), solid (graphite purified from the raw sample), colloidial
suspension (graphite in water or other liquid carrier), or the raw sample it-
self. Standard chemical techniques as employed by conventional AMS are
used to prepare and purify the sample in the form of C02, graphite, etc.
Several options are available to volatalize the sample in the gas carrier (usu-
ally argon) of the ICP (2) source:
~ the gas form of the sample (eg C02) may be injected directly into the
carrier stream (this is a standard ICP technique)
~ a liquid form may be injected into the gas-carrier stream as an areosol
using a suitable nozzle (this is a standard ICP technique)
~ a solid form suspended in a carrier liquid (usually water, but other
carries liquids may be used) may be injected into the gas-carrier stream
as an areosol (this is a standard ICP technique)
2131942
~ a solid or liquid sample may be completely volatalized by laser ablation
within a chamber containing the gas-carrier stream (this is more recent,
but now standard ICP technique)
~ other standard ICP techniques, such as spark, glow discharge, hot fila-
ment, graphite (or other material) crucible furnace, etc. may be used,
if necessary and appropriate (eg. a graphite crucible furnace would not
be used in the measure of C-14 in a sample because of obvious carbon
contamination)
The preferred sample most suitable for radiocarbon dating (where 14C/1zC
ratios are well under lppt) is in the form of C02 using standard chemical
techniques to prepare and purify the sample into this form. This provides
maximum final sensitivity in the measurement of 14C/'ZC by reducing ini-
tial levels of nitrogen, hydrogen, etc. that form molecular and atomic mass
isobars, (eg 14N,'ZCHZ, ~2CD, ~3CH, etc.) as well as minimize accidental con-
tamination from other sources of 14C during handling. Also, C02 is the most
suitable form when raw samples are physically too small (eg under lmg) to
physically manipulate in a practical manner. (Alternatively, a graphite form
of the sample may be prepared; however, this requires initial transformation
into C02 before graphitization, with a slightly greater risk of contamination
due to greater handling.)
2. The ICP is used to generate positive ions with energy of order a few to
tens
of KeV and to dissociate molecular ions to the 1 part per trillion level. The
ICP source electrically "floats" at a few to tens of KV from the remaining
system, which is at ground potential; this potential difference is used to
accelerate the ions.
3. Standard ICP vacuum baffles and electrostatic lenses are used to isolate
the
relatively high pressure in the region of the ICP from the high vacuum of the
remaining system and to transfer the ions of interest from the source into
the remaining system
4. 1 or 2 Einzel lenses, with finai focus at infinity (not critical)
5. an electrostatic analyzer (<30cm central radius of curvature, 90°
bend, but
these values are not critical)
6. slit system
7. drift length (<30cm, dimensions not critical, except to match analyzers)
8. a magnetic analyzer (<30cm central radius of curvature, 90° bend,
but these
values are not critical)
9. slit system, including:
~ positionable Faraday cups to monitor mass 12-16 beams
6
2131942
~ slit and removable FC system to pass only central (mass 14) beam
10. An Electrostatic quadrupole doublet or triplet is used to reshape the beam
to give a point focus simultaneously in x and y directions at the center of
the "adder" foil or gas canal
11. A standard acceleration tube (aperture with insulator) is used to acceler-
ate beam to a few KeV/nucleon and to electrically isolate the "adder-foil"
(below). (The accelerator "tube" may even be in the form of a simple drift
space in vacuum, with acceleration potential provided by the "adder-foil".)
12. A 0.5 - 2 ~g self-supporting carbon foil ( "adder-foil" ) is used to:
~ charge exchange mass 14 beam from positive to negative with 5-10%
efficiency ~13-16)
~ remove r4N from further analysis as nitrogen can not form negative ions
~ further reduce abundance of mass 14 molecular isobars (by factor of
1000)
Alternatively, a. supersonic gas jet or even a differentially pumped gas canal
( "adder gas-jet" ) may be employed for the charge exchange process.
The increase in x*B phase space by straggling a.nd beam divergence during
charge exchange is minimized by focusing the beam impinging upon the
"adder-foil" as tightly as possible.
13. Standard acceleration tube (aperture with insulator) to accelerate beam an
additional few ICeV/nucleon as well as electrically isolate the "adder foil"
or "adder gas-jet" ) The accelerator "tube" be may even be in the form of
a simple drift space in vacuum, with acceleration potential provided by the
"adder-foil" (above). The additional acceleration is also used to reduce the
effective "temperature" of the beam caused by the straggling and angular
spread introduced by the previous charge exchange process, thereby facili-
tating subsequent beam analysis by the following E- and B- analyzsers.
14. An electrostatic Einzel lens or quadrupole doublet or triplet is used to
re-
shape beam to give final focus at infinity (not critical)
15. a cylindrical electrostatic analyzer (<30cm radius, 90° bend, but
these values
are not critical)
16. slit system
17. drift length (<30cm, dimensions not critical)
18. a magnetic analyzer (<30cm radius, 90° bend; values are not
critical)
19. slit system
7
2131942
20. An ion detectors) to count the final (mass-14) ions, or an ion detectors)
to
count the final ions and measure their energy (E) and / or energy loss (dE);
the detectors) may also include passive regions (i.e. absorbers to range out
unwanted species, when applicable)
21. A reference scale length of 30cm is shown, which corresponds to the
preferred
dimensions of the energy and momentum analyzers.(Note that for the lighter
isotopes, 3H and 14C, that the pair of analyzers upstream from the adder foil
may even be reduced by well over 50%.)
22. Trajectory of the central ions, from initial source (2) to detector (20)
23. accelerating high voltage (up to a few tens of kVDC) applied to ICP source
24. accelerating high voltage (up to a few tens of kVDC) applied to electron
adder (12), via an insulated feedthrough. Items 23) and 24) illustrate the low
values of ion acceleration voltages and energies compared with conventional
AMS
Standard high vacuum pumping systems are used.
Computer control, data acquisition, and data analysis systems are used to
control
the physical selection of samples into the ICP, power supplies, analyze the
data
and provide the final results. (The computer systems are similar to those em-
ployed elsewhere in AMS, ICP and other facilities worldwide and their
particular
configurations are not critical. )
The ratio of radio to stable isotopes (eg 14C/'2C) is obtained by comparing
the
number of ions counted at the final detector with the integrated current from
the
first (Low Energy) magnetic analyzer and correcting with suitable transmission
efficiency factors. Because the concentration of the radionuclide is measured
rela-
tive to its stable isotope, the final result is relatively independent of
initial source
intensity, sample matrix, and other initial conditios. The efficiency
correction fac-
tors are determined at regular intervals by using sample "standards" of known
iaC concentration a.s well as by retuning both magnetic analyzers for transmis-
sion of 12C to the final detector (this may include reducing the beam
intensity
using a mechanical chopping wheel, insertable grid-aperture, electrostatic or
mag-
netic "bouncing" (i.e. transient retuning between different parameters and
back),
etc.). Because several high resolution mass filtering stages are used, only
simple
detection and data analysis is necessary (c.~ conventional AMS which uses low
to medium resolution filtering with complex detection (energy, energy loss,
range)
and particle identification algorithms).
Other features of the invention include:
~ The ion source ca.n easily handle solid, liquid or gas samples
~ The ion source can easily switch samples to over 1000 samples per day
8
21319%2
~ The ability to use COZ instead of graphite saves valuable time and money and
reduces risk of contamination, especially for "very old" (ie 14C/12C«lE-12)
samples
~ The ability to analyze "young" or enriched samples directly without chemical
preparation
Only small sample sizes, from O.lmg to lOmg, are necessary, with the smaller
sizes well suited for COZ extracted from difficult environments (such as in
glacial and arctic ice pack studies); small sizes also are useful in reducing
un-
necessary radiation exposure to patients (animal as well as human) undergo-
ing radiocarbon diagnostics and to radiation workers handling radio-tagged
compounds or radio waste.
~ There is an obvious potential as a microprobe using laser ablation
~ Overall detection efficiency is comparable or slightly greater than conven-
tional AMS
~ Final ion count rate is comparable or greater than conventional AMS
Sample char~geover time is considerably shorter than for conventional AMS;
there is also no lengthly sample "burn-in" period, which can be a major
bottleneck for conventional AMS.
~ Short total distance reduces probability of residual gas scattering
~ Low energy reduces probability of slit scattering
~ Although molecular isobar rejection is not 100% exactly, the amount is more
than adequate. (Note that for conventional AMS, which is tuned to se-
lect charge states q>2+, molecular and atomic mass isobars still need to
be discriminated in the final detectors due to secondary processes: eg, 14N
may be accelerated as '9NH-, stripped to 14N+a slit or gas scatter t0 19N+3
and appear as a background under 14C+3 that must be removed using range,
energy-loss; ~2CH+2 may scatter to end up with the same momentum/charge
~ iaC+3 and rescatter after momentum analysis to give same energy/charge,
etc.; so ineffect, stripping is not completely 100% either.) The use of foils
and 2 pairs of E- and B- analyzers in addition to chemical preparations and
an ICP source results in much greater sensitivity, and reduction and/or elim-
ination of mass isobar backgrounds (by several orders of magnitude) than
can be achieved with simple ICP-MS.
The preferred embodiments of the invention have been illustrated above by way
of
example. However, it is apparent that modifications and adaptations of those
em-
bodiments will occurr to those skilled in the art. Such modifications may
include
(but are not limited to) use of different ion analyzers (eg, WIEN (crossed mag-
netic and electric field), time-of-flight, Radio-Frequency (~uadrupole(s)
(RFC),
etc.) and/or combinations with and/or without the above; beam optics;
different
9
_ 2131942
charge-exchange mediums (thin films, gas canals, supersonic gas jets, etc.);
changes
in the acceleration voltages; changes in dimensions; changes in sample
chemical
preparation; changes in sample volatilization and / or ionization; changes in
ICP
configurations; etc. It. is to be expressly understood that such modifcations
and
adaptations are within the spirit and scope of the present invention.
2131942
References
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Bennett, M.R. Clover and E. Sondheim, Rev. Phys. Appl. 12 (1977) 1487
( 2J D.E. Nelson, R.G. Korteling and W.R. Stott, Science 198 (1977) 507
3J Proc. First Conference on Radiocarbon Dating with accelerators, ed., H.E.
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( 6J Proc. Workshop on Techniques in Accelerator Mass Spectrometry, eds.,
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( 9J Workshop on AMS Requirements in Canada, ed., D.B. Carlisle, Canada Center
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(lOJ Low-energy fusion cross sections of d -f- d and d ~- 3He reactions ", A.
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p150
(12J "Biomolecular tracing through A.M.S.", J.S. Vogel & K.W. Turteltaub,
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Radioat. Isot. vol 43 #1/2, pp61-68, 1992
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Bren-
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(14J "Range and Stopping Powers of heavy ions", L.C. Northcliffe and R.F.
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(15J "Angula.r Distributions of ions scattered in thin carbon foils", G.
Hoegberg
and H. Norden, Nucl. Instr. and Meth. 90 (1970) p283
11
~1 3194 2
~16~ "Equalibrium Charge State Distributions", K.Shinxa, T. Mikumo and
H.Tawara,
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~18~ "Ion motion in a small low energy cyclotron" ; Kirk J. Bertsche, Nucl.
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~19~ "The possibilities of cosmogenic isotope invcatigation by mean of mass -
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Relevant patents are:
US4973841. 111990, Purser:" Precision ultra-sensitive trace detector for
carbon
14 when it is at concentration close to that present in recent organic
materials"
US4037100, 71977, Purser: "Ultra-sensitive spectrometer for making mass and
elemental analyses"
US4489237, 121984, Litherland et al.: "Method of broad band mass spectrometry
and apparatus therefor"
1z
