Nuclear Engineering

Accelerator Mass Spectrometry

One important challenge in radioactive dating process is the detection and quantification of radioactive element present in any sample of very small amount. Using AMS technique, concentrations of very small amount of samples, of the order of a milli-gram (mg) and less can be measured within few hours. AMS differs from other forms of mass spectrometry as it accelerates ions to high kinetic energies before mass analysis, and is used to determine the ratio of the abundant to rare isotopes of beryllium, carbon, aluminum, chlorine, iodine and many others. The basic principle of a mass spectrometer is to produce ions from the sample, separating them according to their mass, followed by detection and generation of mass spectrum. The use of accelerator in mass spectrometry was first demonstrated by L. W. Alvarez and Robert Cornog of USA in 1939. AMS is particularly useful for isotope ratio analysis of very rare cosmogenic isotopes such as Be-10, Cl-36, Al-26 and C-14. The sensitivity of the system is expressed in terms of the ratio of number of rare nuclide to the more abundant isotope in the sample. In typical AMS measurements, count rate of rare ions is quite small (e.g. 1/100^th count per hour), while that of more abundant isotope is much higher (e.g. 10¹³ ions/sec). The ratio of these two rates, which is the measure of the relative abundance of the two isotopes, reaches the sensitivity limits of the order of 10^-15. Thus, AMS gives the ability to discriminate and measure very rare isotopes against a high background. A small amount of sample (to the order of few mg) contains enough number of rare isotopes. For example, let us consider the isotope ¹⁴C, which has a half life of 5730 years. It is estimated that organic sample of carbon form has a ¹⁴C/¹²C ratio of 1.2 x 10^-12. Hence, a typical AMS sample of 1 mg would contain 6 x 107 atoms of ¹⁴C, and using AMS facility, thousands of these atoms could be collected in an hour.
The capability to count rare isotopes is related to the ability of separating, identifying and measuring the rare isotopes. The basic principle of AMS is to convert the sample material into ion-beam (as in conventional mass spectrometry), and after acceleration, separate and detect the different isotopes by charge and mass. An AMS system consisting of a tandem van-de Graff generator is schematically represented in Fig. m4.3. Negative ions are produced from the sample in a cesium sputter source (I) pre-accelerated to 30-200 keV. These ions are mass analyzed by an analyzer magnet. Mass analyzers separate the ions according to their mass to charge ratio. In an electromagnetic field the force on a charged particle is given by the Lorentz’s force law:

where F is the force applied to the ion, m is the mass of the ion, q is the ionic charge, E is the electric field and v x B is the vector cross product of the ion velocity and the magnetic field. Combining it with Newton’s law of motion

we can write

where a is the acceleration of the charged particle under the force F. Therefore, the job of analyzer is to select charged particles of different m/q and bent them in different trajectories before entering the acceleration stage. This particular mass analysis is done in injection stage (II).

FIGURE m4.3 Block diagram of an AMS
These pre-accelerated ions are now injected in the strong electric field of the accelerator (accelerator stage, III). At the centre of the accelerator, a terminal is maintained at very high potential [tens of MeV]. Negative ions get accelerated to the positive terminal. On reaching the terminal, the ions encounter either a thin carbon foil or low pressure gas. Several electrons may be stripped off, thus transforming the negative ions to multiply charged positive ions. Now the positively charged ions are further accelerated back to the ground potential in the second stage of the tandem accelerator. A subsequent magnetic analysis (analyzing stage, IV) selects the ions of interest with a well-defined combination of charge state and energy and directs them to the detection system. In this case the analyzer magnet selects C-14 for detection and further analysis and rejects other isotopes as depicted in Fig. m4.4.

Fig. m4.4 Selection of C-14 by analyzer magnet in
The final identification of the isotope under interest is dependent on the sensitivity, efficiency and resolution of the detection system (V). Sensitivity of detection for most of the radio isotopes is limited by the background from stable isobars. Therefore, the important task is to separate their signals and hence highly sensitive detectors like multi-anode gas ionization chamber, gas filled magnet etc. are used in the present day AMS. Finally, the chemistry of sample preparation is extremely important, where the output of radio-isotope of interest has to be maximized, and backgrounds from other stable nuclei eliminated as much as possible. Here we will discuss AMS studies of few isotopes. Now I am going to illustrate some more important examples of radioactive dating:

Beryllium-10 (T_1/2 = 1.5 million years) dating:
¹⁰Be has a half-life time of ^~1.5 million years and is produced by cosmic ray spallation of atmospheric nitrogen and oxygen in the stratosphere. After several years of residence in the atmosphere, the ¹⁰Be precipitates to earth or deposit in the ocean, accumulating in marine sediments and manganese nodules. The ¹⁰Be deposition rates have remained constant over the past 7 million years or so, and hence it has been suggested that ¹⁰Be could be used as a good dating tool for determining the growth rates of manganese nodules and sedimentation rates of deep sea sediment. In AMS study, BeO^– is selected for analysis, as Be does not form a stable negative ion. Currents of several microamps are obtained from modern high-intensity sources. Boron-10 is a stable isobar of ¹⁰Be, and since BO^– ion is formed as readily as the BeO^– ion, ¹⁰B ions accompany the ¹⁰Be ions after acceleration. Despite the best efforts of the chemist, counting rates of these unwanted ¹⁰B ions are generally well beyond the capabilities of ionization detectors. This problem could be solved with higher energy ( 20 MeV or more) accelerators. At this final energy, the difference in range of ¹⁰B and ¹⁰Be ions is such that ¹⁰Be ion still has ^~40% of its initial energy after ¹⁰B has been brought to a halt. Hence, the ¹⁰B ions may be stopped in a gas cell or a foil before the detector, while the ¹⁰Be ions continue to deposit their residual energy in the detector. Discrimination between ¹⁰Be ions and other species such as ⁹Be and ⁷Be may be improved by deriving two or more energy loss signals. Because ⁷Be ions from ¹H (¹⁰B, ⁷Be) ⁴He reaction can constitute a serious background when ¹⁰B fluxes are high, it is necessary to avoid any hydrogeneous component of the gases and windows of the absorber cell and the detector. In many cases, Ar is usually used as gas absorber cell. An advantage of using a gas cell as the ¹⁰B absorber is that it may be configured as an ion chamber, and the current produced due to boron flux can be used to tune the AMS system for optimum beryllium transmission.

Aluminium-26 (T_1/2 = 7.2 x 10⁵ years):
AMS study of ²⁶Al is basically used for tracing elements in biological sample. It is a long-lived isotope, having a half-life of 7.2 x 10⁵ years. In spite of its large abundance in the earth crust, its concentration in living tissues is extensively low. In Al measurements, the injection of Al^–beam is done by sputtering a mixture of Al₂O₃ and Ag powder. Eventually, they are separated by other ions (pre-dominantly C and O). One of the interesting studies being carried on is the study of aluminum toxicity and possible related problems (e.g. Alzheimer disease).

Chlorine-36 (T_1/2 = 3 x 10⁵ years):
³⁶Cl has important uses in geology, hydrology and environmental studies. Its half-life is comparable to the residence time of water in many aquifers and because of chlorine’s hydrophilic properties, it is well-suited as a tracer for hydrological processes. ³⁶Cl is produced in the atmosphere by three reactions – spallation of heavier nuclei, principally Ar, K and Ca; the ³⁶Ar(n,p)³⁶Cl reaction induced by evaporation of neutrons from the spallation process; and thermal neutron activation through the reaction ³⁵Cl(n, γ)³⁶Cl. Since cosmic radiation is strongly attenuated by the atmosphere and lithosphere, the first two reactions occur primarily in the atmosphere and upper layer of the earth’s crust respectively. Although chlorine can readily exist as a negative ion, and can therefore be detected using a tandem AMS system, nature also provides a stable isobar ³⁶S, which can exist as a stable negative ion. Thus, any sulphur contamination in the sample produces a background of isobaric ions in the final detector which cannot be removed by any combination of electric and magnetic fields. Because ³⁶S has only a 0.014% natural isotope abundance ratio, it might be anticipated that backgrounds from this species could be made small if clean chemical procedures were used during target preparation. However, in practice, sulphur is a very widely distributed element and AMS is so exquisitely sensitive that experimentally it has been found to be impossible to eliminate ³⁶S contamination at the ³⁶Cl concentrations found in some underground aquifers. To achieve adequate ³⁶S – ³⁶Cl separation, it has been found necessary to take great care during chemical preparation and also to accelerate the ions to energies of at least 30 MeV before they are directed into an ionization detector.

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Accelerator Mass Spectrometry

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