chapter 13 analytical applications of nuclear …oregonstate.edu/instruct/ch374/ch418518/chapter 13...
TRANSCRIPT
Chapter 13 Analytical Applications of Nuclear Reactions
As mentioned previously (chapter 4), one of the compelling reasons to use nuclear
analytical methods is their high sensitivity. The radiation from the decay or
excitation of a single nucleus can be readily detected. Even when one has to have
the intervening step of a nuclear reaction to produce or excite the decaying species,
the ability to detect very small quantities of material still occurs. This chapter deals
with those nuclear analytical methods (activation analysis, particle-‐induced x-‐ray
emission (PIXE), Rutherford backscattering (RBS)) in which a nuclear reaction is the
necessary first step in the analysis procedure. The techniques to be discussed are
known for their sensitivity, the ability to do non-‐destructive analysis of a large
number of samples, sometimes quickly and the ability to analyze the surfaces of
materials. All these techniques are elemental analysis techniques and do not, in
general, give information about the chemical form of the element, any attached
ligands, etc. This lack of speciation information is a drawback of these methods.
13.1 Activation Analysis
13.1.1 Basic description of the method
Activation analysis is an analytical technique that allows one to determine the
amount of a given element X contained in some material Y. The basic steps in the
activation technique are as follows:
1. Irradiate Y with a source of ionizing radiation so that X will change into X*, a
radioactive isotope of X.
2. Using chemical or instrumental techniques, “isolate” X and X* from all other
elements in Y (not necessarily quantitatively) and measure the activity of X*.
Chemical “isolation” of the activity of interest is performed simply by separating
it chemically from all other activities. Instrumental “isolation” of the activity of
interest involves the detection of radiation that can uniquely identify the nuclide
in question.
3. Calculate the amount of X present.
These basic steps are shown schematically for neutron activation analysis in Figure
13-‐1.
How does one calculate the amount of x present, knowing the activity of X*
produced in the irradiation? It can be shown that
(13-‐1)
where AX* is the activity of X* present at a time td after the end of the bombardment,
NX is the number of X nuclei present initially, σ is the nuclear reaction cross section,
φ is the flux of activating particles, ti is the length of the irradiation and λX* is the
decay constant of X*. From this equation one could calculate Nx from AX*, knowing
all the other variables. (The above equation for AX* is valid for “thin targets”, i.e.,
samples that absorb < 5% of the flux of activating particles).
This method of analysis is called absolute activation analysis and is done
rarely. The reasons for this are the need for detailed knowledge of the flux and
energy of the bombarding particles in the sample, the compounding of the
uncertainties of our knowledge of cross sections, decay branching ratios, etc. in the
final results. A simpler technique is to irradiate and count a known amount of pure
X under the same conditions used for the mixture of X inY. Then
(13-‐2)
This is known as the comparator technique and is the most widely used
method of activation analysis. It depends on irradiating and counting standards of
known amounts of pure material using the same conditions as the samples being
analyzed.
13.1.2 Advantages and Disadvantages of Activation Analysis
Since we know that A=ελN where A is the measured radioactivity, λ is the
decay constant, N is the number of radioactive nuclei present, and ε is a constant
representing the detection efficiency, we know that just a few radioactive nuclei
need to be present to give measurable activities. Use of activation analysis can lead
to measurement of elemental abundances of the order of 10-‐6 to 10-‐12g. The actual
detection sensitivities for activation analysis of various elements, as practiced by a
commercial activation analysis service, are shown in Figure 13-‐2. One can detect µg
levels of over 2/3 of the elements using activation analysis.
Although the high sensitivity of activation analysis is perhaps its most
striking advantage, there are a number of other favorable aspects as well. Activation
analysis is basically a multielemental technique. Many elements in the sample will
become radioactive during the irradiation; and if each of these elements can be
“isolated” chemically or instrumentally, their abundances may be determined
simultaneously. Activation analysis can be a nondestructive method of analysis.
Numerous tests have shown that with careful experimental manipulation, activation
analysis is an accurate (~1 % accuracy) and precise (~ 5% precision) method of
measuring elemental concentrations.
Activation analysis is not without its drawbacks, however. Among them are
the need to use expensive equipment and irradiation facilities, the inability to
determine the chemical state of the elements in question, the need to work with
significant levels of radioactivity, with their attendant radiation safety and legal
problems, the long times needed to complete some analyses, and complex analysis
sometimes needed to unscramble the γ-‐ray spectra in a given experiment.
The ultimate test of the utility of activation analysis as an analytical
technique is whether there are competitive technologies that have the advantages of
activation analysis with fewer drawbacks. One candidate for this designation is
inductively-‐coupled-‐plasma-‐mass-‐spectroscopy (ICP-‐MS).
The detection limits in ICP-‐MS are shown in Figure 13-‐3 and are certainly
equal to those achieved by activation analysis. In addition, ICP-‐MS apparatus is
frequently connected to ordinary chemical separation apparatus, such as liquid
chromatography (LC) thus allowing a sensitive determination of both the amount
and chemical species present for both metals and non-‐metals. (Figure 13-‐4)
In recent years, there has been increasing use of ICP-‐MS techniques to
replace those of activation analysis although there still are a large number of
applications of activation analysis each year, especially in the geological sciences.
13.1.3 Practical Considerations in Activation Analysis
To better understand the practical details of how activation analysis may be
applied to a given problem in elemental analysis, let us consider the various aspects
of a typical activation analysis problem. To make our discussion more concrete, let
us consider a specific problem, the measurement of the aluminum content of rocks
and meteorites [2]. The choice of this problem as an example was dictated by its
pedagogic simplicity and the fact that conventional chemical analyses of aluminum
in rocks are known to be inaccurate for low aluminum concentrations and, in
general, not very precise.
The first step in an activation analysis procedure is sample preparation. The
unknown and known samples (sometimes referred to as the unknown and standard
samples) should have the same size, composition, and homogeneity insofar as
possible, to insure that any attenuation of the incoming radiation, or the sample
radiation before counting, or any count rate dependent effects are exactly the same.
In practice, this step is accomplished by making sure that the unknown sample and
known sample have the same physical volume, are irradiated in a homogenous flux,
and are counted under exactly the same conditions (geometry, detector, etc.) Pre-‐
irradiation treatment of the sample should be kept to a minimum so as to lessen the
possibility of sample contamination. The standards are either aqueous solutions of
the elements in question or multi-‐elemental standard reference materials whose
composition is certified by a national or international agency (IAEA, US NIST, etc,).
The second step in an activation analysis concerns the choice of nuclear
reaction to change X into X*, plus the irradiation facility in which the reaction will be
carried out. In addition, the length of irradiation and decay prior to counting must
be chosen so the produced X* activity is enhanced relative to all other activities
produced. Most activation analysis is done with thermal neutrons produced in
nuclear reactors for the following reasons:
1. Many elements have high cross sections for the absorption of thermal
neutrons in (n,γ) reactions.
2. Copious fluxes of thermal neutrons (φ~1012 n/cm2/sec) are available in
nuclear reactors.
3. Neutrons penetrate matter easily, and therefore there are few problems
related to attenuation of the neutron flux in the sample.
4. The major elements, carbon, nitrogen, and oxygen, are scarcely activated by
thermal neutrons, making detection of other elements easier.
Although most activation analysis is done with reactor thermal neutrons,
several other nuclear reactions and irradiation facilities can be used. Spontaneous
fission of 252Cf furnishes 3.8 neutrons per fission, and fluxes of up to 109 n/cm2/sec
are available from 252Cf isotopic neutron sources. Cockroft-‐Walton accelerators can
be used to accelerate deuterons to energies of ~ 150 keV, and then, using the 3H(d,
n) reaction, ~14 MeV neutrons can be produced (fast neutrons). Typical neutron
generators of this type give fluxes of ~109 n/cm2/sec of 14 MeV neutrons. These
fast neutrons are useful for activating the light elements, such as silicon, nitrogen,
fluorine, and oxygen, via (n, p) or (n,α) reactions, leading to sensitivities of 50-‐200
ppm and thus, is complementary to slow neutron activation analysis.
Charged particle or photon-‐induced reactions can also be used for activation.
The typical charged particles used are protons, deuterons, 3He and α-‐particles.
Charged particle activation analysis (CPAA) is frequently complementary to neutron
activation analysis (NAA). NAA has poor sensitivity for the lighter elements while
CPAA has good sensitivity. Because of the limited penetrating power of charged
particles in matter, CPAA either requires a thin sample or is used for surface
analysis. This attenuation of the primary radiation by the sample puts especially
stringent requirements on sample preparation.
Activation by photons (PAA) usually takes place via the (γ, n) reaction
although other reactions like (γ, p), (γ,α), etc. are possible. Of special interest is the
determination of lead by PAA with a detection limit of ~0.5 µg. (Lead is very hard to
detect using NAA (Fig. 13-‐2)). Photon sources are usually electron accelerators,
which produce high energy photons through the bremsstrahlung process when the
electrons strike a heavy metal target.
For the sample problem of determining the Al content of rocks, the activating
nuclear reaction was chosen to be 27Al (n,γ)28Al, with the irradiation source being a
nuclear reactor. The 28Al decays with a 2.2 min half life and emits a β-‐particle and a
high energy (1.78 MeV) γ-‐ray.
Even if you have chosen to irradiate a sample with thermal neutrons from a
nuclear reactor, you may be surprised to learn that several other neutron energies
may be present and cause reactions. For the popular TRIGA design of reactor, only
~25% of the neutrons at a typical irradiation position are ‘thermal” neutrons
(0<En<0.05 eV). The rest of the neutrons have higher energies, with neutrons with
0.05 eV< En < 0.1 MeV being called epithermal neutrons and neutrons with 0.1 < En <
15 MeV being called fast neutrons. The capture cross sections for epithermal
neutrons frequently involve resonance capture (Chapters 10 and 11) and can
involve very large cross sections (>104 barns). Usually one uses epithermal
neutrons as the activating particle when one wants to avoid interfering activities in
the sample due to thermal neutron capture. For example, suppose a sample has a
large content of sodium. Sodium is easily activated via the 23Na (n,γ) reaction giving
rise to copious quantities of 15 hr 24Na in the sample, which may interfere with the
detection and measurement of other activities. How do we get rid of this sodium?
We can surround our sample with a metallic cadmium cover (~0.1 cm thick).
Cadmium has a very large capture cross section for neutrons in the energy region
below 1.0 eV and effectively “cuts off” or removes these neutrons. The resulting
neutron flux in the sample consists of the higher energy (epithermal) neutrons.
Frequently one measures a “Cd ratio” for activation of a specific element to get some
idea of how much of the produced activity is due to epithermal activation. This Cd
ratio, R, is defined as
(13-‐3)
Typical values of R range from 2-‐1000 depending on the reactor
irradiation position. Epithermal activation is advantageous for Ag, As, Au,
Ba, Cs, Ga, In, Mo, Pt, Rb, Sb, Se, Sr, Tb, Th, Tm, U, W, Zn, and Zr among
others.
Once a nuclear reaction and an irradiation facility have been selected, the
possibility of interfering reactions must be carefully considered. This term means
that quite often, although X will change to X* during the irradiation, some other
element Z may also change to X* during the irradiation. Thus the activity of X* is
proportional to the abundances of Z and X in the sample, not just X. This effect is
referred to as an interfering reaction or interference, and a correction must be made
for it. In the case of the aluminum analysis, there is a very important interference—
namely the occurrence of the 28Si(n,p)28Al reaction whereby silicon in the rock is
converted into 28Al by reactions involving fast neutrons present in any reactor
(along with the desired thermal neutrons). Thus the measured 28Al activity will be
due to the activation of 27Al and 28Si. By irradiating a known amount of silicon and
counting it, and from the well-‐known Si abundances of rocks, a correction for the
28Al produced by the 28Si(n,p)28Al reaction can be calculated. Other possible
interferences are the fission of any uranium in the sample, or the occurrence of two
nuclides that emit γ-‐rays that have similar energies that cannot be resolved.
The final decision concerning irradiation conditions involves the
determination of the flux and irradiation duration. A rough rule is that the longer
one irradiates the sample and the longer one lets the sample decay before counting,
the greater the activity of the long-‐lived species relative to the short-‐lived species.
One must keep in mind the saturation properties of irradiations are such that it
rarely pays to irradiate any material for a time corresponding to more than two half-‐
lives of the desired activity. (In the Al analysis, a sample irradiation time of 1.0 min
and a neutron flux of 5 x 1010 n/cm2/sec were used.)
Frequently multiple irradiations of a sample are made. The first irradiation
is short (minutes) to determine the short-‐lived radioisotopes (of Ag, Al, Ba, Br, Ca, Cl,
Co, Cu, Dy, F, I, In, K, Mg, Mn, Na, Se, Sb, Si, Sr, Ti, U, and V) and the subsequent
irradiations (hours) are to determine the intermediate (As, Au, Br, Cd, Ga, Ge, Hg, Ho,
K, La, Mo, Na, Pd, Sb, Sm, U, W, and Zn) or long-‐lived (Ag, Ce, Cr, Cs, Co, Eu, Fe, Hf, Hg,
Lu, Nd, Ni, Rb, Sb, Sc, Se, Sn, Sr, Ta, Tb, Th, Tm, Yb, Zn, and Zr) radionuclides. In the
long irradiations, it is common to let the sample “decay” for several days to get rid of
the 15 hr 24Na.
The next major step in any activation analysis procedure is the selection of a
method of “isolating” the activity of interest, X*, to measure it. Two methods of
“isolating” X* are commonly used—instrumental activation analysis (IAA) and
radiochemical activation analysis (RAA). In instrumental activation analysis, the
characteristic energies of the γ-‐rays emitted by the radionuclides in the activated
sample are used to identify them, and the corresponding photopeak areas give a
measure of the activities. Instrumental activation analysis is non-‐destructive,
allowing further use of the sample. Furthermore, it permits the use of short-‐lived
activities to identify various elements that might not be possible if a lengthy
chemical separation would precede the counting. Also, instrumental activation
analysis (IAA) lends itself to automation and reduces the time spent per sample in
the analysis. The use of Ge semiconductor detectors with excellent energy
resolution has made IAA the preferred method of activation analysis.
Although most investigators prefer to use IAA, in some situations
radiochemistry must be done prior to counting the sample, to isolate the activity of
interest. An example of the need for radiochemistry is the determination of trace
elements in biological materials, such as blood, which have a very high sodium
content. Large quantities of 24Na are produced via the 23Na(n,γ)24Na reaction, and
they tend to “mask” the trace element activities in the blood by creating a large
Compton background in the region where the photopeaks of other trace-‐element
activities are found (see the discussion in Chapter 18 on gamma ray detectors). One
solution to this problem is to separate the sodium chemically from the irradiated
blood (using ion exchange with hydrated antimony pentoxide) and then to
instrumentally analyze the purified blood. This example does illustrate a feature of
modern radiochemical activation analysis—that of not completely separating the
element of interest, but of making a group separation of a relatively small number of
activities and further resolving these activities by γ-‐ray spectroscopy.
All of our discussions up to now have focussed on detecting the γ-‐rays from
the decaying activation products. There is another approach that has been used in
some cases. This approach is called prompt gamma ray activation analysis (PGAA)
in which one detects the prompt γ-‐radiation emitted during the activating reaction.
For neutron activation via the (n,γ) reaction, one detects the γ-‐rays emitted during
the neutron capture. Such analyses must be carried out with beams of activating
particles (such as neutrons) and usually involves detecting high energy (> 5 MeV) γ-‐
rays. Because of these constraints, this rapid analysis method is restricted usually to
the determination of the major elemental constituents of the sample.
13.1.4 Applications of Activation Analysis
The applications of activation analysis are almost innumerable. In the physical
sciences, activation analysis is used in trace-‐element analysis of semiconductor
materials, metals, meteorites, lunar samples, and terrestrial rocks. In most cases,
the multi-‐elemental analysis feature of activation analysis is used to measure the
concentrations of several trace elements simultaneously. From these detailed
studies of trace element abundance patterns, one has been able to deduce
information about the thermal and chemical history of the earth, moon, Mars, and
meteorites, as well as the source or age of an object.
The use of activation analysis in criminal investigations (forensic activation
analysis) is also well-‐established. The basic idea here is to match the trace-‐element
distributions found in bullets, paint, oil, and so on found at the scene of a crime with
the trace-‐element distributions in objects found with criminal suspects. Such
identification is rapid and nondestructive (allowing the actual evidence to be
presented in court). Moreover the probability of its correctness can be ascertained
quantitatively. Other prominent examples of the use of forensic activation analysis
involve confirmation of the notion that Napoleon was poisoned (by finding
significant amounts of arsenic in hair from his head) and the finding that the
activation analysis of the wipe samples taken from a suspect’s hand can reveal not
only if he or she has fired a gun recently but also the type of gun and ammunition
used.
Applications of activation analysis in the environmental sciences are routine.
Determinations of the trace element content of urban atmospheres, lakes, streams,
and similar areas have been used to trace the flow of pollutants in various
ecosystems. In addition, a few of the trace elements whose abundances have been
measured by activation analysis have turned out to be biologically significant by
themselves. The classic example is mercury and the significant mercury
concentration in fish and other foodstuffs revealed by activation analysis. A
particular combination of activation analysis and radiotracer methods has found
important applications in the environmental sciences. This combination involves
the use of stable isotopes instead of radioactive isotopes as tracers in various
systems, with activation analysis of the samples collected after tracer dispersal
being used to measure the tracer concentrations. Such a technique avoids the need
to introduce radioactive materials into a system (such as the environment with its
subsequent health and legal complications) and yet retains the selectivity and
sensitivity of radiation measurements. The stable isotopes are called stable
activable tracers. Kruger has described their use [1].
In summary, activation analysis is a multi-‐elemental, non-‐destructive, very
accurate method of analysis. The best-‐case sensitivities are pg/g with an irregular
variation from element to element. It is best suited for the analysis of solid samples
and can be “tuned” using changes in irradiation conditions, particles, etc., and post-‐
irradiation sample treatment. Disadvantages are the long analysis times, the need
for access to an irradiation facility, (usually a reactor), the need to handle
radioactivity, the labor-‐intensive nature of sample counting, and the inability to get
speciation information.
13.2 PIXE
Particle-‐induced x-‐ray emission (PIXE) is an analytical technique based upon
observing fluorescent x-‐rays. As such, it really is not a nuclear technique, since it
involves an atomic process, x-‐ray emission. But the atomic electron shell vacancies
that are filled when the x-‐ray is emitted are created using particle-‐accelerator
beams and one uses typical semiconductor radiation detectors, Si (Li) detectors, to
detect the x-‐rays.
The essential features of a PIXE setup are shown schematically in Figure 13-‐
5. A beam of charged particles from an accelerator, typically 2-‐4 MeV protons,
impinges on a thin sample in a vacuum chamber. The protons collide with the
electrons in the material and some eject inner shell electrons from the atoms in the
sample. A Faraday cup is used to collect the charge deposited by the incident
protons and this is integrated electronically to give the beam current. The sample is
typically a thin, uniform deposit of the material to be analyzed on a thin backing
material. The characteristic x-‐rays from the sample are detected with a Si (Li)
detector. A typical spectrum is shown in Figure 13-‐6. The spectrum consists of
discrete x-‐ray peaks superimposed on a continuous background of bremsstrahlung.
One can see the Kα and Kβ lines of the lighter elements (from the filling of the K shell
vacancies) and the L lines of the heaviest elements. The peaks corresponding to a
given element are integrated to give peak areas and the amounts of that element
obtained either from a knowledge of the absolute ionization cross sections (~1-‐104
barns), fluorescence yields (0.1-‐0.9), beam current and geometry or by comparison
to the results obtained from a thin elemental standard. The term fluorescence yield
refers to the fraction of the electron vacancies filled by x-‐ray emission vs. the
ejection of Auger electrons.
Typical detection limits for various elements in a biological sample are
shown in Figure 13-‐7. Typically PIXE has sensitivity at the ppm level for many
elements. About 25% of the applications of PIXE are in biology and medicine. The
light element matrices lead to smaller continuous backgrounds and many trace and
toxic elements are easily detected by PIXE. (There are no “holes” in detection limits
as there are in activation analysis as all the elements emit some x-‐rays).
Considerable attention has been and must be devoted to the preparation of thin,
representative samples. Note that PIXE is only sensitive to the elemental
composition of the sample and not to the the isotopic composition.
One of the most successful applications of PIXE has been in the analysis of air
pollution particulate matter. Atmospheric particulate matter is typically collected
by impaction on a filter paper, which provides an ideal thin sample for PIXE
analysis. Another aspect of PIXE that is very important for the analysis of aerosol
samples is the ability to analyze a large number of samples in a short time. PIXE
analyses typically take less than a minute and the entire irradiation, counting,
sample changing and analysis procedure can be automated.
An important variant on PIXE is micro-‐PIXE. By using a proton beam whose
spatial dimension is ~0.5 µm (rather than the usual 10 mm), one can determine the
trace element content of a small portion of the sample, giving one a “trace-‐element
microscope.” This application is important in probing samples of medical interest.
A related technique is used in the electron microprobe where the ionization is
caused by electron impact.
13.3 Rutherford Backscattering (RBS)
One of the earliest experiments in nuclear physics was Rutherford’s
demonstration of large angle scattering of α-‐particles by gold nuclei. This
experiment established the existence of a small nucleus within the atom (Chapter
10). The force acting in this process, called Rutherford scattering, is the repulsive
Coulomb force between the positively charged nuclei. A schematic diagram of the
phenomena is shown in Figure 13-‐8.
Rutherford scattering is an elastic event, i.e., no excitation of either the
projectile or target nuclei occurs. However, due to conservation of energy and
momentum in the interaction, the kinetic energy of the backscattered ion is less
than that of the incident ion. The relation between these energies is the kinematic
factor, K, which is given by the expression
(13-‐4)
where M1 and M2 are the masses of the incident and target atoms, respectively and
θ is the angle between the direction of the incident and scattered ions. Note the
relative shift in energy in the collision depends only on the masses of the ions and
the angle of the detector. If one measures the scattering angle and the energy shift,
one can calculate the mass (identity) of the scattering atom. The largest change in
energy occurs for θ = 180° where
(13-‐5)
A geometry that allows detection of the scattered α-‐particles at very large angles is
usually selected.
The probability or cross section for Rutherford scattering (Chapter 10) is
given (Segre) as
(13-‐6)
where x = M1/M2, e2 is the square of the electronic charge and E is the energy of the
incident ion. Note the probability of scattering goes as (Z1Z2)2 and as 1/E2. If this
were all that went into Rutherford backscattering, we would expect a spectrum of
backscattered particles that consisted of a peak for each element in the sample with
a relative height (area) ∝Z2. The elemental abundances could be calculated using
the relation
(13-‐7)
where N is the number of target atoms, D is the number of detected events and F is
the incident ion flux. This is the situation if one has a very thin film as the target
material or if one scatters particles from the surface of a thick sample.
In reality, the situation is usually more complicated because the incident ions
lose energy as they penetrate into the sample thus continuously changing the
probability of scattering and the energies of the scattered particles. The resulting
spectrum for scattering from a single element at varying depths is shown in Figure
13-‐9, where the incident ion energy is E0, the energy of ions scattered from the
surface is KE0 and the energy of ions scattered from a depth x is E1. In this situation,
the energy loss in traversing (into and back out of ) a foil of thickness Nx is
(13-‐8)
(13-‐9)
where εin and εout are the energy dependent stopping cross sections (Ziegler) on the
inward and outward paths of the ion.
Rutherford backscattering is an important method for determining the
composition and structure of surfaces and thin films. In Figure 13-‐10, we show the
results of a RBS measurement with 2.0 MeV 4He incident on a Si surface with a Co
impurity that was diffused into the bulk material. One can clearly detect the Co and
its depth profile.
Another important application of this technique has been to determine the
elemental composition of the lunar and Martian surfaces. Turkevich, et al. [3]
constructed a rugged device to measure the backscattering of α-‐particles from the
lunar surface, which flew on three Surveyor missions in 1967-‐68 and yielded the
first complete and accurate analysis of the lunar surface. The α-‐particles came from
a radioactive source (242Cm) that was part of the instrument package. The results of
these experiments, which showed an unexpected and comparatively high
abundance of Ti, were confirmed by laboratory analysis of lunar samples gathered
in the Apollo missions. Since then, this technique has been used to study Martian
rocks and soil.
References General references about nuclear analytical methods 1. W.D. Ehmann and D.E. Vance, Radiochemistry and Nuclear Methods of Analysis,
(Wiley, New York, 1991) One of the best general references on nuclear analytical methods.
2. D. Brune, B. Forkman and B. Persson, Nuclear Analytical Chemistry (Chartwell-‐Bratt, London, 1984).
3. Chemical Analysis by Nuclear Methods, Z.B. Alfassi, ed. (Wiley, Chichester, 1994). A series of essays on various aspects of nuclear analytical chemistry. Most of them are quite good.
References about activation analysis 1. P. Kruger, Principles of Activation Analysis (Wiley, New York, 1971). The best
textbook approach to activation analysis. 2. D. de Soete, R. Gijbels, and J. Hoste, Neutron activation analysis (Wiley, New
York, 1974) An encyclopedic work. 3. D.J. Hughes, Pile neutron research (Addison-‐Wesley, Cambridge, 1953) The
bible (old testament) of reactor neutron physics. References about PIXE 1. S.A.E. Johansson and J.L. Campbell, PIXE:A Novel Technique for Elemental
Analysis (Wiley, Chicester, 1988) 2. S.A. E. Johansson, J.L. Campbell and K.-‐G. Malmqvist, PIXE, (Wiley, New York,
1995). References about RBS 1. Ion Beam Analysis, J.F. Ziegler, P.J. Scanlon, W.A. Lanford, and J.L. Duggan, eds., (North-‐Holland, Amsterdam, 1990). Specific references 1. W.R. Corliss, Neutron Activation Analysis (USAEC, 1963). 2. W. Loveland., R.A. Schmitt, and D.E. Fisher, Geochim. et Cosmochim. Acta 33, 375 (1969). 3. A. Turkevich, E.F. Franzgrote, and J.H. Patterson, Science 165, 277 (1969). 4. Anal. Chem. 5. S.A.E. Johansson and T.B. Johansson, Nucl. Instr. Meth. 137, 473 (1976). 6. K. Ishii and S. Morita, Int. J. PIXE 1,1 (1990). 7. E. Rauhala, in Chemical Analysis by Nuclear Methods, Z.B. Alfassi, ed. (Wiley, Chichester, 1994). 8. J. Saarilahti and E. Rauhala, Nucl. Instr. Meth. Phys. Res. B, B64, 734 (1992).
Problems 1. For each of the following analyses, indicate what role, if any, activation analysis
could or should play. Be sure to clearly state the reasons for your choice. (a) determination of the oxygen content of steel (b) verification of the
authenticity of ancient paintings (c) determination of the radionuclides present in fallout from nuclear weapons testing. (d) determination of the extent to which radionuclides leaking from nuclear waste storage facilities contaminate the water of nearby streams. (e) determination of lithium impurities in thin films of GaAs.
2. (a) Calculate the activity (in microcuries) of 49Ca produced when 2.7 grams of CaO are irradiated in a flux of 3 x 1012 n/cm2-‐sec for 10 minutes. (b) Repeat this calculation for the situation when the bombarding particle is 21 MeV deuterons, and the deuteron beam current is 10 microamperes. Assume the (d,p) cross section is 50 mb.
3. Using the Chart of the Nuclides as a guide, estimate the sensitivity (minimum quantity that can be detected) of neutron activation analysis for europium using a thermal neutron flux of 3 x 1012 n/cm2-‐sec. Assume no irradiation may last more than 1 hour and the minimum detectable activity is 10 dpm.
4. For the following analyses, indicate whether radiochemical neutron activation analysis would be preferred to instrumental neutron activation analysis. If radiochemistry is indicated, briefly sketch the separation procedures to be used. (a) the determination of ppm levels of Mo in flathead minnows. (b) the determination of the trace element content of agricultural field-‐burning particulate matter. (c) the use of stable activable tracers to determine flow patterns in an ocean estuary. (d) the determination of Dy in pine needles.
5. Consider you want to trace the deposition of particulate matter using the stable activable tracer In. The dilution factor between the point of release and the point of sampling is 106. Assume the samples that are collected are activated in a thermal neutron flux of 3 x 1012 n/cm2-‐sec for 10 min. Further assume a 1% efficiency for detecting the emitted photons. Determine the minimum amount of In that must be released to insure the uncertainty in the measured sample concentrations is 5%.
6. Consider the following results obtained by neutron activation analysis of lake water samples for their Mn content. Assume the sample volumes are 1 liter.
Sample # EOB Activity (cp5m) 1 1204 2 1275 3 940 4 1350
10 mg Mn standard 5000 What is the Mn content of the lake water and its uncertainty? 7. Two thin 1 mg samples of dysprosium are irradiated and counted in a similar
manner, except for the use of a Cd cover foil on one sample. A Cd ratio of 7 is measured, with the bare foil saturation activity of 1 x 104 dpm. Calculate the thermal neutron flux at the irradiation position in the reactor.
8. Devise an activation analysis scheme for determining the concentration of nitrogen in a sample of plant material. Assume the analysis must be non-‐destructive and rapid. Suggest an appropriate reaction, irradiation and counting conditions and indicate possible interferences in your analysis.
9. Compute the “advantage factor” for using a reactor pulse to produce 20 s 46Scm compared to the activity produced by steady state irradiation. Assume the reactor is of the TRIGA type and produces a 15 ms 3000 MW pulse with a peak instantaneous flux of 21 x 1015 n/cm2-‐sec. Assume steady state operation is at 1 MW.
10. Imagine you wish to detect ppm levels of Al in a matrix containing iron, calcium and silicon. Assume you have access to a modern nuclear reactor. Describe an activation analysis procedure to do this analysis. Be sure to describe the irradiation conditions, any pre-‐ or post-‐irradiation chemistry and the counting strategy. Indicate how you would deal with any interferences in the analysis.
Figure Captions
Figure 1. A schematic representation of activation analysis. From Corliss
[1].
Figure 2. Table of activation analysis sensitivities as offered by General Atomic
Company, San Diego, California.
Figure 3. Detection limits with ICP-‐MS. ppt≡parts per trillion. Reproduced by
permission of VG Elemental.
Figure 4. Detection of the metallic species of As and Se by LC-‐ICP-‐MS. From [4].
Figure 5. A schematic diagram of a PIXE setup. Reproduced from Ehmann and
Vance.
Figure 6. PIXE spectrum of a rainwater sample. From [5].
Figure 7. Detection limits in a PIXE analysis of a biological sample. From [6].
Figure 8. A schematic diagram of Rutherford backscattering. From [7].
Figure 9. Energy depth scale in Rutherford backscattering. From [7].
Figure 10. Rutherford backscattering for 2.0 MeV 4He ions incident on a Si (Co)
sample. The dots represent the experimental data while the solid line is a simulated
spectrum. Scattering angle Θ = 170°, with θ1=θ2=5°. From [8].