NEUTRON ACTIVATION ANALYSIS
Prepared by
Dr. Zs. Molnár
CONTENT
2.2. Kinetics of activation [2]
2.3. Choosing the
appropriate procedure [4]
2.3.2. Measurement of radioactivity
2.3.3. Experimental
parameters
2.4. Methods of
standardisation
2.5. Measurement and
evaluation [6]
2.5.1. Analysis of the gamma spectra
5. Data processing –
uncertainty estimation
1. Introduction[1]
Neutron Activation
Analysis (NAA) is a quantitative and qualitative method of high efficiency for
the precise determination of a number of main-components and trace elements in
different types of samples. NAA, based on the nuclear reaction between neutrons
and target nuclei, is a useful method for the simultaneous determination of
about 25-30 major, minor and trace elements of geological, environmental,
biological samples in ppb-ppm range
without or with chemical separation.
In NAA, samples are
activated by neutrons. During irradiation the naturally occurring stable
isotopes of most elements that constitute the rock or mineral samples,
biological materials are transformed into radioactive isotopes by neutron
capture. Then the activated nucleus decays according to a characteristic
half-life; some nuclides emit particles only, but most
nuclides emit gamma-quanta, too, with specific energies. The quantity of
radioactive nuclides is determined by measuring the intensity of the
characteristic gamma-ray lines in the spectra. For these measurements a
gamma-ray detector and special electronic equipment are necessary. As the
irradiated samples contain radionuclides of different half-lives different
isotopes can be determined at various time intervals.
Although the development
of analytical techniques has led to the expansion of new methods (ICP-AAS,
ICP-MS, etc.), which can also be widely applied in analytical chemistry, NAA is
still competitive in many areas. The indisputable advantage of the method is
its sensitivity and accuracy especially in respect of some trace elements. The
method is of a multielement character, i.e. it enables the simultaneous
determination of many elements without chemical separation. In the case of instrumental determination, the preparation
of samples involves only the preparation of representative samples, i.e.
pulverization or homogenization in most cases, and this reduces the danger of
contamination to a minimum and accelerates the whole analytical process. If the
determination of some special elements or groups of elements can be carried out only through chemical
separation, it is possible to carry out after irradiation. Thus the pollution
caused by the different chemicals will not get activated, the chemical yield
can be measured by feeding inactive carriers and the chemical processes can be
better controlled. During NAA the neutrons get into interaction with the
nucleus, therefore, the chemical composition and crystal structure of the substance
under analysis will have an effect on the result only in exceptional cases.
The development of the
method has contributed to the elaboration of some very simple and accurate
methods of standarization, which lead to a surpassingly accurate analysis.
The widespread application
of NAA is hindered, however, by some conditions. Among the different fields of
application, the Instrumental Neutron Activation Analysis (INAA) following a
reactor irradiation is the most competitive. In view of the increasing protest
against nuclear energy, a number of research reactors have been shut down;
therefore, the possibilities of irradiation are limited in many countries. The
equipment needed for the analysis is rather expensive and requires special
laboratories and a highly qualified staff.
2. Principles of the method
In the process of NAA the
neutrons interact with the stable isotopes of the target element converting
them to radioactive ones. The so-called compound nucleus emits gamma rays
promptly with extremely short half lives in the order of ms and these can be
measured during irradiation through a technique called prompt gamma activation
analysis (PGAA). In most cases, the radioactive isotopes decay and emit beta
particles accompanied by gamma quanta of characteristic energies, and the
radiation can be used both to identify and accurately quantify the elements of
the sample.
Subsequent to irradiation,
the samples can be measured instrumentally by a high resolution semiconductor
detector, or for better sensitivity, chemical separations can also be applied
to reduce interferences.
The qualitative
characteristics are: the energy of the emitted gamma quanta (E) and the half life of the
nuclide (T½).
The quantitative
characteristic is: the I intensity, which is the
number of gamma quanta of energy E measured per unit time.
1. step of the analysis: sample preparation
means in most cases only pulverising, homogenising, mass determination, packing,
as well as the selection of the best analytical process and the preparation of
the standards, if any.
2. step of the analysis: for irradiation one
can choose from the various types of neutron sources according to need and
availability.
3. step of the analysis: after the irradiation
the analysts face the dilemma whether or not a chemical separation should be
carried out for better sensitivity. If it is needed, the measurement shall be
made after the separation. If there is
no chemical treatment, the measurement is performed after a suitable cooling
time (tc).
4. step of the analysis: measurement, evaluation and calculation
involve taking the gamma spectra and the calculating trace element
concentrations of the sample. The most widely used gamma spectrometers consist
of germanium based semiconductor detectors connected to a computer used as a
multichannel analyser for spectra evaluation and calculation.
2.1. Irradiation facilities
2.1.1. Neutron sources
Isotopic neutron sources
[2]
In the case of the most
frequently used isotopic neutron sources an alpha emitting radioactive material
is mixed with beryllium and an (,n) reaction generates the
neutrons.
Isotopic neutron sources:
|
-emitter |
Half life |
Neutrons s-1 Ci-1 emitted |
Average neutron energy [MeV] |
|
227Ac |
22 y |
1.5x107 |
4 |
|
226Ra |
1620 y |
1.3x107 |
3.6 |
|
239Pu |
2.4x104 y |
1.4x107 |
4.5 |
|
210Po |
138 d |
2.5x106 |
4.3 |
The spontaneous fission of
some artificially produced transuranium isotopes can be applied as a small
neutron source. E.g. 252Cf (half life 2.6y) undergoes fission,
producing 3.76 neutrons of 1.5 MeV per event. One milligram of 252Cf
emits 2.28x109 neutrons per second.
The major advantage is
that the isotopic neutron sources can be made portable and generate a stable
neutron flux. But, as the neutron flux is rather low in comparison to a nuclear
reactor their use in NAA is limited to the determination of elements of high
activation cross section which are present in major concentrations.
2.1.2. Neutron generators
These neutron sources are
accelerators where a convenient target material is bombarded by accelerated
charged particles and the neutrons are produced in a nuclear reaction.
In the most frequently
used and commercially available neutron generators, deuterons are accelerated
and the target material is tritium. The nuclear reaction carried out is 3H(d,n)4He.
The energy of the produced monoenergetic neutrons is 14 MeV. The typical
neutron yields of about 1011 neutrons s-1 mA-1
means a neutron flux of approximately 109 neutrons cm-2 s-1.
Due to the emitted fast
neutrons, in NAA the neutron generators are used for the determination of
elements of high cross section in this energy region.
Examples of elements of
geochemical interest determined by the fast neutrons of generators [3]:
Magnesium: 26Mg(n,)23Ne T½
= 37.6 s
Aluminium: 27Al (n,p)27Mg T½ = 9.5
min
Silicon: 28Si (n,p)28Al T½ = 2.3 min
Titanium: 46Ti (n,p)46mSc T½ = 18.7 s
Iron: 56Fe (n,p)56Mn T½= 2.58
h
Zirconium: 90Zr (n,2n)89mZr T½
= 4.2 min
Nickel: 60Ni (n,p)60mCo T½ = 10.5
min
2.1.3. Nuclear reactors
Owing to the high neutron
flux, experimental nuclear reactors operating in the maximum thermal power
region of 100 kW-10 MW with a maximum thermal neutron flux of 1012-1014
neutrons cm-2 s-1 are the most efficient neutron sources
for high sensitivity activation analysis induced by epithermal and thermal neutrons.
The reason for the high sensitivity is that the cross section of neutron
activation is high in the thermal region for the majority of the elements.
There is a wide
distribution of neutron energy in a reactor and, therefore, interfering
reactions must be considered. In order to take these reactions into account,
the neutron spectrum in the channels of irradiation should be known exactly.
E.g. if thermal neutron irradiations are required, the most thermalized
channels should be chosen.
2.2. Kinetics
of activation [2]
In the case of nuclear
reactions induced by neutrons the radioactivity of the examined isotope depends
on the flux of the neutrons and the cross section of the given nuclear
reaction. The cross section and the neutron flux highly depend on the energy of
neutrons, and therefore the usual activation equation is:
, (1)
N: number of interacting
isotopes
s (E): cross-section [in cm2] at neutron energy of E [in eV]
f (E): neutron
flux per unit of energy interval [in cm-2 s-1 eV-1]
R: reaction rate
In nuclear reactors the
integral in Equation [1] is usually replaced by the sum of two integrals
separating the thermal and epithermal regions, the lower limit of the
epithermal component of a neutron spectrum most commonly is 0.55 eV:
R = N (fth ·sth + fe · I0 ) (2)
sth: conventional
thermal neutron flux [in cm2]
fth : effective
thermal neutron cross-section [in cm2]
fe: conventional
epithermal neutron flux [in cm-2 s-1 eV-1]
Io: resonance integral cross section (in epithermal
region), for 1/E epithermal spectrum [in cm2]
The activity (A) of the
isotopes depends on time. During irradiation the activity of the radioactive
isotope produced grows according to a saturation characteristic governed by a
saturation factor S=1-e-lti.
Subsequent to the irradiation the isotope decays according to the exponential
law: D=e-l
td:
Where ti : time
of irradiation; td : time of
decay; l
: decay constant
A = ( 
o) 
(3)
NAv: Avogadro number
fi : isotopic abundance
m : the
mass of the irradiated element
Arel : atomic mass of target
element
The intensity of the
measured gamma line is proportional to the activity. The measured parameter is
the total energy peak area (NP) at a particular energy given by
(4)
The efficiency (eg) of a semiconductor
detector varies with gamma energy. The emission probability of a gamma photon
at a given energy is the fg,
tm is the measuring time.
2.3. Choosing
the appropriate procedure [4]
When solving an analytical
problem by means of activation analysis, or any other method, the analyst must
select an appropriate procedure. In attempting an optimisation, one must
consider a number of aspects. A set of experimental parameters must be chosen
for adjustment, the others being fixed by practical considerations.
A number of elements have
more than one isotope which can be activated by neutrons. Each activation
product has its own cross-section, isotopic abundance and decay scheme. The
first decision is to choose the most selective nuclear reaction in order to
optimise the procedure.
2.3.1. Irradiation conditions
The incident flux of the
irradiating particles directly affects the level of radioactivity produced,
generally the neutron self absorption is negligible.
In a nuclear reactor the neutron flux cannot be freely changed,
but there are some possibilities to choose. The effective cross-section of a
nucleus depends on the energy of the bombarding particles. Some reactions,
“threshold reactions”, do not occur below certain energy. Some interferences
can be avoided by proper selection of the energy range. In nuclear reactors
there are several irradiation channels with different neutron energy spectra.
The use of a thermal
neutron filter is an important option of selection. This type of analysis is
called Epithermal Neutron Activation Analysis (ENAA), where Cd or B is used as
a filter. ENAA is advantageous in cases when the nuclide has a high Io/sth
ratio. Some elements can be determined with higher sensitivity by ENAA, e.g.
As, Br, Rb, Sr, Mo, Sb, Ba, Ta and U.
Some interferences can
also be avoided by this way. Sometimes, the same radionuclide can be produced from
two different elements, e.g. 28Al which is produced from 27Al(n,g)28Al and 28Si(n,p)28Al.
The (n,g))) reactions are
initiated mainly by thermal neutrons, while for (n,p), (n,a) reactions, fast neutrons are
required. In this case, the samples can be activated twice, with and without
cadmium filter, in order to determine both Al and Si in the sample.
2.3.2. Measurement of radioactivity
Generally, the activation
product emits more than one gamma quanta, each of different energy and emission
probability. To choose the proper analytical gamma line(s) the gamma abundance,
the efficiency and the possible interferences must be considered, in
exceptional cases self absorption can be the main problem. Sometimes, it is
also possible to use more than one gamma line in order to improve the accuracy
and the reliability of the measurement.
The region of gamma ray
spectrum bellow 150 keV is generally quite complex, and it is frequently
advisable to count the samples with a Low-Energy Photon Detector (LEPD). In
geological samples, the use of alternate gamma lines in the low energy region
is more effective in the case of Ba, Nd, Sm, Tb, Hf, Th and U, while the
determination of Gd, Ho, and Tm can be successfully realized only by measuring
the photons at low energies.
Measuring geometry: the
efficiency of the measurement depends on the solid angle presented by the
detector to the sample. Therefore, the detection efficiency is highly affected
by the shape of the sample and its distance from the detector. The counting
rate can also be changed in this way.
In the case of shortage of
samples (analysis of meteorites,
special minerals etc.), very low concentrations in biological samples the use
of a well-type detector is desirable.
2.3.3. Experimental parameters
By optimising the
irradiation, decay and measuring times a lot of elements can be determined with
higher sensitivity. A number of interferences can be avoided in this way, too.
For elements with short
half lives the shortest irradiation and cooling times are determined by the
technical limitations. In this case some special methods are required, e.g. the
process of cyclic activation [5]. In this method the samples are repeatedly
irradiated and counted, and then, the spectra are summed. The transfer,
irradiation and counting times, as well as the number of cycles should be
optimised.
The radioactive isotopes
of long half lives produced after irradiating the elements for long time (e.g.
eight hours) in a thermal channel of the reactor are measured several times.
The cooling times are one week and one month or longer in special cases. By
this way, usually 25-30 elements can be determined in different types of
samples.
2.4. Methods
of standardisation
The two features of
neutron induced reaction - high penetrability for neutrons and gamma radiation
- ensure that its standardisation is potentially easy and accurate. As the
signal to concentration ratio is nearly matrix independent, the sample
preparation is rather easy; therefore, the risk of systematic or random errors
is reduced.
The analytical procedure
can be made faster and more economical by simplifying the standardisation
procedure.
2.4.1. Absolute method
Since the theory of NAA is
well founded, an “absolute” standardisation procedure can be applied. The
quantitative measurement can be effected by determining the neutron flux and
counting the absolute gamma rays. The direct calculation of concentration is
made by applying nuclear constants according to Eq. 3.,4.
According to Girardi‘s
investigation [6], systematic errors up to tens of percent may be the
consequences of the uncertainties of nuclear data taken from literature,
especially those on decay schemes and activation cross-section. Although
recently the conditions have much improved, the factor of uncertainty is still
considerable compared to the relative methods. Irrespective of this, the
ability to estimate the quantity of an element in an unknown sample is a very
important feature of NAA.
2.4.2 Classic relative method
The method is based on the
simultaneous irradiation of the sample with standards of known quantities of
the elements in question in identical positions, followed by measuring the
induced intensities of both the standard and the sample in a well known
geometrical position.
A relative standardisation
can be performed by means of individual monoelement standards, or by using
synthetic or natural multielement standards. The calculation of the unknown
quantity (m) is made according to Eq.5.
, 
; (5)
(st) refers to the
standard.
The accuracy of the
relative method depends on the standard preparation procedure (e.g. non
stoichiometry of the standard compound, dilution and micropipetting
uncertainties).
The disadvantage of the
classic relative method lies in the multielement application. The procedure of
the standard preparation and counting is rather laborious, and this is coupled
with the occasional loss of information if an unexpected element appears for
which no standard has been irradiated.
The use of home-made
multielement standards can be an answer to these problems, but this raises the
question of homogeneity and stability.
There are commercial
multielement Standards Reference Materials (SRM) available. They aim at
improving the accuracy of the measurements and allowing a proper quality
assurance in the laboratories.
2.5. Measurement
and evaluation [6]
The modern gamma measuring
systems consist of a gamma detector, usually a HPGe type and sometimes NaI(Tl)
scintillation crystals. The detectors are connected to a multichannel analyzer
(MCA) by an appropriate electronic system (preamlifier, spectroscopy amplifier,
etc.). Nowadays, the MCAs are computer based systems with the ability of an
automatic spectrum evaluation.
The most important
parameters of the detectors are the efficiency and resolution. The efficiency
of the NaI(Tl) detectors is quite high, but their resolution is poor comparing
to the semiconductor germanium detectors. The typical resolution of a 3x3”
NaI(Tl) detector at 1333 keV energy is 90 keV and the same for a HPGe detector
with a 30% relative efficiency (related to a 3x3” NaI(Tl) in similar
circumstances) is 1.9-2.2 keV.
2.5.1. Analysis of the gamma spectra
The usual objective of the
measurements by gamma ray spectrometers is the determination of the number and
energy of the photons emitted by the source.
The peak location and the peak area in the spectra have to be determined.
The peak location is a measure of the gamma energy, while the peak area is
proportional to the photon emission rate. For the energy measurement the pulse
height scale must be calibrated with standard sources emitting photons of known
energies. In order to calculate the activities, the full-energy-peak
efficiencies of the source- detector system have to be determined by using
sources of known activities.
For the determination of
the peak areas the background under the peak interval has to be subtracted. The
net count (Np) results from NP = Nint - NB, (Nint
integral under the peak and NB refers to the background).
The peak area can also be
calculated by computer programs which fit an analytical function to the peak.
The shape is described basically by a Gaussian function, modified by suitable
auxiliary functions. Thus all the peaks
including also the multiplets can be automatically analysed.
2.5.2. Counting statistics
The nuclear decay
processes occur at random, and follow a Poisson distribution, where the
standard deviation (s) equals
to N1/2 (N is the observed
number of events). In gamma spectrometry, the peak area is the measured
parameter. The standard deviation is: s = (N+2NB)1/2 where the confidence level is 68%.
The counting statistic is
only one of the possible sources of errors in NAA, the overall value depending
on a number of different factors (e.g. sample preparation, weighing, and
uncertainty of standardisation).
3. Equipment and materials
- sample for analysis (chosen by the students: hair
sample, soil or steel)
- standard solutions (Ca, Mn, Na standard solutions)
- analytical balance
- micropipette
- reactor for irradiation
- HPGe detector, spectrometer
4. Procedure
1. Before starting the irradiation the following
have to be answered:
- How can we determine Ca, Na, Cl in the samples in
the presence of interfering components (e.g. Mg, Al, Si):
- Choose the proper - nuclear reaction
- analytical gamma line
- irradiation, decay and measuring times,
- Calculate the quantity of the elements to be used
for standardization.
2. Sample preparation:
- weigh the samples into
polyethylene bags using analytical balance
- prepare standards using micropipettes
3. Irradiation of the samples using pneumatic
system of the reactor
4. Measure the gamma-spectra, evaluate the spectra
(determine the peak areas at the given gamma-lines)
5. Identify the isotopes in the spectra using
gamma library. Determine the elemental concentrations and their uncertainties
using standard method (Eq. 4.).
5. Data processing – uncertainty estimation
Thermal neutronflux (at
10kW): 2,4·1011 neutron/cm2sec,
measurable intensities at the chosen gamma line: 10 counts/sec.
|
Element |
Nuclear reaction |
s Barn |
Isotopic abundance % |
T1/2 |
Gamma energy keV |
Gamma abundance % |
Efficiency* % |
|
Na |
23Na(n,g )24Na |
0.513 |
100 |
14.959
h |
1368.6 |
100 |
0.246 |
|
Mg |
26Mg(n,g)27Mg |
0.0372 |
11.01 |
9.458
m |
843.8 1014.4 |
71.4 28.6 |
0.371 0.318 |
|
Al |
27Al(n,g)28Al |
0.226 |
100 |
2.24
m |
1778.9 |
100 |
0.193 |
|
Cl |
37Cl(n,g)38Cl |
0.423 |
24.23 |
37.21
m |
1642.4 |
32.5 |
0.208 |
|
Ca |
48Ca(n,g)49Ca |
1.12 |
0.187 |
8.719
m |
3084.4 |
91.7 |
0.0996 |
|
Mn |
55Mn(n,g)56Mn |
13.2 |
100 |
2.578
h |
846.8 1810.7 |
98.9 27.2 |
0.37 0.189 |
·
Efficiencies of the HPGe PopTop detector at
50 mm distance.
6. Knowledge check
1. What is the difference
between absolute and relative methods?
2. How can we determine
e.g. Ca in lead (why it is difficult)?
3. How can we measure Al
in soil?
7. References
[1] A.Vértes, S. Nagy, K:
Süvegh, Nuclear Methods in Mineralogy and Geology, Plenum Press, New York and
London (1998)
[2] D.Soete, R. Gijbels,
J.Hoste, Neutron Activation Analysis, Wiley Interscience, New York, 1972
[3] H.Das, A.Faanhof, H.
Van Der Sloot, Radioanalysis in Geochemistry, Elsevier, Amsterdam, 1989
[4] Z. Alfassi, Activation
Analysis, CRC Press, Boca Raton, 1990
[5] M. Al-Mugrabi, N.
Spirou, J.Radioanal.Nucl.Chem. 110,67 (1987)
[6] F.Girardi, G. Guzzi,
J. Pauly, Anal. Chem.36(8), 1558 (1964)