1. DOI: 10.1007/s10967-007-6977-6 Journal of Radioanalytical and Nuclear Chemistry, Vol. 275, No.1 (2008) 89–95
0236–5731/USD 20.00 Akadémiai Kiadó, Budapest
© 2007 Akadémiai Kiadó, Budapest Springer, Dordrecht
Epiboron NAA: An option to analyze unfavorable matrices
R. Sz ke,* I. Sziklai-László
MTA KFKI Atomic Energy Research Institute, P.O. Box 49, H-1525 Budapest, Hungary
(Received January 30, 2007)
Boron filter with 500 mg/cm2
surface density has been constructed and used for epithermal neutron activation analysis of geological and biological
materials. Bare and boron-covered irradiations were performed to determine the boron activation ratios (RB) and improvement factors (IFB) for 23
nuclides. Biological and geological SRMs were also analyzed to demonstrate the practical use of this technique. Experiments have proved that
epiboron activation extends the applicability of NAA to samples with unfavorable matrices (i.e., Na, Ca, P, Sc, etc.).
Introduction
When applying instrumental neutron activation
analysis (NAA), induced activities of major components
can occasionally dominate the resulting spectrum
hampering the detection of trace elements in a sample to
be investigated. For example, most irradiated biological
materials need at least 1 week cooling time to get rid of
the excessive 24Na activity before measurements. If the
determination of the 75Se is also aimed at, an additional
2–3 months waiting time is required to eliminate the
disturbing high energy -radiation and associated
bremsstrahlung of 32P. This bremsstrahlung shows up
in the gamma-spectra as a continuous background
and affects the measurement precision of nuclides,
particularly at low energies. These effects could result in
an unfavorably long turn-around time of the analysis. A
solution to the above mentioned problems is the
application of selective irradiation.
Selective irradiations are mostly accomplished by
irradiating samples in a Cd box. Cadmium has a large
resonance absorption cross section at 0.178 eV (19910
barns for 113Cd), resulting in a sharp cut-off for neutrons
with <0.5 eV kinetic energy. The main disadvantages of
Cd are the low melting point (320.9 °C), and the high
residual activity after irradiation (115mCd T1/2 = 45 day).
Since Cd is a soft metal, this further complicates
handling procedures and eventually rules out the use of
the same Cd box in subsequent irradiations.
Boron and boron compounds were also used as
neutron absorber by several authors.1–3 The main
advantages of boron filters are: the high melting point
(2450 °C), the high thermal neutron absorption cross
section (3837 barns for 10B) and the low residual
activity induced by the impurities in boron. The filter
cutoff energy depends on the boron thickness and can be
varied between 10–300 eV.
Since the application of boron filters usually needs a
unique irradiation channel in any core arrangements, a
reusable boron container was constructed for the rotating
* E-mail: rszoke@sunserv.kfki.hu
channel No. 17, which is the largest in diameter at the
Budapest Research Reactor.
In the present paper, the experimentally determined
boron activation ratios and calculated improvement
factors are reported. Occasionally, measured Cd ratios
are also listed. The accuracy and precision of the new
technique was tested via the analysis of IAEA Soil-7
standard reference material.
Experimental
Boron filter construction
Figure 1 shows the schematic diagram and a picture
of the boron container constructed in our laboratory.
The 99.4% purity B4C powder (10B 19% min.) used
for shielding material was commercially available from
Alfa Aesar (maximum metallic impurities: Fe 0.05%, Si
0.15%, Al 0.05%, others 0.35%). The double-walled
container was made from aluminum cylinders of 2 mm
wall thickness. The B4C powder was filled into the
container using a hand-operated press, with a pressure of
approximately 15 MPa. An optimum natural boron
thickness giving 500 mg/cm2 surface density was
derived from the work of ROSSITTO et al.4
Since the channel No. 17 is cooled by the reactor
coolant (~50 °C) the sample decomposition is reduced to
a minimum, therefore, the heat-sensitive biological
samples remain in shape after irradiation.
Standards, sample preparation
Standards and reference materials used for the
determination of boron and cadmium ratios (Table 1)
were obtained from the following institutions: Institute
for Reference Materials and Measurements (IRMM);
National Institute of Standards and Technology (NIST);
International Atomic Energy Agency (IAEA);
GoodFellow Ltd.; Alfa Aesar.

2. R. SZ KE, I. SZIKLAI-LÁSZLÓ: EPIBORON NAA: AN OPTION TO ANALYZE UNFAVORABLE MATRICES
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Fig. 1. Schematic diagram and a picture of the boron container
For the analysis of geological and biological
materials 150–200 mg were sealed in high purity quartz
vials (Suprasil AN, Heraeus) and then packed in the
boron shielded container.
Irradiation
The bare and boron-shielded samples and flux
monitors were irradiated for 24 hours in the Budapest
Research Reactor at a thermal neutron flux density of
1.5.1013 n.cm–2.s–1, thermal-to-epithermal neutron flux
ratio f = th/ epi = 49 and = 0.015, where the
epithermal flux is represented by the 1/E1+ function
and is a form factor. Following boron-covered
irradiations, all samples were measured after a minimum
delay of approximately 1 hour on account of the
substantial reduction of matrix activities 24Na
(T1/2 = 14.95 h), 32P (T1/2 = 14.28 d) and 82Br
(T1/2 = 1.47 d). Following bare irradiations, the first
measurement of the samples were carried out after 4–5
days. Counting times varied between 1800 seconds and
10 hours. Gamma-spectrometric measurements were
performed with a Canberra HPGe detector (energy
resolution of 1.74 keV and relative efficiency of 36% for
the 1332.5 keV 60Co line), and associated linear
electronics consisting of an 8K ADC and an
ACCUSPEC/B type 16K MCA board. The spectrometer
was equipped with a Westphal-type Loss-Free Counting
(LFC) module operated in Dual Spectrum Mode
providing full compensation for all kind of counting
losses. The gamma-ray spectra were evaluated by the
program HYPERMET-PC,6 involving automatic peak
search, energy calibration, net peak counts computation.
The quantitative evaluation of the INAA multi-element
measurements was based on the k0 standardization
method using gold and zirconium flux monitors co-
irradiated with the samples. An in-house program
NAACNC was used for isotope identification and
elemental concentration calculations.7
The IAEA-Soil-7 reference material was analyzed by
ENAA to test the accuracy of the implemented
technique.
Results
Determination of boron activation ratios
The activation ratios were calculated for individual
(n, ) reactions using the specific activities of samples
induced in bare and filtered irradiations.

3. R. SZ KE, I. SZIKLAI-LÁSZLÓ: EPIBORON NAA: AN OPTION TO ANALYZE UNFAVORABLE MATRICES
91
The improvement achieved by filtered irradiations may
be expressed as the so-called improvement factor:1
)1(
)2(
B
B
B
R
R
IF ,
where RB(1) refers to the determinant, and RB(2) refers to
the dominant interfering element . Table 2 lists the
nuclear data for the radionuclides used, RCd and RB and
the calculated IFB for interferences of 24Na and 46Sc.
Sodium and scandium were chosen as typical
interferences always present in biological and geological
samples.
Effect of the boron filter on the neutron flux distribution
Boron’s absorption cross section follows the 1/v law
over a wide energy range (from about 0.001 to a few
hundred eV). The resulting EB = 15.2 eV cutoff for
500 mg B/cm2 surface density allows a very effective
depression of strongly activating 1/v and low resonance
target isotopes while reducing only slightly the
activation of analytically important, higher than 10 eV
resonance elements such as As, Sb, Th, Zn, U (Fig. 2).
An additional advantage of epiboron NAA is the
significant reduction of the analysis turn-around time.
Using a boron shield, the measurements could be started
after about 1 hour waiting time. Consequently, some
short-lived trace elements as Mn, As, Mg, Ti, ,etc. can
be determined instrumentally with higher accuracy and
minimum delay (Fig. 3).
Fig. 2. Excerpt of a MathCad program showing the approximate neutron flux distributions in the
rotating channel No. 17 of the Budapest Research Reactor using 1 mm Cd and 500 mg/cm2
natural boron filters

4. R. SZ KE, I. SZIKLAI-LÁSZLÓ: EPIBORON NAA: AN OPTION TO ANALYZE UNFAVORABLE MATRICES
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5. R. SZ KE, I. SZIKLAI-LÁSZLÓ: EPIBORON NAA: AN OPTION TO ANALYZE UNFAVORABLE MATRICES
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Table 2. Nuclear data, Cd B ratios and improvement factors for some (n, ) reactions of interest
Nuclear data Activation ratio Improvement factor
Element Nuclear reaction
r, eV I0/ 0 RCd RB SD, % IFB(Na) IFB(Sc)
Ag 109
Ag(n, )110m
Ag 6.08 16.7 – 26 (3) 0.58 0.95
As 75
As(n, )76
As 106 13.6 4.2 6.2 (2) 2.38 3.91
Au 197
Au(n, )198
Au 5.7 15.7 3.9 26 (1) 0.56 0.92
Br 81
Br(n, )82
Br 152 19.3 – 4.7 (1) 3.16 5.16
Ce 140
Ce(n, )141
Ce 7200 0.83 – 115 0.13 0.21
Co 59
Co(n, )60
Co 136 1.99 – 45 (4) 0.34 0.54
Cs 133
Cs(n, )134
Cs 9.27 13.2 – 15 (4) 1.06 1.73
151
Eu(n, )152
Eu 0.448 0.87 – 440 (4) 0.03 0.06Eu
153
Eu(n, )154
Eu 5.8 5.66 – 40 (1) 0.37 0.61
Fe 58
Fe(n, )59
Fe 637 0.97 50 92 (2) 0.16 0.26
La 139
La(n, )140
La 76 1.24 – 69 (2) 0.22 0.35
Na 23
Na(n, )24
Na 3380 0.59 – 220 (5) – –
Rb 85
Rb(n, )86
Rb 839 14.8 – 4.7 (3) 3.15 5.16
121
Sb(n, )122
Sb 13.1 33.0 – 7.6 (5) 1.94 3.20Sb
123
Sb(n, )124
Sb 28.2 28.8 – 6.1 (3) 2.46 4.08
Sc 45
Sc(n, )46
Sc 5130 0.43 103 589 (5) – –
Se 74
Se(n, )75
Se 29.4 10.8 – 13 (6) 1.16 1.91
Th 232
Th(n, )233
Th/233
Pa 54.4 11.5 – 8.8 (2) 1.68 2.75
64
Zn(n, )65
Zn 2560 1.91 26 30 (6) 0.49 0.79Zn
68
Zn(n, )69m
Zn 590 3.19 16 18 0.81 1.34
94
Zr(n, )95
Zr 6260 5.31 10 9.3 (2) 1.60 2.63Zr
96
Zr(n, )97
Zr/97m
Nb 338 251.6 1.2 1.4 (2) 10.6 17.3
U 238
U(n, )239
U/239
Np 16.9 103.4 – 4.4 (2) 3.36 5.51
Fig. 3. A typical spectrum region of a geological sample irradiated for 24 hours in a boron filter and measured after 30-minute waiting time

6. R. SZ KE, I. SZIKLAI-LÁSZLÓ: EPIBORON NAA: AN OPTION TO ANALYZE UNFAVORABLE MATRICES
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Fig. 4. Gamma-ray spectra of the SRM NIST -613 Trace Element in Glass samples irradiated with and without a boron filter. Upper spectrum:
without B-filter ti = 24 hours, td = 53 hours, tc = 1800 s; lower spectrum: with B-filter ti = 24 hours, td =6 hours, tc = 3600 s
Table 3. Analysis results of SRM IAEA-Soil-7 by INAA and ENAA
Analysis results, mg/kg (mean SD%)
Element Certified value
by IAEA
INAA ENAA
As 13.4 (6) 14.3 (5) 14.3 (5)
Ce 61 (11) 57 (5) 57 (5)
Co 8.9 (10) 8. 7 (5) 8.9 (6)
Cr 60 (21) 67 (5) **
Cs 5.4 (14) 5.4 (14) 5.2 (10)
Br* 3–10 5.4 (6) 5.3 (5)
Hf 5.1 (7) 4.9 (5) **
La 28 (4 ) 28 (5) 25 (7)
Rb 51 (9) 52 (7) 49 (8)
Sb 1.7 (12) 1.8 (10) 1.6 (5)
Sc 8.3 (13) 8.4 (4) –
Sm 5.1 (7) 4.9 (10) **
Ta 0.8 (25) 0.7 (10) **
Th 8.2 (13) 7.9 (6) 8.2 (6)
U 2.6 (21) 2.4 (6) 2.5 (5)
Yb 2.4 (15) 2.2 (5) **
Zn 104 (6 ) 107 (6) 106 (6)
Zr 185 (6 ) 198 (12) 185 (6)
Tb 0.6 (33) 0.7 (19) **
Eu 1 (20) 0.9 (7) 0.8 (8)
Ca 163000* 160140 (6) **
Fe 25700* 25484 (4) 27336 (9)
* Information value.
** No ENAA results due to the lack of RB factors.

7. R. SZ KE, I. SZIKLAI-LÁSZLÓ: EPIBORON NAA: AN OPTION TO ANALYZE UNFAVORABLE MATRICES
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Table 4. Comparison of the detection limits of INAA and ENAA for some elements
in SRM IAEA-Soil-7
LD, mg/kg
Element
with INAA with ENAA
As 0.03 0.001
Cs 0.04 0.005
La 0.01 0.001
Rb 1.2 0.82
Ta 0.05 0.002
Tb 0.01 0.001
Th 0.02 0.01
U 0.01 0.005
W 0.07 0.001
Zn 2.0 0.1
Irradiation time: 24 hrs 24 hrs
Cooling time: 6.2 days 0.3 days
Counting time: 120 min 80 min
Samples of the SRM NIST-613 Trace Element in
Glass were also irradiated with and without a boron
filter. In the case of bare irradiation the 1368 keV
photopeak for 24Na is still dominating (Fig. 4 upper
curve) after 53 hours decay time. Following boron-
filtered irradiation the activity of 24Na is significantly
reduced, consequently the background of the spectrum
decreases. Therefore, the analytical sensitivities are
much better for the isotopes with short half-lives (lower
curve).
To test the applicability of this technique SRM
IAEA-Soil-7 material was analyzed by ENAA, and for
comparison, parallel samples were measured by INAA.
Table 3 gives the element concentrations determined
with both techniques.
The initial sensitivity of the determination by ENAA
depends among other things on the ratio of resonance
integral to the thermal cross section, and on the levels of
the elements whose neutron induced radionuclides
produce the Compton continuum background below the
analytical peak of interest. The obtained detection
limits8 (LD) for some elements in IAEA Soil-7 reference
material measured by INAA and ENAA are presented in
Table 4.
The experimental results confirm that the accuracy
and precision of the two methods are comparable, so the
recently developed epiboron NAA can be integrated into
the routine NAA work of our laboratory.
Conclusions
If an NAA laboratory is engaged in routine analysis
of geological and biological materials, the use of
selective irradiation is unavoidable to unfold the
resulting complex spectra. Cd filters were generally used
to depress the activation of the so-called 1/v isotopes,
but several disadvantages of Cd make the method
unfeasible. In this study a large volume boron filter has
been constructed and tested. The filter is reusable and
following subsequent irradiations, no mechanical
damages have been observed so far.
The results of this study proved, that by using boron
irradiation filters significant improvements in detection
sensitivity were obtained for those (n, ) reactions having
strong and >10 eV resonances in the epithermal region.
The reliability of epiboron NAA is comparable to
conventional thermal neutron activation analysis, and
the number of the elements that can be determined
instrumentally in biological and geological materials is
considerably extended.
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