Journal of Radioanalytical Chemistry, Vol. 10 (1972) 83--88

E X A M I N A T I O N OF SOME B I N A R Y M I X T U R E S BY I N E L A S T I C N E U T R O N S C A T T E R I N G WITH A RADIOISOTOPE NEUTRON SOURCE T. B. PIERCE, C. R. BOSWELL,* K. HAINES Analytical Sciences Division, A . E . R . E . ,

Harwell, Nr. Didcot, Berks. (England)

(Received May 24, 1971)

Magnesium, aluminium and iron have been determined in sand at levels of > 1 by inelastic neutron scattering. Neutrons were obtained from a 1 Ci polonium-beryllium neutron source and analytical determination was based on measurement of the characteristic ~-lines emitted during nuclear de-excitation.

Introduction The usual techniques of neutron activation analysis are based on the measurement of the induced radioactivity of radioactive isotopes, formed as a result of neutron irradiation of the sample. This necessarily restricts application of the method to the determination of those elements which form measurable radioisotopes, under neutro~~ irradiation. Neutron activation may be still further limited by restriction on the type of neutron source that can be considered for a specific application since interactions occurring between target nuclei and incident neutrons will be dependent on neutron energy, which in turn will be a function of the method of neutron production. When suitable radioisotopes are not formed f r o m the element to be determined, or when the relevant decay radiation cannot be reliably measured because of nuclear or other interferences, a possible alternative may be to measure the p r o m p t y-radiation resulting from capture, inelastic scattering or other nuclear reaction processes. High background caused by the close proximity of neutron source to the y-ray detector complicates measurement of y-radiation emitted during irradiation, but despite this difficulty p r o m p t y-techniques can provide the basis of a satisfactory analytical method, and are particularly useful for the determination of certain light elements, such as boron and carbon, which cannot be easily measured by other neutron techniques. Analytical methods based on inelastic neutron scattering have so far received little attention, although carbon, oxygen, 1 aluminium, silicon and magnesium z have been determined with neutrons from an accelerator source and inelastic scattering has been assessed for extraterrestrial 3 analysis. Since both neutrons and y-photons are penetrating radiation, analytical results can be obtained f r o m * Present address: Massey University, Palmerston North, New Zealand. 6*

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a relatively large volume of sample, so that inelastic neutron scattering is potentially useful for the analysis of bulk materials which are relatively inhomogeneous, and for which analysis of a small volume of material would give unsatisfactory results without careful sampling. Thus, an obvious application is to provide analytical information for the purposes of quality control from moving, solid streams which are part of a plant production process. Accelerator neutron sources can be large and expensive and may require substantial maintenance, so that there can be considerable attractions, particularly for long-term operation, in devising sensors for process control applications which are based on the use of radioisotope neutron sources. Neutron output of those radioisotope neutron sources generally available is lower than can be obtained from accelerator sources, but in some cases radioisotope source neutrons can excite y-lines of sufficient intensity from major constituents to provide a practical means of analytical measurement. The 4.4 MeV y-line from the reaction 12C(n,nt)12C has been excited by neutrons from an americium-beryllium neutron source, and the line intensity measured to determine carbon in sinter mix and fly ash, a while a variety of p r o m p t y-techniques have been examined to provide a method for elemental analysis in boreholes. ~ This paper describes the determination of magnesium, aluminium and iron in simple binary mixtures of solid materials by inelastic scattering of neutrons from a 21~ neutron source.

Experimental Neutrons for inelastic scattering were obtained from a 1 Ci 21~ neutron source giving an output of 2.5 9 106 n 9 sec-~; total y-rays were detected with a 3" x 3" NaI(T1) scintillator and output pulses from the scintillator were passed, after amplification, to a small digital computer programmed to operate as a multichannel pulse-height analyser. Shielding between neutron source and detector was necessary, not only to limit direct interaction of the neutrons with the detector itself, but also to reduce the total background in the detector from the 7-radiation emitted by the isotope source. The 4.4 MeV y-rays, emitted during the de-excitation of 12C formed in an excited state by the reaction 9Be(e, n)12C occurring within the source, were particularly troublesome, and a 6" shadow bar of lead or tungsten was inserted between source and detector to attenuate the total signal recorded by the detector in the absence of sample. Spectra were usually accumulated for 10 min and after accumulation in the computer memory, data was read out, stored on magnetic tape and subsequently processed on an IBM 360/75 computer by a least-squares fitting procedure. Calculation required information from standard spectra of all elements contributing y-lines to the y-spectrum, and consequently pure oxides were irradiated to obtain the necessary information. Sample size was generally 1 - 2 kg although larger quantities of sample could be examined if necessary. J. Radioanal. Chem. 10 (1972)

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Results and discussion

Analysis of samples by inelastic neutron scattering is based on the measurement of 7-lines emitted during the de-excitation of nuclei raised to their first or higher excited state by interaction with energetic neutrons, and the low energy-level density of light nuclei often results in well defined, characteristic 7-lines which are suitable for measurement. Inelastic scattering is therefore well suited for the determination of light elements which in fact are often difficult to determine by wellestablished methods of solid analysis such as X-ray fluorescence and must often be measured in plant analysis of minerals and raw materials. Neutron techniqt es are particularly valuable for the analysis of inhomogeneous solids since the relatively large volume of sample examined reduces errors which may be introduced by any heterogeneity of the sample material. The work reported here was intended to investigate the possibility of measuring some of the elements commonly present in mineral feeds by inelastic scattering, with neutrons from a radioisotope neutron source, in order to assess the potential for development of a sensor suitable for long-term operation. Silica was chosen as providing a realistic l~ase material for measurements and iron, magnesium and aluminium, presel~t at levels of 1 ~ or greater, were determined in different mixtures. y-Lines on which measurement was based, together with their origin are shown in Table 1. In all cases lines of major intensity were well separated from the 1.77 MeV line emitted by silicon and instrumental resolution provided no problem. More troublesome was the background induced in the detector by neutrons or by y-photons from the source itself. Neutron capture in the detector leads to p r o m p t y-ray emission and to production of the radioactive isotopes 24Na and 128I, SO that the induced activity of the detector will increase during use. Inelastic scattering to levels of 23Na and 127I will further add to the detector background as will recoils produced in the detector. The neutron source yields y-photons, not only from the decay of the alpha emitter, but also from de-excitation of the excited states of 12C formed from 9Be by the (c~, n) reaction so that scattered y-rays also add to the measured background. In addition, capture y-rays from the sample, and radiation resulting from interactions occurring in the scintillator housing will Table 1 y-Lines of major intensity emitted by magnesium, aluminium, silicon and iron by inelastic scattering of radioisotope source neutrons Element

Interacting nucleus

E>,

Transition

1.37

(1) ~ (o) (1) ~ (o)

Magnesium Aluminium

24Mg

27A1

0.84

28Si 56Fe

1.01

(2) ~ (o)

Silicon Iron

i .77 0.84

(1) ---, (o)

(1) ~ (o)

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also be detected to complicate further the accumulated spectra. Complete compensation for background by running a blank of some kind is difficult since all properties of the sample affecting the accumulated y-ray spectrum should be reproduced save that of characteristic y-ray emission. However, satisfactory spectra could be obtained, and spectra accumulated during irradiation of alumina, magnesia and ferric oxide as well as silica are shown in Fig. 1. Aluminium, magne0.8/*

1.37 I

1.01 I

i

1.77 I

I I

-... ......i.~.....~. "-,..

i".. ':' .

~~

-

9' ~ .

"

i

~---'AXc .,.

..

--B -ira,.-

Energy,

MeV

Fig. 1. v-Ray spectra resulting from the inelastic scattering of source neutrons from A -- alumina, B - - magnesium oxide, C - ferric oxide and D - - silica sium, iron and silicon can all be seen to give those characteristic y-lines expected from the reactions shown in Table 1 and analytical determination was based on the measurement of line intensity. The effect of sample size on y-ray yield was investigated to assess the response of the experimental system. A cylindrical sample holder was placed on the sodium iodide scintillator and the neutron source positioned centrally in the cylinder on a shadow bar in the shape of a truncated cone 6" long. The intensity of the 1.77 MeV y-line was measured from cylindrical samples of sand varying in both diameter and depth and results obtained are given in Fig. 2. The decreasing sensitivity of response occurring with increasing sample height is clearly shown and demonstrates the restrictions on gains in 7-line yield that can be achieved by increasing the sample size. J. Radioanal. Chem. 10 (1972)

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The experimental configuration employed for measurement of 7-1ine intensities from mixtures is shown in Fig. 3. Boron or 6Li filters could be placed between sample and detector, if necessary, to absorb thermal neutrons, and no spectra were accumulated for quantitative measurement until the source-detector configuration had been established for at least 24 hrs. tn

I

o

o

.

0

2

o~

4

.

,

i i 6

9

~

oB

_

8 Sample height, inch

F i g . 2. V a r i a t i o n o f i n t e n s i t y o f t h e 1.77 M e V ~/-line f r o m s i l i c o n w i t h size o f s a n d s a m p l e s . S a m p l e d i a m e t e r s : A - - 6", B - - 8", C - - 11" f

......

///3 ~7/,

/////"//~

Fig. 3. E x p e r i m e n t a l c o n f i g u r a t i o n e m p l o y e d f o r the m e a s u r e m e n t o f x - l i n e intensities o f m i x t u r e s , A - - sample, B - - r a d i o i s o t o p e n e u t r o n source, C - - s h a d o w bar, D - - s c i n t i l l a t o r and p h o t o m u l t i p l i e r h o u s i n g

Spectra from samples and standards were accumulated for l0 and 20 min, respectively, and a background counted for a much longer period, usually several hours, with source and detector in the configuration as shown in Fig. 3, but in the absence of a sample giving rise to characteristic 7-lines. No attempt was made to distinguish between the different components of the background spectrum, but essentially no difference could be found in the results calculated by a least squares fitting procedure, whether the background spectrum was inserted into the calculations as a constant or fitted as an unknown. J. Radioanal. C h e m . 1 0 ( 1 9 7 2 )

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Q u a n t i t a t i v e results calculated f r o m y-spectra a c c u m u l a t e d d u r i n g the i r r a d i a t i o n o f b i n a r y mixtures o f the oxides o f iron, m a g n e s i u m or a l u m i n i u m with silica are presented in Fig. 4. F o r all mixtures, a linear relationship was f o u n d between the y-ray yield derived f r o m the a d d e d element a n d the k n o w n c o n c e n t r a t i o n of t h a t element p r e s e n t in the mixtures. Precision o f the m e a s u r e m e n t s was governed by the intensity o f the y-line to be m e a s u r e d which was a f u n c t i o n n o t only of

B

10

20 Percent

Fig. 4. Calibration curves of the variation of ~-line intensity with concentration obtained from binary oxide mixtures. Matrix silica, A - - alumina, B - - ferric oxide, C - - magnesium oxide

c o n c e n t r a t i o n b u t also of the type o f SiO2 used in the matrix. The q u a n t i t a t i v e m e a s u r e m e n t s p r o v i d i n g the results shown in Fig. 4 were o b t a i n e d with w a s h e d sand, a n d in this m e d i u m i r o n was d e t e r m i n e d at the 5 ~ level, with a coefficient o f v a r i a t i o n o f ~ 10 %.

References 1. T. C. MARTIN, I. L. MORGAN, T. D. HALL, Proc. Intern. Conf. on Modern Trends in Activation Analysis, Texas A and M University, College Station, Texas, 1965, p. 71. 2. T. B. PIERCE, P. F. PECK, D. R. A. CUFF, J. Radioanal. Chem., 4 (1970) 305. 3. J. A. WAGGONER, in Analytical Chemistry in Space (R. E. WAINERDI, Ed.), Pergamon, Oxford, 1970, p. 165. 4. R. F. STEWART, I. S. zff. Transactions, 6 (1967) 200. 5. A. K. BERZIN, D. F. BESPALOV,V. M. ZAPOROZHETS, S. A. KANTOR,D. I. LEIPUNSKAYA, V. V. SULIN, I. I. FELDMAN,Y. S. SHIMELEVICH,At. Energy Rev., 4 (1966) No. 2, 59. 9L R a d i o a n a l . C h e m . 10 ( 1 9 7 2 )