Use of neutron-energy moderation for resolving interferences in fast-neutron activation analysis

Talanta,

1968,

Vol.

15, pp. 1153 to 1158.

Permtton

Press.

Printed

in Northern

Ireland

USE OF NEUTRON-ENERGY MODERATION FOR RESOLVING INTERFERENCES IN FAST-NEUTRON ACTIVATION ANALYSIS T. B. PIERCE, J. W. EDWARDS and K. HAINE~ Analytical Sciences Division, A.E.R.E., Harwell, Nr. Didcot, Rerks, U.K. (Received 29 February 1968. Accepted 25 March 1968)

Summary-Four elements, vanadium, chromium, manganese and iron, have been determined in nickel-base samples after measurement of the intensity of two y-lines; the 0.85-MeV y-ray emitted by 66Mn (produced from both Mn and Fe), and the 1*44-MeV y-ray from Y (arising from V and Cr). The two elements contributing to each y-peak were assayed separately from data obtained by irradiating each sample twice with neutrons of different energy distributions. ACCELERATOR neutron sources have found application in neutron-activation analysis either when nuclear reactors have been inaccessible, or when fission neutrons have been incapable of inducing suitable nuclear reactions in the element to be determined. In particular, neutron generators producing 1PMeV neutrons by the reaction 3H + 2H + 4He + ln + 17.6 MeV

(1)

have received considerable attention, as high neutron outputs can be obtained at relatively low accelerating voltages of 100-150 kV. Highly endoergic reactions can take place with 14MeV neutrons and a number have proved to be of considerable analytical importance (e.g., leO(n, p)laN) but nuclear interferences frequently occur between near-neighbours in the periodic table as the same radionuclide may be formed from more than one target element. These nuclear interferences often impose major limitation on the usefulness of fast-neutron activation analysis, particularly when the interfering elements cannot be determined separately and in certain cases only the sum of the required activity and that from interferences can be assayed. Fast neutrons from a neutron generator employing the D-T reaction [equation (l)], together with the slow neutron component formed by moderation of the fast flux in the generator and surrounding materials, may induce (n, p), (n, a), (n, 2n), (n, n’) and (n, y) reactions. Thus the nuclide zAM, where A represents mass number and Z the nuclear charge, may be formed by the following routes: *JlM(n,

*;lM(n,

2n) zAM

(2)

14z*M

(3)

s*M(n,

n’) zAM

(4)

&N(n,

P) zAM

(5)

$ziO(n,

a)

zAM

There is no change of atomic number in reactions these reactions will all serve to reinforce the contribution 1153

(6) (2), (3) and (4) and therefore of zAM formed from element

1154

T. B.

PIERCE,J. W. EDWARDS and K. HAINES

M. Consequently zAM may be produced from three source elements M, N and 0 and the intensity of the characteristic radiation from zAM will thus be made up of contributions from all three if these are present in the sample in appreciable quantities. This may be written Cr = s3r w,, -I- SN w, + Sow, (7) where Cr is the total count of the radiation from zAM (say measured over a y-ray peak), S, is the specific activity of the sample derived from element X in counts/g under a given set of irradiation and counting conditions and W, is the weight of element X in the sample. The mode of decay of the radionu~lide xAM is independent of its method of formation and therefore provides no means of assessing the proportion of the yields derived from different target nuclei, but the specific activity S, is not only characteristic of element X but is also a function of neutron energy. It has already been shown1 that if the neutron energy is chosen carefully from a knowledge of reaction thresholds and Coulomb-barrier restrictions, interferences in fast-neutron activation analysis can be subs~ntialIy reduced. An accelerator capable of functioning as a versatile neutron source and producing neutrons of various preselected energies must necessarily be more complex and expensive than a simple 14MeV neutron generator, but an alternative method of obtaining neutrons with an energy of less than 14 MeV would be to reduce the energy of the neutrons from the generator by moderation. Neutrons with more than one energy distribution could therefore be used to irradiate the sample and equation (7) would then be rewritten C,A = s,*

w, + s,*

w, + s,*

wo

(7A) where the superscript A is used to distinguish the different conditions available for irradiation of the sample. Thus three irradiations at different neutron energies would yield data for three simul~neous equations of the form of equation (7A) which could be solved for W,, W, and W. once values for the specific activity constants had been found from standards. For solution of these equations it is clearly important that the product Sx W, should be significant when compared with C, in at least one of the equations for each element. A multiple irra~ation procedure will clearly take longer to complete than activation analysis involving a single irradiation and counting cycle but the time penalty may not be severe if the additional irradiation and counting periods are kept short Insertion of moderator between neutronand provide extra useful information. emitting target and sample will reduce the flux through the sample but in certain cases the relative changes in specific activity which occur in moving the sample to the lower flux positions may reduce masking of small quantities of activity from one element by large quantities from another. As an example of the application of the technique we describe here the determination of four elements, vanadium, chromium, manganese and iron, in nickel-base samples by measurement of the yields of two radionuclides manganese-56 and vanadium-52. EXPERIMENTAL Neutron generatop. The installation used in this work was based on a seaIed-tube unit with a maximum neutron output of 10’” n/set totai (Elliott Electronic Tubes Ltd.). The system, which has been described in detail elsewhere,* was capable of controlling irradiation, cooling and counting times, flux monitor operation etc.

Resolving interferences

in fast-neutron

activation analysis

1155

Preparation for irradiation Samples were in the form of powder or turnings and were always pressed into cylinders with a 30-ton hydraulic press before irradiation, to ensure constant geometry and to avoid errors due to compacting during transfer. Dies were manufactured so that the pressed samples were of the correct diameter to fit into the standard polyethylene capsules used with the generator; the thickness of the compacted samples was controlled by pressing a measured weight of material. Standards were made up to have a composition which was as similar to that of the samples as possible. Known quantities of the elements to be determined were added to nickel powder and carefully agitated in a special mixer before pressing. y-Ray spectroscopy. A 3 x 3 in. thallium-activated sodium iodide scintillator was used in conjunction with a 512-channel pulse-height analyser for y-ray spectrometry. Data manipulation and calculation was carried out in a laboratory data processor. RESULTS

AND

DISCUSSION

The products of the (n, p), (n, a), (n, 2n) and (n, r) reactions with the most abundant isotopes of vanadium, chromium, manganese and iron are shown in Table I together with cross-sections for the production of the isotopes and the half-lives and y-ray energies of the products. The cross-sections for the (n, p), (n, m) and (n, 2n) reactions are for neutrons with an energy of En = 14 MeV;3 capture cross-sections apply to thermal neutrons. The difficulty of determining the elements listed in Table I in the presence of each other is immediately apparent. Chromium can be determined by the reaction %!r(n, P)~~Vbut vanadium and manganese interfere by the (n, 7) and (n, a) reactions respectively, while the (n, r) reaction on manganese interferes with the determination of iron by the (n, p) reaction. The reactions of cobalt are not included in Table I as the quantity of this element in the samples was known to be below the level that could be detected with the neutron generator, but at higher concentrations the presence of cobalt would interfere with iron and manganese determinations by the production of 56Mn by the reaction 59Co(n, a)56Mn. Vanadium in the sample could be separately assayed from the intensity of the 0.32-MeV 61Ti peak provided that the yield of low-energy y-rays and Compton background was not high in the same region of the y-spectrum, but for samples with high chromium content, allowance would be necessary for the 0.32-MeV contribution TABLE L-REACTIONS

OF GENERATOR NEUTRONS WITH MOST ABUNDANT ISOTOPES OF VANADIUM, CHROMIUM, MANGANESE AND IRON

Abundance, Element

%

Vanadium

99.76

Chromium

83.76

Manganese

Iron

100.0

91.68

Reaction

61V(n, p)51Ti 61V(n, u)%c 61V(n, 2r@W W(n, r)? Wr(n, p)“W Wr(n a)40Ti Wr(n: 2n)Wr %r(n, y)Wr 65Mn(n, p)Wr 6SMn(n, @*V S5Mn(n, 2n)64Mn 65Mn(n, y)SsMn 6BFe(n, p)68Mn 66Fe(n, Cr)Wr &OFe(n,2n)55Fe 66Fe(n, y)57Fe

Cross-section, mbarns

27.0 28.6 4500 78 285 75 30 825 13,300 103 00 -

Half-life of product

Er of product, MeV

5.8 min 44 hr Long 3.76 min 3.76 min Stable 27.8 d Stable 3.5 min 3.76 min 303 d 256 hr 2.56 hr Stable 2.7 yr Stable

0.32 0.99 1G 144 0.32 no Y 144 0.84 0.85, 1.81, 2-l 0.85, 1.81, 2.1 no Y -

1156

T. B. PIERCE,J.

W. EDWARDSand K. HAINES

Moderator thickness,

cm

FIG. 1. Relative variation of neutron flux with distance from neutron-emitting target.

of %!r produced by the reaction 52Cr(n, 2n)51Cr. However, the multiple irradiation technique described here permitted vanadium to be determined from the combined vanadium and chromium contributions at 1.44 MeV. The absence of cobalt from the samples greatly simplifies analytical determination since suitable equations can be derived for irradiations carried out in two positions, one in which fast-neutron and the other in which thermal-neutron reactions predominate in the sample. The energy distribution of neutrons in the irradiation position will depend on the design of the neutron generator and its surroundings, but Fig. 1 shows the relative neutron flux at different distances from the neutron-emitting target of the sealed-tube generator used for this work. Data were obtained by securing monitors to special carriers which controlled the distance of nearest approach of the sample to the neutron source; the ordinate gives the counts under the 144-MeV y-peak of ssV obtained from different target elements normalized to the same neutron dose. Figure 1 shows that the fast flux, as monitored by the reaction 52Cr(n, P)~~V, decreases rapidly with distance from the face of the neutron emitting target, but the capture reaction 61V(n, Y)“~V is less sensitive to position. Consequently the relative yield of the capture to the (n, p) reaction increases with distance, and conditions for the solution of the equations cl? = scr* wc, + sv* w, c,u = s crB wc, + svB WV

(8) @A)

are satisfied if the sample is up against the neutron-emitting target in position A and in a higher relative thermal flux in position B; C, is the total counts measured over

1157

Resolving interferences in fast-neutron activation analysis TABLEII.-DIPERMINATIONOP

VANADIUM AND CHROMIUM IN NICKEL-BASE MIXTURES

OR MANGANESEANDIRON

Chromium

Vanadium

Sample

Expected, %

Found, %

Expected, %

Found, %

5.0 5.0 ;:;

49 46 51 2.7 1.0

150 100 5.0 2.5 1.0

151 10.1 5.0 2.4 1.0

1 2 3 4 5

1.0

cycle (1) Cycle (2)

TI = 1 mh,

TD = i mh, TC = 4 mh Tr = 2 min, TD = 9 mh, Tc = 4 min

Iron

MallgiUh% Sample

Expected, %

Found, %

Expected, %

Found, %

5.0 5.0 25 7.5

4.8 4.7 2.9 75

10.0 15.0 2.5 15.0

10.0 15.7 2.4 14.7

1

2 3 4 Cycle (1) Cycle (2)

TI = 1 min, TD = 30 min, Tc = 10 min TI = 2 min, TD = 30 min, Tc = 10 min

the 144-MeV peak of vanadium-52. In this work 4.3 cm of hydrogenous moderator were placed between sample and neutron-emitting target to obtain irradiation position B. This was achieved by the simple expedient of rotating the sample carrier through 180”. A similar pair of equations to (8) and (8A) can clearly be obtained for manganese and iron, based on the measurement of the intensity of y-rays from manganese-56. In order to test the reliability of a double irradiation procedure for solving equations of the form (8) and @A), samples containing either manganese and iron, or chromium and vanadium, in a nickel base were irradiated. The specific activity constants required for solution of the equations were found from the irradiation of standards consisting of single elements in a nickel matrix. Results obtained are given in Table II and can be seen to agree with expected values; also included in Table II are irradiation TABLEIII.-DETERMINATION

OF VANADIUM,

Vanadium

Sample

CHROMIUM, SAMPLES

Chromium

MANGANESE

AND

Manganese

Expected, %

Found, %

Expected, %

Found, %

Expected, %

Found, %

S

4.9

5.6

20.8

19.3

T U V W X Y Z

4.1 3.5 2.0 4,7 3.7 1.2 1.3

4.3 3.1 2.3 4.2 3.6 1.5 le.5

26 4.9 9.5 1.4 7.5 14.2 17.6

;:; 9.2 1.4 7.6 13.6 16.6

4.2 2.3 l-6 3.0 1.2 2.6 2.0 3.6

4.7 2.9 1.6 4.1 1.2 2.8 1.4 4.0

Cycle (1) Cycle (2)

TI = 1 min, TD TD TI = 2 min, TD TD

= = = =

4 min, Tc = 4 min 30 min, Tc = 10 min 4 min, TC = 4 min 30 min, Tc = 10 min

IRON IN MCKEL-BASE

Iron Expected, %

5.4 ::; 2.4 58 3.9

Found, % 3.5 2.4 5.4 1.6 3.4 2.7 6.1 3.7

1158

T. B. PIERCE, J. W. EDWARDSand K. HAINES

(Tr), cooling (decay) (Tu) and counting (Tc) times for each irradiation cycle. The first of the two irradiations carried out on each sample took place in the lower flux position to minimize the residual activity in the sample. The 3*76-min activity of vanadium-52 was always allowed to decay before the sample was irradiated for a second time but a correction for the longer-lived manganese-56 was often required when totalling the counts in the 0*85-MeV y-peak after the second irradiation and was calculated from a knowledge of the first count and the time between first and second counting periods. In order to determine the four adjacent elements vanadium, chromium, manganese and iron in the same samples, the procedures for the two pairs of interfering elements were combined. Samples were counted twice in each activation cycle, once after a cooling time of TD = & min to obtain the 52V count and again, when the short lived activity had decayed away, at T, = 30 min to measure the 56Mn activity which was usually of lower intensity. The manganese and iron concentrations in the sample were calculated first and the manganese figure used to apply a correction to the 1*44-MeV y-yield for 62V formed from manganese by the reaction 55Mn(n, a)52V. Results obtained are shown in Table III, each figure being the mean of determinations carried out on at least 4 samples of each material and can be seen to show reasonable agreement with the values expected. Zusammenfassung-Vier Elemente, Vanadium, Chrom, Mangan und Eisen wurden in Proben mit Nickel als Hauptbestandteil nach Messung der Intensitlt zweier y-Linien bestimmt: der 035 MeV-y-Strahlung von 5sMn (aus Mn und Fe) sowie der 1,44 MeV-y-Strahlung von “*V (aus V und Cr). Die zu jedem Y-Peak beitragenden zwei Elemente wurden einzeln bestimmt an Hand von Daten, die durch zweimaliges Bestrahlen jeder Probe mit Neutronen verschiedener Energieverteilungen erhalten wurden. R&sum&-On a dose quatre elements, le vanadium, le chrome, le manganese et le fer dans des echantillons a base de nickel apres mesure de l’intensite de deux raies y: la raie y 0,85 MeV Bmise par le sBMn (produit par Mn et Fe) et la raie y 144 MeV du s2V (provenant de V et Cr). Les deux elements contribuant a chaque pit y ont Bte determines s6parement a partir de don&es obtenues en irradiant chaque echantillon deux fois avec des neutrons de repartitions d’energie diffkrentes. REFERENCES 1. E. L. Steele, Modern Trends in Activation Analysis, in Proc. 1965 Intern. Conf. College Station, Texas, U.S.A., 19-22 April 1965, p. 102. 2. T. B. Pierce, J. W. Edwards and D. Mapper, U.K. At. Energy Estab. Rept., 5616 (1967). 3. B. T. Kenna and F. J. Conrad, Sandia Corporation Research Report SC-RR-66-229.