LITERATURE CITED (1) E. P. Bertin, “Principles and Practice of X-ray Spectrometric Analysis”, Plenum Press, New York-London, 1970. (2) W. Slavin, “Atomic Absorption Spectroscopy”,Interscience Publishers, New York, N.Y., 1968. (3) 0 . Erdtman and W. Soyka, Nucl. Insfrum.Methods, 121, 197-201,(1974). (4) C. M. Lederer, “Table of Isotopes”,6th ed., Wiley and Sons, New York, N.Y.. 1967. (5) B. A: Roscoe, “Acquisition and Analysls of Neutron Activation Data”, Master of Science Thesis In Nuclear Science and Engineering, VPI & SU, 1976. (6) B. A. Roscoe and A. K. Furr, Nucl. fnstrum. Methods, 137, 173-178 (1976). (7) B. A. Roscoe and A. K. Furr, Nucl. Instrum. Methods, 140, 401-404 (1977).
(8) Y. DeAisenberg, I. M. Cohen, R. 0. Korob, and M. D. Rudelli, Nuclear Activation Techniques in the Life Sciences, IAEA-SM-157/72. (9)G. H. Simmons, “A Training Manual for Nuclear Medicine Technologists”, Public Health Service, BRH/DMRE 70-3. (10) A. M. Selby, Standard MathematicalTables, 21st ed.,The Chemical Rubber Co., Cleveland, Ohlo, 1973. (11) P. Kruger, “Principles of Activation Analysis”, Wiley-Intersclence,New York, N.Y., 1971. (12) P. F. Zweifel, Nucleonics, 18 (ll),174-175 (1960). (13) E. J. Cohen, Nucl. Instrum. Methods, 121, 25 (1974).
RECEIVED for review February 3,1977. Accepted June 6,1977.
Determination of 13 Elements with Atomic Numbers between 12 and 47 by 14-MeV Helium-3 Activation Analysis C. S. Sastri, H. Petri,” and G. Erdtmann Zentralabteilung fur Chemische Analysen, Kernforschungsanlage Julich GmbH, 5170 Julich, West Germany
Nuclear reactions for the trace determination of the elements Mg, AI, TI, V, Cr, Mn, Fe, Ni, Zn, Zr, Nb, Mo, and Ag by activation analysis with 14-MeV 3He ions were Investigated. For these reactions, thick target yields were measured and interference-free detection limits were calculated. For an irradiation of 1 h or 1 half-life, whichever is shorter depending on the product nuclide, at 2 PA, the detection limits are in the range 1-50 ppb for Ai, Ti, V, Mn, Ni, Zn, and Nb; 50-100 ppb for Mo; and 100-500 ppb for Mg, Cr, Fe, Zr, and Ag.
metals and other elements with 2 > 42 can be investigated for their low 2 impurities. In previous papers (18,19), the determination of impurities in the matrices Nb, Ta, and W, by activation analysis with 14-MeV 3He-particles was described. At this energy, the commonly observed nuclear reactions are (3He, a),(3He,2p), (3He, p), (3He,p2n), (3He,n) and (3He,2n). In the present work, 13 elements between 2 = 12 and 2 = 47 were irradiated and from the y-ray spectra of the irradiated targets, the optimum detection reactions having high specific activities and low nuclear interferences were found and the detection limits based on these reactions were calculated.
Charged particle activation analysis has gained great importance in the detection of light elements at the sub-ppm level. The particles that are commonly used are protons, deuterons, tritons, helium-3 and helium-4 ions (1-5). Protons have been used (6-8) also to find heavy element impurities in high purity metals, minerals, etc. Sometimes other techniques like proton activation followed by x-ray counting and PIXE (particle induced x-ray emission) have been used (9,10) for trace element study. Markowitz and Mahony (11) were the first to suggest 3He ions for activation analysis of light elements. Following this, to a limited extent, this technique has been used for investigating heavy element impurities (12, 13). Ricci and Hahn (14) have calculated sensitivities for elements from Be to Ca for 18-MeV 3He ions. Kormali and Schweikert (15) have measured thick target yields for elements from Zr to Cs with 40-MeV 3He ions. In our laboratory the light elements C and 0 in metals are being investigated by the 14-MeV helium-3 activation technique (16,17). Concurrently systematic studies have been made to see if these irradiation conditions are useful also for the determination of heavy elements by nondestructive analysis. This energy corresponds to the Coulomb barrier of 14.2 MeV between 3He nucleus and a target nucleus with 2 = 42, assuming both the nuclei to be spheres with radii of 1.4 X A1/3 cm. This means that elements lighter than molybdenum will undergo nuclear reactions and therefore can be determined. With heavier elements, reactions take place to a very limited extent because of the tunnelling effect. Thus,
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High purity metal foils of natural isotopic composition (suppliers: Goodfellow Metals and Ventron) of the size 20 X 20 mm (approx.) were irradiated in the form of thick targets. The range of the 14-MeV 3He ions was from 30 to 60 mg/cm2 in the targets used; the target thickness varied from 100 to 150 mg/cm2, in all cases being thicker than the range of the beam in the material. Most of the irradiations were made in the internal beam of the isochronous cyclotron JULIC at Kernforschungsanlage Jtilich. The irradiations were made with 14-MeV 3He ions at currents ranging from 50 to 500 nA and for times ranging from 10 to 30 min. It has been found that these irradiation conditions can be reproduced without difficulty. As a check, the thick target yields for some of the standards were measured periodically. Short irradiation times of a minute or less were avoided to minimize the errors due to occasional fluctuations in currents, lasting a few seconds, that are likely to happen with the machine. If for some reason a large fluctuation in beam current had occured during an irradiation, such a measurement was discarded. A current integrator was not available for the present measurements. Because of the high magnetic field in the cyclotron, Fe and Co targets could not be irradiated in the internal beam of JULIC. They were irradiated in the external beam of the Compact Cyclotron CV 28 of Kernforschungsanlage Julich. Irradiations were made for 5 to 10 min at 400 nA to 1 WAcurrent. At the isochronous cyclotron JULIC, the internal beam (-2 mm diameter) strikes the rectangular target onto a side. In the compact cyclotron CV 28, the external beam (- 10 mm diameter) strikes the rectangular target at the center. In both cases, the
Table I. Calculation of Detection Limits for 14-MeV 3He Ions
Element Mg A1 Ti
V
Cr Mn Fe
Nuclide 17Mg
Half-life" 9.48 m
"A1 48Cr
2.246 m 23.0 h
49Cr
41.9 m
szmMn 52gMn
21.3 m 5.6 d
53gFe 56Mn
8.51 m 2.576 h
TO 5
7
~
~
17.9 h 270 d 71.3 d 3.41 h
Ni
58C~ 6'cu
Zn
65Ga
15.2 m
67Ga
78.1 h
Zr Nb
"Nb %gTc
72.0 m 4.88 h
7-Ray, MeV" 0.843 1.014 1.779 0.112 0.308 0.090 0.153 1.434 0.744 0.935 1.434 0.378 0.846 1.810 0.931 0.122 0.810 0.283 0.656 0.115 0.153 0.093 0.184 0.300 0.657 0.703 0.850 0.140 0.215 0.876
Net photopeak Thick target counting rate, 7-emission rate, P,countslminb A , r/minc 2.3 x 7.4 x 2.4 x 5.5 x 2.1 x 1.2 x 6.7 x 1.1 x 2.7 x 2.2 x 1.4 x 6.6 X 4.3 x 4.6 x 2.1 x 2.5 x 2.2 x 3.7 x 1.1 x 7.4 x 1.2 x 1.4 X 5.6 x 2.2 x 6.0 x 2.7 X 2.1 x 2.1 x 2.0 x 3.7 x
lo6 105 10' 105 105
lo8 107 107
105 105
105
lo6 lo6 105 105 105 104
lo6 lo6 107 10' lo6 105 105
105
lo6 lo6 lo6
2.9 x 1.2 x 6.3 x 5.5 x 6.4 X 1.2 x 7.8 x 2.4 x 2.8 x 3.1 X 3.0 x 2.7 X 5.4 x 1.2 x 2.9 x 2.7 X 2.6 X 1.0 x 1.0 x 7.4 x 1.4 X 1.4 X 9.0 x 6.3 X 5.5 x 2.7 x 2.6 X 2.6 x 3.8 X 4.9 x
10'
10' 109
lo6 lo6 109
lo8 109 107 10' 107 10'
lo8 lo8 107 lo6
lo6 lo8 lo8 lo8 lo8 10' lo6 lo6 107
lo8 10' 107
Minimum detectable yemission rate, R , r/mind 65.0 77.7 111.6 1.1 0.90 3.5 3.6 53.7 6.4 8.3 7.2 33.3 20.9 19.5 8.0 1.2 0.71 4.2 10.3 9.1 8.3 1.1
1.8 2.3 17.7 9.3 11.2 1.9 1.9 21.8
Detection limit, ppbe 220 650 17 200 140 3 5 22 230 270 240 120 40 160 280 440 27 0 40 100 10 60 80 200 370 320 30 40 70 500 440
9ymTc 6.02 h "Ru 2.88 d 105 lo6 Ag 1"gIn 58 m 105 10' " Half-lives and y energies are taken from Ref. 20. Photopeak counting rate (counts/min) at the end of an irradiation of 1h or 1half-life, whichever is shorter at 2 PA current and measured with a GETAC Ge(Li) detector which has 0.50% absolute efficiency (see text). Measurements done with another detector were normalized to this efficiency. yemission rate According to Equation 11. e According to Equation 1. A = P / E ;P from column 5. E = photopeak counting efficiency. Mo
target-holders were water cooled. After irradiation, the samples were measured with a ?-ray spectrometer consisting of a Ge(Li) detector and a Nuclear Data 4096 channel analyzer. Most of the measurements were made with a 50 cm3 Ge(Li) detector (Getac, Mainz, Germany) and a few were made with a 30 cm3Ge(Li) detector (Canberra,Meriden, Conn.). Their characteristics were as follows; resolution 2.3 and 3.1 keV FWHM, resDectivelv, for the 1332-keVy line of @'Co and absolute efficiencies 0.50% and 0.18%, respectively, for the 1332-keV y line of 6"Co and for the geometries used. The peak area evaluation was done with the program ND 411007 developed by Nuclear Data Inc. From the peak areas, the thick target yields at the end of irradiation were calculated by correcting for the decay between the end of irradiation and start of measurement and for the decay during measurement. As different irradiation times and fluxes were used for different targets, the thick target yields obtained were normalized t o the conditions of 2 FA beam current and an irradiation of 1 h or 1 half-life, whichever was shorter, and for 100% detection efficiency.
RESULTS AND DISCUSSION From the activation experiments, two values referring to the sensitivity of the method were calculated for each element and are given in Table I: the thick target yields and the detection limits. The thick target yields are the average values of 2 to 4 irradiations for each element and are expressed as gammas/min instead of disintegrations per min and therefore are named "thick target y-emission rates". They were obtained by dividing the observed photopeak counting rates (counts/min) at the end of irradiation, by the efficiency of the Ge(Li) detector for the y ray concerned of the given nuclide. While measuring the activated targets, their positioning was such that the irradiated side always faced the
detector in order to minimize the absorption of, particularly low energy, y rays in the target itself. The detection limit is defined as follows:
Detection limit (ppm) = [minimum detectable 7-emission rate (R)X lo6]/[thick target y-emission rate ( A ) ] (1) The minimum detectable y-emission rate is obtained from the minimum detectable net peak count rate which in turn is determined by a photopeak which is clearly contrasted from the background. In all practical cases the background under a photopeak is composed of three parts: (1) the "natural" background including all contributions from external sources, (2) the "intrinsic" background arising from the Compton, scattering, and bremsstrahlung continua of the radionuclides due to the element to be determined, and (3) the "matrix" background arising from all other radionuclides produced in the sample. The matrix contribution cannot be generally estimated since it strongly depends on the nature of the matrix and the purity of it. For this reason we have calculated "interference free" detection limits. It can be shown by a simple mathematical treatment that, of the two remaining types of background, the natural background alone plays the major role in deciding the interference-free detection limits. Assume that the minimum detectable peak area is given by
c = 3UB
(2) where C is the number of net counts in the peak, UB is the standard deviation due to the statistical error of the total
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Table 11. Nuclear Interferences to the Applied Reactions with 14-MeV 'He Ions Useful nuclear Q-value, Interfering Interfering nuclear Q-value, Interference Element reaction MeVa element reaction MeVa level, %b Mg 26Mg('He,2~)~'Mg -1.3 A1 27A1(3He,2p)ZBA1 0.0 Mg 26Mg(' H ~ , P ) ~ ~ A I + 8.3 < 10 t 5.6 Cr 50Cr(' H e , ~ n ) ~ ~ C r Ti 46Ti(3He,n)48Cr -3.0
