Use of a 60-MeV Linac for Fast and Energy Neutron Activation Analysis Peter E. Wilkniss Nat.al Research Laboratory, Washington, D.C. 20390 The relative intensity distribution of photoneutrons and bremsstrahlung around high-2 element converters irradiated with an electron beam is discussed. The photoneutrons are distributed almost isotropically, while the bremsstrahlung intensity is strongly peaked forward. The use of such photoneutrons for fast neutron activation analysis of 0, F, Mg, AI, P, and Ti was investigated. Variable-energy neutron activation analysis is also possible and allows one to determine F without interference from 0. Possible application of photoneutrons for prompt radiation techniques and as a source of interference in photon activation analysis is outlined.
were exposed, and the relative bremsstrahlung intensity distribution was obtained by counting the 18F produced via '9F(y,n) 18F. The position of the sample rotator during the irradiations was at 90" with respect to the bremsstrahlung beam axis and 3.5 cm from the center of the thin tantalum converter. When the thick tungsten converter was used, the position was 135" from the bremsstrahlung beam axis and 5.5 cm from the converter center. Polyethylene capsules were used to pack the samples for irradiation. Usable capsule volume was 0.8 ml.
RECENTLY, activation analysts have shown considerable interest in the use of photons for activating samples ( I ) . Highenergy photons are obtained as a bremsstrahlung beam produced by the reaction of the electron beam from a linear accelerator with a high-Z element converter such as tantalum. In addition to the photons, considerable numbers of energetic neutrons are produced by the interaction of the photons with the converter. Since under favorable conditions a total yield of approximately l O I 3 neutrons/sec can be obtained, an investigation of the use of such sources for fast-neutron activation analysis seemed to be warranted.
RESULTS .4ND DISCUSSION
EXPERIMENTAL Instrumentation. We have used the NRL 60-MeV Linac, which is described in detail by Godlove et al. (2). Two different neutron sources were used. One was a "thin" converter, with 9.76 g/cm2 tantalum, which was irradiated with 80 PA (average) beam current from a 45" deflected beam. The other was a "thick" converter, containing 56 g/cm2 of gold-plated tungsten, and was irradiated with 145 pA (average) beam current from the undeflected linac beam. A pneumatic transfer system and sample rotator (3) were used to facilitate sample irradiations. A 5 X 6-inch NaI(T1) scintillation crystal and a 512-channel analyzer were used to detect the high-energy gamma rays from NL6. Some of the other activities produced during the experiments were counted using a 3 X 3-inch NaI(T1) crystal or a lithiumdrifted germanium detector in combination with a 4096channel analyzer. The method of detection used for each radionuclide will be indicated in the results. Subsequent irradiations were compared either via the neutron intensity, monitored during irradiations by a BFI tube, or via the electron beam current on the converter which was registered by a recorder. The reactions 27Al ( n , ~ ) ~ ~(threshold Mg -2 MeV) and 27Al(n,a)24Na(threshold MeV) were used t o measure the relative neutron intensity distribution around the converter. For this purpose, aluminum squares were irradiated in different positions. Together with these aluminum squares, pieces of Teflon foil (1) [EUR 3896 d-f-e], Proc. 2nd Conf. on Practical Aspects of Activation Analysis with Charged Particles, H. G. Ebert, Ed.,
Liege (Belgium), September 21-22, 1967.
(2) T. Godlove, R. Tobin, and J. McElhinney, Report of NRL Progress, January 1964, p 1, U. S. Naval Research Laboratory, Washington, D. C. 20390. (3) P. Wilkniss, J. I. Hoover, and R. E. Leighton, Nucl. Instrum. Methods, 56, 120 (1967).
Neutron Distribution from Converters. Our results show that the relative neutron intensity distribution is nearly isotropic around the thin tantalum converter. These measurements were carried out at a bremsstrahlung energy of 22 MeV maximum. It was observed that at all angles 90% of the neutrons detected had an energy below 6 MeV. These results agree basically with much more sophisticated experiments in reference ( 4 ) . For the thick tungsten converter we found that the neutron intensity is 2.5 times higher at backward angles (>90°) than a t forward angles (2 MeV) = 5 x lo9 n/cm2sec, at 30 MeV bremsstrahlung energy for both converters. We were not able to place flux monitors closer than a few centimeters to the actual neutron producing regions of the converters because of their design. Calculations indicate that redesigning for optimum sample-source geometry should yield a flux @ 4 > 2 MeV) of about 1O1O n/cm2sec. Bremsstrahlung Distribution from Converters. The bremsstrahlung distribution was only measured for the thin tantalum converter at 22 MeV maximum bremsstrahlung energy. Our results, which agree with similar measurements by Murray (3, show that the relative bremsstrahlung intensity decreases rapidly from 100% in the forward direction (0') to about 0.07% in backward directions (135"). Our results concerning the relative intensity distributions of neutrons and bremsstrahlung then indicate that a sample should be placed as close as possible to the converter between 90" and 180" if a photoneutron source is to be used for activation analysis. This is shown schematically in Figure 1. Activities Obtained from Different Elements Using Photoneutron Activation. The elements listed in Table I were irradiated in a photoneutron flux of %(>2 MeV) = 4 X lo8 n/cm2sec obtained from the thick tungsten converter at 35 MeV maximum bremsstrahlung energy. The calculated re-
(4) D. B. Gayther, and P. D. Goode, J . Nucl. Energy, 21,733 (1967). ( 5 ) K. M. Murray, Nucleonics, 22, No. 2, 61 (1964). VOL. 41, NO, 3, MARCH 1969
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Figure 1. Simplified angular distribution of bremsstrahlung and photoneutrons from tantalum converter irradiated with a high energy electron beam sults of Table I apply to the irradiation of 1 g of the element for one half life of the radionuclide produced. The count rates have been corrected for decay back to the end of the irradiation. It should be noted that photonuclear reactions (particularly the y,p type) on other stable isotopes of the elements being determined may contribute to the product nuclide activities. For example, the y,p reaction on 170, Z5Mg,and 49Tiwould interfere with reactions 1,4, and 9 in Table I. Variable-Energy Neutron Activation Analysis. This discussion will be based on the interference-free determination of fluorine via 19F(n,a)16N(threshold 3 MeV) in the presence of oxygen which gives 16N via l60(n,p)I6N (threshold 10.5 MeV). The latter reaction cannot occur if the maximum bremsstrahlung energy is less than 18.1 MeV, the sum of the threshold energy (10.5 MeV) and the energy required to separate a neutron from lS1Ta(7.6 MeV). For the experiments, it is important that an electron beam analyzed with respect to energy be used. This was the case for our thin tantalum converter. The sample rotator was at 90” with respect to the bremsstrahlung beam axis, as described before. We have irradiated 0.5-g HzO samples in this position with bremsstrahlung resulting from electron
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beam energies between 18 and 22 MeV. Up to 22 MeV, no 16N from 160(n,p)16N was detected. From 21 to 22 MeV electron beam energy, the background at 5-7 MeV y-ray energy increased slightly, but the activity was not positively identified as 16N. It is to be noted that, although the electron beam energy substantially exceeds the threshold value of 18.1 MeV, the energy distribution of the bremsstrahlung is such that very few photons have sufficient energy to produce neutrons of an energy greater than 10.5 MeV. Fluorine, on the other hand, is readily activated according to ‘gF(n,~t)~~N under these conditions. The irradiation of 1 mg fluorine for 35 sec at 22 MeV gave a yield of 3.1 x l o a cpm (background subtracted, 70 cpm) 16N. (The counting started 3 seconds after irradiation end.) This result shows that we can determine fluorine interference free in the presence of oxygen using photoneutrons. Interfering Photonuclear Reactions. A good starting point for this discussion is again the determination of fluorine via ‘ ~ F ( ~ , C Y ) ~According ~N. to the results in the preceding paragraph l60(n,p)l6N is not induced by photoneutrons produced by bremsstrahlung of maximum energy 20 MeV. When we irradiated a 0.5-g H 2 0 sample directly in the bremsstrahlung beam at a maximum bremsstrahlung energy of 20 MeV, we obtained 400 cpm of 16N. This 16N activity can only be explained by photonuclear activation according to ”O(y,p)l6N and 180(y,d)16N. However, 170and I8O have very low isotopic abundances, which explains why we have no photonuclear activation interference in fluorine determinations carried out at 90” from the converter, where the bremsstrahlung intensity is a factor of 1000 lower than at 0”. If the relative abundances of isotopes giving interfering photonuclear reactions are high, as for reactions No. 4 and 9 of Table I, then interference from photonuclear reactions can be serious. Analytical Applications of Photoneutrons. GENERAL CONSIDERATIONS. The most critical factor in the application of photoneutron activation analysis is the interference from photonuclear reactions. Proper sample positioning, source design, and the selection of a favorable bremsstrahlung energy can eliminate the interference, but in practice it will be the limiting factor. Each case will have to be evaluated experimentally, as little information on photonuclear cross sections is available. Photoneutron spectra in general (except the one from uranium) resemble the fission spectrum (4). The photoneutron intensities are peaked between 1-2 MeV. The highenergy tail falls off rapidly, so that few neutrons with energies greater than 10 MeV are found. Therefore, low sensitivities will result for nuclear reactions which only have considerable cross sections above 10 MeV.
Table I. Photopeak Count Rates (Background Subtracted) at End of Bombardment for Radionuclides Produced in Irradiation of 1 Gram of Element Indicated. No. Element Reaction Ey-photopeak, MeV (CPm) Detector 1 0 ‘60(n,p)leN 5.0-7.0 6 X IO4 5” x 6” NaI/TI F lgF(n,a)1eN 5.0-7.0 3 x 106 5” X 6” NaI/Tl 2 0.83 2 x 105 5” X 6” NaI/Tl 26Mg(n,y)27Mg 3 Mg 5 ” X 6” NaI/TI 4 Mg 24Mg(n,p)z4Na 2.75 8 x 105 3” X 3” NaI/T1 5 A1 27Al(n,y)z8A1 1.78 4 x 105 6 A1 2’Al(n,p) 27Mg 0.83 1 x 106 3” x 3” NaI/TI 2.75 3 x 105 3” X 3” NaI/Tl 7 A1 27A1(n,c~)~‘Na 3 x 106 5” X 6” NaI/Tl 1.78 8 P 31P(n,a)Z*Al 9 Ti 48Ti( n , ~ ) ~ ~ S c 0.178 3 x 103 Ge-Li The thick tungsten converter was used, maximum electron beam Irradiation times are equal to one half life of the product nuclide. energy 35 MeV, beam current 145 PA. A %(>2 MeV) of 4 X 108 n/cm2sec was measured in the irradiation position. 422
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An important economic consideration is that photon activation analysis and photoneutron activation analysis can be carried out simultaneously. This is possible because photon activation analysis is performed directly in the bremsstrahlung beam, while the best position for photoneutron activation analysis is at angles of 90’ or more with respect to the bremsstrahlung beam. Photoneutron activation analysis is especially suited for product radionuclides with long half lives, as the target life is essentially unlimited and the machines are often operated for long periods at a time. In the past, fast-neutron (>2 MeV) fluxes of 5 X IO9 n/cm2sec have been obtained, which compare favorably with the ones provided by small 14-MeV neutron generators or cyclotrons. Under optimum conditions, a flux of 1O1O n/cmzsec is now possible. Even higher fluxes are anticipated for the high-intensity linear accelerators now under construction. FAST-NEUTRON ACTIVATION ANALYSIS.The experience gained with 14-MeV neutron generators and Van de Graaffs, that fast-neutron activation in general is limited in its applications, also applies to photoneutron activation. One can predict the use of photoneutron activation for purely instrumental techniques and for elements which occur at the minor or major constituent level in a sample. The results given in Table I were obtained in a flux of 4 X lo8 n/cm2sec. As we can obtain fluxes of 1O1O n/cm2sec the specific activities of Table I will be a factor of 25 higher under optimum conditions. The calculation of detection limits for the elements listed has to be carried out for each individual case, considering interfering reactions, matrices, etc. VARIABLE-ENERGY NEUTRON ACTIVATION ANALYSIS.This problem has been discussed in detail for the determination of fluorine in the presence of oxygen. Of course variable-energy photoneutron activation can be applied in general to all cases where there is a difference of several MeV between the energy threshold of the reaction of interest and that of an interfering reaction. So far the number of interesting cases to which this principle might be applied is limited (6). The problem of interference-free fluorine determination was of special interest to us because we have a program in the marine geochemistry of this element. This requires the determination of fluorine at considerably different concentration levels, at ppm and less in seawater and atmospheric samples and at the per cent level or fractions thereof in sediments and marine animals. Obviously a combination of photon activation analysis which is carried out at a bremsstrahlung energy of approximately 20 MeV and has a detection limit of 10-8 g with photoneutron activation, with a detection limit of 0.1 would cover this range of fluorine concentrations. As was discussed earlier, both techniques can be carried out simultaneously. Indeed the application of photoneutron activation analysis to the determination of fluorine in rocks and marine shale proved successful as the following results show. We have analyzed the materials listed in Table 11. They were supplied by Dr. Paul Greenland of the U. S . Geological Survey and had been preanalyzed for fluorine by emission spectrography. The sample with the highest fluorine value was used as standard. The results in Table I1 show that photoneutron activation analysis and emission spectrography agree satisfactorily. The relative standard deviation given for the activation analysis data is *lo%. It is based on five individual determinations for each sample. It should be noted here that (6) E. L. Steele, Proc. 1965 Int. Conf. Modern Trends in Activa’ tion Analysis, College Station, Texas, USA, April 19-22, 1965, p 102.
Table 11. Determination of Fluorine in Different Rocks by Photoneutron Activation Analysis. Fluorine. Z Emission Photoneutron Material spectrograph activation Tourmaline granite 0.24 =t 0,036 0.24 (standard) Migrnatite 0.12 =t 0.018 0.13 =t 0.013 Marine Shale 0.07 =t 0.01 0.10 27 0.01 Samples preanalyzed by emission spectrography using CaF bands by Dr. Paul Greenland, U. S. Geological Survey, Washington, D. C.
there is much room for improvement, for instance, none othe improvements worked out by the NBS group for fastf neutron activation analysis (7,8) have been used. At present, the detection limit for the fluorine determination for our method is 0.05% when one-gram sample is used. PHOTONEUTRON PULSES FOR CAPTURE GAMMA RAYTECHNIQUES. One of the major applications of photoneutron sources in nuclear physics is in investigations of capture gamma rays. The application of such prompt radiation to the solution of analytical problems has been considered in recent years by activation analysts, and the progress made was reviewed by Smales (9). It seems to the author that the use of a linac by chemists for this purpose might open new possibilities for this application, especially in light of the fact that excellent experimental setups can be found at many installations. POSSIBLEINTERFERENCE OF PHOTONEUTRONS IN PHOTON ACTIVATION ANALYSIS.From the discussion and results given in this paper, it is obvious that whenever a photon beam from a linac is used for activation analysis, considerable numbers of photoneutrons will also interact with the sample. If bremsstrahlung of 35 MeV maximum energy is used, then we can expect neutrons with energies in excess of 20 MeV. On the other hand, any moderating material, such as cooling waters, will give rise to thermal neutrons. Both types of neutrons can induce interfering reactions which would change the results of photoactivation analysis. In most cases, photoneutrons will not interfere.’ It is hoped, however, that the data presented herein will be helpful in evaluating interference by photoneutron activation when it must be considered. ACKNOWLEDGMENT
The author is indebted to the staff of the NRL Linac Branch for many very useful theoretical discussions and for assistance with the irradiations. He is grateful to Dr. Paul Greenland of the U. S. Geological Survey, Washington, D. C., who provided and preanalyzed rock samples, and to Dr. Evans Howard, Linac Division, National Bureau of Standards, Washington, D. C., who loaned the tungsten converter to NRL. RECEIVED for review August 13, 1968. Accepted December 11, 1968. (7) S. S . Nargolwalla, M. R. Crambes, and J. R. DeVoe, ANAL. CHEM. 40, 666 (1968). (8) F. A. Lundgren, and S. S. Nargolwalla, ibid., p 672. (9) W. Wayne Meinke and Bourdan F. Scribner, “Trace Characterization, Chemical and Physical,” National Bureau of Standards, Washington, D. C., Monograph 100,April 28, 1967, p 323. VOL. 41, NO. 3, MARCH 1969
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