The Recoil Technique and Its Possible Use in Activation Analysis Richard L. Hahn Oak Ridge National Laboratory, Oak Ridge, Tenn.
THEUSE OF CHARGED particles in activation analysis, especially for the determination of low-Z elements, has been of interest in recent years (1-6). Essentially all of the work done with these particles has involved the assay of the target after irradiation, and possibly after chemical purification, for the radioactive products of interest. However, in a few instances, such as that described by Ollerhead et al. (7),a different approach to activation analysis, borrowed from nuclear physics research, has been used. To study the diffusion of 1 7 0 through zirconium oxide, they counted the alpha particles emitted during the nuclear reaction, 170(3He,a)160. In an experiment that is complementary to detecting such light emitted particles, one can collect and later assay the radionuclides that recoil out of the target during an irradiation. This technique is also extensively used in nuclear research (8); the details of the nuclear reaction determine the energies and emission angles of the recoils. In this brief note, some of the principles of recoil phenomena are discussed. With a few illustrative experiments using copper-oxygen samples, we attempt to indicate how the recoil technique may be of possible use in activation analysis.
4.0 4 . Cu RECOIL IN 0
3.5
1
2 . Cu RECOIL IN Cu
1
3. 0 RECOIL IN 0
4. 0 RECOIL I N Cu
PHYSICAL CONSIDERATIONS
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Recoil Energy. For a nuclear reaction, such as 3He -,18F p, in which the final state consists of two bodies, the kinetic energies of the products are uniquely determined from kinematic relations (9). At zero degrees with respect to the incident beam, the maximum energy of the recoil nucleus can be expressed as
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l 6 0
where subscripts 1, 2, 3, and 4 refer, respectively, to the incident particle, target nucleus, observed recoil product, and unobserved product. M i s mass; E, kinetic energy; and Q,the energy released in the reaction. Because the sum of masses, ( M 3 M 4 ) ,appears in the denominator of Equation 1, the maximum recoil energy of the product nucleus from a reaction involving light nuclei, like 160(3He,p)18F, is generally, for a fixed incident-particle energy, greater than that from a reaction with heavier nuclei like 6 j C ~ ( ~ H e , a ) ~ ~ C This u . effect is exemplified by the values in Table I for a wide range of nuclear masses and Q-values.
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(1) S. S. Markowitz and J. D. Mahony, ANAL.CHEM., 34, 329
(1962). (2) J. D. Mahony, University of California Repf. UCRL-11780,1965 (unpublished). (3) E. Ricci and R. L. Hahn, ANAL.CHEM.,37, 742 (1965). (4) Ibid., 39, 794 (1967). (5) C . Engelmann in “Proceedings of International Symposium on Radiochemical Methods of Analysis,” International Atomic Energy Agency, Salzburg, 1965, pp. 358, 417. (6) E. Schweikert and Ph. Albert, ibid., p. 338. (7) R. W. Ollerhead, E. Almqvist, and J. A. Kuehner, J. Appl. Phys., 37, 2440 (1966). (8) B. G . Harvey, Awl. Reo. Nucl. Sci., 10, 235 (1960). (9) R. D. Evans, “The AfomicNucleus,” McGraw-Hill, New York, 1955, p. 408.
ENERGY OF RECOIL (MeV)
Figure 1. Approximate range-energy curves for oxygen and copper recoil nuclei. Values were calculated from the curves of Lindhard et al. (10)
Table I. Maximum Recoil Energies for Reactions Induced by 20-MeV 3He Particles& Reaction Q,MeV Ea,MeV” +2.0 7.4 2.5b -4.8 5.0 +10.7 +6.5 2.4 +3.9 2.4 -7.9 2. 3b +9.7 5.0 +1.9 15.2 +10.0 17.1 +4.9 13.8 +6.3 5.1 +8.3 9.1 +5.5 1.2 2.7 +11.4 0.7c $4.0 +14.4 1.6c “The recoil has maximum energy when it emerges from the target at zero degrees with respect to the beam. Values for the (3He,2n) reaction were calculated assuming both neutrons were emitted at the same angle, 180”. The classical Coulomb barrier has a value of 23 MeV for the reaction 3He 238U. Because of quantum mechanical effects, the reaction will proceed at 20 MeV.
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Recoil Range. The recoil range, or distance that a recoil nucleus travels through matter, depends upon the recoil energy. In Figure 1, approximate range-energy curves, calculated for copper and oxygen recoils in copper and oxygen media from the universal range-energy plots of Lindhard et ai. (IO), are presented. The trends of the data are clear. For a fixed recoil energy, the lighter recoil nucleus has a larger range in a given material. And, for a given nucleus with fixed recoil energy, the recoil range increases as the atomic number and mass of the medium increase. Thus, at a Fxed recoil energy, the range of oxygen recoils in copper is greater than either the range of copper recoils in copper or the range of oxygen recoils in oxygen. Combined Effects. The dependence of both recoil energy and recoil range upon the mass of the recoil nucleus results in a pronounced effect. At a given bombarding energy, the fact that the maximum recoil energy of a light nucleus (oxygen) is generally greater than that of a heavier recoil (copper) signifies that the recoil range of the oxygen should be appreciably greater than that of the copper. Thus, it is expected that for a copper-oxygen target the ratio, such as IsF/G6Ga,of radionuclides produced from oxygen to those formed from copper, will be larger for a catcher foil in which recoils have been deposited than for the target itself. This effect should be enhanced for an oxide layer on the surface of a metal because the recoils from the metal will have a larger distance to traverse before escaping than will the recoils from the oxygen. EXPERIMENTAL
To demonstrate these effects, we prepared copper-oxygen targets by heating 1-mil copper foils at -900" C ; the CuO layers were uniform and adhered strongly to the foils. The weight fraction of oxygen to copper in the targets was -0.14, while in pure CuO, this ratio is 0.25. The targets thus were composed of copper metal, covered by a deposit of CuO. Irradiations were carried out with 25-MeV 3He particles from the Oak Ridge Isochronous Cyclotron. Aluminum absorbers were used to decrease the beam energy to values from 20 to 12 MeV; energy losses in the targets ranged from -3 MeV at 25 MeV to > 5 MeV at 12 MeV. Also in the target assembly were the copper-oxygen foil and an aluminum catcher foil, with diameter 1.5 times that of the target, placed -0.006 inch downstream from the target. All of the irradiations, 3 minutes long at an average 3He beam current of 1.5 PA, were performed under cyclotron vacuum. The gamma rays from the radioactive products were counted with a 3-inch X 3-inch NaI(T1) detector coupled to a multichannel analyzer. Eighty mils of copper were placed on each side of each sample to ensure complete annihilation of the emitted positrons. Counting was begun approximately 1 hour after irradiation, so that none of the short-lived products were detected, and continued for -36 hours. RESULTS .4ND DISCUSSION
The main radioactivities detected, as expected from the excitation functions ( I , I]), were the products of the 160(3He,p)18F and W u ( 3He,2n)66Ga reactions. Because the aluminurn catchers were also exposed to the 3He beam, possible contributions to the **Factivity from the aluminum or its impurities must be considered. The cross section (2) for the 27A1(3He,3a)18Freaction at 20 MeV is
