3 347
CHEMICALLY SIGNIFICANT DETAILS OF NUCLEAR REACTIONS
The Chemically Significant Details of Some Nuclear Reactions by C. H. W. Jones Chemistry Department, Simon Fraser University, Burnaby 2 , British Columb(a, Canada
(Received March 9, 1970)
The chemical effects of nuclear transformations have been widely studied in solids. The aim of the present paper is to reexamine the sequence of events as they occur following thermal neutron capture in a solid. Experimental evidence recently reported in the literature is reviewed. I n particular, attention is focussed on the possible existence in the y cascade following neutron capture, of delayed states with half-lives of nanoseconds or greater, and which may be highly internally converted. The stopping time for energetic atoms recoiling from nuclear reactions in solids is also discussed, and relevant experimentalevidence from Mossbauer spectroscopy and perturbed angular correlation studies is reviewed. The implications of the reported experimental work for models of the chemical reactions occurring following the (n,r) reaction are discussed.
Introduetion Studies of the chemical effects of nuclear transformations in solids have been principally concerned in the past with investigations of the (n,y) reaction.lv2 A model which set out to describe the physical and chemical processes occurring during and following the recoil event was formulated by Harbottle and Sutin3in 1958. The purpose of the present paper is to reexamine, in the light of experimental evidence accumulated in the literature in recent years, some of the important features of the (n,y) reactions for several specific cases. I n particular, attention is focussed on the possible existence in the prompt y cascade, following thermal neutron capture, of delayed states with half-lives of nanoseconds or greater, and which may have high internal conversion coefficients. The time over which the recoil event takes place is also examined, and relevant experimental evidence, obtained from hlossbauer spectroscopy and perturbed angular correlation measurements, is reviewed. In the hot-zone model for recoil events in ~ o l i d sit, ~ was visualized that in an (n,y) reaction the daughter atom would acquire a kinetic energy of perhaps 100’s of eV as a consequence of the emission of high-energy y rays in the y cascade. The recoil atom was then envisaged as losing its kinetic energy in ca. lO-’3 sec in collisions with surrounding substrate atoms and molecules, and in so doing the radioactive daughter atom could become chemically recombined. The recoil energy transferred to the lattice was visualized as producing 5 or 6 local “hot-spots” in the crystal surrounding the recoil atom. These hot-spots would then coalesce in ca. 10-l2 sec to form a hot zone in which the crystal would be essentially molten. The hot zone was then pictured as cooling down below the melting point of the lattice in ca. lo-” sec. During the lifetime of the hot zone it was proposed that further chemical reactions could occur involving the recoil atom. The Harbottle and Sutin model did not take account of the spectrum of recoil energies made available as a consequence of the vector cancellation of y-ray momenta in
the complex y cascade. I n one or two instances, where the main details of the y cascade were sufficiently well established, calculations of the recoil energy spectra have been attemptedU4J For those specific cases, the calculations show that in a very large fraction of events the recoil kinetic energies are considerably greater than chemical bond energies, or the energy required to displace an atom in a solid lattice. A second point, of probably greater significance, is whether the processes occurring during the recoil event are dominated simply by the kinetic energy of the recoil, or whether electronic excitation and ionization of the recoiling species will play a significant roles6 At the recoil kinetic energies available in (n,y) reactions, ionization will not occur through the autoionization process. However, internal conversion of y rays in the y cascade will result in very extensive ionization of the recoiling atom through the Auger charging process. The y transitions which will have high internal conversion coefficients will be low-energy transitions with high multipole character. Such transitions are just those transitions which may involve delayed states with lifetimes of nanoseconds or greater. Several authors have in the past made reference to the possible existence of relatively long-lived states with high internal conversion coefficients in the n, y cascade, including Wexler,6 Campbell, and Wolfgang.’ The latter author has pointed out the possibility that the recoiling atom in the (n,y) reaction could lose its (1) (a) L. V. Groshev, A. M. Demidov, V. I. Pelekhov, L. L. Sokolovskii, G. A. Bartholomew, A. Doveika, K. M. Eastwood, and S. Monaro, Nucl. Data, A3, 4 (1967); A5, 1 (1968); A5, 3 (1969); (b) C. M. Lederer, J. M.Hollander, and I. Perlman, Ed., “Tables of Isotopes,” 6th ed, Wiley, Kew York, N. Y., 1967. (2) (a) G.Harbottle, Ann. Rev. Nucl. Sci., 15, 89 (1965); (b) A. G. Maddock in “Nuclear Chemistry,” Vol. 11, L. Yaafe, Ed., Academic Press, New York, N. Y., 1968, p 186. (3) G. Harbottle and N. Sutin, J . Phys. Chem., 6 2 , 1344 (1958). (4) I. G. Campbell, Adv. Inorg. Chem. Radiochem., 5, 136 (1963). (5) S. Wexler, Act. Chim. EbZ. Radiat., 8, 107 (1965). (6) G. N. Walton, Radwchim. Acta, 2, 108 (1964). (7) R. Wolfgang, Progr. React. Kinet., 3, 97 (1965).
The Journal of Physical Chemistry, Vol. 74,No. 18,1970
C.H.W. JONES
3348 kinetic energy and become chemically bound long before the internal conversion of the long-lived state. The final chemical products incorporating the daughter radioactivity would then be greatly influenced by the Auger ionization and excitation occurring in the primary (n,y) recoil product molecule. Muller8 has further proposed that there may be several rounds of successive ionization and subsequent electronic relaxation following successive internal conversion of several states in the y cascade. In the early 1950's Wexler and Daviesg and Yosim and DavieslO demonstrated that iodine, bromine, and indium (n,y) recoils acquire a positive charge in a certain fraction of events as a consequence of internal conversion. Thompson and Miller'l subsequently investigated the charge states of In, Dy, and Mn recoils and observed that the charge state of the recoil ejected from the surface of an irradiated foil or film was independent of the chemical composition of the target. They concluded that the charge was acquired, after ejection from the surface, through internal conversion of long-lived states in the y cascade. These experiments have been well reviewed by W e ~ l e r . ~ The above experiments do not provide a very detailed picture of the recoil event. This can only be obtained by the direct measurement of the energy levels populated in the y cascade, the lifetimes, and the relative number of events occurring through these states. Such information is slowly emerging from nuclear spectroscopic studies of the prompt y cascades following thermal neutron capture and other nuclear reactions, and from related spectroscopic studies of radioactive decay processes. In addition, direct experimental evidence as to the stopping time for energetic recoil atoms in solids is becoming available from Mossbauer and angular correlation work, together with measurements on the electronic relaxation time accompanying the Auger process. A review of information available from the literature is now presented for several nuclei of interest and the implications of the data for theories concerning the chemical effects of the (n,y) event in solids are discussed.
Table I: Delayed States
Nuclide 3 2 P
Wr 66Mn
Delayed state, MeV
Halflife, nsec
0.077 0.750
0.5 10.8 5.1 11.0 1600 7.4
0.109
0.023 FZn
0.054
WBr
0,037
K-shell internal conversion coeffa
Very small Verysmall (0.1-1.0) (ca.3) (9)
1.6
NO. or
evenW100 neutron capturesb
Rei for halilives
45
12 15 18, 19 18, 19 22 26
72 20
25
... 100 (Isomeric transition)
1281
0.132
8.0
(0.5)
42
18, 27,
0.030
8.6
...
...
18
28
a Values in parentheses are deduced values. b For the (n,?) reactions, the values quoted are the intensities of the y rays directly observed in the prompt spectrum.
transition. The internal conversion coefficient will then be very small as indicated by the theoretical calculations of Sliv and Band for the internal conversion coefficient as a function of 2, the atomic number, the energy, and the multipolarity of the transition. l3 The (n,y) cascade has been studied in some detail14 and an examination of the intensities of the y transitions indicates that the cascade proceeds through the 0.077-MeV level in ca. 45 events per 100 neutron captures. Since the internal conversion coefficient will be small, the net effect of the delayed 0.077-MeV transition will be that the phosphorus will acquire a further recoil kinetic energy of 0.1 eV, given by
Evidence for Delayed States
in eV, where E, is the photon energy in MeV and M is the atomic mass. Chromium-5i. Chromium has several stable isotopes, but the one reaction widely studied in recoil chemistry is the 50Cr(n,y)61Cr reaction. Bauer, et ~ 2 have . studied ~ ~ the ~ prompt y radiation observed in the 61V(p,ny)61Crand 48Ti(a,ny)Wr reac-
The data for several nuclei of interest are abbreviated in Table I and the data are discussed in detail below for each isotope. Phosphorus-3.2. Phosphorus is monoisotopic and the reaction 31P(n,~)3~P has been one of the most widely used to produce recoil atoms in solids, and is therefore of some interest. Mendelsohn and Carpenterlz have recently studied the prompt y rays emitted in the 31P(d,p)82Preaction and have determined the lifetime of the 0.077-MeV first excited state in 32P to be 5 X 10-lo sec. The transition to the ground state is in all probability an M1
(8) H.Muller, Proceedings of the Symposium on Chemical Effects of Nuclear Radiations and Radioactive Transformations, International Atomic Energy Agency, Vienna, 1964,Vol. 11, p 359. (9) S. Wexler and T. H. Davies, J. Chem. Phys., 20, 1688 (1952). (10) 8. Yosim and T. H. Davies, J. Phys. Chem., 56, 599 (1952). (11) J. L. Thompson and W. W. Miller, J. Chem. Phys., 38, 2477 (1963). (12) R. A. Mendelsohn and R. T . Carpenter, Phys. Rev., 165, 1214 (1968). (13) L.A. Sliv and I. M. Band, ref lb,p 580. (14) Reference l.a, Nucl. Data, A3 (1967). (15) R.W. Bauer, J. D. Anderson, and J. L. Christensen, Phys. Rev., 130, 312 (1963).
The Journal of Physical Chem
