Hot Atoms Produced by Nuclear Transformations A Convenient Tool for the Study of Chemical Reactions in the High-Energy Region Emmanuel A. Gasparakis Meg. Kalivia Hlgh School, 16 Amallas Street, 42 100 Trlkala, Greece
In a nuclear transformation the emission of particles or quanta imparts to the nucleus a recoil energy (Law of Conservation of Momentum). Let us consider three simple cases.
Racoll Energles In Some Nuclear Transformatlono ( 6 )
6Li(nn)3H 3He(n.~)3H "N(n,p)"C
Partlcle Emlsslon
In the case where a radioactive decay process the nucleus emits a particle (e.g., a-particles) the recoil energy R, is calculated by
where m and E, are the mass (in amu) and energy of the emitted particle, respectively, and M is the mass (in amu) of the recoil nucleus. Example
The radioactive 228Th emits a-particles of energy 5.423 MeV (I). Accordingto eq 1thevalue of the recoil energyisR, = 41228 5.423 MeV = 0.095 MeV = 95 keV. Gamma-Ray Emlsslon
Recoil Energy
Reaction
2.73 MeV 192 keV
45 keV 1O2-lo3 eV 10~-10~ ev
3'Cl(n,r)38CI B'Brin.~PBr
where E;- is the maximum value of Eg- in MeV. For the p+decay the recoil energies Rg+and Rs+c,,, are calculated by equations analogous to eqs 4 and 5, respectively. Example
The decay scheme of 24Naincludes @--particles emission withmaximum energy of 1.39 MeV (I). According to eq 5 the value of maximum recoil energy is
If the radionnclide emits a photon of energy E,, the recoil energy R, is calculated hy = 7.48 X
where E, is the energy of emitted photon, M is the mass (in amu) of the recoil atom, and c is the speed of light. If the energy is expressed in MeV and c in cm s-', then eq 2 is transformed to
-
Example
The nucleus of s2Br emits photons of energy 1474.8 keV ( I ) so the value of recoil energy according t o eq 3 is
or R, = 14.2 eV. We must note that for the calculation of R, we did not take all the y-ray energy spectrum into account. Beta Particles Emlsslon
If the radioactive nucleus is deexcited via 8--decay, then the recoil energy Rg- is calculated by
where E,- and E; are the energies of emitted electron and antineutrino respectively, mo is the rest mass of electron, M is the mass of recoil atom, c is the speed of light, and 9 is the angle between the electron and antineutrino. The maximum value of Rg- in MeV is given (2) by
MeV = 74.8 eV
Recoll Energy and Chemical Bond
In 1934 L. Szilard and T. A. Chalrners (3)irradiated ethyl iodide by slow neutrons and found that part of the radioactive lZ8Iformed by the nuclear reaction 1271(n,y)1281,could be extracted from the ethyl iodide with water. Fermi et al. (4) proposed that the carbon-iodine bond was broken when the excited 1281nucleus emitted photons. Because the neutron-binding energy for the reaction '271 (n,y)lZ8Iis 6.6 MeV, an isolated lZ8I atom releasing this energy as a single gamma ray acquires, according to eq 3,182 eV of recoil energy. Although only part of the recoil energy is available for the rupture of the carbon-iodine bond, the bond ruptures because the carbon-iodine bond energy is only 2.3 eV (5).Table 1lists some values of recoil energy (6). Because most chemical bond energies do not exceed 0.5 eV, the recoil energy is sufficient in many cases for the bond rupture. Mechanisms favoring nonbond rupture are (6-8): 1) When the atom recoils into the molecule in a direction close to the molecular axis, then the recoil energy is dissipated into bonds other than that binding the transformed atom. 2) In some (n,y)activation processes (e.g., 79Br (n,y)mBr)the neutron-binding energy is released as a y-ray cascade resulting in oartial cancellation of v-rav . , momenta. As a conseouence of this canrtliaticm some of the atmm receive a net IWOII energ" that is less than that which is required for hmd rupture.
Nevertheless, i t may he concluded that if hydrogenous substances are excluded, the recoil energy is sufficient t o rupture more than 99% of the molecules, when the transformed atom is bound by only one bond (6). Volume 63
Number 10
October 1986
847
These recoil atoms with high translational energy are called "hot atoms", and the study of hot atoms reactions is called "hot atom chemistry". In some cases the bond rupture is due not t o recoil energy b u t t o supplied charge, like i n nuclear reaction 80mBr(IT)80Brin CH2Br. where the charee suvvlied on MBr is redistributed on the whole m o ~ e c u l e r e s u i t hin~ bonds rupture which are due to strong repulsion Coulomb forces (9). The Hot Atoms In Chemlcal Comblnatlons At the moment of its birth the hot atom receives a high translational energy (see the table), and i t is not in thermal equilibrium with the surrounding medium. The hot atom loses its hieh enerw in collisions with molecules of the surrounding medium, and, if the activated atom enters stable chemical combinations before i t becomes thermally equilibrated, the reactions are termed as "hot reactions". In the case where the atom does not combine while it still has excess energy, i t becomes thermally equilibrated and takes part in reactions which are termed as "thermal reactions". A hot or thermal reaction can occur in gas, liquid, or solid phase. For example (6)a hot tritium atom strikes a molecule of C4Hloa t the site of a hydrogen and the following reaction takes place
The reaction does not occur with thermalized tritium atoms but only with hot atoms. With ordinary "thermal" techniques the study of chemical reactions is restricted t o a low-energyregion in the vicinity of activation energy of reaction. Calculations with Maxwell-Boltzmann equation show that in a chemical system of ordinary size (i.e., 1020 molecules) and a t a temverature of 1000 K the number of molecules'with energy m&e than 4 eV is no more than five! But if we irradiate 1g of CCL with slow neutrons for only 1min in a neutron flux of 10'2 n cm-2 8-1 more than 10" radioactive atoms of W l will be produced (ignoring self-shielding). The reaction processes in which the 38C1 atoms take part are understood through analvsis of the irradiated material using radiorhemical separation techniques, and the yields of individual recoil produrts (e.g., CC1,3'CI) are determined by measuring their radionctivities. The figure (10) shows a hypothetical reaction cross section dependence on energy for the reaction of atomic hydrogen (D) with an arbitrary compound RH. As the figure shows, in "thermal" or quasi-equilibrium systems only the threshold reaction path leading to HD formation will be of some significance, but other possible reaction paths and products (e.g., the reaction D RH RD H) characterized by higher activation energies have insignificant cross section values. H'ot atoms produced by nuclear transformations (and by other methods (11)) serve as a convenient tool for the studv of chemical rea&i&s over the entire kinetic energy range. This extension of enerw ranee enables us to imvrove the view of chemical reactivity, as ~ u r b i n(12)notes, '
.
-
+
+
-
The fmdimg that a rcch and efficient chemistry takes place at relative translational energies even above typical bond dissociation energies is a dramatic demonstration that kineticists often gain a distorted view of chemical reactions by sampling only the thermal range from the vast range of energy throughout which chemical reaction can occur. Blologlcal Effecls ol Hot Atom Readlons (13-17) The possible consequences of a radioactive incorporation into a biomolecule in t h e course of biological experimentation, medical work, or from the environment (13)are mainly: 1) A change in the rate of a chemical reaction where the labelled
compound participates (14).For example, the rate of incorpora848
Journal of Chemical Education
Hypahetical reaction cross senion dependence on kinetic energy fa the reaction of atomic hvdrwen ID) The . . . wim an arbibalv comoound RH lim. , don& line indicates me Maxveli.Botumann diswlbutionat 1000 K. A hot atom 01 glvsn energy E w l i react by a I lhe indcated paths. The reaction pams are ind cat& by symDol P..
-
.
-
~.
~
P,: D + R H - H D + R P2: D + R H - R D + H P.: D RH R'D R" P,: D t R H - R " D + R ' + H P.: D RH D tragments
+ +
- + -+
tion into DNA in vivo, of 5-iododeoxyuridine(a thymidine analog) labelled with'2SIis a b u t six timee less than that of thymidine (18). 2) Cell deathdue to irradiation hy the incorporated radionuclide. It was found (19)that in DNA ahout 5%of the decay of 3ZP leads to double-strand breaks and the efficiencyof production of singlestrand breaks was at least one per decay. 3) The rupture of the bond(s) in which the transformed atam participates. The rupture may be caused by the recoil energy of nucleus or by charge transfer from the daughter nucleus to the carrier molecule (18). In this case the bond ruptnre is due to repulsion Coulomb forces and not to recoil energy (9).It has been found (20) that in some phages a double-strand break of DNA was produced for every second 12SIdecay. Note that whereas single-strand breaks of DNA are efficiently repaired, doublestrand breaks are probably lethal events (18). The available results (18) are difficultto explain solely by radiation effects. For the explanation of the biological effects of lz5I incorporated into DNA, i t has been suggested (18)that the charge on the daughter nucleus of T e P 5 1 (IT)lZ5Te)is transferred to the carrier molecule resulting in the appearance of strong repulsion Coulomb forces and conseauently in the ruvture of bonds.
Conclusions Atoms with very high kinetic energy (the so-called hot atoms) produced by a nuclear transformation provide a very useful and convenient tool for the study of chemical reactions in the high-energy region (hot reactions). The study of hot reactions is possible in gases, liquids, and solids. Besides, nuclear transmutations of radionuclides (e.g., lZSIor 14C)incorporated into biomolecules (e.g., DNA, RNA, proteins) may alter drastically the structures and functions of these molecules, resulting in mutations or cell death. Lnwature CHed !I) Frwd1anocr.C. .Kennedy.J.W.,Msriaa.E S.:Miller,J. M."Nurlsauandredilwhem. Irrv "3rd ed : H d c y . N n York. 1981. 21Lihhy W.F.J A m r r Chem Snr 1317.69.25W. (3) Lilard, L:Chslmera,T.A. Notwe 1934,234,462. (41 AmaJdi, E.: D'Agastmo: Femi. E.:Pontwrro, B.; Raaetti, F.: Se@. E.Rac. Ray. Sor. (London) 1935. A149.522. (51 W8apt.R C., Ed."Handbookaf Chemi~tnrandPhysir.,"47thed.:ChwicdRubber Co., 1966. (6) Maddock, A. G.; W o l f g w , R. "Nuclsa. Chemktry"; Y&&, I.Ed.: Academic: New York, 1968: Vol2.p 166. (71 Gordua, A. k: Hsiung, C. J. Chern.Phya. 1%2,36,954.
(8) Campbell. I. 0. Adu. Inorg. Ckm. Redimhem. 1968,5.135. (9)Weder. S . h . Symp.on Chem.Eff cts olNveleor Trarulormotio~(IAEA. V i a n d 1961 1.115. (10) Wolfgang, R. PIogre~~R.octionKinefiesIWS, 3.97. (11) Tominsgs, T.; Tachihwa, E. "Madem Hot Atom Chemishy and its Application9; Sorimer-Vdae: Heidelhers 19R1. Chan 1
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Volume 83
Number 10 October 1986
849
