Walter Loveland Oregon State University Cowallis. 97331
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Textbook Errors, 135:
Nuclear Beta Decay
Most general chemistry textbooks devote a chapter to the discussion of the suhiect of nuclear chemistrv. Unfortunatelv. .. over 90% of these chapters contain serious conceptual errors in their treatment of fundamental nuclear nrocesses. It is the purpose of this note to point out the nature of one of these errors and to present hrieflv a correct treatment of the suhiect material invoived. The error chosen for discussion in this note is the neglect of neutrino emission during nuclear betadecay and its consequences. In nuclear heta decay, a nuclear neutron (or proton) is transformed into a proton (or neutron). In most general chemistry textbooks, these transformations are usually written in the following incorrect equation form
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:'N OleQ C' ffNa :iNe + ?e+ o_,e- + g7Bi gq'Pb
8- decay B+ decay Electron Capture
(1) (2)
(3)
T h e correct representation of these decays is as follows
where the symbols v. and & represent the neutrino and the antineutrino, respectively. T h e e subscript denotes that these particles are emitted in heta decay as opposed to the type of neutrino emitted in the decay of another subatomic particle, the muon. In a hit of pedantry, the symbols P-or B+ are used to denote electrons of nuclear origin, reserving the symbol efor extranuclear electrons. T h e differences between eqns. (4)-(6) and (1)-(3) may be taken to represent a very fascinating epoch in nuclear science in which scientists' faith in the truth of certain fundamental conservation laws was put to the test hy seemingly contradictory experimental evidence. A full account of this story may he found in any standard nuclear chemistry or physics texthook.' What 's wrong with eqns' (1)-(3)? The answer t' that question lies in considering a typical example of the 6decay process, the decay of +(tritium) I to form 3He. If we write this decay (in a manner similar to eqns. (1)-(3) as
-
3H 3He+ 8(7) we find two fundamental nrohlems (which were known orior to 1930). They are (a) The enernies of the B- particles emitted in the decay range from O ke\. t o a maximum rnergy corresponding to the mass difference htween .'Hand ''He ut 1R.6 keV with no other radiation being found in careful investigations with calorimeters. Since the decay is a transition between two definite states, one would expect particles of constant kinetic energy to he emitted. Otherwise, one would have a violation of the law of conservation of enerev. -. (h) T h e principle of conservation of angular momentum and the rule for the statistics of a composite system annear to he violated. The angular momentum of W is 112 while the angular momentum of the (3He P-) is 0 or 1from the spins, plus an integer from any possible orhital angular momentum. ~
~
+
250 1 Journal of Chemical Education
T h e statistics of the system are similarly confused since 3H is a fermion and ("He 8-) is a boson. Thus heta decay as described in most general chemistry books and as known prior to 1930 represents an apparent violation of two fundamental conservation laws. In the early 1930's, Wolfgang Pauli proposed a solution to this dilemma by postulating that a second particle, the neutrino, was emitted along with the electron in heta decay. The energy of transition is divided among the kinetic energies of the electron and the neutrino such that
+
This solved the nrohlem of enerev conservation and hv nostulating a spin of'112 for the neut&o (a fermion), the problem of angular momentum conservation (statistics) was solved also. Fermi, in a joking aside to other Italian physicists, suggested the name "neutrino" for this new particle standing for "a small neutral object" and the name achieved wide spread usage2. Conservation of charge dictated that the neutrino was neutral and detailed considerations based upon the shape of indicated its rest mass was
