El zAEl p Ve y1332

Negatron emission occurs when there is a transition between a discrete energy level in the emitting nucleus (zEl) and a discrete energy level in the product nucleus (z+1El). The difference between the two levels is fixed and determines the energy available to the negatrons. When many nuclei undergo a given transition, the negatrons are not monoenergetic but have energies between zero and the maximum available and, thereby, fail to conserve energy. They have a characteristic, continuous energy spectrum that goes through a broad maximum and then drops to zero at a finite maximum kinetic energy

24 There are three types of neutrino. Each has its antiparticle (antineutrino), which differs from the neutrino with respect to the direction of its intrinsic spin. The electron neutrino (ve) is associated with positron emission (Section 13.5.2.2) and electron capture (Section 13.5.2.3).

Kinetic energy FIGURE 13-12 Example of a negatron spectrum.

£max (the "end point energy") as shown in Figure 13-12. The energies listed for P particles in tables and charts of nuclear data are Emax values. Details of the spectral shape vary somewhat from one P emitter to another, depending on the changes in nuclear properties that occur in the transition. The average energy Eave, which must be used in calculating the dose a person receives from exposure to P radiation (Section 13.12.2), is 30-40% of Emax, depending on the actual shape of the spectrum.

According to the generally accepted theory of P decay, the continuous energy spectrum of negatrons can be attributed to the simultaneous creation and emission of an electron antineutrino, which has no charge and a negligible rest mass (perhaps a very small fraction, e.g., 6 x 10~8 to 6 x 10~6, of the rest mass of an electron.)25 Both particles have continuous energy spectra and share the P-transition energy, keeping the total transition energy (sum of the kinetic energies of the two particles) constant. Thus, the law of conservation of energy is not violated.

Antineutrinos participate not only in the conservation of energy in negatron decay but also in the conservation of nuclear angular momentum, which

25 The rest mass rae of an electron is 5.485 x 10 4 amu, which is equivalent to 0.511 MeV.

requires that the nuclear spin I change by 0 or an integer in a nuclear reaction. Because a negatron (as an electron outside of the nucleus) has an intrinsic spin quantum number of 1/2, its emission would result in a half-integer change in I. Simultaneous emission of an antineutrino, which also has an intrinsic spin quantum number of 1/2, results in the required integer change in I.

When negatron decay occurs, the system loses me the rest mass of an electron particle emitted by nuclide A), but it also gains me because nuclide B contains one more orbital electron than nuclide A. The total energy change is equal to the sum of Emax for the negatron (the energy of the particle plus that of the accompanying electron antineutrino) and the recoil energy acquired by nuclide B. If nuclide A decays to one or more excited states of the nucleus of nuclide B, each transition is followed by one or more 7 transitions as the nucleus de-excites to the ground state. The total transition energy + 7) is the sum of Emax for a given transition and the energy of the photon(s) (or conversion electrons) emitted immediately after the transition plus recoil energy. For example, the + 7) transition energy (0.970 MeV) for the decay of 131I to mXe (Figure 13-10) is equal to

+ 75; P2 + 74' P3 + 73; and P3 + 71 + 72. Because of experimental errors in the particle and 7-ray energies, the sums may differ slightly. The discrepancy between the frequency of emission of 72 and 71 results from the high degree of internal conversion of 71.

Negatron decay for three environmentally important, pure negatron emitters is illustrated in Figure 13-13 for 14C, and the parent-daughter fission products, 90Sr and 90Y. The decay scheme for tritium (3H) is like that for 14C except that the maximum negatron energy is 0.0186 MeV. The more common mode in which negatron decay is followed by 7 emission is illustrated by 60Co and 131I in Figures 13-9 and 13-10, respectively.

Calculation of the recoil energy given to the product atom following emission of a particle is more complex than for 7-ray emission. Two examples of the maximum Erecoil from pure negatron emitters are as follows:

1. About 3.5 eV (3.4 x 102kj/mol) for 3H (12.32 y, Emax = 0.0186MeV), which decays to 3He (s)

2. About 45 eV (4.3 x 103 kj/mol) for 90Y (Figure 13-13)

Although these recoil energies, like those from 7 rays, are not large enough to significantly reduce the net energy of the radiation, they can cause disruptive chemical effects.

13.5.2.2 Positron Emission

A positron (a positively charged electron, e+, written here as p+), and a neutrino, ve, are created and emitted from a nucleus when a proton is transformed into a neutron. The positron is an antielectron, which is one of the

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FIGURE 13-13 Decay schemes for (a) 14C and (b) 90Sr -90 Y. Energy is in MeV.

antiparticles that make up antimatter. This decay process occurs in nuclei having a ratio of neutrons to protons that is too low for stability. In decay by positron emission Z decreases by unity, N increases by unity, and A remains constant as the nuclide moves from a position below the stability band in Figure 13-1 toward the stability band. The equations analogous to equations (13-31) and (13-32) are n + P++ ve (13-33)

Like negatrons, positrons have continuous spectra with an Emax, but with the broad maximum shifted toward higher energy. The role of the neutrino is analogous to that of the antineutrino in negatron emission. Because of their very small mass and the absence of a charge, neutrinos and antineutrinos have a negligible probability of interacting with matter and are, therefore, extremely difficult to detect. Since, however, the sun is a strong source of these particles, they are not in short supply.

In positron emission the rest mass of the system decreases by at least twice the rest mass of an electron. One electron is the P+. The other is the extra-nuclear electron no longer needed to balance the nuclear charge of the product

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