Page 89 - Radiochemistry and nuclear chemistry

78 Radiochemistry and Nuclear Chemistry

occur through the emission of a "y-ray of 0.049 MeV, giving the daughter a recoil energy
of only 0.016 eV. Still the deca x leads to the emission of 80Br from ethyl bromide, when
the parent compound is C2H58umBr, even though the bond strength is 2.3 eV. This is
because the -y is highly converted and as the electron "hole" is filled, Auger emission
occurs. Bromine ions from Br + to Br 17+ have been observed through mass spectrometric
analyses of the ethyl bromide gas phase.

4.10. Closed decay energy cycles

The masses for many short-lived nuclei are unknown although their decay modes and
energies have been determined. From this the nuclear masses may be calculated, and
consequently Q-values of different unknown decay modes can be obtained. This can be
done through the use of closed decay energy cycles.
Suppose we need to know if 237U can decay to 233Th through a-emission. Of course this
is a simple calculation if the masses of 237U and 233Th are known, but let us assume they
are not. We have data that 237U decays through/~-emission (Ema x 0.248 MeV) followed by
-decay (E~ 0.267 MeV). 233Th decays through B-emission (Ema x 1.230 MeV) directly to
3pa. 237Np undergoes a-decay to 233pa with Ea = 4.79 MeV. We may construct a closed
cycle including these decay energies

et 4.79 MeV
233pa .. 237Np 014~237.048030)
2.1x106 y
A
1.23o MeV ~1- 0.248 MeV
22.2 mia B C I, 0.267 MeV
6.75 d
D
et ?MeV
233111 -. 237 U

The Q-value for branch D is; Q = -931.5 (M233T h + MHe - M237u ).
For branch A we can calculate (4.16); Q = E a M z/Mz_ 2 = 4.79 x237/233 = -931.5
(M233p a + MHe - M237Np). By introducing values for MHe and M237N p we obtain M233p a
= 233.040 108.
For branch B we calculate; M233T h = M233P a + 1.230/931.5 = 233.041 428.
For branch C one obtains; M237u = M237N p + (0.248 + 0.267)/931.5 = 237.048 581.
Thus all information is available for calculating branch D. The Q-value is found to be
4.23 MeV, and the E a = 4.23 • or 4.16 MeV. Although spontaneous a-decay is
energetically possible, it has not been detected. The systematics of a-decay (w
indicates an expected half-life of > 106 y. Because the/~-decay rate is much faster (tl,~ =
6.75 d), too few a's are emitted during the life-time of 237U to be detected.