Comparison of the Beta Spectra ofB12andN12

Abstract

The observed near equality of the vector beta coupling constant given by the decay of O 14 and the muon decay coupling constant led Feynman and Gell-Mann 1 to postulate the conservation of the vector part of the beta-decay current. Gell-Mann 2 pointed out that a consequence of this conserved vector current (CVC) theory is an additional coupling in the beta interaction analogous to the anomalous magnetic moment of the nucleon ("weak magnetism"). The interference between this term and the Gamow-Teller transition amplitude for a l + -*0 + , AT = 1 beta transition then results in a deviation from the allowed shape of the B 12 and N 12 spectra. The magnitude of the effect can be calculated 2 ' 3 from the rate of the analogous Ml gamma transition in C 12 . If S(E) is the spectrum of each transition, divided by the corresponding allowed Fermi spectrum, then the CVC theory predicts S(E, B 12 )/S(E, N 12 ) = const[l + (A + 6A)E]f(E),where the constants arises from this interference term. The quantity/(£) is the inner bremsstrahlung correction. The expected value for A is (1.33 ± 0.15)% per Mev with an electromagnetic correction 4 6A of (-0.25± 0.15)% per Mev. The uncertainty in A comes from the uncertainty in the C 12 gamma-decay rate and the uncertainty in 6A is an estimate of the error from using the shell model. Without the CVC theory hypothesis, A is estimated to be roughly 5 times smaller. 4 ' 5The beta spectra of B 12 (# max = 13.369 Mev) and N 12 (£ max = 16.43 Mev) were analyzed with the iron-free single-lens magnetic spectrometer described by Hornyak et al. 6 The baffle system was modified for the high electron energies involved. B 12 (20.3 msec) was produced by bombarding targets of natural boron with 1.65-Mev deuterons, and N 12 (12.5 msec) was produced by bombarding targets of enriched (96%) B 10 with 2.75-Mev He 3 ions. Both beams were pulsed at 60 cps at the ion source of the electrostatic ac-.celerator. The targets consisted of approximately 0.3 mg/cm 2 of boron deposited by cracking diborane on foils of thickness 0.5 mg/cm 2 and 3 mg/ cm 2 for B 12 and N 12 , respectively. The beta rays were detected in a 10-mm thick anthracene scintillator. This is sufficiently thin (approximately 2.3-Mev range for electrons) to give nearly identical pulse-height spectra for all energies in the range studied (5-13 Mev). The use of a thin scintillator allows amplifier gain and discriminator settings to be left constant, reduces background as a result of its small volume, and produces a negligible small pulse tail extending into the noise background. 7 The activity produced on the target was monitored by the reaction protons from B 1C V, p)B n for B 12 and B 10 (He 3 ,£)C 12 for N 12 . A silicon p-n junction counter mounted in the spectrometer near the target served as a proton detector.The spectra were sampled at 0.5-Mev intervals in the range 5 to 10.5 Mev for B 12 and 5 to 13 Mev for N 12 , the data being collected at alternating high-and low-energy points to minimize effects of target deterioration and instrumental drif...

Precision Lifetime Measurements on Positive and Negative Muons

“…The closeness of the predicted and previously observed lifetimes is certainly enough to establish the essential validity of the theory, especially in view of the recent verification as observed in the decay spectra of B 12 and N 12 . 8 It is in fact the precise deviation of the two numbers (2±0.5% after radiative corrections)^that may teach us something new, be it the nuclear physics of O 14 or intermediate boson corrections to the lifetime of the muon. Figure 1 shows the arrangement of counters and hydrogen target.…”