A high-luminosity, high-resolution study of the end-point behaviour of the tritium β-spectrum (II). The end-point energy of the spectrum. comparison of the experimental axial-vector matrix element with predictions based on PCAC.

Abstract

An evaluation of the end-point energy of the tritium β-spectrum is described. The experimental data employed are basically identical with those involved in a previous paper concerned with an inspection of the end-point region of the tritium β-spectrum evidence of neutrino mass and neutrino degeneracy. By using combined electrostatic and magnetic β-spectroscopic methods, allowing an improvement in luminosity of about three orders of magnitude, a much more detailed exploration of the end-point behaviour of the tritium spectrum than previously possible was achieved in that paper. The improved accuracy in the experimental data on the shape of the spectrum in the end-point region also forms a favourable starting point for the realization of an improved accuracy in the determination of the magnitude of the end-point energy. The initial error in the end-point energy, expressing the purely statistical uncertainty in the fitting of the straight line in the Kurie plot, is ±2 eV. The present paper is essentially concerned with the analysis and experimental measures required for retaining as much as possible of the initial accuracy in the extrapolated end-point energy in the final statement about the magnitude of this entity. The result arrived at for the end-point energy of the tritium spectrum is E β, max = 18.610 ± 0.016 KeV. Several other measurements, of about five times smaller accuracy, are compatible with this value. Our result refers to the end-point energy of the spectrum from the free tritium atom. The magnitude stated implies a value of 529.562 ± 0.017KeV for the 3H− 3He nuclear mass differences and a value of 18.651 ± 0.016 KeV for the 3H− 3He atomic mass difference. The inferred ft-value for the tritium decay becomes 1148 ± 3 sec, from which a value |M A| 2 = 2.91 ± 0.05 can be established. Writing | M A| 2 = | M A 0| 2(1+ δ) 2, where M A 0 is the theoretical axial-vector matrix element disregarding exchange effects, this experimental value of | M A| 2 implies a positive value on the exchange parameter δ, preserving the discrepancy between experiment and the calculations by Blin-Stoyle and Tint previously pointed out by these authors. The present experimental result for δ implies agreement to within roughly a one-percent accuracy with recent calculations by Blomqvist, taking one-pion exchange processes into account. Because of a certain covariance between the resulting experimental and theoretical values of δ this agreement can be claimed essentially irrespective of the uncertainty in the structure of the triton.