Precision measurements of the electron energy distribution in nuclear beta decays

Although nuclear β decays have been studied for almost a century, there are still questions that can be answered through precision measurements of the energy distribution of emitted electrons. The shape of the β spectrum reflects not only the weak interaction responsible for the decay. It is also sensitive to the strong interaction which confines the decaying quark in the nucleon. The electromagnetic force also plays a role since the ejected electron interacts with the charged nucleus. Therefore, measurements of the β spectrum shape are an important tool in understanding all the interactions combined in the Standard Model (SM) and high precision is required to disentangle the effects of different interactions. Moreover, these measurements can even shed some light on New Physics (NP), if discrepancies between the shape of a measured spectrum and the one predicted by the SM are observed.

One of the open questions is the validity of the V A form of the weak interaction Lagrangian. Measurements of the β spectrum directly address this issue, since the exotic currents (i.e. other than V and A) affect the spectrum shape through the so-called Fierz interference term b. An experiment sensitive to NP requires a precision at the level of 10−3 to be able to compete with energy frontier measurements performed at the Large Hadron Collider.

When precision is at stake,  the success of an experiment is determined by a proper understanding of its systematic uncertainties. The main limitation of the precision of spectra measurements comes from the properties of the energy detector being used in the experiment. In particular, from the accuracy of the detector energy response and of the energy deposition model in the detector active volume. The miniBETA spectrometer was designed to measure β spectra with a precision at the level of 10−3. It combines a plastic scintillator energy detector with a multi-wire drift chamber where electrons emitted from a decaying source are traced in three dimensions.

Using the plastic scintillator minimizes systematic effects related to the model of the energy deposition in the detector active volume. Due to the low Z of the plastic and a choice of the detector thickness, the probability that the energy is fully deposited is highest. Keeping the probability of other processes (i.e. backscattering, bremsstrahlung, and transmission) low reduces the influence of their uncertainties on the predicted spectrum. Particle tracking filters the measured spectrum from events not originating from electrons emitted from the source. It also allows for a recognition of backscattering events, further reducing their influence.

The thesis describes the commissioning of the miniBETA spectrometer. Several gas mixtures of helium and isobutane were tested as media for tracking particles. Calibrations of the drift chamber response were based on measurements of cosmic muons. The mean efficiency of detecting an ionizing particle in a single drift cell reached 0.98 for mixtures at 600 mbar and 0.95 at 300 mbar. The spatial resolution of a single cell, determined from the drift time, reached 0.4 mm and 0.8 mm, respectively. The resolution determined from the charge division was roughly an order of magnitude worse. Several corrections dealing with systematic effects were investigated, like a correction for a signal wire misalignment sensitive to shifts as small as 0.02 mm.

It was shown that particle tracking improves the precision of spectrum measurements, as the response of the spectrometer depends on the position where the scintillator was hit. Preliminary calibration of the energy response of the spectrometer was performed with a 207Bi source. The calibration was based on fitting the measured spectrum with a spectrum being a convolution of simulated energy deposited in the scintillator and the spectrometer response. The energy resolution of the prototypical setup is ρ1MeV = 7.6%. The uncertainty of the scale parameter, hindering the extraction of energy dependent terms (i.e. the Fierz term and weak magnetism) from the spectrum shape, is 5 *10−3.   These values will be significantly improved in a new setup, with an optimized scintillator geometry and better homogeneity of the energy response.