1. β-Decay and Neutrino Mass The neutrino – a tiny, electrically neutral, almost undetectable particle – presents one of the greatest mysteries in modern physics. For many years it was thought that the neutrino had no mass whatsoever, and although recent evidence shows this not to be the case it has thus far eluded measurement. Although small, the absolute mass of the neutrino has far-reaching implications for physics and cosmology and nuclear β- decay provides an excellent tool for probing it. Figure 1: β-Decay – Cartoon, Feynman Diagram & Nuclear Equation β-Decay is a process, occurring naturally within the nucleus of some unstable atomic species, in which a Neutron decays to a Proton accompanied by the emission of an electron and a neutrino (Fig. 1). Figure 2: The β–spectrum Figure 3: The Kurie Plot, a convenient linearisation of the β-spectrum Aims and Proof of Principle The AMBER prototype has demonstrated, in principle, the ability to measure the neutrino mass but there is much work still to be done. J.A.Thornby – Department of Physics, University of Warwick, CV4 7AL Measuring the β Spectrum with AMBER To accurately measure the end-point energy of β-decay a high precision charge spectrometer with excellent resolution near to Q is required, to avoid the smearing effects shown in Fig. 3. Enter AMBER. Figure 4: The AMBER apparatus – schematic drawing and prototype AMBER employs a levitating ball bearing in a 1 × 10 -4 mbar vacuum. The ball is perfectly electrically insulated, meaning accumulated charges cannot escape. The principle behind AMBER is to charge the ball up using β electrons, while simultaneously and continuously measuring its electrical potential. Figure 5: The charge collection/repulsion process As electrons are collected the ball will become negatively charged, thereby repelling electrons. Only electrons energetic enough to overcome the repulsion can be collected. As the ball’s potential increases the retarding force increases, making it progressively harder for subsequent β-electrons to be gathered. A point will be reached at which even the most energetic β-particles are insufficient to overcome the repulsion of the ball. At this point the ball’s potential is equivalent to the end-point energy. Acknowledgements Dr. Yorck Ramachers, Mr. Adrian Lovejoy & AMBER logo courtesy of Mr. Chris Allen. υeυe duddud duuduu e-e- W -W - N P N → P + e - + υ e K(E) Zero neutrino mass Finite neutrino mass Effect of:  Background  Energy resolution  Excited final states  Q-  E Q (dN/dE) dE  2(  E/Q) 3 Q-m ν c 2 Q The electron has a spectrum of energies (Fig. 2), up to a maximum, fixed by the Q-value of the decay. When E e = E max (“end-point energy”) E υ = 0, as total energy is fixed by Q. If E max ≠ Q then the discrepancy corresponds to the energy required to produce the neutrino. This is seen more clearly in a Kurie Plot (Fig. 3) Electron Energy Relative Decay Probability 1 0.8 0.6 0.4 0.2 0 End-Point Energy E max Vacuum Flange Mounting 1.38T Support Magnet Electromagnetic Coil 1.38T Support Magnet Kelvin Pickup Plate Levitating Ball e-e- Incoming β -electron negative Retarding force AMBER aims to measure the collected charge, inferred from the change in the ball’s potential. From this an integrated β- spectrum (Fig. 7) can be plotted. The true β-spectrum and end-point energy can later be recovered from this integrated spectrum. dV/dt large dV/dt small In order to accurately determine the β end-point energy, AMBER is required to measure the ball’s potential, V, very precisely. Fig. 6 shows data taken over 24 hours and demonstrates: Perfect ball insulation & stability Voltage resolution of ±1mV It is also necessary for AMBER to demonstrate that the charge collection process will work on the ball a in a vacuum. Fig. 8 demonstrates this process, using a variable high voltage electron source. ΔV 0 =ΔV 1 +ΔV 2 +ΔV 3 2 mV ΔV0ΔV0 ΔV3ΔV3 ΔV2ΔV2 ΔV1ΔV1 Figure 6: Ball voltage stability and resolution Figure 7: Integrated β- spectrum of 63 Ni Figure 8: Demonstrating charge collection in the vacuum Increasing electron source voltage