The goal of this project is to perform a very precise measurement of mass difference between two types of chlorine atoms: Chlorine-35 and Chlorine-36. These two isotopes have the same number of protons and electrons, so they have the same chemical properties, but chlorine-36 has one extra neutron, so they do not have the same mass. However, the mass difference is not simply the mass of the neutron. This is because, according to Einstein's famous equation, E = mc2, some of the mass of the extra neutron is given up as nuclear binding energy. Hence, by measuring the mass difference between chlorine-35 and chlorine-36, and accounting for the mass of the neutron, the chlorine-36 neutron binding energy can be determined. The mass difference measurement gives the right hand side of Einstein's equation and can be compared with other, independent measurements of the neutron binding energy that give the left hand side of Einstein's equation. These other measurements use precise spectroscopy techniques to measure the energy of gamma-rays (high energy electromagnetic radiation similar to x-rays) that are emitted just after a chlorine-35 nucleus captures a neutron to become chlorine-36 and releases the excess energy. The comparison of the mass difference and gamma ray energies then provides a direct test of E = mc2.
This project will use high-precision Penning trap mass spectrometry to determine the mass difference between chlorine-35 and chlorine-36 by measuring the ratio of cyclotron frequencies of singly-charged ions containing these isotopes in the 12 Tesla magnetic field of the Central Michigan University High Precision Penning Trap (CHIP-TRAP). The cyclotron frequency will be determined using the image charge detection technique in which the image currents induced in the trap electrodes by the oscillating ion are detected and amplified with low noise cryogenic circuits. CHIP-TRAP will consist of a novel, double measurement trap structure that will enable the two ions whose mass ratio is to be determined to be simultaneously stored for long measurement times. Ultimately, the cyclotron frequency measurement for the two ions will be performed simultaneously, greatly reducing the effect of temporal magnetic field fluctuations. This technique should enable a precision in the mass ratio of 10 parts per trillion or less to be reached. Ions will be produced outside of the Penning trap using either a laser ablation ion source or a plasma ion source, and will be transported to a capture trap before being transferred to the precision measurement traps. This will reduce the amount of the long-lived (300,000 year half-life) chlorine isotope required for the experiment.
