Development of a neutron spin filter based on protons dynamically polarized using photo-excited triplet states

Abstract

A successful test of principle experiment of a neutron spin filter based on polarized protons has been carried out. The proton polarization was obtained through dynamic nuclear polarization (DNP) using the short-lived photo-excited triplet state of pentacene molecules in a host crystal of naphthalene. The specific technique of DNP we used to polarize the spin filter is the integrated solid effect (ISE) where a microwave pulse is applied while simultaneously sweeping the external magnetic field, transferring the electron polarization to the protons. In contrast to classical DNP we have been able to operate the spin filter at relaxed conditions. This was possible due to the high electron spin polarization of the triplet state which is independent of magnetic field and temperature. A maximum proton polarization of 13% has been recorded by nuclear magnetic resonance (NMR) at a magnetic field of 0.3T and a temperature of 100K. This corresponds to a polarization enhancement of 40000. For the purpose of this thesis, an X-band DNP polarizer has been set up at the Paul Scherrer Institute (PSI) in Switzerland. A laser system, consisting of a frequency tripled Nd:YAG and an optical parametric oscillator (OPO), has been installed. The system delivers laser pulses of 600 nm, optimized for the zero-phonon absorption line of the pentacene molecule, at a repetition rate of 30Hz. The preparation of samples required special equipment. We have built a zone refinement apparatus to purify the naphthalene and equipment for crystal growth using the Bridgman method. With these combined tools several spin filter crystals of excellent quality have been produced. Using the X-band DNP polarizer the crystals grown have been thoroughly characterized, and the parameters of the ISE technique have been explored and optimized. Once positioned on a neutron beam at PSI, the sample with its polarized protons was used to demonstrate spin filtering of the neutrons. For a cubical sample with a volume of 125mm we obtained an analyzing power of 0.17. The polarization of the protons was then measured by the neutron beam, confirming the results of the NMR measurements. In a final experiment, we used the neutron beam to map the polarization distribution throughout the spin filter. Most surprisingly, our experiment showed that the polarization distribution was homogeneous. This was the case even at low laser pulse powers. This result directly contradicts the existing theoretical model, calling for a new description of the photo-excitation process.