Observation of Nuclear Excitation of 19F by Synchrotron Radiation

Abstract

Fluorine (F) has the lowest atomic number and atomic weight of the members of the halogen family. Only the isotope with atomic weight 19 is stable. Fluorine is the most electronegative element and the most chemically energetic of the nonmetallic elements. The reactivity of the element fluorine is so high that chemical processes take place at about 500 C; a good sample is the reaction of fluorine and uranium dioxide leading to the production of uranium hexafluoride. F nuclei with a nuclear spin of 1/2 and a 100% natural abundance presents favourable nuclear magnetic resonance (NMR) characteristics. The application of the F NMR to metabolic studies of fluoropyrimidine drugs in clinical use; such as the anticancer drug 5-fluorouracil in human biofluids, have emphasized the usefulness of the technique for the quantitative detection of novel and unexpected metabolites. The use of synchrotron radiation as a source for nuclear resonant scattering experiments has led to many new studies of physical problems such as nuclear Bragg scattering, nuclear forward scattering, and nuclear inelastic scattering that can not be investigated satisfactorily with radioactive sources in the conventional Mossbauer spectroscopy. The unique characteristics of synchrotron radiation are the collimation, the polarization, the pulsed nature, and the high brilliance. The discrimination between the scattering from electrons and the scattering from nuclei has been solved by taking advantage of the instantaneous character of the atomic prompt nonresonant scattering compared with the relatively long lifetime of the nuclear excited states. Avalanche photodiodes allow the observation of delayed events after a few nanoseconds, even in the presence of a huge prompt background. High-resolution monochromators allow us to select energy and tune by energy bands of the order of 1meV width. The third generation synchrotron radiation storage rings, ESRF, APS, and SPring-8, were constructed and equipped with beamlines dedicated to nuclear resonance scattering experiments. The nuclear resonance scattering of synchrotron radiation has been experimentally observed in various nuclei, i.e., Ta (6.21 keV), Tm (8.41 keV), Kr (9.4 keV), Fe (14.41 keV), Eu (21.54 keV), Sm (22.49 keV), Sn (23.88 keV), Dy (25.65 keV), K (29.83 keV), and Sb (37.13 keV). The use of parentless Mossbauer isotopes, e.g., K, has become a reality. Other isotopes such Ni (67.41 keV), Au (77.35 keV), and Gd (79.51 keV) with high-transition energies above 50 keV have been excited and their fluorescence decay has been detected. Excitation energies above 100 keV of nuclei have not yet been reported. In this paper, we report the observation of the F nuclear excitation (109.89 keV) by the use of synchrotron radiation. The experiment was performed at the undulator beamline (BL09XU) at SPring-8. The undulator gap was tuned such that the seventeenth harmonic of the undulator radiation corresponds to the F resonance energy for the transition from the 1=2þ ground state to the 109.89 keV 1=2 excited state. Figure 1 shows our optics and detecting system for the F nuclear excitation and deexcitation. The double-crystal monochromator operates with diamond (333) reflection, while the X-rays from diamond (111) reflection were eliminated using a Ge(111) crystal. X-ray pulses with a time width of 50 ps [full width of half maximum (FWHM)] excited the F nuclei, using a several-bunch operation of the storage ring. The system with a silicon avalanche photodiode (APD) recorded the time spectra of -rays emitted from the 109.89 keV 1=2 state of the F nuclei. The details of the fast counting system with the APD detector were reported by Kishimoto. The APD has a 4 2 array of 3 5 0:15mm pixels and a depletion layer 150 mm thick. An efficiency of 0.6% at 100 keV was estimated, corresponding to absorption in 150-mm-thick silicon. The time resolution is 1.3 ns (FWHM). The APD was mounted under the sample. We used a fast amplifier to obtain fast negative pulses of the APD outputs, which were processed with a constant fraction discriminator (CFD). The outputs from the CFD were used as the start signals into a time-toamplitude converter (TAC). The veto signals into the CFD and the stop signals into the TAC were supplied through some divider and delay modules by 508.58MHz signals from the accelerator. The time spectra were recorded by the TAC and a multichannel analyzer. The prompt radiation outputs were inhibited during the first 5 ns by veto signals. Delayed signals started to be detected after the first 5 ns of the prompt radiation. For the study of the first excited level of the F nuclei, a lithium fluoride sample (LiF, 20 20 10mm) was used as a scattering sample. Figure 2 shows the time dependences of