Nuclear-resonance excitation in F^(19) and Mn^(55) with a germanium bent-crystal monochromator

Abstract

A precision bent-crystal gamma-ray monochromator with stationary source has been built. The instrument consists of three physically independent units: The line source (a radioactive source or the anode of an x-ray tube), the two-meter-radius bent-diffraction-crystal with its pivot and sine-motion mechanism, and the heavy-duty curved-track framework which supports the collimator, detector, and shielding. The motions of the crystal-pivot unit and of the detector-carriage unit are linked together in such a way that the reflection condition is satisfied. Results are presented showing that the precision of the monochromator for measurement of gamma-ray wavelengths is 0.003 x-units. The line width at half-maximum observed when the (800) planes of a bent germanium-crystal are used is 0.080 x-units. The resolution which has been attained with the germanium crystal is illustrated by a measurement of the 244.264 kev, 246.056 kev, and the previously unobserved 245.237 kev gamma line in the decay of W^(183). Nuclear-resonance scattering from the first-excited states in F^(19) and Mn^(55) has been observed with the bent-diffraction-crystal monochromator. The experiment was performed by observing the scattered radiation from nuclei exposed to nearly monoenergetic x-rays selected by crystal diffraction from the bremsstrahlung spectrum of an x-ray tube. Gamma rays scattered at 135° from samples of lithium fluoride and manganese placed in the diffracted beam were observed as a function of the incident photon wavelength. With the lithium fluoride sample three measurements were made under different experimental conditions. In each case pronounced resonance peaks 10 to 15 percent above background were observed. A least-square analysis of the data gives 109.894 ± 0.005 kev for the energy position of the first-excited level in F^(19). From the observed yield the width of this level was deduced to be (5.1 ± 0.7)10^(-7) ev. Measurements with a Mn^(55) scattering sample gave 125.95 ± 0.01 kev for the position of the first-excited level and (1.1 ± 0.3)10^(-6) ev for the resonance width.