The recoil-free coherent resonant scattering of the 14.4-keV Fe 67 gamma rays has been investigated by employing the techniques of the Mossbauer effect together with those of total reflection. Of particular interest are the observed large interference effects attributed to the coherent superposition of the several resonant nuclear contributions and the nonresonant electronic contribution to the scattering amplitude in the forward direction. Gamma rays from a 300-mCi Co 57 source, diffused into the thin edge of a stainless steel plate, were reflected at small glancing angle 0 by an optically flat Fe 67 mirror through a narrow slit into a Nal scintillation detector. The resonant frequency spectrum of the Fe 67 nuclei in the mirror was scanned by varying the source velocity v. Values of the absolute reflectivity R (0,v) were obtained by comparison of the direct and reflected measured intensities. In an auxiliary experiment R(v) was measured with an external magnetic field applied to the mirror. The results of these measurements were compared with computerprogrammed calculations based on the classical optical theory of total reflection. Appropriately averaged quantum-mechanical expressions were introduced for the coherent resonant nuclear scattering amplitude in the forward direction. Separate computations were made for each polarization state of the incident radiation; these were averaged to give R(9,v) appropriate for an unpolarized source and a polarizationinsensitive detector. Suitable integrations were performed to take into account the frequency spread and angular width of the incident radiation. For 0 = 4 mrad, R(v), computed with the six resonances of normal ferromagnetic Fe 57 , agreed in shape and magnitude with the observed reflectivity, when the resonance width was assumed to be four times the natural width. For 0 -2 mrad, where the gamma-ray penetration depth is small, anomalies in the magnetic and chemical environment of the Fe 67 nuclei near the mirror surface required the introduction of additional resonances for FeO and for nonmagnetic Fe.

Measurements are reported on the total neutron cross section of isotopically enriched Pb 208 (99.75%) for neutron energies in the range from 720 to 1890 keV with an energy spread of about 3 keV. In this region at least 85 resonances are observed, of which 24 are analyzed to give tentative spin assignments and reduced widths. The reduced-width estimates furnish evidence that the J = 5/2 resonances at 723 and 821 keV have even parity. Differential cross sections of normal lead measured with 50-keV energy spread at 1.2, 2.2, and 3.2 MeV were used in estimating the in-scattering correction for the total cross-section data and are included in this paper.

where all quantities are known except e(E 2 ). Because of the high efficiency of the Nal(Tl) detector for the low-energy E 2 photons, e(EJ) is determined primarily by the ability of the fast coincidence circuit to accept the photomultiplier pulses resulting from these photons. To measure this efficiency, a source of annihilation photons was placed between the two detectors and the pulse-height distribution from the E 2 detector was observed with and without gating the 20-channel analyzer from the output of the fast coincidence circuit. After normalization the ratio of these two pulse-height distributions gives e(E 2 ) as shown in Fig. 8. Upon integration, the predicted rates for

No abstract

No abstract