Cold moving mice: A microfoil internal conversion electron detector for low and intermediate energy Mössbauer transitions

A microfoil internal conversion electron (MICE) detector is described which permits direct Doppler shifting of resonance radiation up to velocities of ± 20 cm/s, and gives large improvements in signal-to-background ratios for many Mössbauer isotopes, when compared with transmission geometry. The detector described has efficiency of nearly unity, and it allows for cooling the reciprocating microfoil module to 100 K, which improves signal-to-background substantially over room temperature operation. We give an analysis of the signal-to-background that can be expected for this MICE detector, and for a corresponding transmission experiment. In a table of neutron produced isotopes we find more than 10 cases which are favorable to the MICE approach as compared with the more conventional transmission geometry. The signal-to-background enhancement predicted for several Mössbauer isotopes is substantial for the MICE geometry compared to transmission geometry. Direct measurements of the Mössbauer conversion electron spectrum for the 46.5 and 99.1 keV transitions of ¹⁸³W and the 100.1 keV transition in ¹⁸²W are reported and compared with our analysis. In the case of ¹⁸³W (46.5 keV) we observe over 500% signal-to-background, and this experimental result agrees well with our analysis of the expected size of the effect. Satisfactory agreement is also found for the 99.1 keV ¹⁸³W and 100.1 keV ¹⁸²W spectra. Based on the analysis given it is possible to determine nuclear resonance cross sections and thereby infer internal conversion coefficients for the resonance transitions. Thus, we are able to determine the internal conversion coefficient for the 46.5 keV transition in ¹⁸³W to be α = 13 ± 3.