Measurements of the ballistic-phonon component resulting from nuclear and electron recoils in crystalline silicon

We present measurements of the ballistic-phonon compontent resulting from nuclear and electron recoils in silicon at similar to 380 mK. The detectors used for these experiments consist of a 300-mu m-thick monocrystal of silicon instrumented with superconducting titanium transition-edge sensors. These sensors detect the initial wavefront of athermal phonons and give a pulse height that is sensitive to changes in surface-energy density resulting from the focusing of ballistic phonons. Nuclear recoils were generated by neutron bombardment of the detector. A Van de Graaff proton accelerator and a thick Li-7 target were used. Pulse-height spectra were compared for neutron, x-ray, and gamma-ray events. A previous analysis of this data set found evidence for an increase in the ballistic,phonon component for nuclear recoils compared to electron recoils at a 95% confidence level. An improved understanding of the detector response has led to a change in the result. In the present analysis, the data,an consistent with no increase at the 68% confidence level. This change stems from an increase in the uncertainty of the result rather than a significant change in the central value. The increase in ballistic phonon energy for nuclear recoils compared to electron recoils as a fraction of the total phonon energy (for equal total phonon energy events) tvas found to be 0.024(-0.055)(+0.041) (68% confidence level). This result sets a limit of 11.6% (95% confidence level) on the ballistic phonon enhancement for nuclear recoils predicted by ''hot spot'' and electron-hole droplet models, which is the most stringent to date. To measure the ballistic-phonon component resulting from electron recoils, the pulse height as a function of event depth was compared to that of phonon simulations. We find that the ballistic-phonon component resulting from electron recoils is consistent with phonon quasidiffusion simulations that give the fraction of total phonon energy propagating ballistically as similar to 0.9% (150-mu m event depth) and marginally consistent with up to similar to 25% ballistic phonons.