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

Abstract

We present measurements of the ballistic-phonon component resulting from nuclear and electron recoils in silicon at \ensuremath{\sim}380 mK. The detectors used for these experiments consist of a 300-\ensuremath{\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 $^{7}\mathrm{Li}$ target were used. Pulse-height spectra were compared for neutron, x-ray, and \ensuremath{\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 are 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) was found to be 0.${024}_{\mathrm{\ensuremath{-}}0.055}^{+0.041}$ (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 \ensuremath{\sim}0.9% (150-\ensuremath{\mu}m event depth) and marginally consistent with up to \ensuremath{\sim}25% ballistic phonons. \textcopyright{} 1996 The American Physical Society.