No abstract

Simpson and Tuzzolino 1 (ST) have reported that polyvinylidene fluoride (PVF 2 ) can be used to detect individual energetic heavy nuclei such as uranium. They suggest a mechanism whereby an incident nucleus transfers to the sample an energy AE which appears as a heat pulse along the particle trajectory and spreads by heat conduction throughout the sample. The pyroelectricity of PVF 2 then produces charge A Q on surface electrodes. The calculated AQis 9.2AE/L, with Aisin megaelectronvolts, the sample thickness L in microns, and the charge in electron charges. In this Comment we suggest another contribution to the measured AQ-namely, the irreversible depolarization through melting of the material in a cylinder of radius -12 nm about the particle trajectory for A£-30 GeV.The standard theory of energy loss of heavy ions by electron excitation and ionization processes gives a reasonable account of the observed energy losses if the U ion is completely stripped of its electrons during its passage through the sample. The energy transfer to an electron a distance b from the track is proportional to 1/6 2 , and b has a lower bound b r » in = ze 2 /mv 2 , and anHere ze is the charge of the U ion, v its velocity, m the electron mass, and / the minimum energy that can be transferred to an electron; the last can be approximated by the energy gap of the polymeric insulator, which is roughly 10 eV. High-energy primary electrons first produce electron-hole pairs of lower energies; these, along with the primary electrons of lower energy, transfer their energy to the lattice via the electronphonon interaction. Experiments by Pfluger et al 2 on the polymeric dielectric polyethylene have shown thermalization to occur principally through electron-LOphonon scattering. The rate ha) LO y L at which an electron of energy E loses energy to the lattice by Frohlich scattering for E » ft(o LO is given by 3with m* the effective electron mass, co LO the LOphonon frequency, and e 0 and e^ the static and highfrequency dielectric constants, respectively. Since #w L0 -0.37 eV in PVF 2 it follows that a 10-eV electron has a mean free path of -0.8 nm and thus loses energy to the lattice in a cylindrical region of radius -2.5 nm around the nucleus trajectory. Because the energy transferred to an electron is proportional to l/b 2 , roughly j^ of the total energy ( -2.5 GeV) is lost by the nucleus to electrons with kinetic energies < 10 eV. The resulting temperature rise in the cylindrical region would be -10 000 K. Treating this initial heat pulse as a 8 function centered at the nucleus track, and solving the diffusion equation, we can calculate the volume V m of the small region of destroyed polarization, and the resulting charge PV m /L, where L is the sample thickness and P its polarization. We find A0 ^ 74A£'/L, where AE' is -^ of the total deposited energy. The contribution to the measured charge is thus at least y that of the pyroelectric effect caused by the uniform small rise in temperature in the rest of the sample. The measured pyroelectric c...