Recent experiments with X-rays and high energy electrons have shown that image recovery from diffracted intensities is possible for non-periodic objects using iterative algorithms. Application of these methods to biological molecules raises the crucial problem of radiation damage, which may be quantified by Q = ΔE σi/σe, the amount of energy deposited by inelastic events per elastic event. Neutrons, helium atoms and low energy electrons below most ionization thresholds produce the smallest values of Q (see for TMV imaged at 60 eV). For neutrons (λ = 10-2Å, and deuterated, 15N-abelled molecules) Q is ∼3000 times smaller (∼50 times for λ = 1.8Å) than for electrons (80- 500keV) and about 4x 106 times smaller than for soft X-rays (1.5Å). Since σe for neutrons is about 105 times smaller than for electrons (and about 10 times smaller than for soft X-rays), a 105 times higher neutron dose is required to obtain the same S/N in a phase contrast image compared with electrons, if other noise sources are absent.
Side-by-Side Comparison of DNA Damage Induced by Low-Energy Electrons and High-Energy Photons with Solid TpTpT Trinucleotide
