18. Which statement about the Hartree-Fock (HF) method is correct? (A) The HF method gives the exact electronic energy because it includes full electron correlation. (B) The HF method approximates the many-electron wavefunction as a single Slater determinant, enforcing antisymmetry. (C) The HF method treats electron-electron repulsion only as a constant and ignores exchange effects. (D) In the HF method, the orbitals are fixed and never optimized; only the total energy is minimized. (E) The HF method is a semi-empirical method because it requires experimental parameters.

60. Positron Emission Tomography is a highly advanced imaging technology in modern medicine. PET is primarily used to observe cellular metabolism; this is especially useful for cancer. After the isotope in the radiopharmaceutical enters the body, it migrates to the lesion site and emits a positron. As the positron moves through the tissue, it strikes an electron within the body. When the two collide, annihilation occurs, converting matter into energy and emitting two gamma rays of equal energy. The detectors then use these two rays to calculate the precise location of the lesion. Assume that both the electron and the positron carry a kinetic energy K=0.23m0c2, where m0 is the rest mass of an electron and c is the speed of light. What is the wavelength of the gamma rays produced after a head-on collision? [Compton wavelength is λ0=h/(m0c), where h is Planck's constant] (A) λ = 2.83 λ0 (B) λ = 1.62 λ0 (C) λ = 0.81 λ0 (D) λ = 0.40 λ0 (E) λ = 0.25 λ0

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