The Effect of Rotational Freedom in Several Ammonium Salts and Dimethyl Acetylene on the Inelastic Scattering of Slow Neutrons

Recent neutron cross-section measurements have shown that certain ammonium salts and methyl substituted compounds may be useful as moderators for cold neutron sources. In the present experiments slow-neutron total cross sections for polycrystalline NH₄PF₆, (NH₄)₂S₂O₈, and (NH₄)₂CrO₄ have been measured at 80°K and 296°K as a function of neutron wavelength. In addition cross sections have been measured at 23°C for NH₄SO₃F and CH₃C≡CCH₃. Cross sections vs. temperature for NH₄PF₆ and (NH₄)₂S₂O₈ were also obtained at λ_n = 8 A (0.0015 ev) from 80°K to room temperature. The long-wavelength slopes in b/A of the scattering cross sections per hydrogen atom at 23°C are: NH₄PF₆, 12.7; (NH₄)₂S₂O₈, 9.6; NH₄SO₃F, 10.0; (NH₄)₂CrO₄, 5.5; CH₃C≡CCH₃, 12.4. The slope for NH₄PF₆ approaches that for free NH₃ gas indicating a rotational barrier of ∼0.2 kcal/mole for the ammonium ion. The high slope for CH₃C≡CCH₃ indicates a low barrier to rotation of the methyl groups in this compound. The lower slopes for (NH₄)₂S₂O₈ and NH₄SO₃F show that the ammonium ions in these compounds have higher barriers of the order of O.7 to 1 kcal/mole. Slopes measured at 80°K are: NH₄PF₆, 2.5 b/A-H; (NH₄)₂S₂O₈, 1.0 b/A-H; (NH₄)₂CrO₄, 0.0 b/A-H. In addition the cross section vs. temperature curve for NH₄PF₆ indicates no abrupt change in rotational barrier down to 80°K. These low-temperature results show the presence of significant rotational freedom in NH₄PF₆ even at 80°K. The presence of rotational states at low temperature for exchange of energy with neutrons, together with the comparatively low absorption cross sections, make ammonium salts such as NH₄PF₆ (N¹⁵ enriched) and methyl substituted compounds like CH₃C≡CCH₃ of interest as moderators for cold neutron sources.