Instrument description

Located in ILL7 on branch H153 of the cold neutron guide H15, SHARPER is an inelastic spectrometer using the time-of-flight technique. The instrument is designed to access dynamic modes in condensed matter by measuring the energy exchange between a monochromatic neutron beam and the sample.

From the continuous, "white" beam delivered by the H15 neutron guide, a specific wavelength (and thus incident energy, E_i) is selected via Bragg reflection on a monochromator. A Fermi chopper pulses this monochromatic beam, setting the time t₀ of its interaction with the sample. A timer then measures the time t taken by the scattered beam to reach detectors positioned a known distance d from the sample. The velocity v = d / (t - t₀) of the neutrons provides a direct measure (E_f = 1/2·m·v²) of the scattered beam's energy. The energy transfer E_f – E_icorresponds to the energy of a dynamic mode within the sample.

A set of three disk choppers located upstream of the monochromator eliminates harmonics (integer sub-multiples) of the chosen incident wavelength that may not be fully filtered by the monochromator.

A retractable guide between this disk chopper system and the monochromator allows switching between "time focusing" (TF, guide retracted) and monochromatic mode (MM, guide in the beam). The TF mode enhances energy resolution within a narrow energy transfer region while maintaining high flux. The MM mode provides constant resolution over a broader energy transfer range but at the expense of flux reduction.