Detector hardware, optics, mechanics

The detector arm basically consists of a chassis and a movable carriage, the latter representing the actual detector. The chassis is equipped with feet designed to match holes in the heat sink, usually taking screws for mounting the can. Thus, no additional holes are needed to mount the chassis, and four mounts in total provide a firm seat.

The carriage consists of two parts: the projection part below the rotor and the detector part above the rotor. Both are connected with a beam, thus both parts, aligned to one another, move synchronously on the rails on top on the chassis.

The projector is conceived differently to the classical absorbance optics: in the latter, the complete area of a measurement cell is illuminated; the light source is fixed and does not move during scanning. For MWA, light is focussed into the 2/3rd plane of the measurement cell into a spot smaller than 100 µm. This provides a maximum of light intensity, sufficient to saturate the detector with a single flash. Focussing is achieved by means of a quartz lens, located in the projector tube beneath the rotor level. It is fed with a fiber, carrying the light through a vacuum seal from the outside of the chamber. Thus, the light source is easily accessible, located on the floor of the centrifuge, beneath the rotor chamber. After passing the lens, the beam is directed vertically by a 45° mirror.

The detector holds an ocular lens, focussing the light emerging from the cell onto the 20 µm entrance slit of a commercially available spectrometer. As the lens is located on 1/3rd of the distance, a magnification factor of 2 will project a 10 µm event horizon onto the slit, yielding 10 µm as the optical resolution of the device. An adjustable x-y-table carrying the spectrometer allows to align the detector part to the optical axis.

A linear actuator moves the detector assembly against the resetting force of a spring. Positioning resolution is 0.05 µm, accuracy is 15 µm. The assembly can be moved with a maximum speed of 8 mm/s, needing only several seconds to cover the typical radial range of 13 mm.

The Xenon flash lamp, the actuator, and the spectrometer are high quality products purchased from renowned manufacturers, as well as lenses and fibers. All metal parts are high precision components made in Germany.