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Manual for the Use of the Brookhaven Light Scattering Instrument
Background
LASER, or Light Amplication by the Stimulated Emission of Radiation, is a device that creates a narrow, intense, coherent light beam. The Coherent Innova 90 argon 10W laser provides the light source for the light scattering. It is located on a track and should be properly aligned. There is no need to touch the laser. Because of the heat generated, it must be water cooled to prevent overheating and destroying the laser.
This laser is rated class 4 and defined as an eye hazard if the beam is viewed directly, by reflection or by diffuse reflection. It also poses a skin hazard and a fire hazard.
The figure below shows the major parts of the goniometer. The goniometer is a 200 mm turntable mounted on a circular base mounted to a supporting table. Selection of angles can be done manually or motor driven in steps of 0.01°.
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| Number |
Part |
| 1 |
Laser rail and mounts, optional |
| 2 |
Precision-machined base |
| 3 |
Turntable |
| 4 |
Rigid rotating arm |
| 5 |
Upright for alignment aperature and neutral density filter |
| 6 |
Detector rail |
| 7A,B |
Center of rotation adjustment screws |
| 7C |
Center of rotation locking screw |
| 8 |
Center of rotation adjustment table |
| 9 |
Beam focusing and steering lens assembly |
| 10 |
Sample cell assembly |
| 11 |
Beam stop |
| 12A |
Lens adjustment, horizonal |
| 12B |
Lens adjustment, vertical |
| 13A |
Slit adjustment, horizontal |
| 13B |
Slit adjustment, vertical |
| 14 |
Mirror adjustment |
| 15 |
Eyepiece |
| 16 |
Pinhole wheel |
| 17 |
Filter wheel |
| 18 |
Photomultiplier housing |
| 19 |
Angle adjustment |
| 20 |
Clutch release |
| 21 |
Support ring |
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Below are labeled photos of the goniometer and the laser
Click on image for a larger view.
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Ten easy, beginning steps in the use of the Brookhaven Light Scattering Instrument with the laser. REMEMBER, safety is a key issue.
Before proceeding, read the section concerning sample preparation and use of cuvettes. Samples with air bubbles, dust, or other particles that scatter light are not satisfactory for light scattering measurements. Similarily, dirty cuvettes will scatter light and likewise are unsatisfactory.
Turn on the main water valve
Turn on the water valve to the laser. The laser must be water cooled at all times to keep it from burning out.
Ensure that the laser is blocked by setting the dial in the front of the laser to 'C'. THIS IS REQUIRED FOR SAFETY REASONS. When not taking measurements, this dial should always be in the closed position. Turn the key on the laser control box to the 'On' position.
Ensure the interference filter wheel is set to the 'C' position. Bursts of light will damage the photomultiplier tube. The positions marked 488, 514, and 633 refer to the wavelength of the laser. 'O' signifies there is no filter. The filters allow experiments to be run in room light, however, the filters do reduce the amount of scattered light reaching the detector. Use of the 'O' position increases the signal, but the room lights must be turned off. Again, close the filter wheel when not taking measurements.
Close the mirror in the detector optic down (counterclockwise).
If temperature control is necessary, turn on the circulating water bath and allow to equilibrate for seveal hours before beginning measurements.
Turn on the decalin pump. Decalin is a toxic organic compound, so avoid inhalation and contact with body surfaces. It is used because it has a refractive index close to that of glass. It must be kept dust free, thus it is pumped throughout the sample chamber after filtration. Allow the decalin to pump several minutes. Then turn the pump off; otherwise, pump vibrations will interfere with measurements. If dust is not removed during this process, the filter may need to be changed.
Choose the 200µ pinhole turret on the detector which corresponds to one coherence unit and is optimal for measurements. For a weak signal, the 400µ pinhole may have to be used. Conversely, for strong signals, the 100µ pinhole may have to be used. Never use the 1, 2, or 3 mm pinholes.
When you are ready to begin measurements, unblock the beam in the front of the laser, open the interference filter to the correct wavelength (or 'O'), and rotate the mirror in the detector optic to up (clockwise). Again, be careful of the laser beam.
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