PRINCETON UNIVERSITY
BOREXINO PROJECT
LOCATING THE INNER VESSEL
Preliminary Evaluation of REFLEKS PM 30 Laser Range Finder
May
1998E. de Haas
SUMMARY. Tests in the one-meter tank have shown that the instrument is sufficiently precise and accurate for our purpose. The device remains a viable candidate for future tests on the one-quarter scale prototype in Gran Sasso (Italy).
1. INTRODUCTION
The Borexino Project Consists of a nylon balloon with a diameter of 8.5 meter, filled with pseudocumene (trimethylbenzene, p.c. for short), which floats near the center of a spherical stainless steel vessel with a diameter of 13.7 m that is also filled with p.c. The purpose of the installation in a tunnel east of Rome, Italy is to measure neutrinos that originate in the sun. For experimental reasons it is desirable to know where the balloon is at any time, even when parts of it may have slowly moved away from their original positions by 20 or 30 or more centimeters.
The steel sphere is surrounded by a second vessel with a diameter of 18 meters and the volume between the two vessels is filled with water. That makes it impractical to "look in" on the balloon through glass windows in the 13.7 meter sphere. Instead we have considered a laser range finder, manufactured by Phase Laser Systems of Scottsdale, Arizona, made available to us by Mr. M. Brubacher. The general idea is to select five locations on the balloon, for example two near the top and bottom and three around the equator, and place reflective targets on them.
Those targets would each be viewed by three range finders and the exact place in space determined by triangulation. The total series of measurements would be made once every day in the beginning and less frequently after a while.
2.TEST ARRANGEMENT
The range finder contains a laser transmitter that sends out a light signal to a target. Part of the light is reflected back to a receiver. The device measures the difference in phase angle between the outgoing and reflected light and converts that to a distance, which is shown on the screen. The instrument operates at 670 nm, which is still in the visible range. That facilitates aiming the instrument at a target.
The tank is 1 meter long and 20 cm wide and high. It contains a pyrex glass window at one end, so that the instrument, which is not made for immersion in a liquid, can stay outside in air, while the target is placed in the fluid in the tank. The target is a flat piece of aluminum covered with a reflecting material with corner cubes. See the sketch. The range finder was connected to one of our computers with software supplied on a floppy disk by the company.
3.RESULTS
Initially we had much trouble with reflections from the glass window, which confused the receiver and led to spurious answers. We avoided that problem by placing the range finder at about 7.5 degrees from the axis of the tank (1eft sketch) and later by placing the window at the same angle to the rest of the tank (right sketch). We moved the target over known distances and read the instrument. A typical series of results is attached. Each reading was repeated four times, making five readings on the instrument for every distance.
The tests were done first with the tank empty and then filled with p.c. The latter has an index of refraction of 1.5 and one might therefore expect that moving the target 10 inches in the p.c. would show a difference in the reading of 15 inches. That was approximately the case. The table shows differences of up to 0.05 inch among individual readings, which means that the instrument had an imprecision of that magnitude. [However in preliminary tests, done before the main series, we had found that the mean of five or ten readings was considerably better, so that the accuracy of the instrument appeared to be better than 0.01 inch.
The graph shows the main results for a typical test series. The instrument apparently can follow the movement of the balloon, if a reflecting target is fastened to the nylon and if the instrument is calibrated in advance. The target was placed at various distances from the window and the corresponding readings on the instrument screen were noted. The latter were shown in inches and decimal inches, down to three places behind the decimal point. In principle the movement of the balloon could be recorded in three axes through the use of three range finders and simple triangulation. if we later found that the balloon could be located in space through the use of five targets (say), 15 range finders would be needed.
4.DISCUSSION
If eventually the method with the laser range finder is selected, then that will require glass ports in the stainless steel sphere with the exact locations determined later. But because the sphere will be manufactured during the summer of 1998 and the drawings were approved in April 1998, we have included in those drawings a total of 150 5-centimeter diameter holes, strategically spread over the surface. Those will be in addition to the 2000 plus holes needed for the photo multiplier tubes.