VOLUME
24 A
The
New
GEE - CLIP Test
Tubes Held Always Upright
A Small Sturdy Stainless Steel Clip w i t h a Hundred Uses!
For any size tube from less than 3/s" to 1" diam.
Here are just a
few!
4 Gee Clips hold round bottom as well as flat bottom test tubes erect, when transferring f r o m burette to test tube. Test tubes remain ψ upright when being heated in a beaker of water or other liquid if you use . Gee Clip.
\ i fe^g
* No need to hold a Pyrex test tube by hand when heating. Gee Clips will retain test tubes vertically over burner. Test tubes are al- ψ ways upright when held by Gee Clips. Keep one handy by the balance.
ψ Keep a Gee Clip in the laboratory coat pocket. P r e v e n t burns ψ when pouring acids, hot liquids, or harm ful chemicals. Use a Gee Clip as a sup port and one as a pair of tongs.
Specify No. J-7825
$ 3 0 0 Dozen
LAPINE
ARTHUR S.
& COMPANY
Wll.llll.ll»^lltllUilllJl^.U«1lJI.IH4JIMI«.l!»IIJlE* 109 W. Hubbard St. Chicago 10,
19, N O .
11
INSTRUMENTATION in the case of Raney catalyst the gas adsorption as well as micro scopic estimation gave values three orders of magnitude larger. It was pointed out in this instance that the x-ray method is re stricted to crystallites and the fact emphasized t h a t wide dis agreement of two methods may well reveal important differences in structure and nature of a system. The search for new radiation detectors is an important and un ending quest. This treasure hunt is governed by several rules; what is sought must be sensitive, but it must also compete with alternative radiometers in its adaptability to amplification, re cording, and incorporation in modern instruments. An ex tremely useful and promising device is the photomultiplier detec tor as described by Fitz-Hugh Marshall and J. W. Coltman of Westinghouse. They have provided the well-known R.C.A. photomultiplier tube with a fluorescent screen and efficient, but simple, optical system arranged to register scintillations produced by individual photons of x-rays or nuclear radiations. This simple device has a much higher counting rate than the Geiger counter and has a negligible window thickness. Many investigators have coated phototube envelopes with fluorescent materials to render them sensitive to shorter wave lengths, and indeed Westinghouse had employed this principle for the measurement of x-ray dosage. Further work in this direction, with photomultipliers, indicated to them its value as an extremely sensitive detector. The obvious limit of sensitivity is set by the signal-to-noise ratio of the multiplier and the fact that the thermal emission of electrons is fairly high at room tempera ture. Refrigeration of the cell is possible, but is considered in convenient for a rugged commercial device. So far it has been necessary to select suitable tubes for high signal-to-noise ratio and R.C.A. is engaged in a program to improve the offering. I t would be unkind and ungrateful to minimize the thorough re search and development which went into the early work on multipliers by R.C.A., although it must have been disconcerting when radar researchers employed them as noise generators. Some discrimination between incident photons and noise electrons is possible on the basis of amplitude selection, but at the lower radiation levels noise pulses may exceed those produced by pho tons. Some further discrimination between short noise pulses and the longer signal pulses has been achieved by setting the amplifier input and output time constants equal to the 20-microsecond de cay constant of the fluorescent screen. The technique is very simple; according to the speaker, the detection of alpha-particle* is a high-school experiment. The approach to the extreme or limiting sensisivity obviously requires attention to the details just described. The applications are numerous, as might be imagined. I t has been possible to scan electron diffraction patterns and present them on an oscillograph, and many other uses may well be based upon the fact that counting can occur at prodigious speeds. It was pointed out that an earlier announcement of this device led to an avalanche of orders and inquiries. I t is not commer cially available and the present policy is directed toward further development work. A paper on this subject has been published by Marshall and Coltman [Phys. Rev., 72, 528 (1947)]. New methods and new equipment appear almost daily. North American Philips has announced a new thin mica window Geiger counter which may be used for tracer work employing C 14 , P 3S , S 35 , and for use with soft x-rays. It is 6 inches long by 1 inch in diameter and the window averages 3.5 mg. per square cm. It is self-quenching and with suitable circuits will handle in excess of 10,000 counts per second. The quantum efficiency for soft x-rays is about 70 to 80%. The threshold voltage is 1350 and the oper ating plateau exceeds 300 volts. Normal operating potential is 1450 volts. The same manufacturer lists a new x-ray diffraction
