INSTRUMENTATION by Ralph H. Müller
Electronic field mapping and spark counters advance while automobile instrumentation lags
of electrical fields is imM APPING portant in a number of scientific and technical problems. For a long time the electrochemist has used tank and probe systems to delineate the field in the region of odd-shaped electrodes. In electron optics, electron trajectories can be traced in similar fashion. The electrolytic tank and probe have even been mechanized, and with suitable servosystems the probe position and its response can be plotted automatically. A high speed electronic field mapper has now been described, in which a complete field can be mapped in 0.04 second and in which continuous repetition gives the effect of a moving picture. As described by R. B. Burtt and J. Willis [J. Sci. Instr. 34, 177 (1957)], the electrolytic analog tank is built on the end of a cathode ray tube so that the tube screen forms the bottom of the tank. When a high frequency alternating potential is applied to the electrolytic analog, an alternating current is superimposed on the steady secondary emission current returning to the final anode of the cathode ray tube. This current is proportional to the potential appearing just over the spot and is of sufficient magnitude to allow easy detection and amplification. The spot is deflected by suitable sweep circuits to
trace out a raster of just over 100 lines in synchronism with a display oscilloscope. After amplification, the signal is divided up by triggers which feed pulses at equipotential values onto the grid of the display tube to show a series of equipotential lines. The tube which the authors used had the disadvantage that its screen was far from flat. This would seem to be a drawback which is fast disappearing, because a number of large-diameter tubes with flat screens are now available. This paper is an excellent example of the manifold uses and possibilities of cathode ray tube presentation. One is constantly struck by the infinite possibilities of this device in presenting information. Modified Spark Counter The Greinacher spark counter, now some 22 years old, is being revived and improved. It is extraordinarily sensitive to light and to ionizing radiations. In the original form it consisted of a pointed electrode facing a flat plate. With a high potential across the electrodes, insufficient to cause continuous sparking, but producing a faint corona, the gap becomes very sensitive to ionizing radiations—particularly alpha par-
ticles. Under such excitation, pulses are readify generated and can be counted. If the plate is selected to be photoelectrically sensitive to ultraviolet light, it makes an excellent photon counter. Chang and Rosenblum modified the Greinacher technique by using a flat metal cathode; and a fine wire anode stretched 1.5 mm. in front of the cathode. Multiple-wire anode assemblies have also been used. A recent paper by X. K. Saha and X. Xath of Delhi, India, describes operating characteristics of the spark counter and clears up some anomalies which have been reported in the literature [Nucleonics 15, Xo. 6, 94 (1957)]. Counters as described by the authors can be made highly specific for aparticles and indifferent to β-particles and gamma rays. By using a boron cathode, the spark counter will give one count for every !80 neutrons per sq. cm. and less than 1 count per hour in a gamma field of 1 roentgen per second. The neutron counting in this case de pends upon an n,a reaction in the boron plate. The authors have made careful measurements both on counting and the initial corona conditions and developed equations for the effective counter voltage and corona current as a function of total applied voltage. VOL. 29, NO. 8, AUGUST 1957
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53 A
INSTRUMENTATION
BACKGROUND LESS THAN 1 CPM with t h e
Jracerlat)
A similar development is described b y M . J. Swetnick a n d N . G. Anton [Nucleonics 15, N o . 6, 93 (1957)]. This is a boron trifluorido spark counter. A 2 X 3 inch stainless steel plate is used as cathode a n d the anode array consists of five parallel 0.005-inch stainless steel wires, in a plane 1 m m . from the cathode. The counter is enclosed in an air-tight stainless steel cylinder filled with boron trifluoride to 45 cm. of mercury pressure. T h e boron in the gas is 9 6 % enriched in boron-10. Detection efficiency is proportional to the probabilit} 7 of slow-neutron capture in boron-fu and the probability of alpha-particles or lithium recoils from the boron-10 (n,a) lithium-7 reaction reaching the sensitive corona region of the electrode assembly. The slow-neutron-sensitive spark counter can be converted into a fastneutron-sensitive spark counter by encasing the detector in a paraffin shield to moderate fast neutrons a n d covering the exposed surface of t h e paraffin with a 2-mm. cadmium sheet to absorb incident slow neutrons.
l o w - b a c k g r o u n d beta counting system •
Plateau slope less than 1% per 100 volts
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Ultimate sensitivity of 0.05 cpm
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Window thickness less than Img/cm
Tracerlab's new Low-Background Beta Counting System offers the ultimate in sensitivity for solid samples of beta-active materials, h is the first complete beta counting system ever offered commercially with a background of less than one count per minute. This remarkably low background allows the assay of samples with very low specific activity. For instance, the CE-14 Low-Background Beta Counting System offers a sensitivity to carbon-14, a factor of 10 greater than a window less flow counter. The sensitivity ratio is even greater for isotopes with higher energy beta radiations. The plateau slope of less than 1% per 100 volts means unusually stable operation for periods of months or years, with excellent reproducibility of results. The central beta counters are designed for continuous gas flow to insure maximum long-term reproducibility. The CE-14 Low-Background Beta Counting System is complete with two central flow counters and mercury isolation shield, inside a dust-tight steel cover which also houses the anticoincidence umbrella. The massive shield, gas flow system and special three-channel anticoincidence scaler are included as an integral part of the complete system.
A detailed description of this LowBackground Beta Counting System is available. Write for bulletin CE-14.
Tracerlab 1 6 0 6 Trapelo Rood, Waltham, Mass. 2 0 3 0 Wright Ave., Richmond, Calif. Offices in principal cities throughout the world Circle No. 54 A an Readers' Service Card, page 73 A 54 A · ANALYTICAL CHEMISTRY
Automobile Instrumentation We continue to be excited a n d awed by the fantastic rate of instrumental development and then chagrined by t h e degree to which these things fail to touch our daily lives. One of the most discouraging things is the current status of automotive instrumentation. Let us face it—styling is the thing which sells our American motor cars; its simple instrumentation must be in t h e hands of hair stylists or couturières. W e happen to drive a heavy car of 1957 vintage; hence presumably it is n o t a museum piece. T h e dashboard instruments must be dedicated to some unknown gridiron hero—they are all elliptical. T h e tiny ellipse representing a clock bears four digits—3, 6, 9, a n d 12, with intermediate dimples for the other hours. I t takes no more t h a n 40 seconds to decide whether it is 10:10 or 11:10 A.M., and yet there is a red sweep second h a n d which sweeps around in elliptic glee for accurate interpolation. At first, we kept reciting to ourselves t h a t χ1/a1 + y'2/b2 = 1, b u t we have discarded conic sections a n d substituted a small sand-filled hour glass. T h e fuel gage, also dedicated to Descartc's later studies, bears four lines in a fan-shaped p a t t e r n with the letter Ε on the left a n d F on the right, neither of which aligns with a n y division. A red lollipop pointer dances over this sem blance of a scale. We rely on the im perishable dictum of Charlie M c C a r t h y t h a t " t h e gas is somewhere between e m p t y a n d positively!" Fuel indica tion turns out to be a function of
vehicle or engine velocity a n d fails to indicate a t all unless the engine is running. T h e simple alternative of inserting a dry stick in the gas t a n k is foiled by the streamlined body. Other information, for which one might have casual interest, such as temperature and oil pressure, is afforded by signal lamps illuminating an appropriate legend. T h e cardinal reference points for the temperature scale are " h o t " and "cold." When starting, a green sign announces "cold," which is not very informative. When the cooling system reaches tem peratures which are too high, a red sign indicates " h o t . " What goes on in between these limits is presumably none of the drivers' business. When the starter b u t t o n is depressed, a red light indicates " g e n " — a state of affairs which has already been evident b y the audible whine of the starter. Another sign reading "oil" might mean the presence of t h a t essential fluid, t h a t it is a t proper pressure, or none a t all. All these indicators eventually blank out, which means one of two things: T h e operating conditions are proper, or a fuse is blown in the indicator lamp circuit. This is instrumentation worthy of a C u b Scout. The speedometer is an elliptical sun burst with divisions a t 5-miles-perhour intervals. More accurate inter polation is easily achieved b y solving the simple quadratic equation. This becomes unnecessary at several speed ranges, where the speedometer is totally obscured by t h e steering wheel. T h e push button-controlled radio, which we rarely use, is superbly designed, b u t this is a product of a progressive and imagi native industry. T u r n indicators flash prominently on the dashboard and will continue to do so even if the rear lamps are not operating. A major advance in automotive instrumentation will come about when these lamps are connected in series rather t h a n in parallel. T h e mileage indicator is fixed a n d can be reset only by a shrewd used car dealer. Resettable trip mileage in dicators apparently went o u t with the acetylene head lamps and fringed t o p . We have been told t h a t t h e woman driver is responsible for this advanced instrumentation—that she does not want to be bothered with these gadgets and all their implications. We refuse to accept this slander. We believe t h a t she, like other drivers, likes to know what time it is, how fast she is driving, and with reasonable certainty t h a t the car will continue to run for a finite period. As long as annual advances are restricted to major shifts in chromium trim and sales are abetted by seductive Powers and Conover models, we shall have to step on the starter a n d hope for the best.
