Flash Burst Operation of an Electronic StroboscopeAllen D. Wood Citation: Review of Scientific Instruments 41, 1091 (1970); doi: 10.1063/1.1684701 View online: http://dx.doi.org/10.1063/1.1684701 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/41/7?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Improvement in time and space resolution of stroboscopic micrography using high power xenon flash Rev. Sci. Instrum. 69, 325 (1998); 10.1063/1.1148519 Circuit for a flash stroboscope for examining vocal cord functions J. Acoust. Soc. Am. 89, 2481 (1991); 10.1121/1.400841 The thermonuclear flash model for Xray bursts AIP Conf. Proc. 115, 263 (1984); 10.1063/1.34530 Stroboscopic Scanning Electron Microscopy at Gigahertz Frequencies Rev. Sci. Instrum. 42, 251 (1971); 10.1063/1.1685056 Electrical Characteristics of Stroboscopic Flash Lamps J. Appl. Phys. 12, 848 (1941); 10.1063/1.1712876

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NOTES 1091

FIG. 3. Temperatures of "He and sample as a

T I K)

function of microwave O.5f'-___ .-.=_--~---power; ~- sample, - - - resistance.

0.2

50 100 200 Microwave Power (pW)

A mercury diffusion pump (Edwards 2M4A )was used to reach temperatures down to 0.37 K. These temperatures were measured in absence of microwave power by means of a carbon resistor placed at the bottom of the cryostat close to the Mylar tail. In presence of microwave power, large temperature gradients have been found between sample, resistor, and liquid 3He surface. In order to reduce the gradients due to the poor thermal conductivity of liquid 3He, about 20000 copper wires (20 fJ.) were placed in the cryostat. In addition, the bottom part of the sample holder rod was made of copper. But there still remain temperature gradients. Figure 3 shows the respective temperatures of the carbon resistor and a sample of ethylene glycol as a function of microwave power, the pressure on the 3He surface being constant and corresponding to 0.48 K. The temperature of the sample has been inferred from its proton spin-lattice relaxation time whose strong tempera­ture dependence is known.· The results show that for future large size polarized targets below 1 K, the cooling of the sample may be limited by thermal exchange as well as pumping speed.

* Permanent address: Department of Physics, Nagoya University, Chigusaku, Nagoya, Japan.

1 The glue used was 50% Epikote 828 (Shell Chemical Co.) with 50% Versamid 125 (Generall\1i11s, South Kensington Rd., Kankakee, IlL).

2 Hydrogenless polyethylene, made by M. W. Kellogg Co., N. J. 3 H. Gliittli and A. Masaike (to be published).

Flash Burst Operation of an Electronic Stroboscope

ALLEN D. WOOD

Lockheed Palo Alto Research Laboratory, Palo Alto, California 94304

(Received 3 March 1970)

A SIMPLE electronic modification of a General Radio type 1538-A Strobotad stroboscope is described that

permits operation in a burst mode. The instrument then becomes a useful stroboscopic light source for multiflash photography of a transient event and the need for me­chanical switches or shutters is eliminated.

FIG. 1. Modification of a General Radio type 1538-A Strobotac for flash burst operation.

PUl..'iE INPUT

}-c-lO-35V

G

1 Mil

TOA.T.22

TOA.T.66

The circuit shown in Fig. 1 connects two points in the Strobotac which are normally unconnected except when changing ranges. Under quiescent conditions the field effect transistor (FET) has a sufficiently low on-resistance to pull the control for the charging circuit (A.T. 22) to nearly the -15 V level of A.T. 66 and this prevents the instrument from flashing. The application of a 10-35 V pulse sharply increases the FET on-resistance and the Strobotac will operate normally for the pulse duration. The requisite 10-35 V pulse is most conveniently obtained directly from the plus gate output of an oscilloscope. Oscilloscopes are generally available, provide the necessary triggering, single-sweep, and delay functions, and the width of the gate (which is slightly longer than the sweep duration) is easily adjustable. The normal operation of the vertical channel of the oscilloscope is unimpaired and it may be used to display other information about the event.

Operation of the oscilloscope-Strobotac combination results in a series of light flashes inside the envelope of the sweep duration. The flashing rate during the burst is determined by the rpm dial of the Strobotac. The maxi­mum rate of 150000 rpm for the type 1538-A corresponds to a period of 400 iJ.sec between flashes. An example of a single flash burst is shown by Fig. 2 where two oscilloscopes were used to demonstrate the sudden turn-on and turn-off of the Strobotac. On the 24 00(}-·150 000 rpm range, the -delay is about equal to the period between flashes and t.he first and last flashes may not be as bright as the remainder. These effects occur because the plus gate timing is un­related to the Strobotac oscillator and are inherent dis­advantages in this simple system. They are unimportant when many flashes occur in a single burst.

The modified Strobotac has been successfully used at its highest flashing rate in an 1/8 schlieren system. The images were recorded on a 125 cm length of 35 mm

FIG. 2. Single burst of 25 flashes at highest flash rate. Strobotac driven by a second oscilloscope set for a 10 msec gate following a 4 msec delay.

t LIGHT OUTPUT

... TIME (2 m .... ..;cm)

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1092 NOTES

Tri-X film in a rotating drum camera. While this type of camera is ordinarily considered to be a streak camera, very little blurring was caused by the film motion because the flash duration (0.5 .usec) is very short compared to the 400 .usec minimum period between flashes. The resulting film was essentially a motion picture containing about SO frames taken at a rate of 2500 frames/sec. This is but one of many possible applications2 of this instrument.

The author acknowledges the assistance of C. W. Gilbreth in designing the FET circuit.

1 Strobotac is a registered trademark of the General Radio Co. 2 C. E. Miller, Handbook of High-SPeed Photography (General

Radio Co., West Concord, Mass., 1967), 2nd ed.

Signal-to-Noise Ratios for Ion and Metastable Detection of Neutral Molecular Beams

R. SINGH

Aeronautical Sdences Division, University of California, Berkeley, California 94720

(Received 17 February 1970)

I T is useful to have an estimate of the signal-to-noise (S/N) ratio expected from a beam system. This note

briefly discusses the chief sOurce of noise and gives a simple formula for estimating SIN. Under some conditions, a superior SIN ratio is obtained with metastable detection of weak beams,! for reasons that appear not to have been made explicit in the literature.

In metastable detection the exciter and the detector are physically apart. Excited molecules must, therefore, drift before being detected. This has relatively little influence on the detected beam flux, but only the fraction A/47rD of excited background molecules is detected­where A is the detector slit area and L is the exciter­detector separation. In ion detection, on the other hand, all excited molecules are detected. Since typical values of A and L are 10 mm2 and 100 mm, respectively, metastable detection reduces the background flux by a factor of 10-4

•

The sensitivity of ion beams to stray electric fields and their tendency to spread due to self-repulsion make it difficult to employ the separation technique with ion detection.

Noise is inherent in both techniques owing to the graini­ness of the input (a particle flux) and Poisson statistics describe the result of time integrated measurements of the multiplier output, as shown by Robben.2 For a steady signal, delivering a mean R counts/sec and measured with an instrument of response time t, the signal "'Rt, and the Poisson noise (rms fluctuations)",(Rt)t. Thus SIN =C(Rt)t, where C is a constant of 0(1). For repetitive signals averaged by means of a multichannel signal averager the appropriate t is the integrating time per channel.

It is easily shown that for a chopped beam S/ N =CBt i /

(B+G)!, where Band G are the mean counting rates at the multiplier due to ionized (or excited) beam and back­ground molecules, respectively. Band G are given by B, G=0.18PlB •GnB.Gi, where P is the ionization (exci­tation) probability, IB the electron path length through the beam (cm), IG the electron path length through the background (em), nB.G the respective densities (em-3),

and i the ionizing (exciting) current (mA). An order of magnitude estimate for Band G can be obtained, for many commonly used gases, by taking P=3. For metastable detection, B must be reduced by the ratio of the detector slit area to the beam cross sectional area in the excitation region, and G by A/471'D, as already stated. Detec­tion efficiencies for ions and metastables appear to be comparable.

From S/N=CBti/(B+G)~ it is clear that, when nB

and nG are comparable, little is gained by going to meta­stable detection and reducing G. In this case the simpler ion detection is preferable. Metastable detection is also not desirable when the detection of slow molecules in the beam is important, since these are deflected by electron impact.3 •

4

Signal-to-noise values obtained in our laboratory, using ion detection and signal averaging, agree quite well with the estimated values; however, the SIN ratio quoted by Winicur1 appears to be too low for unknown reasons. The estimated SIN ratio is representative of the total SIN if the Poisson noise is much greater than the noise of the following electronics. This is readily achieved by using a particle multiplier of adequate gain and by making B much greater than the dark rate of the multiplier. Under these conditions the multiplier adds little noise.

Grateful acknowledgement is made to Dr. Frank Robben for advice and discussions. Thanks are also due to Dr. Daniel Winicur and Dr. Eldon Knuth for several suggestions.

1 D. H. Winicur, W. E. Rodgers, and E. L. Knuth, Rev. Sci. lnstrum. 40, 9 (1969).

2 F. Robben, "Noise in Measurement of Light with Photomulti­pliers," Aktiebolaget Atomenergie, Sweden, Rep. No. AE-323.

3 W. E. Lamb and R. C. Retherford, Phys. Rev. 79, 594 (1950). 4 J. C. Pearl, D. P. Donnelly, and J. C. Dorn, Phys. Lett. 30A, 145

(1969).

Temperature Control Circuit for Simultaneous Evaporation from Two Sources*

E. J. WHITE AND J. R. CLARKE

Department of Electrical Engineering, University of Virginia, Charlottesville, Virginia 22901

(Received 20 February 1970; and in final form, 9 March 1970)

I N the deposition of thin films of compound semicon­ductors by simultaneous deposition of the constituents,

it is necessary to maintain close control over the source temperatures. The necessity to position the two sources

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