Undesirable Effects of Cooling PhotomultipliersAndrew T. Young Citation: Review of Scientific Instruments 38, 1336 (1967); doi: 10.1063/1.1721100 View online: http://dx.doi.org/10.1063/1.1721100 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/38/9?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Compact Photomultiplier Housing with Controlled Cooling Rev. Sci. Instrum. 43, 641 (1972); 10.1063/1.1685712 Generating Cold Gas for Photomultiplier Cooling Rev. Sci. Instrum. 41, 916 (1970); 10.1063/1.1684725 Photomultiplier Tube Cooling Device Rev. Sci. Instrum. 36, 232 (1965); 10.1063/1.1719533 Simple Photomultiplier Cooling Apparatus Rev. Sci. Instrum. 35, 413 (1964); 10.1063/1.1718837 Photomultiplier Cooling Chamber Rev. Sci. Instrum. 34, 1281 (1963); 10.1063/1.1718218

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1336 NOTES

20% in polystyrene doped with either DPPH or Galvin­oxyl, and 28% in Parow ax doped with DTBN. A polariza­tion of 28% in Parowax is the highest measured polariza­tion in a CH2 type material and is possibly useful as a polarized proton target. With further study of hydro­carbons doped with free radicals, even larger polarizations should be possible.

We wish to thank Dr. C. F. Hwang for his helpful sug­gestions, C. C. Putnam and M. I. Forsberg for preparation of samples, and W. A. Mayoros for polarization mea­surements.

* Work performed under the auspices of the U. S. Atomic Energy Commission.

1M. Borghini and A. Abragam, Compt. Rend. 248, 1803 (1959). 2 P. L. Scott, thesis, Univ. of California, Berkeley, 1961. 3 R. J. Wagner and R. P. Haddock, Phys. Rev. Letters 16, 1116

(1966). • A. K. Holiman, A. M. Feldman, E. Gelblun, and W. C. Hodgser,

]. Am. Chern. Soc. 86, 639 (1964). 5 C. D. Jeffries, Dynamic Nuclear OrienJation (Interscience Pub­

lishers, Inc., New York, 1963). 6 A. Moretti, S. Suwa, and A. Yokosawa, Proceedings of the Second

International Symposium on Polarization Phenomena of Nucleons, Karlsruhe, Sept. 1965, P. Huber and H. Schopper, Eds. (Birkhauser, Basel, 1966), p. 128.

7 S. Suwa, A. Y okosawa, N. E. Booth, R. J. Esterling, and E. R. Hill, Phys. Rev. Letters 15, 560 (1965).

8 Private communication from R. P. Haddock to A. Yokosawa. 9 C. F. Hwang, B. A. Hasher, D. A. Hill, and F. W. Markley (to be

published in Nucl. Instr. Methods).

Undesirable Effects of Cooling Photomultipliers

ANDREW T. YOUNG

The University of Texas, Department of Astronomy, Austin, Texas 78712

(Received 24 March 1967)

THE effects of cooling an EMI 6256 photomultiplier (S-13 spectral response) to about lOOoK with

liquid nitrogen were recently discussed by Lipsett.! He stated that the signal-to-noise ratio was increased by this cooling for wavelengths as long as 700 m,u.

Lipsett took the ratio of signal current to dark current at the anode to be the signal/noise ratio. However, any dc component of the dark current can be subtracted or balanced out of the signal current; the noise is really the fluctuation of the current. If the signal and dark currents consist primarily of pulses, the fluctuation (rms or standard error) is proportional to the square root of the number of pulses counted.

If we neglect the change in dynode gain with tempera­ture, the number of pulses is proportional to the anode current. In Lipsett's case, the reduction of ten times in dark current corresponds to about a threefold reduction in noise. However, at 700 m,u Lipsett's data indicate a decrease in signal by more than a factor of four. Therefore,

at this long wavelength the effect of cooling is to degrade the SIN ratio of his tube.

The above discussion does not consider the very dif­ferent pulse-height spectra of light and dark pulses, or their changes with temperature. In fact, it can be shown2

that the noise limitation in the 6256 is set by cosmic rays, and that this limit is reached at about O°C (273°K) for dc operation. Further cooling increases the cosmic-ray noise slightly, owing to the increase in blue sensitivity of the tube.

A quite different hazard of excessive cooling has been discussed by Keene.3 As shown by Murray and Manning,4 the resistivity of the cathode film becomes high enough at low temperatures to cause significant voltage drops across the tube face, distorting the electric field distribution be­tween cathode and first dynode. This can cause seriously nonlinear response.

Finally, there is the possibility that large temperature differences may cause a structural failure in the graded seals between the glass base and silica window of the 6256.

In general, blue-sensitive end-window photomultipliers need not be cooled much below the ice point to achieve maximum SIN ratio. Further cooling is undesirable.

1 F. R. Lipsett, Rev. Sci. Instr. 38, 278 (1967). 2 A. T. Young, Rev. Sci. Instr. 37, 1472 (1966). 3]. P. Keene, Rev. Sci. Instr. 34, 1220 (1963). 4 R. B. Murray and J. J. Manning, IRE Trans. Nucl. Sci. NS-7,

No. 2-3, 80 (1960).

Ionization Gauge Circuit for Detection of Multiple Shock and Reflected

Detonation Waves* ROBERT F. FLAGG

Institute for Aerospace Studies, University of Toronto, Toronto 5, Ontario, Canada

(Received 19 April 1967)

THE classic ionization gauge circuit of Knight and Duff! shown in Fig. 1 (A) has been used for many

years to detect the time of arrival of shock waves and detonation waves. In operation, the high conductivity of the gas behind the waves closes the circuit through the ionization probe and discharges capacitor C! through resistor R!. The circuit produces a single pulse per probe per run. In principle, this circuit detects repeated waves provided the time constant for recharging capacitor C! is less than the time between signals from that probe. For the circuit of Fig. 1, this time is R 2C! or ~22 msec. The high resistance of R2 (2.2 MQ) is required primarily to isolate the sections from one another. The long time constant also provides blanking for extraneous signals for this period and may be desirable for some measurements, for example, where only the initial wave needs to be detected.

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NOTES 1337

-300 VDC

-300 VDC

R2

4.7K

PROBES

A

R2

4.7K

IN39~ IN39A IN39A

PROBES

Rl

30K TO SCOP~'

Rl

200 TO SCOPE

FIG. 1. Ionization probe circuits, (A)-classical ionization probe circuit of Knight and Duff'; (B) ionization probe circuit as modified to detect multiple waves.

In a recent experimental study,2 it was necessary to determine the trajectory of an incident gaseous detona­tion wave and its reflection from a solid surface or the incident wave and the strong reflected shock wave gener­ated by the detonation of thin layers of solid explosives by the incident wave. A simple modification to the classical circuit makes it possible to detect these repeated waves.

By substituting a smaller resistor and a diode for the high resistance R 2 , as shown in Fig. 1 (B) where R2 is typically 4700 Q, the sections remain isolated from one another due to the high reverse resistance of the 1N39A diode, but allow the sections to recover rapidly (",5 fJ.sec) and detect subsequent waves. Rl is also reduced to dis­charge C1 in a time short compared to the re,covery time.

The values shown in the figure were obtained by trial and error and give readable signals for the present experimental conditions. For other conditions the values may have to be varied to obtain optimum results. To be exact, the probes are not acting as "on-off" switches, but rather are detecting changes in contact resistance as if resistors of different values were being switched in and out and the contact resistance of the gas may have to be considered in some cases.

This circuit has been used successfully in the study of initiation of solid explosives by gaseous detonation waves.2

Three probes were spaced 3.81 cm apart, in a 3.81 cm diam, 10 cm long chamber. The last probe was 1.17 em from the

FIG. 2. Typical oscillograph record of the time of arrival of an incident detonation wave and a reflected shock wave obtained using the modified circuit. 1, 2, and 3 are the incident detonation wave signals, and 1', 2', and 3' are the reflected shock wave signals.

surface of a thin layer of explosive placed at one end. A plane detonation wave in a stoichiometric oxygen-hydrogen mixture initially at 14 kg cm-2 was initiated at the other. A typical record is shown in Fig. 2. The time of arrival marks for the incident detonation wave are shown as breaks 1, 2, and 3. The time of arrival marks for the reflected wave, which in this case is an unstrengthened shock wave as the explosive did not detonate, are breaks 3', 2', and 1'. The record shows that capacitor C1 for probe 3 has re­covered enough charge in '" 12 fJ.see to give readable signal. Capacitors for probes 2 and 3 have recovered essentially the equilibrium charge for the experimental conditions.

This circuit has proved very useful in the study of the initiations of primary and secondary explosives by gaseous detonation waves. The circuit would also be useful in determining the reflected wave trajectories in shock tubes, shock tunnels, and in detonation tubes.

* This research was supported by the U. S. A. F. Aerospace Re­search Laboratories under Contract No. AI" 33 (615)-2766.

'H. T. Knight and R. E. Duff, Rev. Sci. Instr., 26, 257 (1955). 2 R. F. Flagg, "The Application of Implosion Wave Dynamics to

a Hypervelocity Launcher," Institute for Aerospace Studies, Univer­sity of Toronto, UTIAS Report No. 125 (1967).

Propagation of Plasmoids at High Pressures* DAVID ALLEN FREIWALD AND ALI BULENT CAMBELt

Gas Dynamics Laboratory, Department of Mechanical Engineering and Astronautical Sciences,

Northwestern University, Evanston, Illinois 60201

(Received 3 February 1967; and in final form, 2 June 1967)

EXTE~SIVE pl~smoid studies have been reported b~ Bostick and hiS co-workers. I-a The purpose of this

note is to describe laboratory observations wherein pi as­moids have been operated at pressures higher than those previously reported. In particular, it was of interest to

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