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UCRL-8227 ' -

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UNIVERSITY OF CALIPORNIA I

Radiation Labor ator y Berkeley, California

Contract No. W-7405-eng-48

A GENERATOR OF FAST-RISING LIGHT PULSES FOR PHOTOTUBE TESTING

Quentin A. Kerns , Frederick A. K;ir e_hen, and Gerald C. Cox

March 24, 1958

Printed for the U. S . Atomic Energy CZ

. , -1-

DISCLAIMER

This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency Thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

DISCLAIMER

Portions of this document may be illegible in electronic image products. Images are produced from the best available original document.

T h i s r e p o r t was p repared as an accoun t o f Government sponsored work. N e i t h e r the Uni ted S t a t e s , n o r t h e Com- m i s s i o n , n o r any pe r son a c t i n g on b e h a l f o f t h e Commission:

A. Makes any war ran ty o r r e p r e s e h t a t i o n , e x p r e s s o r i m p l i e d , w i t h r e s p e c t t o t h e a c c u r a c y , com- p l e t e n e s s , o r u s e f u l n e s s o f t h e i n f o r m a t i o n c o n t a i n e d i n t h i s r e p o r t , o r t h a t t h e u s e o f any i n f o r m a t i o n , a p p a r a t u s , method, o r p r o c e s s d i s c l o s e d i n t h i s r e p o r t may n o t i n f r i n g e p r i - v a t e l y owned r i g h t s ; o r

B. Assumes any l i a b i l i t i e s w i t h r e s p e c t t o t h e use o f , o r f o r damages r e s u l t i n g from t h e u s e o f any i n f o r m a t i o n , a p p a r a t u s , method, o r p r o c e s s d i s - closed i n t h i s r e p o r t i

A s used i n t h e above, "pe r son a c t i n g on beha l f o f t h e omm mission" i n c l u d e s any employee o r c .on t rac to r o f t h e Commission ' t o t h e e x t e n t t h a t such employee o r c o n t r a c t o r p r e p a r e s , hand les o r d i s t r i b u t e s , o r p r o v i d e s a c c e s s t o , any

c o n t r a c w i t h t h e

A GENERATOR OF FAST-RISING LIGHT PULSES FOR PHOTOTUBE TESTING

Quentin A. Kerns, Frederick A. Kirsten, and Gerald C. Cox

Radiation Laboratory University of California

Berkeley, California

March 24, 1958

ABSTRACT

The nied by elec usual repeti

UCRL light-pulse generator provides flashes of light accompa- trical pulses generaied at an impedance level of 50 ohms. The tion rate i s 60 cps. The primary features of interest are

(a) that the 1070 to 90% rise, both for the light and for the electrical pulse, i s less than 0. 5 nlpsec,

(b) that the light pulse r ises and then falls to 5070 peak amplitude in less than 1. 5 mpsec,

(c) that the time relation between the light and electrical pulses i s fixed,

(d) that the electrical pulse power available i s large.

In conjunction with an oscilloscope, the light-pulse generator has . proved useful in the testing and evaluation of multi lier phototubes and low - light-level image devices. Time resolution of 10-yo second i s typical of measurements made with conventional fast oscilloscopes (e. g. , Tektronix 517), while elaboration of technique permits relative time measurements that a re better in some cases by at least three orders of magnitude. The light i s emitted from a region a few mils in diameter, and thus may often be considered to come from a point source. An S4 photosurface subtending 0.1 steradian at the light source emits photoelectrically about l o 7 electrons per steradian per pulse. For convenience, three decoupled electrical out- put channels a re provided, together with a polaroid attenuator for the light pulse.

A GENERATOR OF FAST -RISING LIGHT PULSES FOR PHOTOTUBE TESTING

Quentin A. Kerns, Frederick A. Kirsten, and Gerald C. Cox

Radiation Labor ator y University of California

Berkeley, California

March 24, 1958

INTRODUCTION

The UCRL mercury-capsule light-pulse generator described in this report was developed for use in testing and evaluating the high-speed aspects of light-sensitive devices. These devices include multiplier phototubes used in scintillation and Cherenkov counters and in coincidence detectors, and low-light-level image tube s . With this pulser , tests and measurements of afterpulsing, transit time, multiplier transit-time spread, cathode transit- time spread, multiplier chrrent iaturation, etc. , have been made. Thq maximum Light amplitude is great enough to current-saturate low-gain mul- tiplier s, or i t may be so reduced as to release an average of one photoelec - tron or less per pulse from a photocathode.

The light pulses and the electrical trigger pulses a re generated simul- tanesusly in an arc discharge, therefore the e1e.c trical p ~ l s e i s conveniently used as a time reference for the light pulse.

This report describes some of the features of construction and oper- ation of the light pulser, A number in parentheses following the f i rs t men- tion of any part of the pulser or any accessory or related device refers to the fabrication drawing. A list of pertinent drawings i s given on pages 32 and 33.

Electrical and Mechanical Assembly

A photograph of the light pulser (4V5572) (Fig. 1) shows the rear of the pulser (the light is.emitted from the front). The light pulses a re pro- duced in a mercury-wetted contact relay capsule such as i s used in Western Electric 275C or Clare HG1003 relays. The source of light is the arc formed a t the contacts of the capsule._ The pulser i s partially disassembled to show certain features in Fig. 2. The electrical assembly of the light-pulse gen- erator is shown in Fig. 3 (4V5603B), and a schematic drawing of the light pulser in Fig. 4.

'The capsule S- 1 operates in a vertical position. Connectors PG-5 and PG-6 a re up and PG-1, PG-2, PG-3, and PG-4 down when the generator i s correctly oriented. In this position the contacts a re normally open and can be closed by the magnetic field accompanying a current in the driving

a loop (4V 568 1 ), which i s a metal strap making a single - turn secondary winding on T - 1. The light-pulse generator operates mast satisfactorily in a region

' free from strong magnetic fields or mechanical vibrations.

Fig. 1. A rear view of the light pulser. (Light pulses a re emitted from the other side. ) A, permanent magnet; B, mercur y-cap- sule driving coil; C, PG-5 monitor signal output; D, PG -6 high voltage input; E, polaroid adjustment handle; Fa remov- able cap for high-level coaxial connection; G, PG-4 51-ohms trigger pulse; H, PC-3 driving-coil input; I, PG-2 51-ohms trigger pulse; J, PG-1 125-ohms trigger pulse.

Fig, 2. A rear view of the light pulser with permanent magnet, driving coil, base plate, and rear half of metal shell removed. A, mercury capsule; B, rear half of metal shell; C , beeswax.

SHIELDED SINGLE conwcmR CABLE. SHIELD REMOVED FOR

AT THESE COWTACT

TOP UlEV

Fig. 3a

DRIVING COIL .010 FISH R P E R

SECTION A-A (ASSEMBLED)

(b)

Fig. 3b

Fig. 3. Electrical assembly of the light pulser: (a) top and end views showing placement of components; (b) side view and section through metal half shells, showing details of mounting of mercury capsule and driving coil.

p E R ~ n ~ llAClET f H9, E-D 0 TI DRIVIWC CURREWl

SWITCH

TRlCCER PULSE OUTPUT8

Fig. 4. Schematic drawing of the light pulser.

A metal shell (in two halves) (4V5612 and 4V5652) encloses S-1 to form the outer conductor of a coaxial circuit; the contacts of S- l form the inner conductor of the circuit. .

Large-amplitude high-frequency currents accompany the light pulses; these,.of course a r e the electrical trigger pulses previously mentioned. The currents a r e confined to the inner surfaces of the metal half shells ' (4V5681 and 4V5612) because of the small skin depth of the metal for high frequencies. The shielding i s adequate to confine these pulses (which may exceed.100 kilowatts peak power) to the trigger channels so that the pulses do not inter- act with sensitive circuits that may. b,e in the vicinity.

The. magnetic field of the driving loop alternates a t a low frequency, usually 60 cps, and penetrates the metal half shells with negligible attenua- tion. T- 1 trarlslvrms the driving-loop impedance to a convenient value. The small permanent magnet bucks out a par t of the built-in spring tension in the moving contact of S-B. The base plate on which the unit i s assembled i s a magnetic shield and should be in place when the unit i s operated. ,This driving s ys tem gives excellent decoupling be tween the driving and signal circuits, plus convenient single -turn assembly and dismantling.

Electrical Pulse Geometry

Energy for both the light pulse' and the electrical trigger pulse i s stored in the capacitance of the stationary contact of S - l and the s'ho,r:t..ledd of the 500-megohm res is tor , R-13, a s indicated by DE in. Fig. 5. . When. the

, contacts a r e open, this capacitance i s recharged thr0ugh.R-13 which also isolates the high-voltage charging supply and cable from the discharge cix- cuit. The energy-storage circuit DE has a capacitance of 2.8 ppf and thus a charging RC time constant of 500 megohms X .2.8. ppf, or about 1. 5 milli- secondsb . When the contacts a r e driven toward the closed position with volt- age applied, an a r c forms when s'ome critica! potentia1,gradient (which de- pends upon pressure in the capsule) i s reached. The resulting. electrical pulse i s propagated down the transmission linevGH to the resistive attenua-

, tors a t Plugs cl, 2, and 4. . Since the light output can be modulated by electr i- cal reflections, the conditions have been chosen to minimize these, although the geometry of. S-1 permits only an approximation to the ideal impedance- matched .configuration. The mercur y-wetted contact i s , however, nearly ideal f rom the standpoint of electrode erosion; the liquid mercury i s re-formed to a reasonably smooth surface before each pu.lse. This process favors the attainment of a high gradient just prior to breakdown an.d a.corresponding high ra te of r i s e of current. Comparable dry-contact gaps have been'observed to.develop surface irregulari t ies with time, wher.eas mercury light pulsers of the type described in this report have been in service for more than two years with no apparent deterioration.

Dl AME ABOUT

. KnOIELECTRIC CONSTANT TO PC-5 , 1 TO PC-6 A T IO? C Y C L E S I S E C . .

Fig. 5. Electrical pulse geometry of the light pulser. A l l dimen- sions a r e in centimeters.

Power Supplies

Two basic power supplies a r e required for operation of the light- pulse generator. One, the regulated high-voltage supply, i s connected to PG-6. It should be adjustable from B O O to 5000 volts dc, Current drain i s less than 20 microampheres. . Either high or high minus operation i s permissible, although the data in, this report refer only to the high plus con-

. nection, for which the resulting electrical output trigger pulses at PG-1, PG-2, and PG-4 a r e positive. The other required power supply i s an ad- justable source of about 3 volts a t half an ampere a t 60 cps for the driving :

coil T-1, connected.to PG-3. Figure.6 show's a suitable supply for 60-cps operati'on (2x81 81). Figure 7 shows a supply that has been used for lower ra tes and when freedom from 60-cps coupling was desired (2x8791). Each supply has provision for adjusting.the amplitude of the driving-coil current. It is.found that a slight readjustment of driving current for each high-volt- age setting may be requi-red.for maximum amplitude stability.

Monitor Signal

The signal at PG-5 i s the output of a capacity divider which consists of the 0. 002-pf capacitor ,(C-1 of Fig. 4) and the capacity (approximately 0. l ppf) between the unshielded end of the cable attached to PG- 5 and the energy storage circuit, DE of Fig. 5, which i s not accessible otherwise. The ratio of the divider i s thus about 0. 1 ppf divided by 0. 002 pf, or 1:20, 000 for an open circuit a t PG-5. An amplifier of 1-megohm input impedance shunting PG-5 modifies the picture appreciably, but the actual wave shape can be inferred to suff.icient accuracy for monitoring purposes. An ampli- f ier of purely capacitive input impedance will give about the cor rec t wave shape except for dc level. Figure 8 shows the wave form as seen on a scope of B meg and 40 ppf input impedance. -The hv supply setting i s 2 kv. When the driving-coil supply of Fig. 7 i s used, point B occurs about 3 to 5 milli- seconds after the r i se of the driving-current pulse, depending on i t s ampli- t ude .

Pulse G r o u ~ s

As suggested in the legend of Fig. 8, there i s a s tructure associated with P.oint B in.the figure. Microscopic observation of the a r c shows that

J 8 i t occurs a t a contact separation rou.ghly proportional to voltage; the estimate'd cri t ical gradient i s 200 kilovolts/cm. . (The hydrogeri.pres sure in- the capsule S-1 i s perhaps 10 atmospheres. Capsules hold off from 8 to 10 kilovolts a t maximum contact separation, and experimental pulser s have operated sat is - factorily at this level. ) The a r c thus occurs we'll before the mercury sur - faces a r e close enough to form a metallic bridge. Since the mechanical mo- tion of the moving electrode i s relatively slow, the electrodes may be con- sidered stationary on the time scale of the a r c (millimicroseconds): Oscil- lographic observation of the voltage .on.DE (Fig. 5) shows- that the a r c that f i r s t occurs does not completely discharge the energy-storage circuit:DE. Instead, following the fir s t a rc , the contacts continue to move toward each other, at reduced voltage, and eventually a second a r c occurs, and so on

TYPE 2 0 0 8

TO PC 3 OF LIGHT PULSER

A C AMMETER X AND Y ARE 0 - I A # P. CURRENT-WAVE-FORM

X q CONNECTIONS

PC-2 A

Fig. 6 . Light pulser driving-coil power supply for 60-cps opera- tion. Optimum operating current for the driving coil i s in the

. range of 0.25 to 0.7 amp (rms). *

I

e l C -

. .

NO CONNECTION

' TEKTRONIX 5 1 2 SCOPE

DELAYED

OUTPUT

LIGHT-PULSER DRIVING COIL

-------

----------- SCOPE MONITOR VOLTAGE

I I I

~ P G - A SCOPE-MONITOR C U R R E N T

P C - I I I I M E R C U R Y C E L L .

Fig. 7 . Light,-pulser driving-coil'power supply for adjustable repetition rates. The repetition rate i s fixed by the sweep- repetition rate of the oscillosc,ope.

DRIVING CURRENT AT PG-3

MONITOR SIGNAL AT P C - 5

TIME (MILLISECONDS)

Fig. 8. Monitor-signal wave form with 60 cps driving current a s observed on an oscilloscope with 1-meg 40-ppf input termina- tion. Point B i s the time at which the a r c occ.urs. The de- tailed structure following B i s best observed oh .a.shorter time base. On. this basis, one may say that the contacts a r e closed from B to C, they open again at C, and remain open until the time corresponding to B on the next cycle. ABC i s a complete c ycle .

The polarity of T - 1 i s such that electron flow into pin C of P G - 3 closes the contacts with the minimum current, ,i. e . , the driving field aids the permanent-rnagne t field.

until the mercury forms a metailic bridge; the entire process may take 100 microseconds. The voltage on DE thus decays in a ser ies of steps 'of de- creasing size, and a group of light and electrical pulses i s produced on each mechanical cycle. The nature of the group depends both on the hv setting and on the driving current (see Fig. 9) . The pulses a r e well separated (the second pulse i s de.layed-2 microseconds or more after the f i r s t pulse) and may'be used to trigger separate oscilloscope sweeps. The groups have been used.in various ways in conjunction with auxiliary circuits to form gates, e tc. Alternatively all signals after those derived.from the f i r s t pulse may be gated out.

The ptllse data given for both light and electrical pulses in the r e - maining pages refer to the f i r s t or pr imary pulse in the group. Below about 1 kv, usually only the pr imary pulse occurs.

Gate Signals

It i s sometimes desired to pulse various equipment on in advance of the light pulse, which occurs a t Point B (Fig. 8) of the monitor-signal wave form. Since the precise time of the pulse i s not predictable, there will be a jitter time associated with such a gate signal. The magnitude of the jitter and the method of obtaining the gate depend upon the required advance in time.

As the monitor-signal w'ave form shows, time advances of the order of millisecnnds a re obtsincd by gerlerating a gate timed with . respect to the driving-current wave form. . In this way .the time of the flash can be p re - dicted milliseconds in advance. There i s a probable e r r o r of 20 microsec- onds with optimum driving-current settings.

. Gate signals required.to occur a few microseconds or l ess in advance of the light pulse can be generated .from the monitor signal with more ac- curacy than from the driving signal. In this case, the timing prediction con- s is ts in measuring the capacitance increase between the contacts a s they mechanica1l.y approach each other prior to the breakdown. . Figure 10 shows wave forms obtained in the circuit of Fig. 11 1 (2X8'171), designed to utilize this capacitance change to.form a gate signal. When the monitor signal i s used.for this purpose, the capacitor C-1 in Fig. 13. i s disconnected a t point I I ~ ~ I to give the maximum signal to the amplifier. The resulting gate signal can be'adjusted to turn on from zero to 10 microseconds ahead of the light pulse, with a probable e r r o r of about 0.2 microsecond. .(Similar anticipator circuits have been applied to the mercury-switch electrical pulse^)

Trigger Pulse

Figure 12 shows the trigger pulse available a t PG-1, PG-2, or PG-4 (Points I, J, and K in kig. 5) as viewed on a 5XPll cathode-ray tube adapted for coaxial cable connections direct to the vertical plates. Figure 13 shows a schematic diagram (2x8801) for arrangement of 125-ohm coaxial connec- tions to a Tektronix 517 oscilloscope. (Slightly altered values s f R-2 and

\ . --

TIME AFTER FIRST PULSE ( MICROSECONDS 1

Key: Pulses occur w i t h i n n tending toward the position associ- ated with the driving current (in m a a t 60 cps.) written beneath. Pulses may be made to occur within - - by forcing the driving current outside the range where stable pulses occur.

Fig. 9. Pulse groups versus high-voltage setting and driving- coil current for a typical light pulser.

C3 a t-

TlME (MILLISECONDS)

TlME (MICROSECONDS)

F i g . 10; Wave forms of light-pulse anticipator. (a) monitor '

signal from PG-5 of light pulser; (b) amplified monitor sig- nal applied to PG-1 of pulse shaper (Fig. 11); (c) output from PG-2 of pulse shaper. to gated equipment; (d) stretched pulse from PG-2 of light pulser on a 'scope trace .triggered by the .anticipator output. .

I 1El;TRONlX I12 1 AMPLIFIER

POSITIVE PULSE TO GATED EQUIPMENT

I I I I I I I

I I \ V 1 6 V - I L,,,-----J

PULSE SHAPER

Fig. 11. Light-pulse anticipator.

TIME (MILLIMICROSECONDS)

'

Fig. 12. 'Trigger pulse wave for'm aS observed on a 5 ~ ~ ' l l cathode- r ay tube.

Light-pulser high-voltage supply setting 2400 v dc.

Signal deJ.ayed ,through 125 millimicroseconds of RG 6 3 U.

Signal. amplitude at both the 50-ohm and the 125-ohm connectors i s the s a n e when coax lines of the appropriate impedance a r e connected.

~ i ~ n a l ' w a v e form at the 50-ohm and the 1.25-ohm outputs i s also the' same.

, . The arrival time of the electrical pulse at a particular .

.connector i s slightly delayed with respect to the arr ival time of the light pulse a t the aperture in the base plate. The electr i- cal pu1se:arrives f i r s t a t PG-2, and about 10-lo sec later a t PG-1 and PG-4. The electrical pulse arriving at PC-2 may. be considered to be delayed by about 3 X 10-lo sec with respect to the arr ival time of the light pulse at the aperture in the base plate.

PC-I UPPER DEFL. PLATE R C - 6 3 u

- n R G - 6 3 U ?p3pt.t 6P56r\43 (4) - 1 7 2 . 5 m p 1 . A - I -

VERT. R - l P 0 S I ' I ' I O N I N C ~ - - - . - - CONTROL 4.7 M

USE 1 2 5 Q COAX. SWITCH, DRAWING NO. 3V9133 OR I C.R.T.

/ EQUIVALENT.

A L L RESISTORS ARE $WATT. VERT.

POSITIONING : *AN P* I CONTROL R - 4

4 .7H

r, n R G - 6 3 U PC-4

R G 6 3 U n -g .- --U 2 . 5 ~ 1 ~ " 2 . 5 m p s LOWER -

a - - L O W t H

M U-I 5,245

Fig. 13. Schematic diagram of 125-ohm connections to Tektronix 517 oscilloscope vertical deflection plates. With care in min- imizing s t ray capacities a%id inductances a r i s e time of about 1 mysec can be achieved with a 5XPll cathode-ray tube.

R-3 a r e appropriate to the 51 7A, and the apparent r i se time i s somewhat slower than that indicated in Fig. 12. ) The actual trigger pulse i s shorter than Fig. 12 would indicate. Figure 14 gives the measured peak voltage of the trigger pulse a s a function of the high-voltage power-supply setting. The actual peak pulse voltage for Fig. 12 i s about 41 volts, whereas the scope picture (which i s almost the impulse response of the scope) indicates about one-fifth this amplitude and a broader pulse. Figure 15 ~ e p r e s e n t s the peak voltage available when the various attenuating res is tors a r e dis- connected from Point H in Fig. 5 and a single 50-ohm coax line i s connected. This output i s termed the high-level pulse. It i s sufficiently energetic (ap- proximately 100 kw peak power a t 5 kv setting) to provide sweep and unblank- ing wave forms directly, and has been used in this way to make sweeps of 3 X 1 0 - l ~ sec/cm synchronized with the light pulse. In this way, small time variations in the optical path can be measured down to 10- l3 sec. The tr ig- ger pulse i s short enough so that i ts amplitude decays to about 1/2 the initial amplitude after traversing 100 ft of RG - 9/u coaxial cable. 2 -

Light Output

Figure 1.6 shows .the light output in ter'ms of the re$ponse of an.S-11 photocathode a s a function of th'e high voltage for several. units of the present design. For this curve, the polaroid'attenuator i s se t for minimum atten- . uation. The light output increases approximately a s the fifth power of the voltage at low voltages, and approximately as the square of. the voltage at high voltages. The photoelectron yield i s that integrated over a time of 5 mpsec and depends on the choice of driving current. . Figure 17 shows the approximate light-pulse wave shape a s measured .by a . special phototube with control grid gated-by the' high-level pulse. . The trigger pulse i s as observed on an oscilloscbpe employing the Dumont . ~ 1 4 2 1 traveling-wave cathode - ray tube. 4

The relative timing of the pulses shown i s somewhat arbitrary; timing depends on the relative optical and electrical path lengths in a given case. Fo r this purpose, the dimensions and dielectric constants of Fig. 5 a r e helpful; Fig. P 2 indicates the approximate tiniing. The j i t ter time be tween the light and electrical pulses i s known to be quite small, and allows meas- urements in.the 10-13 -second region when the high-level pulse i s used directly to generate the oscilloscope sweep.

Observed under a medium-power microscope, the a r c has an inter- esting and.complicated structure in space and.time. . However, for the pur- poses of this report, one may consider the a r c to have an effective,diameter of a few thousandths of an inch, and.for many p.urposes i t may be considered a point source. . The useful light output from the .light-pulse generator i s confined to an irregular cone of half angle, approximately 10 degrees. When the mercury capsule i s mounted; the attempt i s to make the axis of this P I cone of best visibility" normal to the base plate; for a g'iven unit, this i s approximately true, but often a somewhat better axis may. be chosen by experiment. For best stability of the light-pulse amplitude the phototube should be 1ocated.somewhere on this empirically determined axis. In view of the approximately point-source optics, lenses and m i r r o r s may be used to obtain high li ht intensity; photoelectron current densities of sever a1 pz amperes per cm , a re attainable.

Fig. 14. Trigger -pulse output voltage v s l ight-pulser high-volt- age sett ing m e a s u r e d a t PG-1, PG-2 , or PG-4'.

LIGHT- PULSER VOLTAGE (VOLTS D.,C.)

Fig. 15. High-level output pulse vs light-pulser high-voltage supply setting. The high-level pulse is obtained by 'r emoving the attenuating res i s to r s associated with PG- 1, PG-2, and

.. PG-4and'connectinginsteada50-ohmcoax.

LIGHT PULSER HIGH-VOLTAGE SUP PLY (VOLTS D.C.)

Fig. 16. The yield of photoelectrons per pulse from an S-11 cathode subtending 0.005 steradian at the light source. The polaroid attenuator i s set for minimum attenuation.

I I I L IGHT-PULSE AMPLITUDE 0 I 2 3 (RELATIVE PHOTOCURRENT) T I M E (MILLIMICROSECONOS)

I TRIGGER-PULSE ANPLITUDE ' 3 ( R E L A T I V E VOLTAGE

Fig. 17. (Upper) Wave shape of the light pulse as measured with . a special gated phototube.

. (Lower) Wave shape of the trigger pulse as observed on an oscilloscope utilizing a K1421 traveling-wave deflection cathode - r ay, tube.

he spectrum of the light emitted from the pulser i s shown in Fig . 18.

Polaroid Attenuator

The polaroid attenuator, consi'sting of a stationary and a rotatable disc of Polaroid film, provides a simple way of adjusting the light ampli- tude without distwrbing any electrical settings. It should be noted here that the.' light from 'the 'light puls.,er i s par tially polarized as generated; Table I1 i l lustrates th.is:

Polarization of light from the mercury capsule

Fraction of light h.avinp vertical ~o l a r i za t i on

High voltage setting dVults1

The stationary Polaroid film i s oriented to pass vertically polarized light in order to take advantage of this fact. Thus when set for minimum attenuation, the attenuator readily transmits vertically polarized light. For other attenuator settings the polarizer is' rotated w.liile thc analyzer remains fixed, so that the outgoing light i s always vertically polarized. The pobaroid f i lms may be removed or changed by using the Waldes Truarc plier #1 to handle the retaining rings. Figure 19 i s a plot of the ratio. of incident to transmitted .light vs the angle between polarizer and analyzer, a s observed by an S - l l photocathode. The plot follows a l /cos2 function closely except near maximum attenuation when the transmitted light i s attenuated by a factor of 100 over minimum attenuation.

Dark Enclosure

. .

Figure 20 (2x8493, 2x8501) shows a dark enclosure which has proved useful in connection with the light-pulse generator. 'i'he box i s made of 3 4- L inch plywood .for which the optical attenuation factor i s approximately l o 2 . The light-pulser plugs and polaroid attenuator handle a r e outside the light- tight box. High-voltage leads and 125-ohm coaxial signal cables are ter - minated at the walls of the box with connectors mounted so as to be lighttight. For controling focus-electrode and other voltages from outside the box',

WAVELENGTH IN ANGSTROMS

Fig. 18. (a) The spectrum of the light from the pulser as photo- ' graphed on a Kodak type F- 1 spectrographic plate, which i s most sensitive between 4500. and 6900 A. A spectrograph with a 4 1/2-inch .transmission grating was used. The two views.

. a re identical except for the contrast, which was changed in order to'reproduce both ends of. the spectrum more clearly. The polar'oid attenuators were removed for this exposure. The calibrating marks a r e mercury lines a t (left to right) 4358, 5461., and 5791 A, (b). The response of an S-11 photo- cathode' i s shown on the same scale.

Fig. 19. Relative attenuation factor of the polaroid attenuator used on the light pu l s t r , The measurements were made with a ' phototube having an S- 1 1 photocathode.

'

Fig. 20. A dark enclosure used in conjunction with the light pulser for phototube testing. The pulser is seen attached to the left end of the box.

In this box, the phototube is mounted on a movable cart so that the light incident on the cathode can be varied in accordance with the inverse-square law. Where fixedphoto- tube mounts are adequate, shorter boxes are used.

RG- 58 cables a re passed through holes drilled slightly undersize and sealed with opaque plastic material. All lighttight seals a re carefully made so that the ambient light inside the box causes a photocurrent several orders of mag- nitude less than the photocathode dark current.

Tmical Test S e t u ~

Figure 21 shows a typical test setup (2x0812). A microswitch (not shown) on the dark enclosure cuts off the phototube hv-supply when the door i s opened, Several tubes may view the light source through various optical paths if desired. One of the trigger pulses may appear a s a fiducial mark on the oscilloscope sweep (Cable B), another may be used for z - axis mod- ulation, etc. The transit time of the pulse through the phototube plus the delay of Cable A should be a t least 90 millimicroseconds longer than the de- lay through the cable that triggers the 517 scope, to allow time for the sweep circuits to start the trace. This time i s also considered in determining the length 0% Cable B.

Sliding lines (line stretchers) a re often helpful in measuring incre- mental time delays of tubes in the test setup. The General Radio type 874-LK or 874-LT constant-impedance adjustable lines (and other line ele- ments) a re convenient in the 50-ohm channels. (UCRL drawing # 2x6134 shows a line stretcher for use in 125-ohm impedance lines. )

ACKNOWLEDGMENT

The expert assistance of Raber t Reynolds, who cans true ted the light pulser s, i s gratefully acknowledged,

Fig . 21. Typical phototube-test se tup using the light-pillse gene ra to r .

Lis t of UCRL Drawings Pertaining to the Construction and Use of the. Light Pu l se r

Drawing number Item

A. Fabricat ion Drawings

4v 5572 Elec t r ica l schem.atic drawing of light pulser .

4V5593 Instructions for corn-pleting the mechanical assembly of the light pulser .

'. 4V5603 . Instructions for completing the e lec t r ica l assembly of the light pulser .

4V5612 ~ n s t r u c t i o n s for joining 4V5632, 4V5642, and.4V5652.

4V5622 Base-plate i'abrication.

4'6r5632 . Four -receptacle mount fabrication (supports PG-1, PG-2, PG-3, and PG-4) .

: 4V 5642 . . Two-receptacle mount fabrication (supports PG- 5 and PG-6).

4V 5 6'5 2 Metal half -shell fabrication. '

4V566P Fabricat ion of p a r t s for adjustable light attenuator.

4V5671 Permanent-magnet mounting fabrication.

~ 4 V 5 6 8 1 Driving-loop fabrication.

4V569P Adjustable light-attenuator plast ic scale fabrication.

4V5701 Modifica.t.inn of standard type N connectors for PG -2'arsd P G - 4 . , I

4V5711 Modification of Cannon 5-kv connector for PG-6.

4V5721 Modification of 125-ohm connector for PG- 1.

. '1 4V5731 Modification of W e s tinghouse % P 2 pulse t ransformer for driving coil T-1.

8. Accessory-Equipment Drawings

2x8181 Driving-coil power supply for 60-cps operation.

/ 2x8791 Driving-coil power supply for adjustable repetit ion r a t e s .

2x8771 '- / Light-pulse anticipator.

k

' . . ' . . . . , . . . . . . . . .

Lis t of UCRL Drawings pertaining to the: ~oi i s t ruc t i 'on ' . , . ..

I and Use of the Light Pu l se r

Drawing number .I tem

i B. . ~ c c e s s o r ~ - ~ ~ u i ~ & e n t Drawings (contd. ) . . .:. I

2x8801 125-ohm connections to 517 oscilloscope. . .

3Y 91 33 < 125-ohm coax switch f o r 51'7 oscilloscope.

2x8493 J Dark enclosure for testing ,phototubes up to 5 in. in d iameter .

2x13501. : Sugges Led wiring e n t r into da rk enc losu re for testing photo- tube s . . . ,

2x8812 / T y p i c a l phototube tCst setup with light-pulse generator . . .

2x61134 d'. 125-ohm line s t retcher . ' .

'.' 1x6693 ~ e r c u ~ ~ - s w i t c h e lec t r ica l pulser

Light-Pulser Pa r t s List . . . . . . . . . .

. . . - . . . " P a r t . .

number .... ,' . Specification . . . . . . . . . . ., .

. . . .

002-.mf 600-v ceramic condenser. .. .

Amphenol 82-885, modified (see. 4V5721) (UCRL 125-ohm connector).

PG-2, PG-4 Amphenol U G ~ ~ A / U , modified (see .4V5701).

Amphenol. 97-31 O Z A - 14s- 1 ~ - 6 3 9 3-pin male chassis receptacle

Amphenol SO-239 female. receptacle.

Cannon 3102A-18-16P, modified (see 4V5711)

24-ohm 1/2-w carbon res is tor .

110-ohm 1/2-w.carbon res is tor . I .

(1.

39-ohm 1/2-w carbon .resistor .

130-ohm 1/2-w carbon res is tor .

500 meg 1/2-w S.. S.. White res is tor .

Clare HG-1003 mercury switch (or equivalent).

Westinghouse 1P2 pulse transformer rr~odified (see 4V5731).

NOTES AND REFERENCES

1; References i l lustrating various uses for the light pulsey a r e available in r epor t s prepared for internal distribution a t the University of Calif- ornia Radiation Laboratory, a s follows:

a . Quentin Kerns and F rede r i ck A . Kirsten, Resul ts of P re l imina ry Tes ts of Two RCA LE 59 Photomultiplier Tubes, UCID-18 ,(CRG- 2), Nov. 1955.

b. F rede r i ck A. Kirs ten, Resul ts of Trans i t -Time Measurements on T h r e e . C 71.8.7A Photomultiplier Tubes, . UCID- 19 .(CRG- 3 ) , Dec. 1955.

c . Quentin Kerns, Resul ts of Tes t s on.Two C.7232.Experimental 16- Stage Multiplier Phototubes, UCID-97 :QCRG-7), Nov. 1956.

d. Quentin Kerns, Improved Time: Response in Scintillation Counting, IRE Transactions NS-3, - No. ~ . ( N o v . 1956), P a r t I.

. ' : 2'. Regarding the distortion of pulses t ransmit ted . in coaxial t ransmission - lines:

a . Quentin Kerns, Improved Time Response in Scintillation .Counting, IRE Transact ions NS-3, - No. 4,(Nov. 1956), P a r t 11.

b. UGRL Caunt i l~g IIai-ldbuuk, CvuriCing NotenCC2 - 1.

3. F rede r i ck A. Kirsten, Measurement of the'shape of the Light Pu l se Generated by Mercury Light P u l s e r 4V5572, Engineering Note.EE548, April 1958.

4.. C. orm man. winnings tad, A Fractional-Millimi'crosecond Oscililoscope Sys tem; Utilizing Commercial ly Available Components, UCRL-3778, Sept. 1957.