An Image Intensifier for Track Recording

An Image Intensifier for Track Recording 0. GTLIIEMEIBTER ttnd R. GIESE I’lt,ilsiknliaches Itrstifirl! der 1 Jiiivcrstt(ifBo)rri, (&rrn)cI)iy

INTRODUCTION In 1955 Zavoiskii et a1.l described a scintdlnt8ionchamber using a fivestage image converter for the ir~t~ensification of the luminous tracks produced by ionizing particles in n scint,illat,or. The main advantage of a sciiit>illationchamber over the bubble chamber is the possibility of gabing the image intensifier by rrpecial events for a time duration down to less tjhnn 1 psec, with t~ dead t,inie of less than 20 msec. Very rare special events can t.herefore be picked out. of a large number of background events. For this reason tjhe development, of an image intensifier was stJartedin 1956 at)Bonn 1Jniversit)y. The construct,ion is essenthlly similar to that used by t,he Russian group. In principle it is a multis h g e converter with 1 : 1 electronic imaging by means of a homogeneous ningnet>icfield and with optical contact from stage to stage by means of t,hin light,-tmnsniittiiig foils. This kind of electronic imaging is preferable t,o imaging with elect,rost;Lt:ic lenses, since it leads to freedom from dist,ortions, oonstibnt resolution over t)hewhole image area and insensit,ivity to low nmgnebic s h y fields. As has been shown by Zavoiskii and other authors, an image int,ensifier for use wit#ha scintillation chamber should have sufficient gain that the light, int,ensit#yper unit area of bhe light spots on the output screen produced by single phot,oelect,ronsfrom the input phot.ocathode is high enough to be recorded on a photographic emulsion. Since the light intensity per unit are,a of these light spottuis proportional to the square of the resolving power (expressed in lp/nini), good space resolution reduces the required light amplification. The dark current problems are not, serious. Owing t,he short time of emission of scintillation light. the signal intensity is fm great’er than bhe intensifier background. EXPERIMENTAL ’J’tJBE The presenthefront window photocathode and t h e output screen is Rubdivided by three foils into four equal parts. Each foil consists of a 10 p Kovar sealing glass diaphragm sealed to a Kovar metal ring. Before being mounted in the tube, the foil is covered on one side with zinc sulphicle phosphor backed by an aluminium film. The wall of the tube consists of one cylindrical and three bulged glass 113

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rings per stage sealed to intermediate Kovar metal rings by eddy current heating. The bulging of the glass rings increases the wall length, thereby preventi'ng discharges along the inside wall during operation. Three glass tubes sealed to each cylindrical ring provide access to each stage for a sliding antimony oven, a caesium ampoule and an oxygen

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32cm

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PIC.1. Four-shge image iiiten&er tube.

source for the production of the caesium-antimony photocathodes on the front window and the rear surfaces of the three foils. One side-tube is connected to the pump and there are pump channels from stage t o stage by-passing the foils. All the twelve connections are sealed off after the photocathodes have been finished. The diameter of the image area is 3 cm and the length of the tube is 32 cm. During operattion the metal rings are connected to a voltage divider. EXPERIMENTAL RESULTS The following data indicate the performance of this intensifier. (1) The quantum efficiency of the first photocathode a t wavelength 4620 A is about 16%. (2) The gain has been evaluated for input light of wavelength 4620 if using an Sb-Cs photomultiplier to measure the light input and output, and with constant accelerating potential applied to the four stages. The gain is 1-2 x lo5 a t 38 kV overall voltage, and 2-4 x 106 a t 56 kV overall voltage. This gain

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decreases more or less when t h e later stages of the tube are gated. because of the long decay time of the phosphor. The background equivalent, dark ciirrent of the input. photocathode is less than A/cni2 at 38 kV overall voltage, and 5 x A/cm2 a t 56 kV overall voltage. ( 3 ) For simplicity no r e d foousing of the electrons frmi cathodes to screens has been used. 'I'he spiral diameter of the electron tracks is small enough at a suffioient,lyhigh magnetic field strength. At 500G the image diameter on the output screen produced by a single electron from the first photocathode is about 0.2 mm. At still stronger fields it decreases to about 0.1 m m over the whole image area. (4)As expected, no distortions due to the homogeneous matgnetic field imaging can be seen. ( 5 ) The lifetime can be expected to be many years. Wit>hin1 0 weeks after finishing of the tube no dccrease in nmplificat,ion could be det,ected. A preliminary test of thc tube has just been made with cosmic rays. This is a, good test in so far w the t'ube has to be triggered by the

FIG.2. Scirrt.ill&ion cliainher cosmic ray exporinimt..

penetrating particles, and the particle energy is in the region of minimum ionization. The arrangement is shown in Fig. 2, Above and below a CkI crystal of 5 nini thicknew there are two scintdlation counters giving it coincidence signal when a pmt'icle penetrates. Two f / l lenses coupled front-to-front image the particle track from t,he cryst>alon t.o the input, photocathode. Normally rt const.ant.volt,a.geis applied only to the first, and trhirdstage. The image is st,ored in the first screen. Because of tJhe decay t,iirie of 1.1 psec of C d t'he coincidence signal is delayed by 5 p e c and amplified. It triggers two pulse generators. The first produces a 12 kV negative pulse of about, 0.3 msec duration fed to the

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first foil, so gating on the second stage and gating off the first. The other pulse generator produces an 18 kV positive pulse of 10 msec duration, fed to the output screen to gate on the last stage. This gate determines the exposure time of the film. The output image is viewed by the camera by means of an f l l . 5 lens of 8.5 cm focal length coupled to anf/5 lens of 30 cm focal length producing a virtual image a t infinity.

FIG.3. Cosmic ray particle tracks.

This combination demagnifies about four times. Coaxial brass tubes form the high-voltage connections inside the coil to the rings which support the foils. For high-background experiments tfheinput photocathode instead of the first foil can be pulsed negative. This is possible if the image is stored in the crystal for a t least 0.1 psec. Therefore a fast vacuum tube pulse generator has been built producing a pulse of 1-5 p e c duration and 10-15 kV negative amplitude. The sum of trigger delay and risetime is less than 0.1 p e c . Figure 3 shows two photographs of typical cosmic-ray particle tracks (probably p-mesons) taken with the apparatus described above. Image intensifier tubes of the type desoribed could be produced without complications up to at least 10 cm image diameter. The resolution could be improved by the use of a phosphor of h e r grain,

ACKNOWLEDGMENT We would like to thank Professor W. Paul for his constant encourage-

ment and generous support throughout the course of this research.

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REFERENCE 1. Zavoisltii, $1. K., Rmolkin, G. E., Plakhov, A . C., and Butslov, M. ilk&. A’fZUk SA’A‘12 100, 241 (1055).

M.,Ilotl.

DISCUSSION J . JOHNSON: How effective was the use of tho corrugated g l a ~ s walls in retlming field emission and electrical discharges along the walls of the tube? 0. GILDEMEISTER: With the comagated walls, the voltage per stage could be increased by a facttor of about two before discharges appearod. R. B. OWEN: Could you please esplain the reason for the 5 psec delay bet,weon t h e coincidence of the trigger circuits t,o operate the tube? O . GILDEMEISTER.: The “fast” negat.ivc>gate of 0.3 msec duration is applied t.0 the first foil (sitting between t.he first and second stage of t.he intensifier t.ribe). While this gut.e switches on the second st.age, i t switches out the first. For this reason one has to delay t.liis gate for if tiine which ia long compared with the tlway tiine of the scintillator. The cleoay time of Ch1 is 1.1 psec. J . A . LODGE: In view of tht: reported retluction of phosphor effiriency at, low light levels, at what level were t,he gain measurements made? 0. GILDEMEISTER: The current. in the first, &age was about 3 x 1O-Iy .4/cni2 corresponding to a ciu’rent in the last stage of 2 x to 2 x IOP A/crnZ. It is possible that in t,he last. stages the measured gain is higher than the gain for I m v , short illuminations. N. A. BLAIN: You have quot,ed ciifforent dark current. figures atjdifferent applied potentials. Can you say what is the true t,hwrr\ioniceniissiont 0.m L D E M E I s i m t : The lower value of A/cm* is essentially true therrnionio emission. J . n. MCGEE: It is understood t,hat the elclot,ron image is not focused in each stsage. Is it, this and not the thickness of t,he cascade screen that limits thtl definition? 0 . GILDEMEISTER: The limit of resolution with high magnet.ic fields is given by t,he t,hicltnessof phosphor screen arid glass foil. This limit is about. 0.1 mm. I! probably coiiltl be imprnved if a phosphor. with xrnaller grain size, were ~ i n r c l .