Fast large aperture camera and data acquisition system with applications in astrophysics, particle physics and nuclear medicine

Nuclear Instruments and Methods 211 (1983) 179-191 North-Holland Pubhshmg Company

179

FAST LARGE APERTURE CAMERA AND DATA ACQUISITION SYSTEM IN ASTROPHYSICS, PARTICLE PHYSICS AND NUCLEAR MEDICINE D . J . F E G A N , D. M c L A U G H L I N , J. C L E A R , M . F Physics Department, Untversl O' College, Dubhn, Ireland

CAWLEY

WITH APPLICATIONS

and N.A. PORTER

Received 1 October 1982

A fast large aperture multlelement imaging camera has been developed to record Cherenkov light spots produced in the atmosphere by cosmic ray showers The camera may have possible applications m other branches of astrophysics, In particle physics and in nuclear medicine The camera is capable of simultaneously dlgmzmg the transient outputs (20 ns) of 37 phototubes, each into an 8 b~t byte while also capturing a further 27 bytes of system housekeeping data The 64 bytes are written onto magnetic tape In 114 ,us The throughout rate of the camera is therefore very high The hardware of the imaging system and tts associated data acquisition devices are described m detail

1. Introduction Recent advances in various semiconductor technologies have resulted m a prohferatlon of optical and infrared dewces which allow reahlme processing of two dimensional ~mages provided the image is small Generally, imaging devices have evolved around charge coupled devices or hnear arrays of photodlodes such as are used m spectroscopy There are however applications m nuclear medicine, particle physics and astrophysics where it is reqmred to electronically capture a transient ~mage focussed onto an array winch may be two to four orders of magnitude greater than the sensmve area of a sohd state imaging device For such apphcatlons a conc e n m c array of p h o t o m u l t l p h e r tubes located at the prime focus of the imaging system represents the best comprormse m terms of plxel size, senslttwty, speed of response, dynamic range and rehabfl~ty of operation Below we consider a new development in the atmospheric Cherenkov techmque using a camera for two dimensional lmagmg; although p n m a r d y developed for Ingh energy gamma ray astronomy, the camera may have appllcatmns in related fields where photon countmg systems are used to produce two dimensmnal lntens~ty profiles. The 10m optical reflector on M o u n t H o p k m s m southern Arizona has been converted to a two dimensional imaging detector using an array of fast phototubes. The images of interest are the Cherenkov light generated by small cosrmc ray and gamma ray showers m the earth's atmosphere Fast dxgmsatmn techniques are bemg used to give a 37 p~xel image of each shower (full field of 3.5 °) Since the showers arrive at r a n d o m t~mes the camera must be mggered by the hght s~gnal The m i n i m u m resolwng time (mtegratmn 0 1 6 7 - 5 0 8 7 / 8 3 / 0 0 0 0 - 0 0 0 0 / $ 0 3 . 0 0 ,,5~ 1983 N o r t h - H o l l a n d

time) 1s 10 ns but the camera may also be adapted to study slower pulse p h e n o m e n a if so desired. The primary purpose of the camera wdl be Ingh energy gamma ray astronomy m the 1011-1013 eV region. At 1012 eV primary energies the gamma ray fluxes are small (1 per m o n t h per m 2 of detector) so that satelhte detection is not feasible The earth's atmosphere is opaque to all electromagetlc radlatxon of energy greater than 10 eV, at very high energies the g a m m a ray interacts at an altitude of 20 km and produces a secondary cascade of p a m c l e s and photons. The relatwlStlC electrons m the cascade (sometimes called an air shower) radxate Cherenkov hght winch penetrates to ground level as a disk of hght of thickness 1 m and of diameter 200 m The angular spread of the hght as seen m a wide angle camera is 2 ° full width, tins angular spread is caused by the Cherenkov emission angle in air (1 o), the scattering of particles and the trajectory of the air shower relative to the detector. Although the flux of optical photons is weak (50 p h o t o n s / m 2 ) , the shower can be easily detected at a dark site with a simple hght detector because the d u r a t m n of the light pulses is very short (10 ~ s) Charged p a m c l e s in the cosmic radmtlon of the same energy continuously produce similar air showers, since these are 103 t~mes as numerous as g a m m a rays, the only hope of identifying a gamma ray source is as a directmnal anisotropy using a light detector system with high angular resolution The camera consists of an array of 37 5 cm phototubes each of which acts as an i n d e p e n d e n t detector (plxel) with a wide dynamic range The output of each p h o t o t u b e is simultaneously dlgmzed and written onto magnetic tape The size of the p~xels is chosen to match the angular resolution of the 10 m reflector, ~t also matches the

180

D J Fegan et a l /

Camera and data acqulsttton ~wtern

scale of meaningful hght variations across the shower image Because of the fmlte size of the C h e r e n k o v hght image (spot), the detector optics does not need to have angular resolution better than 20 mln of arc To detect lower energy g a m m a rays the collection area should be as large as possible The S m i t h s o n l a n Astrophysical Observatory's tessellated optical reflector of 10 m aperture at its Whipple Observatory site (2 3 k m altitude) in southern Arizona is the largest i n s t r u m e n t of its kind specifically built for g a m m a ray a s t r o n o m y The focal plane scale is 12.5 c m / d e g r e e winch allows it to be easily coupled to an array of large fast photomultlpllers T h e large scale a n d f / 0 . 7 optics do not p e r n n t the reflector to be easdy coupled to the fast imaging systems used in conventional a s t r o n o m y The optical quahty of the reflector is somewhat better than the conventlonal large reflectors built for solar a s t r o n o m y

2. Overview of the electronic system design philosophy The f u n d a m e n t a l objechve of the electronic compon e n t of the camera ,s to rapidly record the state of the system on c o m m a n d from a master trigger pulse ( M T P ) T h e composite state of the system is determined at any time by the state of various s u b - c o m p o n e n t s each performing a particular task a n d generating its own data set. The complete d a t a set ts stored m digital form as 64 8 bit bytes of i n f o r m a t i o n Each of the 64 bytes ,s stored in d u p h c a t e on tristate latches to allow two i n d e p e n d e n t d a t a acquisition devices to interrogate the system along two i n d e p e n d e n t 8 bit bus hnes The primary tristate latch highway is used to rapidly a n d automatically o u t p u t the data set to a dual buffered magnetic tape t r a n s p o r t system for each M T P received The secondary tristate latch highway is used to o u t p u t the current c o n t e n t s of the system to a m i c r o c o m p u t e r which IS used as a n o n h n e m o n i t o r of the camera's operation t h r o u g h o u t any exposure This device is m u c h slower t h a n the magnetic tape system a n d for that reason is not used for bulk data storage. The design philosophy beh i n d the various sub c o m p o n e n t s of the system is described briefly in (a) to (f) below a n d is summarized as seven interconnected functional blocks in fig 1 (a) The M T P m a y be initiated from any of nine i n d e p e n d e n t subtrIgger sources using priority encoding techniques. This offers the user the c a p a b l h t y of triggering o n a variety of different source signals, e g. (1) genuine multiple coincidence optical C h e r e n k o v signals from a n u m b e r of the p h o t o m u l t l p h e r tubes (PMT); (2) r a n d o m l y injected control signals, (3) trigger pulses from ancillary optical detectors which might be used in association with the 10 m reflector; (4) signals from a radio frequency time transmission service used to inject absolute time markers; (5) triggers derived from the roll over of the system clock every 1000 s, (6) triggers de-

rived from a laser b e a m or triggered lamp used to illuminate the 37 tube array for c a h b r a t l o n purposes Each individual subtrlgger source is uniquely encoded by the priority encoder for future identification (b) U p o n receipt of a M T P the 37 P M T outputs are digitized. This is a c c o m p h s h e d by initially subjecting the lnd,vldual voltage pulses to a m p h f l c a t l o n and pulse height discrimination using wide b a n d w i d t h Le Cro) nuclear physics integrated circuitry The 37 amplified analog pulse outputs are then integrated m Le Croy charge to time converters The conversion process is used to produce an 8 bit binary digital word proportional to the P M T o u t p u t pulse, for each of the channels The words are then stored o n 37 pairs of latches, p r i m a r y a n d secondary (c) The M T P also initiates the recording of the arrival time of the subtrlgger on a 9 decade oven controlled quartz crystal system clock The time is recorded to a resolution of 1 # s The state of the clock is recorded in Binary C o d e d Decimal (BCD) into 9 primary and 9 secondary latches The actual trigger source l d e n m ) is also derived from the priority encoder at this time and fed to another pair of latches, also in B C D (d) The system automatically m o n i t o r s the rate of events from the 37 PMTs where the amplitudes are above the individual discrimination settings for each channel. Tins Is d o n e on a cyclical basis, only 3 out of the 37 rates being counted during any particular 1, 5, or 10 second sampling interval chosen by the operator The rates in q u e s h o n are counted in 3 i n d e p e n d e n t 32 bit scalers. A c o n t i n u o u s pseudo-cyclical rate s a m p h n g procedure d u m p s the contents of the 3 channels m t o 12 p r i m a r y a n d 12 secondary latches A n o t h e r pmr of latches (one primary, one secondary) are used to identify winch group of three is currently being interrogated a n d what the s a m p h n g interval IS This means that for a reasonably Ingh tugger rate (50 200 m - I ) there is a high probability that all tubes will have h a d their rates recorded at least once every 5 min In tins way the state of the sky brightness can be reconstructed subsequently for each of the 37 channels. In one particular mode of operation it is envisaged that the camera system might be used with other i n d e p e n d e n t optical reflectors m close proxirmty F o r tins reason a single latch has been reserved for such possible coincidence or correlated observations m order to set flags (e) A dual buffer 9 track magnetic tape t r a n s p o r t interfaces to the primary latch systems and constitutes the bulk data a c q m s m o n system Each buffer is capable of storing 1024 bytes of data, equivalent to 16 succeslve M T P d u m p s U p o n completion of fIlhng of the first buffer, the buffer contents are written o n t o tape. The second buffer is s,multaneously switched to the latch highway In this m a n n e r the writing dead time is slgmficantly reduced. I n p u t t i n g of the individual 64 byte records to the buffer Is controlled by the system's 1

D J Fegan et al / Camera and data a~qutsttlon system

181

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EAST LARGE APERTURE CAMERAFOR

I ITP

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FLg 1 Functional block dmgram of the electronic system

M H z clock and the primary latch commutator. The digitization of the 37 tubes takes 50 bts while the interrogation of the latches takes 64 /zs. The effective writing into buffer dead time for any MTP is therefore 114/zs In practice and under burst c o n d m o n s a maximum of 32 successive events could be inputted into the pair of buffers in 3.64 ms while the buffer contents may be written onto the magnetic tape at a byte rate of 19 kHz (f) A PET microcomputer with an IEEE-488 interface box is used to interface to the secondary latch system The MTPs continuously update the secondary latches until such time as the operator asks to see the state of the system. W h e n this occurs the PET automatxcally disables any subsequent updating of the secondary latches The secondary latches are interrogated under microcomputer control via the secondary latch commutator. The resident data is collected byte by byte via the IEEE-488 interface. This process takes about 10 S and the state of the system is displayed on the screen The data set format corresponding to each MTP may be summarlsed with reference to fig. 2

3. Photomultiplier tubes

and pulse amplification (block l)

The Cherenkov image ,s formed onto an array of 37 PMTs located at the focus of the 10 m dxsh To detect the flashes of light the detectors must possess a fast response characteristic, wxth rise times significantly shorter than 10 ns. They must also be most sensitive m the UV and blue regions of the spectrum since the n u m b e r of Cherenkov photons emitted per unit path per umt wavelength interval xs proportional to X 2. The R C A 4518 ten stage 2 inch dmmeter tube has been selected to meet these criteria. The outputs of each of the 37 RCA 4518 PMTs are fed along 50 I2 cable for amphficatlon. Throughout the system all cabling was accomplished using RG174AU in conjunction with Sealectro 50 ~2 gold plated sub mlmature plugs and sockets. The pulse rlsetlmes are limited by the transit t~me spread across the 10 m reflector, being typ,cally 5 - 7 ns The pulse durations are somewhat longer being between 10 and 35 ns depending on whether the shower has been lmtlated by a proton or a g a m m a ray and also

182

D J Fe,gan et al / Camera and data acquisition sIgtem

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d e p e n d i n g upon geometrical considerations. The voltage pulses are ac coupled to Le Croy MVL100 chaps for a m p h f i c a t i o n and discrimination Topologically the a r r a n g e m e n t which has been adopted as one whach employs three 16 c h a n n e l a m p h f l e r / d i s c r i m m a t o r cards (Le Croy models 7790N), each of which contaans 16 taghtly spaced MVL100 chips. The layout of these b o a r d s has been designed for speed of operation. Each chap consists of a wade b a n d w i d t h amphfier, a fast comparator a n d a monostable, fig 3 The inputs are dlfferentaal wath the 500 M H z a m p h tier stage generating a × 10 analog o u t p u t pulse and a pa~r of c o m p l e m e n t a r y × 100 analog si_gnals The latter

are ac coupled to a very fast c o m p a r a t o r stage whach has externally controlled voltage threshold The threshold control slope has a value of 200 tLV/V allowlng the input threshold to be varied continuously from 200/~V to 3 2 mV W h e n fired, the c o m p a r a t o r s produce c o m p l e m e n t a r y ECL o u t p u t transitions of whach 37 are used to acnvate the rate m o m t o r s a n d a selected 16 are used to actavate the optical Cherenkov coincadence umt The durations of the ECL o u t p u t transataons from the m o n o s t a b l e s deterrmne the overlap resolvang time of the coincidence u m t a n d may be set by a comblnataon of R C control voltage Vpw a n d RC taming network In p r a c n c e this d u r a n o n as chosen based on a n u m b e r of

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D J Fegan et a l /

183

Camera and data acqulsttlon system

conslderattons such as the rtse time response of the photomultlpher tubes, transit ttme spread within the mdtwdual photomultlphers, dtspersxon in signal cables from end amphfiers and on considerations of the random coincidence rate. In practice this would be set somewhere between 10 and 100 ns. The × 10 analog pulses become the input to the charge to &g~tal clrcmtry, after feeding along appropriate lengths of delay cable to prevent the signal reaching the >tlzatlon circuitry before the arrival of the M T P pulse.

number of phototubes simultaneously. In general noise should show httle or no correlation from tube to tube with the exception of hghtnmg flashes, meteor trails or satellites moving through the field of view On the other hand the umque Cherenkov Image profiles may be picked out with a high degree of confidence, by demanding a simultaneous multiple-coincident photoelectron signature from a number of tubes For a resolvmg time interval ~- and for n channels taken m coincidence, the random rate R is given by t=n

R=2(.-1)~ 4. Optical coincidence unit, priority encoder and master trigger pulse (MTP) generator (block 2) Within ttus unit all the possible subtngger sources are brought together, encoded and made to generate the multiplicity of M T P pulses used throughout the entire camera system Of all the subtrtgger sources the Cherenkov coincidence pulse is the most fundamental When operational, the 37 phototubes continuously detect both signal and noise from the mght sky A genuine Cherenkov image will be expected to dlumlnate a large

"-I nU,,

(~< 1 s).

with N representing the individual counting rates for each channel It was considered sufficient that only 16 of the 37 channels would take part m the formation of the optical coincidence signal. Which 16 of the 37 tubes are used to form the coincidence is a free parameter wtuch depends upon the geometry of the array of tubes located at the focus. C o m m o n sense would suggest that at least the central seven tubes should partake in the coincidence. The other 9 might be determined by trial and error.

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184

D J Fegan et al / Camera and data a~qut~mon svatem

Several techmques exxst for producing a n a n o s e c o n d coincidence pulse [1] The one a d o p t e d here revolved the linear s u m m a t i o n of the ECL logtc tranmuons from 16 MVL 100 chips across a remstor network and comparing the resulting analog sum w~th a varmble bins threshold at the inputs of a very fast c o m p a r a t o r (AM685). By moving the threshold bins level the multiplicity may be set anywhere between 1 and 16, fig. 4 The c o m p a r a t o r output fires a pmr of cascaded ECL F h p F L o p s (MC1670) configured as a pair of m o n o stables [2] The first one, MS1, has a period of 114/~s whtch ts exactly as long as the data set read out txme Ttus acts therefore as a blocking mhxbtt preventing the lmtmhzat~on of a d~glt~zatxon sequence untal the current one has been fully processed. Provided the D input Is held high by the manually acuvated cross coupled N A N D gate (CCN) an output from the A M 685 will trtgger MS1 at ~ts input C2. Th~s mgnal constitutes an optical comcldence and lmmedmtely triggers MS2 which has ~ts output duration set by an RC network. After statable fan out and level conversion [3] its output becomes the MTP whtch begins the charge to Ume conversion process described below The durataon of

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MS2 IS important since it determines the duration for which the charge to time conversion chips are activated Its duration may be set by a potentlometer to be m the range 10-100 ns. The M T P is also used to activate the system time clock, the rate multiplexer and after a 50 gs delay, the output sequence. It should be noted that the output of MS2 also feeds back to the priority encoder A n o t h e r useful feature as that the C C N can be made to set D low on MS1 and therefore to allow absolute manual inhibit of the system if demred Several submdtary sources are used as m p u t s to the priority encoder A suitably decoded ume mgnal such as W W V B or L O R A N - C triggers the system clock to reject absolute ume markers, m order to absolutely cahbrate the system clock The 1000 s clock rollover is also fed in as are two C C N source trtggers whxch simultaneously i n m a t e either the firing of a laser beam or a hydrogen spark gap located near the 10 m reflector, for cahbration purposes, All of the submdlary triggers are coupled into the system via an O R gate to C1 on MS1 The trigger sequence is then as it was for input C2 A priority encoder is a device which encodes the highest order input data hne into a BCD code irrespec-

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D J Fegan et al / Camera and data acqmsmon system

uve of all lower order inputs. In our application we employed the SN74147 giving highest priority to the clock rollover a n d next highest priority to the Cherenkov signals. A version of the M T P was used to latch the 4 bit identification code for the trigger source m question into latch position 50, for b o t h primary a n d seco n d a r y data acquisition systems

5. Charge to digital conversion (block 3) Various techniques exist for converting an analog charge package to a suitably encoded digital equivalent [4] For each of the recording channels we have performed this conversion of the × 10 analog C h e r e n k o v pulses as a two stage process. In stage one the individual pulses are used to charge a capacitor to a voltage which is p r o p o m o n a l to the a m o u n t of charge passed through a h n e a r gate. This voltage is then r a m p e d to zero by a c o n s t a n t current source With a differential amplifier m o n i t o r i n g the capacitor voltage a TTL logic transition (Q to t, charge to time) is generated of d u r a t i o n p r o p o r u o n a l to the total charge integrated d u r i n g the interval for which the gate is open. The second stage involves using the T T L (Q to t) t r a n s m o n to effectively gate a 5 M H z signal, derived from the master clock, into 37 Individual 8 bit T T L scalers The complexity i n h e r e n t in this approach, especially for the short d u r a t i o n charge packages generated by the camera, would make th~s quite a difficult task but for the existence of the Le Croy Q T I 0 0 B charge to time converter chip, fig 5a U p o n receipt of a M T P this chip performs the function described in stage one. Typical waveforms are shown in fig 5b

185

As the charge d u m p e d at the input to each charge to time converter v a n e s from tube to tube and from event to event so therefore does the d e p t h of each ramp and the o u t p u t d u r a t i o n of each TTL transition The saturau o n level represents an o u t p u t transition of 50 #s corres p o n d i n g to a charge of 256 pC deposited at the input. A pedestal voltage may be added to the input s~gnal to change the time for whtch the c o m p a r a t o r fires to produce the o u t p u t T T L (Q to t) t r a n s m o n The pedestal voltage requires a dc level between 0 and - 18 V. This results in the injection of a charge of 0 25 m A / V . For the application of a gating ~ulse of duration T the total charge stored is 0 25VT. Fig. 6 shows the interface electronics used to couple the M T P input, the test input a n d the × 10 analog signal into a given Q T I 0 0 B [5] In order to e n h a n c e the flexibdlty of operation of the individual chips the test, ramp, gain and bias facdmes were also exploited. The test input may be used to inject 0.625 m A / V into the charge storage capacitor within the chip This facility may be used to ensure that the response of individual charge to time converter chips can be m a d e the same for constant s u m u h The ramp connection may be used to allow rap~d termination of the output if desired. The gain input may be used m order to facIhtate the standardization of the conversion gain of each chip, that is, to adjust the c o n s t a n t of p r o p o m o n a h t y between the input stage a n d the o u t p u t T T L duration The o u t p u t of the c o m p a r a t o r changes state for the d u r a t i o n during wluch the capacitor voltage exceeds the threshold bias as shown on the timing diagram of fig. 5b In order to simplify the layout of the system it was decided to fabricate seven i n d e p e n d e n t printed c~rcuit boards for the charge to time converter part of the system Each such b o a r d contained a single

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Fig 7 Transfer funcnon for the charge to time conversion process unit of control electromcs for routing the × l0 analog, M T P a n d Pedestal test i n p u t signals to be fed in parallel to 6 charge to time converter chips. Potent~ometers were p u t on the pedestal, conversion gain and bias inputs allowing a p p r o p n a t e voltage levels to be v a n e d over the full d y n a m i c range quoted by the m a n u f a c t u r e r s This also facditates t r i m m i n g up all 37 tubes This a p p r o a c h

considerably reduced the overhead in control section building a n d worked without any problems of loading or crosstalk The test input was designed into the system such that when triggered by a s t a n d a r d N I M pulse a fixed charge is d u m p e d from capacitor C onto the test pin 2 input of the QT100B, provided the gating input is s~multaneously activated Fig 7 indicates the transfer function for the charge to time converters The second stage of the conversion involves t r a n q a tlon of the T T L (Q to t) logic transition to an 8 bit b i n a r y word This is accomplished by gating the 5 M H z system clock into a pair of series c o n n e c t e d 4 bit binary counters (SN74LS163), one pair per channel. How this is achieved m a y be u n d e r s t o o d with reference to fig. 8 A n M T P enables a fl~p flop which opens gate G1 allowing the passage of 250 cycles of the 5 M H z clock into a dlwder. After an elapsed u m e of 50/zs the falhng edge of the 250th pulse generates an o u t p u t which flre~ MSI whose o u t p u t performs the following functions (a) resets the flip flop (b) generates enable pulses for b o t h the primary and secondary latch systems (assuming G2 has not been deactivated by the microcomputer) and (c) triggers MS2 to reset all 37 8 bit counters to zero. In this m a n n e r the 37 8 bit latches, which connect with the counter outputs along individual 8 bit bus hnes, are u p d a t e d 50 /~s after digital conversion has b e g u n Immediately after thts the contents of the counter are reset to zero a n d m a d e ready for the next stage of dlgmzatlon which wdl take place on the next M T P pulse received after the latches have been interrogated

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D J Fegan et al / Camera and data acqutsmon system

6. System master clock, timing and synchronization (block 4)

187

respect to an external time signal. This procedure would be performed at the beglnnang of a m g h t ' s observation T h e 1 M H z clock rate thus generated is sent to the b a n k of 9 cascaded decade counter modules. Each module consists of a T T L B C D counter (SN7490), a T T L 4-bit date latch (SN7475), a T I ' L B C D to 7-segment dec o d e r / d r i v e r and a 7-segment incandescent &splay, all m o u n t e d on mdavadual printed circuit boards. At the i n p u t to each decade counter (D1 to D9) is an exclusive O R gate one of whose inputs receives the clock rate from the previous decade while the other as connected to a push b u t t o n cross-over switch (S1 to $9) through a p m r of cross coupled N A N D gates to e h m l n a t e contact b o u n c e As a result of th]s a r r a n g e m e n t a pulse can be entered into any decade eather from the o u t p u t of the previous decade or by pushing the appropriate switch once This is a simple but powerful &agnostic feature which can be used b o t h to facahtate the operator to find possible tlmEng faults during setting-up or, for example, to set a specific n u m b e r on the clock before beginning a c o u n t sequence In this way the c o u n t ripples through the nine decades In n o r m a l operation the clock &splay xs held fixed while the counter updates at 1 MHz. It therefore always displays the time (with respect to the system clock) at winch the last M T P was rece]ved. The total cycle tame for the clock is 1000 s. ThEs produces a

A high precision oven controlled quartz crystal oscdlator ( H C D research model 126) has been incorporated into the system, fig. 9, The crystal produces a 25 M H z sine wave o u t p u t which is stable to one part in 108 over any 24 hour period. The low level 50 I2 o u t p u t was amplified a n d converted to a T T L logic level an order to perform a n u m b e r of system tasks The major task revolves the synchronazation of data transfer between the primary latch system and the digital magnetic tape. In order to facdltate this, the level shifted 25 M H z o u t p u t is & w d e d d o w n to 1 M H z using a cascaded pair of T T L counters (SN74LSI63) Buffered outputs at 1 M H z are used to feed both the 64 h n e address decoder a n d digital tape record hne as shown later m Block 6 A 1 M H z buffered o u t p u t ~s also produced for system test purposes. The 1 M H z clock rate produced here is also used as the basic clock signal for the 9 decade system clock It ~s taken through a N A N D gate, one of whose inputs is controlled by a push b u t t o n swatch. If the b u t t o n is pressed it grounds its input to the N A N D gate a n d forces the o u t p u t high. This action then "steals' pulses from the clock a n d is used to adjust Its phase w~th

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188

D J Fegan et a l /

Camera and data acqutsltmn ~ stem

rollover pulse which may be fed back into the system to m a i n t a i n a tag on the total elapsed time F o u r lines from each decade counter (D1 to D9) are taken to the a p p r o p r i a t e tristate latch b o a r d s on each of which there are nine of the 64 system latches dedicated to the clock output. The B C D bytes from the 9 time digits are stored In this m a n n e r on latches L41 to L49 for b o t h primary a n d secondary latches systems. In order to slmphfy the identification of each night's data set we have dedicated three latches for this purpose Three i n d e p e n d e n t 10 position B C D encoded t h u m b w h e e l switches may be set by the user to any value between 000 a n d 999. The particular value chosen is routed to latches 38 to 40 and becomes written as part of each subtrigger's data set.

7. Signal multiplexers and ratemeters (block 5) The C h e r e n k o v pulses from the night sky are detected agamst a b a c k g r o u n d of starlight, airglow and

variable haze By Increasing the b a n d w i d t h of the system the sensitivity of a C h e r e n k o v detector m a y be improved since the n u m b e r of b a c k g r o u n d p h o t o n s per sampling interval a n d hence the fluctuation upon this n u m b e r are reduced By exploiting the coincidence technique the r a n d o m distribution may be reduced to virtually zero By using closed loop optical feedback servo systems [6] the rates may be stabilized to _+ 5% of the n o m m a l c o u n t i n g rate but this technique leads to a continuously varying threshold energy which in our application is undesirable. For this reason the c o u n t i n g system being described has been designed to operate without any servo control It ~s nevertheless desirable to be able to continuously m o m t o r the individual p h o t o n rates for each tube a~ a function of time W i t h a 37 channel system th~s is not however realistic, due to the volume of data which 1~ p r o d u c e d A compromise solution has been reached whereby all 37 tubes are continuously m o m t o r e d but only three generate an o u t p u t at any one time How th~s is a c c o m p h s h e d may be understood with reference to fig 10

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D J Fegan et al / Camera and data acqut~mon system

Pulses from the 37 E C L c o m p a r a t o r outputs of the M V L 100 chaps are level shifted a n d fed to 37 of the 48". inputs of three parallel T T L 16 bit data selectors (SN74150). Locations 38 to 48 are unconnected. The three rate outputs from the data selectors are each connected to the inputs of three 32 bit binary scalers (4 × SN74LS163) where the events are counted over intervals of either 1, 5 or 10 s. How this is accomplished ts that the three data selectors are simultaneously addressed by a 16 address generator (SN7493) The address generator is itself activated by a 1 Hz clock which may be manually switched through either a 1, 5 or 10 counter. D e p e n d i n g upon the switch settings the o u t p u t fires a pair of cascaded monostables MS1 a n d MS2 ( = 100 ns) every 1, 5 or 10 s. The first m o n o s t a b l e MS1 enables the u p d a t i n g of three groups of four primary latches ( L 5 1 - L 6 2 ) configured using T T L SN74 LS373 devices This is done automatically at the end of each parttcular counting interval and is immediately followed by MS2 resetting the scalers. As this is h a p p e n i n g the address counter augments by one a n d three new tubes are routed to the scalers In this m a n n e r particular groups of tubes (e.g 1, 17, 33; 2, 18, 34, etc.) are continuously being monttored t h o u g h not necessarily outputted. The actual o u t p u t t l n g of the event rates to the magnetic tape system is controlled by the primary electronic c o m m u t a t o r which itself is activated by a MTP. Comm u t a t o r pulse C51 disables the u p d a t i n g of the latches, a m a n d a t o r y condition during any o u t p u t sequence, while pulse C64 re-enables them for subsequent count u p d a t i n g D u r i n g the disable period appropriate sequen-

tlal latch signals cause L51 to L62 to o u t p u t their data byte by byte o n t o the primary data haghway for writing o n t o tape. This m e t h o d of o u t p u t t l n g rate data for each M T P means that the particular trio of tubes sampled by any one M T P is a hat or mass affair but that for the event rates envisaged in this experiment there is a high p r o b a b l h t y that each tube will have its rate outputted at least once every 5 roan. A ( C C N ) inhibit line allows m a n u a l setting of the flip flop or total inhabit of the latch updating, if so desired. The 63rd latch is used to encode the address of the particular inputs being interrogated on the data selectors, in order to identify which tube's contents are currently in the 12 o u t p u t latches. It also encodes the position of the 1, 5, t0 switch The 64th latch is in effect a spare, hats m a y be set within it by external devices or experiments The secondary latch systems data, which is used as o u t p u t to the PET microcomputer, is treated in virtually the same m a n n e r b u t for the sake of economy is not shown in fig. l0 The only major difference is that the o u t p u t is controlled by the secondary electronic commutator system

8. The primary data latch system and the magnetic tape transport (block 6) How the 64 bytes of data have been acquired by the system has been described above in relation to blocks 3, 4 a n d 5 This data must now be transferred to the p r i m a r y storage latch system for p e r m a n e n t recording. This is accomplished by using the primary latch enable

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D J Fegan et al / Camera and data acqmvltton system

pulse (fig 8) which ~tself has been generated by the M T P After fanout this pulse is fed mmultaneously to 64 tmstate latches (SN74LS373) which become m o m e n t a r fly (100 ns) t r a n s p a r e n t a n d therefore update, fig 11 T h e input redes of the latches are fed from the counters (37 bytes), the system clock (10 bytes), the r u n identification encoder (3 bytes), the ratemeters (13 bytes) and also from a single spare byte. The next step revolves m~tmtton of the sequentml o u t p u t t m g of the data For p e r m a n e n t storage the data ~s written out o n t o a 9 track Dtgl D a t a magnetic tape system comprising a s y n c h r o n o u s transport, a f e r m a t t e r and dual buffer with control electromcs In fig 11 the enable o u t p u t pulse ~s shown opening a flip flop which ~s used to gate a stream of 64 write pulses through G1. U n d e r normal circumstances these pulses will be at 1 M H z but they may be fed through the O R gate at 4 Hz or m single shot mode for test purposes T h e o u t p u t of G I is fed to a pmr of T T L counters (SN74LS163) which generates 64 u m q u e addresses before the fhp flop ~s closed. These addresses are decoded by what ~s m effect an electromc c o m m u t a tor unit T h e u m t [7] comprises four 4 to 16 h n e decoders (SN74154). As each address is decoded the o u t p u t 64 lanes go low one at a time for one half clock cycle a n d this alters the outputs of the particular latch addressed (latch N ) from the high i m p e d a n c e o u t p u t state to the normal T T L states As all of the latches share the same data bus h n e this ensures that the c o n t e n t s of only a single latch loads the bus hnes at any one t~me Simultaneously with each c o m m u t a t o r control pulse the " r e c o r d " h n e m the formatter is pulsed In thts way as the c o m m u t a t o r pulses the " r e c o r d " h n e tn the formatter at 1 M H z it synchronously causes the data stored m each latch to be put into successive locations m the buffer Each M T P also triggers an inhibit M o n o stable of d u r a t i o n 114 p+s at the systems front end The lninblt prevents data pileup a n d overwriting during the r e a d o u t cycle The process of w r m n g the data into the tape machine buffer takes 64 kts a n d it is this interval plus the data acqms~t~on t~me (50 /~s) winch sets the total dead time of the system to be a b o u t 114/Ls. The tape system has a sophlsttcated formatter winch controls the dtrect writing of data onto the 9-track dlgttal tape The data on the 8-bit bus ~s loaded into one of two buffers m the formatter, xt Is written byte by byte at 1 iMHz by the record h n e Each buffer is 1024 bytes long so that It takes mxteen events each conststmg of 64 8-bit wide bytes to fill it. W h e n this buffer is full the next event Is switched to the second buffer while the c o n t e n t s of the first buffer are written at 19 K H z onto tape Additional demgn features m c n r p o r a t e d into the system but not shown in fig 11 allow the user to read the tape byte by byte m single shot mode Each byte value m a y be read from tape a n d displayed upon a three decade decimal display u m t The procedure is slow but

useful m checking the fidelity of data written o n t o tape at the start of a m g h t ' s observation

9. T h e s e c o n d a r y l a t c h s y s t e m and t h e m i c r o c o m p u t e r i n t e r f a c e ( b l o c k 7)

It as not generally possible to vemfy the fidehty of large quantities of data written o n t o magnetic tape unless one uses an online printer and mixes b o t h read a n d write modes This as undesirable in astronomical applications where the exposure tames (and therefore writing times) are quite long O n h n e m o n i t o r i n g with our primary data acquisition would not have been easily i m p l e m e n t e d For this reason we have incorporated a secondary 64 latch system which acts as a slave back up to the primary system, deriving its identical data set from those sources which feed the primary latch system Tins secondary system was then used to c o m m u m c a t e with an online C o m m o d o r e (PET 8032) m i c r o c o m p u t e r system winch could m o n i t o r and report upon the state of the system whenever tins was desired. No attempt was made to bulk record any data w~th th~s system It was designed simply to display u p o n c o m m a n d , the digitized outputs of the 37 tube Intensities, the u m e of arrival of the subtrigger in question, the p h o t o n count rates in the ratemeters for the s a m p h n g interval m question a n d the run ~dentiflcatlon code The operation m a y be understood by e x a m i n a t i o n of fig 12. The 64 latches are u p d a t e d simultaneously by the

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D J Fegan et al / Camera and data acqutsmon system

secondary latch enable pulse. This process is an ongoing one until such time as a R U N c o m m a n d , issued to the microcomputer, disables any further u p d a t i n g of the latches by virtue of b r l n g m g the m i c r o c o m p u t e r disable lane low in fig. 8. The R U N c o m m a n d also begins the process of data transfer by initiating a software routine m the m i c r o c o m p u t e r which communicates with the 8 bit data acquisition device along the IEEE 488 interface higway The device has been designed to accept data into its input port from the tristate bus As with the primary latch system tins data is strobed one byte at a time by virtue of the 64 line electronic c o m m u t a t o r which successively activates each of the secondary latches A twin line h a n d s h a k e link between the 8 bit acquisition unit and the 64 way address generator effectively controls this data interchange. Each pass of the software routine through its loop brings back a single byte of data. The total readout time for the complete data set is a b o u t 10 s The scope and capability of this secondary data acquisition system could be considerably extended with manor software changes if so desired

10. Conclusions The major advantage of two dimensional imaging is the i m p r o v e m e n t in angular resolution winch preh m l n a r y estimates indicate will result in a reduction of the cosmic ray b a c k g r o u n d by a factor of a h u n d r e d and lowering of the m i n i m u m flux sensitivity (at 1012 eV) by a factor of 10 There are, however, other very i m p o r t a n t features of tins technique which would make it worthwhile even if there was no angular resolution i m p r o v e m e n t These include e n h a n c e d collection area, e n h a n c e d estimates of the total energy in the showers which are imaged a n d some degree of discrimination against tsotropic b a c k g r o u n d p r o t o n induced showers T h e applicability of the imaging techniques to g a m m a ray a s t r o n o m y has recently been reviewed by Porter [8] The camera system winch we have described here was developed and tested at University College D u b l i n

191

It has recently been moved to the Winpple Observatory at M t Hopkins, Arizona a n d has been interfaced into the 10m optical reflector located there Calibrations and test procedures have been completed and 19 of the proposed 37 pixes are now operational The remaining outer ring of 18 will be incorporated into the system in 1983 To date the system has functioned as expected with all design criteria specifications having been met. Isophotes of images taken so far follow the generally expected p a t t e r n for showers and they will be c o m p a r e d with computer simulations of p r o t o n a n d g a m m a ray induced cascades in order to pick out the g a m m a ray showers. It has also become a p p a r e n t that some of the design features may have potential use in other areas of short time c o n s t a n t a s t r o n o m y and astrophysics, as well as having possible a p p h c a t l o n s in particle physics and nuclear medicine W e would like to t h a n k D r K E. Turver and Dr. K.J Orford for useful discussions on charge to time conversion techniques. We greatly appreciate the encouragem e n t and support of Dr T.C Weekes, Miss C H a n d ley, Miss G McNeill a n d Mr T M c K e n n a in the development of the camera system This project was supported by a grant from the N a t i o n a l Board of Science and Technology of Ireland

References [1] F A Kirsten, IEEE Trans Nucl Scl NS-20 (1973) 22 [2] Borghesl, A , Goggl, G and Nardo. R Nucl Instr and Meth 137 (1976) 605. [3] Palm, W A, Electromcs (June, 1978) 148 {4] Henry, T W , IEEE Trans Nucl So NS 20 (1973) 52 [5] Stubbs, R J and Waddoup, W P Nucl Instr and Meth 146 (1977) 561 [6] Fruln, J H and Jelley, J V , C a n J Phys 46 (1968) l l l 8 [7] Douce, A , Semiconductor c i r c u i t design, vol 11 (Texas Instruments, 1973). [8] Porter, N A , Proc. Workshop on High energy gamma-ray astronomy, Ootacamund, In&a (1982) p 64