A high speed driftchamber readout system: “Tirus”

Nuclear Instruments and Methods 217 (1983) 351-356 North-Holland Publishing Company

A HIGH SPEED

351

DRIFTCHAMBER READOUT SYSTEM: "TIRUS"

J.H.J. DISTELBRINK M1T Bates Linear Accelerator

a n d B.H. C O T T M A N *,

P.O. Box 846, Midd/eton. MA, 01949

2846. USA

TIRUS (Time Interval Readout Using Sealers) is a fast and accurate, digital wire chamber readout system which is capable of operating under high background and high real count rate conditions. The system can be configured for a variety of experiments in medium energy physics, including multiple arm spectrometer setups. Drift time is measured directly by a 500 MHz counter for each wire. Timing resolution is 1 ns, readout speed is 30 to 60 ns per event and relative channel efficiency variation is less than 3%. No delay adjustments are necessary. Current results from a 28 wire prototype are presented.

I. Introduction Wire c h a m b e r s in the focal planes of medium energy magnetic spectrometers can be required to operate in a high background and real event rate environment. The readout system should be able to handle these rates, distinguish and store multiple events and conserve maxi m u m position resolution. A n additional i m p o r t a n t feature is that it maintains constant channel width with respect to the position measurement. Finally, it should be able to operate in a multiple arm setup. In this paper, a new readout system, T I R U S [1], is presented which satisfies these requirements.

2. Design considerations

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In m e d i u m energy experiments the focal plane spectra can have a large degree of structure. For example, electron scattering energy loss spectra may contain m a n y inelastic states (fig. 10). These states are analyzed by peak fitting routines which are very sensitive to variations in channel width. It is therefore i m p o r t a n t to keep the dispersion in this quantity minimal. For multiple arm experiments, the coincidence trigger rate is usually a factor of 1000 or more lower than the single arm rates (local triggers). It is therefore necessary for the coincidence trigger to be available to the readout system. Usually the coincidence logic is located in the control room because of limited access to the spectrometer area during the experiment and rapidly changing experimental conditions. At the Bates Accelerator, the average distance between the detectors a n d the experimental control area is 80 m. The fronted or fast part (fig. 3) of the readout s y s t e m * Work supported in part by US DOE contract DE-AC027GER03069. 0 1 6 7 - 5 0 8 7 / 8 3 / 0 0 0 0 - 0 0 0 0 / $ 0 3 . 0 0 © 1983 N o r t h - H o l l a n d

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J.H.J. Distelbrink, B.H. Cottman / High speed driftchamber readout

352

From fig. 1 we see that for the local configuration, the coincidence trigger has to be transmitted back in order to accept or reject the event in the local electronics. This signal will arrive at a time interval, ~, after the particle has traversed the detectors because of the distance traveled, flight time differences in the spectrometer arms and the finite processing time of the coincidence logic. At Bates, r~, can be as long as 1 /xs. One has to delay every wire signal by % in order to avoid deadtime. This is very unattractive for systems of more than 100 wires. For the remote configuration the problem is effectively the same. In this case, every wire signal from the chambers has to be transmitted to the control area. There is a second solution for the local configuration which avoids the wire delay problem. Every event can first be labeled at the spectrometer with a time marker Later, the arrival times that correspond to a coincidence can be selected by a binary time comparator [2].

3. T I R U S configuration The configuration of T I R U S is shown schematically in fig. 2c. The digitizer part of the frontend is mounted close to the wire chambers. Every event that satisfies the local trigger is sent to the control area. Optionally the system can transmit all unreduced wire chamber data if

a local trigger is not available. The fronted logic, frequently used in fast drift chamber readout systems [4], is shown schematically in fig. 3a. In this arrangement, the registers store the clock value at the time of arrival of the wire signal. In T I R U S (fig. 3b), the registers are replaced by 500 MHz counters (for safe operation at 500 MHz we found it necessary to select the F100131 prescaler lC's - fig. 5). This arrangement has several advantages: (1) The number of cables is reduced because wire number and drift time are fully encoded. (2) High quality cables are not needed because the timing of the binary data is not critical. (3) Multiple arm experiments with flight time differences of up to 200 300 ns (dependent on the maximum drift time) are possible without extra delay in the data lines. (4) The wire signal starts the counter rather than stopping it. A simple delay of the trigger, instead of every wire, will avoid the problem of a stop occurring before a start. (5) Only the oscillator signal and not the entire set of outputs of the central counter has to be distributed over the system. (6) No adjustments are necessary to correct for delay differences in these outputs. (7) No verniers and associated adjustments are needed.

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(8) The period of the oscillator determines the drift time channel width directly. The variation in the channel width will be small because the period is not affected by the distribution mechanism. The counters will not be able to register multiple events at the same wire in a time interval smaller than

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the maximum drift time. For medium energy experiments, unlike those in high energy, this is unlikely to happen because at each trigger all but one of the registered events are uncorrelated (background). Fig. 4 shows the three separate parts of the system and its intercohnections.

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354

J.H.J. Distelbrink, B.H. Couman / High speed drifichumber readout

4. Drift time measurement and data transmission

After amplification, the wire signal starts the 10-bit a s y n c h r o n o u s binary counter by setting the S T A R T F F (fig. 5). The S T A R T F F keeps the counter r u n n i n g until it receives a trigger signal from the local backup detector. The backup detector signal is delayed by the maxim u m drift time. If no trigger appears, the counter is stopped and reset to zero by the overflow (carry) signal. The Q6 through Q9 outputs of the last scaler section can replace the overflow signal for shorter drift spaces. Optionally, the counter can be configured to reset at every wire signal. This mode is not r e c o m m e n d e d because of possible multiple pulsing. The contents of the counter are loaded into the data latch approximately 15 ns after the trigger. The counter can be restarted immediately afterwards. The phase ( 0 ° - 1 8 0 ° or 180 ° 360 ° ) of the oscillator is determined by the two left flip-flops in the FI00131 package shown in fig, 5. This is done in order to achieve a timing accuracy of 1 ns from a 500 M H z clock. The o u t p u t s Q0 trought Q9 appear as though produced by a 10-bit 1 G H z counter. However Qo only measures the phase of the oscillator at the time the counter is started. The resulting probability distribution of determining the correct time interval is shown in fig. 6(b). The fwhm of the distribution can be reduced to 1 ns by also measuring the oscillator phase at the time the trigger stops it. This is accomplished by decrementing the counter outp u t by one if Qi ~ Q0 ~ Q~ (fig. 5) is true. The probability distribution of this effective 1 G H z counter is s h o w n in fig. 6(c). This option reduces the range of the counter to 512 ns because there are only 10-bits available. This m e a n s that the advantage m e n t i o n e d under (8) in §3 is lost for the least significant bit. Four of the channels shown in fig. 5, together with the readout logic, are m o u n t e d on a 12 × 16 cm Scaler M o d u l e (SM). This b o a r d also contains a voltage regulator IC that can be used to switch off the approximately 20 W of power fed to the b o a r d between b e a m bursts. This option reduces the power c o n s u m p t i o n of the system by at least two orders of magnitude for present Bates b e a m conditions.

A real event is defined by at least a 3 adjacent wire hit in the c h a m b e r [2,3]. For this reason the four 10-bit outputs in a SM a n d an 8-bit module n u m b e r are fed in parallel to a 48-bit bus. Via ECL line drivers and receivers, this bus runs directly to the M e m o r y Module in the control room as shown in fig. 4. A 33 M H z oscillator, located in the Driver Module (fig. 4), drives the readout circuits in the SMs. The Driver Module transmits only the non-zero 48-bit words every 30 ns until every SM has been read. A priority encoder scheme searches for the next module while the previous word is fed to the bus. The memory has a 10 ns access time and can store 256 words. Because of the finite readout speed a n d the priority encoder scheme, there will be a module n u m b e r d e p e n d e n t deadtime. However, because of the d e r a n d o m i z i n g effect of the data latch and high readout speed, wire c h a m b e r losses due to finite drift time and ion charge effects will usually be more severe.

5. Protot)rpe results Seven SMs were connected to 28 wires of a vertical drift c h a m b e r (VDC) [3]. The V D C was m o u n t e d in the focal plane of the Bates energy loss spectrometer [3]. The operating frequency of the system was reduced to 450 M H z to simplify the selection process ({}3). This resulted in a loss of wire c h a m b e r resolution of less than l gc for this specific setup. A pair of commercial C A M A C memory modules were used because the T I R U S Memory Module was not yet completed at the time of the experiment. The electron b e a m was run at an energy of 200 MeV, at a repetition-rate of 300 Hz and average b e a m current of 30 I-tA. The energy loss spectrometer was set at an angle of 60 ° . Following the methods pointed out in ref. 3, the resolution of the V D C in c o m b i n a t i o n with T1RUS was determined by measuring the dispersion in the quantity A:o.4, where A = t I - 2t 2 - t~ (fig. 7) and 04 = 2 % g is

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J.H.J. Distelbrink. B.H. Cottman / High speed driftehamber readout 32O [

s h o w n in fig. 8b. As expected, no decrease in wire c h a m b e r resolution due to the finite frequency of the counters is found. In fig. 9 a flat or " w h i t e " spectrum is shown. This is the energy loss spectrum for BeO(e,e') in the quasi-elastic region. The cross section varies very slowly with energy in this region. F r o m this spectrum, the dispersion in channel width is found to be less than 3%. The position is calculated using the algorithm

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To avoid truncation errors due to the integer division, the drift times ( t ) were extended by setting the least significant bits randomly. In fig. 10 the energy loss spectrum for inelastic electron scattering from 42Mo is shown. This spectrum has not been corrected for radiative loss in the target or focal plane abberations. The resulting overall resolution is 0.02%.

the intrinsic cell accuracy. The measured distribution is plotted in fig. 8a. Assuming a drift velocity of 50 f f m / n s , the intrinsic cell accuracy (o, = 1.9 ns) corresponds to 95 /~m. For reference, a similar distribution measured with a commercial high resolution T D C is

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mm). States are identified by spin, parity and energy (MeV). XI. ELECTRONICS

356

J, tt.J. Distelbrink, B.H. Cottrnan / ttijch .speed driftchamher readout

6. Conclusion

References

T h e p r o t o t y p e h a s s h o w n t h a t a drift c h a m b e r r e a d o u t s y s t e m u s i n g U H F c o u n t e r s to m e a s u r e t h e drift t i m e s c a n o p e r a t e reliably in t h e focal p l a n e of a spect r o m e t e r . A 256 wire s y s t e m is p r e s e n t l y u n d e r c o n s t r u c tion for u s e in t h e n e w B a t e s s p e c t r o m e t e r M E P S .

[1] J.H.J. Distelbrink, Proposal for VDC (X) Time Interval Readout Using Sealers (TIRUS), Bates Internal Report (1981). [2] J.H.J. Distelbrink, E. Kok, H. Blok, J.L. Visschers, P.K.A. de Witt Huberts, to be submitted to Nucl. Instr. and Meth. [3] W. Bertozzi et al., Nucl. Instr. and Meth. 141 (1977) 457. [4] W. Farr and D. Weiskat, Nucl. Instr. and Meth. 190 (1981) 35.

W e w a n t to t h a n k D r W. H e r s m a n for h e l p f u l disc u s s i o n s a n d P. P e n g e r o t h for his e s s e n t i a l c o n t r i b u t i o n s to t h e d e s i g n a n d c o n s t r u c t i o n of t h e electronics.