Introduction

1. Introduction What is there to gain from using a Beetle1.3MA1 and a 12 dynode pmt compared to a Beetle1.3 and 8 dynode pmt in the RICH detectors. • Benefits in noise, etc. • Modification needed to remove spillover/overshoot • and simulations. • 3. Beetle1.3MA1 conclusions. • 4. Chip submission considerations. Nigel Smale University of Oxford

2. Consider the Front-end as two parts: The Pre-amp & Shaper Only two external adjustable components that we need to think about for pmt shaper Pre-amp Nigel Smale University of Oxford

3. Zfeedback Vin Zload Vout Pre-amp. Just an integrator:X talk. Zfeedback must be << Zload ∴ Cfb must be large At the working frequency Zfeedback must be << Zload so that all of the charge is collected on Cfb and not shared with Zload. Shared charge is X-talk. MA feedback capacitor is 2 times bigger than the B1.3 Nigel Smale University of Oxford

4. Pre-amp. Just an integrator: Noise. Zfeedback must be << Zload ∴ Cfb must be large If Rfb and Zload are large compared to ZCfb. Then Zfeedback Vin If Zload is comparable then the gain = Zload Vout Now noise voltage source is magnified by the new gain factor. MA feedback capacitor is 2 times bigger than the B1.3 Nigel Smale University of Oxford

5. Effects of Dynamic Range Vmean-single-photon-pulse-height (Vmspph) The dynamic range = Vout-max/ Vmspph. Vmspph depends on the HT of the pmt. • Dynamic range effects: • Pile-up at the Pre-amp; Large Vmspph reduces allowable occupancy. • Overshoot at the Shaper output ;worse for large Vmspph. • The S/N ; improves with larger Vmspph. Nigel Smale University of Oxford

6. Compare the dynamic range of B1.2 with MA0 From MA0. A dynamic range of 9 From B1.2. A dynamic range of 5 Mean signal is 128mV. S/N ~18 Mean signal is 324mV. S/N ~10 Measured at pipeline Nigel Smale University of Oxford

7. Adjust the HT for a MA0 dynamic range of 5 (same as B1.2) MA0 Dynamic range of 9 MA0 Dynamic range of 5 Much better S/N Mean signal is 128mV. S/N ~18 Mean signal is 204mV. S/N ~30 Measured at pipeline of MA0 Nigel Smale University of Oxford

8. Effects of Dynamic Range Is pile-up and overshoot o.k when using the MA0 in a dynamic range of 5. Nigel Smale University of Oxford

9. Zfeedback Pre-amp. Just an integrator: Pile-up. R2 only determines how fast the pre-amp returns to base. For a channel occupancy of 10% this must return within 250ns else the pulses add up until saturation occurs. Vout Pile up of output from Pre-amp Output of Pre-amp Nigel Smale University of Oxford

10. Zfeedback Pre-amp. Just an integrator: Pile-up. BUT! Make the gradient large and overshoot will be seen on the shaper output (see next slide). Make the gradient small then pile up will occur and the Pre-amp will saturate. The only place where pile-up occurs is here. Pile up is not seen by the shaper until saturation. Vout Pile up of output from Pre-amp Output of Pre-amp Nigel Smale University of Oxford

11. Shaper. Pre-amp effects on Shaper output integrator The long term overshoot is a voltage representing the negative gradient of the pre-amp. This is not accumulative in the sense of pile up. However a baseline shift will occur depending on the occupancy and average pulse height. The base line will then find an average. See next slide. Ac coupling or differentiator. Vout Output of Pre-amp Output of Shaper Nigel Smale University of Oxford

12. This is an extreme case where a single photon is received @ 100% channel occupancy. In this case there is no degradation in pulse amplitude ( no pile-up subtraction) only a shift in baseline of 30% of a single photon response. Depending on the average occupancy and pulse height a mean pulse baseline will be found which will be stable within a few %. Nigel Smale University of Oxford

13. R2 only determines how fast the shaper returns to base. Make R large and overspill will occur. Make R2 small and the shaper output will represent the input closely, in this case a differentiator. Therefore if Pre-amp has large negative gradient the output of the shaper will have an overshoot. Shaper. Just a differentiator and integrator. integrator Ac coupling or differentiator. Vout Output of Shaper Output of Pre-amp Nigel Smale University of Oxford

14. Modifying the MA to Remove Spill-over. Another way to reduce noise is to make the pulse shape from the shaper as wide as possible I.e Integrates the noise for a longer period. From LHCb specifications 30% of the pulse height can be remaining (overspill) 25ns after the peak. However this does not seem acceptable for a pmt solution because of ghost hits. The following slides shows simulation results of a modified MA to remove overspill. The single photon pulse height has been chosen to give a dynamic range of 5. Nigel Smale University of Oxford

15. To modify the MA) These Rs are FET devices. The necessary circuit time constants can be achieved by reducing the length of these devices. Very simple. shaper Pre-amp Nigel Smale University of Oxford

16. Optimised FE output pulse. ~800K-e input. 25ns Rt=4.5ns Pk to pk=50mV S/n~35? Overshoot =0 Undershoot=0 Narrow pulse will increase noise slightly. Nigel Smale University of Oxford

17. Linearity and remainder • Input 1 to 5 photons • Input measured@25ns • 0% of single photon response I.e 50mV • 12% • 32% • 46% • 62% 25ns Long term overshoot represents gradient of large pulses from pre-amp Due to R2 of shaper, can be tuned for a particular pulse height but not all of them. Nigel Smale University of Oxford

18. Why do the larger pulses have overshoot The Pre-amp feedback resistor is a FET device and is not a fixed R value due to the way it is used. Therefore bigger Pre-amp output pulses make the FET less resistive. R being smaller gives a larger gradient on the output and therefore the Shaper output has a larger overshoot to represent this. However the feedback cap discharges quicker and therefore gets back to the normal gradient within 25ns. Nigel Smale University of Oxford

19. Response to single photon @ 10% Channel occupancy 250ns 2500ns Shaper output Pre-amp output Nigel Smale University of Oxford

20. Zoom 8% of a single photon response is remaining after 25nS. This is not accumulative but because of a different gradient now. 25ns Shaper output Pre-amp output Remainder can be tuned out when occupancy is known. Nigel Smale University of Oxford

21. Conclusion of MA • The saturation effects are not as harsh as the B1.2. • The MA is more linear at the sampling point. • It does not suffer to the same degree with load capacitance. • The noise is less • No overshoot for average single photon pulse height. • No overspill for average single photon pulse height. • Cross talk is less than the B1.2. • S/N is 2-3 times better than B1.2 Note: 30% overspill was designed in to reduce noise, this can easily be removed on the next iteration with a very low risk factor. The increase in noise is expected to be only a few % which will be more than recovered with the increase pulse height. Nigel Smale University of Oxford

22. Chip submission considerations • Beetle1.3 will be submitted for Engineering/production run near the end of Q1 2004. • Around 800 chips/wafer for a Beetle size chip. This gives 23K chips (assuming an 60% yield). LHCb groups need 10K (including RICH demands for a Beetle1.3). • LHCb RICH can submit a Beetle1.3MA1 on the same wafer for no cost to the RICH group. This would give ~11.5K MA1chips to RICH and 11.5K B1.3 chips to the other groups. • Modification risks for MA1 are very very low. In the worst case there would still be enough Beetle1.3s for the RICH. Nigel Smale University of Oxford