Possible Upgrades for CNI Carbon Polarimeter

1. Possible Upgrades for CNI Carbon Polarimeter G. Atoian, R. Gill, B. Morozov 1

2. Introduction Main problems with present system 70 KHz/10mm2 0.3 MeV Carbon Detectors and Front End: • Large changes in leakage current (0.02μA to 4μA) • Poor resolution (>50 KeV) • Poor high rate response (“ghost” peak) • Probably “Dead layer” instability during run • Expensive custom silicon strips • Low dynamic range of the Preamps (~11MeV) • Preamp connections can damage vacuum feed-through • Large shaper constant (~ 35 ns) • Noise pick up due to long distance (~100m) between Preamp and Shaper and cable type Event filtering (Digitizer and “DAQ” Problems): • 8-bit WFD limits dynamic range • Rate dependence of resolution, gain and efficiency for pulser • Adjacent polarimeters give varying results • Observe unexplained bunch-to-bunch asymmetry • FPGA code limitations 0.3 MeV carbon “ghost” peak 2

3. Ultra thin Carbon ribbon Target (5mg/cm2) 6 1 Detectors & FrontEnd Detectors: Hamamtsu Single PIN photodiode for direct detection (S7509) Each detector has 10mm x 2mm active area and ~120 μg thickness. The typical dead layer is 60 μg/cm2 8 detectors wereplaced on the existing 6 vacuum ports. Light diodes were used for monitoring purpose Front End: Charge sensing Preamps & Shapers(MSI-8) (in one box) wereconnected to the detectors throughthe0.5 m long low capacitance coax. The Shaper has two outputs: “fast” - 20 ns rise time and “slow”- 140 ns rise time. Dynamic range: “slow” - 35 MeV “fast” - 60 MeV 18cm 2 5 8 Si single PIN detectors (TOF, EC) 3 4 3

4. Test results (Detector 1cm x 1cm, Hamamatsu S3590-19)The prototype of the setup has been tested on Tandem & RHIC run’09. 4

5. Test results (Hamamatsu S3590-19, collimated by 4mm x 8mm) • RHIC Run’09 – single bunch Carbon Spectroscopy: • low Inverse Bias current (from ~0.02μA to ~0.2μA after four months run. Compare: 0.02μA to 4μA for BNL-strip) • excellent and stable energy resolution • (<20 KeV) over all run. • good intrinsic time resolution (.4ns) carbon mass separation with mass resolution almost three times better than BNL-strip. 5

6. Simplified Schematic 6

7. Digitizers & Trigger Digitizers: The Peak sensing ADC(MADC-32) is used for deposit energy measurement -11 bits - 0.8 μs dead time - 10 ns Time Stamp - VME The Dead Time-less TDC (V767A) is used for TOF - 0.6 ns bin width - 10 ns double pulse resolution - VME Triggers: The CFD discriminator (V812B) & fast logic(SiS3820) were used for trigger - with “fast” shaper output - protection against multiple pulsing - bunch#0 time synchronization - “Prompt” suppression at the beginning of each bunch. 7

8. DAQ update While the detectors and the front end modifications are well motivated by test results, the DAQ system modification is not so obvious. The suggested DAQ is a conventional DAQ based on the Peak Sensing ADC. It is simple and easy to maintain. The present CNI-DAQ has more advanced architecture based on WFD, but: • it samples pulses from shaper, not from preamps output; • it works, so-called, in ADC mode – pulse peak and time obtained “on-the-fly” by using FPGA code, which has limitation for modification. In fact, we were unable to handle the monitoring pulses which have a slightly different shape compared to “Carbon” pulse; • DAQ modification based on WFD requires a lot of man-power as well as time. There are two main drawbacks using conventional ADC: a possible pile-up and dead time losses. A pile-up comes from the fact that the rise time of the digitized pulse is ~140 ns (“slow” shaper output). It gives a pile-up of ~0.7% at the rate of 50 KHz/channel (compared to ~0.2% for present CNI-DAQ). During the test we use the “slow” output for purpose to suppress high frequency induced noise only. (In fact the 11-13-bits Mesytec MADC32 can digitize pulses with rise time of 20 ns at 11-bits mode of operation.) Unfortunately, our efforts to suppress the high frequency induced noise by using another methods were unsuccessful so far. Thus, conservative strategy is to keep the rate low. MADC32 has 0.8 μs dead time and one gate for 16 channels. It will give ~40% losses at the rate of 50 KHz/channel. The suggested passive splitter will reduce the loss value to ~15%, which is acceptable. It should be noted that the pile-up reduction is important, because pile-up “introduces” directly systematic shift to polarization value, while dead time losses increase the measurement time for given statistics only. 8

9. DAQ update cont. The VME can handle 80Mbytes/sec, but it is, probably, hard to realize in practice even MADC32 and TDC use some buffering technique. The ~50Mbytes/sec is more realistic number. The total number of detector is 48 (6 ports x 8 detector). At assumption that background/signal ratio is 1 (run’09 result) and 14 bytes event size (ADC(2) + TimeStamp(4) + TDC(4) + Spare(4)), the data rate is ~70Mbytes/sec at 50KHz carbon rate. It is safe to split system into two VME crates. The buffer of the VME interface has 2Gbyte, which is enough even for ramp mode operation. 9

10. Cost Estimation Total for entire system ~$200000 10

11. Summary During last the years we tested several detectors for alpha and carbon spectroscopy: Si-strips, Si PIN Photodiode Strips with different terminal capacitance, LYSO scintillators, Si diamond and Si PIN Photodiode Single. Also, several Front End configurations have been tested: Charge Sensing Preamps&Shapers (with different form factors, shaping constants and dynamic ranges), current preamps with 4K and 40K amplifications. Single SI PIN Photodiode with the charge sensing preamps&shapers (20ns/140ns shaping and 60MeV/35MeV) one box form factor gives the best performance in terms of the energy and time resolutions, dead layer uniformity, rate behavior and noise suppression. It is very robust set up, easy to handle and… also cheap. On our strong opinion it is the best choice for CNI polarimeter so far. The conventional DAQ with modern Peak Sensing ADC (thanks to MADC32) is also simple and well suitable for our purpose, especially, if one uses the shaper filtering and prompts noise suppression on the level of the events triggers. Besides that it is to programming without an any “magic touch” technique. Estimate less than 1 man-year for DAQ software development. The detectors set up and DAQ are based completely on commercial available devices. Concerning the RHIC polarimeters we still have two main problems, and therefore, we have to pay attention: to the rate control and the system monitoring. 11