Luminosity Monitor and Test Run Results. Presented by: Luis Mercado UMASS Amherst 7/23/08

Transcription

1 Luminosity Monitor and Test Run Results Presented by: Luis Mercado UMASS Amherst 7/23/08

2 Overview Luminosity Monitors Lumi Setup Components PMT Linearity Motivation/Formalism Experimental Test Setup Acceptable Usability Range Data Acquisition Modified 16-bit ADC Lowered Pedestal Noise Long-term Drifts January Test Run Lumi Test Run Summary Analysis Results Sources of Noise

3 Luminosity Monitor Setup Used as diagnostic for experimental setup (beam-line, instrumentation and target). Located 7m from the target. Consists of eight symmetrically placed detectors.

4 Luminosity Monitor Setup

5 PMT Linearity - Motivation Need to understand usability range of Photomultiplier Tubes being used as Luminosity Monitors. Want to find conditions for good linearity better than 2%. For PREX, would like to have linearity better than 0.5-1%.

6 PMT Linearity - Formalism A PMT s non-linear response can be expressed as follows: N ± PMT ( ± ) 1+ β Experimental asymmetry can be approximated: + ± = N N N N A = PMT PMT = A 1 N exp true N + + N 0 PMT PMT ( + β )

7 Experimental Test Setup 2 LEDs (1 kept constant, 1 pulsed at 30Hz) used to generate small asymmetry. Filter Wheel attenuates both light signals after passing through a diffuser. There are six filter settings (0,20,40,60,80 & 100%). PMT output is integrated by a 16 bit HAPPEx ADC.

8 Experimental Test Setup

9 Studies of Good Usability Range For lower photocathode currents, observed HV range that produces good linearity. Typical I max ~50uA. Results good to sub-1%, but residual instabilities preclude making conclusions at better than the 0.5% level.

10 HV scan for 10nA HV (-V) I (ua) IT (*2.5us) Beta*N_0 (%)

11 HV scan for 5nA HV (-V) I (ua) IT (*2.5us) Beta*N_0 (%)

12 HV scan for 2nA HV (-V) I (ua) IT (*2.5us) Beta*N_0 (%)

13 Photon Statistics Method Used to calibrate input photocathode current in experimental test setup. N is number of pairs per integration window. More accurate than previous method: gain curves collapse to one. I ~ σ Ne 2 lumi N = 15Hz

14 Lumi PMT Gain Curves Using low-gain PMT Gain Curves for PMT Gain 1000 "2nA" "5nA" "10nA" "20nA" HV (-V)

15 Lumi PMT Gain Curves Using Photon Statistics Gain Curves for PMT Gain nA 2.7nA 6.5nA 15.5nA HV (-V)

16 Gain Curves for all Lumis Gain Curves with Error Bars for All Lumis 1.E+05 Gain 1.E+04 1.E+03 Blumi1 Blumi2 Blumi3 Blumi4 Blumi5 Blumi6 Blumi7 Blumi8 1.E HV (-V)

17 Data Acquisition

18 Modified 16-bit HAPPEx ADC The input range, or gain, of the ADC can be adjusted by changing C1 and C2. We end up with two gain settings 1:Integrates with C1 2:Uses C1+C2 Should verify linearity before using.

19 Lowering PMT Pedestal Noise Old Configuration: New Configuration has RC on ground. Pedestal Noise was minimized with R= 10kOhm and C= 1000pF.

20 Lowering PMT Pedestal Noise Old Configuration:

21 Lowering PMT Pedestal Noise New Configuration in Hall A:

22 Examining Long-term Drifts Less than a 1% drift over a period of 2hrs.

23 PREX Test Run January 08

24 January 08 Test Run Used several beam energies/currents on Carbon and Lead targets. New Lumi design was implemented which included a filter box. Some of the goals were to study backgrounds, diagnose noise and understand linearity of Lumi setup (PMTs and ADCs).

25 Test Run Configuration , 3, 5 & 7 see full signal. 2 & 6 have 10% filters & 8 were blinded with aluminum sheets Bottom three were shielded by Lead bricks.

26 Analysis of 08 Run #10296 Data acquired on 01/25/08 with beam current of 60uA on a Thin Lead target. RMS values are regressed with respect to position and current. Lumi ID Reg. RMS [ppm]

27 1 08 RMS Values I (odd)

28 RMS Values II (even)

29 08 Combo Noise V D1 C Sum H D2 X Ave

30 Analysis of 08 Run (cont.) D2 H V D1 Statistics for 4-Lumi combo is ~50 ppm Joining 1+5 and 3+7 gets rid of position noise. When averaging Lumis, width does not scale like 1/sqrt(N). Lumi ID Reg. RMS [ppm] V 55.7 H 62.8 D D C 51.7 X Sum Ave 126.3

31 What are the source of noise? Statistical noise. Blinded Backgrounds. PMT pedestals. Correlated noise. Noise in BCM and BPM.

32 PMT and BCM Pedestal Noise Pedestal noise from BCM is estimated to be ~30 ppm for each Lumi. Each Lumi has its own pedestal noise, which range from ppm. Also found a small 2 CH correlation between PMT pedestals, adding about 20 ppm more.

33 Sources of noise Unshielded Shielded

34 Summary Have found ~100ppm of extra noise due to position. Achieved noise floor of ~50 ppm. Shielding is essential for reducing backgrounds, which kill us when signal is attenuated. Need further tests and analysis Joining runs to get grand averages. Must invest time on OS runs.

35 Extra Stuff

36 Calibrating Input Photocurrent

37 Calibrating Input Photocurrent Is done by using very low-gain tube of order unity. I out plateaus after a certain HV. Calibrated values can be verified by using photon statistics. Can now get gain curves for each PMT.

38 Non-linearity of Board 13 (Left) ADC response to CW LED w.r.t. Integration Time. (Center) Residuals after fitting data above 25K CHs. (Right) Residual Percentages.

39 16-bit ADC Linearity Gross non-linearity is no longer a problem.