1. Hellenic Open University
Physics Laboratory
School of Science and Technology
Hellenic Open University
George Bourlis
PMT Readout and Floor Triggering
Charge estimation using the times over the thresholds
Event Building and Triggering +
multiplicity
My talk is about the ReadOut Electronics developed for the HELYCON Experiment and the Time Over Threshold Technique adopted for the charge estimation of the photomultipliers. I will also briefly refer to *** Event Building and Triggering ***

2. ReadOut Electronics 5 PMT Signal Inputs Trigger Output USB Port HPTDC
32 channels (LR) – 8 Channels (HR)
25ps (HR) to 100 ps (LR) accuracy
Self Calibrating
Let’s start with a schematic of the ReadOut Electronics. The card bears 5 signal inputs. The signals are amplified and then go through a set of comparators. The outputs of the comparators are driven to the heart of the system, which is a High Precision Time to Digital Converter developed at CERN. The TDC’s outputs are then led along with the GPS signal to the FPGA (a Xilinx one) which includes filter algorithms, trigger logic and performs the event building algorithms. The communication between the FPGA and the hosting computer is handled by a Cypress USB Controller.
The HPTDC has an accuracy of 25ps with 8 channels or 100ps with 32 channels.
The outputs of the HPTDC is the times the input pulses cross the 6 programmable thresholds in either direction (leading and trailing edge) and our aim is to estimate the charge of the pulses using this information.
25ps accuracy TDC
GPS Input

3. ReadOut Electronics
This is a snapshot of the card, where u can see the HPTDC, the FPGA and the USB Controller along with the USB port of the card.

4. EZ-USB FX2 HPTDC CONTROL
This is the interface of the program we currently use to communicate with the electronics and perform data acquisition. We can enable or disable channels and set the thresholds for each channel (there are 6 channels for each input corresponding to the 6 thresholds).

5. These are results of the initial tests performed
These are results of the initial tests performed. You can see a pulse produced by a function generator and the reconstructed pulse using the times of the threshold crossings.

6. Card Debugging Configuration
Offset of each input
Scale of each input
Timing among input channels
Trigger
Input
splitter
discriminator
Currently the card is under extended testing aiming at determining the offset of each of the channels, the exact value of the factor by which each input signal is amplified and determining the timing among the input channels. The current version of the card bears different multiplication factors for each channel, so that we can figure out the best option. In particular, we need to balance the desire for a higher factor so us to be able to reconstruct large pulses and that for a small factor that will allow the reconstruction of small pulses.
I should mention here that no algorithms has been yet programmed in the FPGA, which is just giving us the times of the threshold crossings. Our current testing configuration is using the signal of one HELYCON detector, whose signal is split and driven to the card and a high precision Tektronix oscilloscope. A discriminator is also used for the triggering of the oscilloscope, whose signal is also led to the card to be used for the identification of pulses that have been recorded by both the card and the oscilloscope.

7. Sample Pulses
Input 1
These are some pulses as recorded by the oscilloscope (the line) and the red dots correspond to the data acquired by the electronics. ***

8. Sample Pulses
Input 1
These are some pulses as recorded by the oscilloscope (the line) and the red dots correspond to the data acquired by the electronics. ***

9. Sample Pulses
Input 1
These are some pulses as recorded by the oscilloscope (the line) and the red dots correspond to the data acquired by the electronics. ***

10. Gain vs HV Calibration @ “nominal” H.V. gain: ~ 4 105
<charge>/p.e. ~ 0.07pCb
<pulse height>/p.e. ~ 1.05mV
Rise Time: 1.2 ns
The Photomultiplier Tube: PH: XP1912
Charge (in units of mean p.e. charge)
At the Detector Center
Data
- Monte Carlo Prediction
Charge (pCb)
Single p.e
Gain vs HV
Calibration
PMT Characteristics

11. Charge versus Time Over Threshold
Charge (pC)
Time Over Threshold (s)
1st Threshold only
1st & 2nd Threshold
1st, 2nd & 3rd Threshold
In order to evaluate the performance of the Time Over Threshold technique for charge estimation, we have used data from a HELYCON detector. The pulses’ times over 3 threshold (4, 15 and 50mV) were determined and here’s the distribution of charge versus time over threshold using only the 1st threshold, using the 1st and the 2nd threshold and using all 3 thresholds.
50mV
15mV
4mV

12. Charge Parameterization
1st & 2nd Threshold
1st, 2nd & 3rd Threshold
Then, the charge was parameterized using a polynomial function of the times over threshold. These are the results using the 1st & 2nd threshold and using all 3 thresholds.

13. Charge Estimation - Estimated Resolution ~10%
1st & 2nd Threshold
11%
1st, 2nd & 3rd Threshold 8%
Here you can the distribution ***
- Estimated Resolution ~10%
- Better if all thresholds are crossed

14. Charge Estimation - σ=(1.01 ± 0.01) (2 thresholds)
1st & 2nd Threshold
1st, 2nd & 3rd Threshold
Charge Estimation
- σ=(1.01 ± 0.01) (2 thresholds)
- σ=(1.1 ± 0.1) (3 thresholds)

15. Time (ns)
Trigger
Input
Configuration for measurements *****

16. Card Debugging Photo

17. Card Debugging photo

18. Card Debugging photo