Implementing digital QDC in Sci-Compiler

Introduction

Digital charge integration, also known as QDC (charge-to-digital conversion), is a technique used to measure the energy of particles or photons detected by photomultiplier tubes (PMTs) or silicon photomultipliers (SiPMs). It involves converting the analog charge signal generated by the detector into a digital representation for further analysis and measurement. The digital samples obtained from the ADC are then integrated over a certain time window to calculate the total charge accumulated during that period. This integration can be performed using simple numerical techniques, such as summing the digital samples.

This technique to calculate the charge is suitable for fast detector. The charge integration is not a shaper indeed, if the signal is long, pile-up will corrupt the measure of the energy.

Experimental setup

We used the following setup to test the charge integration.

We use Cobalt60 as signal source and a Photomultiplier tube (PMT), to detect the gamma rays.

The PMT is connected to the SCI DK DT1260 board and the HV Power Supply Module.

It is connected to the digitizer DT1260 to channel 1 with a LEMO cable . It is connected to the HV Module, the DT5533E, to channel 1 with a BNC cable.

Both the modules are also connected to the computer. The digitizer is connected with a micro USB cable. The HV Module is connected with a USB type B cable.

SciCompiler firmware

We use in particular these boxes:

Trigger LE

The Trigger LE block generates an output signal when the input signal amplitude exceeds a threshold. The threshold can be specified in the Treshold entrance.

The output signals are Delayed Data, Trigger and TOT (Time Over Threshold).

• the Trigger output generates a signal every time an input signal exceeding the defined threshold (size 1);
• the TOT output stays HIGH for all the time that a trigger signal exceeds the threshold (size 1);
• the analog Delayed Data output reports the input signal with a delay (size 1..32).

Charge Integration

The Charge Int block performs the charge integration of the input signal IN.

The inputs are:

• Trigger indicates the presence of a signal whose area has to be measured, so it is used to start the integration process;
• Int Time represents the time window for the signal integration, that has to be set properly in order to cover all the signal length;
• Pre Int input indicates how much time before the trigger signal the integration window has to start. This is necessary because the trigger could occur during the rise time of the signal to be studied. In this way all the signal will be integrated;
• Pile-up Inib input indicates the time interval during which the integration process is inhibited if another trigger occurs. This gate starts from the trigger signal and allows to discard the events occurs during its length. In this way the area information will not be affected by an error;
• Gain input indicates the factor of digital gain that has to be applied to the resulting signal area;
• Offset input indicates the offset to be added to the resulting signal area;
• Baseline input indicates the baseline value to be subtracted to the total signal area.

The outputs are: Energy, Energy VLD, INT, PILEUP and BUSY.

• ENERGY is the result of the integration process, is the area of the input signal;
• ENERGY VLD is a digital output that indicates when the integration process has produced a result;
• INT indicates the integration gate;
• PILEUP indicates when a pileup event occurred during the integration;
• BUSY indicates when the integration calculation is occurring.

Baseline Restorer

The Baseline Restorer block performs the baseline calculation of the input signal IN. The baseline is calculated by averaging the signal values for a certain amount of time.

The baseline calculation starts when the input signal TRIGGER is HIGH. The LENGTH input signal indicates the number of samples to be averaged, while the HOLD input signal indicates the time interval for which the baseline is not calculated (for a correct baseline calculation the HOLD should be equal to the signal duration).

The BASELINE outputs are Baseline, Valid, Run/Hold and Hold Time.

• BASELINE is the signal of the baseline value;
• VALID output indicates that a signal has been processed;
• RUN/HOLD output is HIGH when the baseline calculation is occurring;
• HOLD TIME output indicates the time in which the baseline is not calculated.

The example shows how to integrate the analog input signal to measure its energy.

At first, we put an Analog Input box and select A0 as the analog signal input. It is connected to a Polarity Inverter box (input bins: 16, effective # bins: 12): the signal of the PMT has a negative polarity so we invert it, because it is easier to work with positive signals. We add a Register Read box (pol) to set the inversion.

The output signal of the polarity inverter is connected to the Oscilloscope box, to an analog entrance (A0), and to the Trigger LE box, to the In entrance.

To set the threshold of the trigger, we put a Register Read box (trgthrs).

The signal to be integrated is given by the Delayed Data output. This is connected to the Baseline Restorer box and to the Charge Integration box, both to the IN entrance. This output is also connected to the Oscilloscope box, to an analog entrance (A2).

The Trigger output gives the trigger to both Baseline box and Charge Integration box, to start the process. It is also connected to the Oscilloscope box tho a digital entrance (D0_2) and to Start entrance to give the external trigger to the Oscilloscope.

The baseline is calculated for a number of samples giving by the Register Read box (Nsamples). The time window for which the baseline is not calculated (Hold) is given by the Register Read box (time_hold_bs).

The baseline value is provided to the Charge Integration box to be subtracted to the signal area (to the Baseline entrance), and to the Oscilloscope box to be displayed (to the analog entrance A1). Also the Valid and the Run/Hold outputs are connected to the Oscilloscope to the digital entrance D0_1 and D1_1, to see if the signal is processed and when the baseline calculation is occurring.

The Charge integration is performed when the trigger signal arrived. To set all the parameters of the integration, we use some Register Read boxes: time_integr, to set the integration interval, pre_integr, to set the pre-integration gate, gain and Offset_int. The pileup rejection time is by default 0.

The resulting value Energy is connected to the Oscilloscope box to the analog entrance (A3), to a Spectrum box and to a List box.

Also the output Energy VLD, INT and PILEUP are connected to the Oscilloscope box to the digital entrances D0_3, D1_3 and D2_3.

The Spectrum block calculates the histogram of the input values and stored it in the internal memory. This box receives the Energy signal and the Energy VLD signal. The Energy VLD signal enables the data writing.

The List box represents a list of data that could be buffered and downloaded with the specific List Module tool in the Resource Explorer. It receives the Energy signal as input.

Readout data with Resource Explorer

Opening Resource Explorer, it is possible to view the oscilloscope, to set the parameters of the registers, to download the data from the List and to see the spectrum of the data.

We set the polarity (pol) as 1, to invert the signal. We set the threshold of the trigger (trgthrs) al 2000, to see an effect on the signal. We set the trigger sources of the oscilloscope to External, so we get the same trigger as the Trigger LE box.

We set the number of samples for the baseline calculation (Nsamples) as 7. This parameter is the exponent of 2, so the number of samples is 2^7 = 128. We set the time for which the baseline is not calculated (time_hold_bs) as 100, to exclude all the signal from the calculation.

We set the integration time (time_integr) as 76, to integrate the signal for 76 samples. we set the pre-integration time (pre_integr) as 5, to start the integration 5 samples before the trigger. we set the gain (gain) as 10000 and the offset (offset) as 0, to have the signal area in ADC counts.

To see the histogram of the data, we click on the Acquire button and the following window will be shown.

It is possible to set the Rebin factor (16384), as the number of channel to be used. The acquisition mode can be set as Free, Counts end Time. In the field Refresh it is possible to set the refresh rate of the spectrum. The spectrum can than be read as a standard memory and each bin is represented by a word.

To get the data from the List, we click on the Download button and the following window will be shown.

In this tool, it is possible to select the Format in which the data will be downloaded (decimal or binary). The fields File Name and File Folder allow to specify the name and the path of the file downloaded.

The number of data in words is specified in the Target Size field. It is possible to set the unit from the menu on its right side (word, kW, MW). In the Packet Size field it is possible to set the number of words.

The download process can be started and stopped with, respectively, the Start and Stop buttons.

In the chosen directory, a file “.txt” will be created with the data.

Readout data using SciSDK

It is possible to read and to plot the data using the following code.

Python

In the Python directory there are the following directories:

• Read_spectrum: it contains a code to read the spectrum data. Set the registers and the spectrum parameters. It waits for the acquisition to get some data (50s). Then it reads the data and save the spectrum data in a file spectrum_read.txt with two columns: the first one is the channel number and the second one is the number of counts.

• Read_list: it contains a code to read the list data. Set the registers and the list parameters. It allocates the list buffer. Then it reads the data of the list and save them in a file list_read.txt with two columns: the first one is the progressive number, from 0 to info.valid_samples/4, and the second one is the number in the list. In the terminal it is possible to see the number of valid samples.

• Plot_chargeintegration: it contains the following files:

  • Plot_spectrum.py: it contains the code to set the registers and the spectrum parameters. With matplotlib, it plots the spectrum. In the window it is possible to start, stop and restart the acquisition; and also to set the parameters of the registers. To actually change the parameters, it is necessary to press “enter” after the value is set.
  • Plot_spectrum_PyQt.py: it is the same of Plot_spectrum.py but with PyQt5.
  • Plot_oscilloscope.py: it contains the code to set the registers and the oscilloscope parameters. With matplotlib, it is possible to plot the oscilloscope. In the window it is possible to start and stop the acquisition; and also to set the parameters of the registers. With a checkbox it is possible to set which chanel is plotted. Every channel has an analog signal and some digital signals.
  • Plot_spectrum_oscilloscope.py: it is a combination of the three previous files: with PyQt5, it is possible to plot the spectrum and the oscilloscope in the same window; start, stop and reset the acquisition; set the parameters of the registers; and choose which channel to plot in the oscilloscope.

• Spectrum_Analysis: it contains:

  • Spectrum_PyQtgraph.py: it contains the code to plot the spectrum and the oscilloscope channels in the same window. It is possible to set the Parameters of the registers, and to save the changes it is necessary to press “Save State” button. It is possible to start, stop and reset the acquisition of the spectrum. With a checkbox it is possible to set which channel is plotted in the oscilloscope plot. In the “Lines” menu there are two line with a checkbox; if it is checked a “LinearRegion” is plotted in the spectrum plot. It is possible to move and to change the dimension of the region. In the region a gaussian fit is made and ploted. Under every line there are the parameters of the fit (centroid, sigma, resolution). Int he last menu “Peaks” there are two writeable boxes to set the value in keV of the two centroid of the line regions. Pressing “Calibration” button the calibration is made and next to each parameters of the fit there is the value in keV.

  • Charge_integration_class: it is a directory that contains three files:

    • Main.py: it is the main file that contains the code to plot the spectrum, the oscilloscope and to made the analysis. It made the same things of the file Spectrum_PyQtgraph.py. It is also possible to add as many LinearRegion as you want. In the “Peaks” menu it is possible to choose which line use for the calibration with a dropdown menu; set the value of the centroid in keV and press the button “Calibrate” to calibrate the spectrum. Unlike Spectrum_PyQtgraph.py only the spectrum plot and the analysis menu are in the main window.

    

    

    To plot the oscilloscope channels and to set the parameters it is necessary to press the “oscilloscope” button. This program is based on different classes defined in the other two files.

    • Oscilloscope_parameters_class.py: it contains two classes: channels_class and registers_class. The fist one has a dictionary in which all the channels are defined. The constructor generate a Group of parameters as a menu and set the legend in the plot. This class has “get” methods for every channel, and a method to “update” the channel value in the dictionary. The second class constructor takes as inputs the dictionary with the value of the registers and the sdk; it generate a Group Parameter. It has two methods: “update” to change (not save) the value of the registers and “save” to only save the state when the “Save State” button is pressed.
    • Region_class.py: it contains five classes:
      • Analysis: it is a class for the analysis of the spectrum. The constructor generate a group parameter with centroid, sigma and resolution. The class has “get/set” methods for every parameter. It is used associated to a LinearRegionItem in the Line class.
      • Line: it is a class for the lines of the spectrum. The constructor generate a Simple parameter. Every line has an associated Analysis class; a plot, which is the fit of the spectrum; a peak, which is the peak of the gaussian fit; an interval which is the edge of the LinearRegionItem associated to the line. It has “get/set” methods for every parameter. It is used to generate a Group parameter in the Region class.
      • Region: it is a class for the regions: LinearRegionItem. It generate a Group parameter of Line class. It has a plot associated to it, which is the plot of the spectrum. It has two methods: addLine, to add a line in the list, and updateLines, it creates the LinearRegionItem.
      • Peak: it is a class for the peaks of the spectrum. It generate a List parameter with the name of the possible lines to choose to calibrate the spectrum. It has a region associated to it, which is a LinearRegionItem. Every item in the list is associated to a Line class. It has three methods: “update_limits”, which update the list of the possible lines; “on_list_param_change”, which is the method called when the user chooses a line; and “change_peak_value”, which sets the value of the peak with which do the calibration.
      • Peaks: it generate a Group parameter of Peak class. It has two Peak parameters, “Peak 1” and “Peak 2”, and the “Calibrate” button. It has two internal parameters, a and b, which are the parameters of the linear calibration. It has a method “calibrate” to calibrate the spectrum using the two peaks.

C#

In the C# directory there are the following directories:

• ReadSpectrum: it contains a code to read the spectrum data. Set the registers and the spectrum parameters. It waits for the acquisition to get some data (50s). Then it reads the data and save the spectrum data in a file spectrum.txt with two columns: the first one is the channel number and the second one is the number of counts.
• Read_Oscilloscope: it contains a code to read the oscilloscope data. Set the registers and the oscilloscope parameters. Then it reads the data and save the oscilloscope data in four different files: analog.txt, digital0.txt, digital1.txt and digital2. Each file has four columns, each for a different analog channel.
• Plot_spectrum: it contains the code to plot the spectrum. It is possible to set the parameters of the registers and of the spectrum. The spectrum is plotted with OxyPlot. It is possible to start, stop and restart the acquisition.
• Plot_Oscilloscope: it contains the code to plot the oscilloscope. It is possible to set the parameters of the registers and of the oscilloscope. The oscilloscope is plotted with OxyPlot. It is possible to start and stop the acquisition. With a checkbox it is possible to set which chanel is plotted. Every channel has an analog signal and some digital signals. It also possible to set the parameters of the registers from the window.
• Plot_ChargeIntegration: it contains the code to plot the spectrum and the oscilloscope in the same window. It is possible to set the parameters of the registers, the spectrum and the oscilloscope. The spectrum and the oscilloscope are plotted with OxyPlot. It is possible to start, stop and restart the acquisition. With a checkbox it is possible to set which chanel is plotted. Every channel has an analog signal and some digital signals. It also possible to set the parameters of the registers.

C++

In the C++ directory there are the following directories:

• Read_Spectrum: it contains a code to read the spectrum data. Set the registers and the spectrum parameters. It waits for the acquisition to get some data (50s). Then it reads the data and save the spectrum data in a file spectrum_one_time.txt with two columns: the first one is the channel number and the second one is the number of counts. After that it enters in a while loop to read every second the data and save them in a file spectrum.txt with the same two columns. The loop stops when the user press the ‘Q’ key.
• Read_Oscilloscope: it contains a code to read the oscilloscope data. Set the registers and the oscilloscope parameters. Then it reads the data and save the oscilloscope data in four different files: analog.txt, digital0.txt, digital1.txt and digital2.txt. Each file has four columns, each for a different analog channel.