The heart of the data acquisition system is the desktop digitiser DT5780 of the company CAEN shown in figure 3.10. The acquisition software that was developed for this work is based on Cern’s ROOT–framework which enables efficient data handling, also for high data rates.
The DT5780 is basically a 14–bit 100 MSmp/s digitiser with a field programmable gate array (FPGA) which allows on-line digital pulse height analysis (DPHA). It has two independent channels and, especially for usage with scintillation and semiconductor detectors, two programmable high voltage power supplies (up to 5 keV) plus two Sub–D connectors for powering the pre–amplifiers. The DPHA of the system assigns a time stamp and a channel number to the incoming signal in accordance with the collected charge in the detector which is proportional to the deposited energy. The advantage of a digitiser is that the system can also be used as a digital oscilloscope as it records the waveform of the input signal. This is a useful feature for the optimisation of the parameters for the DPHA.
The digitiser also supports safety functions such as the disabling of the high voltage supply if the detector temperature exceeds a critical value. This provides an effective protection of the sensible HPGe detector, especially its field effect transistor (FET) which is located in the evacuated detector capsule.
Figure 3.10: Shown is a rear– and front–view of the desktop digitizer CAEN DT5780 which was used for data acquisition1.
1http://www.caen.it/documents/work_EcommerceProduct/756/MCA_G.jpg
3.2 DATA ACQUISITION 25 The software for data acquisition, data handling, presentation and the user interface was developed in the frame of this work for a Microsoft Windows® platform using Visual Studio 2012 (VC++2011) in the programming language C++. The software for the file container, graphical representation during the measurements and the data management was developed using the classes supplied by the ROOT [19] framework. It enables highly efficient data handling and automatic data analysis. The software is modularly designed allowing easy extensions and modifications afterwards. An overview over the modules and data exchange between these modules is given in figure 3.11.
Online Hist
Online Scope
Status Display DAQ
Setting Dialog Parameter
File-Containter GUI
Digitizer USB
Time & Energy
Figure 3.11: Simplified data stream diagram of the data acquisition software.
The class module DAQ manages the connection to the digitizer and the entire com-munication like update of settings for the signal processing, enabling or disabling the high voltage supply and also the acquisition and management of the incoming data. The digitiser supports the connection of peripheral by USB or PCI optical link bridge which is also supported by the class module DAQ. An uninterrupted data stream was realised by the run of different parts of the DAQ module on individual threads. The data acquired is directly written into a ROOT TTree structure wrapped in a TFile container with time stamp and channel number of the event.
The class DAQ was developed with the intention to provide modularity so that it can be used as a basis for other custom acquisition applications with hardware from CAEN.
For this purpose, relevant register settings of the digitizer were wrapped into class methods with human readable parameters.
The class modules Online Hist, Online Scope and Status Display are mainly graphical representation classes which are not involved in processing the raw data, and
hence, only act as receivers. The graphical interfaces of the software are shown in figure 3.12.
Online Hist generates the frequency spectra in real time during the measurement.
It shows the number of γ–counts as a function of the channel number. A monitoring of the temporal change of the spectra is possible at any time during the acquisition.
To avoid a procedural interference with the data acquisition and utilise the capacity of multi–core processors, the frequency spectra is also generated in an own independent thread. Furthermore, a tool–box is available which enables fitting various functions to the recorded frequency spectra. The frequency spectra can be saved at any time to a file in various file formats. In addition, the x–axis of the frequency spectra can be calibrated by using a linear function.
The class module Online Oscilloscope enables the monitoring of the waveform of the input signal, the applied trapezoidal fit to the input signal. This feature is useful for adjusting the signal processing parameters of the DPHA to a specific connected detector to achieve its maximum performance and energy resolution.
Figure 3.12: A screenshot of the graphical front end of the data acquisition software.
The class moduleSetting Dialogmanages the settings of the digitiser, parameters for the analogue–digital converter, DPHA, high voltage supply, as well as settings for the on-line histogram and oscilloscope. All settings can be stored to a file which can be reloaded by the Setting Dialog. This ensures that measurements can be carried out with the same set of parameter values. The value of the high voltage can be set in steps of100 mV. The voltage ramp for switch–on and shut-down is independently adjustable in steps ranging from 1V/s to (50)V/s. For safety, a maximal allowable voltage and current can be set. If the current reaches the threshold, the high voltage power supply will be ramped down
3.3 ENERGY CALIBRATION 27