# Cosmo ArduSiPM: An All-in-One Scintillation-Based Particle Detector for Earth and Space Application
## 3 Overview of the Discrete Analog Functional Blocks of ArduSiPM
### Optical Test
#### 4.2.1 SiPM Dark Count Evaluation and Dependency of the Threshold  
The dark count is an important feature of SiPM. Due to the discrete nature of the SiPM signal, the switching on of each SiPM cell gives an additional and quantifiable contribution to output signal amplitude. This effect is magnified thanks to the LNA amplification.  
Based on Cosmo ArduSiPM applications, it is very useful to know the expected dark count rate as a function of the selected threshold, as well as the number of SiPM cells that must switch on simultaneously to reach the overthreshold condition. To study this dependence, the SiPM was placed inside the lightproof box, at room temperature (25 \({}^{\circ}\)C), without any light source [22]. A custom Python 3.7 script adjusts the 12-bit DAC value within the range of 0 to 1000 while measuring the time required to reach at least 400 threshold-crossing events. For threshold values where the dark count rate exceeds 400 Hz, the measurement duration is set to 1 s; this approach ensures the attainment of a statistically substantial sample size maintaining a measurement error below 5%. The process aims to determine the repetition rate of dark counts. The last value of 1000 for the DAC was chosen based on the time required to reach 400 events that grow with the threshold. In Figure 12, the result of this measurement is presented.  
As is evident, there is a clear separation between dark count events switching on one or more cells of SiPM. Whenever the threshold becomes greater than the typical amplitude of one single cell signal, we can see a steep decrease in the counting rate, followed by a flat zone corresponding to a double amplitude of the signal. The separation is quite constant for each additional cell.  
Figure 11: Experimental setup for optical test, including LED head and SiPM placed in the lightproof box. Below, the LED light reflecting on the white holder.  
This measurement facilitates the selection of a threshold value within the plateau region between consecutive inflection points, enabling the identification of events associated with the simultaneous activation of a specific number of SiPM cells, thereby achieving stability against gain fluctuations. Thanks to this procedure, we can easily characterize the dark count of the utilized SiPM and determine the optimal threshold based on the application requirements. Additionally, if necessary, we can subtract the expected dark count from the final rate measurements. For example, in bioluminescence applications, where we attend to a high number of photons with a flat time distribution, we can use a \"single cell\" (i.e., a single p.e.) threshold to measure the increase in overthreshold events rate [23; 24]. Moreover, as a cosmic ray counter, where we expect a low rate of events yielding a lot of photons inside the scintillator, a threshold that allows one to overlook the dark count rate is preferable. In particular, the presence of a scintillator coupled with the SiPM would cause the count rate to be dominated by the cosmic rays rate for thresholds greater than three SiPM-lit cells (i.e., three photoelectrons).