# A Review of Requirements for Gamma Radiation Detection in Space Using CubeSats
## 3 Gamma Radiation Detection Strategy and Reference Detector
### Photosensors  
To scrutinize and select the appropriate photosensing technology for the mission, the detector parameters must be carefully evaluated and contextualized based on the typical constraints (power consumption, size and weight, reliability, and compatibility to space environment) posed by CubeSat missions. A typical choice is the use of standard, compact Photomultiplier Tubes (PMT). Recently, a more compact, lightweight option has been made available through Multi-Pixel Photon Counters (MPPC) or Silicon Photomultipliers (SiPM). For the purpose of this paper, the two sensing technologies have been evaluated. To obtain an efficient mechanism of light collection and energy conversion, the spectral sensitivity of the ideal photosensor is required to match the emission spectrum of the scintillating target. Nowadays, most of the manufacturers can provide devices sensitive to a wide range of wavelengths. For example, the typical spectral response of the Hamamatsu S13361-6050 NE-04 MPPC series (Figure 6) is in the range of 320 to 900 nm and peaked at 450 nm. Narrower spectral responses are typical of PMTs instead: the Hamamatsu R11265-200 (in Figure 7) is sensitive to 300-650 nm, with maximum sensitivity at 400 nm. Figure 6: The S13361-6050 NE-04 MPPC series manufacturer by Hamamatsu: the device consists of 16 independent cells, each one of 6 mm \(\times\) 6 mm sensitive area. Figure 7: The R11265-200 PMT manufactured by Hamamatsu. In Table 3, the most relevant device parameters are reported. From the optical point of view, both PMT and MPPC offer very similar performances. However, due to the stringent mission requirements, their intrinsic characteristics need to be carefully investigated. The physical properties of the device can play a role in constraining the size and weight of the target. For the reference detector under discussion, a R11265-200 PMT-based readout would take 12% of the available volume, contributing with about \(\sim\)100 g to the overall mass budget, posing severe constraint on the selection of the target in terms of size and weight. The lightness and compactness of S13361-6050 NE-04 MPPCs instead make this device the ideal choice when heavier and larger detection targets are needed. From the mechanical point of view, a MPPC might be the ideal candidate for a gamma-ray detector mounted on a CubeSat spacecraft; however, its electrical and physical properties pose severe limitations on its use. The major concern when operating a MPPC is the Dark Counting Rate (DCR) that can easily be of the order of several MHz per detection channel at room temperature and at operating conditions (typically a few V above the breakdown voltage). Afterpulses (AP) and Crosstalk (CT) typical of such kind of device might contribute in worsening the detection performance too. A further aspect to take into account when operating MPPCs is the variation of the functional parameters with temperature: as a consequence, a temperature-driven feedback is recommended in order to prevent strong gain fluctuations along the various region of the orbit. Although this action can happen at the level of the spacecraft operations by implementing an active system to control and stabilize the payload temperature, the CubeSat power constraint forbids the use of such adaptive control. Therefore, a more sustainable approach is to actively correct the biasing voltage implementing the temperature feedback at the level of payload electronics. Note that DCR strongly depends on the temperature variations and cannot be constrained by implementing actions based on temperature feedback. One additional key parameter to be taken into consideration is the power supply system used to operate the photosensors. In this respect, the use of MPPCs is generally smoother because these devices are typically biased at less than 60 V. The use of PMTs instead is in general considered risky because high voltage is required to operate the device and to reach adequate gains.